ENHANCING THE ELECTRICAL PERFORMANCE OF ORGANIC … · 2015-08-27 · manuscript revisions, the constant encouragement, and the incredible flexibility to pursue my coaching career,

ENHANCING THE ELECTRICAL PERFORMANCE OF ORGANIC FIELD-EFFECT

TRANSISTORS THROUGH INTERFACE ENGINEERING

BY

JEREMY WILLIAM WARD

A Dissertation Submitted to the Graduate Faculty of

WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES

in Partial Fulfillment of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY

Physics

August 2015

Winston-Salem, North Carolina

Approved By:

Oana D. Jurchescu, Ph.D., Advisor

Ioannis Kymissis, Ph.D., Chair

Abdessadek Lachgar, Ph.D.

Akbar Salam, Ph.D.

Richard T. Williams, Ph.D.

ii

Dedication

To Alexis,

My wife and best friend for life!

Partners in Family,

Teammates in Soccer,

Companions in Adventure.

iii

Acknowledgements

My experience as a student at Wake Forest University and as a resident of the city of Winston-Salem, NC

would not have been as productive, educational, or enjoyable without the presence and support from a number

of people and organizations. I worked hard to maintain a strong diversity between school and outside

experiences and in doing so the past five years have been amazing.

First of all, I want to thank my academic and research advisor, Professor Oana Jurchescu. Together, we

both embarked on a new, and uncertain, stage in our careers with which I have grown immensely. Her

dedication in ensuring that I experienced a well-rounded and well-traveled research education was vital to

my growth as a scientist and to the shaping of my mentoring philosophies. Her constant encouragement to

present my work, participate in research experiences outside Wake Forest University, both domestically and

internationally, and her willingness to allow my involvement in the Organic Electronics Research Group

created a rich environment for my development on many fronts. For the countless hours of proposal and

manuscript revisions, the constant encouragement, and the incredible flexibility to pursue my coaching

career, I am deeply grateful.

As a member of the Organic Electronics Research group, we have developed a culture of excellence that

is maintained by all members involved, graduate and undergraduate students alike. I want to personally

acknowledge the individual members of my research group. I want to thank Katelyn Goetz for my initial

introduction and training within the field of organic electronics and for the great Iowa-based banter! I want

to thank Yaochuan Mei for numerous conversations and for being a great teammate on the Intramural Soccer

Championship squad – Physics et al. During the final stages of my research, my projects became closely

aligned with the work of Zach Lamport, for whom I want to thank for the many deep thoughts and discussion

on self-assembled monolayers and spray-deposition equipment fabrication as well as for the constant

encouragement to enjoy the outdoors through golf outings and intense Frisbee play. Each year we, as a group,

dedicated time to being photographed for our website, and for that, alongside much additional laboratory

support and photography guidance, I want to thank Eric Chapman. I want to acknowledge Pete Diemer as

my teammate and accomplice in LabVIEW ventures. Frequently, I found myself enthralled by the

contraptions and ideas that he developed, often times migrating from my own work to his just to chat. I want

to thank Pete for his constant availability for conversation, programming and ‘Blendering’ support, and for

his general presence as an electronics and science aficionado.

Acknowledgements iv

Our research group greatly values the efforts and involvements of all of our undergraduate students and

I hope that I have helped in some way to shape your ideas of science and education. Specifically, I want to

thank the first undergraduate student to trust in my mentoring, Abdul Obaid. While unaware of what he was

getting into upon agreeing to work with me, he diligently pressed on and greatly help to shape multiple

portions of my projects, resulting in two publications. I wish him the best in his continued education and

career. Most recently, I want to thank Hannah Smith for also placing in me her trust to help shape her

undergraduate research career. Her progress has been swift and I thank her in advanced for her likely

continuation and completion of our research ventures.

I want to thank Professor Richard Williams of the Wake Forest University Physics Department,

Professors Akbar Salam and Abdou Lachgar of the Wake Forest University Chemistry Department, and

Professor Ioannis Kymissis of the Columbia University Electrical Engineering Department for their

willingness to be on my dissertation committee.

To the faculty members at my alma mater, Simpson College, I want extend my gratitude. I thank my

academic advisors, Professors Deb Czarneski and David Olsgaard, for their encouragement to consider

graduate studies as the next step in my education and for the numerous letters of reference and referrals that

they have drafted in my support. Professor Rick Spellerberg, it is much to your credit that I became excited

in Simpson College and your convincing words that encouraged me to pursue a dual major. Thank you for

being a strong advocate for me to this point and in the years to come.

Extending beyond the classroom and laboratory, I have spent hours, if not days, at the amazing fields of

BB&T Soccer Park, developing my personal skills as a youth soccer coach and working with some incredible

young athletes. I thank the entire Twin City Youth Soccer Association (TCYSA) staff and community for

their great support along my coaching education progression and in trusting me to develop within their staff.

Specifically, I want to thank the former Youth Academy Director at TCYSA and coaching educator, Nathan

Williams, for instilling in me a coaching mentality and for creating an opportunity for me within the Twins

organization. I have no doubt that my progress as a coach would not be where it is today without him. Also,

I want to thank the executive director of Twins, Scott Wollaston for always supporting my balancing act

between coaching and academics and for constantly helping me to pursue coaching education opportunities.

While I have interacted with many players and families within the organization, I specifically want to thank

the 2000 Lady Twins Blue and 2002 Lady Twins Silver players and families for working with me to grow as

a coach. I hope that they will look back and value the time they spent with me. Finally, within the Twins

organization, I want to thank a great personal friend and fellow coach, Katherine Skarbek, for numerous

‘soccer talks’ and constant reflection of training and match ideas. I wish the best of luck to her as she begins

a new chapter in her career!

Some of the most enjoyable experiences that I have had involved the numerous backpacking trips with

close friends in the Appalachian Mountains of North Carolina and Zion National Park in Utah. Out of these

experiences, I have learned how to disconnect from school and coaching and I have developed a hobby that

Acknowledgements v

I hope to carry forward in my life. I have greatly enjoyed being able to always count on gathering for weekend

game nights, sitting around a fire for s’mores, and creating some amazing carved pumpkins! Thank you to

everyone that has helped me to thoroughly enjoy the last five years of my life and to develop lasting

friendships.

Lastly, I want to thank my family for constantly acknowledging how proud they are of my

accomplishments and for supporting my decisions in life and in academics. First and foremost, I want to

thank my parents, Mike and Kim Ward, for raising me with the values that I maintain on accountability,

responsibility, and for seeing things through to the end. Specifically, I want to thank and congratulate my

mother for her many years of services as a high school vocal educator and for imparting in me an

understanding and respect for the arts. I want to thank my father for the years of coaching he gave to me and

for continuing this passion even after I left. He has impacted the lives of numerous young athletes in the

Norwalk, IA area and in doing so, he has continued to spread “The Beautiful Game”. I want to also thank

him for passing along the Husker traditions and express my hope for the view of many football games to

come. Having been married to my wife, Alexis, for nearly five years and together for an additional ten, her

family has been an integral part of my life. I want to thank Tracy, Carla, Elizabeth, and Emma for their

continued acceptance of me into their family and for the trust they have each placed in me with their family.

I want to thank my Grandma Ward for her travels to the east coast to visit Alexis and I in our North Carolina

home and for helping me to learn the value of being responsible during the many summer trips that I spent

with her in Nebraska. Last but not least, I want to thank my grandparents, Paul and Emily Deutscher. I have

no doubt that I have gained much of my problem-solving abilities, creativity, and work ethic from my

experiences on “The Farm”.

While writing this dissertation, our family experienced a sudden and great loss. I want to take a moment,

now and often, to remember Grandma Judy Kuehl and the amazing impact that she had on everyone around

her, especially on her family. I sincerely wish to carry on the genuine love and heartfelt compassion that she

passed along to everyone she met.

Jeremy William Ward

August 2015

vi

Table of Contents

LIST OF FIGURES AND TABLES ............................................................................................. viii

CHAPTER

1. INTRODUCTION .............................................................................................................. 1 1.1 Progress in Electronics Beyond Silicon ........................................................................ 2

1.2 Development of Materials for OTFTs........................................................................... 5

1.2.1 Organic Semiconductors .................................................................................. 5

1.2.2 Organic Dielectrics ........................................................................................ 10

1.2.3 Solution-Processable Electrodes .................................................................... 13

1.3 Low-cost Manufacturing to Exploit OSC Properties .................................................. 14

1.3.1 Texture Control .............................................................................................. 14

1.3.2 OTFT Patterning ............................................................................................ 16

1.3.3 Large-Area Electronics .................................................................................. 21

1.4 Conclusion .................................................................................................................. 25

1.5 Thesis Outline ............................................................................................................. 26

1.6 References ................................................................................................................... 27

2. ORGANIC FIELD-EFFECT TRANSISTOR FABRICATION AND

CHARACTERIZATION .................................................................................................. 41 2.1 Introduction ................................................................................................................. 41

2.2 Contact Fabrication by Photolithography ................................................................... 41

2.3 Self-Assembled Monolayers ....................................................................................... 43

2.3.1 Composition and Function of SAMs ............................................................... 44

2.3.2 Growth and Characterization of SAMs .......................................................... 45

2.3.3 Interface Energetics and Electrical Characterization of SAMs...................... 46

2.4 Organic Semiconductors ............................................................................................. 48

2.4.1 Growth and Film Morphology of OSC Layers ............................................... 48

2.4.2 Structural Characterization of OSC Layers ................................................... 50

2.5 Electrical Characterization of OTFTs ......................................................................... 51

2.5.1 Figures of Merit of OFETs ............................................................................. 51

2.5.2 Contact Resistance ......................................................................................... 55

2.4 References ................................................................................................................... 57

3. STRUCTURAL PHASE TRANSISTIONS IN ORGANIC SEMICONDUCTORS AND

THEIR EFFECTS ON DEVICE PERFORMANCE AND STABILITY ......................... 61 3.1 Introduction ................................................................................................................. 62

Table of Contents vii

3.2 Experimental ............................................................................................................... 63

3.2.1 Calorimetric and Structural Characterization of the OSC ............................. 63

3.2.2 Field-Effect Transistor Fabrication and Characterization ............................ 63

3.3 Results and Discussion ............................................................................................... 64

3.4 Conclusions ................................................................................................................. 71

3.5 References ................................................................................................................... 72

4. TAILORED INTERFACES FOR SELF-PATTERNING ORGANIC THIN-FILM

TRANSISTORS ................................................................................................................ 75 4.1 Introduction ................................................................................................................. 76

4.2 Experimental ............................................................................................................... 78

4.2.1 SAM Fabrication and Characterization ......................................................... 78

4.2.2 Field-Effect Transistor Fabrication and Characterization ............................ 79

4.2.3 Structural Characterization of the Organic Semiconductors ......................... 79

4.3 Results and Discussion ............................................................................................... 81

4.4 Conclusions ................................................................................................................. 95

4.5 References ................................................................................................................... 96

5. RATIONAL DESIGN OF ORGANIC SEMICONDUCTORS FOR TEXTURE

CONTROL AND SELF-PATTERNING ON HALOGENATED SURFACES ............ 101 5.1 Introduction ............................................................................................................... 102

5.2 Experimental ............................................................................................................. 104

5.2.1 Self-Assembled Monolayer Treatment .......................................................... 104

5.2.2 Field-Effect Transistor Fabrication and Characterization .......................... 104

5.2.3 Structural Characterization of the Organic Semiconductor ......................... 104

5.3 Results and Discussion ............................................................................................. 105

5.3.1 Structural Characterization .......................................................................... 105

5.3.2 Interface Interactions Analysis ..................................................................... 110

5.3.3 Electrical Characterization .......................................................................... 111

5.4 Conclusions ............................................................................................................... 114

5.5 References ................................................................................................................ 116

APPENDIX A: MANUSCRIPT COVER GRAPHICS ........................................................ 119

SUMMARY ................................................................................................................................. 122

LIST OF PUBLICATIONS ......................................................................................................... 125

LIST OF PRESENTATIONS ...................................................................................................... 127

CURRICULUM VITAE .............................................................................................................. 128

viii

List of Figures and Tables

Chapter 1

Figure 1 – Evolution of the field-effect mobility through time ............................................................... 3

Figure 2 – Schematic representation of field-effect transistor architectures ........................................... 4

Figure 3 – Energy-level diagram of organic semiconductor-metal electrode system .............................. 6

Table I – Small-molecule organic semiconductors exhibiting high electrical performance .................... 7

Figure 4 – SAM dielectrics and SAMFET device characteristics ......................................................... 12

Figure 5 – Schematic and device characteristic of non-patterned and patterned OTFTs ...................... 17

Figure 6 – Non-relief patterning process flow ....................................................................................... 18

Figure 7 – Surface-energy patterned blade coating process flow .......................................................... 19

Figure 1 – Differential microstructure-based patterning ....................................................................... 20

Chapter 2

Figure 1 – Schematic representation of OFET fabrication process flow ............................................... 42

Figure 2 – Chemical structure of common SAMs used in OFET fabrication ....................................... 44

Figure 3 – Image of water droplet on untreated and PFBT-treated Au surfaces ................................... 46

Figure 4 – Schematic energy-level diagram SAM-treated electrodes ................................................... 47

Figure 5 – Schematic representation of a Kelvin Probe instrument ...................................................... 48

Figure 6 – Schematic illustration of representative OSC deposition methods ...................................... 49

Figure 7 – Schematic of OFET electrical transfer and transport characteristics ................................... 53

Figure 8 – Circuit diagram and IV characteristics of the Gated-TLM method...................................... 55

Chapter 3

Figure 1 – DSC spectrum of diF-TES ADT .......................................................................................... 64

Figure 2 – Representative current-voltage characteristics of diF-TES ADT device ............................. 65

Figure 3 – Transfer characteristics for each polymorph ........................................................................ 66

Figure 4 – Evolution of linear mobility with temperature ..................................................................... 66

Figure 5 – Evolution of saturation mobility with temperature .............................................................. 67

List of Figures and Tables ix

Figure 6 – Optical micrographs of diF-TES ADT single crystal ........................................................... 68

Figure 7 – Powder diffraction of diF-TES ADT for each polymorph ................................................... 69

Figure 8 – Evolution contact resistance with temperature and transport characteristics ....................... 70

Chapter 4

Figure 1 – Schematic representation of diF-TES ADT and fluorinated SAMs ..................................... 77

Figure 2 – Images used for goniometric analysis .................................................................................. 78

Figure 3 – Schematic representation of the μGIWAXS setup ............................................................... 80

Figure 4 – PM-IRRAS of fluorinated SAMs and neat spectra .............................................................. 82

Figure 5 – Optical micrographs of diF-TES ADT on SAM-treated electrodes ..................................... 83

Figure 6 – Representative μGIWAXS characteristics of diF-TES ADT films in OTFTs ..................... 84

Figure 7 – GIXD of films of diF-TES ADT on SAM-treated gold surfaces ......................................... 86

Figure 8 – GIXD full pole figure comparing (001) peak intensities for each treatment ........................ 87

Figure 9 – Device transfer characteristics and OTFT mobility for each SAM treatment ...................... 88

Figure 10 – Work function measurements for each SAM-treated electrode ......................................... 89

Figure 11 – Energy-level diagram at the interface of SAM and OSC ................................................... 90

Figure 12 – Contact resistance as a function of gate-to-source voltage for each SAM ......................... 91

Figure 13 – Evolution of device mobility with channel length ............................................................. 92

Table I – Summary of measurements .................................................................................................... 93

Figure 14 – GIXD on films of diF-TSBS ADT ..................................................................................... 95

Chapter 5

Figure 1 – Chemical structures of organic semiconductors ................................................................. 103

Figure 2 – Microbeam GIWAXS patterns ........................................................................................... 106

Table I – Summary of crystalline orientations .................................................................................... 107

Figure 3 – Structural map of BC-OTFTs determined from microbeam GIWAXS ............................. 108

Figure 4 – Relative “edge on” orientation for ADT, PDT, and Pn films ............................................. 109

Figure 5 – Schematic drawing of the relationship between OSCs and fluorinated SAMs .................. 110

Table II – Comparison of geometrical relationship between OSCs and SAM surface ....................... 111

Figure 6 – Transfer characteristics of each OSC on each electrode surface ........................................ 112

Figure 7 – Transport characteristics of each OSC on each electrode surface ...................................... 113

CHAPTER 1 1

INTRODUCTION

This chapter was published as Jeremy W. Ward, Zachary A. Lamport, and Oana D. Jurchescu

ChemPhysChem, DOI: 10.1002/ cphc.201402757 (2015), and is adapted and reprinted with permission

(License Number: 3605560738854, John Wiley and Sons Publishing).

Chapter 1. Introduction 2

1.1 Progress in Electronics Beyond Silicon

Progress in the technologies addressing the constantly evolving demands of consumers, driven by

the development of new materials and processing techniques, has opened the door to an array of

new opportunities. The field of organic electronics has emerged as a viable technology to meet very

diverse consumer needs due to the synthetic versatility and the processing flexibility of carbon-

based compounds. One of the vital components necessary for emerging technologies is the organic

field-effect transistor (OFET). OFETs can become active components within active-matrix organic

light-emitting diode (AMOLED) displays, electronic paper, smart bandaging and biological

sensors, radio frequency identification tags (RFID), textiles, etc.[1,2] While of prime interest for

technology, organic materials also raise interesting questions in terms of fundamental science.

Native to most organic systems is the presence of weak van der Waals interactions resulting in

narrow electronic bandwidths, strong interactions between charge carriers and the lattice, high

susceptibility to defect formation and strong dependence on the processes taking place at interfaces

with contacts and dielectrics. The nature of charge transport remains unclear and significant effort

is aimed towards elucidating the microscopic and macroscopic parameters affecting it. This

knowledge is critical in further improving the performance of organic devices.[3, 4]

The electrical properties of organic materials range from insulating to superconducting, with

the majority having semiconducting character.[5] The figure of merit for the organic semiconductors

(OSCs) is the charge carrier mobility μ, which defines the charge velocity in a given electric field.

This parameter is very complex, depending strongly on the materials’ purity and degree of

structural order,[6] but can also depend on the environmental conditions such as temperature,[7] and

the details of the measurement such as electric field strength.[8] While, in principle, all organic

semiconductors could exhibit both p-type (hole) and n-type (electron) conduction, provided that

the electrodes can facilitate injection, most OSCs showed predominantly hole conduction.

Recently, several electron-conducting materials with good environmental and operational stability

were developed. Figure 1 shows the highest reported mobilities for different classes of materials

through time and was adapted and updated from ref. [4]. The largest currently reported mobilities

for solution-processed semiconductors are: 43 cm2V-1s-1 (small-molecule p-channel, blue),[9] 11

cm2V-1s-1 (small-molecule n-channel, violet),[10] 10.5 cm2V-1s-1 (polymer p-channel, green),[11] and

7 cm2V-1s-1 (polymer n-channel, maroon).[12] Similarly, for vacuum-processed OSCs the largest

reported mobilities are 23 cm2V-1s-1 (p-channel, red)[13] and 10 cm2V-1s-1 (n-channel, black).[14] Note

that the best n-type performance was achieved in ambipolar transistors.[12, 14] Several recent reports,


however, suggest that some of these values may be overestimated owing to the fact that standard

mobility extraction models were used in conjunction with non-ideal current-voltage characteristics

in the corresponding devices.[15, 16] Nevertheless, the significant increase in the electrical

performance with time for each class is clear, and this is a result of the development of new

materials and device structures.

Single crystals provide an effective platform for the investigation of the intrinsic properties of

materials, and while they generally offer the best performance achievable by a certain compound

because of their high purity and minimal defect density, they are not capable of facilitating high-

throughput, low cost, and market-ready devices.[17, 18] Thin films, on the other hand, are compatible

with large-area and fast-deposition methods, but typically exhibit a lower degree of order, resulting

in inferior electronic performance. For this reason, developing and implementing fabrication

methods that allow better control of the organic film quality and uniformity are of key importance.

Thus, in addition to progress with material design, intense effort is also dedicated to designing

novel device structures which take advantage of the unique fabrication opportunities offered by

organic compounds and allow processing at amiable conditions on flexible substrates through

large-area-compatible fabrication techniques, without compromising material performance.

All field-effect transistor devices included the same functional layers, depicted in Figure 2,

consisting of a gate electrode (black), semiconductor (orange), source and drain electrodes (gold)

Figure 1 Evolution of the field-effect mobility of p-channel and n-channel transistors. The

highest mobilities for solution-processed organic semiconductors are shown in blue (small-

molecule p-channel), violet (small-molecule n-channel), green (polymer p-channel), and maroon

(polymer n-channel). Vacuum-processed OSCs are shown in red (p-channel) and black (n-

channel). Adapted and updated from ref. [4].


to inject charges into the semiconductor layer, and an insulating gate dielectric (grey). Depending

on the order by which each of these layers is deposited onto a substrate (blue), four different

structures can be acheived, summarized in Figure 2. Most devices included in this work will focus

on the structure similar to the one presented in Figure 2d, as we exploit the ability to treat the

source/drain electrodes of this structure prior to the semiconductor deposition.

The fabrication of organic thin-film transistors (OTFTs) made entirely using solution-based

techniques is of particular interest because of the simple processing since, in general, most of a

product's final price stems from the steps required to fabricate it, not from the direct cost of the

materials used. This chapter also outlines the current state of the field of solution-processed organic

thin-film transistors, with an emphasis on the simplicity of processing. In Section 1.2 we discuss

the development of materials that form the electronically active layers in OFETs. In Section 1.3 we

focus on the device design and fabrication while providing several examples of techniques used to

take full advantage of the versatility offered by the organic compounds.

Figure 2 Schematic representation of the possible field-effect transistor geometries. Labels of S

and D denote the source and drain electrodes, respectively. The structures are denoted as (a)

Top-gate, top-contact (TGBC); (b) Bottom-gate, top-contact (BGTC); (c) Top-gate, bottom-

contact (TGBC); (d) Bottom-gate, bottom-contact (BGBC).


1.2 Development of Materials for OTFTs

The list of organic materials currently available for the fabrication of optoelectronic devices has

grown substantially in recent decades. Their rich electronic properties allow their use as conductors,

semiconductors, and insulators in a variety of device architectures. Manufacturing processes taking

advantage of a material’s solubility include inkjet printing,[19–21] spray deposition,[22–25] and roll-to-

roll processing,[26, 27] among others, but require deposition from inks, which presents challenges

related to the control of morphology and uniformity. Specific approaches to address these

challenges are discussed in Section 1.3.1.

In addition to the materials needed for the construction of the OFET structure, materials such

as self-assembled monolayers (SAMs), additives, orthogonal solvents, etc. are often used in the

fabrication and manufacturing process to create optimal semiconductor film morphology and

texture,[28–32] good injection, improved environmental stability, or film patterning.

1.2.1 Organic Semiconductors

Available OSCs are composed of two classes of materials: small molecules and polymers. Each

class, in turn, is divided into three categories: p-type, n-type, and ambipolar. The tendency of a

material to conduct positive charge carriers (hole, p-type) or negative charge carriers (electron, n-

type) is dependent upon both the choice of semiconductor and electrode material. The energy-level

diagram presented in Figure 3 depicts the pathway for charge carriers in p-type (left) and n-type

(right) transistors. While the injection of charges from the electrode into either the highest occupied

molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) of the

semiconductor is, in principle, possible, most OSCs operate in unipolar mode. This implies that the

charge injection of one carrier type is often more efficient becasue of a lower energetic barrier and,

thus, determines the type of device operation.

Here, our discussion will be limited to small-molecule OSCs; for a review on polymeric

semiconductors, please refer to refs. [33–35]. Since the first reports of vacuum deposited p-type

small-molecule OSCs by Kudo et al. in 1984, the field-effect mobility has shown great

improvements from ~1.5 × 10−5 cm2V-1s-1 [36] to greater than 10 cm2V-1s-1, as we will detail below.

