Molecular Organic Semiconductors for Electronic Devices

Molecular Organic Semiconductors

for

Electronic Devices

Oana Diana Jurchescu

MSC PhD thesis series 2006-15

issn: 1570-1530

isbn: 90-367-2708-1

The work described in this thesis was performed in the research group Solid State

Chemistry of the Materials Science Centre at the University of Groningen, the

Netherlands.

Printed by: PrintPartners Ipskamp B.V., Enschede, the Netherlands

Rijksuniversiteit Groningen

Molecular Organic Semiconductors for

Electronic Devices

Proefschrift

ter verkrijging van het doctoraat in de

Wiskunde en Natuurwetenschappen

aan de Rijksuniversiteit Groningen

op gezag van de

Rector Magnificus, dr. F. Zwarts,

in het openbaar te verdedigen op

vrijdag 6 oktober 2006

om 14:45 uur

door


geboren op 1 maart 1979

te Timisoara, Roemenie

Promotor: Prof. Dr. T.T.M. Palstra

Beoordelingscommissie: Prof. Dr. Y. Iwasa

Prof. Dr. G. G. Malliaras

Prof. Dr. P. Rudolf

Contents

1 Introduction 1

1.1 Organic conductors - general aspects . . . . . . . . . . . . . . . . . 1

1.2 Charge transport in organic semiconductors . . . . . . . . . . . . . 3

1.3 Organic electronic devices . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.1 Field effect transistors - operation principles . . . . . . . . . 8

1.3.2 Fabrication of OFETs . . . . . . . . . . . . . . . . . . . . . 11

1.3.3 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.4 Organic molecular crystals . . . . . . . . . . . . . . . . . . . . . . . 15

1.5 Single crystals - model systems for investigation of intrinsic properties 18

1.6 Aim of the present research . . . . . . . . . . . . . . . . . . . . . . 18

1.7 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 19

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2 The Effect of Impurities on the Mobility of Single Crystal Pen-

tacene 27

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2 Control of the defects and impurity states in organic crystals . . . 28

2.2.1 Spectroscopic evidence for the presence of impurities . . . . 29

2.2.2 Purification of the starting material . . . . . . . . . . . . . 30

2.2.3 Single crystal growth . . . . . . . . . . . . . . . . . . . . . . 31

2.2.4 Quantitative analysis of 6,13-pentacenequinone content . . 32

2.3 Electronic response to structural and chemical properties . . . . . 35

2.3.1 Space charge limited currents . . . . . . . . . . . . . . . . . 35

v

vi Contents

2.3.2 Evaluation of the bulk mobility of the high quality crystals 38

2.3.3 Band transport in pentacene single crystals . . . . . . . . . 39

2.3.4 Origin of traps . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3 Interface Controlled High-mobility Organic Transistors 47

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.2 Current status of the field . . . . . . . . . . . . . . . . . . . . . . . 48

3.3 Novelty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.4 Single crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.5 Surface morphology of pentacene single crystals . . . . . . . . . . . 50

3.5.1 Surface mapping . . . . . . . . . . . . . . . . . . . . . . . . 50

3.5.2 AFM measurements . . . . . . . . . . . . . . . . . . . . . . 50

3.6 Comparison between different FETs . . . . . . . . . . . . . . . . . 53

3.6.1 FETs fabricated with conventional dielectrics . . . . . . . . 53

3.6.2 FETs fabricated with pentacenequinone dielectric . . . . . 54

3.7 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.8 Properties of FETs fabricated with

pentacenequinone gate dielectric . . . . . . . . . . . . . . . . . . . 56

3.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4 Cross-over from 1D to 2D - Space Charge Limited Conduction

in Pentacene Single Crystals 65

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2 Space-charge-limited-current theories for semiconductors . . . . . . 66

4.2.1 Mott-Gurney model . . . . . . . . . . . . . . . . . . . . . . 66

4.2.2 Geurst model . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.3 Space charge limited currents in pentacene single crystals with pla-

nar contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.3.1 Experimental procedure . . . . . . . . . . . . . . . . . . . . 68

4.3.2 Charge transport in the SCLC regime . . . . . . . . . . . . 69

4.3.3 Evaluation of mobility for sandwich-type and gap-type struc-

tures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.3.4 Resistivity anisotropy effects - effective thickness . . . . . . 72

4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Contents vii

5 Electronic Transport Properties of Pentacene Single Crystals

upon Exposure to Air 77

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.3 Exposure to dry and ambient air . . . . . . . . . . . . . . . . . . . 79

5.3.1 I-V characteristics in the dark - in vacuum and after expo-

sure to air . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.3.2 Gas diffusion in pentacene single crystals . . . . . . . . . . 80

5.3.3 Effect of Oxygen and water in dark . . . . . . . . . . . . . 84

5.3.4 Effect of Oxygen and water in light . . . . . . . . . . . . . 86

5.3.5 Effect of high O2 pressure . . . . . . . . . . . . . . . . . . . 87

5.4 O2 and H2O effects - experimental observations . . . . . . . . . . . 88

5.4.1 Electrical measurements . . . . . . . . . . . . . . . . . . . . 88

5.4.2 UPS spectroscopy . . . . . . . . . . . . . . . . . . . . . . . 89

5.5 Mechanism of oxygen doping . . . . . . . . . . . . . . . . . . . . . 91

5.5.1 Formation of the charge transfer complex between pentacene

and O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

5.5.2 Reversible oxidation of pentacene . . . . . . . . . . . . . . . 92

5.5.3 Calculation of the oxygen-induced dipole moment . . . . . 93

5.6 Quantitative analysis of the effect of O2 and H2O . . . . . . . . . . 96

5.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6 Low Temperature Crystal Structure of

Rubrene Single Crystals Grown by Vapor Transport 103

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

6.2 Polymorphism in rubrene . . . . . . . . . . . . . . . . . . . . . . . 104

6.3 Growth of rubrene single crystals . . . . . . . . . . . . . . . . . . . 105

6.4 Single crystal diffraction . . . . . . . . . . . . . . . . . . . . . . . . 106

6.4.1 Experimental details . . . . . . . . . . . . . . . . . . . . . . 106

6.4.2 Orthorhombic rubrene . . . . . . . . . . . . . . . . . . . . . 106

6.5 Relation between molecular stacking and electronic mobility . . . . 109

6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

A Crystal structure of pentacene 117

B Crystal structure of rubrene 121

viii Contents

Summary 131

Samenvatting 135

List of Publications 139

1Introduction

1.1 Organic conductors - general aspects

Organic semiconductors are a fascinating class of materials, with a wide range

of properties [1–7]. The interest in organic electronics is motivated by increasing

demands of supplementing Si-based electronics with materials that offer easier

processing methods completed with functionalization by chemical manipulation.

Typical deposition techniques for organics are readily available or not very difficult

to implement (spin-coating, drop-casting, direct printing via stamps or ink-jets).

Their compatibility with light-weight, mechanically flexible plastic substrates,

completed by new, innovative fabrication routes, make them possible candidates

for future electronic devices. Moreover, due to different molecular tailoring pos-

sibilities via chemical synthesis, organic materials present an infinite variety in

functionality. The design of the molecular structures can be engineered to enhance

particular properties (solubility in different solvents, color of light emission, crystal

packing). In particular, addition of polar groups in polymers (e.g. polyvinylidene

fluoride and its copolymers with trifluoroethylene and tetrafluoroethylene) leads

to rich systems for investigation of ferroelectricity [8]. By incorporating them in

transistors (ferroelectric field-effect transistors - FeFETs), combined functionality

can be achieved for non-volatile memory devices that are compatible with flex-

ible plastic substrates [9]. Modification of the chemical end-groups also allows

fabrication of large transistor arrays for organic sensors. Different methods are

developed to increase the sensitivity and selectivity of organic sensing transistors

to different chemical or biological species. Applications include environmental

issues (e.g. air pollution), pH indicators [10], food freshness, toxic compounds,

1

2 Chapter 1. Introduction

stress and pressure indication in clothes [11].

Rapid progress is being made in the industrial development of the organic

electronic devices. Organic devices can be implemented on large scale application

area when their operation offer high performance, reliability, stability, long life

time, good control and reproducibility. Impressive steps have been accomplished

towards incorporation of organic conductors in devices, as constituents of plastic

electronic components, by continued innovation in materials, processing methods

and device design. For example, Philips and Polymer Vision already presented

their prototype rollable display based on pentacene FETs that will be used for fab-

rication of mobile phones [12]. OLED1 technology has already been incorporated

in commercial applications such as small displays for mobile phones, car radios,

and digital cameras [13]. It was suggested that if ”the field continues to progress

at its current, rapid pace, electronics based on organic thin-films materials will

soon become a mainstay of our technological existence” (S. Forrest [14]).

In the academic community, the interest in electrical properties of organic ma-

terials has grown enormously in the last years [16–26]. Different materials and

structures are actively being investigated. On the one hand, the research is mo-

tivated by new applications for organic (plastic) electronics that can overcome

limitations of inorganic materials. On the other hand, the effort is driven by

the intellectual challenges that the investigations of new generation of functional

materials offer. This implies addressing new questions, concepts and models that

allow the manipulation of chemical, structural, physical properties and device

performance. The field is moving very fast; at the moment researchers are able to

control the material properties at the molecular level. They can impose different

molecular packing during growth, add side groups that enhance particular prop-

erties, as a result of the understanding of the various microscopic contributions to

the charge transport. Past investigations were hindered by material instabilities

(to environmental conditions and/or processing methods) and structural defects

that prevented the measurement and understanding of the intrinsic properties of

the organic materials. In the last years, by careful design of new methods that

enable the exploitation of the fascinating properties of carbon-based semiconduc-

tors, good control of the structural properties and charge density distribution in

organic materials is achieved (both by electrostatic and chemical doping). The

degradation is avoided as a consequence of a better understanding of the material

behavior in different conditions (temperature, moisture, light) and interactions

during processing.

1OLED - organic light emitting diodes

1.2. Charge transport in organic semiconductors 3

1.2 Charge transport in organic semiconductors

Organic semiconductors are wide-band-gap and small bandwidth semiconductors.

The HOMO2-LUMO3 gap is in the range of 1-4 eV [27]. With such a large gap,

it might be expected that the organic materials are insulators (an electron would

have to acquire a large thermal energy to make the jump from the valence band

to the conduction band). There are some effective methods that can generate

charge carriers in the organic semiconductors:

⋄ injection of carriers from metallic electrodes;

⋄ optical excitation (creation of electron-hole pairs);

⋄ electrostatic or chemical doping.

Different organic molecular structures are of current interest for the contin-

ued search for novel and useful properties. Carbon nanotubes (CNTs) are one-

dimensional, cylindrical systems that are attractive for nanometer-sized electron-

ics because they combine the high electrical and thermal conductivity with good

mechanical strength and flexibility [28]. The physical properties of nanotubes

are imposed by their diameter, length, and chirality, or twist. For example, they

can be both metallic and semiconducting, depending on the chirality [29]. Nan-

otubes are attractive also for the field emission properties [30] because they are

long and very thin, conductive, with high mechanical strength, and have a low

work-function. As the CNTs are generally accepted as being the bridge between

the nano- and microscopic length scales, a step further was done in coupling them

to biological systems aiming the fabrication of biosensors [31, 32]. More recently,

superconductivity has been observed in nanotubes [33].

When the carbon sheet is not rolled into a tube, it forms a graphene sheet.

Graphene is a candidate for nanometer-scale electronic devices that combine the

properties of carbon nanotubes with the compatibility with established micro-

electronics manufacturing techniques. Low scattering yields mobilities up to 104

cm2/Vs [34]. This value gives ultra-fast-switching transistors for electronics. Also,

the quantum Hall effect was recently reported in graphene [35].

The most common 3D structure in the fullerene structural family is C60,

the spherical buckyball (Fig. 1.1(d)). Because of its particular electronic struc-

ture, C60 exhibits semiconducting properties, but it reveals one amazing prop-

erty after another upon doping [36]. It can be tuned to a metallic or insulating

2HOMO - highest occupied molecular orbital3LUMO - lowest unoccupied molecular orbital


state by transferring charge on the molecule through doping. At low temper-

ature this metallic state often exhibits superconductivity, with transition tem-

peratures only exceeded by the high-temperature superconducting copper ox-

ides [37]. This is not an unique type of organic superconductor. Several organic

superconductors have been identified, such as the quasi-one-dimensional Bech-

gaard salts ((TMTSF)2X4 and (TMTTF)2X

5, with X= PF6−, FSO3

−, ClO4−,

etc.), and quasi-two-dimensional salts derived from the donor group BEDT-TTF6

(Fig. 1.1(a)) [38]. Organic superconductors are charge transfer compounds, un-

conventional superconductors, in which the interchain transfer integral can be

changed with pressure, temperature, and composition (the nature of the anion

X controls the chemical pressure), leading to a delicate balance of interactions

that gives rise to a rich variety of physical phenomena. These include charge or-

dering, spin-Peierls ground state, antiferromagnetism, spin density waves (SDW),

metallicity and superconductivity [38].

In polymers, the electrical properties are dominated by disorder that localize

the charges at low temperature. Still, these materials present fascinating proper-

ties that make them attractive for organic electronic devices [1,2,6]. We presented

in Section 1.1 the potential of polymers, that were already incorporated in com-

mercial devices, as components of the OLEDs. Their semiconducting properties

are also used in FETs and solar cells [16–18]. In OLEDs and solar cells an exciton

(excited electron - hole pair) is generated in the organic material. In OLEDs

light is generated through radiative recombination of electrons and holes and it

is emitted through the transparent electrode. Two distinct processes, equally im-

portant, govern the operation of the device: charge transport and recombination.

The color of the emitted light is tuned by the band gap of the active material that

is used. In solar cells light is absorbed through the transparent electrode and the

photons absorbed in the semiconductor create mobile excited electron-hole pairs.

These excitons subsequently undergo dissociation yielding free electrons and holes

that are collected at the contacts. The efficiency of the solar cell is given by the

photon absorption efficiency, exciton dissociation efficiency and charge collection

efficiency. Organic photovoltaic devices are limited by exciton lifetime, and low

charge carrier mobilities; only a fraction of the light-induced excitons contribute

to the current generation. Tang et al. demonstrated that the efficiency can be in-

creased considerably if a heterojunction of a electron donor and electron acceptor

material is manufactured [39]. To enhance the quantum separation efficiency, a

4TMTSF stands for tetramethyltetraselenafulvalene5TMTTF stands for tetramethyltetrathiafulvalene6BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene


Figure 1.1: Chemical formulae of organic molecules with different functionalities.


network of internal heterojunctions, with a large contact area between donor and

acceptor species (referred to as ”bulk heterojunctions”) was proposed [40].

Despite the fast developments in the field of organic electronics and dramatic

improvement in knowledge and manipulation of the charge injection and trans-

port in organic semiconductors, a reliable relation between microscopic properties

and their effect on the physical properties is still under development. There is

significant work done in understanding the mechanism of conduction in organic

semiconductors. Correct correlations between morphology, molecular packing and

the resulting electronic properties are essential, in order to elucidate fundamental

questions regarding charge transport in these materials. Small molecule con-

ductors allow these type of studies. Because of a higher degree of order than

solution-processed polymers, they can be structurally characterized straightfor-

ward using diffraction, and microscopy techniques. Complementary to this, the

theoretical modelling can be easier implemented because of the lower complexity

of these systems.

In organic molecular solids, the intermolecular forces are weak (van der Waals

and electrostatic type). A detailed description of these issues will be developed

in Section 1.4. The bonding energies are considerably lower than in covalent and

ionic inorganic semiconductors [27,41]. For this reason, the mechanism of charge

transport is fundamentally different. In organic conductors, the charge carriers

interact strongly with the lattice environment leading to polarization effects and

tendency of charge carrier localization. The weak van der Waals interactions result

in a small electronic bandwidth, strong electron-lattice interaction, and polaron

formation. For example, calculations performed on organic molecular crystals

that are free of defects, yield bandwidths in the order of 0.1-0.5 eV [42–47]. This

is more that one order of magnitude lower than the bandwidth in silicon (∼ 10

eV, [48]). The small bandwidths is reflected in low charge carrier mobilities (10−5-

10 cm2/Vs [16–26] for organic semiconductors, compared to 50-500 cm2/Vs in

silicon [49]), and strong interactions between free charge carriers and the lattice.

This interaction facilitates the localization of the charge carriers and narrowing

of the bandwidths even further, and thus are expected to crucially affect the

transport properties. Considerable effort is involved in describing the polaron

dynamics, lifetime, binding energies, and diffusion in the lattice [50, 51].

The mobility of the charge carriers reflects the drift velocity of the charges in

the lattice, thus it is influenced by all interactions that it encounters:

µ =vd

E(1.1)

where µ is the drift mobility, vd is the drift velocity and E the electric field.


Scattering by defects, impurities and phonons (lattice vibrations) will decrease

the drift velocity of charges, thus the electronic mobility. The conductivity of the

material is given by charge carrier density n and charge carrier mobility µ.

σ = neµ (1.2)

where e is the elementary charge. The mobility is influenced by the scattering

events, so that [52]:

µ =eτ

m∗(1.3)

where τ is the time between two consecutive scattering events, and m∗ is the

effective mass. This provides a direct relation between the morphology of the

materials and their mobility. In polymers the conduction (µ = 10−8 − 10−4

cm2/Vs) [16, 17] is limited by disorder and hopping of charges between polymer

chains, and thus is lower than in oligomers (µ = 10−3 − 10 cm2/Vs). Even for

the class of oligomer devices, the mobility increases with the degree of structural

order. Structural defects and chemical impurities present in the crystal lattice

promote the localization of the charge carriers. They can either form new states

in the semiconductor band-gap, leading to electronic traps, or scatter the charges.

Both processes result in a decrease of the mobility.

The value of the mobility directly affects the performance of the material in

devices, as it is related with the switching speed. Dramatic improvement in con-

trol and understanding of the transport mechanism in organic materials, on the

molecular level, together with the knowledge on the influence of intrinsic and ex-

trinsic factors on a good performance, has been achieved lately (the value of the

mobility increased 5 orders of magnitude in the last 15 years, reaching the value

of amorphous silicon [49]). This is very important, as some of the possible ap-

plications, like switching devices for active-matrix flat-panel displays (AMFPDs)

based on OLED displays, active-matrix backplanes of OTFTs (organic thin film

transistors) for ”electronic paper” displays, or radiofrequency identification tags

(RFID) require mobilities greater than 1 cm2/Vs [53], values already exceeded in

devices built on organic single crystals [19]. Parallel to organic electronic devices,

organic-inorganic hybrids emerge as new applications by coupling high carrier

mobilities inorganic semiconductors with the flexibility of organic materials [54].

Most organic semiconductors behave as either p-type or n-type semiconduc-

tors (they have either holes, or electrons as majority carriers) (see Fig. 1.1 c, d).

The absence of ambipolar behavior is a severe limitation for the organic electronic

devices for the fabrication of CMOS7-like circuits. Different strategies were pro-

posed to overcome this problem. Most of them involve separate steps to fabricate

7CMOS - complementary metal oxide semiconductor


n-type and p-type transistors [17,55]. However, for many materials it is generally

accepted that this is not an intrinsic property, but rather a result of trapping of

one type of charge carriers [16] or due to a high energetic barrier for either elec-

tron or hole injection from the metal electrodes, which is caused by the relatively

large bandgap of organic semiconductors.

1.3 Organic electronic devices

1.3.1 Field effect transistors - operation principles

Organic semiconductors are active materials in different devices. Field effect tran-

sistors, light emitting diodes and solar cells are intensely studied. Organic mate-

rials are soft, fragile and relatively reactive, thus the conventional semiconductors

device fabrication technologies are not always compatible with these compounds.

For this reason the intrinsic electronic properties could not be reached in de-

vices for a long period of time. They were explored with different techniques

(e.g. time of flight [56]). Only lately, using innovative approaches, the fabrication

of organic electronic devices was successful and a good reproducibility between

research groups was achieved [19, 24, 25].

Figure 1.2: Structure of a field effect transistor (FET) with organic semiconductor

as active material. The source electrode is connected to the ground.

This convention is valid for all the devices presented in this thesis.

1.3. Organic electronic devices 9

The field effect transistor (FET) is a three terminal device. The three contacts

are referred to as gate (G), drain (D) and source (S, connected to ground). The

schematic picture of a a FET is drawn in Figure 1.2. The active channel forms at

the semiconductor-insulator interface. The gate insulator acts like a capacitor and

the electric field applied at the gate electrode determines the density of the charge

carriers accumulated at the interface. The current between source and drain is

modulated by the gate voltage (VG). The operation principle of a FET was

introduced by Lilienfeld [57], in 1930, and later developed by Shockley and Pearson

[58]. In 1947 Bardeen, Brattain and Shockley (Bell Laboratories) discovered the

transistor effect and fabricated the first device [59]. They were awarded the Nobel

Prize in physics in 1956 for their discovery. The first metal-oxide-semiconductor

field-effect transistor (MOSFET) was introduced in 1960 by Khang and Atalla

[60].

MOSFETs, built at the surface of inorganic semiconductors, were intensively

studied, and they are incorporated in integrated circuits [41]. Owing to similar

experimental I-V characteristics between organic field-effect transistors (OFETs)

and MOSFETs, the theory developed for MOSFETs is used as starting point

in modelling the OFET behavior. However, the electrical transport in organic

semiconductors is different than in the covalently bonded inorganic semiconduc-

tors. Some attempts to describe the operation principle in OFETs were per-

formed [61, 62].

The transistor channel is active only when the gate voltage (VG) value exceeds

the value of the threshold voltage (VT ). Below this point, the transistor is turned

off, and there is no conduction between drain and source (sub-threshold region).

In the operation of a FET, two distinct regimes can be distinguished (Fig. 1.3(a)).

In the linear regime (small drain-source VD voltages, VD ≪ VG−VT ), the current

between drain and source (ID) depends linearly on the applied voltage (Eq. 1.4).

The device acts as a gate voltage - controlled variable resistor. The value of the

drain current, ID, is given by:

ID =W

LµCi(VG − VT )VD (1.4)

where L is the channel length, W is the gate width, Ci is the capacitance per unit

area of the gate insulator. This model assumes constant velocity, electric field,

and inversion layer charge density between the source and the drain. A more

realistic approach accounts for the variation of the inversion layer charge between

source and drain, and yields the following expression for the current:

ID =W

LµCi

[

(VG − VT )VD −V 2

D

2

]

(1.5)


Figure 1.3: I−V curves for a FET operation. (a) Output characteristics (ID−VD)

for different applied gate voltages (VG). The curves are obtained from

simulating the I − V curves that correspond to expression 1.5. The

linear and saturation regimes are indicated. The dotted line points

the pinch-off that separates the linear region of operation on the left

from the saturation region on the right. (b) Transfer characteristics

(ID − VG) for different drain voltages. The curves correspond to ex-

perimental points for a pentacene transistor.

At higher VD voltages, the channel is not continuous, but a depletion area forms at

the drain contact. The onset of this region is called pinch-off (Fig. 1.3(a)). Beyond

this point, the operation regime is referred to as saturation regime. The drain

current is now relatively independent of the drain voltage, being only controlled

by the gate voltage, and varies quadratically with the field:

ID =W

2LµCi(VG − VT )2 (1.6)

Equations 1.4, 1.5, and 1.6 represent expressions that yield the value of the mo-

bility of the semiconductor. The mobility can also be estimated from the gate

voltage sweep (Fig. 1.3(b)), at low drain voltages VD. Here, the expression for

the transconductance (gm) is:

gm =∂ID

∂VG(1.7)

From this equation, the value of the mobility can be extracted:

µ =L

W

1

CiVD

( ∂ID

∂VG

)

VD→0(1.8)


However, all these expressions assume a field independent mobility, and that all

the charges induced by the gate voltage are mobile.

Key parameters in the operation of a FET are the electronic mobility of the

charge carriers and the on/off ratio. The first determines the switching speed and

the maximum current, and the later impose the switching of the device from a

non-conducting (off ) to a conducting state (on).

