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Charge Carrier Transport in Conjugated Polymers

Permanent Link: http://ufdc.ufl.edu/UFE0021825/00001

Material Information

Title: Charge Carrier Transport in Conjugated Polymers
Physical Description: 1 online resource (78 p.)
Language: english
Creator: Wilson, Bryan E
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: carrier, charge, homo, lumo, mobility, oled, organic, pled, polymer, semiconductor
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Current-Voltage measurements and charge transport properties of poly(3-hexyl-thiophene) (?P3HT?), poly(3,4-propylenedioxythiophene) (?PProDOT?) and poly(3,4-propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring (?PProH?) films have been studied. The zero-field hole mobility (mu sub h) was determined from current-voltage data by iterating curve fitting parameters in the space charge limited current model which was derived from Child?s Law, also known as the Mott-Gurney Law. To measure hole mobility, hole only devices were constructed with indium-tin-oxide (ITO) anodes and gold cathodes (very large electron injection barrier) on a glass substrate. A hole transport layer of Poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (?PEDOT?) was spin coated between the ITO and sample polymer film in order to reduce the energy barrier for injection of holes. The effects of spin coating speed on film thickness, and subsequently on the electronic properties of the materials was also investigated. Device preparation in a glove box using an argon ambient with oxygen and water concentrations of < 5 ppm was found to be critical for reproducible electrical data. Spin coating speeds of 700 ? 1000 RPM for 30 seconds resulted in thin films ranging between 10-90 nm as measured by atomic force microscopy (AFM). Hole transport in films of PProH was space charge limited for voltages in the range of 0-5V, with mobilities of 1.6x10-6 cm2/V-s. In contrast, hole transport in films of PProDOT was trap limited. The origins of the traps were speculated to be residual impurities and/or structural deformations.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Bryan E Wilson.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Holloway, Paul H.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021825:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021825/00001

Material Information

Title: Charge Carrier Transport in Conjugated Polymers
Physical Description: 1 online resource (78 p.)
Language: english
Creator: Wilson, Bryan E
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: carrier, charge, homo, lumo, mobility, oled, organic, pled, polymer, semiconductor
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Current-Voltage measurements and charge transport properties of poly(3-hexyl-thiophene) (?P3HT?), poly(3,4-propylenedioxythiophene) (?PProDOT?) and poly(3,4-propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring (?PProH?) films have been studied. The zero-field hole mobility (mu sub h) was determined from current-voltage data by iterating curve fitting parameters in the space charge limited current model which was derived from Child?s Law, also known as the Mott-Gurney Law. To measure hole mobility, hole only devices were constructed with indium-tin-oxide (ITO) anodes and gold cathodes (very large electron injection barrier) on a glass substrate. A hole transport layer of Poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (?PEDOT?) was spin coated between the ITO and sample polymer film in order to reduce the energy barrier for injection of holes. The effects of spin coating speed on film thickness, and subsequently on the electronic properties of the materials was also investigated. Device preparation in a glove box using an argon ambient with oxygen and water concentrations of < 5 ppm was found to be critical for reproducible electrical data. Spin coating speeds of 700 ? 1000 RPM for 30 seconds resulted in thin films ranging between 10-90 nm as measured by atomic force microscopy (AFM). Hole transport in films of PProH was space charge limited for voltages in the range of 0-5V, with mobilities of 1.6x10-6 cm2/V-s. In contrast, hole transport in films of PProDOT was trap limited. The origins of the traps were speculated to be residual impurities and/or structural deformations.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Bryan E Wilson.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Holloway, Paul H.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021825:00001


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CHARGE CARRIER TRANSPORT IN CONJUGATED POLYMERS


By

BRYAN E. WILSON
















A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2007

































2007 Bryan Wilson

































To my mother, Isabel. I work hard because she's worked harder.