The onset of solution-processable small-molecule OSCs was realized in 1999 by Herwig and

Müllen as a multi-step process of depositing a pentacene precursor and subsequently converting

the film into an active pentacene layer through heating from 130 to 200 °C.[37] Since then, the


availability of p-type solution-processable small-molecule OSC materials has expanded

significantly, and n-type OSCs were also developed. The chemical structures and typical mobilities

of a few notable compounds are summarized in Table I.

p-Type Small-Molecule Organic Semiconductors

In the late 1990’s, Anthony et al. synthesized, for the first time, triisopropylsilylethynyl pentacene

(TIPS Pn).[38, 39] The addition of substituent groups to the pentacene backbone made the acene

soluble in common organic solvents, allowing for low-cost solution deposition of this compound,

shown in Table I. In subsequent years, this quickly became a benchmark material, and devices

based on it were fabricated using methods such as spin coating, both traditional and off-centered,[9,

31, 40] drop casting,[31, 41] solution shearing,[42] FLUENCE,[43] electrospray deposition,[44] and inkjet

printing.[45] Different modifications of the silylethyne-functionalized acenes have subsequently

been used to fine-tune crystal packing and enhance electrical properties.[46–50] In parallel, this

synthetic platform has been extended to produce an entire set of acene and hetero-acene p-type

semiconductors with varying substituent groups,[24, 38, 51–55] for example, the fluorinated

triethylsilylethynyl-substituted anthradithiophene (diF-TES ADT),[24, 56] triethylgermylethynyl-

substituted anthradithiophene (diF-TEG ADT),[57] and difluoro-(tri-sec-butylsilyl ethynyl)

Figure 3 Energy-level diagram in p-type transistors (left) and n-type transistors (right).

Alignment between the Fermi level (EF) of the electrodes and semiconductor result in a curvature

of the semiconductor’s lowest unoccupied molecular orbital (LUMO) and highest occupied

molecular orbitals (HOMO) near the electrodes. The electrodes’ workfunction (WF) is shown

relative to the vacuum level.


anthradi-thiophene (diF-TSBS ADT).[55, 58] The nature and size of the substituent group affects both

the film texture and the crystalline packing of the semiconductor and therefore alters the material’s

performance. Additionally, Goetz et al. reported on the effect of increasing the backbone length of

partially-fluorinated, stable, soluble heteroacenes.[55] An increase in backbone length yielded a

more favorable crystalline packing and higher mobility: 1.8 cm2V-1s-1 for tri-sec-butylsilylethynyl

pentacenedithiophene (diF-TSBS PDT), compared to 2 × 10-3 cm2V-1s-1 in diF-TSBS ADT.

Other classes of soluble OSC materials have also shown excellent electronic performance. For

example, alkylated derivatives of [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) exhibit good

solubility and efficient charge transport properties.[59] Specifically, the compound 2,7-

dioctyl[1]benzo-thieno[3,2-b][1]benzothiophene (C8-BTBT) has shown excellent electrical

performance when deposited by a wide range of solution-based methods, such as spin-casting,[60]

directional crystallization,[61] and inkjet printing.[62] Minemawari et al. used anti-solvent

crystallization in conjunction with inkjet printing to produce OFETs exhibiting average mobilities

of 16.4 cm2V-1s-1, with a maximum mobility of 31.3 cm2V-1s-1,[62] six times greater than previous

results[63] and more than an order of magnitude greater than its first application in solution-

processed OFETs.[60] Utilizing a new technique called off-center spin-coating, Yuan et al. achieved

a field-effect mobility of 43 cm2V-1s-1 from a blend of C8-BTBT and polystyrene.[9] Additional

R = Ph: DPh-BTBT, μh = 2.0 cm

2V

-1s

-1 R = C8H17: C8-BTBT, μ

h = 43 cm

2V

-1s

-1

R = H: DNTT, μh = 8.3 cm

2V

-1s

-1 R = C10H21: C10-DNTT, μ

h = 7.9 cm

2V

-1s

-1

R = Ph: DPh-DNTT, μh = 3.7 cm

2V

-1s

-1

Class Structure of Benchmark Performance Compounds

p-t

yp

e Functionalized

Acenes

Thienoacenes

n-t

yp

e

Functionalized

Acenes

Perylene Diimides

RRR

R

X

X

X

X

X'

X'

X

X

X

X

X'

X'

X'

CN

R R

NC

PDI-CN2: R = , μe = 1.4 cm

2V

-1s

-1

PDI8-CN2: R = n-C8H17 μe = 0.1 cm

2V

-1s

-1 PDIF-CN2: R = n-CH2C3F7, μe

= 0.6 cm2V

-1s

-1

X’ = F, X = H, μe = 0.03 cm

2V

-1s

-1 X’ = H, X = F, μ

e = 0.2 cm

2V

-1s

-1 X’ = F, X = F, μ

e = 0.1 cm

2V

-1s

-1 X’ = H, X = Cl, μ

e = 0.56 cm

2V

-1s

-1

X’ = F, X = F, μe = 0.1 cm

2V

-1s

-1 X’ = H, X = Cl, μ

e = 0.05 cm

2V

-1s

-1 X’ = F, X = F, μ

e = 0.41 cm

2V

-1s

-1

R = Si/Pr3, μh = 11 cm

2V

-1s

-1 R = Ge/Et3, μh = 2.2 cm

2V

-1s

-1

Table I Small-molecule organic semiconductors exhibiting high electrical performance when

processed from solution-methods. Examples of p-type and n-type compounds discussed in this

review are provided and references can be found throughout the text.


thienoacene-based organic semiconductors, derivatives of BTBT or π-extended versions thereof,

have recently become the subject of intense study and the reader is directed to the excellent

summary included in the work by Takimiya et al.[64] Of particular interest is the compound

developed in 2007, dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT)[65, 66] and its more

recent derivatives C10-DNTT[67] and DPh-DNTT[68] due to their high field-effect mobility and

exceptional stability[69] when incorporated in flexible, low-voltage devices.[70–72]

While these materials display auspicious electrical characteristics, there exist several

drawbacks which must be addressed before their introduction into mainstream electronics. Apart

from the alkylated Cn-BTBT compounds, the solubility of these materials is quite poor, limiting the

possible deposition techniques.[67, 73] Another major impediment associated with these materials has

been the elaborate synthesis steps, along which comes higher cost and lower yield, however new

synthetic techniques have been developed for both symmetric and asymmetric BTBT

derivatives.[74, 75]

n-Type Small-Molecule Organic Semiconductors

In parallel with the development of organic p-type materials, n-type OSCs are also necessary for

integration in complementary architectures for application in logic devices.[76–78] Challenges faced

during the development of n-channel materials have included low Ion/Ioff ratios and large threshold

voltages when characterized in ambient conditions.[79] In the early 2000’s, Katz and co-workers

developed a set of small-molecule compounds based on a naphthalenetetracarboxylic diimide that

performed as n-channel FETs, however, they observed a large decrease in performance when

characterizing the devices in the presence of oxygen.[80] The inability to fabricate devices in ambient

environments greatly minimizes the choice of compatible fabrication techniques and adds to the

processing and manufacturing costs. From a molecular design perspective, the objective is to

overcome these limitations by developing compounds with a high electron affinity and strong

intermolecular orbital overlap to facilitate both ambient stability and high charge injection and

transport.[81] In 2007, Marks, Facchetti, and co-workers realized a breakthrough by developing a

new class of n-channel materials based on anthracene-dicarboximides that showed comparable

performance in both air and vacuum for up to 4 months of exposure to ambient air.[82]

Similar to the development of p-type OSCs, the n-type materials have transitioned from

vacuum- to solution-deposition. In 2007, N,N’-bis(2,2,3,3,4,4,4-heptafluorobutyl)-3,4:9,10-

perylene tetracarboxylic diimide (PTCDI-C4F7) was originally reported with an electron mobility


of 0.72 cm2V-1s-1 and showed minimal degradation after 50 days in air.[83] Through modifications

of the semiconductor-dielectric interface using octadecylsilanes (OTS), the mobility of this

compound was improved to 1.4 cm2V-1s-1.[84] This work also adds the advantage of solution-based

fabrication. In 2012, Qiao et al. showed that direct modification of the π-conjugated quinoidal core

to include small molecules of 2-ethyl-hexyl and 2-hexyl-decyl promoted electron-only transport in

diketopyrrolopyrrole (DPP)-based materials for the first time.[85] Synthesized at Polyera,

fluorocarbon-substituted dicyano-perylene-3,4:9,10-bis(di-carboximide) (PDIF-CN2) and N,N′-

bis(n-octyl)- x:y, dicyano-perylene- 3,4:9,10-bis(dicarboximide) (PDI8-CN2) compounds showed

electron mobility values as high as 0.6 and 0.1 cm2V-1s-1 with excellent ambient stability, when

deposited using spin-coating and inkjet printing, respectively.[86, 87] Examples of compounds that

exhibit n-type behavior and are displayed in Table I.[88] In 2009 Bao and co-workers examined a

set of functionalized acenes and observed the transition from p-type to n-type OSCs through the

addition of electron withdrawing groups.[89]

Ambipolar Small-Molecule OTFTs

Two routes towards achieving simultaneous hole and electron transport are pursued in parallel: 1)

the use of p-type and n-type material blends or bi-layers and 2) the implementation of materials

that exhibit ambipolar conduction when used in conjunction with an appropriate electrode and bias

condition.[90] In 2003 Meijer et al. fabricated OFETs by blending p-and n-type polymeric

semiconductors to facilitate both hole and electron conduction.[78] Shortly after, Rost et al. also

demonstrated an ambipolar device using a bilayer of pentacene as hole-transport and N,N’-

Ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) as electron-transport material

in combination with gold and magnesium electrodes.[91] A hybrid design approach combined the

air-stability and solubility of hole-transporting OSCs with complimentary electron-transporting

characteristics found in oxide semiconductors yielding air-stable performance with hole and

electron mobilities greater than 2 cm2V-1s-1.[92] Another approach has been based on a co-planar

bisphthalocyaninato copper:[93] Shi et al. designed and synthesized a copper complex with energy

levels tuned to the range of an air-stable ambipolar OSC in a single material and fabricated OFETs

using a quasi-Langmuir-Shäfer process for thin-film deposition, achieving electron and hole

mobilities of 1.7 × 10−1 and 2.3 × 10−4 cm2V-1s-1, respectively. The applications based on ambipolar

FETs have expanded to include organic light-emitting transistors (OLETs). OLETs operate

similarly to OFETs with the same three-electrode device structures, however, OLETs use an


electroluminescent, ambipolar semiconductor where holes injected from one electrode recombine

with electrons injected from another electrode to generate photons. A background on the OLETs

can be found in a 2007 review by Cicoira and Santato.[94]

Undoubtedly, the development of new hole- and electron-conducting materials will continue

to promote improvements in the performance of unipolar and ambipolar OFETs. In parallel with

the semiconductor material design, the optimization of the interfaces of the OSC with the contacts

and dielectrics are also important, as we will be discussed in the sections to follow.

1.2.2 Organic Dielectrics in OFETs

The dielectric layer influences the operating voltage of the OTFT and can affect the semiconductor

film formation in the bottom-gate device geometries. One of the most ubiquitous dielectric

materials in both organic and inorganic TFTs is thermally oxidized silicon (SiO2) due to the use of

highly doped silicon as the substrate and subsequent gate electrode. This, however, introduces a

few glaring disadvantages: a relatively high gate leakage current requiring a thick oxide layer,

incompatibility with flexible and large-area electronics, and deposition techniques generally

requiring the use of high temperature and vacuum. To reduce the cost of developing TFTs and

increase their applications, much research has been done in the field of solution-processed polymer

dielectrics.

Polymer dielectrics offer the versatility of a wide range of deposition techniques including spin

coating,[95] spray coating,[96] inkjet printing,[97] and dip coating.[98] There exists a variety of

insulating organic polymers that have been examined in the context of OFETs such as polyvinyl

alcohol (PVA),[99] poly(methyl methacrylate) (PMMA),[100] benzocyclobutene-based (BCB)

polymers,[101–103] poly-4-vinylphenol (PVP),[104] CYTOP,[105] and other fluoropolymers.[106] These

materials have attracted substantial attention because of their outstanding film formation, high

solvent resistance, flexibility, and stability. Due to the range of solvents used and the properties of

this class of insulating polymers, there exists a viable choice for an incredible array of applications

based on solvent orthogonality and device structures. Another advantage held by polymer

dielectrics over inorganic, rigid insulators is that many exhibit fewer dangling hydroxyl groups or

other charge-scattering polar end-groups which have been shown to reduce charge mobility of an

organic semiconductor.[107] An intriguing concept that gained momentum in the late 1990’s is the

idea of using a monolayer of densely packed, chemically adsorbed, and self-assembled molecules

as the dielectric layer in a TFT.[108–110] Such a self-assembled monolayer (SAM) dielectric presents


its own challenges in deposition, as well as in achieving high molecular densities, but these are

outweighed by the overwhelming advantages inherent when using an ultra-thin (e.g. ~2 nm),

covalently bonded dielectric. Devices constructed using ultra-thin monolayer dielectrics can

operate in the saturation regime at a drain-source voltage of approximately 2 V due to the greatly

increased dielectric capacitance: an areal capacitance greater than 1 µF cm-2 can be achieved,[111]

which is two orders of magnitude higher than the value corresponding to a 200 nm SiO2 dielectric

(17.3 nFcm-2). Since the first proof of concept by the Vuillaume group in 2000, significant strides

have been made. In 2004, Halik et al. provided the first example of an OTFT utilizing a SAM

dielectric with a field-effect mobility of 1 cm2V-1s-1 and a sub-threshold slope of 100 mVdec-1 at a

gate-source voltage as low as -2.5 V.[112] Interestingly, the current density through a SAM dielectric

has been shown to increase upon deposition of the organic semiconductor, prompting the search

for a monolayer with a higher packing density which was achieved using a phenoxy-terminated

molecule in order to increase the π-π interactions between molecules and create a pseudo second

layer.[109, 110] They then used the phenoxy-terminated trichlorosilane monolayer on an oxygen

plasma treated silicon wafer with a thermally evaporated pentacene semiconducting layer as shown

in Figure 4, panels a-d. The same group expanded this work to a phosphoic acid-based SAM on

an oxygen plasma-treated aluminum bottom electrode.[113] They demonstrated the feasibility of

their system by creating an inverter using evaporated pentacene as the p-type semiconductor and

evaporated copper hexadecafluoro-phthalocyanine (F16CuPc) as the n-type semiconductor. A

recent example of SAM dielectrics used in OTFTs has shown the further applicability of such

devices with solution-processable semiconductors. Liu et al. used cyclohexyl-terminated

monolayers on the high-k dielectric AlOy/TiOx to achieve field-effect mobilities of up to 2.7 cm2V-

1s-1 for p-type TIPS Pn and up to 1.44 cm2V-1s-1 for n-type 6,13-bis((triisopropylsilyl)ethynyl)-

5,7,12,14-tetraazapentacene (TIPS-TAP) when measured in air.[114] Another innovative use of

monolayer dielectrics involved a bifunctional SAM where one section of the monolayer functions

as the semiconductor and the other segment forms a hybrid dielectric with an oxygen plasma-

treated aluminum oxide layer (Figure 4, panels e-g).[115]

1.2.3 Solution-Processable Electrodes

In many organic electronics applications, RF-sputtered indium tin oxide (ITO) is used as an

electrode because it is highly conductive and optically transparent.[116] However, ITO remains a

costly option due to the vacuum-processing required, as well as the limited supply and rising price


of indium, thus, leading to a concerted effort to find a lower-cost alternative which retains similar

conductivity and transparency in addition to being compatible with large-area fabrication. Solution-

processed conductors offer the unique opportunity to place device electrodes inexpensively and

without the use of vapor-deposited metals observed to damage the soft layers underneath.[117–119]

Possibly the most prevalent of these materials is poly(3,4-ethylenedioxythiophene) polymerized

with poly(4-styrene-sulfonate) (PEDOT:PSS) which is used extensively in OFETs[120] as well as

organic photovoltaics (OPVs).[120–122] This material is water soluble and relatively inexpensive,

although lower conductivities have hampered its development for use in devices requiring

extremely efficient charge transport.[123] Fortunately, various treatments of PEDOT:PSS with

solvents and acids such as dimethyl sulfoxide (DMSO),[124–126] ethylene glycol,[122, 127] sulfuric

acid,[128] zwitterions,[129] ionic liquids,[130] anionic surfactants[131] and combinations thereof, have

been shown to increase the conductivity to within an order of magnitude of that obtained using RF-

sputtered ITO, while remaining transparent and flexible.[132] For example, the conductivity of

unmodified PEDOT:PSS is generally reported between 0.05 - 10 Scm-1[123] while treatments with

ethylene glycol show an increase in the conductivity to as high as 1418 Scm-1[122] and treatments

with sulfuric acid reach even higher to 3065 Scm-1.[128] Efficient solar cells can be fabricated with

Figure 4 (a) Molecular structure of (18-phenoxyoctadecyl)trichlorosilane (PhO-OTS); (b)

Molecular structure of pentacene; (c) Schematic of a OTFT with SAM dielectric; (d) Transport

characteristics (left), transfer characteristics (right); (e) Molecular structure of [12-

(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)dodecyl]phosphonic acid (BTBT-C12-PA); (f)

Transfer characteristics of devices fabricated with BTBT-C12-PA on flexible substrates; (g)

Schematic of SAMFET device. Adapted with permission from refs. [112] and [115].


power conversion efficiencies up to 13.3% due to their already high transparency and this greatly

increased conductivity.[133] A creative use of PEDOT:PSS was introduced by Akkerman et al. to

increase the yields in molecular electronics applications.[134] In order to protect against pinholes

created by vapor deposited top electrodes,[117, 118] they devised a method using a stack consisting of

a layer of gold, photo-lithographically patterned photoresist, a monolayer attached to the gold,

PEDOT:PSS, and a final gold top electrode. This design allowed for the examination of large-area

molecular junctions without subjecting them to the damage generally caused by metallic top

electrodes. Additionally, it has been demonstrated that PEDOT:PSS is compatible with roll-to-roll

processing, a prerequisite for industrial applications of organic electronics. Lim et al. implemented

the method of spray deposition to fabricate PEDOT:PSS electrodes, setting the stage for its use in

large-area electronics.[135]

A viable alternative to PEDOT:PSS exists in the use of single-walled carbon nanotubes

(SWCNTs) and multiwalled carbon nanotubes (MWCNTs).[136–140] A particularly interesting

technique used arrays of CNTs in n-type OTFTs as their source and drain electrodes.[141] Cicoira et

al. designed titanium contacts with CNTs along the edges which resulted in improved charge

injection by allowing charge carriers to more efficiently cross the Schottky barrier at the electrode-

semiconductor interface. These n-type FETs using CNT contacts exhibited mobilities of 2 × 10−2

cm2V-1s-1 compared to 1 × 10−3 cm2V-1s-1 obtained using gold contacts. Random networks of CNTs

can show a high optical transmittance as well as high conductivity,[138, 139] however in the absence

of laborious separation procedures,[142, 143] even purified SWCNTs contain a mixture of metallic

and semiconducting CNTs due to the inherent variability in structure, and the semiconducting

CNTs will absorb light without contributing to the conductivity.[144, 145] This mixture decreases the

overall conductivity of the thin film unless the layer is made considerably thicker, resulting in a

decreased transparency. The costly fabrication and purification required led many researchers in

pursuit of an alternative electrode material. The search for an electrode that is both highly

conductive and transparent continued on to random networks of metal nanowires, generally made

of silver because of their extremely high conductivity. Lee et al. demonstrated that a mesh of silver

nanowires can have similar conductivity and transparency to ITO, leading to a similar power

conversion efficiency in solar cells while remaining solution-processable.[146] A recent result from

Kang et al. introduced the method of spray coating silver nanowire films, which presents the

advantage of being compatible with large-area roll-to-roll processing.[147] Another system that has

received considerable interest is that of organic metals based on charge-transfer (CT) complexes,

which consist of co-crystals of donor and acceptor semiconductor materials.[5, 90, 148, 149] They can


form excellent electrodes, yet the poor solubility of CT complexes has prohibited their use in large-

area electronics. Hiraoka et al. were able to overcome this challenge by inkjet printing the two

soluble components, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ),

sequentially, which then combined on the substrate to form the CT complex TTF:TCNQ.[150] This

method allows for simultaneous deposition and patterning of electrode materials by solution

processing of the CT material, greatly increasing the array of practical applications.

1.3 Low-cost Manufacturing to Exploit OSC Properties

Because of the susceptibility to degradation upon exposure to heat, light and/or many chemical

compounds, the use of methods developed for the inorganic materials processing is not always

suitable for organic compounds. Such materials demand new approaches to achieve appropriate

film texture, effective device patterning, and high quality electrodes. A large number of creative,

cost-effective techniques based on rapid, simple manufacturing schemes were designed and

implemented for the fabrication of organic devices. Techniques such as inkjet printing,[21, 126] spray

deposition,[125] solution shearing,[42] and dip coating,[151] among others, take advantage of the

solution processability and allow film deposition at a low cost per unit volume. Separately or

together, these techniques offer considerable advantage over the energy-intensive practices such as

RF sputtering, thermal or electron beam evaporation, chemical vapor deposition, and atomic layer

deposition which require high temperature, high vacuum, or both.

1.3.1 Texture Control

The nature of the intermolecular interactions found in organic materials allow the opportunity to

modify the film structure by tuning the processing parameters, but also create many challenges to

produce optimized films for efficient charge transport. In addition, in a device configuration one

must consider not only the optimization of each layer, but that the inter-layer interactions at each

interface play a vital role in determining device properties. Different methods were developed to

modify the morphology, microstructure, and crystallinity of the OSC, with the aim of improving

charge transport. One method used to tune crystalline film formation includes the use of SAMs to

influence the microstructure of crystalline domains. For example, increased grain size was observed

in thin films of diF-TES ADT deposited on HMDS-treated substrates, compared to untreated


SiO2.[

152] The larger grains resulted in increased charge carrier mobility and reduced low-frequency

noise. A more sophisticated approach involves manipulation of chemical interactions at interfaces:

by taking advantage of fluorine-fluorine and sulfur-fluorine interactions at the interface of a

halogenated-SAM-treated electrode and a halogenated OSC. In 2008 Gundlach et al. demonstrated

a controlled self-assembly process of the OSC molecules, in which the film microstructure can be

tuned such that it is highly conductive on and in the vicinity of an OTFT electrodes whilst

maintaining insulating characteristics in the region between neighboring devices.[30] The

conductive regions were realized through the growth of molecules in a purely (001) orientation,

which corresponds to the molecules arranging “edge-on”, promoting charge transport in the high

conductivity direction.[153] This is in contrast to the observed mixture of (111) and (001) orientations

found in the regions where such interactions were prohibited. The mechanism for this intriguing

microstructure formation was elucidated by studying combinations of fluorinated SAMs and

fluorinated OSCs that allow fluorine-fluorine and sulfur-fluorine interactions of varying strengths.

This phenomenon was then applied for OTFT self-patterning, as discussed in Section 3.2.[28, 58] In

2013 a similar film morphology was observed by Kymissis et al. for an un-fluorinated OSC (TIPS-

Pn) in combination with the fluorinated-SAM, pentafluorobenzenethiol (PFBT), on copper and

gold electrodes, and this was described in terms of electronic coupling between the SAM and the

OSC.[31]

Polymer additives, including nucleating agents,[45] introduced in the small-molecule OSCs

solutions prior to or during film crystallization have also been shown to improve film morphology.

For example, Madec et al. investigated the evolution of the TIPS Pn film morphology, upon the

introduction of polymer binders, through x-ray diffraction and electrical characterization.[154] They

observed that the mobility of TIPS Pn-based OTFTs, in combination with poly(α-methylstyrene)

(PAMS), amorphous polystyrene (PS), isotactic polystyrene (iPS), and isotactic poly(α-vinyl

napthalene) (iPVN), depends on the weight fraction of the binder up to a 50% concentration. The

roughness of the OSC/binder film was observed to decrease with increased binder concentration.

A few years later, in 2010, Smith et al. observed a decrease in activation energy with decreased

grain size diF-TES ADT blended with poly(triarylamine) (PTAA) explained by deeper trap levels

from higher angle grain boundaries or increased surface roughness.[155]

The methods described thus far have included the chemical modification of a surface or the

addition of materials to the OSC solution to alter the morphology or structure. Much success has

also been found in methods that apply an external stimulus during or after the film formation.

Diemer et al. applied gentle vibration to the substrate during the solvent evaporation, which resulted


in a significant reduction of the density of traps at the OSC/dielectric interface and devices

approaching the performance of single crystals of the same materials, with a mobility of 3 cm2V-

1s-1 and subthreshold slope of 0.43 V·dec-1.[156] They explored such fabrication parameters as the

amplitude and frequency of vibration and determined the parameter space that yields high

performance.