Good performance organic field-effect transistors (OFETs) were fabricated at

the surface of the organic single crystals using deposition techniques that min-

imize damage at the interface [19, 24]. Fabrication of devices with competitive

characteristics remains an ambitious task for large-scale applications.

1.3.2 Fabrication of OFETs

Owing to the fragility of organic materials, processing to incorporate them in

electronic devices represents a challenge at this early stage. However, they trigger

interest to develop revolutionary methods that are simple and efficient to be used

for large scale applications. In this section we will describe recent advances in

organic electronic devices, focusing on OFETs fabricated on molecular crystals.

Different device structures were proposed to study the electrical transport at the

surface of organic crystals, in order to measure a high intrinsic mobility, that

is not diminished by disorder introduced during processing. There are many

factors involved in the good performance of the device. In spite of the better

reproducibility that is archived lately, the results reported by different groups

are still not always consistent. This can be attributed in part to the fact that

the performance of the electronic devices depends critically on the quality of the

crystals and the interfaces, as well as on the extrinsic factors (like, for example

the environmental conditions in which the experiments were performed). Organic

semiconductor quality, dielectric properties, contacts, and interface properties are

equally important.

Deposition of the organic semiconductor

The most common method used for the deposition of small molecule conductor

films is vacuum sublimation. The macroscopic electronic properties of the films

are imposed by their crystallinity [53]. Better crystallinity and larger grain size

facilitates higher mobilities. Values of 1 cm2/Vs were reached in vacuum subli-

mated pentacene TFTs, after optimization of the fabrication process [85].

We mentioned in Section 1.1 that the highest impact that the organic elec-

tronics can produce over traditional Si-based technology, is the relatively easy


processing techniques that their deposition requires. Polymers are attractive be-

cause they are soluble in organic solvents, thus they can be spin coated or printed

on flexible substrates, forming amorphous or polycrystalline films. Still, the high-

est mobilities are achieved in devices build with small molecules. A drawback is

that their solubility is limited and they require deposition methods like vacuum

sublimation, or physical vapor deposition. These demands are not straightforward

to accomplish. Because a high electronic mobility is not sufficient for a material

to be competitive for large scale applications, scientists develop different methods

to facilitate the compatibility with cheap and easy solution processing techniques.

Herwig et al. proposed a synthetic concept for fabrication of a soluble pentacene

precursor [63]. The precursor is converted to pentacene via thermal [63] or ir-

radiative [64] treatment, and the obtained thin film transistors (TFTs) exhibit

mobilities of 0.2 cm2/Vs. A different route to increase the solubility of oligomers

is the attachment of flexible side groups. At the molecular design, careful atten-

tion is payed to the interplay between the degree of solubility and the molecular

stacking that the side group induces. Field-effect transistors with maximum mo-

bilities of 0.01 cm2/Vs were fabricated from quaterthiophene and hexathiophene

end-substituted with 3-butoxypropyl groups [65].

The above mentioned directions represent a compromise, a balance between

performance and cost, because the mobility of materials deposited from solution

remains lower than that of the thermal evaporated material [19, 20]. Moreover,

in polycrystalline films [20, 21, 23], the mobility is lower than in single crystals

[19, 24–26]. This is partially caused by a large grain boundary resistance.

Gate dielectric materials

In field-effect transistors the conduction takes place at the surface of the semicon-

ductor, thus the performance is limited by the quality of the interface between

organic and dielectric, and only in part by the bulk properties. This is evident

from experiments that demonstrate that the gate insulator can modify the charge

density at the interface, having a crucial effect on the operation of both poly-

mer [66] and small molecule [67] devices. Particularly important are the roughness

of the semiconductor/dielectric interface, and the density of defects and impuri-

ties present in this region. Different treatments of the dielectric were proposed in

order to decrease the trap density (e.g. OTS8 treatment [68]).

General requirements for a high quality dielectric include several parameters.

The introduction of a large capacitance, that governs the magnitude of charge

8OTS denotes octadecyltrichlorosilane


Figure 1.4: Different geometries of the OFETs. Source (S), drain (D), and gate

electrodes are indicated. The transport takes place at the interface

between semiconductor layer and gate insulator layer. (a) bottom-

gate geometry; (b) top-gate geometry.

induced in the channel by gate effect, can be done using high-k dielectrics or by

varying their thickness. Besides this, a good insulator in FETs should account

for a large breakdown voltage and low leakage currents, excellent thermal and

chemical stability. There are three classes of dielectric materials incorporated in

OTFTs [69]: inorganic dielectric materials (e.g. SiO2, Ta2O5, Al2O3), organic

dielectric materials (e.g. parylene, PS9, PMMA10, Fig. 1.1(e)) and self-assembled

mono-and multilayers. In FETs with top-gate configuration (Fig. 1.4(a)), the

deposition of inorganic dielectrics partially damages the surface of the organic

crystals and introduces chemical and structural disorder, due to violent process-

ing methods (sputtering, e-beam, plasma-enhanced chemical vapor deposition).

This is the reason why organic dielectrics are used only in bottom-gate devices

(Fig. 1.4(b)). The deposition of an organic dielectric requires lower temperatures

(typical deposition techniques are spin-coating, casting, and printing), are not

destructive, and yield remarkably high mobilities (8 cm2/Vs in rubrene single

crystals transistors with parylene gate dielectric [24]).

Contacts

Contact issues are amply discussed in the field of molecular electronics. Different

models are proposed, as it is believed that the charge transport is either limited by

injection problems due to poorly defined contacts, or by bulk conductivity. Baldo

9PS denotes polystyrene10PMMA denotes polymethylmethacrylate


et al. elaborated an interface injection model, that accounts for two distinct

injection steps [70]. During the first step, charges are injected from the contacts

into an interface region that contain a broad distribution of states induced by

interface dipoles. In the second, limiting step, the charges migrate from the

interface to the bulk region.

Two types of device geometries are used in the fabrication of the field-effect

transistors: top-contact and bottom-contact structures. In the first case, it was

observed that the deposition of contacts on the organic semiconductors inflicts

with their chemical and mechanical fragility, and introduce traps locally, at the

metal/organic interface [71]. Different deposition methods are used, depending

on the nature of the contact. Metal contacts are usually evaporated or sputtered

using shadow masks, or painted (the latter method is also applied for colloidal

graphite paste). The choice of the metal contact gives the value of the Schottky

barrier, thus the deviation from the ohmic regime, and also the type of the major-

ity carriers. Alternative contact materials, with specific deposition requirements

were proposed and successfully applied. Room temperature lamination of metal

coated elastomeric stamps (e.g. PDMS11 [19]) represents a non-destructive, high

resolution method to fabricate contacts. The polymer PANI12 and the copolymer

PEDOT:PSS13 (Fig. 1.1(b)) are currently widely used in organic devices, espe-

cially OLEDs and solar cells because of their excellent transparency in the visible

region, but also in FETs due to good electrical conductivity, and environmental

stability [72].

1.3.3 Outlook

Reviewing the recent advances in materials, methods and emerging applications,

two very important issues can be mentioned. Firstly, organic semiconductor re-

search is an exciting and interdisciplinary area of current research activity that

raised the interest of theorists, chemists, physicists, and device scientists and is

developing extremely fast. Secondly, organic materials impose a new way of think-

ing because they exhibit a wide range of electrical properties, being tunable from

insulator (in gate dielectrics) to semiconductor (as active layers in devices), to

metal, and even superconductor. A scheme with the wide range of functionalities

that the organic materials offer can be found in Figure 1.1(a-e). These diverse

properties are spectacular and give the opportunity of building ”all-organic de-

vices”.

11PDMS denotes polydimethylsiloxane12PANI denotes polyaniline13PEDOT:PSS denotes polyethylene(3,4-dioxythiophene)/polystyrene sulfonate

1.4. Organic molecular crystals 15

1.4 Organic molecular crystals

Characteristic of organic semiconductors are the intramolecular bonds. The

alternation of single (σ) and double (π) bonds, referred to as conjugation, is typ-

ical for organic conductors. The Carbon atoms involved in this type of bonding

are sp2 hybridized. The three hybrid sp2 orbitals form the σ bonds, with the

σ-electrons being highly localized. The remaining nonhybridized pz orbitals of

adjacent Carbons form the π- orbitals perpendicular to the σ bonds and delocal-

ized over the molecule. The σ bonds are very strong (they are positioned lower

in energy). The molecular orbitals that are filled (π-bonding orbitals) form the

valence states. The filled orbital with the highest energy is called the Highest

Occupied Molecular Orbital. The vacant orbitals (π∗-antibonding orbitals) form

the conduction band, with LUMO being the Lowest Unoccupied Molecular Or-

bital. The π-electrons are responsible for an important part of the intermolecular

conduction in these materials. Besides this, the molecular packing in the solid,

imposed by the physical interactions, determine the physical properties, e.g. the

electronic transport.

The intermolecular forces are weak, and result from the cooperation and

competition between π − σ and π − π interactions. The interaction energy be-

tween molecules originates from several contributions. The electrostatic inter-

action between permanent multipoles (usually quadrupoles), and the dispersion

(van der Waals) forces between induced dipoles represent the dominant force.

The interaction between the permanent multipoles and the induced multipoles,

called induction, is generally treated as a second order term. In a simple pictorial

model, the van der Waals forces between molecules originate from the instan-

taneous polarization of the neutral molecules into dipoles [27]. The temporary

generated dipoles promote the formation of new induced dipoles in neighboring

molecules and spread in the lattice. For interplanar separation that characterize

organic molecular crystals (d > 3.4 A [32]), the van der Waals forces are always

attractive. The quadrupolar interactions emerge from the coupling between the

permanent quadrupolar moments of the molecules [73, 74], and can be attractive

or repulsive, depending on the relative orientation of the quadrupoles.

The crystal structure is determined by an interplay between van der Waals

forces and quadrupolar interactions. The two competing interactions promote

different molecular stacking that minimize the energy. The van der Waals inter-

actions are optimized for ”face-to-face” orientation of the molecules, that result

in the maximum π− overlap. On the other hand, the presence of quadrupolar

moments favor the ”edge-to-face” stacking, in which the hydrogen on one ring


Figure 1.5: Crystal structure of pentacene single crystal. Left panel: view along

the [100] axis (the layered structure can be seen). Right panel: the

herringbone arrangement within the layer, view along the long axis of

the molecule. The unit cell orientation is also drawn.

encounter the π− network of the adjacent molecule. The most common crystal

packing that results from the afore mentioned mechanisms is the herringbone-

arrangement (Fig. 1.5). Oligomers often crystallize in a layered fashion, and

the molecular arrangement within the layers is imposed by the van der Waals

and quadrupolar interactions. Gavezzotti et al. distinguished four possible her-

ringbone modes in which polynuclear aromatic hydrocarbons pack: herringbone

structure (naphthalene, anthracene, pentacene ), sandwich herringbone structure

(pyrene, perylene), γ structure (benzopyrene, coronene), β structure (trybenzopy-

rene, tetrabenzoperylene) [75]. Cofacial π-stacking in molecular crystals is hardly

ever found. Anthony et al. synthesized these types of structure by substitution

of different groups to pentacene backbone [76]. They showed that different type

and amount of π-overlap can be controlled by the nature, size and position of the

substituent, leading to various stacking motifs. Although seldom encountered,

cofacial configurations are particularly attractive because they accommodate the

largest electronic splitting in the HOMO and LUMO levels, promoting the high-

est theoretical predicted charge carrier mobilities [77]. In the local picture (as

opposed to the band picture) of the charge transport in organic conjugated ma-

terials, the efficiency of this process reflects how easy the charges are transferred

1.4. Organic molecular crystals 17

between neighboring sites, and, as expected, is very sensitive to orientation of the

molecules with respect to each other. The electronic coupling between adjacent

molecules, quantified by the transfer integral t, is modulated by the molecular

arrangement and directly associated with the electronic mobility [45, 46, 77]. In

the framework of these calculations, the amplitude of the electronic coupling is

influenced by the intermolecular separation distance, the molecular overlap, the

length of the molecule, and, in the case of herringbone structures, the rotation

of molecular planes [77]. Additionally, the relevance of thermal motions in the

modulation of the electronic coupling between molecules in an an organic solid

was demonstrated [45].

Owing to the weak interaction forces between molecules in the solid, small

variations in the crystal packing can be present, leading to polymorphism. The

term polymorphism refers to the existence of more than one crystal structure

for a particular compound. This phenomenon is frequently displayed by organic

crystals and is driven by the growth conditions and/or subsequent treatment. In

rubrene, different polymorphs can be obtained, depending on the pressure in the

system in which the starting material is sublimed (see Chapter. 6 in this thesis

and the references therein). In quaterthiophene (α−4T) [79] and sexithiophene

(α−6T) [80,81] different molecular arrangements are induced by the source tem-

perature. The general picture is even more complex in pentacene films, where

four polymorphs were detected [78, 82]. The obtained polymorph is dictated by

the substrate type, the substrate temperature and the thickness of the film. For

pentacene single crystals, only one polymorph exists [78].

The existence of different polymorphs for some molecular crystals provides

a unique opportunity to understand the influence of the crystal packing on the

electronic properties, since these systems only differ with the orientation of the

constituent building blocks with respect to each other, the molecule being the

same [42, 46, 81]. For example, Troisi and Orlandi performed band structure

calculations on the four pentacene polymorphs and found that the mobility tensor

is highly anisotropic for three of the four considered polymorphs [83]. This result

is relevant for understanding the fundamental mechanisms of charge transport in

organic crystals.


1.5 Single crystals - model systems for investiga-

tion of intrinsic properties

The progress in the field of organic electronics requires a good fundamental under-

standing of factors that influence the electronic behavior. This leads to a better

control of the microscopic properties that determine the conduction of the mate-

rials used in the devices. A correct insight on the interplay between the effects of

chemical structure and molecular orientation on the transport process can only

be accomplished when single crystals are used.

Single crystals are not meant to be incorporated in applications, but to provide

a well defined structure, in which the intrinsic electronic properties of the material

can be measured [19,24–26,84]. They serve as model systems, for which structure-

properties correlations can be explored.

Although thin films (polycrystalline or amorphous) are more attractive for

organic electronic devices, here the intrinsic properties are generally masked by

grain boundaries where structural defects localize and trap the charge carriers.

The major limitation of thin-film transistors (TFTs) comes from the fact that their

performance is severely dependent on the fabrication conditions. Moreover, even

films that are grown in an identical manner can exhibit very different electrical

properties [85].

The electrical properties of organic single crystals have been successfully probed

by time-of-flight (TOF) [56] and space-charge-limited current (SCLC) methods

[26], as well as field-effect transistor (FET) experiments [19, 24, 25].

1.6 Aim of the present research

Motivated by the various properties that organic semiconductors reveal, we eval-

uate in this thesis a critical analysis of the different factors that determine the

electrical conduction in these materials. Although the progress is fast, both in

performance and reliability, there are still numerous questions that lack an answer,

or for which inconsistent explanations were given.

The questions we address in this thesis concern intrinsic and extrinsic factors

that determine the charge transport in organic conductors. In order to answer

these questions we focus our work on single crystals (the advantages of working

with single crystals are outlined in Section 1.5). We systematically study the effect

of structural defects and impurities (Chapter 2, Chapter 3), geometrical factors

(Chapter 4), molecular stacking (Chapter 6) and exposure to ambient conditions

1.7. Outline of the thesis 19

(Chapter 5).

While the organic field-effect transistor characteristics were improved in the

past years by implementation of novel processing techniques, the charge carrier

mobility remains limited. In this context, our efforts focus on the understanding

of the microscopic processes that determine the value of the mobility in molecular

crystals, in particular the generation and migration of defects. The work aims

not only at improving the performance of organic materials for possible incor-

poration in devices, but also at satisfying the pressing need for insight into the

relevant physical processes that govern the electrical conduction in these materi-

als. We show that even small defect densities are critical and act as charge traps,

consequently decreasing the mobility significantly. By systematic measurements,

we demonstrate that the origin of defects is of dual nature. The trapping sites

such as crystal defects and chemical impurities are created during crystal growth.

Additional traps are introduced during processing or exposure to ambient con-

ditions. Moreover, we propose successful methods that allow the study of high

quality materials with desired properties.

Furthermore, we incorporate the investigated materials into field-effect de-

vices. Here, the conduction channel is limited to the vicinity of the interface. We

show that the conductivity is strongly influenced by disorder and charge traps

near the interface, and propose a reliable method to overcome this limitation.

Hence, our efforts can broadly be defined as contributions in the area of fun-

damental and applied studies on the organic molecular crystals properties. By

means of correlations between electrical measurements and structural properties,

determined via X-ray diffraction, or thermodynamic response via thermogravi-

metric experiments, we wish to cover important features of the chemical, physical

and technological problems that arise in these materials.

1.7 Outline of the thesis

This thesis is organized as follows:

Chapter 2 emphasizes the importance of the control of defects and impurity

states in organic molecular crystals in order to obtain a high electronic mobil-

ity. We measure record electronic mobilities as a result of the reduction of the

number of traps by careful crystal growth and subsequent handling. The temper-

ature dependence of the mobility is consistent with the band model for electronic

transport.

In recent studies, little attention is paid to the concentration, distribution

of impurities and their consequences on electronic properties. Thus far, it has


been generally assumed that the impurities are evenly distributed throughout

the lattice. In Chapter 3 we will show that this is not the case for pentacene

single crystals, where the impurities are are located preferentially at the surface.

Moreover, we describe a new, reliable method, and demonstrate that in our field-

effect transistor devices the impurity states at the interface can be made inactive

by incorporating them into the dielectric gate barrier.

In Chapter 4 we give a geometrical description of the electric field distribution

in organic crystals. We report the cross-over from 1D to 2D - space charge limited

conduction in pentacene single crystals with planar contacts. Furthermore, we

establish the parameters for which the conduction is dominated by bulk charges

and show that the transition to a surface-governed transport takes place gradually.

These results incorporate corrections for the anisotropic resistivity.

Chapter 5 is dedicated to the effect of air exposure on the electronic prop-

erties of pentacene single crystals. We show experimentally that air can diffuse

reversibly in and out of the crystals. This process is reflected in the electrical

properties. We are able to distinguish two competing mechanisms that modulate

the electronic transport: the doping effect of oxygen and the trapping caused by

the presence of water vapor. By combining the gravimetric and electric measure-

ments, we can describe quantitatively these effects.

The investigations that yield the results presented in Chapters 2, 3, 4, and 5

focused on the study of pentacene single crystals, the first organic crystalline

material already incorporated in prototypes of commercial devices [20]. The mea-

surements discussed in Chapter 6 were motivated by intriguing changes observed

in the values of electronic mobility of rubrene single crystals at low temperatures.

We relate this changes with the structural transformations in the material.

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2The Effect of Impurities on the Mobility

of Single Crystal Pentacene∗

We have obtained a hole mobility for the organic conductor pentacene (in the

single crystal form) of µ = 35 cm2/Vs at room temperature increasing to µ = 58

cm2/Vs at 225 K. These high mobilities result from a purification process in which

6,13-pentacenequinone was removed by vacuum sublimation. The number of traps

is reduced by two orders of magnitude compared with conventional methods. The

temperature dependence of the mobility is consistent with the band model for

electronic transport.

∗This Chapter is adapted from O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Appl. Phys.

Lett. 84, 3061 (2004).

27

28 Chapter 2. The Effect of Impurities on the Mobility

2.1 Introduction

Organic materials are presently being investigated and incorporated in semicon-

ductor devices for a new era of the electronics industry. The understanding of the

electrical conduction mechanism in these materials and at their interfaces repre-

sents a challenge, for which various, often conflicting models have been proposed.

Molecular crystals are formed by relatively weak van der Waals interaction be-

tween molecules, and the molecular packing determines the electronic behavior.

Thus the charge carrier transport must be described using completely different

models than for covalently bonded semiconductors. Of the many molecular con-

ductors, pentacene is a promising candidate for future electronic devices and an

interesting model system. Recent improvements in electronic applications showed

that this material exhibits mobilities higher than 1 cm2/Vs for TFTs made from

highly ordered films [1,2]. A mobility up to 8 cm2/Vs was measured in single crys-

tals of rubrene using a complete organic field-effect transistor [3]. The anisotropy

of mobility, and a band-like temperature dependence was demonstrated in rubrene

FETs with PDMS flexible elastomeric stamps as substrate, and air-gap as gate

dielectric. This fabrication method yields mobilities as high as 15 cm2/Vs at

room temperature [4] and 20 cm2/Vs using the 4-probe measurements [5]. The

above mentioned values for mobilities reflected hole transport. By using the last

mentioned approach, electron mobilities of 1.6 cm2/Vs were measured for TCNQ

(7,7,8,8-tetracyanoquinodimethane) single crystals [6]. The importance of impu-

rities for the limitations in device performance has been emphasized during the

last few years. However, little quantitative analysis concerning the consequences

of impurities is incorporated in recent studies [7].

2.2 Control of the defects and impurity states in

organic crystals

We report a mobility of µ = 35 cm2/Vs at room temperature increasing to µ = 58

cm2/Vs at 225 K for ultra-pure pentacene single crystals. The crystals were

obtained by vapor transport growth in argon flow after purification of the material

by a vacuum sublimation technique designed to remove pentacenequinone. The

content of the quinone impurity in pentacene was determined using high pressure

liquid chromatography technique (HPLC), indicating a reduction by almost one

order of magnitude. The structures of pentacene and 6,13-pentacenequinone,

respectively, are shown in Figure 2.1.

2.2. Control of the defects and impurity states in organic crystals 29

2.2.1 Spectroscopic evidence for the presence of impurities

Figure 2.1: Molecular structure of pentacene (a) and 6,13-pentacenequinone (b).

Figure 2.2: Left panel: IR spectrum of pentacene powder without any additional

purification. The presence of the peak around 1697 cm−1 is due to

quinone impurities. Right panel: mass spectrum of the starting ma-

terial. The peak at m/z = 278 corresponds to pentacene and the one

at m/z = 308 is a result of the presence of 6,13-pentacenequinone.

The starting material for the experiment was pentacene obtained from Aldrich.

Infrared absorption measurements (Nicolet Nexus spectrometer) show that 6,13-

pentacenequinone is present as an impurity. The evidence for this is the absorption

peak at 1697 cm−1 which is assigned to a C=O bond vibration (Fig. 2.2 - left

panel). The pure material does not have significant absorption in this region. The

infrared experiments were complemented by mass spectrometry analysis, which


confirmed that the C=O vibration originates from a pentacenequinone molecule

(Fig. 2.2- right panel).

2.2.2 Purification of the starting material

We have used vacuum sublimation under a temperature gradient as purification

method. This technique is effective for the separation of impurities from a solid

if these impurities have a vapor pressure that is sufficiently different from the

desired product [8]. The cleaning tube is shown in Figure 2.3. The pentacene

powder is placed in an alumina boat inside a glass tube that is thoroughly cleaned

chemically, and then heated in a furnace under vacuum to remove the solvents

used for cleaning. A controlled temperature gradient is applied along this tube.

The purification takes place at T = 430 K for 70 h under a dynamic vacuum

of a membrane pump. Special attention is paid to avoid contamination due to

vacuum connections. The sublimated molecules will condense in the cold zone

of the tube. The entire set-up is placed in the dark to prevent UV degradation

of the acene molecules. The carbonyl groups at each side of the middle ring

reduce the sublimation enthalpy compared to the host molecule, thus at T = 430

K pentacene will not sublime and only quinone will be removed. This can be

detected as a brown powder on the walls of the tube. The violet powder that

did not sublime is purified pentacene that is used as the starting material for

the single crystal growth. We repeat this purification process several times to

minimize the impurity content. We will present in Section 2.2.4 the effect of the

different sublimation steps on the degree of purity.

Figure 2.3: Schematic overview of the vacuum sublimation set-up used to remove

the 6,13-pentacenequinone from pentacene.


