Novel Polymer Thin Films for

Application in Flexible Organic

Light-emitting Diodes

Kyung Min Lee

A Dissertation

Presented to the Faculty

of Princeton University

in Candidacy for the Degree

of Doctor of Philosophy

Recommended for Acceptance

by the Department of

Electrical Engineering

Adviser: Professor Barry Rand

September 2019

© Copyright by Kyung Min Lee, 2019.

All rights reserved.

Abstract

Organic light-emitting diodes (OLEDs) offer potentially cost effective and energy effi-

cient alternatives to their inorganic counterparts. Superior mechanical flexibility and

relaxed processing conditions have allowed OLEDs to become widespread in displays

and mobile devices. Next generation OLEDs are emerging, which include flexible

white OLED lighting and biodegradable electronics. These applications have practi-

cal implications for reducing energy consumption and electronic waste as electronics

have become ubiquitous.

In this thesis, we explore two emerging areas of research in which OLEDs can

be used to meet the increasing energy demands. First, we demonstrate a practical

method to increase luminous efficacy of white OLEDs. This involves developing a

process that spontaneously forms micro- and nano- pores in an optically transparent

and high-index plastic. The porous polymer films are optimized to enable broadband

outcoupling from white OLEDs.

Second, we introduce the properties of low ceiling temperature polymers and their

potential as plastics with high recyclability. By incorporating photocatalysts in the

polymer and integrating an OLED directly atop, we demonstrate a proof-of-concept

device that can realize room-temperature depolymerization reactions.

iii

Acknowledgements

Wow! I cannot believe this day has finally come. Last five years were truly interesting

and time well spent. I got to make things glow, developed really good night vision

running experiments in the dark, and learned some valuable life lessons. And this is

also where I get to thank everyone who made it possible. First and foremost, this

work would not be possible without unconditional love and support from my family. I

would also like to thank my advisor Prof. Barry Rand and my mentors Profs. Claire

Gmachl, and Craig Arnold, and Antoine Kahn for your support.

I had the greatest privilege to work closely with many talented scientists. I want

to say special thanks to Dr. Tae–wook Koh for being an amazing OLED guru and an

amazing mentor. Dr. Romain Fardel taught me how to zap things with lasers and

good lab practices. Many thanks to Dr. Nakita Noel, Dr. Lisa Lin, Dr. Kwangdong

Roh, Zhuozhi Yao, and Amanda Du; your scientific expertise and unwavering enthu-

siasm are truly inspiring. Andrew Ma and MC Otani, you two were the best mentees

I could ask for, and you brought so much fresh perspective.

I would also like to give special thanks to the members of Cool Kidz and AT&T

Family, Taylor, Gabi, and Piper for their companionship. Thank you Dr. Nan Yao,

John Schreiber, Colleen Conrad, Roelie Abdi, Jess Johnson, Lidia Stokeman, and

Linda Dreher for your technical and administrative support. Finally, this work

received funding from the DOE EERE SSL Program and DARPA Young Faculty

Award.

iv

To my family.

v

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

1 Introduction to organic light-emitting diodes and current challenges 1

1.1 Background on organic light-emitting diodes . . . . . . . . . . . . . . 1

1.2 Light outcoupling in OLEDs . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.1 Analysis of OLED efficiency . . . . . . . . . . . . . . . . . . . 13

1.2.2 Dipole radiation near interfaces . . . . . . . . . . . . . . . . . 14

1.2.3 Previous work on OLED outcoupling . . . . . . . . . . . . . . 18

1.3 Opportunities for lighting and transient electronics . . . . . . . . . . 20

2 Experimental determination of OLED efficiency and comparison

with an electromagnetic model 25

2.1 Measurement and characterization of OLEDs . . . . . . . . . . . . . . 25

2.1.1 Comparison of model with experiment . . . . . . . . . . . . . 31

2.2 Increasing outcoupling efficiency via substrate modification . . . . . . 37

2.2.1 Preparation and characterization of porous scattering films . . 38

3 Substrate light outcoupling in flexible white OLEDs 47

vi

3.1 Colorless polyimide-silver nanowire composite as a flexible substrate

for white OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.3.1 Characterization of scattering polyimide film . . . . . . . . . . 57

3.3.2 Outcoupling enhancement in green and white PHOLEDs on

scattering substrates . . . . . . . . . . . . . . . . . . . . . . . 58

4 Application of OLEDs in transient electronics 67

4.1 Background on transient electronics . . . . . . . . . . . . . . . . . . . 67

4.1.1 Low ceiling temperature transient polymers . . . . . . . . . . 69

4.2 Transient substrate for OLED . . . . . . . . . . . . . . . . . . . . . . 75

4.2.1 Experimental details . . . . . . . . . . . . . . . . . . . . . . . 90

5 Conclusion and outlook 93

A Publications and presentations 98

A.1 Publications contributing to this thesis . . . . . . . . . . . . . . . . . 98

A.2 Presentations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

Bibliography 100

vii

List of Tables

1.1 Performance of various TCEs in literature. . . . . . . . . . . . . . . . 6

1.2 Performance of white OLEDs published in a DOE report. . . . . . . . 22

2.1 Relative powers radiated into each mode. . . . . . . . . . . . . . . . . 34

2.2 Comparison of simulated outcoupling efficiency and measured EQEs. 40

4.1 Transient materials and devices reported in literature. . . . . . . . . . 68

4.2 Ceiling temperatures of various polymers. . . . . . . . . . . . . . . . . 69

viii

List of Figures

1.1 Molecules and structure used in the first OLED. . . . . . . . . . . . . 2

1.2 Cross-section and energy diagram of a bottom-emitting OLED. . . . . 3

1.3 Energy diagram of a guest-host system. . . . . . . . . . . . . . . . . . 4

1.4 Highest EQEs reported for fluorescent, phosphorescent, and TADF

emitters by year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.5 Ray optics representation of losses in a bottom-emitting OLED. . . . 8

1.6 Dispersion relation for a stratified OLED. . . . . . . . . . . . . . . . 9

1.7 Propagation of surface plasmon polariton along a metal-dielectric in-

terface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.8 Patterns of power emitted by a dipole with various orientations. . . . 15

1.9 Dipole radiation near interfaces. . . . . . . . . . . . . . . . . . . . . . 16

1.10 Thin-film interference effects in OLEDs. . . . . . . . . . . . . . . . . 17

1.11 Luminous efficacy of various lighting elements. . . . . . . . . . . . . . 21

1.12 Photograph of electronic waste. . . . . . . . . . . . . . . . . . . . . . 23

2.1 Photopic sensitivity function. . . . . . . . . . . . . . . . . . . . . . . 26

2.2 Measurement geometry for OLEDs in this work. . . . . . . . . . . . . 27

2.3 Chemical structures of iridium phosphorescent emitters. . . . . . . . . 28

2.4 The CIE 1931 color map with x, y coordinates of the iridium emitters

used in this work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.5 Structure and optical properties of the OLED for simulation. . . . . . 32

ix

2.6 Simulated power radiated into each wavevector at emission peak. . . . 33

2.7 Simulated power spectral density plotted over wavelength and in-plane

wavevector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.8 Comparison of measured and simulated spectra at two viewing angles. 35

2.9 Comparison of the measured EQEs of thick-ETL OLEDs and simulated

outcoupling efficiencies. . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.10 Performance of thick-ETL OLEDs. . . . . . . . . . . . . . . . . . . . 36

2.11 Fabrication of a porous polyimide film by precipitation immersion . . 38

2.12 Characterization of a pristine Kapton polyimide and a porous Kapton

polyimide film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.13 Performance of green OLEDs on glass/ITO with and without a scat-

tering film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.14 Angle-dependent performance of green OLEDs on glass/ITO with and

without a scattering film. . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.15 Performance of white OLEDs with and without a scattering film. . . 44

2.16 Angle-dependent performance of white OLEDs on glass/ITO with and

without a scattering film. . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.1 Proposed mechanism for extracting substrate loss from flexible OLEDs. 48

3.2 Outcoupling efficiencies simulated for OLEDs on two different substrates. 49

3.3 Synthesis of Kapton polyimide. . . . . . . . . . . . . . . . . . . . . . 50

3.4 Synthesis of colorless polyimide. . . . . . . . . . . . . . . . . . . . . . 51

3.5 Comparison of Kapton polyimide and colorless polyimide. . . . . . . 52

3.6 Thermogravimetric analysis of colorless polyimide. . . . . . . . . . . . 53

3.7 Fabrication scheme for flexible scattering substrates. . . . . . . . . . . 54

3.8 Device structures of the green and white PHOLEDs. . . . . . . . . . 55

3.9 Electron microscope images and optical characterizationof a scattering

substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

x

3.10 Performance of green OLEDs on various substrates. . . . . . . . . . . 60

3.11 Performance of white OLEDs on various substrates. . . . . . . . . . . 62

3.12 Angle-dependent spectra of various WOLEDs. . . . . . . . . . . . . . 63

3.13 Angle-dependent CIE 1931 coordinates of green and white PHOLEDs

on various substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.14 Angle-dependent CIE 1931 coordinates and CCT for WOLEDs on var-

ious substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.15 Normalized outcoupling efficiency over many bending cycles. . . . . . 65