Diao et al. have induced a laminar flow to an OSC solution to make highly-ordered, high

performance, single crystal OFETs with average mobilities of 8.1±1.2 cm2V-1s-1 and as high as 11

cm2V-1s-1.[43] They used a modified solution-shearing method with a micropillar-patterned printing

blade to induce recirculation of the OSC solution to enhance the crystal growth. They attributed the

success of the technique, coined FLUENCE, to the elimination of grain boundaries, improved

domain alignment and a degree of crystallinity.

Post-processing treatments were also employed to improve the morphology of organic

semiconductor films. For example, solvent annealing of triethylsilylethynyl anthradithiophene

(TES ADT) films, has been shown to convert a poor electrically performing amorphous layer to a

high performing, highly crystalline layer.[157] Dickey et al. observed two order of magnitude

increase in film mobility when it was annealed under dichloroethane or toluene vapors and they

observed that the threshold voltage shifts depend on the polarity of the annealing solvent. They

suggest that the solvent vapor penetrates the organic film, altering the surface potential at the

OSC/dielectric interface.

1.3.2 OTFT Patterning

Most solution-deposition techniques yield full-surface coverage, however, incorporation of OTFTs

in transistor arrays require the insulation of individual devices (e.g. thin-film patterning). Patterning

is necessary in order to minimize the cross-talking between neighboring devices, which reduces the

power consumption and improves the Ion/Ioff ratio. Figure 5 shows a schematic and electrical

characteristics for both a non-patterned (left) and patterned (right) array of devices. The patterned

devices show sharper turn-on (red curves) and higher on/off ratios because of the reduced value of

the off-current (see Chapter 2, Section 2.5 for a detailed description of the electrical properties).

For inorganic semiconductors this is accomplished through multi-step photolithography techniques


that involve developing and etching using solvents. These processes, however, result in large

amounts of chemical expense and waste, use complicated alignment procedures, involve intricate

equipment and are often performed using high intensity light sources. While some of these

techniques can be modified to be compatible with organic materials, the availability of solution-

processable organic materials open opportunities for creative, simple solutions to pattern OFETs,

not accessible to their silicon counterparts. Patterning techniques compatible with organic materials

are primarily performed in one of two ways: 1) the simultaneous deposition and patterning of the

material (additive patterning) or 2) deposition followed by patterning (subtractive patterning).

Additive processes typically require fewer steps, simplifying the fabrication process and will, thus,

be the focus of this section. Successful examples include direct evaporation through a shadow

mask,[158] inkjet printing,[62, 159] selectively treated surfaces,[20, 160] and differential microstructure.[28,

30, 31, 58, 153]

Inkjet printing is a technique in which individual droplets from a nozzle merge on a substrate

to deposit and pattern a material. This method has emerged as one of the most competitive

processing techniques in part due to its low-cost, its small environmental impact and its

compatibility with large-area and flexible electronics. First introduced in the fabrication of organic

Figure 5 Schematic representation and electrical characteristics of non-patterned (left) and

patterned (right) OTFTs.


light-emitting (OLED) devices by Sturm and co-workers in 1998[161] and later developed for use in

polymer-based transistors in the early 2000’s,[21, 159] inkjet printing methods have become a

common practice in many research environments. A thorough description of the most important

developments in the inkjet printing technology can be found in refs. [162] and [163] and progress

related to large-area electronics is discussed in Section 3.3.[162, 163]

Challenges associated with inkjet printing are the relatively large limit of printing resolution

due to the tendency of low viscosity inks to naturally spread on surfaces and the inability to print

on hydrophobic surfaces, such as fluoropolymer dielectrics. In 2008, Someya et al. observed a 4-

fold increase in mobility of pentacene-based OTFTs as the droplet size of the silver contacts was

reduced from 15 pl to 1 pl volumes.[164] They further improved the resolution, achieving channel

lengths as small as 1 μm through tuning the applied wave-form to a conducting wire within the

printing nozzle such that sub-femtoliter droplets can be produced.[19]

Another viable method for patterning OTFTs implements selective surface treatments using

SAMs. Lowering the surface energy in regions surrounding OTFTs through SAM treatment has

provided versatile avenues to confining material to localized regions while maintaining resolutions

comparable with other photolithographic techniques.[160] In 2008 Jackson and co-workers

selectively exposed OTS SAMs to deep UV light through shadow masks to obtain regions of

hydrophobic and hydrophilic surfaces ( Figure 6). The organic thin-film deposited from solution

over this surface only adhered to the hydrophilic regions and thus patterning of the OTFTs and

circuits was achieved. The patterned diF-TES ADT devices had FET mobilities ~0.12 cm2V-1s-1,

on/off current ratios > 105, and threshold voltages of -4 to 4 V.

Figure 6 Schematic representation of the non-relief-patterning process flow used to pattern

OTFTs. Reproduced with permission from ref. [160].


The combination of surface selectivity with the use of sintered silver nanoparticles as pinning

agents has produced devices having improved resolution and has made viable the opportunity of

an all-solution based process. This allowed the progress from the fabrication of inkjet-printed

OFETs in which at least one layer was deposited by using other techniques (thermal evaporation,

spin coating, or blade coating) to all-solution processed OFETs,[20, 21, 165] and even a fully-printed

organic transistor by Arias and co-workers in 2014.[20] By combining surface energy patterned

(SEP) blade coating for the electrodes and semiconductor deposition with blanket blade coating of

the gate dielectric and surface tension-guided inkjet printing of the gate electrode, OTFTs of diF-

TES ADT/PTAA blended films were fabricated (Figure 7). The all-printed arrays exhibited

extremely high device-to-device reproducibility with a yield of 96%. In the case of a 1:1

concentration of diF-TES ADT to PTAA, devices exhibited mobilities with an average of

0.68±0.23 cm2V-1s-1 and threshold voltages near -2 V.

Other additive processes for high-throughput patterning of semiconductor films have been

demonstrated. Popular methods include the use of stamping techniques (e.g. PDMS stamping,[166]

printing (μCP),[167, 168] and nanotransfer printing[166]). Generally, stamping uses an element with a

surface relief to selectively apply material. The first demonstration of this technique, by Wilbur et

al. in 1994, produced feature resolutions of ~1 μm and has since been proven as viable approach to

the patterning of OFET components.[169] The simplicity in the μCP method resides in the ability to

create a single high-resolution master that can then be used an unlimited number of times to

accurately recreate the desired pattern.

Another successful patterning method developed recently is based on establishing a differential

Figure 7 (a) Surface-energy patterned (SEP) blading coating process flow; (b) Procedure for all-

printed OTFTs from SEP blade coating of the electrodes and semiconductor, blanket blade

coating of the gate dielectric and surface tension-guided inkjet printing of the gate electrode; (c)

Top view of final OTFT architecture. Reproduced with permission from ref. [20].


microstructure within the OSC films. As prefaced in Section 3.1, the differential microstructure is

achieved through the deliberate selection of SAM-OSC systems that allows or inhibits interactions

capable of templating molecular order. The high mobility regions consisting of (001) oriented

molecules are present on the electrode surfaces and their immediate vicinity (10 - 50 μm).[30] They

are surrounded by low-mobility domains of mixed (001) and (111) orientations that electrically

isolate neighboring devices in OTFT arrays. This patterning technique for the OSC is a purely

additive process that, in contrast to inkjet printing techniques, can generate patterned films through

blanket film techniques such as spin coating. In 2012, through a combination of electrical studies

and structural characterization, the mere presence of a halogenated-SAM was found to be

insufficient in promoting self-patterning of the OSC. On the contrary, specific design features must

be considered in the SAM-OSC combination.[58] Here, the OSC diF-TES ADT was paired with

three mono-fluorinated benzene thiol varieties (Figure 8a) and the per-fluorinated SAM PFBT

electrode treatment. Self-patterning was observed for the case of PFBT and of 4-fluorobenzenethiol

(4-MFBT) and no differential microstructure formation was observed in the cases of 2-

fluorobenzenethiol (2-MFBT) and 3-fluorobenzenethiol (3-MFBT). The effectiveness of this

patterning technique can be seen in the evolution of the field-effect mobility as a function of device

Figure 8 Representation of the interaction between (a) dif-TES ADT and a fluorinated-SAM

treatment allowing two or more interaction pathways and (b) diF-TIPS Pn (Peri) and a mono-

fluorinate-SAM treatment with only one interaction pathway. Optical micrographs of di-F TES

ADT films are shown for a (c) high-mobility (PFBT-treated electrodes) and a (d) low mobility

(2-MFBT-treated electrodes) device morphology. (e) Evolution of the field-effect mobility for

OTFT devices fabricated untreated (black square), 2-MFBT (violet circle), 3-MFBT (blue

triangle up), 4-MFBT (orange triangle down) and PFBT (red diamond) treated electrodes. Panels

a-b) are reproduced with permission from ref. [28]. Panels (c-e) are reproduced and modified

from ref. [58] with permission from The Royal Society of Chemistry.


channel length (Figure 8e) with the onset of two distinct regimes for the PFBT and 4-MFBT

treatments, which is not present for the other treatments. The relationship between fluorinated-

SAM treatments and halogenated OSCs was further investigated with diF-TIPS Pn (Peri) (a 1:1

isomeric mixture of 1,8- and 1,11-difluoro 6,13-bis(triisopropylsilylethynyl) pentacene) and diF-

TSBS PDT (Figure 8b).[28] It was found that the presence of two or more interactions between

halogen sites on the SAM or OSC molecule is necessary and sufficient to promote molecular

anchoring and contact-induced crystallization. These interactions were found to be suitably strong

to induce a preferred texture only when the halogen atoms originating from the OSC and from the

SAM are located closer than about twice the van der Waals interaction distances, highlighting the

importance of tuning molecular interactions in organic-based electronics. Patterning by differential

microstructure is different from patterning by tuning surface energy in that 1) it implies the presence

of a chemical interaction at interfaces and 2) it results in a continuous film, but as the molecules

within this film adopt different orientations depending on the substrate treatment, the film is

electrically patterned.

1.3.3 Large-Area Electronics

Organic large-area electronics can access a variety of consumer applications such as solar cells,

backplanes to light-emitting displays, and sensor technologies. Applications of electronics with

physical dimensions of up to tens of meters, realized through the combination of nanometer- and/or

micron-sized devices implemented into functional arrays, can be made affordable for consumers

by the reduced cost per unit area of organic electronics manufacturing. For example, just the cost

of patterning in traditional silicon-based technologies can be as high as 3.0 $cm-2, which may not

provide an economic route to producing large-area devices.[3] In contrast, organic materials

processing can provide avenues for fast deposition and patterning over large-areas at low cost by

using fast-processing techniques compatible with the highly desired roll-to-roll (R2R) processing.

Roll-to-roll processing is a method of fabricating large-area electronic devices on flexible

substrates that allow for high-throughput device manufacturing. A typical R2R process requires a

substrate to traverse through a series of stages that include the deposition and patterning of

subsequent layers (e.g. electrodes, insulators, semiconductors). While this process is currently more

common for OPVs,[170] recent advancements in the development of components necessary for R2R

OTFTs have been realized.[20, 23, 171, 172] OTFT devices present the unique challenge of patterning


discrete layers and are therefore much more complex than their OPV counterparts. Representative

techniques used for solution-processable OTFT constituents within the R2R framework include the

previously discussed ink-jet printing and spray deposition. Common among spray deposition

processes is the separation of a liquid into an aerosol mist composed of discrete droplets of solution.

This mist is then transferred on to a substrate, which is often heated, to form a continuous film of

a desired thickness. Within the field of organic electronics, three primary methods are used to create

and propel this aerosolized media: 1) gas-propelled spray, 2) ultrasonic spray, and 3) electrospray

deposition.[173–175]

The gas-propelled method consists of the rapid ejection of a liquid through a small orifice using

a pressurized gas (i.e. Ar or N2). This process requires a high pressure of propellant gas to ensure a

fine mist and, in turn, produces a strong directional jet of aerosol spray.[176] In contrast, the

ultrasonic spray technique often uses piezoelectric crystals to induce high-frequency vibrations into

the bulk of a liquid which is then transferred to the air-liquid boundary.[177] This creates constant

compression and decompression of the liquid and results in the creation of very small mist droplets

cast in to the air. Transferred using a carrier gas, the mist is then deposited onto the desired

substrate. Within these two constructs, similar physical and solution parameters require

optimization, however, an increased control of the droplet diameter is realized through a fine-tuning

of the piezoelectric frequency, down to picoliter drop sizes.[178] While directional spray is possible

with deflection of the mist using inert gases, it is not necessary. The third technique typically used

to aerosolize a liquid involves applying a high voltage between the substrate and the nozzle for

ejection of the liquid.[175] Once the electric field reaches a critical value for the solution, the liquid

dissociates to produce droplets of the solution at which point the solvent begins to evaporate while

in flight, leaving a very dense aerosol of the organic compound. The optimization of each process

for electronic applications is quite complex and requires a finely tuned balance between physical

deposition parameters (i.e. spraying distance, travel rate, orifice diameter, propellant media,

substrate temperature, etc.) as well as solution parameters (i.e. viscosity, vapor pressure, solution

concentration, etc.).

Gas-Propelled Spray Deposition

The first demonstration of gas-propelled-spray deposited organic films were realized using

semiconducting polymers in OPVs by two independent results by Ishikawa et al. in 2004[179] and

Mo et al. in 2005.[180] A similar technique was employed by Chan et al. to fabricate OTFT devices


from spray-deposited poly-3-hexylthiophene (P3HT) in a 6:1 chlorobenzene:1,2,3,4-

tetrahydronaphthalene solution on OTS-treated gate dielectrics exhibiting mobilities as high as 0.1

cm2V-1s-1.[173] As they noted, this performance was comparable to the best mobilities observed in

P3HT using other preparation methods.

Azarova et al. used this method to fabricate OTFTs from a small-molecule OSC, diF-TES

ADT, and obtained mobilities of 0.2 cm2V-1s-1, and on/off current ratios of 107.[23] This performance

was realized using 20 times more dilute solution than traditional spin-casting techniques,

dramatically reducing the cost per unit area in large-area manufacturing settings. More recently,

Mei et al. extended this system to demonstrate the spray-deposition of triethylgermylethynyl-

substituted anthradithiophene (diF-TEG ADT).[24] This compound yielded record-high spray-

deposited mobilities of 2.2 cm2V-1s-1, low threshold voltages of 0.5 V, and on/off current ratios of

104.

Khim et al. fabricated spray-deposited OTFTs and complementary inverters using the p-type

semiconductor region-regular P3HT and the n-type semiconductor ActiveInk N1450.[172] High

device-to-device uniformity, suggested to be due to the production of smoother films with increased

solution flow rates, was observed and the recorded charge-carrier mobilities were 0.25 cm2V-1s-1

and 0.01 cm2V-1s-1, respectively. Of distinct interest is the observation of an order of magnitude

increase in device mobility for ActiveInk N1450 as the flow rate increased from 0.8 to 1.6 ml min-

1 while the P3HT performance remained constant. The stark improvement is attributed to the

increase in crystallinity of the ActiveInk compound from the slower evaporation of the solvent.

In 2013, Yu et al. fabricated flexible ammonia (NH3) gas sensors using OTFTs fabricated from

gas-propelled spray-deposited TIPS-Pn on a PMMA dielectric.[181] X-ray diffraction was used to

demonstrate the highly ordered and highly crystalline film. Multiple device characteristics showed

dependencies on NH3 concentration, however the most prevalent was a -20 V shift in threshold

voltage when exposed to a 100 ppm atmosphere of NH3. Multiple cycles of N2 and NH3 exposures

demonstrate the favorable response and recovery characteristics of the sensors.

Ultrasonic Spray Deposition

Even though multiple ultrasonic-based spray techniques have been demonstrated for the

fabrication of OPVs,[182, 183] only recently has the method been used for OFET fabrication. This

process was first discussed by Abdellah et al.[184] and later developed by Shao et al.[185] They

fabricated ITO bottom-gate, Au top-contact OFETs using ultrasonic-based spray deposition of


TIPS Pn and PVP as the active semiconductor and insulator, respectively. A maximum charge-

carrier mobility of 0.35 cm2V-1s-1, threshold voltage of 11.3 V, and on/off current ratio >104 on a

flexbile substrate was observed from tilting a substrate at a small angle of 3° to align the TIPS Pn

crystals, a solution flow rate of 1.2 ml min-1, a nozzle-to-substrate distance of 4.6 cm, and a nozzle

translation of 8 mm s-1. This performance is comparable to devices fabricated using traditional spin-

coating processes, with the added ability for large-area fabrication. The PVP film exhibits low

roughness due to the nature of the small droplet size produced by the ultrasonic waves.

The ability to generate very small particles within a very narrow size distribution facilitates the

formation of uniform and homogeneous films, which is especially important in the fabrication of

very thin dielectric layers. While the technique is typically used in polymer-based materials, the

solubility, ease of purification and synthesis, and compound diversity of small-molecule organic

semiconductors create incentives for further development.

Electrospray Deposition

The electrospray deposition (ESD) technique offers similar advantages over traditional coating

methods to that of gas-propelled and ultrasonic-generated aerosols. However, the use of an electric

field between the nozzle and conductive substrate facilitates small droplet creation, charged media,

improved droplet-size control, and deposition efficiency due to the ability to apply the aerosol to a

substrate without the use of any carrier gas.[174]

In 2013, Onojima et al. fabricated bottom-contact, bottom-gate OFETs based on domains of

crystalline TIPS Pn using the ESD method.[44] Active semiconductor films were deposited from

1,2-dichlorobenzene and acetone at a solution feed rate of 0.6 ml min-1, a substrate temperature

held at 55°/60° C, a nozzle-substrate distance of 5 cm, and an applied voltage of 13 kV between

the solution and substrate. Notably, the deposition times were very fast (~1 min). Electrical

characterizations include a field-effect mobility of 0.12 cm2V-1s-1 at a drain-source voltage, VDS =

−50 V, an on/off current ratio of 104, and a high threshold voltage of −17 V. Upon reduction of the

bottom-contact thickness, an improved threshold voltage of −6.4 V was observed.

In 2014, Yamauchi et al. implemented the ESD technique for the direct wet-patterning and

development of n-channel ZnO-based and p-channel TIPS Pn-based step-edge vertical-channel

FETs (SVC-FETs).[186] This stucture has been shown to acheive a submicron channel thickness.[187]

The linear regime field-effect mobilities were 6.8 × 10−5 cm2V-1s-1 (n-channel, VDS = −10 V) and

1.6 × 10−4 cm2V-1s-1 (p-channel, VDS = −10 V). While this is not a demonstration of benchmark


electrical performance, the results highlight the fabrication of OFET devices from both inorganic

and small-molecule organic semiconductors and the ability to directly pattern the films through

electrically biasing the gate lines.

1.4 Conclusion

Organic-based FETs provide the unique opportunity to place electronics on large-area, light weight,

flexible substrates at a low cost, a process that is inaccessible to most inorganic devices. In addition,

the versatile chemistry of organic compounds allows for fine-structural modifications that result in

a diverse assortment of usable dielectrics, semiconductors, and electrodes that can be deposited by

a rich variety of deposition techniques. The solution-based techniques include, but are not limited

to, doctor blading, spray coating, drop casting, and inkjet printing. While full solution-processing

greatly simplifies production, the inherent disorder in the resulting films has great influence on

their performance. Even so, decades of cross-discipline research in all phases of development has

resulted in device efficiencies worthy of use in modern electronics applications. Investigations

during the infancy of organic field-effect transistor research focused mainly on obtaining and

characterizing high mobility semiconductors, however it was quickly realized that the dielectric

deserved equal attention due to its far-reaching impact on the performance of a device. This led to

the examination of high-k dielectrics, various polymer dielectrics, ultra-thin monolayers, as well as

hybrid multi-layer dielectrics in search of a material that combines low voltage operation with low

leakage currents. The research efforts dedicated to the development of organic electrodes focused

on improving charge injection by reducing contact resistance in the presence of organic/organic

interfaces. Materials that are readily deposited from solution, such that the entire process avoids

high-vacuum environments, are of prime interest. Examples include dispersions of inorganic metals

and carbon nanotubes, metallic polymers and organic charge-transfer complexes. Modern

electronics generally requires device patterning on a particular substrate to reduce power

consumption and signal ambiguity, whereas most approaches to organic electronics involve full-

surface coverage. Organic materials allow for creative solutions to this issue including selective

surface energy treatments, differential microstructures, as well as applications to large-area

electronics.


1.5 Thesis Outline

This thesis is organized in the following manner:

Chapter 2 describes the solution processing and electrical characterization of organic thin-

film transistors and the techniques used in this study to measure the structure of organic

semiconductor films. The assembly, composition, and characterization of self-assembled

monolayers on electrode surfaces is also discussed.

In Chapter 3 we emphasize the impact that structural phase transitions can have on the

performance and stability of OTFTs. A temperature driven phase transition is identified for the

high-mobility organic semiconductor, diF-TES ADT. This transition is studied by variable

temperature electrical characterization, thermal measurements, and structure determination.

In Chapter 4 and Chapter 5 we develop relationships between halogenated self-assembled

monolayers and organic semiconductors that allow us to fine tune film morphology and achieve

self-patterning of OTFTs by differential microstructure. Chapter 4 is dedicated to understanding

the effect that subtle changes in the self-assembled monolayer structure have on the thin-film

growth, device performance, and feasibility of self-patterning. In Chapter 5 we expand these studies

on a broader range of organic semiconductors and SAMs and establish design rules for efficient

OTFT self-patterning.

27

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CHAPTER 2 41

ORGANIC FIELD-EFFECT TRANSISTOR FABRICATION AND

CHARACTERIZATION

2.1 Introduction

As shown in Chapter 1 (Figure 2), the architecture of an OFET requires a set of individual layers

to be assembled in a specific manner to create a functioning device. While a variety of techniques

used to deposit the organic semiconductor were outlined throughout Chapter 1, this chapter will

serve to detail the specific techniques used to fabricate the individual layers for the devices used

throughout this research project. Section 2.2 describes the methods used to fabricate an array of

contacts using photolithography and electron-beam evaporation, Section 2.3 provides details about

the fabrication and characterization of self-assembled monolayers, Section 2.4 describes the growth

and characterization of the organic semiconducting layer, and Section 2.5 details the electrical

characterization of the complete OFET devices.

2.2 Contact Fabrication by Photolithography

Independent features within an OFET can be defined using additive approaches (i.e. printing[1]) or

subtractive techniques (i.e. stamping[2] or shadow masking[3, 4]). An approach used to define the

features of the electrodes on to a substrate of n++Si with thermally oxidized SiO2 is the subtractive

method of photolithography. This method, shown in Figure 1, includes the general process of

coating a substrate with a layer of a photo-reactive material (photoresist) (Figure 1b) and selectively

exposing regions of the film to a high-intensity light through a transparent mask containing the

desired features (Figure 1c). When using a “negative” process, the features that are to eventually

be contacts are realized by the opaque regions of the mask, preventing those regions from

exposure. This process effectively “hardness” the regions illuminated by the light, rendering them

insoluble to the liquid developer used to remove the portions hidden by the mask (Figure 1d). Upon

development of the film, the contact region of the substrate now consists of voids in the photoresist

regions by dissolving the remaining photoresist using a compatible solvent (Figure 1g).

Chapter 2. OFET Fabrication and Characterization 42

Fig

ure

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The procedure used to fabricate the contact-patterned substrates in this research is detailed

hereinafter. A 4 inch wafer of n++Si with a 200 nm thermally oxidized SiO2 film was prepared

using a 4 minute dehydration bake at 90 °C. Since the approach used consisted of a negative

process, the NFR016D2 photoresist was spun on using a two-step process: a 1) 500 rpm sec-1

acceleration to 500 rpm for 5 seconds to spread the liquid uniformly over the substrate and a 2)

1000 rpm sec-1 acceleration to 3000 rpm for 30 sec to thin the photoresist to the desired thickness.

The coated film was baked for 2 minutes at 90 °C in an oven in air prior to a 10 second exposure

through a Cr-patterned quartz mask at a 13.5 mw sec-1 intensity. A final bake for 2 minutes at 90

°C in air completed the curing of the exposed photoresist. The substrate was immersed for 1 minute

in MF-319 developer to remove the unexposed regions of photoresist. The developed substrate was

exposed to a 45 second oxygen plasma ash at 100 W to ensure full removal of the photoresist from

the unexposed regions.