2.2.3 Single crystal growth

Pentacene single crystals were obtained using physical vapor transport in a hori-

zontal glass tube [9] under a stream of argon. The use of ultra-pure argon without

hydrogen as the transporting gas is motivated by the need to prevent the introduc-

tion of other impurities, like 6,13-dihydropentacene in the crystal, which can form

by the hydrogenation of the acene at the middle ring (most reactive positions).

Figure 2.4: Crystal growth setup. The source material is placed in the boat in

the hot part of the tube and it is transported by the inert gas to the

crystallization region. The temperature profile of the growth tube is

shown in the lower panel.

The gas was obtained from AGA, with purity of 99.999%. A drying column

was inserted in the system for additional purification of the gas. We paid attention

to the quality of the transporting gas because the quinone can be re-introduced by

residual water or oxygen as ppm impurities in the carrier gas during growth. The

growth tube set-up is presented in Figure 2.4 (upper panel). Prior to growth, the

inner tube was cleaned with soap, type II water (ρ ∼= MΩcm at 25 C), acetone

and alcohol. The empty tube was heated in argon flow at T = 620 K for 15 hours


to remove possible water and other solvents. 30−40 mg of the source pre-cleaned

material was placed at the end of the tube in an alumina boat. A temperature

gradient was applied by resistive heating of two heater coils around the tube. The

temperature profile is shown in Figure 2.4 (lower panel). The temperature was

controlled using two thermocouples, placed at positions marked with labels T1

and T2. The sublimation temperature, Ts = 545 K, was kept as low as possible,

in order to obtain a low crystallization rate that ensures a minimum formation

of defects and avoids side reactions [10]. The setup was placed in the dark to

protect against oxidation. The molecules, in the vapor phase, are transported by

the argon gas and will crystallize in the cold part of the tube (approx. 30 cm from

the sublimation point. The crystals are platelet-shaped and their dimensions can

be controlled by the growth time. Typical growth time is around 3 days. This

yields crystals of maximum 4 × 4 mm in plane and 50 µm thickness.

After obtaining ultra-pure crystals, they were annealed for 50 h at a lower

temperature (Th = 450 K) than the crystallization temperature (Tc = 490 K).

This treatment diminishes the number of dislocations and the stress in the crystal.

The crystal structure of pentacene single crystals at room temperature is given

in the Appendix A of this thesis.

Without pre-purification of the pentacene, pentacenequinone will sublime to-

gether with the host molecule. Part of it will be introduced in the pentacene

matrix in the crystallization process at the low temperature part of the tube.

2.2.4 Quantitative analysis of 6,13-pentacenequinone con-

tent

We used HPLC (Agilent 1100 LC/MSD) [11] to determine the impurity concen-

tration of the pentacenequinone in pentacene single crystals. Pentacene crystals

were dissolved in 1,2,4-trichlorobenzene by stirring for 24 h at 45C under inert

atmosphere (in the glove-box).

Pentacene (P) and pentacenequinone (PQ) were separated on a silica column.

The selectivity for retention was controlled with a 3 : 1 v/v mixture of 1,2,4-

trichlorobenzene and cyclohexane used as mobile phase, at 80C. The volume

of the sample injected was 50 µl. We used the Beer’s law [12] to determine the

concentration of pentacenequinone impurity in pentacene single crystals. The

absorbance A at a particular wavelength λ is proportional to the concentration c

of the constituent species:

A(λ) = bε(λ)c (2.1)

where b is optical path length and ε(λ) is the extinction coefficient at λ. Com-


Figure 2.5: Retention times for pentacene (P) and pentacenequinone (PQ) at λ =

390 nm. Upper panel: P with 0.4% PQ. Lower panel: P enriched with

30% PQ

bining the expressions for the absorbance of P and PQ, corresponding to the two

peak areas in Figure 2.5, we obtain the relative concentration (c%) of PQ in P.

The expression that allows the calculation of concentration involve the extinction

coefficients (εP , εPQ), peak areas (AP , APQ) and molar masses (MQ, MPQ) of

the two components:

cPQ(%) = 100 ·H

1 + H(2.2)

where we define H:

H =APQ

AP·

εP

εPQ·MPQ

MP(2.3)

The amount of quinone was determined from the integrated intensity of the

chromatogram, using a diode array UV-Vis detector that can be tuned at differ-

ent absorption lines (Fig. 2.5). We have chosen λ = 390 nm because it corre-

sponds to a low extinction coefficient for pentacene, and much higher value for

pentacenequinone [13]:

log(εP ) = 2.75

log(εPQ) = 4.389(2.4)

Figure 2.5 presents typical HPLC measurements from which pentacenequinone

concentration in pentacene was calculated. The retention times for pentacene and

pentacenequinone were ≃ 2 min, and ≃ 11 min respectively. The upper panel

corresponds to 0.4% PQ in P. We deliberately introduced pentacenequinone in


Figure 2.6: 6,13-pentacenequinone concentration in pentacene in different stages

of purification: 1- as received, 2- single sublimation clean, 3- double

sublimation cycle, 4- crystal grown from untreated powder, 5- crystal

grown from doubly cleaned powder.

the solution obtained from the solvation of a pentacene single crystals. This can

be seen as an enhanced signal at time = 11 min in the lower panel of Figure 2.5,

that corresponds to 30% PQ content. Additional peaks, of lower intensity, arising

from other impurities can be detected in both panels of Figure 2.5. However, in

this study we did not concentrate on the quantitative evaluation of their content,

but we focus only on the majority impurity.

The quinone concentration was reduced from the as received material contain-

ing 0.68% in two sublimation steps to 0.17%. Subsequent crystal growth reduces

the quinone concentration to 0.028%(±0.004) compared with 0.11%(±0.006) in

crystals grown from untreated powder (Fig. 2.6). The characteristic absorption

of C=O is observable even for the purest crystals (stage 5 in Fig. 2.6). We will

show in Chapter 3 that this small quantity of pentacenequinone impurity still

present in pentacene single crystals is critical for the fabrication and operation of

the field-effect transistors.

2.3. Electronic response to structural and chemical properties 35

2.3 Electronic response to structural and chemi-

cal properties

2.3.1 Space charge limited currents

We determined the electrical properties of the pure pentacene single crystals using

space-charge-limited current (SCLC) measurements (Fig. 2.7). Gold was used as

hole-injecting electrode. The samples were measured in dark and a vacuum of

2 · 10−7 mbar.

In the analysis of the evolution of the current density J with respect to the

applied electric field E several assumptions are made. Following the standard

analysis [7, 14, 15], we consider:

• a 1-dimensional, unipolar current flow (holes are injected from the contact

placed at position x = 0 and collected at the contact at x = L);

• the contacts are ohmic;

• the mobility of free charge carriers is independent of the magnitude of the

applied electric field;

• the diffusion of charge carriers inside the crystal is neglected;

• the injecting contact is an infinite source of charge carriers;

• the traps are homogenously distributed in space and all correspond to one

discrete energy level;

• the density of free charge carriers (nf ) is described by Boltzmann statistics,

and the density of the trapped (localized) carriers (nt) follows the Fermi-

Dirac statistics, as follows:

nf = NV exp(

−EF (x)

kT

)

(2.5)

nt =h(E)

1 + exp(

Ei−EF (x)kT

) (2.6)

Here NV is the effective density of states in the valence band, EF the Fermi

level, Ei stands for the energy, h(E) describes the energetic distribution of

localized states, and k is Boltzmann’s constant.


Figure 2.7: Current density (J) vs. electric field (E) for pentacene single crystal

at room temperature. The solid lines represent the fits for the three

different regimes. The value of the electric field that corresponds to

the transition to the trap-free regime (ETFL) is shown. The inset

shows experimental configuration of the a, b, and c*-axis, and the

contacts.

Four distinct regimes can be distinguished in the evolution of the current with

the applied electric field (see Fig. 2.7). At small electrical fields the transport is

ohmic, and the current density depends linearly on the electric field. The current

is space charge limited (SCLC) at high electric fields. In the first part of the SCLC

regime, the injected carriers are trapped and the current is reduced by a factor

Θ, which represents the ratio between free and total number of charge carriers

introduced in the solid (Eq. 2.7):

Θ =nf

ntot=

nf

nf + nt(2.7)

where nf and nt are the free and trapped carriers density, respectively, and ntot

is the total carrier density. At VTFL/ETFL (trap-filling voltage) the current rises

abruptly. After this point, all the traps are filled and the trap-free regime is

reached (Θ = 1).

The expression that describes the current-voltage (density of current - electric

field) are obtained upon combining several expressions:


• the continuity equation (Eq. 2.8) [14, 16]:

J(x) = eµnf(x)E(x) (2.8)

where J(x) is the current density at distance x from injecting electrode, e is

the elementary charge, µ is the charge carrier mobility and E is the electric

field,

• Poisson’s equation (Eq. 2.9):

dE(x)

dx=

e

ǫ0ǫrntot(x) =

e

ǫ0ǫr[nf (x) + nt(x)] (2.9)

Combining equations 2.7, 2.8, and 2.9, the expression of the current becomes:

J = µΘǫ0ǫrEdE

dx(2.10)

By integrating this expression, one obtains:

Jx

µΘǫ0ǫr=

E2(x) − E2(0)

2(2.11)

Taking into account the assumption made previously, the electric field at the

injecting electrode (x=0) is E(0)=0, Eq 2.11 becomes:

E(x) =

√

2Jx

µΘǫ0ǫr(2.12)

Integration of Eq 2.12, together with the use of the boundary conditions imposed

in the beginning, yields the expression of the density of current versus electric

field (Mott-Gurney law) [7, 14]:

JSCLC =9

8ǫ0ǫrΘµ

V 2

L3(2.13)

where JSCLC is the current density in the space-charge-limited regime, V is the

applied voltage across a length L, ǫr is the dielectric constant of the conductor (for

which we use the literature value ǫr=3), and Θ the ratio between the concentration

of free carriers and the total numbers of carriers (see Eq. 2.7).

The mobility in our experiments was calculated from the trap-free region of

the space-charge-limited-current regime, using the Mott-Gurney model (Eq. 2.13).

We note that this formula was derived for the sandwich-type electrode geometry,


Figure 2.8: Temperature dependence of the electrical hole mobility for a pen-

tacene single crystal using the actual crystal thickness () and the

effective thickness (•).

whereas we use a gap-type geometry. A description of the two types of structure,

and a detailed study on the evaluation of the current-voltage characteristics will

be presented in Chapter 4. The value of the number of traps Nt can be calculated

from the value of the trap-filling voltage (VTFL):

VTFL =2

3

eL2

ε0ǫrNt (2.14)

The SCLC measurements show that the pentacene single crystals grown after pre-

cleaning of the starting material are very pure, with Nt = 1.74 · 1011 traps/cm−3.

This is almost two orders of magnitude lower than the number of traps obtained

for crystals grown with the conventional procedures [17].

2.3.2 Evaluation of the bulk mobility of the high quality

crystals

If we assume a homogeneous current flow through the sample, the mobility is

µ = 11.2 cm2/Vs. However, as the mobility in the basal plane ab is much larger

than the mobility along the c* -axis (perpendicular to ab plane), the current will

be confined to the contact’s side of the crystal. The measurements of the ohmic


regime of the current-voltage characteristic showed different values for the resis-

tivity for different directions (ρa = 1.3 · 106 Ωm, ρb = 4.7 · 105 Ωm, ρc∗ = 2.1 · 108

Ωm). We have used Montgomery’s method [18, 19] for analyzing anisotropic

materials, transforming an anisotropic sample with resistivities ρa, ρb, and ρc∗

and dimensions x, y, and z, to an isotropic solid with dimensions x′, y′ and z′.

For the isotropic solid, the normalized effective thickness is determined to be

zeff ′/(x′y′)1/2 = 0.7 for normalized sample thickness z′/(x′y′)1/2 = 2.19 and the

ratio for the in-plane directions y′/x′ = 0.535. Using this procedure, the relation

between the effective thickness (zeff ) and the real thickness of the crystal (z) is

calculated. Eq. 2.15 expresses the conversion for the two dimensions:

zeff = 0.32 · z. (2.15)

With the effective thickness introduced in the current density JSCLC,tf in

Eq. 2.13, for the trap-free regime, the calculated mobility increases by more than

a factor of three (see Fig. 2.8). Therefore, a more accurate value for the mobility

is µ = 35 cm2/Vs at 290 K and µ = 58 cm2/Vs at 225 K. We note that the

Montgomery method is only valid in the linear part of the I − V regime. For

I ∝ V 2 the effective thickness (zeff ) should be considered as the upper limit.

Thus, this analysis provides a lower limit of the intrinsic mobility.

2.3.3 Band transport in pentacene single crystals

Fig. 2.8 shows that below room temperature the mobility increases with decreasing

temperature following the relation:

µ = C · T−2.38 (2.16)

This behavior is noticed in several samples and it is consistent with a band model

for charge transport in pentacene [14], with the interaction of the delocalized car-

riers with the phonons, the main scattering process. Above room temperature a

different transport mechanism dominates the mobility. This low temperature de-

pendence was earlier suggested by time-of-flight experiments on single crystals of

naphthalene and anthracene [20]. In this Chapter we present the low-temperature

electrical measurements performed on organic single crystals pentacene, that also

point to a ”band-type” conduction. We were able do reproduce the early TOF ex-

periments in electrical measurements using crystals with a high degree of purity

(Nt = 1.74 · 1011 traps/cm−3). The ”metallic-like” behavior was also demon-

strated in field-effect transistors built at the surface of high quality rubrene single

crystals [4, 5]. Previous electrical measurements on organic crystals were dom-

inated by defects and impurities that severely localized the charges. This was


reflected in a lower mobility than the intrinsic one, and a thermally activated

charge transport, which is also not of intrinsic origin. In the band-like picture

of the transport in molecular crystals (Fig. 2.9(a)), the mobility decreases with

increasing temperature due to electron-phonon coupling. The scattering events

involve interactions with defects and lattice vibrations. The vibration of the lat-

tice increases with temperature, causing more scattering, and thus reducing the

mobility. Moreover, following Holstein’s theory of small-polarons, at low tem-

peratures the electronic mobility decreases with increasing temperature due to

bandwidth narrowing. Hannewald et al. expanded the local approach developed

by Holstein with nonlocal (Peierls-type) couplings using first-principles density-

functional calculations [22, 23]. Their theory is in good agreement with the low

temperature behavior of charge-carrier mobility shown in Figure 2.8. At high

temperature, due to phonon-assisted hopping (Fig. 2.9(b)), the mobility increases

with temperature (as seen in Fig. 2.8 above room temperature) [21].

2.3.4 Origin of traps

In the following, we will focus on the origin of the trapping factor (Θ) in Eq. 2.7.

The traps in the crystal are mainly caused by structural imperfections and chemi-

cal impurities (see the scheme in Fig. 2.10). In our crystals Θ varies from Θ = 0.3

at 225 K to Θ = 0.82 at 340 K. With increasing electric field the density of in-

jected carriers will increase, and above the trap-filled limit voltage (VTFL) the

mobility is not affected by impurity states and defects [24], as shown in Fig. 2.7.

This part of the curve was used to calculate the mobility of pentacene.

Extended defects, such as edge dislocations or screw dislocations modify the

available energy levels in their vicinity, often leading to the presence of accessi-

ble vacant orbitals in the band gap. We minimized the number of traps by a

careful crystal growth and subsequent handling. Heating the as grown crystals in

an inert argon atmosphere will reduce the dislocation density. These defects are

introduced during the crystallization process and are thermodynamically unsta-

ble. Thus their number decreases by annealing. Dislocations will also enhance the

chemical reactivity in their vicinity. Under the influence of light and temperature,

reactions that oxidize pentacene to pentacenequinone will occur preferentially at

dislocations [7]. So, even if the quinone is not present after crystal growth, it can

be formed at defects after exposure to air and light.

We found that 6,13-pentacenequinone is the dominant chemical impurity. We

did not observe C22H15 and C22H13O impurities that were calculated to form gap

states in our pentacene, C22H14 [25]. Moreover, we argue that these molecules


Figure 2.9: Charge transport mechanisms in organic conductors. (a) Band-type

conduction. In a perfect crystal, a free charge carrier is delocalized.

As the temperature is increased, the lattice vibrations scatter the

charges. This limits the charge carrier mobility. The mobility µ for

band transport increases with decreasing temperature. (b) Hopping

conduction. If the carrier is localized (e.g. due to defects), the lat-

tice vibrations promote a carrier to ”hop” over barriers of height Eb

between the localized sites. For hopping transport the mobility µ

increases with increasing temperature. The figure is adapted from

ref. [27].

42

Chapter

2.

The

Effect

ofIm

purities

on

the

Mobility

Figure 2.10: Origin of traps for the injected charges in pentacene single crystals

2.4. Conclusions 43

are irrelevant as these radicals are highly reactive. We were able to prevent the

formation of the dihydropentacene C22H16 by using argon as transport gas during

the crystal growth. We have shown that the reduction of 6,13-pentacenequinone

(C22H12O2) impurities in pentacene by a factor five reduces the number of traps by

almost two orders of magnitude. These impurities have different energy levels from

pentacene, but they are energetically inert as a hole trap because their HOMO

level is positioned below that of the host molecule ( [25] for C22H16 and [26] for

C22H12O2). This is distinctly different from experiments performed on smaller

acenes, where the impurities yield states in the gap [7]. For this reason, the

number of traps in our measurements is different from the number of chemical

impurities. Although the impurity molecules do not act as trapping centers, they

will induce a local deformation by distorting the pentacene lattice locally and

create a scattering center. The quinone molecule is non planar and larger than

pentacene. The middle ring has a flattened-chair shape with the bond length

of 1.216 A and planar inclined at an angle of 3.1 to the molecular plane [27].

Thus, it will induce a local deformation leading to an increase in potential energy

because of changes in molecular density. The quinone will strongly influence the

number of such scattering sites, and thus the charge transport through organic

single crystal.

2.4 Conclusions

In conclusion, we have reduced the impurity concentration of pentacenequinone

in pentacene by a pretreatment consisting of vacuum sublimation of the impurity

under a temperature gradient. The crystals exhibit a trap-free space charge lim-

ited current behavior. The mobility increases with decreasing temperature with a

power law µ ∝ T−n from µ = 35 cm2/Vs at room temperature to µ = 58 cm2/Vs

at 225 K, indicating band transport. These results incorporate corrections for the

effective thickness of the crystal for the anisotropic resistivity, where the effective

thickness is at least three times smaller than the crystal thickness. Our results

emphasize the importance of the control of defects and impurity states in molecu-

lar organic crystals in order to obtain a high electronic mobility, and allow studies

of the band transport regime [28].

References

[1] D. J. Gundlach, T. N. Jackson, D. G. Schlom, and S. F. Nelson, Appl. Phys.

Lett. 74, 3302 (1999)

[2] D. Knipp, R. A. Street, B. Krusor, R. Apte, and J. Ho, J. Non-Cryst. Solids

299-302, 1042 (2002)

[3] V. Podzorov, V. M. Pudalov, and M. E. Gershenson, Appl. Phys. Lett. 82,

1739 (2003)



[5] V. Podzorov, E. Menard, A. Borissov, V. Kiryukhin, J. A. Rogers, and M.

E. Gershenson, Phys. Rev. Lett. 93, 086602 (2004)

[6] E. Menard, V. Podzorov, S.-H. Hur, A. Gaur, M. E. Gershenson, and J. A.

Rogers, Adv. Mat. 16, 2097 (2004)

[7] M. Pope, and C. E. Swenberg, Electronic Processes in Organic Crystals and

Polymers 2nded. (Oxford Univ. Press., New York, 1999)

[8] A. R. McGhie, A. F. Garito, and A. J. Heeger, J. Cryst. Growth 22, 295

(1974)

[9] R. A. Laudise, C. Kloc, P. Simpkins, and T. Siegrist, J. Cryst. Growth 187,

449 (1998)

45

46 References

[10] L. B. Roberson, J. Kowalik, L. M. Tolbert, C. Kloc, R. Zeis, X. L. Chi, R.

Fleming, and C. Wilkins, J. Am. Chem. Soc. 127, 3069 (2005)

[11] The measurements are done in collaboration with P. van ’t Hof, and J. C.

Hummelen (RuG)

[12] J. R. Dyer, Applications of Absorption Spectroscopy of Organic Compounds

Prentice-Hall, Inc. (1965)

[13] http://webbook.nist.gov/chemistry/

[14] K. C. Kao, and W. Hwang, Electrical Transport in Solids, vol. 14, Pergamon

Press, New York, (1981)

[15] S. Nespurek, and J. Sworakowski, Phys. Stat. Sol. (a) 41, 619 (1977).

[16] S. M. Sze, Physics of Semiconductor Devices Wiley, New York, (1981)

[17] C. C. Mattheus, A. B. Dros, J. Baas, G. T. Oostergetel, A. Meetsma, J. L.

de Boer, and T. T. M. Palstra, Synth. Met. 138, 475 (2003)

[18] H. C. Montgomery, J. Appl. Phys. 42, 2971 (1971)

[19] J. D. Wasscher, Philips Res. Repts. 16, 187 (1961)

[20] W. Warta, and N. Karl, Phys. Rev. B 32, 1172 (1985)

[21] T. Holstein, Ann. Phys. 8, 343 (1959)



[24] G. Horowitz, M. E. Hajlaoui, and R. Hajlaoui, J. Appl. Phys. 87, 4456 (2000)

[25] J. E. Northrup, and M. L. Chabinyc, Phys. Rev. B 68, 041202(R) (2003)

[26] G. de Wijs, private communication

[27] A. V. Dzyabchenko, V. E. Zavodnik, and V. K. Belsky, Acta Cryst. B35,

2250 (1979)

[28] V. Y. Butko, X. Chi, D. V. Lang, and A. P. Ramirez, Appl. Phys. Lett. 83,

4773 (2003)

3Interface Controlled High-mobility Organic

Transistors∗

We report the fabrication of Organic Field-Effect Transistors (OFETs) in which

it is possible to reach the intrinsic bulk mobility. This was accomplished by

minimizing the trap density at the semiconductor-insulator interface. We obtain

low defect density interfaces by converting impurity scattering centers into the

gate dielectric. High mobilities (µ= 14−40 cm2/Vs), large on/off rations (Ion/Ioff

= 106) and good reproducibility of the measurements characterize the transistors

fabricated with present method.

∗This Chapter is adapted from O. D. Jurchescu, M. Popinciuc, B. J. van Wees, and T. T.

M. Palstra, submitted

47

48 Chapter 3. Interface Controlled High-mobility Organic Transistors

3.1 Introduction

Electronic devices based on organic semiconductors have gained prominent indus-

trial and academic interest in recent years [1–18]. Organic materials represent new

semiconductors for flexible, low-cost, light-weight electronic devices. The device

functionality remains limited by the relatively low electronic mobility, which for

thin film devices is partially caused by a large grain boundary resistance [19]. Only

single crystal based devices allow the determination of intrinsic properties. Rapid

technological progress has been made in the fabrication of field-effect transistors

(FET) at the organic - semiconductor interface [20–25]. However, the electronic

mobility of such FET devices is typically much smaller than can be obtained

in bulk materials 1 [26–28]. We demonstrate that for pentacene-derived devices

this can be related to surface properties of the crystal. We use a new method

to inactivate the impurity states at the interface in the single crystal devices by

incorporating them into the dielectric gate barrier. Thus, high mobility devices

in which the bulk mobility is reached can be reproducibly constructed.

Recent progress in organic single crystal FET devices has shown that the

semiconductor interface layer, which forms the conduction channel between the

source and drain electrode, contains a broad distribution of interface states [2].

In order to minimize the trap density, organic dielectrics such as parylene have

been used as gate dielectric [1, 20, 21, 26]. Previous attempts using more robust

dielectrics such as SiO2 and Al2O3 involve to many chemical reactions and lead

to structural disorder. Nevertheless, even organic dielectrics often yield limited

mobilities, which is partially attributed to damage below the metal electrodes [6].