3.16 Reproducibility of flexible scattering substrates. . . . . . . . . . . . . 66

4.1 Chemical structures of PHA monomer and cyclic PPHA. . . . . . . . 70

4.2 Schematic of a low ceiling temperature polymer undergoing cyclic poly-

merization and depolymerization. . . . . . . . . . . . . . . . . . . . . 71

4.3 Acid generating mechanism of a Rhodorsil-Faba PAG under UV exci-

tation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.4 Energy diagram of photo-induced electron transfer between a PAG and

a sensitizer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.5 Chemical structure and optical properties of BPET sensitizer. . . . . 74

4.6 Storage modulus of sensitized PPHA before and after photo-exposure. 75

4.7 Acid generation mechanism via photo-induced electron transfer and

proposed mechanism for OLED-induced transience. . . . . . . . . . . 76

4.8 Optical characterization of Yb, Al, and PPHA, and normalized resis-

tance of AgNW embedded in UV-exposed s-PPHA. . . . . . . . . . . 78

4.9 Characterization of a green OLED on a pristine PPHA substrate. . . 79

4.10 Characterization of OLEDs on s-PPHA/glass/ITO and s-PPHA/AgNW

substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4.11 Spectroscopic evidence of sensitizer photobleaching. . . . . . . . . . . 81

xi

4.12 Photographic evidence of OLED-induced transience and mechanical

degradation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

4.13 Evolution of a phototriggered s-PPHA substrate over 72 h. . . . . . . 84

4.14 Quantification of depolymerization via FTIR. . . . . . . . . . . . . . 84

4.15 Photographs of OLED-induced depolymerization and OLED lifetimes

on depolymerizing s-PPHA substrates. . . . . . . . . . . . . . . . . . 86

4.16 Projected transmission for different sensitizer loadings. . . . . . . . . 87

4.17 Performance of a polymer LED fabricated on a pristine PPHA substrate. 88

4.18 Photographs of polymer LEDs on a pristine PPHA substrate and a

s-PPHA substrate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.1 Photographs of porous polyimide directly formed on polymer sub-

strates by spin coating. . . . . . . . . . . . . . . . . . . . . . . . . . . 94

xii

Chapter 1

Introduction to organic

light-emitting diodes and current

challenges

1.1 Background on organic light-emitting diodes

The 1950 discovery of electroluminescence (EL) in organic materials led to the devel-

opment of the first organic light-emitting diode (OLED) 27 years later[1]. This first

OLED consisted of a 75 nm aromatic diamene hole transport layer and a 60 nm elec-

tron transporting fluorescent emitter, tris-(8-hydroxyquinoline)aluminum (Alq3). The

two active organic layers were vapor deposited between an indium-tin-oxide (ITO)

transparent electrode and a Mg:Ag reflective top electrode. This green OLED had

about 1% external quantum efficiency (EQE), which is still roughly the maximum

EQE achievable with similarly structured Alq3 OLEDs today. Since the demonstra-

tion of the first fluorescent OLED, a variety of OLEDs with multilayer structures and

other fluorescent emitters were demonstrated[2].

1

Figure 1.1: Molecular structures of a) Alq3, and b) diamine hole transport layer,and c) structure of the first OLED; reprinted from [1], with the permission of AIPPublishing.

Many high-efficiency OLEDs today are fabricated in a multilayer structure for

better charge balance and exciton confinement, but the basic structure of the OLEDs

remains unchanged from that of the first OLED (Figure 1.1). The basic structure

of a bottom-emitting OLED includes a substrate followed by a transparent conduct-

ing electrode (TCE). Most commonly, an ITO-coated glass serves as a hole-injecting

transparent contact due to its deep work function as well as its excellent electrical and

optical properties. A wide bandgap hole transport layer (HTL) selectively transports

holes into the emissive layer (EML). Electrons are transported by an electron trans-

port layer (ETL) deposited between the EML and the reflective top contact, cathode.

We note that the lowest unoccupied molecular orbital (LUMO) levels of most wide

bandgap ETLs are high (2-3 eV) and therefore, a cathode with a low workfunction

is needed to achieve a low energetic barrier to inject electrons[3]. Other OLED struc-

tures include a top-emitting OLED and a transparent OLED, which differ from the

bottom-emitting OLED by the location of the transparent contact and the number

of transparent contacts, respectively. The physical structure and the light generation

mechanism in a bottom-emitting OLED is shown schematically in Figure 1.2.

In 1993, Kido et al. demonstrated the first white OLED with three active organic

layers containing red, green, and blue emitters[4]. The fluorescent OLEDs had lim-

ited EQEs however, due to the spin statistics in which electrically excited organic

2

Figure 1.2: a) Cross-section of a bottom-emitting OLED, and b) energy diagram ofcharge injection and radiative recombination in the OLED. The top and the bottomof the HTL, EML, and ETL represent LUMO and the highest occupied molecularorbital (HOMO) levels.

molecules form singlet excitons and triplet excitons in a 1:3 ratio. This is because

excited state organic molecules form bound electron-hole pairs (excitons) localized to

each molecule, as opposed to free electron-hole pairs that form in inorganic materials.

Fluorescent emitters with non-emissive triplets therefore can at most achieve 25% in-

ternal quantum efficiency (IQE) in an electrically driven device, and their EQE is well

below 5% due to the low outcoupling efficiency (∼20%) of planar OLEDs. The lim-

ited efficiency of fluorescent OLEDs was first lifted by Baldo et al. in 1998 by use of a

phosphorescent red emitter[5]. The phosphorescent OLED (PHOLED) consisted of a

platinum organometallic compound, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin

platinum(II) (PtOEP) in which the heavy platinum atom facilitates intersystem cross-

ing, allowing the emitter to phosphoresce from its triplet state. By doping a small

concentration of PtOEP (10%) into a host material with larger singlet and triplet en-

ergies, the energy transfer (resonant Förster and Dexter) from the excited-state host

to the dopant becomes energetically favorable and can result in up to 100% capture

rate of the triplet excitons by the dopant. Most PHOLEDs today utilize this guest-

3

host system. The mechanism governing the radiative recombination in PHOLEDs is

illustrated in Figure 1.3.

Figure 1.3: The Jablonski diagram of a guest-host system. Triplet excitons form onthe guest dopant via energy transfers from the host where both singlets and tripletsare first excited.

Various phosphorescent emitters derived from iridium and platinum organometal-

lic compounds are among the highest-performing OLED emitters today, with EQEs

exceeding 38%[6]. These emitters contain rare-earth metals in which the heavy metal

atom increases spin–orbital interactions and facilitates intersystem crossing[5]. This

process is also known as the heavy atom effect and allows for the typically none-

missive triplet excitons–which account for 75% of the initially excited excitons– to

contribute to electroluminescence. As such, phosphorescent emitters can achieve up

to 100% internal quantum efficiency (IQE). Despite the high efficiencies achievable

with the organometallic emitters, there is a large demand to replace them with a more

abundant material as iridium is one of the rarest metals. Only 2 parts per billion of

iridium is known to be located in the earth’s crust[7]. A promising alternative to the

rare-earth compounds includes all-organic compounds engineered to exhibit a very

small triplet-to-singlet energy gap[8,9]. This allows for efficient reverse intersystem

crossing at room temperature, and these compounds are able to convert the nonemis-

4

sive triplets to singlets. The class of such emitters are known as thermally activated

delayed fluorescence (TADF) emitters, and their efficiencies are fast approaching that

of the phosphorescent emitters. As TADF emitters are synthesized with all-organic

constituent molecules, they may provide lower-cost alternatives to the phosphorescent

emitters containing precious metals. The evolution of the reported EQEs in literature

by year is shown in Figure 1.4.

Figure 1.4: Highest EQEs reported in literature by year. Sourced from [10].

Similarly, there exists a large demand to replace ITO with another TCE due to

its mechanical brittleness and high concentration of a rare metal indium. In today’s

literature, the performance of ITO serves as a benchmark; a commercially available

ITO-coated glass substrate exhibits high visible transmission (>87% at 550 nm) and

5

Substrate/TCE Rs (Ω/2) Transmission at550 nm

Depositionmethod

Glass/ITO 10 87% SputteringPET/ITO1 60 84% Sputtering

PET/AgNW[13] 38 95% Bar coatingPolyimide/AgNW(this work)

25 85% Spin or spray coat-ing

PET/CuNW[14] 99.1 86.6% Bar coating

Table 1.1: Performance of various TCEs.

low sheet resistance (Rs ∼ 10 Ω/2). However, high temperature (400 °C) anneal-

ing is necessary to produce highly crystalline ITO films with optimum conductivity

and visible transmission, making ITO incompatible with plastic substrates with much

lower thermal tolerance. As a result, ITO-coated plastic substrates such as polyethy-

lene terephthalate/ITO (PET/ITO) are produced without the aggressive thermal

annealing step and thus exhibit poorer performance with Rs ∼60 Ω/2 and ∼85%

transmission.