In order to ensure a high quality metallization occurs, the substrate was placed in a vacuum

chamber and pumped down to near or below pressures ~10-6 torr. Typically when depositing Au

onto a silicon-based substrate, the use of an intermediate layer (i.e. Ti or Cr) is needed to ensure

good adhesion of the Au to the substrate. In our case, we deposited 50 Å of Ti immediately followed

by 600 Å of Au, both with a rate of 2 Åsec-1. The final steps for the fabrication of the contacts

included the metal lift-off through a 1-2 hour soak in a solvent such as N-Methyl-2-pyrrolidone

(NMP) or acetone. This process can be expedited using elevated temperatures (~60 °C) or

sonication, however this increases the risk of torn contacts that show poor edge definition.

Prior to further deposition of subsequent layers or contact treatments, the 4 inch wafer was

subdivided into identical pieces (~2 cm x 2 cm) using a diamond-tipped pen. Prior to the deposition

of the OSC layer, each of these test-beds were subjected to a thorough degreasing and photoresist

removal process that included a 10 minute soak in acetone and then isopropanol (IPA) at 80 °C

followed by a 10 minute UV-ozone exposure and a 30 second rinse with deionized (DI) water.

2.3 Self-Assembled Monolayers

The processing of organic compounds using solution-based techniques requires a precise control

of the relationship between the surface of the underlying substrate and the composition of the liquid.

One technique to impart improved wettability is through the modification of the surface using a


self-assembled monolayer (SAM). SAMs are available in a wide variety of configurations and are

highly tunable to form strong, chemical, bonds with a range of surfaces. The ability for SAMs to

modify pristine oxide and metal surfaces allows the tailoring of those surfaces to support improved

semiconductor growth and morphology,[5–10] better charge injection at the semiconductor’s

interface with the electrodes,[7, 11–13] and the reduction of charge scattering and trapping sites at the

semiconductor-dielectric interface and thus improving the charge transport characteristics of OFET

devices.[14] The chemical structures and names of a subset of typical SAMs used in OFET

fabrication are shown in Figure 2.

2.3.1 Composition and Function of SAMs

In this work, SAMs terminated with a sulfur atoms, known as thiols, were used to promote a strong

adsorption of the monolayer to the metallic surfaces during the device fabrication process, as shown

in Figure 1h. The incorporation of SAMs into the research presented here served two purposes:

1) Modification of the substrate’s electronic properties at the interfaces between the device

electrodes and semiconducting film to promote improved injection of charge carriers into

the active layer,

Figure 2 Chemical structure of common (a) thiol and (b) silane varieties of SAMs used in OFET

fabrication. Abbreviations: PFBT – pentafluorobenzenethiol, PClBT – pentachlorobenzenethiol,

4-NBT – 4-Nitrobenzenethiol, HMDS – Hexamethyldisilazane, FDTS – Perfluorodecyl-

trichlorosilane, OTS – Octadecyltrichlorosilane.


2) Modification of the substrate’s surface chemistry to promote interactions between the SAM

and deposited organic semiconductor.

In Chapter 3, the fluorinated pentafluorobezenethiol (PFBT), displayed in Figure 2a, was used to

both shift the work function of the electrode and to create an electrode surface suitable with the

growth of large, highly-ordered, and favorably oriented films for effective charge transport. This

choice takes advantage of exploiting the fluorinated SAM and the fluorinated organic

semiconductor to create an environment capable of facilitating interactions at their interface

through halogen-halogen interactions. In Chapter 4 and Chapter 5, the nature of this interaction was

further exploited through the investigation of SAMs with similar composition to PFBT presenting

compounds with a reduced amount of fluorination.

The wetting properties of the SAM-modified surfaces were macroscopically investigated using

goniometry (Section 2.3.2) and the modification of the electronic nature of the modified surface

were characterized using Kelvin Probe measurements (Section 2.3.3).

2.3.2 Growth and Characterization of SAMs

The treatment of substrates using self-assembled monolayers is typically accomplished by either

vapor deposition (in vacuum or ambient) or by immersion in a solution. In this study, all electrode

treatments were performed from a 30 - 50 mM solution of the SAM material diluted in high purity

ethanol. A 3 ml solution was placed in the bottom of a small beaker (~5 ml) followed by the

submersion of the cleaned substrate containing Au-patterned electrodes. After a 30 minute

treatment, the substrate was removed and thoroughly rinsed with and/or sonicated in ethanol to

ensure full removal of the remaining bulk layers not covalently bonded to the substrate. Using this

approach, the timescale of SAM adsorption is dependent on the size of the SAM molecule and can

range from ~10 minutes to ~2 days.[15]

We have performed goniometry using a Rame Hart Model 200 Standard Contact Angle

Goniometer and image analysis using the DROPimage Standard software suite.[16] All goniometry

experiments were performed using the static sessile drop method in which a small, constant, volume

of a DI water was gently deposited on the substrate under test. A bright, diffuse, light was shone

through the droplet and the stage was translated relative to the ccd camera so that its silhouette was

visible on an image of high contrast. Figure 3 shows a representative image used in this analysis

with a water droplet on untreated, cleaned, Au surface on the left and an Au surfaced treated in a


solution of 30 mM PFBT on the right. The high fluorination of the monolayer promotes a shift in

the hydrophobic nature of the substrate. The impact that a SAM treatment has on a surface was

studied by characterizing multiple substrates to gain an understanding of the reproducibility of the

treatment method and characterizing multiple spots per substrate to reveal the spatial uniformity of

the treated substrate.

2.3.3 Interface Energetics and Electrical Characterization of SAMs

A key component that must be considered upon the selection of a SAM for modifying a device’s

electrode stems from the ability of SAMs that exhibit a dipolar nature to shift the energetics of the

electrode, schematically represented in Figure 4.[11] Ensuring a proper alignment between the

HOMO/LUMO of an organic semiconductor and the work function of the electrode surface is vital

to improving and maintaining efficient charge transport in a device. A misalignment creates an

energetic “injection” barrier that charge carriers must overcome prior to being involved in effective

charge transport through the OSC.

A SAM based on alkanethiols or their perfluorinated counterparts adsorbed onto a metal

electrode can be represented as a stacked bilayer of two dipolar layers. One constituent of the total

dipole is due to that formed by the metal-sulfur (M-S) interaction and the other is from the

composition of the monolayer itself. In the case of a non-fluorinated alkanethiol SAM, the dipole

contribution of the metal-sulfur interaction exceeds that of the SAM itself and thus, a smaller,

negative, shift in the workfunction of the electrode is observed relative to the fluorinated variety.

The change in the electrode’s work function can be determined from

Δ𝑊𝐹 = −𝑁 [𝜇⊥,SAM

𝜀0𝜅SAM+

𝜇M−S

𝜀0𝜅M−S] (2.1)

where N is the grafting density of the monolayer, 𝜇⊥,M−S and 𝜇⊥,SAM are the components of the

Figure 3 Image of a water droplet on an untreated Au surface (left) and on a PFBT-treated Au

surface (right) used for goniometric characterization of SAMs.


SAM’s dipole moment and the effective metal-sulfur dipole moment perpendicular to the surface,

respectively, 𝜀0 is the permittivity of free space, and 𝜅SAM and 𝜅𝑀−𝑆 represent the dielectric

constants of the SAM and metal-sulfur interaction, respectively.

Two primary methods to determine the work function of either a pure metal electrode surface

or a modified electrode surface are through using either ultraviolet photoemission spectroscopy

(UPS) or Kelvin Probe measurements. Using a UPS method requires the use of a high-vacuum

environment whereas a Kelvin Probe approach can be done in ambient conditions.[17, 18] As such,

since the majority of the processing of organic materials in this work was accomplished in ambient

conditions, we have chosen the Kelvin Probe method to be more appropriate in mapping the charge-

injection picture of our systems.

We have performed the Kelvin Probe measurements using TREK Model 325 Electrostatic

Voltmeter, calibrated using freshly cleaved highly ordered pyrolytic graphite (HOPG). A schematic

of the probe and surface under test is shown in Figure 5. After the instrument was zeroed using the

HOPG standard, the SAM-treated Au substrate was placed on a grounded metallic stage, capable

precise translation along the vertical axis. The surface under test was brought within one to two

millimetres of the probe head, making sure the two surfaces did not come in physical contact. Due

to the sensitivity of the measurement to static charge in the surround area, the probe head setup was

placed in a grounded Faraday cage. The contact potential difference between the surface under test

Figure 4 Schematic energy-level diagram depicting the effect that self-assembled monolayer

treatments on a metal surface have on the energetics of the interface between the metal and

semiconductor. Diagrams are shown for (a) untreated metal with electron-injection barrier (EB,e

)

and hole-injection barrier (EB,h

), (b) metal treated with a SAM to decrease EB,e

and increase EB,h

,

and (c) metal treated with a SAM to increase EB,e

and decrease EB,h

.


and probe surface (𝑉sample − 𝑉probe), as measured, was used to determine the absolute work

function using

𝑊𝐹 = −e(−(𝑉sample − 𝑉probe) ) + 𝜙HOPG (2.2)

where ‘e’ is the electron charge, 𝑉sample is the signal from sample, 𝑉probe is the signal from the

HOPG and 𝑊𝐹HOPG = 4.48 eV is the work function of HOPG.[19]

2.4 Organic Semiconductors

2.4.1 Growth and Film Morphology of OSC Layers

The methods used to deposit the organic semiconducting films include both techniques intended to

fabricate discrete devices, such as physical vapor transport[20–22] and inkjet printing processes,[23–27,

1, 28] as well as those intended to create more continuous films such as spin casting,[6, 7, 21, 29–32] doctor

Figure 5 Schematic of the Kelvin Probe instrument setup used to determine the contact potential

difference between the SAM-treated electrode and the probe head.


blading,[33–35] and drop casting.[13, 31] As is briefly outlined in Figure 6 for a few representative

deposition techniques, each method presents advantages and disadvantages related to the device

performance and rate of throughput of fabrication. Along the top axis, “slow” refers to a process

that requires a long (~days) to perform the crystal growth process while “fast” denotes a much

quicker process (~seconds). The bottom axis refers, generally, to the relative expected electrical

performance of the organic semiconductor crystals grown when in an OFET devices. The primary

method used throughout this research is that of spin coating. This process is most frequently used

to create very uniform thin films on geometrically flat surfaces. After the substrate is mounted, a

typical deposition process includes four phases: 1) dispensing of a liquid onto the substrate, 2)

rotational acceleration of the substrate to a low speed to facilitate the uniform spreading of the

liquid, 3) further acceleration to a very high rotational velocity to thin the film to the desired

thickness through spinning fluid off the substrate using the outward centrifugal force, and 4)

evaporation of the remaining solvent through a constant rotation of the stage for an extended period

of time. The parameters used in each of the steps are dependent on the properties of the liquid (i.e.

viscosity) as well as the desired thickness of the resulting film.

The organic semiconductor films deposited for the OFET fabrication that is presented in the

following chapters typically included the deposition of ~300 μl of 1-2% by wt. organic

semiconductor dissolved in an organic solvent (i.e. chlorobenzene) onto a 2 cm x 2 cm substrate

adhered by vacuum to the stage of the spin coater. The stage is accelerated at a rate of 100-333 rpm

sec-1 to a maximum spinning speed of 1000 rpm. In most spin coating processes, the magnitude of

the acceleration is less vital to the resulting film formation, however in processes intended to allow

Figure 6 Schematic illustration of a few representative techniques used for the deposition of the

organic semiconductor layer within OFET fabrication. From left to right: Physical vapor

transport, Solvent assisted deposition, Drop casting, Spin casting. General trends related to the

time required for film/crystal formation and electrical performance are depicted above and

below the arrow, respectively.


the interaction between the organic semiconductor and the chemically modified substrates, we

found that a small acceleration is necessary. Solvent evaporation continues for an additional 80

seconds of rotation to ensure its full removal. Since the devices fabricated here were bottom-gate,

bottom-contact devices, following the deposition of the OSC, structural and electrical

characterization was conducted, as discussed in Sections 2.4.2 and 2.4.3. This process creates an

environment suitable for the investigation of interactions between organic semiconductor and

device surfaces, however it does not facilitate a high-throughput technique to take advantage of the

unique applications of OFETs.

2.4.2 Structural Characterization of OSC Layers

The structure and function of a thin film are intimately related, thus a good control of the electrical

performance requires a thorough understanding of the structure of the film and its impact on charge

transport. In this work, we will use x-ray scattering techniques to investigate the structural

characteristics of the semiconductor layer performed as a function of temperature, surface

treatments, and spatial location within a device. X-ray diffraction (XRD) techniques are appropriate

for our purpose for two main reasons: 1) the wavelength of x-rays are on the same length scale as

the interatomic distances in the materials of interest (0.15 – 4.0 nm) and 2) under appropriate

procedures, x-ray scattering techniques are non-destructive to the sample or device under test.[36]

Depending on the information that is to be extracted from the XRD experiment, there exist

many approaches to choose from (i.e. single crystal diffraction, powder diffraction, x-ray

reflectometry (XRR), grazing incidence x-ray diffraction, etc.). However, the basic principles are

similar, regardless of the method, and they will be discussed conceptually here. For a more detailed

discussion and mathematical derivation of these principles see X-Ray Diffraction by B.E. Warren[37]

and Thin Film Analysis by X-Ray Scattering by Mario Birkholz.[36]

An X-ray scattering process used throughout this work includes the following steps: 1) the

production of X-rays, either from a synchrotron source or a Cu radiation source in the lab, 2) the

shaping and directing of the source X-rays towards the sample under test, and 3) the interaction

(i.e. scattering) of the incident waves with the lattice planes of the crystalline solid. When

interacting with the lattice, those waves whose path lengths have been altered by an integer (n)

multiple of the incident X-ray’s wavelength (λ) will interfere constructively and remain in phase.

This condition for constructive interference leads to the Bragg relationship between the lattice

spacing of the solid and the incident wavelength of the X-rays, given by:


2𝑑 𝑠𝑖𝑛(𝜃) = 𝑛𝜆. (2.3)

where d is the interplanar spacing between two adjacent planes of a crystal lattice and θ is the angle

of the incident x-rays. As the X-rays are scattered from the lattice, a diffraction pattern is measured

by the detector. Based on the intensity and location of the detected X-rays incident on the detector,

the structural properties and film characteristics can be obtained.

In Chapter 3, we explored the structural changes induced to the small-molecule diF-TES ADT

using temperature-dependent powder X-ray diffraction. In Chapter 4 we explore the structural

changes within blanket films of diF-TES ADT deposited on SAM-treated electrodes using the

grazing incidence x-ray diffraction (GIXD) technique. In Chapter 5, we use the microbeam grazing

incidence wide angle X-ray scattering (μGIWAXS) to investigate the structure of organic

semiconductor films on components within an actual device.

2.5 Electrical Characterization of OTFTs

The OFET consists of only a few necessary components (see Chapter 1, Figure 2) and the quality

of and relationship between them play a vital role into the overall performance, and thus the

feasibility of the device being used for certain applications. A typical set of figures of merit are

used to determine and communicate the electrical performance of an OFET. The methods used

throughout this thesis to determine the field-effect mobility (μ), threshold voltage (VT), sub-

threshold slope (S), and on/off current ratios (Ion/Ioff) are discussed below. In this section we also

present the algorithm used for the calculation of contact resistance and contact work function. This

section serves as a discussion of the electrical characterization techniques used throughout the

present research.

2.5.1 Figures of Merit of OFETs

Organic field-effect transistors operate by the principles described below. When a gate voltage that

is greater than the threshold voltage is applied, either holes (negative VGS) or electrons (positive

VGS) from the organic semiconductor accumulate at its interface with the gate insulator and the

device operates in accumulation mode. While in this mode, the source and drain electrodes allow

access to the mobile charges.


The operation of a FET consists of two regimes, identified in Figure 7b. A device is in the

linear regime of operation when VDS << |VGS-VT|. Here, the device acts as a gate voltage controlled

variable resistor with the source-to-drain current following

𝐼D =𝑊

𝐿𝜇𝐶i (𝑉GS − 𝑉𝑇 −

𝑉DS

2) 𝑉DS (2.4)

where W and L are the width and length of the FET channel, respectively, μ is the charge carrier

mobility, and Ci is the areal capacitance of the gate insulator.

As the drain voltage increases, a space charge forms, limiting further increase in the drain current

and this is denoted as the pinch-off point. Beyond that point, the device operates within the

saturation regime and the drain current follows

𝐼D =𝑊

2𝐿𝜇𝐶i(𝑉GS − 𝑉T)2 (2.5)

The models discussed above follow from the long channel silicon FET model developed by Van

de Wiele.[38] The remainder of the section details the extraction of useful parameters from typical

device transfer and transport characteristics.

Off-Current

The first stage of a typical cycling of an OFET device for determining device properties begins in

the devices’ off-state. In this stage, the device is biased with a VGS such that a very small, often

negligible, drain current (ID) is observed. A very low current in this regime is desired so that

minimal power is used when the device is supposed to be inactive. In the present research, the

device’s off-current (Ioff) is defined as the minimum ID measured and is denoted in Figure 7a.

Subthreshold Slope

The subthreshold slope is used to characterize the subthreshold swing of a device prior to becoming

active and is determined using

𝑆 = 𝜕𝑉GS

𝜕(log 𝐼D) (2.6)

The subthreshold swing quantifies the speed with which the device turns, where a better device is

identified by a smaller value, and is typically reported in units of mV dec-1. Equation (2.6)

represents the inverse slope of the log (ID) versus VGS curve and is typically determined from the


maximum slope of that curve. Fundamentally, the best room temperature value for the subthreshold

slope is 60 mV dec-1.[39]

Threshold-Voltage

As the VGS in a device is increased, it transitions from its off-state to one in which the channel is

active. The threshold voltage (VT) occurs upon the accumulation of charges at the interface of the

OSC with the gate dielectric and is observed by an appreciable increase in ID. In the present

research, the threshold voltage is determined by extrapolating the linear portion of the √𝐼D versus

VGS curve to the axis with VGS, as shown in Figure 7a. A threshold voltage near zero is ideal,

resulting in an efficient amount of power required to maintain a device in its off-state.

Field-Effect Mobility

After the channel of an OFET becomes active, charge carriers move from the source electrode,

through the semiconducting channel, and into the drain electrode. The effectiveness and rate of this

movement is most commonly discussed in terms of the device’s field-effect mobility (μ) and is

typically reported in units of cm2V-1s-1. The drift velocity of a charge carrier in an electric field is

related by its mobility through

�� = 𝜇 �� (2.7)

Figure 7 Schematic of the evolution of the (a) drain current as a function of the gate-to-source

voltage at a fixed drain-to-source voltage in the saturation regime of operation and (b) evolution

of the drain current as a function of the drain-to-source voltage for various gate-source voltages.


and can be experimentally determined from the FET IV characteristics. Solving Equations (2.4)

and (2.5) for μ, we can use the slopes of the ID versus VGS curve in the linear regime and the √𝐼D

versus VGS curve in the saturation regime with the following expressions:

(Linear Regime) 𝜇lin = 𝐿

𝐶i𝑊

𝜕𝐼𝐷

𝜕𝑉𝐺𝑆 (2.8)

(Saturation Regime) 𝜇sat = 2𝐿

𝐶i𝑊 (

𝜕√𝐼D

𝜕𝑉GS)

2

(2.9)

While this model assumes ideal behaviors that satisfy Equations (2.4) or (2.5), not all OFET

devices exhibit this behavior. Realistically, factors such as a high contact resistance at the interface

of the device’s electrode and organic semiconductor can create non-ideal behavior in the current-

voltage characteristics.[5, 40] In this research, the mobility was extracted using the most linear portion

of the transfer characteristics. We note that the experimental interpretation of mobility can be

greatly enhanced by selectively choosing only a few points that can often originate from noise

regions of device operation. To avoid this, in this research we developed a LabVIEW-based

analysis platform that helped to remove much of the subjectivity in extracting the mobility through

automating the analysis of the measured data. All measurements use a minimum of a five points to

determine the slope by first choosing the set of lines possessing the highest degree of linearity from

which the maximum value was chosen.

On/Off Current Ratio

After the region in which the ID greatly increasing, a final region observed in a typical VGS sweep

is one where the drain current begins to saturate and is identified in Figure 7a. Here, the maximum

ID is measured. We define this maximum drain current as the on-current (Ion). A measure of the

contrast between a transistor’s on- and off-states is the ratio between Ion and Ioff.

2.5.2 Contact Resistance

The resistance that a charge-carrier experiences as it transfers from the elctrode to the

semiconductor is defined as the contact resistance (Rc). The method used in the present research

for extracting this figure of merit is the gated-transmission line method (Gated-TLM).[41] In this


formalism, the resistance in a device is modeled as a series of three resistors (Figure 8a), two for

the contributions of the contact resistance at each electrode and one for the contribution from the

resistance within the channel. For a set of devices, the total device resistance for each VGS sweep is

given by

𝑅on = 𝜕𝑉DS

𝜕𝐼D (2.10)

and is determined in the linear regime of device operation, as shown in Figure 8c. Since the overall

resistance of the device depends on the length of the channel, the evolution of the total resistance

as a function of channel length is determined and is displayed in Figure 8d. Modeling the overall

resistance of a transistor device to include contributions from both the semiconductor channel and

the injection of charge carriers into the channel we have:

𝑅on = 𝑅ch(𝐿) + 𝑅c (2.11)

where Rc is the contact resistance. As L approaches zero, all the resistance of the device now

Figure 8 Circuit diagrams depicting the (a) contact and channel resistance for channel lengths

greater than zero and (b) contact resistance as L0. Representative transport characteristics (c)

and evolution of the total device resistance with channel length (d) used in the contact resistance

analysis.


originates at the electrode-semiconductor interface, thus the contact resistance (Figure 8b).

Extrapolating the evolution of Ron versus L to L = 0, we determine the contact resistance of the

device. As the channel length decreases, so too does the contribution of the channel resistance term

in Equation (2.11). Therefore, the total resistance for devices with short channel lengths is realized

by a greater fraction of Rc relative to the Rch. Since Rc is dependent on the device width, and often

the applied VGS, a common representation is the evolution of the width-normalized Rc as a function

of VGS, shown in Chapter 5. While typically Rc decreases with increasing VGS due to the reduction

of the injection barrier between the charge carrier and the metal electrode as the Fermi level

shifts,[42, 43] we observed VGS-independent RC, similar to that observed by Smith et. al.[18] The origins

for these observations are currently under investigation.

57

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CHAPTER 3 61

STRUCTURAL PHASE TRANSITIONS IN ORGANIC SEMICONDUCTORS

AND THEIR EFFECTS ON DEVICE PERFORMANCE AND STABILITY

This work was published: J. W. Ward, K. P. Goetz, A. Obaid, M. M. Payne, P. J. Diemer, C. S. Day, J. E.

Anthony, and O. D. Jurchescu, Applied Physics Letters 105, 083305 (2014), and is reprinted with permission

(License number: 3605560291075, AIP Publishing LLC).

The use of organic semiconductors in high-performance organic field-effect transistors

requires a thorough understanding of the effects that processing conditions, thermal and

bias-stress history have on device operation. Here, we evaluate the temperature dependence

of the electrical properties of transistors fabricated with 2,8-difluoro-5,11-

bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), a material that has attracted

much attention recently due to its exceptional electrical properties. We discovered a phase

transition at T = 205 K and discuss its implications on device performance and stability.

We examined the impact of this low-temperature phase transition on the thermodynamic,

electrical, and structural properties of both single crystals and thin films of this material.

Our results show that while the changes to the crystal structure are reversible, the induced

thermal stress yields irreversible degradation of the devices.