However, little detailed knowledge is available on the nature of the defect states

and traps.

3.2 Current status of the field

The mobility of high purity organic field-effect transistors, is usually much lower

than the value determined from SCLC measurements. This can be attributed

to the fact that in FET devices the surface conduction is probed, whether in

SCLC the bulk mobility is measured. The mobility in the bulk can be increased

by reducing the number of traps through controlled processing. We have chosen

to investigate pentacene. This material has become a prominent choice, both

for industrial applications and for fundamental studies. Moreover, it is used in

rollable displays in prototypes of commercial devices [3], and exhibits the highest

1By bulk mobility we denote the mobility obtained from SCLC measurements

3.3. Novelty 49

reported electronic mobility: µ= 35 cm2/Vs at room temperature in bulk single

crystals [28]. However, for FET devices built on pentacene single crystals, a

mobility of 0.3 cm2/Vs was measured when using a parylene gate dielectric [26].

This value was improved to 2.2 cm2/Vs for purified pentacene [8]. A mobility of

1.4 cm2/Vs was measured for pentacene single crystal FETs using a SiO2 gate

dielectric treated with self-assembled monolayers [27]. The different mobilities of

the bulk and FET devices built on pentacene single crystals indicate the relevance

of a structurally and chemically clean interface. The morphology, spatial ordering

and roughness at the interface are as important as the purity of the active area

of the devices [14–16].

3.3 Novelty

Our OFETs, fabricated using the method described in this work, have a typical

charge carrier mobility of µ = 15 - 40 cm2/Vs at room temperature, similar to the

value measured in bulk, ultra-pure single crystals [28]. This mobility represents

a record value for the FETs based on organic semiconductors. The on/off ratios

are as high as Ion/Ioff= 106. The large values of the mobility and on/off ratio

result from a reduction in the number of scattering sites at the interface between

the semiconductor and insulator. The method developed in this work leads to

remarkably good reproducibility of the devices.

3.4 Single crystal growth

We have grown pentacene single crystals with a high degree of purity using phys-

ical vapor transport [7]. The starting material was obtained from Sigma Aldrich

and was purified using vacuum sublimation under a temperature gradient. We

have described in Section 2.2.3 of Chapter 2 the details concerning this step.

This growth method yields platelets of pentacene single crystals with in-plane

dimensions of 1 − 4 mm and thicknesses of 10 − 50 µm.


3.5 Surface morphology of pentacene single crys-

tals

3.5.1 Surface mapping

We have previously established that 6,13-pentacenequinone (PQ) constitutes the

largest impurity fraction in pentacene single crystals [28]. The quinone is easily

formed upon oxidation of the relatively reactive conjugated pentacene molecule.

Moreover, pentacene is routinely synthesized from PQ, thus small fractions of

the quinone can remain in the final reaction product. The purity of the material

can be improved by vacuum sublimation treatment of the starting material [28]

or repeated vapor transport crystallization [21]. In typical crystals, PQ may

be present at a concentration of 0.11% and even after repeated distillation we

observed a fraction of 0.028% [28]. Thus far, it has been commonly assumed that

such an impurity is evenly distributed throughout the lattice. We will show that

this is not the case, rather that PQ is located preferentially at the surface. This is

reasonable and can be expected from the dynamics of the crystal growth process.

At the interface, PQ molecules form scattering centers, reducing the mobility or

even preventing transistor action altogether. However, PQ can be incorporated

into the gate dielectric by growing additional quinone layers on the surface. Thus,

the PQ can form a pinhole-free gate barrier and considerably minimize the trap

states.

Pentacene is a layered material characterized by a herringbone pattern within

the layers, while adjacent layers are bonded by van der Waals forces. Pentacene

has several polymorphs, each with a particular d(001)-spacing along the c*-axis

(Fig. 3.1). This length varies from 14.1 to 15.5 A, but for pentacene single crystals

it is unique: dP (001)=14.1 A [29]. This polymorph is already adopted in the first

monolayers. The d(001) spacing is significantly different in 6,13 pentacenequinone:

dPQ(001)=17.79 A [30] (Fig. 3.1). This allows us to distinguish between the two

types of molecules present at the surface by Atomic Force Microscopy (AFM).

3.5.2 AFM measurements

In order to characterize the surface morphology of pentacene single crystals, we

investigated several crystals using the AFM technique and measuring on different

areas of the crystals. The AFM measurements were performed in the tapping

mode using a MultiMode Scanning Probe Microscope, the NanoScope IV, from

Digital Instruments. On the surface of pentacene single crystals we observed

3.5. Surface morphology of pentacene single crystals 51

Figure 3.1: Pentacene crystal packing. Pentacenequinone impurity is located ran-

domly at the surface. The molecular structure, as well as the interlayer

separation for pentacene (14.1 A) and pentacenequinone 17.79 A are

indicated.

Figure 3.2: AFM picture at the surface of pentacene single crystal showing char-

acteristic steps. The step height reveals the inter-layer separation

distance d(00l).


Figure 3.3: Upper panel: histogram showing the occurrence of the 14 A and 18 A

steps at the surface of the pentacene single crystals. The Gaussians

peaking at 14 A and 18 A are a guide to the eye. Lower panel: step

distributions characteristics for the bulk (obtained after cleaving the

crystals), where only the 14.1 A step is observed.

3.6. Comparison between different FETs 53

terraces (see Fig. 3.2), which exhibit steps heights with predominantly two val-

ues: 14 A and 18 A. The former value corresponds to the d-spacing of single

crystal pentacene, and the latter to that of PQ. Although we have no detailed

quantitative analysis over the entire crystal, we observe that the 14 A step oc-

curs approximately five times more often than the 18 A step. The results differ

slightly between crystals, depending on the surface area that is measured. The

occurrence of the 14 A and 18 A steps at the surface of the pentacene single crys-

tals, corresponding to pentacene and pentacenequinone, respectively, is shown in

Figure 3.3-upper panel. The crystals were then cleaved and the new surface was

mapped (bulk material). There, we observed only terraces with 14.1(5) A steps,

characteristic of pentacene (Fig. 3.3-lower panel). We conclude that the impu-

rity PQ molecules agglomerate in patches that are distributed over the surface

of the pentacene crystals. Similar effects were reported for acenes with a lower

number of benzene rings. Quantitative analysis via Gas Chromatography (GC)

on tetracene single crystals allowed the detection of the tetracenequinone surface

concentration larger by one order of magnitude with respect to the bulk [31].

Both in pentacene and tetracene single crystals, because the pentacenequinone

and tetracenequinone, respectively, are preferentially located at the surface, many

more scattering sites are formed in the FET geometry than we determined for the

bulk [28]. This is critical for the performance of the FETs because the traps are

located at the interface with the gate insulator, where the active channel forms.

3.6 Comparison between different FETs

3.6.1 FETs fabricated with conventional dielectrics

In the case of OFETs built on the surface of pentacene single crystals using con-

ventional methods (Fig. 3.4(a)) [8, 26, 27], the charge transport is dominated by

disorder, thus the mobility is lower than the bulk value. This reduction can orig-

inate from the presence of pentacenequinone molecules at the surface. The inter-

actions between the π−systems of the ’host’ and ’impurity’ molecules introduce

additional scattering sites to the inevitable traps formed during the deposition

of the gate insulator. The electric field applied at the gate electrode has to fill

first these trap states. Only a higher gate bias can populate the density of states

(DOS) of pentacene and modulate the drain current for a proper FET opera-

tion. Moreover, random dipoles and quadrupoles [32] are locally introduced at

the interface due to the presence of the impurity molecules. This leads to ener-

getic disorder and an increase in carrier localization by electronic polarization. It


has been shown by Lang et al. [2] that even high quality pentacene single crys-

tals have a relatively broad distribution of states (∼ 1eV ) above the edge of the

valence band. The different intermolecular interactions between pentacene, pen-

tacenequinone and the gate insulator material modulate the energy levels of the

states. This results in a broadening of the DOS and the formation of an increased

number of tail states.

3.6.2 FETs fabricated with pentacenequinone dielectric

We replace the conventional gate dielectrics (SiO2, parylene, polymers) with or-

dered 6,13 - pentacenequinone films (P-PQ OFET: Fig. 3.4(b)). This method

presents the unique advantage of being able to transform the scattering sites

present at the surface of pentacene single crystals into an extremely good layer at

the semiconductor-gate interface. A slow deposition rate ensures a small surface

roughness. Once the pentacenequinone layer at the surface is completed, an uni-

form, extremely good-quality heterointerface is built. This ensures the uniformity

of the conduction path. In this device, the electric field applied at the gate is

able to inject directly into the DOS of pentacene. This is reflected in a smaller

value of the threshold voltage (VT = ± 2 V) of devices built with the geometry in

Figure 3.4(b), compared with devices fabricated with conventional methods (VT

= 5 V in ref. [26] and VT = -10 V in ref. [27]).

3.7 Device fabrication

We paid special attention to the fabrication of the field-effect transistors on the

organic crystal surface, in order to avoid damaging the surface of the semicon-

ductor. Silver epoxy source and drain contacts were painted on the crystal at

different separation lengths, L. A film of 6,13-pentacenequinone was deposited

on top. The deposition of the insulator was carried out in a clean environment, in

high-vacuum (10−7 mbar), and the growth was carefully controlled. The source

material was obtained from Sigma Aldrich. No further purification was done.

The sublimation rate was low (5 A/min) to ensure the homogeneity of the film.

A shutter was inserted between the quinone source material and the pentacene

crystal. First the evaporation rate was stabilized, and only after that the device

structure was exposed to the molecules. The thickness of the film was determined

very precisely during the growth by a quartz crystal balance. The silver epoxy

gate electrode was then painted on top of the device.

3.7

.D

evice

fabrica

tion

55

Figure 3.4: Schematic cross-section of a FET fabricated on a pentacene single crystal. The bond-

line chemical formulae for 6,13-pentacenequinone and pentacene are drawn. (a) Con-

ventional gate dielectric is deposited on the surface of the pentacene crystal. Here,

pentacenequinone molecules present at the interface form scattering sites and de-

crease the mobility. (b) P-PQ OFET: highly ordered pentacenequinone films act as

gate insulators and reduce the number of scattering sites at the interface.


3.8 Properties of FETs fabricated with

pentacenequinone gate dielectric

Field-Effect Transistor measurements were performed in the dark and a vacuum of

10−6 mbar [9], in a home-built probe station. The characterization was done using

a Hewlett Packard 4155B Semiconductor Parameters Analyzer. We investigated

several FETs built on pentacene single crystals obtained from different batches.

The electrical properties of the devices were similar. We performed I − V mea-

surements on pentacenequinone single crystals in order to estimate the electrical

breakdown field of the gate dielectric. We obtained a value of ∼ 3 MV/cm for the

PQ single crystal insulator. Pinhole-free insulation was obtained for film thick-

nesses greater than 200 nm. We performed capacitance measurements on single

crystals of the PQ gate dielectric material and evaluate the dielectric constant

εr(PQ)= 3.5. An Andeen-Hagerling 2500A Ultra Precision Capacitance Bridge

was used for the dielectric measurements performed on pentacenequinone and a

Keithley 237 Source Measure Unit was used to determine the insulator breakdown

voltage.

The procedure we utilize for pentacene may be applied for the fabrication of

FETs of similar materials. Surface oxidation is widely encountered in conjugated

organic materials. Pentacene is highly susceptible to oxidation at the most reac-

tive positions, leading to 6,13-pentacenequinone. The larger acenes are even less

stable. The attachment of substituents on the acene backbone (e.g. phenyl in

the case of rubrene) makes these positions less reactive and protects the molecule

from unwanted oxidation. In this case the interaction of the oxygen with the

crystal is significantly different; the oxidation is reversible yielding endoperoxide

instead of quinones [38].

The transistors built using this method, on the surface of ultra-pure pen-

tacene single crystals, exhibit p-type conduction. We did not observe ambipolar

transport for this configuration. The sharp increase in the drain current ID in

the transfer characteristics presented in Figure 3.5 is a typical feature of a high-

performance field-effect device.

We calculate the field-effect mobility µ from the I−V characteristics presented

in Figure 3.5 and Figure 3.6. We observe typical mobilities in the range µ = 15 - 40

cm2/Vs for the transistors fabricated using the PQ gate dielectric. The variations

in mobility are caused by different crystal quality, contact injection issues, and

contact resistance. The on/off ratio is of the order of 106. We measured 20 devices

built on 8 different crystals and observed consistent behavior. The expression of

3.8. Properties of FETs fabricated with pentacenequinone gate dielectric 57

Figure 3.5: OFET fabricated on a high purity pentacene single crystal with 250

nm thick pentacenequinone film gate dielectric. The geometry of the

device is L × W = 250 µm × 4 mm. The graph shows the variation

of the drain current (ID) with respect to the applied gate voltage

(VG) (relative to the source contact) for VD = 1V and VD = -1V. The

properties of the transistor are included in the inset.

the mobility from transconductance (Fig. 3.5) is:

gm =∂ID

∂VG(3.1)

At low drain voltages VD:

µ =L

W

1

Ci

1

VD

( ∂ID

∂VG

)

(3.2)

where L is the channel length, W is the gate width, and Ci is the capacitance per

unit area of the gate insulator. The expression for the subthreshold slope is:

S =∂(logID)

∂VG(3.3)

The normalized subthreshold slope:

Si = S · Ci (3.4)

has a value Si = 33.7 V nF/dec · cm2. This value is higher by a factor of 6 - 8

than the value obtained for the best rubrene transistors [20].


Figure 3.6: Drain current (ID) versus drain voltage (VD) for different negative

gate voltages VG for a device with L × W = 250 µm × 4 mm, and

250 nm thick pentacenequinone film gate dielectric.

Typical output characteristics of the P-PQ transistors are plotted in Figure 3.6.

The curves are stable with time, even if the device is stored in ambient atmosphere

and light. The current in the channel increases as the negative gate bias (VG)

is increased, and saturates at VD = VG − VT . For each curve of Figure 3.6,

corresponding to a gate voltage, when the drain voltage reaches the value VD =

VG − VT , a depletion region will start to form around the drain contact. As

VD ≫ VG − VT , this region increases and the drain current becomes saturated,

independent of the applied voltage. We attribute the nonlinearities in the low VD

regime to the contact injection issues. This is consistent with previous studies that

assigned the superquadratic-like behavior at low drain voltages to the presence

of the Schottky barrier between Fermi level of the metal contact and HOMO

of the organic semiconductor (LUMO in the case of electron conduction) [33],

the (diffusion-limited) thermionic emission and the recombination at the metal-

organic interface [34], or the presence of a depletion area in the vicinity of the

”top contacts”, with a high density of localized states, that induces a parasitic

resistance in the FET circuit [6, 35, 36]. The quality of the manually deposited

contacts is not very good, and this is reflected even in the I − V characteristics

of the best devices of both VG and VD sweeps. Moreover, the contacts are not

equivalent and the device shows an asymmetry in the VG sweep between VD =

+1 V and VD = -1 V.

3.9. Conclusions 59

Small shifts in the threshold voltage were recorded for different devices, be-

tween positive and negative values. As for devices fabricated with identical meth-

ods, VT is both positive and negative, we suggest that this is not of intrinsic origin

(the negative VT is not caused by bulk defects, and the positive VT is not a result

of bulk crystal doping). More probably, during the deposition of the contacts and

the gate insulator, traps/additional charges are accumulated at the surface of the

crystal, randomly.

The field-effect-mobility estimated from Figure 3.6, from the linear regime,

as well as from the saturation regime (using Eq. 1.5, and Eq. 1.6, respectively),

increases with drain and gate voltage. This feature has also been reported by

Goldman et al. [27] and Podzorov et al. [37]. This can result from a field dependent

mobility, while in all the assumptions we considered that this parameter does not

vary with the electric field. Also, in the part of the graph that corresponds to the

linear regime of the FET operation, the channel resistance (Rc) varies linearly

with the applied voltage for a standard FET. The parasitic resistance introduced

at the contacts is field-independent. The total resistance (Rt) of the channel

is thus: Rt = RS + Rc + RD, where RS , RD are the resistance of the Source,

respective Drain contacts, and will not have a linear dependence of the field. This

will introduce additional variations from the already available models.

3.9 Conclusions

In conclusion, we have been able to incorporate the interface scattering centers of

pentacenequinone into the gate dielectric of Organic Field Effect Transistors built

on pentacene single crystals. Thus, we obtain a semiconductor/dielectric interface

of extremely high quality. The FET mobility µ = 15− 40 cm2/Vs can reproduce

the value obtained in ultra-pure single crystal bulk devices. This demonstrates

that we are able to measure the intrinsic properties of the single crystal by careful

control of the interface properties, which imply minimizing the trap density in

the active channel.

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4Cross-over from 1D to 2D - Space Charge

Limited Conduction in Pentacene Single

Crystals∗

We report the cross-over from 1D to 2D - space charge limited conduction in

pentacene single crystals with planar contacts. The space charge is confined to

the in plane longitudinal direction for L/h ≤ 15, with L the contact separation

and h the sample thickness. For L/h > 250 the space charge is dominated by the

transverse component of the electric field.

∗This Chapter is adapted from O. D. Jurchescu, and T. T. M. Palstra, Appl. Phys. Lett.

88, 122101 (2006)

65

66 Chapter 4. Cross-over from 1D to 2D - SCLC in Pentacene Single Crystals

4.1 Introduction

Small molecule organic crystals can exhibit large electronic mobilities at room

temperature, if local impurities and extended defects, such as grain boundaries,

can be minimized. Thus, single crystals can provide information about the in-

trinsic electronic transport properties [1–6]. Nevertheless, the evaluation of the

transport parameters can be difficult because of the charge injection at the in-

terface, or the non-uniform nature of charge carrier density, current density, and

electric field under non-ohmic conditions. The evaluation of the electronic mo-

bility depends strongly on the correctness of the relationship between electronic

parameters, including mobility, charge density etc., but also on the associated

geometry that is used.

In this chapter we analyze the current flow across pentacene single crystals

with planar contacts. Our study focuses on the electric field E pattern for parallel

planar contacts with different gap separations L, in the space-charge-limited-

current (SCLC) regime. We discuss the effect of the geometric parameter L/h,

with h the crystal thickness, and the anisotropy of the mobility on the electric

field distribution inside the semiconductor. Effects like surface scattering, charge

diffusion, surface traps [7] and surface polarization are neglected. We observe a

gradual transition from a 1- to 2-dimensional (D) space charge limited conduction

as L/h increases.

4.2 Space-charge-limited-current theories for semi-

conductors

4.2.1 Mott-Gurney model

The charge transport in organic conductors is often limited by the emergence of

space charges. There are two limiting geometries to evaluate the conduction. The

Mott-Gurney theory [8] describes the current-voltage characteristics for sandwich-

type contact geometries (Fig. 4.1(a)) for materials with one type of trap (the

majority trap) distributed uniformly in space (density Nt), having one discrete

energy level (Et). This is a 1D theory in which the electric field, as well as the

space charge, are confined to the channel. Mott-Gurney model gives an expression

of the current density versus applied voltage:

J =9

8Θε0εrµ

V 2

L3. (4.1)

4.2. SCLC theories for semiconductors 67

- electrodes- crystal

( a )

( b )

( c )

L

h

L

L

Figure 4.1: Electrode configuration: (a) sandwich-type geometry-side view, (b)

gap-type geometry-side view, (c) gap-type geometry-top view (used

in this study).

Here, J is the current density for the applied voltage V , Θ the trapping factor,

L the electrode separation, εr the relative dielectric constant of the material, and

µ the carrier mobility. A detailed description of this model was presented in

Chapter 2, Section 2.3.1.

4.2.2 Geurst model

Geurst analyzed theoretically space-charge-limited-currents in thin semiconduc-

tors layers for a gap-structure geometry [9] (Fig. 4.1(b)). Here, the thickness of

the film is negligible with respect to the separation between the contacts. For this

2D model the longitudinal component of the electric field is responsible for the

charge transport and the transversal component, perpendicular to the conduction

channel, is determined by the magnitude of the space charge. This analysis leads

to an expression in which the value of the current I depends quadratically on

the applied voltage V , is proportional to the relative dielectric constant of the

material, εr, and inversely proportional to the square of L, the distance between

the contacts:

I =2

πε0εrµ

V 2

L2(4.2)

Zuleeg et al. [10] have introduced the width of the electrodes, W , into the

expression of Eq. 4.2. This model improves the approximation of Geurst of infinite

long contact length.

Thus, the space-charge-limited current at high voltages varies with L−3 in the


Mott-Gurney 1D theory (Eq. 4.1) and with L−2 in Geurst 2D model (Eq. 4.2). The

former was used to estimate the hole mobility in tetracene [11] and rubrene [12]

single crystals with parallel plate electrodes. It was also applied for analyzing

pentacene single crystals [5,13] and TCNQ-coronene cocrystals [14] in a gap−type

geometry, with a conduction channel length comparable to the thickness of the

crystals. The model of Geurst was used to describe the charge transport of thin

layers of crystalline Se [15], in which the proportionality of the current with L−2

was observed.

4.3 Space charge limited currents in pentacene

single crystals with planar contacts

The question we address is for which parameters the 1D-model can be used for

planar contacts, and when the transition to the 2D-model takes place. We show

experimentally that for small L/h ratios, the Mott-Gurney type behavior is ob-

served. Due to the resistivity anisotropy in different crystallographic directions

and short conduction path, the current penetrates only a fraction of the crystal

thickness (heff ), and the electric field is homogenous. We find that for pentacene

single crystals the 1D SCLC model is accurate for L/h < 15, whereas the 2D

model applies for L/h > 250. The transition takes place gradually and follows

the changes in the value of L/h.

4.3.1 Experimental procedure

Platelets of high purity pentacene single crystals were grown from double subli-

mation cleaned [5] powder from Aldrich. Physical vapor transport in a horizontal

glass tube under a stream of argon was used [16], as described in Chapter 2. The

geometry of the crystals is maximally 4 × 4 mm in plane and thickness typically

of 15 − 20 µm. Silver epoxy contacts were painted parallel on the crystal with

a distance varying between 60 µm and 3 mm, limited by the dimensions of the

crystals (Fig. 4.1(c)). We used a Keithley 237 Source Measure Unit to perform

the electrical characterization. All I −V curves were taken at room temperature,

in darkness and a vacuum of 10−6 mbar, to avoid environmental effects [17]. For

each single crystal of thickness h, a voltage Vij was applied across different pair

of electrodes i and j attached to the sample at a distance Lij and the value of

the current Iij was measured. The source contact was kept the same to minimize

contact effects [7], and the drain was changed.

4.3. SCLC in pentacene single crystals 69

Figure 4.2: Double logarithmic plot of the current at constant bias for different

ratios of the planar electrode distance L with respect to the crystal

thickness h. The lines are guides for the eyes and correspond to 1D

model (slope -3) and 2D model (slope -2) respectively. The inset rep-

resents the derivative d(logI)/d(logL) versus L/h. The positions with

values of 2 and 3, indicating the two considered models are marked.

The data were obtained for one crystal with multiple contacts at room

temperature.

4.3.2 Charge transport in the SCLC regime

The I −V curves are ohmic at small biases (IΩ ∝ V ), and quadratic at high volt-

ages(

ISCLC ∝ V 2)

. Figure 4.2 shows the variation of the electrical current I at

constant bias voltage for a crystal of h = 20 µm thickness and different contact

spacings, Lij (60 µm < Lij < 3 mm), in the SCLC regime. The width of the

contacts is approx W = 200 µm. The results are presented as a function of the ra-

tio L/h. The inset shows the logarithmic derivative, d(logI)/d(logL) versus L/h,

with a value −3 corresponding to the 1D conduction, and −2 to the 2D model.