Metal nanowires and metal nanotubes have been presented as promising alterna-

tives to ITO[11,12]. This class of TCEs exhibits superior mechanical flexibility and

can outperform glass/ITO when optimized. In addition, certain metal nanotubes

and nanowires are derived from more abundant materials such as copper (Cu) or

graphene, offering advantages over ITO which relies on precious indium. The com-

parison of ITO and select TCEs is summarized in Table 1.1. In particular, silver

nanowires (AgNWs) gained much attention for favorable processing conditions (solu-

tion deposition in an ambient environment) and superior performance. The AgNW

electrodes have been combined with a variety of plastic substrates to serve as flexible

transparent conducting substrates for OLEDs. While silver is more expensive than

indium, AgNWs are highly compatible with large-area, roll-to-roll processes and are

generally regarded as a more cost effective flexible TCE compared to ITO.

1Available from Sigma Aldrich.

6

Highly optimized OLED materials such as TADF emitters and emitters with spe-

cific dipole orientations are commercially available. Thanks to the high IQE of these

emitters, the efficiency of optimized OLEDs is now mostly limited by outcoupling, a

process in which emitted photons can escape the physical structure of the OLED. To

illustrate this point, a standard bottom-emitting OLED on a glass/ITO substrate is

shown in Figure 1.2. The outcoupled photons must escape the device past the organic

core and the substrate, into air. Outcoupling in this structure is impeded by three

main loss channels. First, the proximity between the emitter molecules and the metal

cathode gives rise to surface plasmon polaritons (SPPs). Once coupled to SPPs, light

propagates parallel to the metal-organic interface before being absorbed by the metal.

Second, the high-index organic core between the reflective metal contact and the low-

index substrate supports waveguiding, trapping light in the organic core. Finally, the

light incident on the organic-substrate interface below the critical angle may enter

the substrate but can still undergo total internal reflection (TIR) at the substrate-air

interface. This loss due to the substrate-air index mismatch is known as substrate

loss. Without modifying the planar OLED structure, for every exciton formed on

the emitter molecule, only about 20-30% result in outcoupled photons[15]. A variety

of structures have been incorporated in OLEDs to outcouple one or more of the loss

channels in planar OLEDs. In section 1.2.3, we will discuss some of the relevant work

in literature.

1.2 Light outcoupling in OLEDs

We consider the optical environment of a simplified planar bottom-emitting OLED in

Figure 1.5 to study outcoupling in similar OLEDs that will be discussed later in this

thesis. We note that the in-plane wavevector kx is a conserved quantity throughout

the structure as the structure is invariant in x̂. For the light generated in the emissive

7

Figure 1.5: A ray optics representation of a bottom-emitting OLED and light prop-agating in air (1), substrate (2), organic core (3), and metal-organic interface (4).

medium with refractive index nem propagating at an angle θem relative to the surface

normal, the in-plane wavevector kx can be written in terms of the wavevector (k) and

the emission wavelength (λ).

kx = |k|sinθ =2π

λnemsinθem (1.1)

Only a select set of θem is able to escape the OLED structure, as large θem or light

incident on an interface at an oblique angle can undergo total internal reflection. Four

different modes are identified in Figure 1.5 in order of increasing θem: air mode (1),

substrate mode (2), organic/waveguided mode (3), and SPP mode (4). The critical

angle at each interface places the upper limit on the value of kx that allows light to

escape each medium (processes 1-3).

We plot the dispersion relation for the same structure, ω(kx) in which the propa-

gating mode in each medium with relative permittivity � satisfies the following rela-

tionship where c is the speed of light:

ω =c|k|√�

=c√�

√k2x + k

2z (1.2)

The linear lines plotted in Figure 1.6 represent the condition, kz = 0, and are

known as light lines. Above the light line, the wave propagates in a given medium

8

with the form, eikxxeikzze−iωt where both kx and kz are real. Below the light line, kz is

imaginary, and it indicates that the wave has an exponentially decaying y-component

(i.e. evanescent). The kx values at which the light lines intersect the light with a

given frequency, ω (3.1 eV or 400 nm in this example, noted by a horizontal line in

Figure 1.6) is directly related to the maximum angle θem below which a propagating

solution is supported in the medium. This corresponds to the critical angle above

which TIR traps light in the organic stack or the substrate. The regions labeled (1),

(2), and (3) are air, substrate-trapped, and organic/waveguided mode, respectively.

Figure 1.6: Dispersion relation for the OLED in Figure 1.2. The horizontal linecorresponds to 400 nm. The lines intersecting the x-axis represent the maximum in-plane wavevector allowed in each numbered region. Note that nsub = 1.5, norg = 1.9.

The SPP excited at a metal-dielectric interface propagates parallel to the interface;

this wave involves oscillation of free electrons on the metal surface. As we have alluded

to earlier, this wave exponentially decays away from the interface. The conditions for

such surface wave to exist can be found by solving Maxwell’s equations and applying

9

boundary conditions for the given interface. We look for the solution in which a

transverse magnetic (TM) wave propagates along the interface formed by a metal (�m)

and a dielectric (�d). The wave propagating in x but confined in z has wavevectors,

~kd = kxx̂+ iγdẑ in the dielectric (z > 0), and ~km = kxx̂− iγmẑ in the metal (z < 0).

Note that γd, γm > 0. We express ~E and ~H for this TM wave in each medium.

Figure 1.7: Surface plasmon polariton along a metal-dielectric interface

First, in the dielectric:

~Ed =< Ex,d, 0, Ez,d > eikxx+γdz−iωt

~Hd =< 0, Hy,d, 0 > eikxx+γdz−iωt

(1.3)

And in the metal,

~Em =< Ex,m, 0, Ez,m > eikxx−γmz−iωt

~Hm =< 0, Hy,m, 0 > eikxx−γmz−iωt

(1.4)

10

We solve the last curl equation in the source-free Maxwell’s equations (Equation 1.5)

where i is for the metal or the dielectric.

∇ · �i ~Ei = 0

∇ · ~Hi = 0

∇× ~Ei = µ0∂ ~Hi∂t

∇× ~Hi = �i∂ ~Ei∂t

(1.5)

At the boundary (z = 0), the tangential components of ~E and ~H, and the normal

component of ~D = � ~E are continuous.

Ex,m = Ex,d

Hy,m = Hy,d

�mEz,m = �dEz,d

(1.6)

Solving the last curl equation in Equation 1.5 and applying Equation 1.6, we arrive

at the condition for SPP to exist at the interface:

γm�m

= −γd�d

(1.7)

We have assumed that γd,m are positive real numbers. For a positive real �d, �m

must be a negative real number, which is possible for an ideal Drude metal with the

dielectric function below its plasma frequency ωp[16]:

�m(ω) = 1−ω2pω2

(1.8)

11

Noting that k2d,m = k2x−γ2d,m, Equations 1.7 and 1.8 can be rewritten for kx as follow:

kx =ω

c

√�d�m�d + �m

=ω

c

√√√√ �d(1− ω2pω2 )�d + 1−

ω2pω2

(1.9)

Without explicitly solving the above expression for ω(kx), we consider the following

limits on ω where ωspp =ωp√�d+1

is the surface plasmon frequency.

(a) ω → 0 =⇒ kx → ωc√�d

(b) ω → ωspp =⇒ kx →∞

The observation indicates that at small ω, the SPP dispersion curve approaches that

of the light line for the dielectric medium (norg). For a large wavevector, the frequency

approaches the surface plasmon frequency, ωspp. This observation is consistent with

the actual dispersion curve shown in Figure 1.6 solved for the interface between alu-

minum and a dielectric with �d = n2org. The plasma frequency of aluminum was taken

from literature to be h̄ωp = 14 eV[16]. The part of the solution that lies below the

light lines indicates that the metal-dielectric interface supports evanescent waves (i.e.

SPP). We see that for 400 nm light, a coupling to SPP takes place for large kx. It also

shows that above the bulk plasmon frequency ωp, the metal becomes transparent, and

hence the solution lies above the light lines. We have assumed an ideal lossless metal

and a dielectric but real materials can have complex dielectric functions. Accounting

for the loss leads to a qualitatively similar result but includes attenuation in x.

It should be noted that SPP propagates with a large in-plane wavevector due to

its evanescent nature. That is, propagating (or far-field) waves in any of the media in

the given OLED structure, cannot excite SPP since their kx is always smaller than the

kx required for SPP. This means that SPP cannot be excited with external sources

such as lasers or lamps without a coupling mechanism such as a prism. However,

SPP can be excited in OLEDs because the OLED emitters are located within tens of

12

nanometers away from the metal surface. Due to the proximity between the emitters

and the metal surface, SPP can be excited via near-field coupling. In this near-field

regime, the OLED emitters may radiate with arbitrarily large kx and hence can couple

to SPP. Once excited, the SPP mode is unable to couple to far-field radiation due to

wavevector mismatch and results in reduced outcoupled light.

1.2.1 Analysis of OLED efficiency

We have shown that OLED emitters in a stratified structure radiate power into various

modes that do not result in far-field radiation in air. The structure inherently traps

light in the high-index media and prevents outcoupling. This limited outcoupling

efficiency is the bottleneck for achieving high EQE in today’s OLEDs. The EQE is a

composite effect of various electrical and optical phenomena in an OLED[5].

EQE = γ η STηeff ηout (1.10)

The first term γ represents electrical efficiency, or the efficiency at which injected

charges form excitons in a single layer, single emitter EML. Typically, a near unity

electrical efficiency can be achieved by doping transport layers and/or by using charge

blocking layers as recombination is a bimolecular process requiring balanced injection

and transport of electrons and holes. In an OLED with multiple EMLs (or emitters)

to achieve broadband emission spectrum, excitons are distributed among the EMLs.