ACKNOWLEDGMENTS

First, I thank my beautiful and wonderful Fiancee, Johanna Talcott, for organizing

everything from my references to my work area. Without her, I'd still be digging through

a pile of papers looking for the perfect reference. Also, I thank her for all those late night

dinners while I spent hours isolated in the office working on this project. She has never

given up hope that I would ever finish my thesis and without that, I may have already

quit.

My parents of course deserve a lot of credit as well. Always trying to keep me

focused on the bright side of things, my mother has provided continued support through

the many challenges I personally faced while trying to complete this work. My father has

also provided support and I'm forever thankful to him for being there for me and always

willing to talk things over.

Many thanks to Dr. Reynolds and his research group in the Department of

Chemistry at the University of Florida for providing all of the materials and some of the

equipment to perform my research.

My interest in the field of electronic materials has its roots in my work at A&N

Corporation in Williston, Florida. A&N provided the flexibility for me to attend graduate

school while keeping my job as an R&D engineer. Special thanks to my former

supervisor, Vern McCoy and to the Vaudreuil family for opening the door to so many

opportunities.









Last, but definitely not least, I'd like to extend my sincere appreciation to Dr. Paul

H. Holloway, my graduate research advisor. From the first moment I approached him

about my interest in the graduate program at the Department of Materials Science and

Engineering to the day before my thesis submission deadline, he has provided many

hours of guidance and an infinite level of patience. When things got tough and I was

ready to throw in the towel, Dr. Holloway always found ways to keep me in the game and

always provided me with new ways of looking at my data and understanding my work.

Additionally, this work was funded in part by the U.S. Army Research Laboratory

under contract W911-NF-04-200023 with additional sponsorship provided by the Air

Force Office of Scientific Research.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

L IST O F T A B L E S ........ ...................................................................... .. viii

LIST OF FIGURES ......... ......................... ...... ........ ............ ix

A B S T R A C T .............................................. ..........................................x iii

CHAPTER

1 INTRODUCTION AND MOTIVATION.................................................................1

2 LITERA TURE REVIEW .......................................................... ..............4

2 .1 In tro d u ctio n ................... ................................................ ............... .. 4
2.2 Organic Light Emitting Diodes................................ ......................... ....... 4
2.2.1 History 4
2.2.2 Organic/Polymer Semiconductor Physics.............................................6
2.2.3 D evice O operation ......................................................... ............ .. 11
2.3 Charge Carrier Transport ............................................................................ 14
2.3.1 Mobility ............................................. .........................14
2.3.2 H all Effect M ethod .... .......... ....................................... 17
2.3.3 Tim e of Flight .............................. ................................ 18
2.3.4 Space Charge Limited Current ............................................ 20
2.3.4 Trap Charge Limited Current ...........................................22

3 EXPERIM ENTAL M ETHODS ........................................ ........................... 29

3 .1 In tro d u ctio n ................................................................................................. 2 9
3 .2 D ev ice P rep aration .................................................................. ................ .. 2 9
3.2.1 Substrate Cleaning and Etching ................................. ................ 29
3.2.2 ITO Surface Treatm ent .............................................. ............... 30
3.2.3 Addition of Hole Transporting Layer ................................................ 31
3.2.4 Addition of Active Layer ......................................... ...............32
3.2.5 Vapor Deposition of Gold Electrodes .............................................33
3.3 Current-V oltage M easurem ents ............................................. ............... 34
3.3.1 K eithley Source M eter ........................................ ...................... 34
3.3.2 Sam ple H older .............................................. ................. ... 34









3.4 Structural Characterization................... ........ ........................... 35
3.4.1 Profilom etry .................................... .................. ......... 35
3.4.2 Atom ic Force M icroscopy.................................. ....................... 35
3.5 Experim ental Procedures ............................................................................ 36
3.5.1 Film Thickness V ariation.................................. ........................ 36
3.5.2 Increased Temperature Exposure..................................36