Chapter 3. Structural Phase Transitions in Organic Semiconductors 62

3.1 Introduction

Soluble small-molecule organic semiconductors exhibit excellent electrical performance when

crystallized by a variety of techniques; these include ink-jet printing,[1, 2] polymer blends,[3, 4]

solution shearing,[5] vibration assisted crystallization (VAC),[6] and spray deposition.[7–10] Charge

transport in these materials is strongly dependent on the crystal packing motif, which is dictated by

the molecular structure: very subtle modifications in the chemistry of the compound yield

significant changes in the electronic couplings[11, 12] and device performance.[3, 6, 10, 13–15] In addition,

such systems are susceptible to variations in crystalline structure upon processing. The existence

of more than one possible crystal structure for a compound is referred to as polymorphism, a

phenomenon that is quite common in molecular crystals given the weak nature of the intermolecular

interactions that are characteristic to such systems.[5, 16–28] With 5,11-Bis(triethylsilylethynyl)

anthradithiophene (TES ADT), for example, the solvent type used during film deposition affects

its crystalline structure and morphology, and alters its field-effect mobility by over one order of

magnitude.[29] In the same material, polymorphism can be driven by post-processing steps such as

solvent annealing,[30] or by sweeping the temperature.[31] The fluorinated version of the

aforementioned compound, 2,8- difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-

TES ADT), also exhibits temperature-dependent polymorphism.[32] In this system an enantiotropic

phase transition was reported near room temperature, which affected its electrical properties, and

the observed changes were completely reversible. In the present study, we explore the properties

of this material over a broader temperature range and discover the presence of another structural

phase transition present at T = 205 K. This transition is completely reversible in terms of crystalline

structure, but it yields irreversible deterioration of the electrical performance in organic thin-film

transistors (OTFTs) fabricated from this material. The transition was studied using differential

scanning calorimetry (DSC), X-ray diffraction (XRD), and OTFT measurements. This study

provides a complete phase diagram of the phases present in diF-TES ADT in the temperature range

120 - 350 K and their effect on charge transport and device stability.


3.2 Experimental

3.2.1 Calorimetric and Structural Characterization of the OSC

DSC measurements were performed in hermetically sealed aluminum pans at a scanning rate of 20

K/min using a Q2000 TA Instruments differential scanning calorimeter. Multiple heat/cool sweeps

were performed over the 180 − 350 K temperature range. Temperature-dependent XRD was carried

out in order to elucidate the structural changes within the material as it passed through the phase

transitions. Single crystal XRD measurements were not successful as the crystal routinely cracked

or shattered upon cooling below 280 K, preventing the collection of sufficient data to determine

the crystalline structure at temperatures below the 205 K transition. We were, however, able to gain

qualitative information about the low-temperature phase by performing powder XRD. Powder

diffraction was acquired on a Bruker D8 DaVinci diffractometer with Cu-Kα radiation and a liquid

nitrogen-cooled low-temperature stage fitted with a Be hemispherical enclosure. The source beam

was conditioned using a 0.1 mm lateral slit and 2.5° Soller slits. The reflected beam was conditioned

with a 0.6 mm lateral slit, 2.5° Soller slits and a Ni filter to remove Kβ contributions. The powder

was lightly mechanically ground and sifted using a fine mesh filter screen to ensure an even coating

on a zero-background Si substrate. After initial measurements were taken at room temperature (T

= 294 K), the powder was sequentially cooled to T = 200 K then T = 150 K, and finally returned to

T = 294 K, all at a rate of 5 K min-1. Scan data set was collected at each temperature for the range

10° < 2θ < 40°.

3.2.2 Field-Effect Transistor Fabrication and Characterization

The device studies were carried out using bottom-gate, bottom-contact field-effect transistor (FET)

structures with heavily doped n++Si as the gate electrode and 200 nm SiO2 gate dielectric. The

source and drain contacts were defined by photolithography and lift-off, with Ti (5 nm)/Au (45 nm)

deposited using e-beam evaporation. The substrates were cleaned with hot acetone and hot

isopropanol followed by UV-ozone exposure and an ethanol rinse. A self-assembled monolayer

(SAM) treatment was performed on the electrodes by soaking the substrate in a 30 mM room-

temperature bath of pentafluorobenzenethiol (PFBT) (Sigma Aldrich) for 30 minutes followed by

a 5 minute sonication in ethanol. The organic semiconductor was spin-coated from a 1.2 wt. %

chlorobenzene (Sigma Aldrich) solution at 1000 rpm. As-prepared devices were placed in vacuum


for at least 24 hours prior to measurement to ensure full solvent removal. Electrical characterization

was carried out during cooling, followed by heating using an Agilent 4155C Semiconductor

Parameter Analyzer in combination with LakeShore cryogenic probestation. The heating/cooling

rates were 5 K min-1 to prevent film cracking.

3.3 Results and Discussions

Figure 1 depicts the DSC results obtained for a typical diF-TES ADT sample. Two peaks can be

clearly distinguished, denoting the presence of two distinct transitions separated by a ΔT = 90 K

interval. The known room-temperature phase transition manifests itself as a peak at T = 294 K,

characterized by a latent heat of ΔH = 1.8 kJ mol-1, in agreement with previous reports.[32] Further

cooling shows an additional peak at T = 205 K, with latent heat twice as large at ΔH = 3.6 kJ mol-

1. Both transitions appear to be enantiotropic in nature. For the remainder of this article, the

individual phases will be denoted from warmest to coolest as α, β, and γ. Slight shifts in the peak

positions are observed during the heating segments of the DSC, but the temperature interval

between the two peaks remains constant.

The devices operate as p-type FETs, and in Figure 2a we present a typical device characteristics

in the saturation regime (VDS = −40 V). We extracted the mobility µ from the slope of the square

root of the drain current (ID) vs the gate voltage (VGS) using the current-voltage relationship:

𝐼D =𝑊

2𝐿𝜇𝐶i(𝑉GS − 𝑉T)2 (3.1)

Figure 1 DSC spectrum of diF-TES ADT displaying the presence of two phase transitions at T

= 294 K and T = 205 K. The heating (bottom, red) and cooling (top, blue) curves are shown and

phases are defined as α, β, and γ. The inset shows the molecular structure of diF-TES ADT.


where W and L are the channel width and length, respectively, Ci is the gate oxide capacitance per

unit area, and VT is the threshold voltage. The plot in Figure 2a corresponds to measurements taken

at T = 295 K and representative transfer characteristics at different temperatures are displayed in

Figure 3 for the saturation regime. The evolution of the FET mobility, as characterized in the

saturation regime of operation, versus temperature is shown in Figure 2b. We measured more than

15 devices, and all showed similar behavior. This plot shows several inflection points at

temperatures that coincide with peak locations in the DSC measurements. For this reason, we

attribute them to the phase transitions. A similar trend was observed in data extracted from linear

regime (Figure 4), however to minimize the effects due to the contacts and to more effectively

access the intrinsic properties of the material, our discussion will be limited to the saturation regime

characterization. The change in the slope of the μ vs. T plot begins at T = 294 K, marking the

transition from the α-phase into a region where the α- and β-phases co-exist, in agreement with

previous results.[32] In the temperature range 220 K < T < 275 K the mobility varies monotonically

with temperature and the film consists of pure β-phase. Upon further cooling, the onset of a region

in which the mobility increases is observed. This marks a phase-coexistence region between the

Figure 2 (a) Typical current-voltage characteristics for a device in the saturation regime of

operation (VDS

= −40 V), taken at T = 295 K. This device has a geometry of W = 1000 μm and

L = 15 μm. The inset displays the schematic of the device that was used. (b) Evolution of the

saturation field-effect mobility (μ) with temperature. The inflection points correspond to the

structural phase transitions. Individual phases are identified by α, β, and γ. The inset shows

representative slopes used in the activation energy calculation.


β- and γ-phases, with the β-phase gradually being converted into the higher mobility γ-phase. At T

< 175 K the film consists of a single phase, i.e. the γ-phase. Small differences in the transition

temperatures between the DSC and electrical measurements are due to the differences in the grain

-40 -30 -20 -10 0 10 2010

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

I D (

A)

VGS

(V)

- polymorph

- polymorph

- polymorph

(a) (b) (c)

(d)

Saturation Regime Characteristics

Linear Regime Characteristics

-40 -30 -20 -10 0 10 20

0.0

1.0x10-3

2.0x10-3

3.0x10-3

4.0x10-3

5.0x10-3

6.0x10-3

- polymorph- polymorphI1

/2

D (

A1

/2)

VGS

(V)

- polymorph

-40 -30 -20 -10 0 10 20

VGS

(V)

-40 -30 -20 -10 0 10 20

VGS

(V)

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

I D (

A)

Figure 3 (a-c) Output characteristics of a device with L/W = 15/1000 and dielectric thickness of

200 nm in the saturation regime, VDS

= -40 V. The data is taken at T = 300 K (α – polymorph,

red), T = 250 K (β – polymorph, black) and T = 150 K (γ – polymorph, blue).

Figure 4 Device linear mobility as a function of temperature. Vertical dotted lines denote the

phase transitions as observed in DSC and saturation regime mobility.

120 140 160 180 200 220 240 260 280 300 32010

-4

10-3

10-2

Lin

ea

r M

ob

ility

(cm

2V

-1s

-1)

Temperature (K)

+ +


size between the organic semiconductor powder used in the DSC measurements and the thin-films

used in the OFET characterization. Other causes for peak shifting in DSC measurements are

variations in heating and cooling rates and the total mass of the sample, though the peak onset

remains the same. In addition, the width of the transition windows varied slightly between

measurements on different samples, which is typical for transitions that proceed via a nucleation

and growth mechanism.[33]

Each of the three phases exhibit activated behavior in the evolution of the mobility with

temperature, as observed in Figure 2. We extracted the activation energies 𝐸A for the temperature

zones where only pure phases are present (see inset in Figure 2) using the Arrhenius relationship

given by:

𝜇 = 𝜇0𝑒−𝐸A (𝑘B𝑇)⁄ (3.2)

where 𝜇0 is the trap-free mobility and 𝑘B is the Boltzmann constant. The corresponding values for

each phase are 𝐸A(α) = 20 meV, 𝐸A(β) = 115 meV, and 𝐸A(γ) = 40 meV. The electrical properties

are completely reversible when the system contains α and/or β polymorphs, but they cannot be

recovered upon heating once the devices reach the γ-phase, as the mobility drops irreversibly

(Figure 5). This result can be better understood upon a closer look at Figure 6, which shows pictures

of a diF-TES ADT single crystal while passing through the two transition points. The high

temperature transition (α- to β-phases) does not induce any notable optical changes within the

crystal (Figure 3a-b), but as the crystal transitions into the γ-phase, it changes color from red to a

Figure 5 Device saturation mobility as a function of temperature for a full temperature cycle.

Cycle include one cooling step (black) and one heating step (red).

100 120 140 160 180 200 220 240 260 280 300

10-3

10-2

10-1

Heating

Sa

tura

tio

n M

ob

ility

(cm

2V

-1s

-1)

Temperature (K)

Cooling


pale pink, exhibits cracking, and in some cases shatters completely. This behavior suggests that in

spite of the fact that the 205 K transition is enantiotropic in nature, as pointed out by our DSC

measurements, the mechanical stress associated with this transformation induces irreversible

damage to the crystal. This explains the degradation in our transistor measurements and highlights

the detrimental effect of such transition in the reliability of devices made on this material when

measured at low temperatures. The β to γ transition is thus very different than the one observed

near room temperature, which resulted in minor, reversible changes in the electrical

characteristics.[32] Encouragingly, the characteristic transition temperature is well below the device

operating temperatures, and while the transition degrades the device performance, it does not pose

a threat for the operation of diF-TES ADT OTFTs in opto-electronic applications. This is unlike

the case of 6,13-bis(triisopropylsilylethynyl) pentacene, where such dramatic transitions occur

slightly above room temperature, yielding film cracking and irreversible performance

degradation.[34, 35]

Because the single crystals cracked when cooled into the γ-phase, we were not able to determine

the crystal structure of the new polymorph. Nevertheless, our temperature-dependent powder XRD

Figure 6 Optical micrograph of a diF-TES ADT single crystal at (a) T = 294 K, (b) T = 223 K,

(c) T = 187 K, and (d) T = 120 K. Cooling of the material at a rate of 5 K/min shows both a

change in color from red (a) to pink (b) and the onset of internal cracking and segmentation of

the crystal (c-d). The return to room temperature resulted in further degradation of the crystal.


measurements (Figure 7) clearly show that the transition observed at low temperature is structural

in nature, and that the phase transformation occurs gradually, with the β and γ polymorphs co-

existing over a temperature interval that vary slightly from sample to sample. The two room-

temperature measurements taken before (red) and after (not shown) the cooling are very similar,

confirming the fact that the transition is reversible. Measurements taken near the transition

temperature (150 K and 200 K) exhibit noticeably different patterns beyond simple shifts in the

peak positions; which are also significantly different from those observed near room temperature.

Based on the mobility data presented in Figure 2, the spectrum taken at T = 200 K corresponds to

a phase-coexistence region between β and γ polymorphs, while at T = 150 K, a single phase (γ) is

present.

The observed changes in the device mobility reported in Figure 2 have two possible origins.

First, the intrinsic material mobility varies as a result of the different molecular packing

characteristic to each of the three polymorphs, promoting different electronic structures.[11] Second,

the changes in the crystalline packing at the surface of the electrode occurring when the system

undergoes the transition cause shifts in the position of the highest occupied molecular orbital

(HOMO) level of the semiconductor. This alters the Schottky barrier at the injecting contact, and

thus the contact resistance. We have shown that a shift of ~ 0.2 eV in HOMO is obtained by

transitioning from a pure (001) to a mixed (111):(001) orientation with respect to the surface, with

Figure 7 Powder diffraction of diF-TES ADT at T = 294 K (Red), T = 200 K (Black), T = 150

K (Blue). Dissimilar patterns indicate a structural change within the material.


no changes in the crystalline structure. This resulted in variation of the contact resistance (Rc) by

more than two orders of magnitude.[36] It is therefore expected that similar or greater shifts will

occur in the case of polymorph transition. To investigate the effect of the structural phase transition

on the contact resistance of our OTFTs, we analyzed it as a function of temperature using the gated

transmission line method (TLM), as described in Chapter 2. For a set of devices of different channel

lengths and fixed widths we measured the total device resistance in the linear regime of operation

using

𝑅on = 𝜕𝐼D/𝜕𝑉DS, (3.3)

where 𝑅on = 2𝑅c + 𝑅ch.[37] Plotting the values of 𝑅on as a function of channel length for each gate-

source voltage VGS and extrapolating the total resistance to L = 0 allowed us to extract the contact

resistance of the FET (see the inset in Figure 8a). In Figure 8a we show the evolution of Rc as a

function of temperature for VGS = −20 V. Typical output curves used for these calculations are

shown in Figure 8b. The increase in the contact resistance with decreasing temperature is typical

for OFETs and is observed as more pronounced deviations from linearity in the low VDS region

Figure 8 (a) Evolution of the contact resistance with temperature for VGS

= −20 V. The

inflections indicate the impact that structural changes in the semiconducting material have on

the injection of charges between the SAM-treated device electrodes and diF-TES ADT. In the

inset, representative curves for the total device resistance as a function of channel length is

shown for several gate voltages. (b) Output characteristics of the same device characterized in

the α-phase (T = 300 K), β-phase (T = 220 K), and γ-phase (T = 120 K).


(Figure 8b). Systems in which the semiconductor undergoes no phase transition exhibit an

exponential dependence of Rc on temperature.[38] Nevertheless, in our measurements we also

observe an inflection point of the Rc vs. T curve as a result of the conversion of the β to γ

polymorphs, in agreement with our hypothesis that the changes in the crystal packing induce shifts

in the HOMO level. Studies are ongoing to describe in detail the impact of the transition on the

band structure of diF-TES ADT.

3.4 Conclusions

In conclusion, we have shown that (2,8- difluoro-5,11-bis(triethylsilylethynyl)

anthradithiophene) undergoes a low-temperature phase transformation at T = 205 K. We discovered

a new phase, which we define as the γ polymorph. This phase exhibits higher mobility in OTFTs,

in spite of the significantly higher contact resistance that results from an unfavourable shift in the

HOMO level. The phase transition is completely reversible in XRD and DSC measurements, but

once the system undergoes the changes, the device performance cannot be recovered. We believe

this effect results from the strain induced in the crystal upon the molecular rearrangement, which

yields cracks within the film. Our results provide a complete picture of the structure-property

relationships as a function of temperature in a high performance organic semiconductor, and

highlight the impact that fine adjustments in crystalline packing can have on the overall device

performance and operational stability.

72

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CHAPTER 4 75

TAILORED INTERFACES FOR SELF-PATTERNING

ORGANIC THIN-FILM TRANSISTORS

This work was published as J. W. Ward, M. A. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony, and O. D

Jurchescu, Journal of Materials Chemistry 22, 190487 (2012) – Reproduced by permission of The Royal

Society of Chemistry and part of this work was published as R. Li, J. W. Ward, D.-M. Smilgies, J. E.

Anthony, O. D. Jurchescu, and A. Amassian, Advanced Materials 24, 5553 (2012), reprinted with permission

(License Number: 3605560123653, John Wiley and Sons). For the Advanced Material manuscript, JWW

fabricated the devices, collected and analyzed the electrical data and assisted with structural data acquisition

and analysis.

Patterning organic thin-film transistors (OTFTs) is critical in achieving high electronic

performance and low power consumption. We report on a high-yield, low-complexity

patterning method based on exploiting the strong tendency of halogen-substituted organic

semiconductors to crystallize along chemically tailored interfaces. We demonstrate that the

organic semiconductor molecules self-align on the contacts, when the halogen-halogen

interaction is allowed by the chemical structures and conformations of the self-assembled

monolayer and organic semiconductor. The ordered films exhibit high mobilities and

constrain the current paths. The regions surrounding the devices, where the interaction is

inhibited, consist of randomly oriented molecules, exhibiting high-resistivity and

electrically insulating neighboring devices. To identify the role of F-F interactions in the

development of crystalline order, we investigate OTFTs fabricated on mono-fluorinated

benzene thiol treated contacts, which allows us to isolate the interactions between the F

originating from the organic semiconductor and the F in each position on the benzene ring

of the thiol, and to selectively study the role of each interaction. Combining the results

obtained from quantitative grazing incidence x-ray diffraction and Kelvin probe

measurements, we show that the surface treatments induce structural changes in the films,

but also alter the injection picture as a result of work function shifts that they introduce.

We show that both effects yield variations in the field-effect transistor characteristics, and

we are able to tune the field-effect mobility more than two orders of magnitude in the same

material.

Chapter 4. Tailored Interfaces for Self-Patterning OTFTs 76

4.1 Introduction

The performance of organic electronic devices is rapidly increasing, but their use in low-cost, large-

area applications is hindered by insufficient device performance and poor reproducibility. Efforts

addressing these issues are focused towards the development of new materials,[1–4] improving

structural order,[5, 6] and patterning thin-films to reduce the crosstalk between neighboring

devices.[7–9] Unfortunately, photolithography techniques developed for patterning inorganic

semiconductors are not compatible with organic semiconductors, as they damage the organic

materials due to the high energy of the patterning beams, or by the chemicals needed for the

processing.[10–12] Several approaches have been taken towards non-destructive patterning of organic

films. They include direct evaporation of organic semiconductors through shadow masks,[13] ink-

jet printing,[14] screen printing,[15] lithographic stamping,[16, 17] selective removal of organic layer by

UV exposure,[18] and thin-film processing using orthogonal solvents.[10] Unfortunately, most of

these methods consist of elaborate processing steps, which can be a barrier for low-cost mass

production of organic electronics.

Recently, self-patterning of organic thin- film transistors (OTFTs) was achieved by exploiting

the ordering of organic semiconductors driven by interactions at interfaces.[19] The observed

patterning was a result of the formation of a differential microstructure driven by the presence of

the pentafluorobenzenethiol (PFBT) self-assembled monolayer (SAM) on the surface of the

contacts and its absence on the dielectric surface. The microstructure consisted of ordered domains

of high conductivity on the contacts surrounded by poorly ordered-highly resistive domains, that

electrically insulate neighboring devices.[20] In this study, we uncover the mechanism responsible

for the observed film formation. This knowledge is indispensable for the practical control of

patterning OTFTs by differential microstructure, as well as for its implementation in complicated

device architectures. We propose that the halogen-halogen (F-F) interactions between the F atom

of the SAM and the F atom present in the backbone of the organic semiconductor diF-TES ADT

(2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene) is a strong interaction capable of

controlling the orientation of the organic semiconductor molecules (Figure 1a). Exploiting F-F

interactions has been previously used as a powerful tool in crystal engineering, allowing for rapid

and enhanced crystallization.[21, 22] We believe this interaction can also promote molecular assembly

at the surface of contacts and/or dielectrics. Additionally, we show that halogen-sulfur interactions

can play an important role in the development of crystalline order (Figure 1a).[23] Validation of the


proposed mechanism may allow the combination of synthetic diversity in molecular design with

device processing to achieve a general and powerful route for low-cost, high-throughput patterning

of OTFTs.

To identify the role of F-F interactions in the development of crystalline order, we study the

mono-fluorinated benzene thiols (MFBTs): 2-Fluorobenzenethiol (2-MFBT), 3-

Fluorobenzenethiol (3-MFBT), and 4-Fluorobenzenethiol (4-MFBT) (molecular structures

presented in Figure 1b). The use of these fluorinated SAMs (F-SAMs) allows us to isolate the

interactions between the F originating from the organic semiconductor and the F in each position

on the benzene ring of the F-SAM molecule, and to selectively study the role of each interaction.

To investigate the effect of the F-SAMs on the diF-TES ADT device properties, we

characterize the organic thin-films deposited on each treatment using field-effect transistor (FET)

measurements, optical microscopy, and grazing incidence X-ray diffraction (GIXD). Our results

a)

b)

Figure 1 a) Schematic representation of the proposed interactions between organic

semiconductor (diF-TES ADT) and fluorinated SAM treated contact (PFBT on Au). b)

Chemical structures of the mono-fluorinated SAMs used as probes for various interactions.

From left to right: 2-MFBT, 3-MFBT, 4-MFBT.


demonstrate that the location of the terminating fluorine atom within the structure of the SAM plays

a critical role in the development of order within the organic film. More subtly, when the F atoms

of the SAM and the semiconductor molecule are in close proximity, the strong interaction between

them induces a preferred orientation of the organic semiconductor, which results in a high electrical

mobility. We show that charge transport in OTFTs with F-SAM modified electrodes results from

an interplay between film microstructure and work-function shifts. Exploring these effects allows

us to achieve patterning by differential microstructure induced on selectively chemically modified

surfaces.

4.2 Experimental

4.2.1 SAM Fabrication and Characterization

SAM solutions were prepared by dissolving the self-assembled monolayers (Sigma Aldrich) in

ethanol (Sigma Aldrich). The SAM treatments were performed by soaking the substrate in a 30mM

room-temperature solution for 30 minutes followed by a 5 minute sonication in an ethanol bath and

a thorough ethanol rinse. The monolayer quality was characterized using the static sessile drop

method with de-ionized water. The contact angles were measured using a Ramé-Hart Model 200

Contact Angle Goniometer and calculated using DROPimage Standard.[24] The measurements were

conducted at room temperature in air and are displayed in Figure 2.

Work function measurements of the SAM modified contacts were evaluated as described in

Figure 2 Images used to measure the H2O angle are shown. The four treated Au surfaces all

exhibit similar wetting characteristics when compared to cleaned Au or SiO2.


Section 2.3.3. All samples were stored in air for about 12 hours to allow for full solvent removal

and film stabilization. PM-IRRAS measurements confirm that even longer air exposure does not

produce degradation of the SAMs used in this study.

Polarized Modulated-Infrared Reflection Absorption Spectroscopy (PM-IRRAS) studies, used

to evaluate the quality of the F-SAMs, were performed with an IR-Spectrometer Vertex 70 from

Bruker equipped with a Polarization Modulation Accessory (PMA 50 unit) and a mercury cadmium

telluride (MCT) detector (700 - 4000 cm-1).[24] SAMs and neat spectra were measured with a

resolution of 4 cm-1 and an incidence angle of 75º by averaging 350 interferograms. Fluorinated

monolayers, 2-MFBT, 3-MFBT, 4-MFBT and PFBT, self-assembled on Au substrates were

investigated. Neat spectra of the four different molecules were also collected within 2 minutes of 2

l deposition of molecules on a clean Au and Si substrate.


The device studies where carried out using bottom contact FETs on highly doped Si gate contact,

with a 200 nm thermally grown SiO2 gate dielectric. The source and drain contacts were defined

by photolithography and deposited by e-beam evaporation (5 nm Ti/ 45 nm Au). We cleaned the

substrates with hot acetone and isopropanol, followed by a UV-ozone and de-ionized water rinse.