In such multi-EML OLEDs, each EML may have a distinct exciton capture rate

depending on its location in the device stack. Modeling multi-layer, multi-emitter

OLEDs requires extracting the exciton capture rate of each emitter[17]. The discussion

in this work assumes a single layer, single emitter OLED with γ = 1. The second

term, ηST , describes the spin selection rule for singlet and triplet excitons that can

radiatively recombine. We assume that this value is 25% for fluorescent emitters, and

13

100% for phosphorescent emitters. The third term, ηeff , is the effective IQE of an

emitter placed in a stratified medium. This differs from the IQE of the same emitter

radiating in free space by the Purcell factor, F whose discussion will follow later.

This is because in a stratified environment such as in an OLED, there is a secondary

field doing work on the source emitter via reflection from interfaces. Generally, if

this reflected field in a particular cavity structure has an interference maximum at

the emitter location, it will increase the emitter decay rate (i.e. resonance). We note

that the intrinsic radiative decay rate, Γr, is modified by F . The non-radiative decay

rate is given by Γnr and is assumed to be small for the emitters of our interest. The

effective IQE is then given by the ratio of the radiative decay rate and the total decay

rate including non-radiative losses.

ηeff =F · Γr

F · Γr + Γnr(1.11)

For very efficient emitters with vanishingly small non-radiative losses (i.e. Γnr → 0,

Γr → 1), the effective IQE approaches 1. This makes intuitive sense as an emitter

with zero non-radiative losses will radiate with 100% efficiency in free space or in a

cavity, hence the effective IQE is unaffected by F . We assume that the phosphorescent

emitters that we will use in this work have 100% intrinsic IQE. Then the expression

for the EQE for a charge balanced phosphorescent OLED with 100% IQE depends

solely on the outcoupling efficiency of the structure.

1.2.2 Dipole radiation near interfaces

In this section, we characterize the OLED emission pattern by studying a dipole emit-

ter near interfaces. An OLED emitter is an oscillating point charge, ~p = pe−iωtδ(~r) p̂,

and generates current density, ~J = d~pdt

= −iω~p. The electric field generated by such

a point source in free space has been established in many early works[18]. The cross-

14

sections of the power emitted by the dipoles with three possible orientations are shown

in Figure 1.8. Each dipole radiates most strongly in the direction perpendicular to its

axis. It is clear why vertically oriented emitters suffer from low outcoupling efficiency,

as it preferentially emits in the plane.

Figure 1.8: Power emitted by a a) vertical dipole (TM), b) horizontal dipole (TM),and c) horizontal dipole (TE).

The total power radiated by a resistive or dissipative dipole is given by the Poynt-

ing flux or its own electric field doing work on the dipole.

Prad =1

2

∫S

Re[ ~E × ~H∗] dS

= −12

∫V

Re[ ~E · ~J∗] dV(1.12)

Since our current source is a point or delta source, the second integral in Equation

1.12 simply reduces to Equation 1.13. The power radiated by a dipole is proportional

to the electric field at the source location (r0) in the direction of the dipole moment,

thus doing work on it.

Prad = −1

2Re[ | ~J | ~Ep̂(r0)] (1.13)

For a source near interfaces such as our OLED, secondary fields ( ~Es) upon reflec-

tion can be added to the total field at the dipole location, ~E(r0) = ~E0(r0) + ~Es(r0)

where ~E0 is the electric field generated by a dipole in free space. Therefore, calcula-

15

tion of the total field at the emitter location is our main task for solving for the power

radiated by a dipole in a stratified medium such as the one in Figure 1.9. In short,

Fresnel reflection and transmission coefficients are computed at each interface, and

by using the transfer matrix method[19], ~Es can be computed at the dipole location.

Figure 1.9: Dipole emitter in medium ne with its primary field and secondary fieldsresulting from reflections from neighboring media ne−1 and ne+1.

The electric field of an oscillating dipole source, ~E, can be written as a Fourier

integral over the normalized in-plane wavevector, u = kxk

= sinθem[20]. For interested

readers, rigorous formulations can be found in literature[20–22]. In this formalism, the

dipole emission can be represented as a set of plane waves propagating in ẑ, and this

allows us to characterize it in the manner similar to the approach we took in section

1.2. Equation 1.13 can be rewritten as an integral of power spectral density per u per

wavelength, K(λ, u). In this representation, we can rewrite the power emitted into

each in-plane wavevector u.

Prad(λ) =

∫ ∞0

uK(λ, u)du (1.14)

Note that K(λ, u) depends on dipole orientation and polarization since the radiated

power (Equation 1.13) depends on the direction of the electric field parallel to the

dipole moment. Following the notations in the work of Furno et al., we decompose K

into horizontal (“h”) and vertical (“v”) dipoles[23]. Note that horizontal dipoles can

16

assume both TE and TM polarizations.

KTM,v =3

4Re[

u2√1− u2

(1 + a+TM)(1 + a−TM)

1− aTM]

KTM,h =3

8Re[√

1− u2 (1− a+TM)(1− a

−TM)

1− aTM]

KTE,h =3

8Re[

1√1− u2

(1 + a+TE)(1 + a−TE)

1− aTE]

(1.15)

Where,

a+TM,TE = r+TM,TE e

2ikzd+

a−TM,TE = r−TM,TE e

2ikzd−

aTM,TE = a+TM,TEa

−TM,TE

(1.16)

A close observation at Equation 1.15 reveals that the numerators of the K ′s refer

to wide-angle interference in which the secondary field interferes with the primary

field. The denominators refer to Fabry-Perot interference that depends on the phase

accumulated over multiple reflections by the same beam[24]. This is illustrated in Fig-

ure 1.10. For a bottom-emitting OLED where only one of the electrodes is strongly

reflective and the other is transparent, the effect of wide-angle interference is dom-

inant. This means that the distance between the emitter and the strong reflective

cathode has a large influence on the microcavity environment in bottom-emitting

OLEDs.

Figure 1.10: a) Wide-angle interference, and b) Fabry-Perot interference.

17

For a dipole with an arbitrary orientation in both horizontal and vertical direc-

tions, the power spectral density is then the sum of the two scaled by the anisotropy

factor, a. An isotropic emitter has a = 13, and a perfectly horizontal emitter has

a = 0.

K = aKv + (1− a)Kh = aKTM,v + (1− a)(KTM,h +KTE,h) (1.17)

Finally, the outcoupling efficiency can be written as the ratio of the power radiated

into air and the total power radiated into all in-plane wavevectors. Recall that only

a small set of kx is within the light line for propgation in air (Figure.1.6). This

condition can be determined by calculating the critical angle between air and the

emitting medium (i.e. ucrit = sinθcrit,air =nairnem

).

ηout(λ) =Pair(λ)

Prad(λ)=

∫ ucrit0

uK(λ, u)du∫∞0uK(λ, u)du

(1.18)

The power radiated into each mode (air, substrate, waveguide) can be obtained by

setting the upper limit on the numerator integral in Equation 1.14 as the maximum

allowed u, or ninem

where ni = nair, nsub, norg. The power radiated into SPP corresponds

to large in-plane wavevectors, u > 1 (evanescent in ẑ). This will be revisited in

Chapter 2 with an example. We also note that the Purcell factor, F , is defined as

Prad normalized by the power that would be radiated by the dipole in free space.

1.2.3 Previous work on OLED outcoupling

We have identified that the prescence of a microcavity surrounding OLED emitters

limits efficient outcoupling of photons to air. We also noted that outcoupling ef-

ficiency is the determining factor of EQE as long as the OLED emitter has very

small non-radiative losses. As a result, various strategies have been shown to out-

18

couple more light from OLEDs. Some improvement is possible for planar OLEDs by

modifying the optical environment; this includes the use of low-index ETLs which

can reduce waveguiding and SPP[25]. Engineering emitter orientations such that the

emitters grow with a preferentially horizontal orientation has also shown a lot of

promise[26,27]. Without any modification to the planar structure, these preferentially

horizontal emitters have higher outcoupling efficiency (∼30%) compared to isotropic

emitters (∼20%).

Outcoupling efficiency can be further enhanced by adopting non-planar structures.

For example, substrate trapped light can be recovered by attaching a hemispherical

lens index matched to the substrate[28]. In this way, the rounded lens edge can be used

to reduce the incident angle, thus preventing TIR. However, this is only effective when

the hemispherical lens is much larger than the OLED making it bulky and impractical.

Mladenovski et al. demonstrated that even higher outcoupling efficiency is possible if

the hemispherical lens is combined with a high-index substrate (n ∼1.8, sapphire)[29].

Compared to the glass+lens OLED, the sapphire+lens OLED is expected to have

reduced waveguided loss as the index contrast between the organic/ITO stack and

the substrate is reduced and the substrate is optically incoherent.

A more elegant solution to extracting substrate loss includes the use of microlens

arrays attached to the substrate backside. Various arrays of shapes (pyramidal, hemi-

spherical, hexagonal, etc) and sizes can be created with lithography[30,31]. Microlenses

are also compatible with flexible substrates unlike macrolenses. Other outcoupling

mechanisms include modification of substrates by incorporating scattering structures,

such as nanoparticles[32] or roughness-induced scattering surfaces[33].