4 EXPERIMENTAL RESULTS ............................................................................39

4 .1 B a ck g ro u n d ................................................................................................. 3 9
4.2 Results ......... ............... .............................40
4.2.1 Physical Characterization........... .......... ............. ............... 40
4.2.1.1 Film preparation for thickness measurements......................40
4.2.1.2 Atom ic force m icroscopy ....................................... .......... 41
4.2.2 Electrical Characterization............. .... ............... ............... 43
4.2.2.1 Current-Voltage (I-V) Measurements.............. .......... 43
4.2.2.2 H ole M obility A nalysis................................ ............... 46

5 C O N C L U SIO N ......... ...................................................................... ......... .. ..... .. 56

LIST OF REFEREN CES ............................................................ .................... 59

B IO G R A PH IC A L SK E TCH ..................................................................... ..................64
















LIST OF TABLES


Table pge

4-1 Chemical Structure of sample polymers ...................................... ............... 47

4.1 Thickness and RMS roughness data for all conjugated polymer films ..................49

4.2 Fitting parameters obtained by iterating the field-dependent mobility equation .....54

4.3 Hole mobilities of various polymers ............................................. 55
















LIST OF FIGURES


Figure pge

2-1 Structure of polymer LED (OLED) ......................... ..................... ...............24

2-2 Progress in LED efficiency. ............................................ ............................. 24

2-3 Ethylene m olecule Lew is Structure................................... .................................... 24

2-4 Ethylene molecule depicting s-oribitals, sigma bonds and p-orbitals...................24

2-5 Depiction 7t-bonds in ethylene molecule ...................................................25

2-6 Summary of carrier transport and recombination in OLEDs ................................25

2-7 Energy band diagram of typical OLED............. ........... ......... ...............25

2-8 Band diagram of an organic electroluminescent layer (OEL) under forward bias. .26

2-9 Three layer device depicting four possible cathode materials. .............................26

2-10 I-V for ITO/MEH-PPV device with various anodes.............................................26

2-11 Basic setup to measure carrier concentration using the Hall effect .......................27

2-12 Tim e of Flight Experim ental Setup........................................ ....... ............... 27

2-13 Time of Flight photocurrent profile at various applied bias voltages. ....................28

2-14 J0.5 vs. V plot for low MW (29.9 kg/mol) poly(3-hexylthiophene).................. 28

3-1 Summary of ITO patterning procedure. ...................................... ............... 37

3-2 Braun Glove Box used for sample preparation...................................................... 37

3-3 Continuation of device preparation .................................. ..................................... 38

3-4 Sample Holder................................ ................................ .......... 38

4-1 Depiction of channel formation for measurement of PEDOT-PSS film thickness..47

4-2 Depiction of channel formation for measurement of total film thickness................48









4-3 Typical AFM surface roughness analysis output with the RMS surface
rou ghn ess in th e red b ox ................................................................ .....................4 8

4-4 Depiction of film thickness measurements. The red arrows appear to the right
and left of a channel wall.produced by scratching ................................................49

4-5 I-V data for PE D O T :P SS. ............................................................. .....................50

4-6 Typical J-V data for a PProH polymer device. Note the exponential character
typical of polym er film s. ...................................... ...... .....................................50

4-7 Dependence of I-V data on thickness for PPrOH films .......................................51

4-8 Linear regression of log I vs log V data for PProH films. .....................................52

4-9 Linear regression of log I vs log V data for ProDOT.............................................53

4-10 Increased J were observed upon heating the 25nm PProDOT samples ...................53

4-11 Effect of baking and relaxation on current density. ............................................54
