As it can be observed from this graph, at small L/h ratios the bulk charges are

dominant and at high ratios the surface charges determine the conduction. For

L/h ≤ 15, there is a good correspondence between the present experiment that we

performed on a crystal with planar contacts and the analytical solution described


by Mott-Gurney for devices with parallel plate electrodes(

I ∝ L−3)

[8]. This

is due to the fact that at small electrode spacing L, the electric field inside the

semiconductor is relatively uniform (Elong) and parallel to the conduction path.

Moreover, when this distance is small, and considering the anisotropy of mobility

in pentacene single crystals, the current is confined to the surface of the conduc-

tor [18]. We demonstrate that the 1D theory (Eq. 4.1) has a relatively limited

range of validity in case of planar parallel electrodes. It is clear from Figure 4.2

that in this geometry the surface contributions become gradually dominant as

L/h increases. The surface charge density gives rise to the normal component of

the electric field (Etrans) and the electric field obtains more pronounced 2D char-

acteristics. Thus the 1D space charge approximation is not valid in this regime

and the field distribution within the single crystal must be taken into account [19].

The electric field E becomes not uniform inside the crystal. With increasing L,

the transverse electric field results in I ∝ L−α, with α ranging from 3 to 2 (see

the inset in Fig. 4.2). We couldn’t reach the I ∝ L−2 regime, corresponding to

Geurst’s analysis (Eq. 4.2) [9], as we were limited by the size of the crystals.

4.3.3 Evaluation of mobility for sandwich-type and gap-

type structures

In the following section we will use the SCLC technique to calculate the value of

mobility in pentacene single crystals. We compare the results obtained for the

mobility in the a−b plane with the two types of contact geometry described in this

study. Pentacene presents one morphology in the single crystal phase: triclinic,

with spacegroup P 1 [20]. The crystals are formed by herringbone-type packing

in layers. The molecular packing determines the electronic behavior. Because of

different bonding and antibonding patterns, the effective mass exhibits different

values for the three crystallographic directions [21]. This results in a strong

anisotropy of the carrier mobility, as is often observed for molecular crystals [1,6].

The mobility is higher along the crystallographic direction in which the bonding

or antibonding is dominant. This anisotropy influences the value of the L/h

parameter at which the transition between 1D and 2D space charges occurs. We

expect that for other molecular crystals the transition from 1D to 2D transport

takes place for different geometrical parameters, due to their different mobility

anisotropy.

Figure 4.3 shows the device configurations and current density J versus applied

bias V for one pentacene crystal within the a− b plane, using sandwich-type and

gap-type architectures. The ratio L/h = 25 in this case sets the system in the first

4.3. SCLC in pentacene single crystals 71

Figure 4.3: Current density J vs. applied voltage V for pentacene single crystal

at room temperature, in the dark and a vacuum of 10−6 mbar for gap

structure contact geometry (), and sandwich structure configuration

().

part of the intermediate regime, where the conduction is dominated by the 1D

space-charges. This allows us, in first approximation, to use Eq. 4.1 to estimate the

value of the mobility (µ) for this crystal. At low voltages, the current is ohmic. At

higher voltages, the square law dependence of the current with the applied voltage

is observed, corresponding to the SCLC regime (Fig. 4.3). Small deviations from

the linear and quadratic regimes can be attributed to the non-linear contribution

of Schottky barriers formed at the metal/pentacene interfaces. As the trap-free

limit was not reached, µ can not be determined, but only the value of the product

Θµ can be calculated. For the gap − type geometry two contacts are deposited

on top of the crystal and the I − V curve is measured. When the measurements

are completed, the parts of the crystal that were covered with metal are removed

and new metal contacts are painted parallel to the thickness of the remaining

crystal (h), for the sandwich-type geometry. The calculation of the mobility with

sandwich configuration is straight forward. The device geometry is similar to

the one of a capacitor and the electric field is constant and oriented along the

direction in which the current is measured. From Eq. 4.1, in which the geometrical

factors of the crystal are introduced (L×W × h = 500× 200× 20 µm), a value of

(Θµ)sandwich = 12 cm2/Vs is obtained. In the case of gap−type structure, we can


see from Figure 4.2 that, as L/h > 15, the electric field will exhibit deviations from

the idealized 1D form, and the use of simple Mott-Gurney theory is not valid.

Moreover, this model underestimates the value of the mobility: (Θµ)gap = 7.2

cm2/Vs. This comes from the fact that the current flow is not homogenously

distributed along the entire thickness of the crystal (h), but confined to the surface

due to the anisotropy of pentacene.

4.3.4 Resistivity anisotropy effects - effective thickness

We use the Montgomery method [18] to determine the effective thickness (heff )

penetrated by the electric field lines in the gap configuration. The input for this

algorithm are the values of the components of the resistivity tensor along the

three crystallographic directions (a, b in-plane and c∗ perpendicular to the plane:

ρa = 1.3 · 106 Ωm, ρb = 4.7 · 105 Ωm, ρc∗ = 2.1 · 108 Ωm), and their correspondent

electrodes separation on the crystal (x, y, z). We transform this anisotropic solid

into the equivalent isotropic system of dimensions x′, y′, z′, using van der Pauw’s

expression [22]:

zeff

(x · y)12

=(ρa · ρb)

14

ρ12c∗

·z′eff

(x′ · y′)12

. (4.3)

Here zeff and z′eff are the effective thickness for the anisotropic and isotropic

system, respectively. The second factor of the expression is the normalized ef-

fective thickness in the isotropic material, which is the abscissa of the graph

proposed by Montgomery (Fig. 2 in Ref. [18]). The normalized sample thickness

in the isotropic space can be calculated from:

z′

(x′ · y′)12

=ρ

12c∗

(ρa · ρb)14

·z

(x · y)12

= 1.036. (4.4)

The effective thickness depends on the ration x/z, in the anisotropic pentacene

single crystal (Eq. 4.4, Eq. 4.5), which corresponds to the order parameter L/h

that we have used. Different curves are obtained for different ratios of the in −plane directions. For this particular geometry:

y′

x′= (

ρb

ρa)

12 ·

y

x= 0.24. (4.5)

Thus, the normalized effective thickness of this system is 0.715. This leads

to the value of zeff = 0.33 · z, corresponding to heff = 0.33 · h. From Eq. 4.1,

the value of (Θµ)gap,heff= 20 cm2/Vs can be calculated [18]. This value should

4.4. Conclusion 73

be considered an overestimate of the mobility. This arises mainly from the fact

that in this regime small deviations appear from the 1D model that was assumed

in the calculations. While the Mott-Gourney theory gives I ∝ L−3, for our L/h

ratio of 25, we should have used I ∝ L−2.7 as better approximation. In the

regime L/h ≤ 15,where the current varies with the cube of the contact separa-

tion (Fig. 4.2), the values of mobility obtained with the two geometries agree

well [5]. The effective thickness that we obtain represents a upper limit. Mont-

gomery’s analysis assumes ohmic conduction in which the equipotential lines are

independent of the magnitude of the electric field, while in the SLCL regime the

current density capacity increases quadratically with the field, leading to a smaller

effective penetration depth.

4.4 Conclusion

In conclusion, we report the cross-over from 1D to 2D type space-charge-limited-

current conduction in pentacene single crystals with increasing L/h for gap-type

contact geometry. For L/h < 15 the gap type contact is well approximated by

1D-space charges, whereas for L/h > 15 2D-space charges should be taken into

account. The 1D-space charges are predominant in pentacene because of the

anisotropy in resistivity.

References



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Rev. Lett. 93, 086802 (2004)


Batlogg, J. Appl. Phys. 96, 2080 (2004)

[4] D. A. da Silva Filho, E. G. Kim, and J. -L. Bredas, Adv. Mater. 17, 1072

(2005)


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[7] R. W. I. de Boer, and A. F. Morpurgo, Phys. Rev. B 72, 073207 (2005)

[8] N. F. Mott and R. W. Gurney, Electronic Processes in Ionic Crystals, 2nd

Edition, Clarendon Press, Oxford 1948

[9] J. A. Geurst, Phys. Stat. Sol. 15, 107 (1966)

[10] R. Zuleeg, and P. Knoll, Appl. Phys. Lett. 11, 183 (1967)

75

76 References

[11] R. W. I. de Boer, M. Jochemsen, T. M. Klapwijk, A. F. Morpurgo, J. Niemax,

A. K. Tripathi, and J. Pflaum, J. Appl. Phys. 95, 1196 (2004)





[14] X. Chi, C. Besnard, V. K. Thorsmolle, V. Y. Butko, A. J. Taylor, T. Siegrist,

and A. P. Ramirez, Chem. Mater. 16, 5751 (2005)

[15] M. Polke, J. Stuke, and E. Vinaricky, Phys. Stat. Sol. 3, 1885 (1963)


(1998)


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[18] H. C. Montgomery, J. Appl. Phys. 42, 2971 (1971)

[19] S. Hirota, J. Appl. Phys. 50, 3003 (1979)


M. Palstra, Acta Cryst. C 57, 939 (2003)

[21] G. A. de Wijs, C. C. Mattheus, R. A. de Groot, and T. T. M. Palstra, Synth.

Met. 139, 109 (2003)

[22] L. J. van der Pauw, Philips Res. Repts. 16, 187 (1961)

5Electronic Transport Properties of Pen-

tacene Single Crystals upon Exposure to

Air∗

We report the effect of air exposure on the electronic properties of pentacene

single crystals. Air can diffuse reversibly in and out of the crystals and influences

the physical properties. We discern two competing mechanisms that modulate

the electronic transport. The presence of oxygen increases the hole conduction,

as in dark each four O2 molecules diffused into the crystal introduce one charge

carrier. This effect is enhanced by the presence of visible light. In contrast, water,

present in ambient air, is incorporated in the crystal lattice and forms trapping

sites for injected charges.

∗This Chapter is adapted from O. D. Jurchescu, and T. T. M. Palstra, Appl. Phys. Lett.

87, 052102 (2005)

77

78 Chapter 5. Electronic Transport Properties upon Exposure to Air

5.1 Introduction

Pentacene is a good material for electronic devices like field effect transistors [1–

8]. However, the results obtained by different research groups are not always

consistent. This can be attributed in part to the fact that the performance of

the devices depends critically on the environmental conditions [9–11]. In many

cases these conditions are not explicitly stated, so a comparison between the

experiments is often not appropriate.

We report the electronic properties of pentacene single crystals under con-

trolled conditions of pressure, gas composition, illumination, and in time. We

provide experimental evidence that the influence of air on the charge carrier con-

duction consists of two separate, distinguishable contributions. Air diffuses into

the pentacene crystals and can be completely removed by re-evacuation. Thus, no

irreversible chemical reactions, like oxidation of pentacene to pentacenequinone,

can be detected during the diffusion process. We demonstrate that oxygen absorp-

tion is responsible for an increase in the conductivity, and its effect is enhanced

by the presence of visible light. In contrast, water present in ambient air will

form trapping sites for the injected charges. This will reduce the number of free

charge carriers, and thus the value of the conductivity. These opposite effects

may account for the contradicting reports on the influence of ambient conditions.

5.2 Experimental

Pentacene single crystals of high purity were grown using the vapor transport

technique [12]. The growth was preceded by the purification of the starting ma-

terial using a two-step vacuum sublimation process under a temperature gradient.

Details about purification and crystal growth are included in Chapter 2 of this

thesis. We performed thermo-gravimetric analysis (TGA) on pentacene single

crystals at room temperature to determine the changes in mass. The increase in

weight at normal pressure in the presence of different gas flows (air, O2, N2, and

Ar) were measured using a SDT 2960 from Thermal Analysis Instruments.

The crystals are platelets with dimensions of approximately 3 x 3 mm in plane

and thickness of 15 − 20 µm. Silver epoxy stripe contacts were painted on the

crystal, with a distance of 3 mm. The electrical measurements were performed in

a home built micromanipulator probe station, connected to a turbo pump and to

gas supplies. The system is equipped with a glass window for optical inspection.

This window was covered for the experiments performed in dark. Otherwise, the

samples are exposed to ambient fluorescent light, with no additional illumination.

5.3. Exposure to dry and ambient air 79

A Hewlett Packard 4155B Semiconductor Parameters Analyzer was used to per-

form electrical measurements. The typical hold and delay time for these scans

were 10 ms in order to avoid charging of the material. All measurements were

performed at room temperature.

5.3 Exposure to dry and ambient air

5.3.1 I-V characteristics in the dark - in vacuum and after

exposure to air

Figure 5.1: Current density J vs. applied voltage V for pentacene single crystal at

room temperature in vacuum and after exposure to dry air (increase

in J) and ambient air (decrease in J). The different curves represent

different exposure times. In dry air : 250 min, 200 min, 150 min, 100

min, 50 min, 10 min, 0 min, in vacuum, in ambient air : 0 min, 10

min, 20 min, 30 min, 50 min, 100 min, 150 min, 200 min, 250 min.


To understand the effect of environmental exposure on the transport in organic

crystals, we separate the two competing effects (doping by O2 and trapping by

H2O), by measuring systematically in dry and ambient air. Figure 5.1 shows

the evolution in time of the current density J versus applied voltage V for single

crystal pentacene in dark, after venting the system with dry and ambient air,

respectively. We associate the behavior at low bias with Ohmic behavior: J ∝V x, with x = 1. The observed values of x range from 0.7 to 1.3 for different

samples, which we relate to the quality of the contacts. In this regime, no marked

changes in the conductivity are induced by the presence of oxygen or water vapor.

Apparently, no free charge carriers are introduced by O2 absorption.

However, the space-charge-limited current regime SCLC (J ∝ V 2) is influenced

by exposure to air. Here, J increases upon exposure to dry air, and decreases upon

exposure to ambient air. We interpret this behavior that the induced carriers are

located in shallow trapped states and are only released beyond a critical field.

The quantitative analysis of the transport changes, introduced by absorbing air,

are performed in the SCLC regime (V = 100 V ). In dry air, the number of holes

introduced by the O2 molecules increases in time, leading to an increase in the

charge carrier density. The system will reach the SCLC regime at smaller bias

voltage.

5.3.2 Gas diffusion in pentacene single crystals

We performed in parallel gravimetric and electrical measurements to be able to

give a quantitative description of the effect of gas diffusion on the properties of

organic crystals. In Figure 5.2 we plot the weight increase of pentacene single

crystals upon exposure to 5N dry air (< 17ppm water vapor), at room tempera-

ture and atmospheric pressure, expressed as a molar fraction of air in pentacene.

For each situation presented in this chapter, we performed 3 experiments and

they are reproducible within an accuracy of 15%. The mass of the accumulated

air in the crystal structure of pentacene was calculated assuming that the change

in mass is caused only by absorption of gases. This assumption is supported by

the observation that the process is completely reversible.

The same trend is observed for the measurements performed in pure O2, N2,

and Ar. The time constants are determined by the size and shape of both crys-

tals and diffusing molecules. As the diffusion coefficients have similar values, we

conclude that there is no selectivity with respect to any of the gases. The satu-

ration values are within experimental accuracy the same for the different gasses:

2.0% ± 0.3%.


Figure 5.2: Evolution in time of the molar fraction of air in pentacene upon ex-

posure of single crystals to dry air at room temperature and the fit

with a 1D diffusion model. This is a lower limit of the molar fraction,

as it is assumed that the gas molecules diffuse homogenously over the

crystal.

We used Fick’s second law (Eq. 5.1) to model the diffusion of gases into pen-

tacene single crystals:∂N

∂t= D(

∂2N

∂x2) (5.1)

Here N is the number of gas molecules at a particular time t and position x.

We use the solution of this equation for the one-dimensional case, constant diffu-

sion coefficient D, and an inexhaustible source. We use two boundary conditions

to obtain a unique solution:

• no gas molecules are accumulated in the crystal in the initial stage (N = 0,

for x > 0, at t = 0), we will add an extra term later;

• at x = 0, Nsource, induced by the gas flow, is maintained constant for t > 0.

These boundary conditions are represented by the following expressions:

N(x, 0) = 0

N(0, t) = Nsource(5.2)


The solution of Eq. 5.1, with the boundary conditions of 5.2, is:

N(x, t) = Nsourceerfc( x

2√

Dt

)

(5.3)

where the erfc (z) function, complementary to the error function, erf(z), is defined

as:

erfc(z) = 1 − erf(z) = 1 −2√π

∫ z

0

e−y2

dy (5.4)

The evolution of the concentration of the gas molecules versus distance, in

time, is shown in Figure 5.3. The distribution of gas molecules, N(x, t), depends

on both distance, x, and time, t. The total quantity of gas molecules in the

Figure 5.3: Evolution of the number of gas molecules absorbed in the crystal

lattice versus distance and in time.

pentacene crystal is obtained by integrating this solution over the length of the

diffusion profile. The result is normalized and expressed in molar fraction of gas

molecules in pentacene. An extra term needs to be added in the solution (Nstart).

This term reflects the fact that the measurement starts at t = 0 with a quantity of

gas that was accumulated in crystal during venting to atmospheric pressure and

subsequent handling. Because of experimental limitations, we could not evacuate

the gases in the TGA-DSC set-up. The equation that models the diffusion of

gasses in pentacene is thus:

N(t) = Nstart + Nsource

[

d(

1 − erf( d

2√

Dt

))

+2√π

√Dt

(

1 − exp(

−d2

4Dt

))]

(5.5)


where N(t) is the molar fraction of gas at time t, d is the length of the crystal

in the direction in which the 1D diffusion occurs, and Nstart is the value of the

molar fraction accumulated before the TGA measurement. For the case of dry air

(Fig. 5.2), fitting the diffusion curve yields d = 17.5 µm, D = 1.8·10−10 cm2/s and

Nstart = −1.37 ·10−3 molecules air/molecules pentacene. Pentacene has a layered

structure with a herringbone arrangement within the layers (Fig. 5.4). Our results

show that the diffusion length is similar to the thickness of the crystal. Therefore,

the diffusion proceeds via the largest surface area, perpendicular to the layers.

We will discuss in Section 5.5 the possible microscopic form adopted by oxygen

molecule in pentacene lattice .

Figure 5.4: The herringbone arrangement of the pentacene molecules in the crys-

tal. The alternation of molecular layers is visible. d is the spacing

between the layers (d=14.1 A for pentacene single crystal). The ori-

entation of the c and a + b direction of the unit cell are drawn.


5.3.3 Effect of Oxygen and water in dark

Figure 5.5: Pressure dependence of the current at fixed bias voltage for single

crystal pentacene in the dark. The effect of dry air is shown in 5.5(a),

and that of ambient air in 5.5(b). The experiments are reproducible

for different samples with an accuracy of 15%.

We investigate the influence of the absorbed gas molecules on the changes in

the electrical current. Prior to the experiments, the samples were outgassed in

vacuum (p = 5 · 10−7 mbar) in the measurement chamber for at least 100 min.

The measurements can be reproduced after re-evacuation, as the gases diffuse re-

versibly in and out of the crystal. In order to study the influence of environmental

conditions on the electrical properties, the pentacene single crystals were exposed

to both 5N dry air and ambient air, using a leak-valve connected to the measure-

ment chamber. For the experiments performed in the dark, where the effect is

weaker, we measured both while outgassing and venting the chamber to ensure

that the changes originate from diffusion and not from charging. The evolution

of the electrical properties in time was recorded immediately after venting the

system to atmospheric pressure. The initial time, t = 0, is taken when pressure

equals atmospheric pressure.

Figure 5.5 and 5.6 present the evolution of the electrical current at fixed bias

versus pressure (5.5) and versus time (5.6) at atmospheric pressure in the dark,


Figure 5.6: Time dependence of the current at fixed bias voltage for single crystal

pentacene in the dark. The effect of dry air is shown in 5.6(a), and

that of ambient air in 5.6(b). The experiments are reproducible for

different samples with an accuracy of 15%.

respectively. The effect of oxygen exposure is studied when venting with dry

air (Fig. 5.5(a), Fig 5.6(a)). As the pressure increases (Fig. 5.5(a)), oxygen will

diffuse into the crystal, and the conductivity increases. The I − V curves are

measured immediately after the desired pressure is reached. Once the system is

at atmospheric pressure, the current continues to increase (Fig. 5.6(a)) because

the diffusion still continues. When venting with ambient air, both water vapor

and oxygen are introduced into the chamber (Fig. 5.5(b), 5.6(b)). We noticed

that in ambient air, the effect of oxygen exposure is counteracted by the exposure

to water. Analyzing the magnitude of the induced changes in the current by

combining oxygen and water effects, one can distinguish two pressure regimes

(Fig. 5.5(b)). At low pressure (p < 10−2 mbar), the conduction is independent

of air pressure. This leads us to conclude that the two effects are opposite and

of the same magnitude. At p = 10−2 mbar, a sudden drop in the current is

observed. Apparently, for p > 10−2 mbar the introduction of scattering sites

exceeds the creation of holes, and this has a major effect on the conduction. This

is reflected also in the results at atmospheric pressure, where diffusion of dry air

causes a current increase (Fig. 5.6(a)), whereas ambient air diffusion cause a drop

(Fig. 5.6(b)).


5.3.4 Effect of Oxygen and water in light

Figure 5.7: Pressure dependence of the current at fixed bias voltage for single

crystal pentacene in the light. The effect of dry air is shown in 5.7(a),

and that of ambient air in 5.7(b). The experiments are reproducible

for different samples with an accuracy of 15%.

Figures 5.7 and 5.8 show the evolution of the current under the same condi-

tions, but now in the presence of light. The effect of oxygen diffusion in time is

stronger than in dark (Fig. 5.7(a), 5.8(a)). As the effect of water is independent

of the presence of light, the total influence of ambient air on the crystal properties

will be different in the presence of light. Consistent with the measurements in

dark, we find two pressure regimes. At low pressure (p < 10−2 mbar), when only

an insignificant amount of water vapor is present, the oxygen doping is the dom-

inant factor of the conduction mechanism. At p = 10−2 mbar, when water vapor

diffuses considerably into the pentacene crystal, the effect of oxygen is decreased,

and therefore a change in slope is observed (Fig. 5.7(b)). However, the presence

of oxygen remains the most important factor, as the value of the current increases

with pressure. After venting with ambient air, the doping caused by oxygen, ac-

tivated by illumination, gives a stronger effect than the trapping of charges by

water. The hysteresis in figures 5.5b and 5.7b is very small. This indicates that

the variation in the values of the current is caused by the diffusion and not by

charging.


Figure 5.8: Time dependence of the current at fixed bias voltage for single crystal

pentacene in the light. The effect of dry air is shown in 5.8(a), and

that of ambient air in 5.8(b). The experiments are reproducible for

different samples with an accuracy of 15%.

5.3.5 Effect of high O2 pressure†

In this section we discuss O2 high-pressure measurements of the electrical prop-

erties of pentacene single crystals. The experiments were performed in situ, in a

cubic anvil press [13]. The system was first evacuated, then O2 with controlled

partial pressure was introduced. 30 min was taken to reach the loading for each

pressure, and then I-V curves were taken. In contrast with experiments per-

formed in air, at 10−6 mbar < p < 103 mbar (Section 5.3.3, 5.3.4), at high O2

pressure (103 mbar < p < 2·106 mbar) an increase in current is observed also in

the linear regime as the O2 pressure is increased. Here, the resistance decreases

with ∼ 20% between the two extreme pressures mentioned above. The relative

changes in the resistance of the pentacene crystal, with respect to the resistance

in vacuum, versus O2 pressure, is plotted in Figure 5.9. Similar to the case of

hole doping by iodine [14, 15], the resistivity decreases, but no metallic state is

reached. Contrarily, when doping pentacene films with electron donors (e.g. alkali

atoms), a high electrical conductivity and a metallic temperature behavior was

reported [16, 17].

†The measurements are done in collaboration with J. Jun, and J. Karpinski (ETH Zurich)


Figure 5.9: Evolution of the resistance with O2 pressure. The results are normal-

ized with respect to the resistance R0 in vacuum. The line is guide

for the eye.

For pentacene loading with O2 at high-pressure, after 2·106 mbar, an precipi-

tous decrease in current is observed, both in the linear and quadratic regime. It

is possible that pentacene is transformed irreversibly to pentacenequinone, as the

electrical characteristics cannot be reproduced after evacuation. Moreover, the

sample becomes amorphous.