The aforementioned nanostructures extract substrate losses by external modifica-

tion of the substrates, and therefore these outcoupling nanostructures do not affect

the electrical performance of the OLEDs. On the other hand, extraction of waveguid-

ing and SPP tends to be more challenging because texturing thin film organic/TCE

19

stack can modify the optical as well as electrical properties of the stack. The inter-

nal modes (waveguiding and SPP) are usually extracted by introducing periodic or

irregular corrugations to the thin-film stack[34,35]. Propagated corrugation is believed

to increase chances of electrical shorts however, as many OLEDs have active layers

with thicknesses (typically

Commercially available white LED bulbs and WOLED panels currently have luminous

efficacies of ∼170 lm/W2 and ∼61 lm/W3, respectively. The discussion of luminous

efficacy will be presented in Chapter 2. If WOLEDs can be produced at lower costs,

Figure 1.11: Luminous efficacy of lighting elements and projections for LEDs andOLEDs. Source: DOE (2012b, p. 38).

they have a real potential for offering higher-efficiency and light-weight alternatives

to current lighting elements. Large area OLED lighting panels published in the 2016

DOE report[40] have already exceeded 50 lm/W, as shown in Table 1.2. The first

flexible white OLED panel is now commerically available from OLEDWorks4. These

flexible WOLEDs are reported to be fabricated on Corning’s 100 µm-thick flexible

2MAS LEDtube 1500 mm UE 21.5 W 840 T8. Available from Philips.3Lumiblade Bright 3. Available from OLEDWorks.4LumiCurve. Available from OLEDWorks.

21

LG Display N6S OLEDWorksFL300

Kaneka Corp.

Panel size 32×32 cm2 5×20 cm2 10×10 cm2Correlated colortemperature

3000 K, 4000 K 2500 K, 2900 K 3000 K 4000 K

Luminous effi-cacy

55-60 lm/W 42-50 lm/W 29-40 lm/W

Table 1.2: Performance of white OLEDs published in the DOE SSL report in 2016[40].

glass for substrates and encapsulation. The company claims that their flexible white

OLEDs offer power efficiencies upwards of 60 lm/W. Despite the continued growth,

WOLEDs have yet to become mainstream, and more work remains to be done to

produce higher-efficiency devices via outcoupling while keeping the processing costs

low. In Chapters 2 and 3, we will discuss our approach to enable a scalable outcoupling

mechanism in flexible WOLEDs for lighting applications.

Other emerging applications using OLEDs include wearable devices such as wear-

able displays[41], medical sensors[42], textiles, and garments. These mechanically flexi-

ble and thin film devices can conform to surfaces such as skin and be worn comfortably

without generating appreciable heat.

The electronics market is changing fast, presenting new and improved technology

annually, and this turns many functioning devices obsolete. This consumption cy-

cle now has turned electronic waste (E-waste) into the fastest-growing waste stream;

most E-waste ends up in landfills[43]. The emergence of inexpensive plastic electronics

is worrisome as plastic materials have extremely poor recycling efficiency. This is be-

cause most plastics today cannot be broken down to pristine monomers that can be

repolymerized into a new product. Typically, a lower-quality recycled material is pro-

duced by mechanically and thermally treating used plastics as these processes result

in lower molecular weight products[44]. Thermoplastics such as polyethylene (PE),

polypropylene (PP), and polyvinyl chloride (PVC) can be melted by heating, and

22

they can be remoulded into another shape. Thermoset plastics such as polyimides,

polyurethanes, and polyester resin undergo irreversible crosslinking processes, and

they typically decompose before melting. Thermoset plastics can therefore be reused

as filler materials after being broken down by grinding or milling. The presence of

impurities such as additives and contaminants also lower the recycling efficiency. In

this current recycling landscape, some estimate that less than 10% of plastics get

recycled[45]. Alternatively, plastics can be incinerated to avoid being landfilled and

recover industrial gases and oil mixtures that can be used to produce fuels[44]. The

heat generated as a by-product of incineration may also be used to drive turbines.

Figure 1.12: Photograph of electronic waste in Fresno in May 2019. Source: photoby Christie Hemm Klok for TIME[43].

Chemical recycling is believed to be a more effective recycling mechanism as it

chemically breaks down a polymer into its monomers by performing chemical reac-

tions at ambient or moderate temperatures. This allows for pristine monomers to

be recovered without much loss in material properties. As such, novel polymers are

being synthesized to exhibit efficient depolymerization chemistry that can turn them

into pure monomers[46–48]. Particularly, low ceiling temperature (Tc) polymers are

gaining attention because they can readily depolymerize at ambient temperatures

23

with specific triggers such as acid. Therefore, this class of polymers are emerging

plastic materials with superior recyclability as the recovery of pristine monomers is

facilitated. In addition to monomer recovery, these polymers are being used as “tran-

sient” materials that can be reintegrated back to the environment without producing

solid waste. For example, transience may be achieved by first triggering a low Tc

polymer to depolymerize into monomers, which can then vaporize. Other modes of

transience include liquification and dissolution in solvents.

In Chapter 4, we will discuss how OLEDs can be fabricated on a transient polymer

substrate. Then, the OLEDs will serve as a triggering mechanism to depolymerize the

supporting substrate. Our work was the first in literature that realized integration of

the triggering mechanism (OLED, in this case) and the transient polymer. Such inte-

grated devices may pave a way to developing compact systems that can be deployed

to remote environments or biological media where transience of obsolete devices is

desired.

24

Chapter 2

Experimental determination of

OLED efficiency and comparison

with an electromagnetic model

2.1 Measurement and characterization of OLEDs

Here, we introduce several terms and definitions that are used to evaluate perfor-

mance of OLEDs, LEDs, and other devices that output light to interact with human

eyes. Instead of using standard radiometric quantities to measure absolute radiant

energy (in units of Watt, Joule, etc), the human perception of the radiant energy is

taken into account as a weighting factor. This practice is known as photometry, and

the conversion between radiometric and photometric quantities is achieved by using

the photopic sensitivity function as a weighting function. Shown in Figure 2.1, the

photopic sensitivity function is an approximation of the average human vision that

describes the relative perceived brightness of visible wavelengths of the same radio-

metric power (Φrad). This means that 1 W of monochromatic light emitting at 555

nm will appear twice as bright as 1 W of 510 nm monochromatic light.

25

Figure 2.1: Photopic sensitivity function.

The measurement geometry for our OLEDs involves an area source OLED and a

photodiode rotating about the OLED to measure the radiant intensity. The photodi-

ode rotates about the OLED from polar angles θ =0° to 90°, and we assume that the

OLED emission is independent of the azimuthal angle, φ. In this setup, the radiant

flux from a Lambertian source received by the photodiode will decrease as a cosine

of the polar angle because the projected area of the OLED decreases by the same

amount. The Lambertian source has uniform radiance however, since radiance (in

unit of W sr−1 m−2) is defined as the radiant flux emitted by a surface per unit pro-

jected area per unit solid angle. The expression for radiance in our setup is defined

in Equation 2.1 where ΦPD(θ) is flux received by the photodiode as a function of

polar angle, and AOLED is the OLED area. Note that the human perception of the

Lambertian source is also uniform as a function of angle as our eyes adjust for the

decreasing projected area with angle.

Lrad(θ) =ΦPD(θ)

AOLEDcos(θ)dΩ(2.1)

26

Figure 2.2: a) Measurement geometry for OLEDs in this work, and b) simplifiedgeometry independent of azimuthal angle.

The radiance of an OLED is often converted to and reported as luminance, a

photometric quantity in units of lumen (lm) obtained by the following formula where

ρ(λ) is the normalized spectrum of the OLED. In this way, the wavelengths that have

greater perceived brightness will result in larger luminance as the OLED spectrum is

normalized by the photopic responsivity function. In other words, 1 W of monochro-

matic light with an emission wavelength of 555 nm has 683 lm of power in photopic

unit.

Lphotopic(θ) = 683lm

WLrad(θ)

∫ρ(λ, θ)f(λ)dλ (2.2)

Another quantity that becomes especially relevant for evaluating the performance of

white OLEDs is correlated color temperature (CCT). Understanding of CCT requires

discussion of color spaces; in particular, we will discuss the first CIE color space

(CIE 1931). The CIE 1931 color space was created by the international commission

of illumination (CIE) in a series of experiments performed on human subjects in

the 1920s. In short, the human subjects were asked to recreate the colors of single

wavelengths in the visible spectrum by tuning the intensities of three primary-colored

laser sources (436 nm, 546 nm, and 700 nm in Smith’s work[49]). The study found

that most single wavelengths were perceptually equivalent to mixtures of the primary

colors except for the wavelengths in the 450-530 nm range (bright greens and cyan).

More details can be found in the original studies[49,50], and here we state some of its

27

findings that will become useful in later chapters. The outer line of the CIE 1931

color space represents a series of pure (or saturated, or single wavelength) colors,

and the inside of the shape is populated with spectrally broad colors that can be

created by mixing more than one colors. The x, y coordinates on the CIE 1931 of

any spectrum can be determined by using empirically obtained weighting functions,

known as color matching functions. The use of the weighting functions represents the

limits of human color perception in which two different spectra can appear the same.