The SAM treatments were performed as described above. The organic semiconductor was

synthesized at University of Kentucky[25] and spin-coated from a hot (55 °C) chlorobenzene

solution (Sigma Aldrich) (1.2% wt.) at 1000 rpm spinning speed. As-prepared devices were placed

in vacuum for 24 hours to ensure full solvent removal and electrically characterized using an

Agilent 4155C Semiconductor parameter analyzer.[24]

4.2.3 Structural Characterization of the Organic Semiconductors

Two complementary structural characterization techniques were used in this study. The µGIWAXS

technique was performed on a typical device structure (with patterned gold contacts on a SiO2

substrate) that provided structural information within a spatial resolution of 10 µm, allowing for a

2-D structural mapping of devices. The second was GIXD performed on diF-TES ADT films

deposited on gold substrates that replicated the electrode region of the transistors for treatment

scenario, as well as on untreated gold. These sets of data gave quantitative information about the

structure of the films.

The µGIWAXS was performed at the Cornell High Energy Synchrotron Source. This


technique, with its ability to resolve microstructural features on the micron-scale, allowed us to

map a 7 × 7 array of 1 × 1 cm2 OFETs of a variety of channel lengths. This work was first-of-its

kind in its ability to perform structural characterization on actual devices and to spatially-resolve

heterogeneous micro-domains within the films.[26] This insight is relevant as it allows us to locate

different structural features within the transistor channels, and to identify microstructural

bottlenecks to charge transport. In Figure 3, the μGIWAXS setup is shown. An intense X-ray beam

was focused onto the substrate, which was an actual FET, at an incidence angle of 2° by an X-ray

focusing capillary,[27] producing a X-ray beam with a footprint of 10 μm in the x direction and 610

μm in the y direction, as verified using a polished cadmium tungstate crystal at grazing incidence

as a fluorescent screen in lieu of the sample. An alignment procedure allowed us to collocate the

marker in the optical micrograph with the center (in x and y) of the x-ray microbeam. The spatial

resolution of the 10 μm microbeam enabled spatially-resolved measurements of the organic

semiconductor on the Au electrode, in the channel area between the two electrodes, and on the

oxide away from the electrodes. All three regions were located on the same OFET device.

Figure 3 Schematic of the μGIWAXS beam scanning a bottom contact device structure. Top

left: typical scattering mapping in the region off the contacts. Top right: footprint of the

microbeam (scale bar = 100 μm).


GIXD was performed at beam line 11-3 at the Stanford Synchrotron Radiation Lightsource

(SSRL). The samples were in a He-enclosed chamber to reduce air scattering and beam damage.

The incidence angle for grazing-incidence measurements was 0.12°. The data was collected using

a 2D image plate detector and is corrected for the polarization and the fixed incidence angle. Pole

figures were constructed using a combination of the grazing-incidence diffraction and a local

specular measurement as described by Baker et al.[28] The local specular measurement involved

rocking the sample incident angle 0.4° about the Bragg angle of the (001) peak during the

exposure. To calculate the orientation fractions, the integration of the pole figures must include the

change in solid angle as a function of . For samples with isotropic in-plane orientation, the total

population of crystals at a given azimuthal tilt goes up with sin while the pole figure measures a

constant solid angle.[29] As a result, the pole figures substantially under-represents the crystal

population at large azimuthal tilts.

4.3 Results and Discussion

Polarization Modulation Infrared Reflections Adsorption Spectroscopy (PM-IRRAS)

measurements carried out on the Au substrates with each of the SAM treatment indicates

preferential orientation of the organic monolayers.[30] Figure 4 shows the IR spectra of the four

SAMs and neat fluorinated molecules (a) 2-MFBT, (b) 3-MFBT, (c) 4-MFBT and (d) PFBT. In the

four neat spectra, three different groups of vibrations can be identified. The benzene ring

compounds commonly exhibit several bands in the region 1600 - 1400 cm-1 attributed to C-C

aromatic stretching modes.[31] Another group of vibrational bands appear in the range of 1300 -

1000 cm-1 and are assigned to C-H bending and C-F stretching modes.[31, 32] Finally, out-of-plane

bending vibrations are observed below 1000 cm-1. The SAMs’ spectra show a reduced number of

vibrational bands compared to neat molecules as a consequence of the IR surface selection rule (i.e.

IR band with transition dipole moment (TDM) oriented parallel to the metal surface are absent).[30]

Therefore, preferential observation of some vibrational modes denotes a degree of molecular order

in the monolayer. The peaks observed at 1600 - 1500 cm-1 are attributed to modes with a TDM

along the S-aromatic ring axis,[33] thus, a molecular geometry lying 100% parallel to the substrate

surface is ruled out for the 3-MFBT, 4-MFBT and PFBT.[34] Interestingly, 2-MFBT SAMs show

peak shift frequency compared to 2-MFBT neat spectra that is not observed in the other 3 SAMs.

This is likely attributed to a higher degree of molecular tilting. IR studies on fluorinated


benzenthiols are rather limited but differences in molecular arrangement for substituted

benzenethiols on a metal surface have been also identified by other techniques.[35] For example,

STM imaging studies on para and ortho-substituted halo benzenes identify significant differences

in molecular packing and ordering of these two molecules. Lower surface coverage and a more

inclined arrangement have been identified for the ortho-substituted molecules attributed to complex

combination of interaction energies (i.e. dipole-dipole, S-F, quadrupole).[36] Preferential orientation

close to tangential has been reported for para-halosubstitued benzene rings and PFBT, in agreement

with our results.[36–38] Out-of-plane vibrations (associated dipole moment perpendicular to the

phenyl ring) are extremely useful for quantitative orientational analysis;[39] however, in our SAM

samples they could not be unambiguously identified (900 - 700 cm-1) because they appear near to

the detector cutoff and this spectral region becomes unreliable. Consequently, in this work we keep

orientational analysis qualitative. Based on our IR results and the STM data available in the

literature,[36, 40] we estimated a tilt angle range from the surface normal of 40º - 70º for the 3-MFBT,

4-MFBT and PFBT and 80º - 90º for 2-MFBT. This is consistent with previous studies on

Figure 4 PM-IRRAS of fluorinated SAMs and the corresponding neat spectra of (a) 2-MFBT,

(b) 3-MFBT, (c) 4-MFBT and (d) PFBT.

800 1000 1200 1400 1600 1800 2000

4-MFBT neat

Int (A

.U.)

Wavenumber (cm-1)

4-MFBT SAM on Au

800 1000 1200 1400 1600 1800 2000

2-MFBT neat

Int (A

.U.)

Wavenumber (cm-1)

2-MFBT SAM on Au

800 1000 1200 1400 1600 1800 2000

PFBT neat

Int (A

.U.)

Wavenumber (cm-1)

PFBT SAM on Au

800 1000 1200 1400 1600 1800 2000

3-MFBT neat

Int

(A.u

.)

Wavenumber(cm-1

3-MFBT SAM on Au

(a)

(d)(c)

(b)


substituted benzenethiols and results from the steric hindrance and the intermolecular interactions

between the aromatic rings.[38, 40] More accurate orientational analysis is impeded by the difficulty

to unambiguously identify out-of-plane vibrations in the SAM’s PM-IRRAS spectra.[30]

Optical micrographs taken on the FETs fabricated on bottom contacts chemically modified

with the SAMs of interest are presented in Figure 5. In Figure 5a we present the case of untreated

contacts, for comparison. This film consists of very fine grains and only a few distinguishable

features. Figure 5(b-c), corresponding to the 2-MFBT and 3-MFBT, respectively, show a gradual

increase in the grain size. Note that in all these cases the morphology of the film on the contact

region is comparable to that observed on the oxide region of the FET. On the contrary, the devices

presented in Figure 5(d-e), fabricated on contacts chemically tailored with 4-MFBT and PFBT,

respectively, exhibit very different crystal habits, consisting of large grains on the contacts,

protruding into transistor channel, and fine grains in the middle of the channel. Figure 5f is a

schematic representation of the differential microstructure present on the source-drain contact

regions and the oxide regions.

We believe the changes observed in the film features originate from the different F-F

interactions that each of the F-SAMs provide, and are not a result of the modifications in surface

energies, as reported in the case of other SAMs.[41, 42] This is supported by our goniometry

measurements (Figure 2), which show that the water contact angles are similar for all treatments.

a) Untreated

50 µm

b) 2-MFBT

50 µm

c) 3-MFBT

50 µm

d) 4-MFBT

50 µm

e) PFBT

50 µm

f) Schematic

Source Drain

Channel

Small GrainsLarge Grains

Figure 5 Polarized optical micrographs showing diF-TES ADT thin-film characteristics on

Untreated contacts (a), 2-MFBT (b), 3-MFBT (c), 4-MFBT(d), and PFBT (f) treated contacts.

The OFET schematic (f) identifies differential microstructure on the contacts and the dielectric

in devices.


The static water contact angle (θH2O) values are summarized in Table I at the end of the chapter; for

comparison, the water contact angle of pristine SiO2 is lower than 5°. This result confirms that, in

this case, the surface energy following SAM treatment does not play a dominant role in determining

the structure of the organic semiconductor film, and the observed effects arise from interactions

between the semiconductor and the F-SAM.

The μGIWAXS measurements allowed us to obtain spatially-resolved microstructural

information from real devices non-destructively and non-invasively. Compared with existing

microscopic methods, such as scanning transmission X-ray microscopy (STXM)[45, 46] and Raman

microscopy,[47, 48] both of which are indirect and highly localized μGIWAXS provides direct and

quantitative microstructural information, including lattice constants, grain size, orientation

distributions and polymorphism averaged along the entire channel width of a working OFET

device. μGIWAXS confirmed that the diF-TES ADT film consists of purely (001) oriented

molecules on PFBT treated Au electrodes, with diffuse scattering just above the attenuator in Figure

3. The film also maintained the same, pure, (001) orientation within the channel for short channel

devices (Figure 6a), however, in the regions away from the electrodes both (001) and (111)

orientations co-exist.

Figure 6 Microstructural mapping of diF-TES ADT devices with channel lengths of (a) 20 μm

and (b) 80 μm. Top shows schematic view of the device structure. Middle is a polarized optical

micrograph of a portion of the device. The bottom graphs are (top) the integrated intensity map

of the diffraction peaks associated to Au, <001> and <111> and (bottom) the out-of-plane

lamellar crystal size and angular full-width at half max (FWHM) determined from the (001)

Bragg sheet of <001>-textured crystal.


In Figure 6 we map the microstructure across a device with a channel length of 20 μm and

across a device with a channel length of 80 μm. We show a side view of the each of these device

structures in the top panel, a polarized optical micrograph of a portion of the device in the middle,

and the bottom graphs are (top) the integrated intensity map of the diffraction peaks associated to

Au, <001> and <111> and (bottom) the out-of-plane lamellar crystal size and angular full-width at

half max (FWHM) determined from the (001) Bragg sheet of <001>-textured crystal. In Figure 6a,

for L = 20 μm, large grains with a pure (100) orientation are shown to extend through the channel

of the device, while in Figure 6b, for L = 20 μm, while those grains extend deep into the channel,

they terminate before they reach the adjacent electrode. The Au (111) intensity maps help to discern

the channel from the buried electrodes, making it possible to distinguish the microstructure of diF-

TES ADT in the channel and on the electrodes. By following the integrated intensity of the (001)

peaks and the sum of the (121 and (122) diffraction peak intensity associated to the <111> texture,

we find that the <001> texture is present everywhere, whereas the <111> texture appears only in

the middle of the channel where there was no PFBT treatment present. The (001) oriented film

extends nearly 25 μm away from the treated contacts, thus allowing a bridge between the contacts

in devices with smaller channel lengths.

GIXD allows us to determine the quantitative structural details of the films, and to establish

the mechanism of the formation of this unique microstructure. Figures 7(a-e) show the 2D-GIXD

measurements taken on diF-TES ADT films spin coated onto untreated Au substrates and substrates

containing each of the four treatments. These substrates replicate the film morphology on the

electrode region of the transistor structure. All films exhibit signatures of high crystallinity, but a

careful inspection points to the fact that the molecular orientations within the films are quite

different. Films deposited on PFBT and 4-MFBT treated Au (Figure 7d-e) display similar structural

features, with all crystallites preferentially orientated along the (001) crystallographic direction. In

this orientation, the molecular backbones are aligned “edge-on”, meaning perpendicular to the

surface, as depicted in Figure 7f, left. This allows for large π-π overlap in the direction of charge

transport, supporting high charge carrier mobility. The films deposited on untreated Au, 2-MFBT

and 3-MFBT (Figure 7a-c) exhibit richer diffraction features. Here, we identify the peaks denoting

(001) orientation, as well as additional ones distinguished by red ovals. The latter peaks can be

indexed to (111) orientation of the same crystal structure where the molecules lay “face-down” on

the substrate (Figure 7f, right).[43] The films containing mixed orientations exhibit low mobility and

high current noise.[44] By using the pole figure comparing the relative intensity of the (001) peak as

a function of the polar angle, χ (Figure 8), we extract quantitative information about the (001)


orientation distribution in each film. The full-width half-maximum (FWHM) of the (001)

orientation is about 2° and the FWHM of the (111) orientation is about 8°. For films deposited on

untreated Au, we found that 66% of the crystals have a nominal (001) orientation relative to the

substrate. The (001) content for films deposited on treated Au is determined to be 55% for 2-MFBT,

94% for 3-MFBT, 100% for 4-MFBT, and 100% for PFBT. These findings clearly indicate that

the films on 4-MFBT and PFBT treated contacts are similar and contain 100% (001) crystals, while

films on untreated, 2-MFBT and 3-MFBT treated contacts contain a mixture of (111) and (001)

crystals in various ratios. The observed structural features for the untreated and PFBT treated films

are in agreement with those reported earlier by Kline et al.[20] Introduction of mono-fluorinated

SAMs in this study, with the fluorine atom located in specific positions, allowed us to selectively

investigate the effect of each F, and to develop a thorough description of the mechanism responsible

for the development of the crystalline order.

The different content of (001) phase detected for the three MFBTs, while having similar surface

wettability, supports our hypothesis that the F-F interactions between the SAM and the organic

d) 4-MFBT

a) Untreated b) 2-MFBT c) 3-MFBT

e) PFBT f) Orientation Schematic

(111)

(001)

Figure 7 GIXD of films of diF-TES ADT on (a) Untreated Au, (b) 2-MFBT, (c) 3-MFBT, (d)

4-FBT, (e) PFBT treated Au. Peaks identified with the circles correspond to (111) orientation,

while un-marked peaks denote (001) orientation. f) Schematic of the molecular orientation on

the Au surfaces for (001) and (111) orientations.


semiconductor are responsible for the observed microstructure. The different location of the F atom

within the F-SAM either allows a geometrically favorable interaction with the F termination of the

organic semiconductor’s backbone, or frustrates this interaction. If either the SAM or the organic

semiconductor does not contain an F atom in their structure, or if this atom is not located in a

position suitable for interaction, self-patterning by differential microstructure cannot occur. This

hypothesis is further supported by the work of Kymissis et al, who examined the effects of PFBT

treatment on un-halogenated materials and concluded that no significant interactions occur between

the two species.[49]

Within this scenario, the highest degree of structural order, with 100% (001) phase, measured

in the case of films deposited on 4-MFBT, is a result of the fact that here the F-F interactions are

permitted by the close vicinity between the F in position 4 on the benzene ring and the F of the

organic semiconductor. The identical structure of the diF-TES ADT thin-films on 4-MFBT and

PFBT treated Au suggests that the dominant F-F interaction between the SAM and the fluorinated

semiconductor occurs at the F located in position 4 on the thiol benzene ring also in the case of the

PFBT SAM (Figure 1a). The films deposited on 3-MFBT treated Au accommodate a smaller

fraction of (001) oriented crystals, but still considerably higher compared to untreated Au. This

may result from the fact that the fluorine located in position 3 within this SAM is geometrically

-80 -60 -40 -20 0 20 40 60 8010

0

101

102

103

104

105

106

Co

un

ts [a

rb.]

Chi [°]

Untreated

2-MFBT

3-MFBT

4-MFBT

PFBT

Figure 8 GIXD full pole figure comparing relative intensities of a (001) peak for each treatment

case.


suitable to only facilitate a weak F-F interaction compared to the previous case, but a F-S interaction

between the fluorinated SAM and the sulfur on the semiconductor is not excluded, and is under

investigation. Additionally, unlike the case of PFBT or 4-MFBT, where due to symmetry the

molecular dipole is always pointing in the same direction, here the F-SAM will present a surface

where the dipoles are pointing in random directions. Since the F may be in either position 3 or 5 of

the benzene ring, regular crystal growth may be disrupted. The films deposited on 2-MFBT treated

Au present a small content of (001) orientation, similar to the untreated case, because the fluorine

atom is located in a position that cannot promote any interaction with the fluorinated

semiconductor. In the absence of these F-F interactions, the first organic semiconductor layer is not

anchored to an edge-on configuration, and cannot template the arrangement of the following layers

in an ordered fashion. While these observations clearly mark the importance of F-F interactions in

assisting the self-patterning process, the nature of these interactions is still under investigation.

The electrical response of the TFTs fabricated on Au electrodes modified with the F-SAMs

investigated in this study is in agreement with the structural differences induced by these

treatments. In Figure 9a we show typical current-voltage characteristics: the evolution of the drain

current versus gate-to-source voltage (ID versus VGS) for a PFBT treated device biased in the

-40 0 40

0.000

0.002

0.004

0.006

I1/2

D [

A1/2]

VGS

[V]

10-9

10-8

10-7

10-6

10-5

10-4

ID [A]

a)

10-3

10-2

10-1

100

sat [c

m2V

-1s-1]

Treatment

5.3

5.2

5.1

5.0

4.9

4-MFBT+Au

PFBT+Au

3-MFBT+Au

Untreated Au

2-MFBT+Au

En

erg

y [e

V]

5.3

5.2

5.1

5.0

4.9

diF

-TE

S A

DT

10

0%

(0

01

)

diF

-TE

S A

DT

(00

1)

+ (

11

1)

0.10

0.18

0.17

b)

c)

Untr

eate

d

2-M

FB

T

3-M

FB

T

4-M

FB

T

PF

BT

Figure 9 (a) Current-voltage characteristics for a representative PFBT treated device, of channel

length 20 µm and channel width 800 µm. Right axis: (log(ID) vs V

GS). Left axis: vs V

GS plot in

the saturation regime. (b) Mobility at L = 5 µm for devices fabricated on untreated and F-SAM

treated contacts. (c) Energy diagram at the interfaces between F-SAM treated Au and the HOMO

corresponding to mixed orientation, (111) + (001), diF-TES ADT (left) and (001) diF-TES ADT

(right).


saturation regime (source-drain voltage, VDS = -40 V). The curve on the left axis represents √𝐼D

versus VGS and was used to calculate the saturation mobility (µsat) using the following the

expression:

𝜇sat =2𝐿

𝑊𝐶i (

𝜕√𝐼D

𝜕𝑉GS)

2

(3.1)

where 𝐶i = 17.3 nF cm-2 is the geometrical capacitance of the dielectric.

In Figure 9b we plot the field-effect mobilities for devices fabricated on each contact treatment.

The measured mobilities span almost three orders of magnitude, from 5.5 × 10−3 (±7.4 × 10−4 )

cm2V-1s-1 for untreated contacts (black), to 3.0 × 10−1 (±5.2 × 10−2 ) cm2V-1s-1 for the PFBT

treated contacts (red). Devices fabricated on MFBTs lay in between these values, and mobility

increases from 1.6 × 10−2 (±1.5 × 10−3 ) cm2V-1s-1 for the 2-MFBT (purple), to 3.8 × 10−2

(±7.7 × 10−3 ) cm2V-1s-1 for the 3-MFBT (blue), and 1.8 × 10−1 (±5.5 × 10−3 ) cm2V-1s-1 for

the 4-MFBT (orange). The averages are obtained over at least 10 devices. While these dramatic

differences in the electrical properties of the same material, promoted by varying the F-SAM

treatment, are a result of both the different grain-size environment and the molecular orientation

induced by the F-F interactions, the polar nature of the F-SAM molecules at the surface of the Au

contacts also results in a shift in the work function of the electrodes and modifies the injection

0 50 100 150 200

5.4

5.2

5.0

4.8 4-MFBT + Au

Wo

rk F

un

ctio

n [

eV

]

Time [s]

3-MFBT + Au

2-MFBT + Au

Figure 10 The work function evolution throughout time as measured by Kelvin probe. The

breaks in the signal represent the grounding of the detector during the movement to a different

spot on the sample.


picture.

Figure 10 shows the work function of the 2-MFBT (Purple), 3-MFBT (Blue), and 4-MFBT

(Orange) treated Au substrates. Uniformity between spots suggests a uniform SAM film on Au,

while the consistency through time points to a stable electrode work function in the presence of the

F-SAM treatments. Our clean Au exhibits a work function 𝑊𝐹Au=4.91 eV, in agreement with the

results reported from UPS studies.[50–52] The measured shifts in the electrode work-function as a

result of SAM treatment are reported in Table I and plotted in Figure 11. The reported values are

averages obtained over 15 measurements. The 2-MFBT treatment results in a negligible shift

from 𝜙Au as the dipole moment vector is likely parallel to the surface. The presence of the fluorine

atom farther from the Au surface accommodates an increased surface normal component to the

dipole, and for this reason, the 3-MFBT treated Au shows a greater shift in work function, ∆𝑊𝐹3-

MFBT = +0.07 eV. For the cases where the greatest component of the dipole moment is normal to

the electrode surface, 4-MFBT and PFBT treated Au, the shift in work function increases further

to: ∆𝑊𝐹4-MFBT = +0.43 eV and ∆𝑊𝐹PFBT = +0.39 eV. The increase in electrode work-function as a

result of F-SAM treatment allows for a reduction in the parasitic contact resistance, as resulting

from our estimations using the gated transmission line method.[53] The contact resistance for each

electrode configuration is shown in Figure 12.

To complete the injection picture, we estimated the position of the HOMO level of the organic

semiconductor using Kelvin Probe. It was shown with other molecular systems that the position of

the HOMO and LUMO levels depends on molecular orientation.[54] Indeed, our films consisting of

-40 0 40

0.000

0.002

0.004

0.006

I1/2

D [

A1/2]

VGS

[V]

10-9

10-8

10-7

10-6

10-5

10-4

ID [A]

a)

10-3

10-2

10-1

100

sat [c

m2V

-1s-1]

Treatment

5.3

5.2

5.1

5.0

4.9

4-MFBT+Au

PFBT+Au

3-MFBT+Au

Untreated Au

2-MFBT+Au

En

erg

y [e

V]

5.3

5.2

5.1

5.0

4.9

diF

-TE

S A

DT

10

0%

(0

01

)

diF

-TE

S A

DT

(00

1)

+ (

11

1)

0.10

0.18

0.17

b)

c)

Untr

eate

d

2-M

FB

T

3-M

FB

T

4-M

FB

T

PF

BT

Figure 11 Energy diagram at the interfaces between F-SAM treated Au and the HOMO

corresponding to mixed orientation, (111) + (001), diF-TES ADT (left) and (001) diF-TES ADT

(right).


pure (001) oriented crystals are shifted with approximately 0.14 eV in the position of the HOMO

level when compared to the films of the mixture of (111) + (001). While the measured shifts are

accurate representations of the differences in the HOMO level locations for the two types of films,

their absolute values should be regarded as estimates, as the Kelvin probe measurements yield the

chemical potential of the organic semiconductor, which is slightly shifted with respect to its

HOMO. Nevertheless, the fact that our films containing mixed orientations have a HOMO of 5.08

eV, in agreement with the results obtained from ultraviolet photoemission spectroscopy combined

with inverse photoelectron spectroscopy (UPS/IPES) by Anthopoulos et al, on films grown on

indium tin oxide (ITO), which likely were composed of a mixture of the two orientations, point to

the fact that our measurements give a good estimation of HOMO level.[55] On the contrary, for the

films containing 100% (001) molecular orientation, we determine a HOMO of 4.94 eV. Combining

the results from GXID and Kelvin Probe measurements, we can now complete the charge injection

and transport picture. The different situations are presented in Figure 11, where the work functions

of the electrodes are represented following the same color scheme used for Figure 9 and the gray

shaded area represents the valence band of the organic semiconductor, where hole-injection occurs.