5.4 O2 and H2O effects - experimental observa-

tions

5.4.1 Electrical measurements

The changes in the electrical properties of the organic semiconductors upon ex-

posure to ambient conditions have not been given much attention in the past.

Only lately, it was stressed that there is a fundamental need to understand these

effects, especially if they are not reversible, because they can affect the perfor-

mance and the lifetime of the electronic devices in products built for large scale

applications [18].

5.4. O2 and H2O effects - experimental observations 89

We relate the changes that we observe in the electrical properties and de-

scribe in Section 5.3.3 and Section 5.3.4 to the diffusion of gases into the crystal.

Our model includes two competing mechanisms associated with the two different

chemical species that influence the conduction, affected by external factors such

as light. On the one hand, water absorbed by exposure to ambient air [19], will

create new trapping sites for the charges that are injected from the electrodes [20].

This will decrease the value of the electrical conductivity. The afore mentioned

effect is, as expected, independent of the exposure to light. The polar nature

of the H2O molecules causes interactions with the injected charges and increases

the energetic disorder [21]. This will lead to a decrease of the conduction. Oxy-

gen present in the air will be incorporated in the crystal structure, as shown

in Figure 5.2 and this process will generate holes (a description of the possible

doping mechanism will be provided in Section 5.5 and a quantitative analysis in

Section 5.6 of this chapter). The density of charge carriers responsible for the con-

duction will thus increase, and accordingly the value of the current will increase.

This process is enhanced by the presence of light. The measurements performed

in pure N2 and Ar under identical conditions show insignificant changes in the

electrical properties, despite a similar amount of absorption.

5.4.2 UPS spectroscopy‡

We use ultraviolet photoelectron spectroscopy (UPS) to investigate the effect of

oxygen and air exposure on the electronic structure of pentacene single crys-

tals [22]. The oxygen doping should induce a shift of the Fermi-level (EF ) in the

organic towards the highest occupied molecular orbital (HOMO). Photoemission

experiments were performed at the end-station SurICat (beamline PM4) at the

synchrotron light source BESSY GmbH. The ultrahigh vacuum (UHV) system

consists of interconnected sample preparation (base pressure: 1 · 10−8 mbar) and

analysis (base pressure: 5·10−10 mbar) chambers. Sample transfer between cham-

bers proceeded without breaking UHV conditions. Pentacene single crystals were

grown ex situ, then mounted on metal sample holders with conducting adhesive

tape, and cleaved in situ (in the preparation chamber) by peeling off the top layers

with a piece of adhesive tape stuck to the crystal surface. Pentacene molecules

crystallize in a layered structure. The interactions between the layers are less

strong than the interactions within the layers, as they are van der Waals interac-

‡The experiments were performed in collaboration with the groups of N. Koch and P. Rudolf

and published as The effect of oxygen exposure on pentacene electronic structure, Eur. Phys.

J. E 17, 339 (2005).


Figure 5.10: UPS spectra of a pentacene single crystal measured under the follow-

ing conditions (and in this time-sequence): (a) 3 · 10−8 mbar partial

O2 pressure, (b) 2 · 10−9 mbar total residual pressure, (c) 3 · 10−8

mbar partial O2 pressure after exposure to 5 · 10−6 mbar O2 for 30

min., and (d) again at 2 · 10−9 mbar total residual pressure.

tions. Therefore, the crystal will cleave easily along the a−b plane (perpendicular

to the c∗ direction).

The valence region UPS spectra (Fig. 5.10) recorded on a pentacene single

crystal under different residual oxygen pressures during the measurement and to-

tal oxygen exposure clearly show no influence of molecular oxygen on the position

of the energy levels at the given conditions. All four spectra in Figure 5.10 ex-

hibit the same characteristic photoemission features of pentacene, comparable to

previous reports [23]. If ”doping” (i.e., p-type) of pentacene by O2 were the case

(to a significant degree), a clear shift of all levels towards lower binding energy

(BE) would be observed. However, the total in situ exposure of the single crystal

to O2 was rather limited (up to a pressure of 3 · 10−8 mbar; for experimental lim-

5.5. Mechanism of oxygen doping 91

itations). Oxygen diffused into pentacene at atmospheric pressure and diffused

out again rapidly after re-evacuation, and does not lead to irreversible changes

(detectable by UPS) in the electronic structure, like, e.g., p-type doping.

In extensive tests we have observed no irreversible reaction of pentacene with

molecular oxygen and water, even if pentacene is optically excited. More im-

portantly, it is found that oxygen diffusion through pentacene single crystals is

reversible, and does not leave behind - after re-evacuation - electrically active

electronic states that would lead to doping of the organic bulk material. This is

evidenced by a lack of energy shifts in UPS spectra of pentacene before and after

exposure to O2 and to air.

5.5 Mechanism of oxygen doping

In the following section we propose a microscopic description of the effect of

oxygen absorption in pentacene. We discuss the nature, on a molecular scale, of

oxygen-induced changes in charge transport properties. Under the experimental

conditions explored here, these changes are reversible. This is fundamentally

different from the experiments in which oxygen creates irreversibly new chemical

species in pentacene, leading to trapping states in the energy gap of pentacene [8].

It should be stressed that oxygen doping is widely observed in organic semi-

conductors. Therefore, it is likely that this interpretation holds for other similar

materials. The exact nature of the origin the phenomena encountered here is not

known. We propose two possible mechanisms for oxygen doping in pentacene singe

crystals. Further experimental and theoretical studies should elucidate which of

them is correct, or if they can coexist.

5.5.1 Formation of the charge transfer complex between

pentacene and O2

When oxygen is inserted in the pentacene lattice, it can generate a charge transfer

complex (CT complex) with the host material. The direction of the charge transfer

can be established by comparing the values of the electron affinities of the two

neutral molecules. As the value of the pentacene electron affinity (Ea (C22H14)

= 2.7 eV [24]) is higher than that of O2 (Ea (O2) = 0.45 eV [25]), pentacene is

electron-donating in this complex, and O2 is electron-accepting. Because of its

electronegativity, oxygen will attract electrons from the electron rich π- system

of pentacene molecules, and this process will generate holes. No net charges are

generated, but a partial transfer of charge occurs, from the donor to the acceptor.


Figure 5.11: The reversible charge transfer formed between pentacene and oxygen.

Figure 5.12: The reversible reaction of pentacene oxidation to endoperoxide. This

reaction can yield irreversibly hydroquinone and quinone.

We expect that the degree of charge transfer in our system is much weaker

that in typical organic charge transfer complexes (e.g. TTF-TCNQ [26, 27]).

Upon formation of the [pentacene-oxygen] charge transfer complex (Fig. 5.11),

the properties of pentacene single crystals are perturbed.

There are several questions that can be raised. How large is the degree of

charge transfer between the two molecules? Does it induce also magnetism,

besides changes in electrical properties? Is it detectable with impedance spec-

troscopy experiments and/or Raman spectroscopy?

5.5.2 Reversible oxidation of pentacene

A second possibility is that molecular oxygen in the crystal forms a 6,13-adduct

(pentacene-endoperoxide), via the reversible reaction presented in Figure 5.12.

This possibility is analogous to the behavior of carbon nanotubes in oxygen [28].

In pentacene positions 6 and 13 are likely affected because they are the most re-

active. This adduct could yield irreversibly, under conditions not explored here,


via the formation of the hydroquinone, the 6, 13-pentacenequinone, which is con-

ventionally the dominant impurity species [8].

The reaction that pentacene undergoes to endoperoxide in the presence of

oxygen is photoinduced [29]. Moreover, it is reversible, in agreement with our ex-

perimental observations that the electrical properties in vacuum can be recovered

after evacuation. Still, Raman studies that we performed on pentacene single

crystals exposed to oxygen do not show the signature of any peak present in

the region ∼ 800 cm−1, that corresponds to the O − O vibration [30]. We find

no evidence for the formation of the endoperoxide, in contrast with the case of

rubrene [29].

5.5.3 Calculation of the oxygen-induced dipole moment

We measure the frequency dependence of the dielectric constant ε(ω) of pen-

tacene single crystals loaded with oxygen and air (Fig. 5.13). We demonstrate

experimentally that a dipole moment is induced in the pentacene crystal, and we

determine its magnitude.

The dielectric function ε(ω) is increased by the polarizability of [C22Hδ+14 ·Oδ−

2 ]

charge transfer. The charge transfer gives rise to dipoles that couple to the ac

electric field. This increases the value of permittivity [31].

We used pentacene single crystals with a high degree of purity, grown from

double sublimated powder. Two contacts were deposited, on top and bottom, in

a sandwich-type geometry, as drawn in the inset in Figure 5.13. The description

of the sandwich-type geometry was explained in Chapter 4. The ac dielectric

measurements were performed in a probe station, in air, using a Agilent 4284A.

The as grown crystals were measured first in ambient air. The dielectric response

is shown in Figure 5.13. A decrease of the dielectric constant by 3% is observed

when the frequency is modified from 1 kHz to 200 kHz. The crystals were then

exposed to pure O2 (AGA, 5N) for a few hours. This is sufficient for maximum

loading, as the diffusion constants for oxygen and air (Fig. 5.2) are similar. The

dielectric constant increases markedly at low frequencies for crystals loaded with

O2 and decreases by 12% from 1 kHz to 200 kHz (Fig. 5.13).

Assuming a Debye response, the dielectric constant will decrease with fre-

quency according to:

ε(ω) = ε(∞) +ε(0) − ε(∞)

1 + (ωτd)2(5.6)

where ε(∞) corresponds to the high-frequency permittivity of the material, and

ε(0) is the static dielectric constant, ω is the angular frequency (ω = 2πf), and


Figure 5.13: Frequency dependence of the dielectric constant of pentacene single

crystals loaded in air and in oxygen. The inset is a schematic rep-

resentation of the sample geometry, showing also the orientation of

the crystal (c* is the axis perpendicular to the a-b plane). The line

is a fit of a Debye response with a single relaxation time.

τd represents the relaxation time. As it can be derived from Eq. 5.6, this model

considers a single relaxation time.

For the measurements performed on pentacene single crystals exposed to pure

oxygen, fitting the curve in Figure 5.13 with Eq. 5.6 yields:

ε(0) = 3.34

ε(∞) = 3.07(5.7)

The fit does not represent very accurately the frequency response of the dielec-

tric constant. At low frequencies, additional contributions increase the permittiv-

ity, and the Debye model does not describe the response accurately. Deviations

from the model can also be ascribed to the presence of different relaxation times,

whereas we assume a single value.

We use the Clausius-Mossotti equation [32], that relates the dielectric constant

ε with the dipole moment µ inside the solid:


ε(ω) − 1

ε(ω) + 2=

Ndµ2

9ε0kBT+

4πNmαm

3(5.8)

Here Nd is the density of dipoles, T is the temperature, Nm is the molecular

density in the solid (Nm = 2.92 · 1021 cm−3 for pentacene), αm is the molecular

polarizability, and ε0 and kB are the vacuum permittivity and the Boltzmann con-

stant, respectively. We assume one dipole created per each absorbed O2 molecule,

thus Nd ≃ 0.02 · Nm. Only 2% of the pentacene molecules form a dipole, cor-

responding to the 2% molar loading. The distribution of the pentacene species

corresponds to the distribution of the oxygen in the lattice (experiments are in

progress to determine the spatial distribution of oxygen in pentacene).

If we assume that the behavior of ε(ω) is only determined by dipole formation

through O2-loading, we find that:

4πNmαm

3=

ε(∞) − 1

ε(∞) + 2(5.9)

Thus, we can determine value of the dipole moment of this charge transfer com-

plex. At room temperature, we obtain µ = 4 D. This allows the calculation of the

degree of charge transfer (q), as µ = q ·d, where d represents the distance between

the charges.

We do not have structural information concerning the position of the O2

molecule in the pentacene crystal lattice. Thus we cannot give an exact value

of the distance between the charges. For this reason, we can only extract an order

of magnitude for the value of the charge transfer. We obtain q ≃ 0.25e, where e

is the elementary charge.

Repeating the same algorithm for the measurements performed in air, and

considering Nd ≃ 4 · 10−3 · Nm (only 20% from the 2% loading represents O2

molecules), we obtain: ε(0) = 3.2, ε(∞) = 3.08, and µ = 5 D. The correspondent

values for the charge transfer is q = 0.3e.

We observed that the values of the dipole moment induce by oxygen in pen-

tacene, calculated for the experiments performed in air and in oxygen are similar,

as expected. We associate the small differences to additional polarization, in-

duced by the interaction of pentacene molecules with other impurity states in

the crystals, and with the metal contacts, and also to experimental errors. The

oxygen-induced charge-transfer mechanism presented here is able to describe the

main features of the frequency dependence of the dielectric constant: the decrease

of the value of ε with frequency, and dependence on the oxygen content.


5.6 Quantitative analysis of the effect of O2 and

H2O

A comparison of Figure 5.2 with Figure 5.6(a) reveals a remarkable consistency.

The time evolution of the electrical current mirrors that of the oxygen absorption.

We can quantitatively assess the influence of O2 and H2O. The value of the hole

current is affected by the number of gas molecules present in the crystal and thus

varies in time:

Jp(t) = eµpp(t)E (5.10)

where e is the elementary charge and µp is the mobility of holes in pentacene.

The term p(t) is the density of charge carriers and consists of several terms:

p(t) = pinj + pdop − ptraps (5.11)

Here pinj represents the hole density injected from the contacts, and pdop is

the hole density induced by exposure to oxygen, which is a fraction of the number

of air molecules absorbed in the solid:

pdop = 0.2ηoxygenN(t) (5.12)

The oxygen efficiency ηoxygen is thus defined as the inverse of the number of

oxygen molecules required to generate one hole. The term:

ptraps = pdef + pimp + pwater (5.13)

is given by the density of traps in the band gap. In vacuum a fraction of the

injected charges are trapped by crystal defects, pdef , and impurities, pimp. In

ambient air, an extra term is added in the trapping sites due to the presence of

water molecules,

pwater(t) = βN(t) (5.14)

which is a fraction of total number of the air molecules N(t) accumulated in the

crystal. Combining the measurements performed in dry air (Fig. 5.6(a), 5.8(a))

with N(t) taken from Figure 5.2, we can calculate the oxygen efficiency ηoxygen,

which determines the number of holes introduced by one O2 molecule (see upper

panel Fig. 5.14).

As it can be observed in Figure 5.14, this number is constant and independent

of time with ηoxygen ≈ 0.25 in the dark and ηoxygen ≈ 0.5 with light exposure.

This means that the efficiency of the oxygen exposure induced doping is indepen-

dent of oxygen loading and it is increased by light exposure. Roughly two O2

5.6. Quantitative analysis of the effect of O2 and H2O 97

Figure 5.14: The upper panel shows the evolution in time of the oxygen efficiency

in the dark () and with light exposure (•). For ambient light in-

tensity, this efficiency is constant and time independent. The lower

panel shows the variation of the trapping factor by introducing water

molecules (Θwater) with time. The values are similar in the dark ()

and with light exposure (•).

molecules are needed in light to create one hole, compared with four molecules in

dark.

In ambient air, the number of trapping sites is proportional to the number of

water molecules that diffuse into the crystal. We introduce the ratio (Eq. 5.15):

Θwater =pinj − pwater

pinj(5.15)

that quantifies the fraction of untrapped charge carriers: Θwater = 1 in vacuum.

After exposure to ambient air, the value of Θwater is reduced. Θwater is calculated

by subtracting Figure 5.6(a)/ 5.8(a) from Figure 5.6(b)/ 5.8(b), using appropriate

geometrical factors. The lower part of Figure 5.14 shows Θwater versus time. The

effects of water in the dark and with light exposure have similar magnitudes, and

they follow the diffusion of water, introduced from ambient air.

The changes of the conductivity with time and pressure are the electronic

response to the diffusion of gases in pentacene crystals. They are fully reversible.

The p-doping by molecular oxygen increases the concentration of charge carriers.


Water molecules form new electronic states in the gap that trap the injected

charges. It should be noticed that experiments under UHV conditions will not

be influenced by exposure to an ambient atmosphere, as the absorbed gas can be

fully removed by evacuation [22]. We expect that these effects are even larger in

thin films, in which the presence of grain boundaries facilitates the diffusion.

5.7 Conclusion

In conclusion, we have investigated the intercalation of gases in pentacene single

crystals and its effect on the electronic properties. We differentiate between the

exposure to oxygen and water. Absorbed oxygen enhances the conduction by

introducing holes near the valance band. Absorbed water molecules create new

defect states, which trap the injected charges. This insight in the relationship

between the macroscopic diffusion of gas in organic conductors and the effect on

the physical properties may lead to an improved control of the device performance.

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6Low Temperature Crystal Structure of

Rubrene Single Crystals Grown by Vapor

Transport∗

The study presented in the following chapter is promoted by reports of a precip-

itous drop in the electronic mobility of rubrene single crystals below 175 K. We

assign this change to a phase transition. We perform the crystal structure deter-

mination for rubrene single crystals, C42H28 (5,6,11,12 - Tetraphenyltetracene),

in the temperature interval 100 K - 300 K. The crystals are grown by physical

vapor transport in an open system. The crystal structure is orthorhombic over

the entire temperature range. We find evidence for a phase transition from Differ-

ential Scanning Calorimetry measurements, but see no evidence for a structural

phase transition in the diffraction experiments.

∗This Chapter is adapted from O. D. Jurchescu, A. Meetsma, and T. T. M. Palstra, Acta

Cryst. B 62, 330 (2006)

103

104 Chapter 6. Orthorhombic Rubrene Single Crystals

6.1 Introduction

The electronic properties of polymers and molecular crystals are of much current

interest due to fundamental questions regarding electron transport and associated

applications. The present focus is in developing novel chemical structures, as well

as improved fabrication techniques that are able to open new fields for electronic

applications based on organic materials.

A number of molecular conductors are of current interest. Rubrene (Fig. 6.1)

exhibits interesting physical properties such as having one of the highest reported

electronic mobilities at room temperature [1] (20 cm2/Vs). Emerging new appli-

cations based on rubrene are organic field effect transistors (OFETs) [1–4] and

organic light emitting diodes (OLEDs) [5, 6].

Figure 6.1: Bond-line formula of rubrene

.

Although the technological development is relatively fast, fundamental ques-

tions concerning the intrinsic mechanism of conduction have still not been an-

swered. Moreover, for the title compound, little attention has been paid to the

interplay between its crystal structure and electronic properties. The interest in

the present work is guided by intriguing changes which have been observed at

low temperature in the mobility of field-effect transistors built on rubrene single

crystals [2, 3].

6.2 Polymorphism in rubrene

In this study, we investigate the structure of rubrene single crystals and correlate

it with the electrical properties. The crystals are formed through competition

6.3. Growth of rubrene single crystals 105

between π- stacking and quadrupolar interactions. Owing to these weak interac-

tions, different growth conditions can lead to different polymorphs. This feature

is widely encountered for molecular crystals. Several polymorphs of rubrene have

been reported. A monoclinic phase was described by Taylor (1936) [7] and a tri-

clinic polymorph was explored by Akopyan et al.(1962) [8]. Unfortunately, these

reports do not mention the growth method. An orthorhombic form of rubrene

(space group Aba2) was reported by Henn et al. (1971) [9] for crystals grown

from vapor in vacuum using sealed ampoules. For crystals obtained in a closed

system, in a two-zone furnace, at ambient pressure, Bulgarovskaya et al. (1983)

reported a second orthorhombic polymorph, with space group Bbam [10]. For all

the polymorphs investigated previously, only the room-temperature structure is

reported, thus no relation with the physical properties at low temperature can be

made.

6.3 Growth of rubrene single crystals

Rubrene single crystals investigated in this work were grown using physical vapor

transport under a temperature gradient, in a horizontal tube [11]. The inner

tube, in which the crystallization takes place, was cleaned and heated afterwards

at T = 600 K under Ar flow for 10 hours to evaporate the solvents. The source

material, with nominal 98% purity, was obtained from Sigma Aldrich. Prior

to crystal growth, the powder was purified using vacuum sublimation under a

temperature gradient process [12], designed to remove the light impurities1. The

purified powder was placed in the hot part of the growth tube in a alumina

boat. The sublimation temperature (T = 553 K) was kept as low as possible

(close to the sublimation threshold). This was done to prevent the sublimation

of heavy impurities and to ensure the slow sublimation rate that yields crystals

with a minimum number of structural defects. Crystals grown at slightly higher

temperatures were of significantly lower quality and gave weak diffraction pattern.

The growth set-up was placed in dark to avoid oxidation. The transport gas was

a mixture of H2 (AGA, 5N) and Ar (AGA, 5N), with a volume percentage of

5.25% H2.

The crystals are orange-colored and needle- or platelet-shaped. The typical

in-plane dimensions are 200 µ m ×1 cm for the needles and 3−5 mm for platelets.

The thickness of the crystals is 3 − 20 µ m.

1We refer to the species present in the system that have lower/higher molecular masses than

the parent compound as ’light’/’heavy’ impurities.


6.4 Single crystal diffraction

6.4.1 Experimental details

The single crystal diffraction experiments were performed on a Bruker SMART

APEX CCD diffractometer. A crystal fragment, cut to size to fit in the homo-

geneous part of the X-ray beam, with dimensions of 0.51 x 0.45 x 0.03 mm was

mounted on top of a glass fiber and aligned on the diffractometer. The diffrac-

tometer was equipped with a 4K CCD detector set 60 mm from the crystal.

Intensity measurements were performed using graphite monochromated Mo-Kα

radiation. The crystal was cooled fast using a Bruker KRYOFLEX (1h was taken

to reach 100 K) and the measurements were performed during heating, after the

temperature was stabilized. For each temperature investigated in this chapter,

a total of 1800 frames were collected with an exposure time of 30.0 seconds per

frame. The overall data collection time was 18.0 h. During this time, the vari-

ations in the temperature were ±0.5 K at 100 K, and at most ±2 K at higher

temperatures.

The structure was solved by direct methods with SHELXS-97. The positional

and anisotropic displacement parameters for the non-hydrogen atoms were refined.

Experimental details about the structure determination, data collection and

refinement are summarized in the Appendix B of the thesis. The data collection

(radiation type, monochromator, data collection method, θ range) and refine-

ment parameters (number of reflections, number of refined parameters, weighting

scheme, goodness of fit) are similar for the structures corresponding to the eight

temperatures investigated in this study.

6.4.2 Orthorhombic rubrene

Crystals grown from physical vapor transport in an inert gas, in an open system

and continuous flow [11] are preferred for electronic applications, because they

present the highest purity and thus the largest mobility. Still, no crystal structure

determination is available for crystals obtained by this method. In this study we

report the crystal structure between 100 K and 300 K of rubrene single crystals

grown from physical vapor transport (PVT). This growth method yields orange

crystals with orthorhombic symmetry, space group Cmca. We notice that the

polymorph of the crystals grown with this method coincides2 with that obtained

2We used the standard settings of the International Tables for Crystallography. The trans-

formation to Bbam (Cmca → Bbam) is: b → a, c → b, a → c. For the crystal orientation in

the electronic measurements [2, 3], the Bbam setting was used.

6.4. Single crystal diffraction 107

Figure 6.2: Perspective ORTEP drawing of the rubrene molecule illustrating the

configuration and the adopted labelling scheme for the non-hydrogen

atoms at 100 K (a), and room temperature (b). Displacement ellip-

soids for non-H are represented at the 50% probability level.

.


Figure 6.3: Lattice parameters a, b and c, and unit cell volume V versus temper-

ature for rubrene single crystals. The lines are guides for the eyes.

for the crystals grown in sealed ampoules by Bulgarovskaya et al. [10].