Figure 2.3: The chemical structures of the iridium phosphorescent emitters used inthis work. The peak PL emission wavelengths are λPL.

Three iridium emitters (Ir(MDQ)2acac, Ir(ppy)2acac, and FIrpic) are commercially

obtained and used in the OLEDs in this work. Their molecular structures are shown

in Figure 2.3. The CIE 1931 x and y coordinates of these iridium phosphorescent

emitters are plotted in Figure 2.4. The triangle formed by the three coordinates

represent all possible colors that can be created by mixing the three emitters at the

vertices. Also plotted in the figure is the Planckian locus, whose coordinates have

been obtained by taking the spectra of an ideal blackbody radiator at temperature,

Tc(K). For a near-Planckian source, a correlated color temperature (CCT) can be

assigned in which the non-Planckian source is perceptually equivalent to the ideal

28

Planckian source[51]. In colloquial terms, the CCT tells a consumer whether a light

source is “warm” or“cool” white.

Figure 2.4: The CIE 1931 color map with the x, y coordinates of three iridium emit-ters.

Returning to our earlier discussion, there are several characterization methods to

evaluate the performance of an OLED. The most widely used performance metric for

an OLED include external quantum efficiency and power efficiency (PE). The EQE

of an OLED is defined as the ratio of outcoupled photons and injected electrons.

The calculation of this quantity requires electrical characterization (current-voltage

relationship) as well as measurement of the generated light that can be collected ex-

ternally from the device (i.e. outcoupled light). The PE is equivalent to the luminous

efficacy discussed in Chapter 1.

For a bottom-emitting OLED, the outcoupled light is measured along the hemi-

sphere surrounding the OLED through the backside of the transparent substrate.

In our experimental setup, a photodiode and a fiber optic cable are mounted on a

29

goniometer to rotate around an OLED to capture the outcoupled power and the out-

coupled spectrum, respectively. We assume that the OLED is homogeneous across its

active area and that its emission characteristic is independent of the azimuthal angle.

Therefore, only several measurements are taken for angles 0 to 90°.

A calibrated photodiode exhibits a near-linear relationship between optical power

impinging on the photodiode and the output current. This relationship is given by the

photodiode responsivity curve, R(λ). The measured photodiode current represents

the cumulative response of the photodiode due to a spectrally broad source; this

quantity alone is independent of emission wavelength. To calculate the contribution

to the photodiode current from each wavelength present in the emitter spectrum, the

spectrum of the source ρ(λ) and the photodiode responsivity curve must be taken into

account. The total power received by the photodiode, accounting for the wavelength-

dependent photodiode responsivity, is given by:

ΦPD(θ) =iPD(θ)∫

ρ(λ, θ)R(λ)dλ(2.3)

To calculate the total flux of photons impinging on a hemispherical surface en-

closing our OLED, we need a differential photodiode power per solid angle. The

relationship can be derived by considering the solid angle that the OLED makes with

the photodiode. Since the measurement is taken by the photodiode with a finite area,

the solid angle that the photodiode makes with the OLED is approximatelyApdr2

. For

a small photodiode area or a large separation, this is a good approximation for the

true solid angle, which requires the area on a curved spherical surface. Then the

differential photodiode power per unit solid angle is given as follows.

dΦPD(θ, φ) =iPD(θ)∫

ρ(λ, θ)R(λ)dλ

r2

APD(2.4)

30

Then the total power received by a hemisphere can be given by integrating the above

expression over the hemisphere. The number of photons per solid angle is then given

by considering the energy of a photon (E = hcλ

) and the emission spectrum.

dn(θ, φ) = dΦPD(θ, φ)

∫dλρ(λ, θ)λ

hc(2.5)

The expression for EQE in terms of empirical quantities is then given by integrat-

ing the above expression over all solid angles and dividing by the number of injected

charges, Ie.

EQE(%) =

∫ 2π0dφ

∫ π2

0sin(θ)dθdn(θ, φ)

Ie

× 100 (2.6)

The expression for EQE can be obtained entirely using radiometric measurements,

but power efficiency is defined in photometric terms which includes the spectral sen-

sitivity of the average human vision. The power efficiency of an OLED is defined

as the total luminance integrated over a hemisphere divided by the electrical power

consumed by the OLED. It has the following form in the unit of lm/W.

PE =683 lm

WAOLED

∫ΩLrad(θ)

∫f(λ)ρ(λ, θ)dλ

IV(2.7)

An ideal monochromatic green OLED emitting at 555 nm can have a luminous efficacy

or PE of at most 683 lm/W, assuming absolutely zero loss. Similarly, theoretical limits

for white OLEDs are 260-300 lm/W, depending on their CCT[52].

2.1.1 Comparison of model with experiment

The electromagnetic model of the OLED emission in stratified media was discussed

in Chapter 1. In this section, we apply the model to the device structure in Figure

2.5. The refractive index measured by ellipsometry is plotted for each thin film

31

in the stack. The small, but nonzero extinction coefficients of the layers were also

measured by ellipsometry and were accounted for in the simulation although they

are not presented here. The photoluminescence spectrum of Ir(ppy)3 is used as an

input to the simulation. The emitter Ir(ppy)3 has been documented to be an isotropic

Figure 2.5: The structure of the OLED (left), and the refractive index and PL mea-surements (right).

emitter when thermally evaporated, with ∼100% internal quantum efficiency. The

emitter orientation is thus accounted for by assigning an anisotropy factor a of 13.

This allows us to understand selective polarization-dependent coupling to SPP and

waveguide modes; it should be noted that SPP coupling (u > 1) only exists for TM-

polarized emission and the two waveguide peaks (0.87< u

Figure 2.6: Power radiated into each wavevector at emission peak (510 nm) for threepossible dipole orientations and corresponding polarizations.

Chapter 1. Here, the light lines are obtained with respect to the emission wavelength.

λ =2π

kx

ninem

where i = air, sub, org (2.8)

Two waveguiding modes and strong SPP coupling can be identified for all wavelengths

of interest. In Table 2.1, the relative powers radiated into the four different modes are

summarized. The transmission of the substrate trapped light back into the organic

stack is considered absorbed.

Some of the simulated results can be compared to the experimental result. The

OLED with the device structure in Figure 2.5 was fabricated and tested for perfor-

mance in an experiment. The portion air mode simulated (8.98%) is equivalent to

the measured EQE for the emitter with near-unity internal quantum efficiency, and

33

Figure 2.7: Natural log of power spectral density, uK, plotted over wavelength andin-plane wavevector, kx.

air substrate waveguide SPP absorption

Portionpower (%)

8.98 47.02 15.01 20.19 8.79

u 0-0.573 0.573-0.87 0.87-1 1-1.2 N/A

Table 2.1: Relative powers radiated into each mode.

we observe that the measured EQE peaks around 8.9% showing good agreement with

the simulation. In addition, the measured angle-dependent spectrum can be com-

pared with the simulation. Typically, planar OLEDs exhibit outcoupled spectrum

that blueshifts with viewing angle; this is because the path length depends on both

wavelength and viewing angle. In this case, shorter wavelengths are being preferen-

tially outcoupled at large viewing angles. The measured spectrum at two extreme

34

viewing angles (0° and 80°) are plotted against the simulated spectra, and it can be

seen that the blueshift of the spectrum with increasing viewing angle can be effectively

predicted.

Figure 2.8: Comparison of measured and simulated spectra at two viewing angles.

The validity of the simulation can be further verified by fabricating a set of OLEDs

with increasing ETL thickness, x. The structure of the OLEDs fabricated in this

experiment is shown in Figure.2.9a.

The simulation (Figure 2.9b) predicts an oscillatory outcoupling efficiency as a func-

tion of ETL thickness. This trend is confirmed by the measured EQEs at various

ETL thicknesses.

It should be noted that the devices with large ETL thicknesses above 100 nm

are nontrivially realized by doping the ETL with a n-type dopant[3]. The doping

efficiency is evident in the current density (J) vs. voltage (V ) data in Figure 2.10 in

which the current density is virtually independent of the ETL thickness. Without the

doped ETL, thick ETLs will cause the current density to decrease more significantly

due to increased series resistance associated with the large ETL thickness. This will

35

Figure 2.9: a) Device structure, and b) portion powers simulated to reside in eachmode. The white dots are experimentally measured EQEs at the corresponding ETLthickness.

Figure 2.10: a) The J − V − L curves, and b) EQE vs J curves of the OLEDs inFigure 2.9a.

not only prevent efficient and balanced charge injection into the emissive layer but

also prevent the OLED from achieving high luminance due to limited current through

the device. However, in our device results, the nearly identical J − V characteristic

confirms that the increase in the device thickness does not increase series resistance.

It also allows us to determine that the varying luminance at each ETL thickness is

the result of an oscillating outcoupling efficiency. This determination is not possible

36

for the devices with thick, undoped ETLs as thicker devices will suffer from hindered

electron injection and transport, resulting in resistive and poorly charge-balanced

devices.