The left panel represents the injection from untreated contacts, 2-MFBT and 3-MFBT treated

-10 -20 -30 -40 -50 -6010

2

103

104

105

PFBT + Au

4-MFBT + Au

3-MFBT + Au

2-MFBT + Au

Untreated AuR

CW

[

m]

VGS

[V]

Figure 12 Contact resistances as a function of gate voltage for bottom contact transistors of

untreated Au contacts (black square), 2-MFBT (purple circle), 3-MFBT (blue triangle up), 4-

MFBT (orange triangle down), and PFBT (red diamond) treated Au contacts.


contacts into the film composed of the (111) + (001) mixture. Here, the hole-injection barriers range

from 0.17 eV for untreated Au to 0.18 eV for the 2-MFBT treated Au and 0.10 eV for the 3-MFBT

treated Au. The low injection barriers suggest that the transport in our devices is not contact-

limited.[56] This is in agreement with the decrease in mobility with increasing the channel length

(Figure 13), a trend consistent with a grain-boundary limited charge transport.[57, 58] The increase in

the mobility values presented in Figure 9b from untreated Au, to 2-MFBT, and 3-MFBT treated

electrodes arise from the cooperating effects of reducing the density of grain boundaries and

increasing the content of (001) phase. The right panel represents the injection from the 4-MFBT

and PFBT treated electrodes, which are shifted to higher work functions because of the stronger

dipole within the SAM. Here, the organic semiconductor films presents pure (001) orientation and

a lower HOMO level. As a result, the energetic barrier vanishes, allowing for efficient hole-

injection. The edge-on molecular orientation further allows efficient charge transport and as a result

the measured mobilities are the highest.

Figure 13a presents the evolution of the mobility against the transistor channel length for TFTs

with various surface treatments and demonstrates the efficiency of the proposed self-patterning

10 100

10-2

10-1

PFBT

4-MFBT

3-MFBT

2-MFBT

Untreated

[cm

2V

-1s

-1]

L [m]

c)

50 µm

b)

50 µm

a)

Figure 13 a) Evolution of µsat

with L for diF-TES ADT bottom contact devices fabricated on

untreated contacts (black square), 2-MFBT (purple circle), 3-MFBT (blue triangle up), 4-MFBT

(orange triangle down), and PFBT (red diamond) treated contacts. Panels b) and c) show

polarized optical micrographs of the films deposited on PFBT and 2-MFBT treated contacts,

respectively.


technique. While for all treatments the mobility decreases with increasing channel length, a few

unique features can be distinguished for each of them. In PFBT (red diamond) and 4-MFBT (orange

triangle down) treated devices, the similar mobilities agree well with the structural and injection

data. These devices exhibit two distinct regimes that can be distinguished by a dramatic change in

the rate at which the mobility decreases as the channel length increases, located at L = 50 µm. At

short channel lengths, large grains consisting of the (001) molecular orientation, which is

preferential to charge transport and the fast crystal growth direction, extend from the contacts to

span the full length of the transistor channel, in agreement with the μGIWAXS measurements

shown in Figure 6a.[20] Here, the most favorable charge injection and transport scenario applies

(Figure 11, right). For TFTs with larger source-drain separation (L > 50 µm) an optically visible

break in large grains is observed (Figure 13b), and this film consists of a mixture of (001) and (111)

orientations, as suggested by Figure 6a. As the F-SAM does not attach to the SiO2 surface, and thus

the F-F interaction is not present here, the film in the middle of the channel consists of fine grains

of mixed orientations, similar to the case of untreated Au. While the injection corresponds to the

right panel of Figure 11, the charge transport in the channel region of the transistor is a combination

of competing effects arising from the presence of large and small grains, as well as the mixed

molecular orientations in the film within the channel. The poorly ordered regions of high resistivity

present in the case of 4-MFBT and PFBT Au both the middle of the channel for long-channel

devices and the regions surrounding all devices, provide electrical insulation between neighboring

transistors. Self-patterning has occurred as a result of the formation of the different microstructure

on the contacts versus their vicinity. On the contrary, when the F-F interactions are inhibited, due

to lack of proximity between the F atoms, or when the F-SAM is not present, self-patterning is not

successful (Figure 13c). This is the case of devices fabricated on 3-MFBT (blue triangle up), 2-

Table I Summary of Measurements: mobility, induced shift in work function, percentage of (001) oriented crystals in thin film and water contact angle for each surface treatment.


MFBT (purple circle), and the untreated devices (black square), where a lower mobility is

measured, as expected from the presence of mixed orientations and the larger injection barriers

(Figure 11, left). The mobility decreases monotonically with increasing channel length. In these

cases, on average, there are fewer large grains present on either the contact or oxide regions of the

transistor structure.

The effectiveness of our patterning technique is also reflected in the values of the on/off ratios

measured for each treatment (data for L = 5 µm is included in Table I). A clear increase from ~104

in the case of untreated devices to ~107 in 4-MFBT and PFBT OTFTs supports our scenario of F-

F interactions inducing self-patterning by differential microstructure. Our results clearly mark the

fact that when the F atoms of a SAM and organic molecule are in close proximity, an exploitable

interaction occurs between them. This interaction is the governing factor in initiating a template

structure for molecular orientation. Once the first layer is formed, the film adopts the same

morphology throughout its thickness.

We suspect that the F-F interactions are weaker than the intermolecular interactions within the

crystals, and the proposed scenario can only occur if the intrinsic crystalline packing of a material

allows such an arrangement. For example, in difluoro-(tri-sec-butylsilyl ethynyl) anthradithiophene

(diF-TSBS ADT),[59] a material with similar chemical structure, but which adopts a sandwich-

herringbone packing, the F-F interactions are not sufficiently strong to induce a co-facial packing

in OTFTs fabricated on F-SAM treated contacts. This is reflected in similar electrical and structural

properties for the case of PFBT treated and untreated contacts (Figure 14).


4.4 Conclusions

Differential microstructure allowing self-patterning of organic thin-film transistors proceeds via F-

F interactions, when the halogen interactions are allowed by the chemical structures and

conformations of the SAM and organic molecule. The patterning we observe as a result of F-SAM

induced differential microstructure is fundamentally different than the patterning resulting from

tuning the hydrophobicity of the contacts and oxide regions. The size obtained for the high mobility

domains (up to 50 µm, reproducibly demonstrated) meets the requirements of most applications,

and is thus technologically relevant. Smaller device size can be easily defined by controlling the

source-drain separation on the substrate using conventional photolithography. Our results provide

insights for the rational design of novel molecular structures targeted towards low temperature

patterning in the presence of SAMs.

Figure 14 GIXD on films of diF-TSBS ADT on a) PFBT treated Au and b) Untreated Au. c)

Evolution of mobility with channel length for diF-TSBS ADT bottom contact devices fabricated

on PFBT treated contacts (black square) and untreated (red circle) contacts.

0 20 40 60 80 10010

-4

10-3

10-2

10-1

PFBT Treated Contacts

Untreated Contacts

Mo

bili

ty [

cm

2V

-1s

-1]

Channel Length [m]

c) Electrical Properties

96

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CHAPTER 5 101

RATIONAL DESIGN OF ORGANIC SEMICONDUCTORS FOR TEXTURE

CONTROL AND SELF-PATTERNING ON HALOGENATED SURFACES

This work was published as Jeremy W. Ward, Ruipeng Li, Abdulmalik Obaid, Marcia M. Payne, Detlef-

M. Smilgies, John E. Anthony, Aram Amassian, and Oana D. Jurchescu, Advanced Functional Materials 24

(32), 5052 (2014), and is reprinted with permission (License Number: 3605560235519, John Wiley and

Sons).

Understanding the interactions at interfaces between the materials constituting consecutive

layers within organic thin-film transistors (OTFTs) is vital in optimizing charge injection

and transport, tuning thin-film microstructure and designing new materials. Here, we

explore the influence of the interactions at the interface between a halogenated organic

semiconductor (OSC) thin film and a halogenated self-assembled monolayer on the

formation of the crystalline texture directly affecting the performance of OTFTs. By

correlating the results from microbeam grazing incidence wide angle scattering

(µGIWAXS) measurements of structure and texture with OTFT characteristics, we find

that two or more interaction paths between the terminating atoms of the semiconductor and

the halogenated surface are vital to templating a highly-ordered morphology in the first

layer. These interactions are effective when the separating distance is lower than 2.5 dw,

where dw represents the van der Waals distance. We demonstrate the ability to modulate

charge carrier transport by several orders of magnitude by promoting “edge-on” versus

“face-on” molecular orientation and crystallographic textures in OSCs. We find that the

“edge-on” self-assembly of molecules forms uniform, (001) lamellar-textured crystallites

which promote high charge carrier mobility, and that charge transport suffers as the fraction

of the “face-on” oriented crystallites increases.

Chapter 5. Rational Design of OSC for Texture Control and Self-Patterning 102

5.1 Introduction

Solution processable OSCs promise to allow mass-production of electronic devices at a low-cost,

on flexible substrates such as plastics, paper and even on human skin.[1–5] The unique properties of

such materials enable new paradigms for device manufacturing that go beyond the limitations of

their a-Si:H counterpart and significantly reduce the cost and complexity of processing. For

example, it has been shown that high performance organic single crystal and thin-film devices can

be simultaneously deposited and patterned using ink-jet printing,[6, 7] and that both the OSC and

dielectric layers can be deposited in a one-step process by controlling their phase-separation from

the mixed solution.[8, 9] In these demonstrations, the relationship between the OSC and its interfaces

within the device was successfully exploited to achieve the desired morphology. However, there is

little insight into the roles of molecular design and interfacial interactions on molecular self-

assembly in the context of heterogeneous nucleation at the solution-solid interface.[10] A recent

development in this area is the successful use of fluorine-fluorine (F-F) interactions between the

conjugated molecule and the surface to control crystallization and engineer film ordering.[11–15]

Fluorine-terminated OSCs can exhibit a differential microstructure when deposited on surfaces

selectively treated with fluorinated self-assembled monolayers (F-SAM): the formation of

crystalline domains consisting of (001)-textured lamellar sheets of molecules, with in-plane π-

stacking capable of effectively transporting charges in the plane of the semiconductor-dielectric

interface, only occurs on and in the vicinity of regions where the F-F interactions are present. The

surrounding regions exhibit a mixture of crystalline textures, which inhibit effective carrier

transport, slowing it down by several orders of magnitude. The interface-driven growth behavior

yields improved device performance by self-patterning the organic thin-film transistors (OTFTs)

over an array of electrodes.[13, 14] Our earlier investigations identified the role that the F-F

interactions between the F-SAM and a fluorinated OSC have on the development of the crystalline

order within the organic film and evaluated their strength with respect to the van der Waals

intermolecular interactions.[12] However, the generality and the limitations of OTFT microstructural

patterning via F-F interactions are not known. This insight is deemed crucial for the rational design

of novel molecular structures coupled with surface treatments to enable high performance bottom-

contact OTFT devices. The identification of appropriate combinations of interfacial layers and

OSCs can also assist in the development of cost-effective, bottom-up self-patterning procedures for

organic electronic structures. We have therefore designed a series of experiments based on several


combinations of F-SAMs and halogenated-OSCs to investigate F-F and F-S interactions.

Displayed in Figure 1 is the chemical structure of the OSCs we employed in this study: (a)

diF-TES ADT (2,8- difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene) referred to as ADT,

(b) diF-TIPS Pn (a 1:1 isomeric mixture of 1,8- and 1,11-difluoro 6,13-bis(triisopropylsilylethynyl)

pentacene) referred to as Pn, and (c) diF-TSBS PDT (2,10-difluoro-5,13-bis(tri-sec–

butylsilylethynyl) pentadithiophene) referred to as PDT. Each of the above molecules crystallizes

into lamellae with in-plane 2D-brickwork packing motif, which is necessary to achieve an “edge-

on” co-facial packing within the OSC layer favoring 2D in-plane charge transport. The packing

motif is crucial, as we have previously shown the interactions between the F-semiconductor and F-

SAM are not sufficiently strong to modify the growth pattern of other packing motifs.[12, 16, 17] The

ADT and PDT promote both F-F and F-S interactions with the F-SAM, while the Pn only allows

the F-F interactions. This chemical difference offers us an experimental platform for the

investigation of a secondary interaction, F-S, that was suggested to impact the microstructure within

the organic films.[18] The length of the heteroacene backbone and its orientation with respect to the

surface normal affect the distance between interacting terminal atoms and provide different

interaction strengths for each of the three cases. The two F-SAMs, pentafluorobenzenethiol (PFBT)

and 4-fluorobenzenethiol (4-MFBT), allow either one (4-MFBT) or multiple (PFBT) F-F

interaction opportunities.

By correlating the crystalline texture from microbeam grazing incidence wide angle x-ray

scattering (μGIWAXS) with electrical properties derived from OTFT measurements, we find that

two conditions are necessary for successfully establishing the differential microstructure which

(a) (b) (c)

Figure 1 Chemical structures for (a) diF-TES ADT (ADT), (b) diF-TIPS Pn (Peri) (Pn) and (c) diF-

TSBS PDT (PDT).


promotes self-patterning and high electronic performance. First, at least two anchoring points must

be established. These atoms originating from either the semiconductor or the F-SAM can connect

with molecular moieties within the consecutive layer via F-F and/or F-S interactions. We believe

this bonding, once established, can prevent any molecular twist in the first layer of the organic film

and thus promotes the formation of a highly ordered templating layer. Second, when the separation

between the interacting atoms is larger than about twice the sum of their van der Waals radii, the

F-F and F-S interactions are too weak to dictate the polycrystalline texture within the film.

5.2 Experimental

5.2.1 Self-Assembled Monolayer Treatment

SAM treatment solutions were prepared by dissolving the respective molecules in ethanol (Sigma

Aldrich). The treatment consisted of soaking the substrate for 30 minutes in a 30 mM room-

temperature solution followed by a 5 minute sonication in an ethanol bath for removal of the bulk

layers. The substrates were then rinsed with ethanol and dried using a stream of N2 gas.


The device characterization was carried out using bottom contact OTFT structures on a highly

doped Si gate electrode with a 200 nm thermally grown SiO2 gate dielectric. The source and drain

contacts (5 nm Ti/45 nm Au) were defined by optical photolithography and deposited by e-beam

evaporation. The substrates were cleaned in hot acetone and isopropanol, followed by a 10 minute

UV-ozone exposure, ethanol rinse, and N2 drying. The SAM layers were subsequently applied, as

described in Chapter 2. The OSCs were dissolved in a 1.2 wt.% solution in high purity

chlorobenzene (Sigma Aldrich).[19, 20] The warm (55 °C) solution was spin cast onto the substrates

at a 1000 rpm spinning speed and the obtained films were placed in a vacuum oven at room-

temperature for 24 hours to ensure full solvent removal. The devices were electrically characterized

in a nitrogen environment using an Agilent 4155C Semiconductor parameter analyzer.

5.2.3 Structural Characterization of the Organic Semiconductor

The μGIWAXS measurements were performed at D1-line, Cornell High Energy Synchrotron

Source (CHESS) at Cornell University. The X-ray beam with a wavelength of 1.155 Å and a wide

bandpass (1.47%) was focused into a 10 x 10 m2 spot using a single-bounce X-ray focusing


capillary positioned on a V groove. The samples were placed on the focal point of the capillary (35

mm away from the capillary tip) and an incidence angle of 2o was chosen to ensure the beam

footprint did not extend beyond the channel width of individual OTFTs. An optical microscope

was located vertically on top of the sample and was used to monitor the beam and sample locations

using a positionally calibrated crosshair. A square beam attenuator was placed between the sample

and the detector to weaken 5 times of the intense (001) Bragg sheet in the (001) texture, thus

allowing longer integration times for other diffraction peaks without saturating the detector. A

Medoptics CCD detector located 98 mm from the sample holder was used with an exposure time

of 30 sec and a lateral sample scan step of 2 m to collect GIWAXS maps of each device

according to a mapping procedure previously developed elsewhere.[21]

5.3 Results and Discussion

5.3.1 Structural Characterization

We have utilized GIWAXS mapping, a method previously developed to map the microstructural

heterogeneities of the OSC within OTFTs,[21] to determine the preferred microstructure and texture

of the OSC on top of the untreated and F-SAM-treated Au electrodes and within the channel of the

transistor, as shown in Figures 2 and 3. The treated Au surfaces maintain uniform, monolayer

coverage for both F-SAMs, in agreement with our previous Polarized Modulated-Infrared

Reflection Absorption Spectroscopy (PM-IRRAS), goniometric and Kelvin probe measurements

(see Chapter 4). As expected, all organic thin films were found to be highly crystalline. In the

absence of a surface treatment, the crystalline texture of the semiconductor thin film consisted of a

mixture of “edge-on” and “face-on”-oriented crystallites on the electrode and in the channel.

However, the microstructure on the F-SAM-treated electrodes consisted of either purely “edge-on”

oriented crystallites or of a mixture of orientations in various relative intensities (quantities),

depending upon the choice of F-SAM, molecule and combination thereof, as will be discussed

below. Note that for all three molecules, the “edge-on” molecular orientation presents the (001)

texture, with the a-b plane and the π-stacking in the plane of the substrate, favoring good in-plane

charge transport. On the contrary, the “face-on” orientation is geometrically ineffective for in-plane

charge transport in OTFTs, present in the (111) texture of diF-TES ADT and (101) texture of diF-

TIPS Pn and diF-TSBS PDT, respectively, as shown in Figure 3d-e. Mixtures of these two


orientations also suffer from lower mobility, depending upon the relative amount of “face-on”

oriented crystallites in the film.[12] Thus, self-patterning by differential microstructure is successful

when the regions containing a pure (001) texture are present on the F-SAM-treated Au electrode

and extend to its immediate vicinity (up to tens of microns) to completely cover the critical channel

of the OTFT, and they are surrounded by regions consisting of a mixture of orientations. As all the

molecules studied herein crystallized in mixed orientations on the oxide regions located far from

the electrodes, we will focus our comparison to the structural development directly on top of the F-

Figure 2 μGIWAXS patterns showing the diffraction pattern on films of Pn on (a) Untreated

gold substrate, (b) 4-MFBT treated gold substrate, and (c) PFBT treated gold substrate. Peaks

circled in yellow are associated with (001)-textured crystals, whereas the peaks identified by the

red ovals are associated with (101)-textured crystals. A section of the intense Au (111) ring is

also visible in the top left corner.

ADT on PFBT/Au ADT on 4-MFBT/Au ADT on UT-Aua) b) c)

(111)

Au

(122)

(121)

(004)

(003)

(002)

(001)

(013)

(012)

(011)

(113)

(112)

(111)

Pn on PFBT/Au Pn on 4-MFBT/Au Pn on UT-Au

(003)

(002)

(001)

(012)

(102)

(011)

(101)

(111)

(101)

(001)

Au

d) e) f)

PDT on PFBT/Au PDT on 4-MFBT/Au PDT on UT-Aug) h) i)

(004)

(003)

(002)

(001)

Au

(101)(103)

(102)

(101)

(101)

Edge-on & Face-on

(001) + (111)

Edge-on

(001)

Edge-on

(001)

Edge-on & Face-on

(001) + (101)

Edge-on

(001)Edge-on & Face-on

(001) + (101)

Edge-on & Face-on

(001) + (101)

Edge-on & Face-on

(001) + (101)

Edge-on & Face-on

(001) + (101)


SAM-treated Au electrodes. In Figure 2, we show representative GIWAXS patterns for each

semiconductor on the electrode regions of the device. Based on such scans on all material-SAM

combinations (Figure 3), we have estimated the fraction of “edge-on” and “face-on” crystallite

orientations across the entire device.[22]

In Figure 4a, we plotted the fraction of “edge-on” oriented crystallites for each combination of

molecule/F-SAM. A summary of the film texture for each molecule-F-SAM combination is also

summarized in Table I and details about the determination of the fraction of “edge-on” orientation

are provided in Table I. As expected, all the materials in question adopt a mixed orientation on

untreated Au electrodes (black). In cases of the Pn and ADT molecules, the “face-on” orientation

of crystallites is fully inhibited in thin films formed on PFBT/Au electrodes (blue). However, unlike

ADT, Pn forms into a mixture of orientations when deposited on 4-MFBT/Au (red), despite the

presence of the F-F interaction, suggesting that there is a threshold of minimum number of halogen

bonds to influence molecular orientation and crystallographic texture. One F-F anchor, as in the

case of Pn on 4-MFBT/Au, does not appear to be sufficiently strong to impede the growth of the

“face-on” orientation. In the cases of Pn on PFBT and ADT on 4-MFBT or PFBT, there are multiple

halogen bonding opportunities, in the forms of F-F and/or F-S (Figures 4b, c), with the F/S atoms

originating either from the SAM or the semiconductor. Such interactions can establish several

active halogen bonds, obstructing molecular conformational changes and possibly anchoring the

molecule onto the fluorinated surface in an orientation that favors growth of <001>-textured

lamellar crystallites. The formation of 100% “edge-on” orientation of Pn crystallites on PFBT/Au

suggests that two bonding opportunities are sufficient, here in the form of F-F interactions (Figure

4c), to successfully achieve a pure preferential orientation of the molecules at the surface of the

treated electrodes. The single F-F path in the Pn/4-MFBT case, possibly together with “surface

Table I Summary of crystalline orientations detected by µGIWAXS of different F-SAM-OSC

combinations.


templating” provided by the aromatic SAM, encourages a higher content of “edge-on” crystallite

orientation in Pn films on 4-MFBT/Au (28 ± 4.3%) compared to the case of untreated

025

50

75

100

0

100 0

100

025

50

75

100

0

100

Un

trea

ted

Au

PF

BT

/Au

Ele

ctro

des

4-M

FB

T/A

u

Posi

tio

n (

m)

Au (111)

Intensity (a. u.)Edge-on% & Face-on%

025

50

75

100

0

100 0

100

025

50

75

100

0

100

Untr

eate

d A

u

PF

BT

/Au

4M

FB

T/A

u

Posi

tion (

m)

Edge-on% & Face-on%Au (111)

Intensity (a. u.)

Ele

ctro

des

025

50

75

100

0

100 0

100

025

50

75

100

0

100

Ele

ctro

des

Au (111)

PF

BT

/Au

Intensity (a. u.)

Edge-on% & Face-on%

Po

siti

on

(

m)

4-M

FB

T/A

u

Un

trea

ted

Au

(a)

(b)

(c)

AD

TP

nP

DT

<0

01

><

111

><

00

1>

<1

01

><

00

1>

<1

01

>

(d)

(e)

(f)

AD

TP

nP

DT

Fig

ure

3

(a-

c) S

truct

ura

l m

ap

s o

f b

ott

om

-co

nta

ct O

TF

Ts

det

erm

ined

fr

om

μG

IWA

XS

anal

ysi

s, i

ncl

ud

ing go

ld

elec

tro

des

(to

p p

anel

s) a

nd

fra

ctio

ns

of

“ed

ge-o

n”

(blu

e) a

nd

“fa

ce-o

n”

(red

) o

rien

tati

ons

on P

FB

T/A

u,

4-M

FB

T/A

u,

and

untr

eate

d (

UT

) A

u (

low

er p

anel

s) f

or

(a)

AD

T,

(b)

PD

T,

and

(c)

Pn.

(d-f

) S

chem

atic

rep

rese

nta

tio

ns

the

<0

01

>

and

<1

11

> m

ole

cula

r o

rien

tati

on f

or

AD

T (

d),

Pn (

e) a

nd

PD

T (

f).


Au (10 ± 1.1%), but it is not sufficiently strong to promote the formation of a purely (001)-textured

film, similar to the case of other OSC-SAM combinations.[23, 24]

We now explore whether the formation of the pure (001) texture can be generalized for

molecular systems containing F and S terminating atoms and deposited on an F-SAM surface. We

performed similar measurements using PDT (Figure 1c), a molecule with a longer backbone,

oriented at an 8.4° angle with respect to the surface normal. We found that in PDT, the presence of

F-SAMs cannot suppress the formation of the (101) textured “face-on” orientation, even when

several F-F and F-S anchors are afforded by the respective SAM and semiconductor molecular

structures (Figure 4b). To understand these results we inspected in more detail the possible

interactions at the F-SAM-OSC interface.

...

(c)

ADT Pn PDT0

10

20

30

40

50

60

70

80

90

100

% <

001>

Orienta

tion

Material

Untreated

4-MFBT

PFBT

(a)

...

(b)

Figure 4 (a) Relative “edge-on” oriented content for ADT, PDT and Pn deposited on PFBT

(Blue), 4-MFBT (Red) and untreated substrates (Black). (b) Schematic illustrating the

relationship between the OSC and the F-SAM for the cases when both F-F and F-S bonding

present and (c) only F-F bonds are present.