The molecule presents 2/m symmetry, with the asymmetric unit consisting of

one quarter of a rubrene molecule. A twofold axis is located along the C1 − C1b

bond. There is a mirror plane perpendicular to the planar tetracene fragment of

the molecule, through the inversion center positioned in the middle of the C1−C1b

bond. The adopted labelling scheme and the molecular geometry are illustrated

in the ORTEP drawings of Figure 6.2(a, b). The plots show graphical representa-

tions of the atomic positions and their anisotropic displacement ellipsoids at the

minimum and maximum temperatures investigated in this study (T=100 K, and

room temperature). These images provide a visual description of the vibrational

motions of each atom around its equilibrium position.

The crystal structure determination reveals a 2.157% volume thermal expan-

sion of the crystal between 100 K and 300 K, in agreement with typical values

measured in organic crystals. The thermal expansion is anisotropic for the three

crystallographic directions (∆a = 0.410%, ∆b = 0.320%, ∆c = 1.562%) and oc-

6.5. Relation between molecular stacking and electronic mobility 109

curs predominantly along the weakest bonding c-axis (see Fig. 6.3). The lattice

parameters together with other crystallographic parameters for all eight temper-

ature structures investigated in this study can be found in the Appendix B of the

thesis.

6.5 Relation between molecular stacking and elec-

tronic mobility

We relate the behavior of electronic mobility versus temperature with structural

changes. Electronic measurements performed on rubrene single crystals show

that the field effect mobility increases with decreasing temperature down to 175

K, consistent with band-like behavior [2, 3] (the results differ slightly between

groups). Below 175 K the mobility drops precipitously. The dramatic decrease

in the field-effect mobility can be a consequence of a structural phase transition

in the material. Our diffraction experiments show that the crystallographic sym-

metry does not change when passing in this temperature interval. It can also

be observed from Figure 6.3 that the unit cell volume and unit cell parameters

are continuous with temperature within our resolution. Also, the evolution of

the fractional coordinates of the Carbon atoms do not signal the presence of a

phase transition. However, differential scanning calorimetry (DSC) performed on

the crystals exhibit the signature of a phase transition around T = 175 K (see

Fig. 6.4). It is likely that there is a phase transition at this temperature, but it

is not observable from our X-ray diffraction experiments.

We use the model proposed by da Silva Filho et al. [13] to explain the changes

in electronic properties as a response to the structural changes in the crystal.

They associate the intrinsic transport properties of rubrene single crystals with the

electronic coupling between adjacent molecules. The measure for this interaction

is the interchain transfer integral, t, which varies with molecular packing. The

electronic coupling is maximal if the molecules are perfect cofacial in the crystal,

oscillates when the molecular displacement increases (with positive and negative

values) and becomes zero at large shifts. This translates to the direct influence of

the molecular stacking on the electronic properties.

It can bee seen in Figure 6.5 that the molecules are not perfectly cofacial, but

they are shifted with respect to each other. We define the molecular displacement

d that quantifies the shift between two molecules. From the calculations of Silva

Filho et al. it is obvious that the molecular displacement, d, is the relevant

parameter, relating the crystal structure with the electronic mobility, similar to


Figure 6.4: Differential Scanning Calorimetry of crushed rubrene single-crystals.

The measurement has been carried out in N2 atmosphere. The en-

thalpy associated with the transition is 3.13 kJ/mole. The arrow

indicates the position or the peak.

the model proposed by Mattheus et al. [14]. This parameter is complementary

to molecular overlap and given by the orientation of the tetracene backbone with

respect to the b - crystallographic direction.

In order to calculate the value of molecular displacement at each temperature,

we show the projection of two rubrene molecules on the b-c plane (see Fig. 6.6).

From geometrical considerations, the value of the molecular displacement (d) is

given by: (Eq. 6.1):

d = r + q (6.1)

where r represents half of the length of the rubrene molecule, and q the distance

from the extremity of the one molecule to the perpendicular taken from the middle

of the molecule to the long axis of the other one. The simplified expression for d

at each temperature is:

d = b · cosΘ (6.2)

where b is the value of the unit cell parameter (Fig. 6.3) and Θ is the angle that

the linear part of the molecule makes with this axis.

The maximum deviation from planarity in the tetracene backbone of the

molecule corresponds to C2 and is 0.0714(11) A at 100 K and 0.0731(14) A

6.5. Relation between molecular stacking and electronic mobility 111

Figure 6.5: Schematic representation of two rubrene molecules in the crystal.

The π- stacking distance is similar to van der Waals distance. The

molecules are shifted with respect to each other along the tetracene

backbone.

.

Figure 6.6: Arrangement of the rubrene molecules in the unit cell: view along

the [100] axis. The drawing includes schematic representation of the

parameters (r, q) used in the calculation of the molecular displacement

(d). Unit cell b and c axis are also indicated. Θ is the angle between

the long axis of the molecule and b-axis.


Figure 6.7: Evolution of the molecular displacement along the long axis of the

rubrene molecule with temperature. The inset represents rubrene

molecular packing viewed along the [100] direction, and defines the

length d of the molecular displacement.

at room temperature. The interplanar separation between two adjacent paral-

lel molecules, along the π-stack, increases from 3.654(3) A to 3.715(3) A between

100 K and 300 K. This length is slightly larger than the typical van der Waals

interaction distance in a C − C π-stack [15].

The molecules slide with respect to each other along their long axis when the

temperature is modified. Figure 6.7 shows the temperature dependence of the

molecular displacement, d along the long axis of the molecule in rubrene single

crystal. The value of the shift increases between 100 K and 300 K (Fig. 6.7). The

changes of d with temperature are dominated by changes in the lattice parameter

b (see Fig. 6.3). However, it can be seen from Figure 6.3 and Figure 6.8, that the

temperature dependence of b and cos(Θ) individually do not present any disconti-

nuity or change of slope at T = 175 K. In spite of the fact that a different picture

is suggested by their combination in the molecular displacement (see Eq. 6.2), we

cannot assign this to a crystallographic phase transition.

The variations in molecular displacement induce changes in the transfer in-

tegral (t), and determine the electronic mobility (see Figure 4, da Silva Filho

6.6. Conclusions 113

Figure 6.8: Variation of the angle Θ between the tetracene backbone of rubrene

molecule and the b crystallographic axis.

et al., [13]). The molecular displacement has a large value due to the effect of

the interactions between phenyl side groups of neighboring molecules. They are

arranged two by two, on different sides of the planar tetracene part of rubrene

and the position is restricted by the inversion center. Their torsion with respect

to the acene linear part ranges from 80.30(5) at 100 K to 80.88(7) at room

temperature. The phenyl rings positioned on the same part of the molecule make

a dihedral angle of 25.32(6) at 100 K and 25.14(10) at 300 K. The connection

between the phenyl rings and the tetracene fragment is the C2 − C6 bond. At

100 K this bond makes an angle of 12.57(8) with the tetracene fragment of the

molecule and 6.13(9) with the phenyl-plane. The corresponding angles at 300 K

are 12.74(9) and 6.00(10), respectively.

6.6 Conclusions

We conclude that rubrene single crystals grown from physical vapor transport

in ambient pressure exhibit an orthorhombic symmetry. We find evidence for a

phase transition from DSC measurements, but see no evidence for a structural

phase transition in the diffraction experiments. Further investigations concerning

the mechanism that drives the dramatic changes in electronic mobility at T = 175

K in rubrene are required.

References



[2] R. W. I. de Boer, M. E. Gershenson, A. F. Morpurgo, and V. Podzorov,

Phys. Stat. Sol. A 201, 1302 (2004)



[4] V. Y. Butko, J. C. Lashley, and A. P. Ramirez, Phys. Rev. B 72, 081312

(2005)

[5] T. Oyamada, H. Uchiuzou, S. Akiyama, Y. Oku, N. Shimoji, K. Matsushige,

H. Sasabe, and C. Adachi, J. Appl. Phys. 98, 074506 (2005)

[6] C. N. Li, C. Y. Kwong, A. B. Djurisic, P. T. Lai, P. C. Chui, W. K. Chan,

and S. Y. Liu, Thin Solid Films 477, 57 (2005)

[7] W. H. Taylor, Z. Kristallogr. 93, 151 (1936).

[8] S. A. Akopyan, R. L. Avoyan, and Yu. T. Struchkov, Z. Strukt. Khim. 3, 602

(1962)

[9] D. E. Henn, and W. G. Williams, J. Appl. Cryst. 4, 256 (1971)

[10] I. Bulgarovskaya, V. Vozzhennikov, S. Aleksandrov, and V. Belsky, Latv.

PSR Zinat. Akad. Vestis, Fiz. Teh. Zinat. Ser. 4, 53 (1983)

115

116 References


(1998)

[12] O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Appl. Phys. Lett 84, 3061

(2004)

[13] D. A. da Silva Filho, E. G. Kim, and J.-L. Bredas, Adv. Mat. 17, 1072 (2005)

[14] C. C. Mattheus, G. A. de Wijs, R. A. de Groot, and T. T. M. Palstra, J.

Am. Chem. Soc. 125, 6323 (2003)

[15] A. Bondi, J. Phys. Chem. 68, 441 (1964)

ACrystal structure of pentacene

Pentacene — T = 100 KChemical formula C22H14Formula weight 278.35

Cell setting Triclinic

Space group P1, 2

a (A) 6.277(1)

b (A) 7.663(2)

c (A) 14.363(3)

V (A3) 669.5(2)

α 76.893(3)

β 88.182(3)

γ 84.326(3)

Z 2

ρcalc (g cm−3)

Crystal form, colour platelet, violet

Crystal size (mm) 0.430 × 0.210 × 0.050

Table A.1: Crystal data and details of the structure determination of pentacene at 100

K

Diffractometer Bruker Smart Apex;

CCD area detector

Radiation type Mo Kα

Monochromator Graphite

Data collection method ϕ and ω scans

θ range () 2.74 − 25.03

Unique data 2310

Data with criterion F0 > 4σ(F0) 1683

Rint 0.0245

Range of h, k, l −7 → h → 7

−9 → k → 9

−16 → l → 17

Table A.2: Data collection for pentacene at 100 K

117

118 Appendix A. Crystal structure of pentacene

No. of reflections 2310

No. of refined parameters 255

Final agreement factors:

Weighting scheme R(F ) 0.0504

wR(F2), 0.1722

Goodness of Fit (S) 1.050

Residual electron density

∆ρmin −0.2 (eA−3)

∆ρmax 0.33 (eA−3)

Table A.3: Refinement for pentacene at 100 K

Atom x y z Ueq(A2)

C1 0.7010(2) 0.40059(1) 0.01612(11) 0.0151(5)

C2 0.5432(2) 0.37090(9) 0.08724(11) 0.0146(5)

C3 0.5778(2) 0.24511(9) 0.17491(11) 0.0155(5)

C4 0.4214(2) 0.21774(9) 0.24535(11) 0.0159(5)

C5 0.4559(3) 0.0927(2) 0.33527(11) 0.0171(5)

C6 0.3002(2) 0.0696(2) 0.40311(11) 0.0189(5)

C7 0.0949(3) 0.1708(2) 0.38655(12) 0.0189(5)

C8 0.0542(2) 0.2906(2) 0.30274(11) 0.0174(5)

C9 0.2132(2) 0.32033(9) 0.22850(11) 0.0150(5)

C10 0.1761(2) 0.44335(9) 0.14271(11) 0.0158(5)

C11 0.6654(2) 0.52590(9) -0.07033(11) 0.0146(5)

C12 -0.1816(2) -0.04326(9) -0.04185(11) 0.0148(5)

C13 -0.1558(2) -0.09971(8) 0.05702(11) 0.0144(5)

C14 -0.3059(2) -0.19924(9) 0.11683(11) 0.0149(5)

C15 -0.2788(2) -0.25490(9) 0.21458(11) 0.0150(5)

C16 -0.4297(2) -0.35691(9) 0.27553(11) 0.0161(5)

C17 -0.3994(3) -0.4106(2) 0.37083(12) 0.0186(5)

C18 -0.2153(3) -0.3661(2) 0.41408(11) 0.0189(5)

C19 -0.0673(3) -0.2710(2) 0.35924(11) 0.0172(5)

C20 -0.0911(2) -0.21086(9) 0.25798(11) 0.0154(5)

C21 0.0573(2) -0.11257(9) 0.20025(11) 0.0155(5)

C22 -0.0321(2) 0.05508(18) -0.10063(11) 0.0143(5)

H1 0.843(3) 0.332(2) 0.0269(11) 0.022(4)

H3 0.720(3) 0.179(2) 0.1853(11) 0.021(4)

H5 0.599(3) 0.026(2) 0.3450(12) 0.030(5)

H6 0.325(2) -0.0141(9) 0.4675(10) 0.007(4)

H7 -0.013(3) 0.154(2) 0.4396(11) 0.019(4

H8 -0.085(3) 0.363(2) 0.2899(11) 0.023(4)

H10 0.030(3) 0.512(2) 0.1321(11) 0.024(4)

H12 -0.308(3) -0.077(2) -0.0728(11) 0.020(4)

H14 -0.434(3) -0.231(2) 0.0867(11) 0.020(4)

H16 -0.555(3) -0.389(2) 0.2455(11) 0.022(4)

H17 -0.496(3) -0.480(2) 0.4135(12) 0.029(5)

H18 -0.199(2) -0.405(2) 0.4857(12) 0.018(4)

H19 0.062(3) -0.240(2) 0.3904(11) 0.028(5)

H21 0.187(3) -0.081(2) 0.2310(11) 0.019(4)

Table A.4: Fractional atomic coordinates of pentacene at 100 K

119

Pentacene — T = 293 KChemical formula C22H14Formula weight 278.33

Cell setting Triclinic

Space group P1, 2

a (A) 6.275(2)

b (A) 7.791(2)

c (A) 14.556(4)

V (A3) 688.7(3)

α 76.434(5)

β 87.585(5)

γ 84.707(5)

Z 2

ρcalc (g cm−3) 1.342

Crystal form, colour platelet, violet

Crystal size (mm) 0.430 × 0.210 × 0.050

Table A.5: Crystal data and details of the structure determination of pentacene at 293

K


CCD area detector




θ range () 2.70 − 25.02

Unique data 2376


Rint 0.0269

Range of h, k, l −7 → h → 7

−9 → k → 9

−15 → l → 17

Table A.6: Data collection for pentacene at 293 K

120 Appendix A. Crystal structure of pentacene




Weighting scheme R(F ) 0.0537

wR(F2), 0.1722



∆ρmin −0.14 (eA−3)

∆ρmax 0.25 (eA−3)

Table A.7: Refinement for pentacene at 293 K

Atom x y z Ueq(A2)

C1 0.7001(3) 0.4027(2) 0.01486(13) 0.0321(6)

C2 0.5428(3) 0.3722(2) 0.08631(13) 0.0313(6)

C3 0.5768(3) 0.2485(2) 0.17321(13) 0.0341(7)

C4 0.4212(3) 0.2209(2) 0.24388(13) 0.0338(7)

C5 0.4551(4) 0.0983(3) 0.33302(15) 0.0420(7)

C6 0.3000(3) 0.0755(3) 0.40070(16) 0.0480(8)

C7 0.0956(4) 0.1745(3) 0.38514(16) 0.0476(8)

C8 0.0554(3) 0.2916(3) 0.30244(15) 0.0410(7)

C9 0.2134(3) 0.3212(2) 0.22806(13) 0.0337(7)

C10 0.1775(3) 0.4426(2) 0.14323(13) 0.0343(7)

C11 0.6654(3) 0.5264(2) -0.07069(12) 0.0309(6)

C12 -0.1807(3) -0.0442(2) -0.04027(13) 0.0317(6)

C13 -0.1557(3) -0.0996(2) 0.05751(13) 0.0300(6)

C14 -0.3056(3) -0.1988(2) 0.11743(13) 0.0327(6)

C15 -0.2795(3) -0.2532(2) 0.21392(13) 0.0331(6)

C16 -0.4302(3) -0.3546(2) 0.27544(15) 0.0389(7)

C17 -0.4006(4) -0.4067(3) 0.36951(16) 0.0472(8)

C18 -0.2173(4) -0.3616(3) 0.41105(15) 0.0464(8)

C19 -0.0691(3) -0.2673(3) 0.35623(14) 0.0407(7)

C20 -0.0920(3) -0.2083(2) 0.25603(13) 0.0344(7)

C21 0.0554(3) -0.1107(2) 0.19804(13) 0.0340(7)

C22 -0.0314(3) 0.0539(2) -0.09933(13) 0.0302(6)

H1 0.842(3) 0.338(2) 0.0273(12) 0.041(5)

H3 0.720(3) 0.184(2) 0.1848(12) 0.041(5)

H5 0.596(3) 0.033(3) 0.3435(13) 0.055(6)

H6 0.326(3) -0.004(3) 0.4650(15) 0.057(6)

H7 -0.008(4) 0.157(3) 0.4381(15) 0.068(7)

H8 -0.082(3) 0.359(3) 0.2904(12) 0.052(6)

H10 0.037(3) 0.507(2) 0.1312(11) 0.041(5)

H12 -0.311(3) -0.075(2) -0.0693(12) 0.041(5)

H14 -0.436(3) -0.228(2) 0.0882(12) 0.044(5)

H16 -0.557(3) -0.387(2) 0.2455(13) 0.053(6)

H17 -0.498(4) -0.481(3) 0.4120(14) 0.056(6)

H18 -0.199(3) -0.401(3) 0.4813(14) 0.053(6)

H19 0.064(3) -0.235(2) 0.3850(12) 0.050(6)

H21 0.186(3) -0.080(2) 0.2262(11) 0.036(5)

Table A.8: Fractional atomic coordinates of pentacene at 293 K

BCrystal structure of rubrene

Rubrene — T = 100 KChemical formula C42H28Formula weight 532.68

Cell setting Orthorhombic

Space group Cmca

a (A) 26.789(4)

b (A) 7.170(1)

c (A) 14.211(2)

V (A3) 2729.6(7)

Z 4


No. of reflections for cell param. 4621

Crystal form, colour Platelet, orange

Crystal size (mm) 0.51 × 0.45 × 0.03

Table B.1: Crystal data and details of the structure determination of rubrene at 100 K


CCD area detector




θ range () 2.9 − 29.3

Tmin 0.952

Tmax 0.998

Total data 10125

Unique data 1424


Rint 0.035

θmax () 26.4

Range of h, k, l −31 → h → 33

−8 → k → 8

−17 → l → 17

Table B.2: Data collection for rubrene at 100 K

121

122 Appendix B. Crystal structure of rubrene




R[F2 > 4(F2)], 0.037

wR(F2), 0.099



∆ρmin −0.21 (eA−3)

∆ρmax 0.26 (eA−3)

Table B.3: Refinement for rubrene at 100 K

Atom x y z Ueq(A2)

C1 0.02737(6) 0.00000(-) 0.00000(-) 0.0135(5)

C2 0.05314(5) -0.14464(16) 0.04914(8) 0.0141(3)

C3 0.02678(5) -0.29578(16) 0.08755(8) 0.0142(4)

C4 0.05191(5) -0.44754(17) 0.13412(9) 0.0170(4)

C5 0.02646(5) -0.58934(17) 0.17534(9) 0.0186(4)

C6 0.10760(5) -0.13448(16) 0.07274(8) 0.0148(3)

C7 0.14292(5) -0.24817(18) 0.02881(9) 0.0185(4)

C8 0.19199(5) -0.25089(19) 0.05979(10) 0.0225(4)

C9 0.20685(5) -0.1389(2) 0.13448(10) 0.0244(4)

C10 0.17219(5) -0.0251(2) 0.17857(10) 0.0233(4)

C11 0.12302(5) -0.02362(18) 0.14816(9) 0.0189(4)

H4 0.0883(5) -0.447(2) 0.1357(10) 0.016(4)

H5 0.0438(5) -0.689(2) 0.2065(10) 0.020(4)

H7 0.1328(5) -0.327(2) -0.0256(10) 0.021(4)

H8 0.2167(5) -0.332(2) 0.0279(11) 0.028(4)

H9 0.2413(6) -0.140(2) 0.1546(10) 0.025(4)

H10 0.1825(6) 0.054(2) 0.2311(12) 0.032(4)

H11 0.0978(5) 0.054(2) 0.1804(11) 0.021(4)

Table B.4: Fractional atomic coordinates of rubrene at 100 K

123



Space group Cmca

a (A) 26.789(4)

b (A) 7.173(1)

c (A) 14.246(2)

V (A3) 2737.5(7)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


Atom x y z Ueq(A2)

C1 0.02723(7) 0.00000(-) 0.00000(-) 0.0156(5)

C2 0.05315(5) -0.14435(18) 0.04909(9) 0.0160(4)

C3 0.02670(5) -0.29541(18) 0.08757(8) 0.0162(4)

C4 0.05192(6) -0.44699(19) 0.13405(9) 0.0193(4)

C5 0.02634(6) -0.58862(18) 0.17511(9) 0.0213(4)

C6 0.10755(5) -0.13436(18) 0.07266(9) 0.0168(4)

C7 0.14287(5) -0.2473(2) 0.02915(10) 0.0214(4)

C8 0.19178(6) -0.2505(2) 0.05983(11) 0.0265(5)

C9 0.20663(6) -0.1387(2) 0.13433(11) 0.0285(5)

C10 0.17202(6) -0.0252(2) 0.17824(11) 0.0281(5)

C11 0.12299(6) -0.0236(2) 0.14796(9) 0.0218(4)

H4 0.0882(6) -0.445(2) 0.1358(10) 0.022(4)

H5 0.0442(5) -0.688(2) 0.2072(11) 0.027(4)

H7 0.1324(6) -0.325(2) -0.0251(11) 0.023(4)

H8 0.2170(6) -0.330(2) 0.0266(11) 0.034(5)

H9 0.2416(7) -0.141(2) 0.1545(11) 0.033(4)

H10 0.1817(6) 0.056(2) 0.2302(13) 0.043(5)

H11 0.0976(6) 0.053(2) 0.1789(10) 0.019(4)





Space group Cmca

a (A) 26.775(4)

b (A) 7.1680(10)

c (A) 14.258(2)

V (A3) 2736.4(7)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


Atom x y z Ueq(A2)

C1 0.02727(7) 0.00000(-) 0.00000(-) 0.0172(5)

C2 0.05307(5) -0.14417(18) 0.04907(9) 0.0177(4)

C3 0.02670(5) -0.29515(18) 0.08757(8) 0.0181(4)

C4 0.05186(6) -0.44662(19) 0.13409(9) 0.0220(4)

C5 0.02630(6) -0.58805(19) 0.17505(10) 0.0242(4)

C6 0.10744(5) -0.13413(18) 0.07267(9) 0.0191(4)

C7 0.14283(5) -0.2470(2) 0.02912(10) 0.0242(4)

C8 0.19169(6) -0.2498(2) 0.06004(12) 0.0306(5)

C9 0.20647(6) -0.1379(2) 0.13425(11) 0.0335(5)

C10 0.17191(6) -0.0249(2) 0.17798(11) 0.0324(5)

C11 0.12289(6) -0.0237(2) 0.14788(10) 0.0252(4)

H4 0.0877(6) -0.448(2) 0.1341(11) 0.023(4)

H5 0.0447(6) -0.688(2) 0.2071(11) 0.031(4)

H7 0.1328(6) -0.324(2) -0.0250(11) 0.031(4)

H8 0.2163(6) -0.330(2) 0.0272(12) 0.038(5)

H9 0.2412(7) -0.142(2) 0.1537(12) 0.043(5)

H10 0.1813(7) 0.055(3) 0.2297(14) 0.048(5)

H11 0.0976(6) 0.053(2) 0.1778(11) 0.026(4)


125



Space group Cmca

a (A) 26.828(4)

b (A) 7.181(1)

c (A) 14.306(2)

V (A3) 2756.1(7)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


Atom x y z Ueq(A2)

C1 0.02718(7) 0.00000(-) 0.00000(-) 0.0197(5)

C2 0.05303(5) -0.14399(17) 0.04909(8) 0.0206(4)

C3 0.02673(5) -0.29472(17) 0.08762(8) 0.0206(4)