2.2 Increasing outcoupling efficiency via substrate

modification

As seen in Chapter 1, substrate loss due to the index mismatch at substrate-air inter-

face presents a significant challenge in developing high-efficiency OLEDs. Thankfully,

there are many outcoupling strategies for extracting substrate loss. This typically in-

volves incorporating scattering particles in the substrate to modify the incident angle

arriving at the interface with air. In 2015, we presented a scalable approach to ex-

tract susbtrate loss by introducing a scattering film on the backside of a glass/ITO

substrate[53]. The scattering polymer comprised of a high-index Kapton® polyimide

(Dupont, referred to as Kapton hereafter) film with embedded air voids that presents

a large index contrast, leading to an optically hazy film. The aim of the scattering

film was twofold. First, it should be fabricated with a low-cost and scalable process

and set of materials for large area lighting applications. Second, the outcoupling

enhancement should be broadband and demonstrated for both monochromatic and

white OLEDs. In this Chapter, we show outcoupling enhancement for rigid glass/ITO

OLEDs assisted with the porous scattering films. In Chapter 3, we will apply this

technique to flexible OLEDs.

37

2.2.1 Preparation and characterization of porous scattering

films

Here, we introduce the fabrication method for producing the aforementioned porous

polymer scattering films. We also carried out optical characterization to assess their

applicability to broadband WOLEDs. Finally we will present the outcoupling en-

hancement obtained by applying the porous scattering films to a glass/ITO OLED.

Figure 2.11: a) Fabrication of a porous polyimide film by phase inversion. b) Dynamicformation of pores in the polyimide film. Reprinted (adapted) with permission from[53]. Copyright (2015) American Chemical Society.

The porous polyimide films are produced via immersion precipitation in which a

film cast from the precursor polyamic acid (polymer and solvent) is introduced to a

coagulation bath containing a nonsolvent. Nonsolvent-induced phase inversion and

solvent-nonsolvent exchange take place, and the polymer solidifies to a membrane-

like porous film[54]. The air voids formed spontaneously in this way are randomly

distributed within the polyimide host, giving rise to broadband haze. In addition,

Go et al. has shown that the choice of nonsolvent has a significant impact on the

morphology and thickness of the resulting porous polymer film, and hence its outcou-

pling efficiency[55]. In this work, we use deionized water as our nonsolvent which is

both scalable and highly miscible with our choice of solvent (N-methyl-2-pyrrolidone,

38

NMP), thus accelerating the immersion precipitation (or phase inversion) process.

The schematic of the phase inversion process is shown in Figure 2.11.

Figure 2.12: (a) Refractive index (n) and extinction coefficient (k) of the Kaptonpolyimide. (b) Transmission (T ) and absorption (A) spectra of an 870 nm thickplain polyimide film and the intrinsic photoluminescence spectra of blue (FIrpic),green (Ir(ppy)3), and red (Ir(dmpq)2(acac)) phosphorescent dopants for the OLEDsemployed in this work. (c) Measured total and diffuse transmission and calculatedhaze spectra for the p-PI scattering layer. (d) Top-down confocal microscopy image(the inset is a photograph of the p-PI layer on a glass substrate), (e) top-down SEMimage, and (f) cross-sectional SEM image of the p-PI scattering layer. Reprinted(adapted) with permission from [53]. Copyright (2015) American Chemical Society.

The porous polymer (p-PI or pPI) film obtained via immersion precipitation takes

advantage of the large index contrast between itself (in this case, Kapton with n >1.7)

39

and nonabsorbing air pockets. The refractive index and extinction coefficient of a con-

trol, pristine Kapton film without pores is measured by ellipsometry and is presented

in Figure 2.12a. As a result, the porous Kapton film exhibits high haze across the

full visible spectrum (Figure 2.12c) and appears visibly hazy (Figure 2.12d). The

microstructures of the non-conductive porous Kapton film can be studied via envi-

ronmental scanning electron microscopy (ESEM). The top-down microscopy images

(Figure 2.12e) show circular pores ranging from nanometers to several microns, which

together with the large index contrast between the Kapton film and voids, accounts

for the large broadband haze. The cross-sectional ESEM image (Figure 2.12f) of

the porous Kapton film shows that the circular pores from the top-down view are

columnar.

Air/EQE Substrate Waveguide SPP Absorption

Simulatedpower (%)

21.1 31.5 15.6 27.2 4.6

ControlOLED (%)

18.2 N/A N/A N/A N/A

OLED+pPI(%)

30 N/A N/A N/A N/A

Table 2.2: Portion powers simulated for a control green OLED and measured peakEQEs of the OLEDs with and without a porous Kapton film. Internal modes (waveg-uiding, SPP, and absorption) could not be measured experimentally, thus some valuesare not available (N/A). Reprinted (adapted) with permission from [53]. Copyright(2015) American Chemical Society.

In addition, we characterized the optical properties of the control Kapton film

prepared without the immersion precipitation step. The total transmission through

the control Kapton film as shown in Figure 2.12b is about 90% in the wavelength

range of the OLED emitter spectra and shows minimal absorption in the visible.

The absorption is especially negligible for thin films (< 1µm); in this particular

case, the control Kapton film was 870 nm, which increases to 2 µm when porosity

is introduced. In Chapter 3, we will address the small but nonzero absorption in

40

the short wavelengths; this will allow us to increase the polyimide thickness without

drastically increasing the absorption in the shorter wavelengths.

We applied the OLED model to the control structure without the porous Kapton

film (Figure 2.13a) to estimate the portion power residing in each mode. Table 2.2

summarizes the simulation results.

Figure 2.13: a) Structure of green OLEDs with and without a porous Kapton film,b) current density-luminance-voltage (J − L − V ) characteristics of the OLEDs, c)EQE vs J curves, and d) power efficiency of the OLEDs. Reprinted (adapted) withpermission from [53]. Copyright (2015) American Chemical Society.

Finally, the outcoupling efficiency of the porous Kapton film was experimentally

measured by fabricating green OLEDs on glass/ITO substrates with and without the

scattering film. The application of the porous Kapton film is external to the OLEDs,

so the J − V curves of the control device and the device with the porous Kapton

film are identical. The luminance increases significantly for the green OLED with

41

the porous Kapton film, resulting in a 65% enhancement in EQE. The peak EQE of

the control green OLED is 18.2% which is slightly lower than the simulated 21.1%

air mode (Table 2.2); this could be due to imperfect electrical efficiency and/or less-

than-unity internal quantum efficiency of the emitter. The increase of 11.8% in EQE

upon introducing the porous Kapton scattering film to the control OLED amounts

for approximatley 44% of the simulated substrate loss (31.5%), indicating that the

porous Kapton film can extract nearly half of the substrate loss for this particular

device structure.

Figure 2.14: a) Angle-dependent emission intensity, and b) angle-dependent spectrafor the green OLEDs with and without a porous scattering layer. Reprinted (adapted)with permission from [53]. Copyright (2015) American Chemical Society.

Angle-dependent measurements provide additional insights about the OLED cav-

ity environment. In Figure 2.14a, we show the emission intensity profiles as a function

of viewing angle for the control green OLED, pPI-assisted green OLED, and a Lam-

bertian surface. The emission pattern of the control OLED is significantly wider than

that of a Lambertian surface, which indicates that this particular OLED microcavity

preferentially outcouples in off-normal directions. With the application of the porous

polyimide film on the glass substrate backside, the emission profile becomes closer to

the Lambertian profile. This means that the strongly angle-dependent OLED emis-

42

sion pattern incident on the glass-air interface is scattered by the porous polyimide

film and redirected to resemble that of an ideal diffuse surface, a Lambertian surface.

Similarly, the angle-dependent spectra of the control OLED and the pPI-assisted

OLED are shown in Figure 2.14b. The spectra of the control OLED exhibit flattened

peaks, as opposed to the spectra of the pPI-assisted OLED which show a prominent

peak around 510 nm and a shoulder at 550 nm. The pPI-assisted spectra resemble

that of the photoluminescence spectrum of Ir(ppy)3 in solution (i.e. free space), hence

free of microcavity effects. In a strong microcavity such as the control OLED, the first

peak at 510 nm is suppressed, compared to the shoulder at 550 nm. The first peak

at 510 nm is recovered upon application of the pPI film, as shown in Figure 2.14b

(right). This means that the control OLED microcavity preferentially traps shorter

wavelengths in the substrate, and the pPI film is able to outcouple such trapped light

by scattering.

To demonstrate the efficacy of the pPI films for broadband applications, we re-

peated the experiment for white OLEDs. The structure of the WOLEDs is shown in

Figure 2.15 in which three distinct emissive layers were used to produce broadband

spectra. Note that the presence of multiple emitters distributed along the device

stack prevents us from using the model we applied to the green OLED. Without

the knowledge of the relative exciton capture efficiency by each emitter, the results

presented here are limited to empirical observations.

The J−V characteristics of the white OLEDs remain unchanged with and without

the porous polyimide film; this is consistent with the observation we made with the

green OLEDs. The J −V −L plot in Figure 2.15b shows increased luminance for the

WOLED with the scattering film, resulting in 60% enhancement in EQE. The EQE

of the control WOLED (11.9%) is increased to 19% when the porous polyimide film

is applied to the glass substrate backside. The PE at 100 cd m−2 was 18 lm W−1 for

the control WOLED, and 32 lm W−1 for the WOLED with the scattering film (Figure

43

2.15d). This corresponds to 78% enhancement in PE. We note that, in electrically

equivalent devices such as the OLEDs with and without the externally applied porous

polyimide films, the enhancement in PE is expected to be larger than that in EQE.