5.3.2 Interface Interactions Analysis

In Figure 5, we present a schematic view of the molecular orientation with respect to the substrate

for a (001)-textured crystal viewed along the b-axis. The backbone orientation relative to the a-b

plane is similar for each molecule and the angle with the substrate is provided in Table II. The

crystal structure was analyzed using CrystalMaker software.[25] We define the F-F and F-S

separations, dF-F and dF-S, as the distances between the F or S termination of the OSC molecule to

the F termination of the SAM, which corresponds to a plane through the highest point of the F-

SAM layer. This assumption is valid for both SAMs, as their orientations with respect to the surface

normal are similar, with the aromatic ring slightly tilted with respect to the surface normal and the

F-atom pointing upwards.[12] The distances for each compound are provided in Table II. By

correlating the estimated F-F and F-S separation distances with the experimental results provided

in Figures 2, 3 and 4, the current study provides a lower limit for the dF-F and dF-S for which the

interaction is strong enough to promote long range order and inhibit the formation of the “face-on”

oriented crystallites. We find that when the separation distance is 1.75•dW or less, where dW is the

sum of the respective van der Waals radii, the formation of a pure “edge-on” orientation is observed

for all cases explored herein.[26] This condition is achieved in the case of ADT and Pn molecules.

In PDT the F-F and F-S interactions are weak as a result of relatively large separation induced by

the backbone orientation (~2.5•dW). As a result, while the formation of (001)-oriented crystallites

is observed, the presence of the F-SAM does not inhibit the formation of the “face-on” oriented

crystallites. These results indicate the upper limit for the interaction distance lies within the range

1.75•dW < dF-F/S < 2.5•dW.

Figure 5 Schematic drawing of the relationship between the OSCs, composed of a molecular

backbone and substituent groups (SG) and the F-SAM.


5.3.3 Electrical Characterization

Organic thin-film transistor measurements were performed as a means to investigate the impact of

the film’s microstructure and texture on the effectiveness of charge transport. In Figure 6, we show

plots of the FET current-voltage characteristics: the evolution of the drain current versus the gate-

source voltage (√𝐼D versus VGS) for representative devices of ADT (Figure 6a), Pn (Figure 6b) and

PDT (Figure 6c) biased in the saturation regime (source -drain voltage, VDS= -40 V). The graphs in

Figure 6 correspond to untreated contacts (black) , 4-MFBT (red), and PFBT treated (blue)

electrodes, respectively. For all devices the channel lengths are sufficiently short that when the

halogen interactions are effective, the crystallization induced from the electrodes into the channel

inhibits the formation of “face-on” oriented crystallites. The slope of the curves presented in Figure

6 yielded the field-effect mobility () using the following relation:

𝜇 =2𝐿

𝑊𝐶i(𝜕√𝐼D

𝜕𝑉GS)

2

(5.1)

where Ci = 17.3 nF cm-2 is the geometrical capacitance of the dielectric and L and W are the channel

length and width, respectively. The reported averages were calculated based on the results obtained

on at least 5 devices. Devices fabricated on ADT films deposited on PFBT and 4-MFBT treated

electrodes showed high currents and produced similar mobilities of PFBT/Au = 1.23±0.09)×10-1

cm2V-1s-1 and 4-MFBT/Au = (9.7±1.2)×10-2 cm2V-1s-1, respectively. Films deposited on untreated

electrodes exhibited nearly two orders of magnitude poorer performance, with Au = (1.8±0.5)×10-

3 cm2V-1s-1. The lower mobility is a result of higher injection barrier at the metal/OSC interface in

the absence of the F-SAM, and due to the presence of the mixed molecular orientations, with the

“face-on” (111)-oriented crystallites hindering in-plane charge transport and intergrain transport

Table II Comparison of molecular backbone-to-substrate angle and relative molecule-to-SAM

distances.


due to large crystallographic misorientation.[12] The evolution of the drain current versus the drain-

source voltage for each semiconductor-treatment pair at various gate-to-source voltages are shown

in Figure 7.

Measurements performed on devices fabricated using Pn films also echo the differences

determined in our structural studies: the OTFTs fabricated on PFBT-treated electrodes showed the

0

5x10-3

1x10-2

2x10-2

2x10-2

3x10-2

diF-TES ADT

diF-TSBS PDT

PFBT

4-MFBT

Untreated

I1/2

D / A

1/2

diF-TIPS Pn (Peri)

0

3x10-4

6x10-4

9x10-4

1x10-3

PFBT

4-MFBT

Untreated

I1/2

D / A

1/2

-40 -20 0 20 40

0

1x10-3

2x10-3

3x10-3

4x10-3

5x10-3

PFBT

4-MFBT

Untreated

I1/2

D / A

1/2

VGS

/ V

(a)

(b)

(c)

Figure 6 Evolution of the drain current, ID, with gate voltage, V

GS, for (a) ADT, (b) Pn, and (c)

PDT thin films on untreated (Black), 4-MFBT treated (Red), and PFBT treated (Blue) gold

electrodes. The devices are measured at VDS

= -40 V with a transistor geometry of L = 5 μm

(ADT) or L = 15 μm (Pn and PDT), W = 1000 μm, and oxide thickness of tox

= 200 nm.


highest mobility with PFBT/Au = (2.1±0.2)×10-3 cm2V-1s-1, owing to a pure “edge-on” molecular

orientation obtained in the presence of favorable F-F interactions. Pn-based devices on 4-MFBT-

treated electrodes and on untreated electrodes showed progressively decreasing performances due

to the decrease in the amount of <001>-textured crystallites phase content (Figure 3b). The resulting

a) ADT on PFBT/Au

0 -10 -20 -30 -40 -50 -60

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

VGS

= -60V

I D(A

)

VDS

(V)

VGS

= 0V

b) ADT on 4-MFBT/Au

0 -10 -20 -30 -40 -50 -60

0.0

1.0x10-4

2.0x10-4

3.0x10-4

4.0x10-4

VGS

= -60V

VDS

(V)

VGS

= 0V

c) ADT on UT-Au

0 -10 -20 -30 -40 -50 -60

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

VGS

= -60V

VDS

(V)

VGS

= 0V

d) Pn on PFBT/Au

0 -10 -20 -30 -40 -50 -60

0

1x10-7

2x10-7

3x10-7

4x10-7

5x10-7

6x10-7

VGS

= -60V

I D(A

)

VDS

(V)

VGS

= 0V

e) Pn on 4-MFBT/Au f) Pn on UT-Au

g) PDT on PFBT/Au

0 -10 -20 -30 -40 -50 -60

0

1x10-5

2x10-5

3x10-5

4x10-5

VGS

= -60V

I D(A

)

VDS

(V)

VGS

= 0V

h) PDT on 4-MFBT/Au

0 -10 -20 -30 -40 -50 -60

0.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

2.5x10-5

VGS

= -60V

VDS

(V)

VGS

= 0V

i) PDT on UT-Au

0 -10 -20 -30 -40 -50 -60

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

1.2x10-5

1.4x10-5

1.6x10-5

1.8x10-5

VGS

= -60V

VDS

(V)

VGS

= 0V

0 -10 -20 -30 -40 -50 -600

1x10-7

2x10-7

3x10-7

4x10-7

5x10-7

6x10-7

VGS

= -60V

VDS

(V)

VGS

= 0V

0 -10 -20 -30 -40 -50 -60

0.0

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

5.0x10-6

VGS

= -60V

VDS

(V)

VGS

= 0V

Figure 7 Output characteristics for (a-c) diF-TES ADT, (d-f) diF-TIPS Pn (Peri), and (g-i) diF-

TSBS PDT on each of the three contact treatments. The devices are measured with a transistor

geometry of L = 5 m (ADT) or L = 15 m (Pn and PDT), W = 1000 m, and oxide thickness

of tox

= 200 nm for VGS

ranging from 0V to -60V.


values for mobilities are 4-MFBT/Au = (7.6±0.4)×10-4 cm2V-1s-1 and Au = (9±8)×10-6 cm2V-1s-1 for

devices based on 4-MFBT-treated and untreated electrodes, respectively.

When the separation distance between the semiconducting molecule and the F-SAM is too

large to promote halogen-halogen interactions, such as in the PDT case, then GIWAXS shows a

mixture of orientations on all electrodes and OTFT devices also yield similar electrical properties

for all types of electrodes. The characteristic mobilities measured for PDT films are: PFBT/Au =

(1.8±0.4)×10-2 cm2V-1s-1, 4-MFBT/Au = (1.1±0.3)×10-2 cm2V-1s-1, and Au = (8.0±0.7)×10-3 cm2V-1s-

1. The slightly higher mobilities obtained on PFBT/Au and 4-MFBT/Au are a result of the lower

injection barrier created by the work function shift of the F-SAM-treated electrode.[12] Nevertheless,

these values are significantly lower than the one reported for PDT OTFTs deposited by solvent-

assisted crystallization (SAC), where a pure (001) oriented film, characterized by = 1.5 cm2V-1s-

1 was obtained because of the slow nature of this crystallization process.[25] The lower mobility in

our thin-films is thus limited by injection into the (001)+(111) phase and not by the transport

through the (001) phase present in the channel (Figure 3).

5.4 Conclusions

In summary, we investigated the role of interfacial halogenation of the substrate on mediating

texture formation and self-patterning of polycrystalline OSC films and the resulting effects on

charge transport properties in OTFTs. We demonstrate that OSCs can either allow or inhibit texture

selection based on whether or not F-F and/or F-S interactions are present at the F-

SAM/semiconductor interface. We report that the presence of two or more interaction opportunities

between F and F/S species per SAM or per OSC molecule are required to promote molecular

anchoring, which can promote the formation of high in-plane mobility textures of the OSC. The

halogen interactions are found to be sufficiently strong to mediate texture only when the halogen

atoms on the molecule and the SAM are located closer than about twice the respective van der

Waals interaction distances. When these conditions are satisfied, the organic film exhibits a high

degree of texture purity, with crystallites oriented with the (001) plane parallel to the surface and

effective in-plane -stacking, resulting in high mobility devices. This study has provided rare

insight directly linking the surface chemistry of the electrode, the terminating atom of the OSC

molecules, the resulting film microstructure and the performance of OTFT devices. This


information is vital to enabling the rational design of OSCs and demonstrates the importance of

considering both the molecule and surface chemistry.

116

References

[1] T. Someya et al., A large-area , flexible organic skin applications pressure transistors

sensor matrix for artificial with. Proc. Natl. Acad. Sci. U. S. A. 101, 9966–9970 (2010)

[2] H. Yan et al., A high-mobility electron-transporting polymer for printed transistors.

Nature. 457, 679–86 (2009)











[8] L. Qiu et al., Versatile Use of Vertical-Phase-Separation-Induced Bilayer Structures in

Organic Thin-Film Transistors. Adv. Mater. 20, 1141–1145 (2008)

[9] C. Liu et al., Solution-processable organic single crystals with bandlike transport in field-

effect transistors. Adv. Mater. 23, 523–6 (2011)

[10] R. Li et al., Heterogeneous Nucleation Promotes Carrier Transport in Solution-Processed

Organic Field-Effect Transistors. Adv. Funct. Mater. 23, 291–297 (2013)


[11] K. Reichenbächer, H. I. Süss, J. Hulliger, Fluorine in crystal engineering--“the little atom

that could”. Chem. Soc. Rev. 34, 22–30 (2005)


Mater. Chem. 22, 19047–19053 (2012)





(2011)

[15] S. M. Huston et al., Role of Fluorine Interactions in the Self-Assembly of a Functionalized

Anthradithiophene Monolayer on Au(111). J. Phys. Chem. C. 116, 21465–21471 (2012)

[16] J. Chen et al., The influence of side chains on the structures and properties of

functionalized pentacenes. J. Mater. Chem. 18, 1961 (2008)



[18] Y. Wang, S. R. Parkin, J. Gierschner, M. D. Watson, Highly Fluorinated

Benzobisbenzothiophenes. Org. Lett. 10, 3307 (2008)

[19] M. M. Payne, S. a Odom, S. R. Parkin, J. E. Anthony, Stable, crystalline

acenedithiophenes with up to seven linearly fused rings. Org. Lett. 6, 3325–3328 (2004)





[22] E. J. Kintzel, D.-M. Smilgies, J. G. Skofronick, S. a. Safron, D. H. Van Winkle, Ultrathin

film growth of p-phenylene oligomers on alkali halide substrates. J. Cryst. Growth. 289,

345–350 (2006)

[23] C. H. Kim et al., Templating and charge injection from copper electrodes into solution-

processed organic field-effect transistors. ACS Appl. Mater. Interfaces. 5, 3716–21 (2013)

[24] H. Lee et al., Rubicene: A Molecular Fragment of C70 for Use in Organic Field-Effect

Transistors. J. Mater. Chem. C (2014), doi:10.1039/C3TC32117G

[25] D. Palmer, CrystalMaker (2010)


[26] A. Bondi, van der Waals Volume and Radii. J. Phys. Chem. 68, 441–451 (1964)

Appendix A: MANUSCRIPT COVER GALLERY 119

Figure A1 Front cover for Advanced Materials, 24 (2012), related to manuscript: “Direct

Structural Mapping of Organic Field-Effect Transistors Reveals Bottlenecks to Carrier

Transport” by R. Li, J. W. Ward, D.-M. Smilgies, J. E. Anthony, O. D. Jurchescu and A.

Amassian in Advanced Materials, 24, 5553 (2012). Cover was designed and produced by the

Amassian Research Group at the King Abdullah University of Science and Technology.

Appendix A: Manuscript Cover Gallery 120

Figure A2 Inside front cover for Advanced Functional Materials, 24 (32) (2014), related to

manuscript “Rational Design of Organic Semiconductors for Texture Control and Self-

Patterning of Halogenated Surfaces” by Jeremy W. Ward, Ruipeng Li, Abdulmalik Obaid,

Marcia M. Payne, Detlef-M. Smilgies, John E. Anthony, Aram Amassian and Oana D. Jurchescu

published in Advanced Functional Materials, 24 (32) 5052 (2014).

Appendix A: Manuscript Cover Gallery 121

Figure A3 Inside front cover for ChemPhysChem, 16 (6) (2015), related to manuscript

“Versatile Organic Transistors Realized from Solution Processing” by Jeremy W. Ward,

Zachary A. Lamport, and Oana D.

122

Summary

Recent decades have been witness to a fast transition into an era consumed with technology.

Ranging from large displays that can now curve to optimize a viewer’s experience to those small

enough to be worn on one’s wrist, the diversity of applications is constantly challenged by

consumer demands. A common thread among all applications is the field-effect transistor (FET); a

small electrical switch that can control the current flow. First proposed in a patent by Julius

Lilienfeld in 1930 and later prepared by Dawon Kahng and Martin Atalla in 1959, the most

common material used in FETs today, from display to processor technologies, is silicon. Materials

used in silicon-based electronics have now begun to be complemented by, and in some case

replaced by, organic-based (i.e. carbon-based) materials. Due to weaker interactions between the

molecules of organic materials, these compounds can be dissolved into a liquid to form an ink. This

ink can be deposited into a film by using low-cost, low-thermal budget methods such as roll-to-roll

printing and spray-processing, thus significantly increasing the manufacturing versatility over the

brittle silicon-based materials. Organic field-effect transistors (OFETs) fabricated from solution-

based methods create pathways to fabricating electronic applications that are: flexible, transparent,

large-area, low-power consuming, and/or even wearable. Since the first OFET based on a soluble

semiconductor was developed by Koezuka in 1987, much progress has been made in designing

new materials, creating new device structures, and developing new fabrication methods, improving

the electronic performance of devices by nearly six orders of magnitude (or 1,000,000 times). Even

in the presence of such great improvement, the adoption of organic electronic devices in industrial

manufacturing is challenged by the following limitations:

Summary 123

#1. The environmental stability of the organic semiconductors;

#2. The low device performance and high operating voltages;

#3. Insufficient understanding about the relation between organic semiconductor

processing, microstructure and its electrical properties.

Through my doctoral research, I addressed these problems, as detailed below:

Challenge #1 was addressed in Chapter 3. We discovered a solid-solid phase-transition at 205 K

in a commonly used organic semiconductor. We investigated the conditions under which the

transition occurred and the implications it had on the thin-film structure and electrical performance.

We found that when the material was in a powder form, the transition was completely reversible,

however, once it was confined to a substrate, in a form of a thin layer, the film cracked upon passing

through the transition point and its electrical properties suffered. This finding contains important

information for this material since it clarifies its response to thermal changes and the stability of

this material. With respect to the consideration for potential commercial manufacturing, since this

transition occurs at very low temperatures, it does not impact the devices’ operation in consumer

applications.

Challenge #2 and Challenge #3 are addressed concurrently through Chapters 4 and 5 by exploring

methods to connect molecular design and processing in order to improve the field-effect mobility

of OTFTs fabricated from organic semiconductors. The approach adopted in this work is through

inducing organic thin-film transistor patterning to electrically segregate individual devices within

an array. In the current electronics industry, the semiconductor films are patterned using multistep

processes that involve harsh chemicals and intense light sources, which are incompatible with

organic materials. In 2008 Gundlach et al. discovered a technique that took advantage of the

chemical structure of the organic semiconductor and its relationship to the surface on which it was

deposited. They demonstrated that by treating the surfaces of the OFET electrodes with a

halogenated (i.e. Fluorine) self-assembled monolayer, they could coerce the semiconductor

molecules in these regions to form a highly conductive film. Patterning was realized since the film

not in contact with the treated-metal surface was a poorly conducting film, thus preventing charges

from leaking to neighboring devices. This technique faced challenges when being transferred to

other materials or treatments as the underlying phenomena was not well understood.

Summary 124

First, by implementing three similar treatments on the metal surfaces and then selectively

choosing other halogenated organic semiconductors, we discovered key components of the

treatment and semiconductor composition that facilitate this type of film growth. We identified the

role of interactions between the fluorine atom on the treatment molecule and the fluorine atom

residing on the organic semiconductor. Specifically, we discovered that these interactions are

effective only when the distance between the interacting atoms within the semiconductor and the

treatment is less than 2.5 times the characteristic bonding length of the halogen atoms involved.

The development of a deeper understanding into the interplay between organic semiconductors and

the surfaces onto which they are placed helps to guide synthetic chemists in making new, high-

performance semiconductors and creates guidelines for device patterning, which are important for

device design.

Throughout the thesis, we addressed Challenge #3 by exploring how different processing

parameters, such as the details of the spinning recipe, interface engineering, or post-deposition

treatments alter the structure of the organic semiconductor film. As the film morphology is

intimately related to its electrical properties, we followed these changes in FET measurements. By

establishing a processing – structure – properties feed-back loop, we gained a great control over

the properties of the resulting devices and consequently improved their performance.

125

List of Publications

* Cover Article † Invited Article

1. Quantitative Analysis of the Density of Trap States at the Semiconductor-Dielectric Interface in

Organic Field-Effect Transistors

Peter J. Diemer, Zachary A. Lamport, Yaochuan Mei, Jeremy W. Ward, Katelyn P. Goetz, Wei

Li, Marcia M. Payne, Martin Guthold, John E. Anthony, and O. D. Jurchescu

Applied Physics Letters (In Revisions)

2. Low-Voltage Polymer/Small-Molecule Blend Organic Thin-Film Transistors and Circuits

Fabricated via Spray Deposition

S. Hunter, J. W. Ward, M. Payne, J. E. Anthony, O. D. Jurchescu, and T. Anthopoulos

Applied Physics Letters, 106, 223304 (2015)

3. † * Versatile Organic Transistors by Solution Processing

J. W. Ward, Z. A. Lamport and O. D. Jurchescu

ChemPhysChem, 16 (6), 1118 (2015)

4. Freezing-in Orientational Disorder Induces Crossover from Thermally-activated to Temperature-

independent Transport in Organic Semiconductors

K. P. Goetz, A. Fonari, D. Vermeulen, P. Hu, H. Jiang, P. J. Diemer, J. W. Ward, M. E. Payne, C.

S. Day, C. Kloc, L. E. McNeil and O. D. Jurchescu

Nature Communications 5, 5642 (2014)

5. Low-Temperature Phase Transitions in a Soluble Oligoacene and Their Effects on Device

Performance and Stability

J. W. Ward, K. P. Goetz, A. Obaid, M. M. Payne, P. J. Diemer, C. S. Day, J. E. Anthony and O.

D. Jurchescu

Applied Physics Letters 105, 083305 (2014)

6. * Rational Design of Organic Semiconductors for Texture Control and Self-Patterning on

Halogenated Surfaces

J. W. Ward, R. Li, A. Obaid, M. M. Payne, D-M. Smilgies, J. E. Anthony, A. Amassian and O.

D. Jurchescu

Advanced Functional Materials 24 (32), 5052 (2014)

7. † Electronics Beyond Silicon

J. W. Ward and O. D. Jurchescu

Circuit Cellar 278, 80 (2013)

8. * Direct Structural Mapping of Organic Field-Effect Transistors Reveals Bottlenecks to Carrier

Transport

R. Li, J. W. Ward, D-M. Smilgies, J. E. Anthony, O. D. Jurchescu and A. Amassian

Advanced Materials 24, 5553 (2012)

List of Publications 126

9. Tailored Interfaces for Self-Patterning Organic Thin-Film Transistors

J. W. Ward, M. A. Loth, R. J. Kline, M. Coll, C. Ocal, J. E. Anthony and O. D. Jurchescu

Journal of Materials Chemistry 22 (36), 19047 (2012)

10. Synthesis and Charge Transport Studies of Stable, Soluble Hexacenes

B. Purushothaman, S. R. Parkin, M. J. Kendrick, D. David, J. W. Ward, L. Yu, N. Stingelin, O.

D. Jurchescu, O. Ostroverkhova and J. E. Anthony

Chemical Communications 48, 8261 (2012)

11. Effect of Acene Length on Electronic Properties in 5-, 6-, and 7-Ringed Heteroacenes

K. P. Goetz, Z. Li, J. W. Ward, C. Bougher, J. Rivnay, J. Smith, B. R. Conrad, S. R. Parkin, T. D.

Anthopoulos, A. Salleo, J. E. Anthony and O. D. Jurchescu

Advanced Materials 23, 3698 (2011)

12. Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and

OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b']dithiophene/2,1,3-benzothiadiazole

Copolymers

R. C. Coffin, C. M. MacNeill, E. D. Peterson, J. W. Ward, J. W. Owen, C. A. McLellan, G. M.

Smith, R. E. Noftle, O. D. Jurchescu and D. L. Carroll

Journal of Nanotechnology 572329 (2011)

127

List of Presentations

1. Low-Temperature Structural Phase Transistion in a Soluble Oligoacene and Its Effect on Charge

Transport

American Physical Society March Meeting

Oral Presentation in Denver, CO (2014)


WFU Physics Colloquium

Invited Talk in Winston-Salem, NC (2014)


Materials Research Society Spring Meeting

Oral Presentation in San Francisco, CA (2013)


Wake Forest University Graduate Student & Postdoc Research Day

Poster Presentation in Winston-Salem, NC (2013)

Award: 1st Runner-Up

5. Tuning the Microstructure and Electronic Performance in Organic Thin-Film Transistors Using

Chemical Modifications at Interfaces

International Semiconductor Device Research Symposium

Oral Presentation in College Park, MD (2011)

Award: Best Student Oral Presentation Award

6. From Nano to Micro-Scale Control of Crystalline Order in Small-Molecule Organic

Semiconductor Films

North Carolina Nanotechnology Commercialization Conference

Poster Presentation in Charlotte, NC (2011)


Semiconductors

Electronic Materials Conference

Oral Presentation in Santa Barbara, CA (2011)


Semiconductors

Wake Forest University Graduate Student & Postdoc Research Day

Poster Presentation in Winston-Salem, NC (2011)

Award: 1st Runner-Up

128

Curriculum Vitae

Jeremy William Ward

04/09/1988 Born in Norwalk, IA

2002 - 2006 Norwalk Senior High School in Norwalk, IA

2006 - 2010 B.A. Physics, Simpson College in Indianola, IA

Advisors: Dr. David Olsgaard

B.A. Mathematics (Honors), Simpson College in Indianola, IA

Advisors: Dr. Debra Czarneski and Heidi Berger

2010 - 2015 Ph.D. Physics, Wake Forest University in Winston-Salem, NC

Subject: Physics

Advisor: Dr. Oana D. Jurchescu

Documents

ENHANCING THE ELECTRICAL PERFORMANCE OF ORGANIC … · 2015-08-27 · manuscript revisions, the constant encouragement, and the incredible flexibility to pursue my coaching career,