C4 0.05186(6) -0.44615(18) 0.13392(9) 0.0251(4)

C5 0.02622(6) -0.58729(19) 0.17496(9) 0.0282(4)

C6 0.10736(5) -0.13391(18) 0.07267(9) 0.0216(4)

C7 0.14277(5) -0.24639(19) 0.02938(10) 0.0278(4)

C8 0.19159(6) -0.2491(2) 0.06005(11) 0.0353(5)

C9 0.20620(6) -0.1376(2) 0.13407(11) 0.0388(5)

C10 0.17173(6) -0.0245(2) 0.17770(11) 0.0379(5)

C11 0.12281(6) -0.0235(2) 0.14762(9) 0.0290(5)

H4 0.0886(6) -0.447(2) 0.1352(10) 0.031(4)

H5 0.0443(5) -0.686(2) 0.2063(10) 0.034(4)

H7 0.1330(6) -0.324(2) -0.0237(11) 0.035(4)

H8 0.2168(6) -0.328(2) 0.0272(11) 0.048(5)

H9 0.2406(7) -0.139(2) 0.1527(11) 0.043(5)

H10 0.1809(6) 0.056(2) 0.2295(13) 0.054(5)

H11 0.0975(5) 0.053(2) 0.178(1) 0.027(4)





Space group Cmca

a (A) 26.838(4)

b (A) 7.181(1)

c (A) 14.332(2)

V (A3) 2762.1(7)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03

Table B.11: Crystal data and details of the structure determination of rubrene at 200

K

Atom x y z Ueq(A2)

C1 0.02718(7) 0.00000(-) 0.00000(-) 0.0215(6)

C2 0.05301(5) -0.14396(19) 0.04907(9) 0.0221(4)

C3 0.02666(5) -0.29437(19) 0.08762(9) 0.0224(4)

C4 0.05171(6) -0.4456(2) 0.13397(10) 0.0275(5)

C5 0.02627(6) -0.5864(2) 0.17497(10) 0.0315(5)

C6 0.10730(5) -0.13351(19) 0.07261(9) 0.0236(4)

C7 0.14271(6) -0.2461(2) 0.02964(11) 0.0310(5)

C8 0.19149(6) -0.2484(3) 0.06023(13) 0.0401(6)

C9 0.20606(7) -0.1373(3) 0.13390(13) 0.0442(6)

C10 0.17164(7) -0.0244(3) 0.17747(12) 0.0430(6)

C11 0.12264(6) -0.0233(2) 0.1473(1) 0.0320(5)

H4 0.0880(7) -0.443(2) 0.1341(12) 0.034(5)

H5 0.0447(6) -0.686(2) 0.2055(12) 0.038(5)

H7 0.1325(6) -0.324(2) -0.0239(11) 0.032(4)

H8 0.2169(7) -0.328(3) 0.0280(13) 0.055(6)

H9 0.2407(8) -0.139(2) 0.1536(12) 0.053(5)

H10 0.1809(8) 0.057(3) 0.2298(16) 0.068(6)

H11 0.0971(6) 0.052(2) 0.1789(12) 0.035(4)


127



Space group Cmca

a (A) 26.818(5)

b (A) 7.174(1)

c (A) 14.348(3)

V (A3) 2760.5(9)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


K

Atom x y z Ueq(A2)

C1 0.02725(7) 0.00000(-) 0.00000(-) 0.0237(7)

C2 0.05282(5) -0.1437(2) 0.04905(11) 0.0246(5)

C3 0.02673(5) -0.2940(2) 0.08752(10) 0.0247(5)

C4 0.05163(6) -0.4448(2) 0.13381(12) 0.0311(5)

C5 0.02624(7) -0.5852(2) 0.17462(12) 0.0355(6)

C6 0.10725(6) -0.1332(2) 0.07259(11) 0.0265(5)

C7 0.14260(6) -0.2455(3) 0.02974(13) 0.0342(6)

C8 0.19131(6) -0.2471(3) 0.06057(15) 0.0459(7)

C9 0.20583(7) -0.1364(3) 0.13366(15) 0.0505(7)

C10 0.17143(7) -0.0242(3) 0.17699(14) 0.0497(7)

C11 0.12234(7) -0.0230(3) 0.14690(12) 0.0364(6)

H4 0.0880(6) -0.443(2) 0.1359(12) 0.033(5)

H5 0.0443(6) -0.682(3) 0.2055(12) 0.042(5)

H7 0.1326(6) -0.324(2) -0.0238(12) 0.032(5)

H8 0.2163(7) -0.325(3) 0.0277(13) 0.055(6)

H9 0.2410(8) -0.137(3) 0.1527(13) 0.060(6)

H10 0.1796(7) 0.052(3) 0.2290(15) 0.060(6)

H11 0.0971(7) 0.054(3) 0.1768(13) 0.041(5)





Space group Cmca

a (A) 26.938(5)

b (A) 7.211(1)

c (A) 14.461(3)

V (A3) 2809.1(9)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


K

Atom x y z Ueq(A2)

C1 0.02714(7) 0.00000(-) 0.00000(-) 0.0294(6)

C2 0.05290(5) -0.14318(18) 0.04901(9) 0.0301(4)

C3 0.02661(5) -0.29299(18) 0.08757(9) 0.0310(4)

C4 0.05169(7) -0.4439(2) 0.13369(10) 0.0387(5)

C5 0.02612(7) -0.5841(2) 0.17468(11) 0.0433(5)

C6 0.10704(6) -0.13275(19) 0.07255(10) 0.0329(4)

C7 0.14248(6) -0.2446(2) 0.03009(12) 0.0425(6)

C8 0.19109(7) -0.2464(3) 0.06060(15) 0.0560(7)

C9 0.20544(8) -0.1361(3) 0.13363(15) 0.0626(8)

C10 0.17115(8) -0.0238(3) 0.17662(14) 0.0599(7)

C11 0.12231(7) -0.0226(2) 0.14648(11) 0.0449(6)

H4 0.0890(7) -0.441(2) 0.1366(12) 0.053(5)

H5 0.0450(6) -0.684(2) 0.2061(12) 0.053(5)

H7 0.1322(6) -0.324(2) -0.0219(12) 0.048(5)

H8 0.2160(7) -0.324(3) 0.0268(14) 0.078(6)

H9 0.2404(9) -0.138(3) 0.1536(15) 0.083(7)

H10 0.1794(8) 0.057(3) 0.2296(16) 0.085(7)

H11 0.0964(7) 0.050(2) 0.1780(13) 0.054(5)


129



Space group Cmca

a (A) 26.86(1)

b (A) 7.193(3)

c (A) 14.433(5)

V (A3) 2788.5(18)

Z 4




Crystal size (mm) 0.51 × 0.45 × 0.03


K

Atom x y z Ueq(A2)

C1 0.02705(7) 0.00(-) 0.00(-) 0.0327(7)

C2 0.05294(5) -0.1429(2) 0.04897(10) 0.0335(5)

C3 0.02658(5) -0.2928(2) 0.08756(10) 0.0341(5)

C4 0.05147(7) -0.4433(2) 0.13347(11) 0.0424(6)

C5 0.02589(7) -0.5831(2) 0.17451(12) 0.0475(6)

C6 0.10704(6) -0.1325(2) 0.07249(11) 0.0369(5)

C7 0.14252(6) -0.2440(2) 0.03033(13) 0.0467(6)

C8 0.19095(7) -0.2455(3) 0.06085(16) 0.0623(8)

C9 0.20534(8) -0.1351(3) 0.13354(16) 0.0694(9)

C10 0.17085(8) -0.0241(3) 0.17627(15) 0.0659(8)

C11 0.12213(7) -0.0226(3) 0.14638(12) 0.0499(6)

H4 0.0875(6) -0.441(2) 0.1339(10) 0.036(4)

H5 0.0444(5) -0.680(2) 0.2050(12) 0.055(5)

H7 0.1318(6) -0.324(2) -0.0213(12) 0.053(5)

H8 0.2165(7) -0.321(3) 0.0286(14) 0.085(7)

H9 0.2409(8) -0.133(2) 0.1504(12) 0.072(6)

H10 0.1794(8) 0.060(3) 0.2267(16) 0.091(7)

H11 0.0955(6) 0.055(2) 0.1763(12) 0.054(5)


Summary

Organic materials are targeted to be implemented as new semiconductors in plas-

tic electronic components for commercial devices. They offer advantages with

respect to traditional inorganic semiconductors regarding processability (they re-

quire simple techniques such as spin coating, printing) and functionality (mod-

ification of the molecules offers enormous variety in functionalization). Rapid

progress is being made in the development of organic electronic devices. The

use of organic field-effect devices for large scale applications requires high perfor-

mance, reliability and stability, long lifetimes, good control and reproducibility.

One measure of the performance of a semiconductor is the electronic mobil-

ity. This is a benchmark for the success of the different organic semiconductors.

The achievement of good transport properties is associated with high mobilities.

The work presented in this thesis aims to take a step forward towards the under-

standing of the microscopic processes that determine the electronic mobility in

molecular crystals: the orientation of the molecules, intermolecular interactions,

defect densities and charge transport. This thesis incorporates the study of the

properties of organic molecular crystals at the materials level (Chapters 2, 4, 5,

6), as well as in devices (Chapter 3). The primary goals of our work are to be

able to relate the molecular stacking, defect density and device geometry with

the electronic properties and further to apply that knowledge in the design of

electronic devices with enhanced performance.

Past investigations have been hampered by low reproducibility of the exper-

131

132 Summary

imental results, and by structural defects that prevented measurements of the

intrinsic properties of organic materials. In recent years, an unprecedented con-

trol of the structural properties at the molecular level and of the charge density

distribution has been achieved by better understanding the properties of the or-

ganic semiconductor material, and by careful design of device structures.

In this general context, we study single crystals of pentacene and rubrene,

molecular conductors with the highest reported mobilities. Although in single

crystal form they may never match the general requirements for incorporation in

large scale applications, they provide unique tool for the investigation of intrinsic

properties and model systems. For the materials that we focus on, we perform the

crystal growth and crystal structure determination by X-ray diffraction. For pen-

tacene we also investigate the chemical properties using IR spectroscopy, UV-Vis

and mass spectroscopy combined with high performance liquid chromatography

(HPLC). We use space charge limited current (SCLC) and field-effect transistor

(FET) measurements to study the charge transport properties.

In Chapter 2 we demonstrate that the purity of the material is critical for high-

performance electronic devices. Impurities can form electrically active states in

the band gap of semiconductors, or distort the charge distribution in the sur-

rounding crystal introducing states in the gap. Thus, they strongly influence the

electronic properties of organic crystals. We perform a quantitative analysis of

the pentacenequinone content, the major impurity in pentacene single crystals.

Furthermore, we show that the reduction of its concentration by a factor of five

(from 0.11% to 0.028%) leads to a decrease of the number of electronically ac-

tive traps by two orders of magnitude (from Nt= 1013 cm−3 to Nt= 1011 cm−3)

and a high mobility (µ = 35 cm2/Vs). In the analysis, we incorporate correc-

tions for the effective thickness of the crystal that result from the anisotropic

resistivity and from the distribution of the electric field inside the material. A

detailed description of the algorithm is incorporated in Chapter 4. In Chapter 3

we demonstrate that the pentacenequinone impurity is located preferentially at

the surface of the crystal and strongly affects the conduction in FET devices by

scattering the charges. We overcame this limitation by converting the impurity

scattering centers into the gate dielectric. The transistors built in this way are

characterized by on/off ratios of 106 and mobilities that reproduce the values

determined for the bulk.

Organic electronic devices need to operate exposed to air. Thus, the influence

of moisture and oxygen are critical in their behavior. Our work concentrates on

improving the performance and reliability of organic based devices. We investi-

gate the physical processes that govern the electrical conduction in these materials

Summary 133

in the presence of air. We demonstrate in Chapter 5 that oxygen/water vapor

present in air diffuses into the crystal. This process generates holes/traps, leading

to enhancement/decrease of the conductivity. The two effects are opposite, they

appear together, and they can have comparable magnitudes. By combining elec-

trical and gravimetric measurements, we obtain quantitative information about

these effects. We find that on average two O2 molecules are needed in the presence

of light to create one hole, compared with four molecules in the dark.

For rubrene single crystals, in Chapter 6 we relate the dramatic changes in

electronic mobility at 175 K to modifications in the crystal structure via the trans-

fer integrals between neighboring molecules. DSC measurements show evidence

for a phase transition at 175 K, but this is not detected by our X-ray diffraction

experiments.

We have obtained high electronic mobilities in organic crystals by careful con-

trol of the material degree of purity. Furthermore, we have successfully fabricated

and measured organic FETs in which we are able to reproduce the high mobilities

evaluated from the experiments that describe the bulk properties. Our interests

involve a broad area of research, and combine crystal growth, crystal structure

determination and charge transport measurements. The work presented in this

thesis gives insight into physical processes such as the generation and migration

of defects in organic crystals, environmental effects and device geometry effects

that influence the electrical conduction in molecular crystals.

Samenvatting

Organische materialen komen in aanmerking voor nieuwe halfgeleidertoepassingen

in plastic elektronica componenten in commerciele apparatuur. Deze materialen

bieden voordelen ten opzichte van de gangbare anorganische halfgeleiders als

het gaat om verwerking (er zijn slechts relatief simpele technieken zoals spin

coaten en printen nodig) en functionaliteit (modificatie van de moleculen biedt

een rijke varieteit in functie). Veel vooruitgang is reeds geboekt in de ontwikkeling

van organische elektronische devices. Grootschalig gebruik van organische

Veldeffecttransistoren houdt in dat hoge eisen moeten worden gesteld aan

hun prestaties, betrouwbaarheid, stabiliteit, levensduur, beheersbaarheid en

reproduceerbaarheid.

Een maat voor de prestatie van een halfgeleider is de beweeglijkheid van zijn

ladingsdragers. Dit is een graadmeter voor het succes van verschillende organische

halfgeleiders. Het tot stand brengen van goede transporteigenschappen is inherent

aan het bereiken van hoge mobiliteiten. Het onderzoek dat in dit proefschrift

aan de orde komt, stelt zich ten doel om te komen tot een beter begrip van de

microscopische processen die ten grondslag liggen aan de elektrische geleiding in

moleculaire kristallen: het verband tussen moleculen, intermoleculaire interacties,

defect dichtheden en ladingstransport. Dit proefschrift omvat naast onderzoek

naar eigenschappen van organische moleculaire kristallen op het gebied van

materialen (Hoofdstukken 2, 4, 5, 6), tevens onderzoek naar hun eigenschappen

wanneer zij zijn ingebouwd in devices (Hoofdstuk 3). De kerndoelen van

135

136 Samenvatting

ons onderzoek zijn enerzijds het in staat zijn om moleculaire stapeling, defect

dichtheid en device opbouw in verband te brengen met elektronische eigenschappen

en anderzijds om die kennis toe te passen in het ontwerp van elektronische

devices met verbeterde prestaties. Onderzoek in het verleden werd bemoeilijkt

door de slechte reproduceerbaarheid van meetresultaten en de aanwezigheid van

structuurdefecten, zodat de metingen aan intrinsieke eigenschappen ontoegankelijk

waren. In de laatste jaren is een ongekende beheersing mogelijk van zowel de

structuureigenschappen op moleculaire schaal als van de ladingsdichtheidsverdeling

door een beter begrip van materiaaleigenschappen van organische halfgeleiders en

een zorgvuldig ontwerp van de device opbouw.

Binnen deze algemene context bestuderen we in het bijzonder eenkristallen

van pentaceen en rubreen, moleculaire geleiders met de tot dusver hoogst

gerapporteerde mobiliteiten. Hoewel dergelijke materialen in de hoedanigheid van

eenkristal nooit aan de algemeen geldende eisen voor toepassing op grote schaal

kunnen voldoen, verschaffen deze eenkristallen ons toch een uniek instrumentarium

voor onderzoek naar intrinsieke eigenschappen en bieden zij ons tevens model

systemen. Van de materialen waar wij ons op richten, groeien we eenkristallen

en bepalen hun kristalstructuur door middel van Rontgendiffractie. Verder doen

wij onderzoek naar de chemische en fysische eigenschappen van pentaceen. De

bepaling van dergelijke eigenschappen berust op het gebruik van infrarood-, UV-

Vis- en massaspectroscopie in combinatie met vloeistofchromatografie met hoog

oplossend vermogen. We maken gebruik van ruimteladingsbegrensde stroom- en

veldeffecttransistor metingen om de ladingstransporteigenschappen te bestuderen.

In hoofdstuk 2 tonen we aan dat de zuiverheid van het materiaal een sterke

invloed heeft op de prestaties van de elektronische devices. Onzuiverheden kunnen

elektrisch actieve toestanden in de band gap van halfgeleiders introduceren, of de

ladingsverdeling in het aangrenzende kristalvolume verstoren. Op die manier

benvloeden zij in sterke mate de elektronische eigenschappen van organische

kristallen. We verrichten een kwantitatieve analyse op pentaceenchinon, de

belangrijkste onzuiverheid in pentaceen eenkristallen. Bovendien tonen we aan

dat het terugbrengen van de concentratie van deze onzuiverheid met een factor

vijf (van 0.11% naar 0.028%) resulteert in een afname met twee ordes van grootte

van het aantal afgevangen ladingsdragers (van Nt = 1013 cm−3 naar Nt = 1011

cm−3) en ook de mobiliteit hoger wordt (µ = 35 cm2/Vs).

In de analyse corrigeren we voor de effectieve kristaldikte die wordt veroorzaakt

door de anisotrope weerstand en de verdeling van het elektrische veld binnen het

materiaal. Een gedetailleerde beschrijving van het algoritme wordt behandeld in

hoofdstuk 4. In hoofdstuk 3 komt aan bod dat pentaceenchinon onzuiverheden

Samenvatting 137

zich bij voorkeur aan het kristaloppervlak bevinden en bovendien daar de

geleiding in sterke mate beınvloeden door ladingsdragers te verstrooien. Door

onzuiverheden die fungeren als strooicentra om te vormen tot gate dielectricum,

kan deze belemmering worden verholpen. De transistoren die op deze manier

vervaardigd zijn, hebben een aan/uit verhouding van 106 en mobiliteiten die

overeenstemmen met de waarden die zijn gevonden voor het bulk materiaal.

Organische elektronische devices moeten hun taak kunnen blijven vervullen

wanneer ze worden blootgesteld aan de lucht. Daarom is de invloed van vocht en

zuurstof van belang voor hun gedrag. Ons werk richt zich op het verbeteren van

de prestaties en betrouwbaarheid van organische devices. Hiertoe onderzoeken

we de fysische processen die de elektrische eigenschappen van deze materialen

bepalen in de aanwezigheid van lucht. We tonen aan in hoofdstuk 5 dat de

zuurstof/waterdamp die aanwezig is in de lucht diffundeert in het kristal. Dit

diffusieproces genereert gaten/vallen, wat leidt tot verbetering/verslechtering van

de geleiding. De twee effecten zijn in competitie, treden gezamenlijk op en kunnen

van vergelijkbare orde van grootte zijn. Door elektrische en thermogravische

metingen te combineren verschaffen we ons een kwantitatief beeld van deze

effecten. We hebben vastgesteld dat er in de aanwezigheid van licht gemiddeld

twee O2 moleculen nodig om een gat te creeren, vergeleken met vier moleculen

in het donker. Hoofdstuk 6 behandelt rubreen eenkristallen. We brengen

de enorme verandering in mobiliteit bij 175 K in verband met veranderingen

in kristalstructuur met behulp van overdrachtsmatrixelement tussen naburige

moleculen. DSC metingen duiden op een fase overgang bij 175 K, maar hiervoor

zijn geen aanwijzingen gevonden in een Rontgendiffractie studie.

We hebben hoge mobiliteiten bereikt door een uitgebalanceerde beheersing van

de zuiverheidsgraad van de gebruikte materialen. Verder hebben we op succesvolle

wijze devices vervaardigd en organische FETs gemeten die in staat waren de hoge

mobiliteiten te reproduceren zoals die gevonden waren in experimenten die de

bulk eigenschappen in kaart plegen te brengen. Onze belangstelling gaat uit naar

een breed onderzoeksgebied en combineert kristalgroei, kristal structuurbepaling

en ladingstransport metingen. Het werk dat in dit proefschrift aan de aan de orde

komt, geeft inzicht in de fysische processen zoals het ontstaan en de migratie van

defecten in organische kristallen, omgevingseffecten en effecten op de electrische

geleiding van moleculaire kristallen die gerelateerd zijn aan de device opbouw.

List of Publications

• B. B. Van Aken, O. D. Jurchescu, A. Meetsma, Y. Tomioka, Y. Tokura,

and T. T. M. Palstra, Orbital-order-induced metal-insulator transition in

La1−xCaxMnO3, Phys. Rev. Lett. 90, 066403 (2003).

• O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Effect of impurities on the

mobility of single crystal pentacene, Appl. Phys. Lett. 84, 3061 (2004).

• O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Electronic transport

properties of pentacene single crystals upon exposure to air, Appl. Phys.

Lett. 87, 052102 (2005).

• A. Vollmer, O. D. Jurchescu, I. Arfaoui, I. Salzmann, T. T. M. Palstra, P.

Rudolf, J. Niemax, J. Pflaum, J. P. Rabe, and N. Koch, The effect of oxygen

exposure on pentacene electronic structure, Eur. Phys. J. E 17, 339 (2005).

• O. D. Jurchescu, and T. T. M. Palstra, Crossover from one- to two-

dimensional space-charge-limited conduction in pentacene single crystals,

Appl. Phys. Lett. 88, 122101 (2006).

139

140 List of publications

• O. D. Jurchescu, A. Meetsma, and T. T. M. Palstra, Low-temperature

structure of rubrene single crystals grown by vapor transport, Acta Cryst.

B 62, 330 (2006).

• O. Ostroverkhova, D. G. Cooke, F. A. Hegmann, J. E. Anthony, V.

Podzorov, M. E. Gershenson, O. D. Jurchescu, and T. T. M. Palstra,

Ultrafast carrier dynamics in pentacene, functionalized pentacene, tetracene,

and rubrene single crystals, Appl. Phys. Lett. 88, 162101 (2006).

• O. D. Jurchescu, M. Popinciuc, B. J. van Wees, and T. T. M. Palstra,

Interface controlled high-mobility organic transistors, submitted.

• A. J. C. Buurma, I. Shokaryev, O. D. Jurchescu, A. Meetsma, R. A. de

Groot, and T. T. M. Palstra, Crystal growth, structure and electronic band

structure of Tetracene-TCNQ, submitted.

• I. Shokaryev, A. J. C. Buurma, O. D. Jurchescu , G. A. de Wijs, T. T. M.

Palstra, R. A. de Groot, The electronic band structure of Tetracene - TCNQ

and similar acene - TCNQ compounds, in preparation.

Propositions belonging to the thesis:

Molecular Organic Semiconductors

for Electronic Devices

1. The purity of the molecular organic semiconductor is crucial to achieve

high-performance and high-reliability electronic device operation.

Chapter 2 of this thesis

2. The greatest challenges at the present stage of the organic electronics

research, is device processability rather than new materials.


3. Oxygen diffuses reversibly into pentacene crystals and on average four O2

molecules induce the formation of one charge carrier in the dark.


4. Intentionally use of unnecessary complicated terminology does not imply

valuable scientific content.

5. Science results as well as open source software are both communal and

competitive: in both cases the property right issue is almost entirely a

matter of respecting the authorship of the original work.

6. Contrary to the expectations of some politicians, global warming is the

greatest threat to humanity during the next millennium.

7. The impact of an article in the field is more important that the impact

factor of the journal in which it is published.

8. In science lately there are more opinions than facts.

9. ”Your focus determines your reality.”

Star Wars

10. The mistakes that are most easy to forgive are your own.


October 2006

These propositions are considered defendable and as such have been approved

by the supervisor, prof. dr. T.T.M. Palstra.

Documents

Molecular Organic Semiconductors for Electronic Devices