This is because the definition of PE takes into account the power (i.e. voltage and

current) while EQE only accounts for current. As a result, the outcoupling-enhanced

OLED is able to reach the same luminance at a lower voltage as well as at a lower

current density, resulting in a larger enhancement in PE.

Figure 2.15: a) Structure of white OLEDs with and without a porous Kapton film,b) current density-luminance-voltage (J −L− V ) characteristics of the WOLEDs, c)EQE vs J curves, and d) power efficiency of the OLEDs. Reprinted (adapted) withpermission from [53]. Copyright (2015) American Chemical Society.

Finally, the angle-dependent behavior of the WOLEDs is shown in Figure 2.16.

The initially broad emission intensity profile as a function of viewing angle becomes

narrower and closer to the Lambertian pattern with the application of the porous

44

Figure 2.16: a) Angle-dependent emission intensity, and b) angle-dependent spectrafor the white OLEDs with and without a porous scattering layer. Reprinted (adapted)with permission from [53]. Copyright (2015) American Chemical Society.

polyimide film. As we discussed earlier for the green OLEDs, this result indicates

that the control WOLED initially has a strongly directional emission pattern due to

the microcavity effects. The porous polyimide film is able to modify this pattern to a

Lambertian pattern which is characteristic of a diffuse scattering surface. The angle-

dependent spectra shown in Figure 2.16b evidence that larger spectral variations are

exhibited by the control WOLED. This is indicated by the arrows in Figure 2.16b

(left) that represent spectral narrowing and the enhanced blue and green peaks with

increasing angle. Such effects are negligible in the angle-dependent spectra of the

WOLED with the scattering film, which show virtually identical spectra irrespective

of viewing angle. This result is especially significant for WOLEDs intended for lighting

applications as ideal lighting elements should appear the same color when viewed from

various angles.

We thus far demonstrated an effective scattering mechanism to extract substrate

loss from rigid glass/ITO substrates. The pPI scattering films were externally applied

to OLEDs, and therefore it allowed us to study outcoupling enhancement in isolation

(i.e. without affecting J − V relationship). In addition, both monochromatic and

45

white OLEDs were realized to highlight the broadband scattering efficiency; the EQEs

of the green and white OLEDs were enhanced by 73% and 60%, respectively. In the

following chapter, we will discuss how the porous polyimide scattering film can be

modified for application in flexible OLEDs.

46

Chapter 3

Substrate light outcoupling in

flexible white OLEDs

3.1 Colorless polyimide-silver nanowire composite

as a flexible substrate for white OLEDs

In Chapter 2, we introduced an inexpensive and scalable process for extracting sub-

strate loss by using porous Kapton films. The amount of available substrate loss and

the portion of the substrate loss extracted were quantified by comparing the simu-

lated substrate loss and the measured EQEs. Despite the efficacy of the outcoupling

efficiency of the porous Kapton film, the application was limited to rigid glass/ITO

substrates, and the weak yet nonzero absorption of the Kapton films in the short

wavelengths resulted in a prominent yellow hue in the films. In this chapter, we will

improve on the previous work by integrating the porous scattering films to flexible

OLEDs and presenting an alternative polyimide with improved visible transmission.

The resulting substrates should be flexible and more transparent than Kapton, mak-

ing them particularly useful for flexible white OLEDs. To achieve this goal, the new

substrate material must be a robust host for a TCE with resistance to scratching,

47

rubbing, or mechanical bending that could take place during fabrication, handling,

and testing. Second, the new material should have negligible absorption across the

entire visible spectrum so that it can be cast as a film of substantial thickness for

handling around a laboratory. Usually this requires a thickness above 20 µm. Be-

low this thickness, the films are difficult to handle and are not mechanically robust.

Third, the material should become porous in a scalable manner to serve as a scat-

tering layer that is index-matched to a nonporous substrate. Figure 3.1 compares

the substrate light extraction mechanism presented in the previous chapter and the

proposed mechanism adopted on a flexible plastic platform.

Figure 3.1: a) A rigid glass/ITO substrate enhanced with a porous Kapton scatteringfilm, and b) proposed flexible scattering substrate with nanowire electrode, substrate,and scattering film. Reprinted from [56].

The elimination of the ITO electrode allows for improved mechanical flexibility

and improved processibility. We proposed the structure in Figure 3.1b in which the

high-index ITO thin film is replaced with metal nanowires, and the low-index glass

substrate is replaced with a high-index polymer. With this structure, a larger portion

of the generated light is trapped in the high-index substrate while waveguiding in

the high-index core is reduced. This is an intuitive result since more reflection at

the substrate-air interface takes place as the index of the substrate increases. The

simulation was performed for a green Ir(ppy)3 OLED on a glass/ITO substrate and

a high index (1.7 at 400 nm) substrate without an electrode as it is difficult to model

metal nanowires. For larger ETL thicknesses above 50 nm, the portion of substrate

48

loss in the high-index case becomes significantly larger compared to the glass/ITO

case. This means that we are starting with larger substrate trapped light available

for extraction. An ideal substrate loss extraction mechanism with 100% outcoupling

efficiency is expected to result in a higher efficiency device on a high-index substrate

compared to a low-index glass substrate.

Figure 3.2: Portion power simulated for a green Ir(ppy)3 OLED as a function ofETL thickness for a) glass/ITO substrate, and b) high-n (1.7) substrate without anelectrode.

The high-index transparent material we chose is colorless polyimide (CPI) and

adopted the same immersion precipitation principle to obtain porous scattering films,

similar to the case for Kapton. Colorless polyimide is an optically transparent mate-

rial that inherits the excellent thermal, mechanical, and chemical stability of aromatic

polyimides. Unlike conventional polyimides such as Kapton that absorb strongly in

the short visible wavelengths, CPIs have improved visible transmission by introducing

alicyclic[57] or semialicyclic[58] substituents or highly electronegative substituents[59] in

the synthesis. These substituents shift the absorption band edge of polyimides by in-

hibiting the formation of charge transfer complexes between the diamine (donor) and

the dianhydride (acceptor) moieties[60].

49

Figure 3.3: Synthesis and imidization process for Kapton polyimide, starting fromprecursors dianhydride and dianiline moieties (top), followed by polymerization intopolyamic acid (middle), and imidization into polyimide (bottom).

In Figure 3.3a, the general synthesis for Kapton polyimide is shown. The polymer-

ization of dianhydride and dianiline moieties form polyamic acid by oxidization of the

dianiline, and the polyamic acid imidizes to polyimide at high temperatures. Follow-

ing the procedures in the work of Spechler et al.[61], our homemade colorless polyimide

is synthesized from polymerizing two precursor moieties, a typical dianhydride accep-

tor and a fluorinated dianiline donor. The synthesis for colorless polyimide is shown

in Figure 3.4. The homemade CPI showed visible improvement in the transmission

and lacks the yellow hue that was especially severe for the films thicker than several

microns.

50

Figure 3.4: Imidization process of colorless polyimide starting from precursors, a typ-ical dianhydride and a fluorinated donor dianiline (top), followed by polymerizationinto polyamic acid (middle), and imidization into colorless polyimide (bottom).

Several thin films with thicknesses 10-20 µm were prepared by spin coating and

imidizing precursors of Kapton and colorless polyimide. The total transmission spec-

tra through these polyimide films with supporting glass substrates were obtained

with an integrating sphere coupled to a monochromator. The transmission spectra

are presented in Figure 3.5a. As the Kapton film thickness is increased from 11.7

µm to 14.5 µm, the transmission across 400-500 nm decreases significantly. However,

the CPI film with a larger thickness than the two Kapton films does not suffer from

severe absorption in the 400-500 nm range, and this can be visually confirmed by ob-

serving the photographs of the freestanding films of Kapton and colorless polyimide

(note that the dark rectangular patterns are densely deposited silver nanowires). As

such, we are able to fabricate thicker CPI films while Kapton films limit the thickness

before significant absorption takes place. This allows us to use the CPI films as a

51

standalone, flexible substrate with thicknesses upwards of 50 µm for handling in a

laboratory setting. A typical scattering CPI substrate is comprised of two parts: a

50 µm thick solid CPI substrate and a ∼8 µm-thick porous CPI film (pCPI). The

ability to achieve substantial thickness without significant absorption combined with

a porous scattering film made from the same CPI precursor creates a smooth and

index-matched interface with minimal reflection loss.

Figure 3.5: a) Transmission spectra of various polyimide films (glass/Kapton andglass/CPI), b) photograph of a patterned AgNW/Kapton substrate, and c) photo-graph of a patterned AgNW/CPI substrate.

One of the advantages of polyimides over other plastic materials such as PET and

polyethylene naphthalate (PEN) is its thermal and mechanical robustness. Our CPI

substrates are thermally imidized at 360 °C and exhibit excellent resistance against

solvents and high annealing temperatures. The thermogravimetric analysis (TGA)

thermal curve of the homemade colorless polyimide shows an onset of weight loss at

560 °C, showing an excellent potential for high-temperature application. The control

Kapton polyimide under the same TGA analysis shows the onset temperature of 557

°C.52

Figure 3.6: Thermogravimetric analysis (TGA) thermal curve of colorless polyimide.

3.2 Experimental details

Colorless polyimide has proven to be an excellent host material for AgNWs, and

composite CPI/AgNW substra