LIST OF ABBREVIATIONS


Eg bandgap

q carrier charge

v carrier drift velocity

Ttr carrier transit time

CY conductivity

J current density

Jx current density in x direction

h Dirac's constant

d distance between repeat units in polymer

E electric field

m electron mass

AE energy change between bands

En energy eigen values

EHOMO energy of highest occupied molecular orbital

ELUMO energy of lowest unoccupied molecular orbital

Y field effect factor

d film thickness

RH hall coefficient

Ey hall field

W hall film thickness

Vy induced potential









kn k-value, as related to energy Eigen values

Bz magnetic field in z-direction

[t mobility

N number of orbitals, number of repeat units

So permittivity of free space

h Planck's constant

L polymer chain length or infinite well length

n quantized states, or number of free carriers

Er relative permittivity

Tc trap characteristic temperature

Nt trap density

V volts

Y wave function















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

CHARGE CARRIER TRANSPORT IN CONJUGATED POLYMERS

By

Bryan E. Wilson

December 2007

Chair: Paul H. Holloway
Major: Materials Science and Engineering

Current-Voltage measurements and charge transport properties of poly(3-hexyl-

thiophene) ("P3HT"), poly(3,4-propylenedioxythiophene) ("PProDOT") and poly(3,4-

propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with

dodecyloxy chains on the phenylene ring ("PProH") films have been studied. The zero-

field hole mobility ([th) was determined from current-voltage data by iterating curve

fitting parameters in the space charge limited current model which was derived from

Child's Law, also known as the Mott-Gurney Law. To measure hole mobility, hole only

devices were constructed with indium-tin-oxide (ITO) anodes and gold cathodes (very

large electron injection barrier) on a glass substrate. A hole transport layer of Poly(3,4-

ethylene-dioxythiophene):poly(styrenesulfonate) ("PEDOT") was spin coated between

the ITO and sample polymer film in order to reduce the energy barrier for injection of

holes. The effects of spin coating speed on film thickness, and subsequently on the

electronic properties of the materials was also investigated.









Device preparation in a glove box using an argon ambient with oxygen and water

concentrations of <5 ppm was found to be critical for reproducible electrical data. Spin

coating speeds of 700 1000 RPM for 30 seconds resulted in thin films ranging between

10-90 nm as measured by atomic force microscopy (AFM). Hole transport in films of

PProH was space charge limited for voltages in the range of 0-5V, with mobilities of

1.6x10-6 cm2/V-s. In contrast, hole transport in films ofPProDOT was trap limited. The

origins of the traps were speculated to be residual impurities and/or structural

deformations.















CHAPTER 1
INTRODUCTION AND MOTIVATION

The increased use of organic and polymer light emitting diodes (OLEDs and

PLEDs) in the solid state lighting and display industries is the motivating force for the

research presented in this work. Currently, conversion of energy from fossil and nuclear

fuels to electricity provides most of the energy required to artificially illuminate living

and working environments. In a recent publication, the United States Energy Information

Administration showed that the energy used to meet domestic residential and commercial

lighting requirements in 2005 was 4.2 quadrillion BTUs [1]. However, because the

currently available sources of light are inefficient, only about 30% of this total energy

was used to actually produce light with the rest being wasted as heat [2]. The limited

quantities and environmental impacts associated with the use of fossil fuel and nuclear

energy points to decreasing the amount of wasted energy with the introduction of new

lighting technologies utilizing efficient, emissive materials.

Coincidentally, the development of new emissive materials and new technologies

in the display industry is also being investigated. Technologies such as the Cathode Ray

Tube (CRT), Liquid Crystal Displays (LCDs) and Plasma Display Panels (PDPs)

currently dominate the display market [3]. However, several factors have limited these

technologies. For example, though the technology is currently the cheapest available, the

bulkiness and weight of CRTs have excluded them from the popular flat panel market

[3]. Also, viewing angle restrictions caused by the birefringence property of liquid









crystals in LCDs has caused the need for development of compensating technologies [3].

Finally, the high energy requirement of PDPs has limited this technology to static (i.e.,

non-portable) applications. The advantages of OLEDs are that they are easy to process,

are characterized by low operating voltages and exhibit wide viewing angles and high

contrast ratios. Furthermore, the mechanical properties of polymer films open the door to

flexible display applications [4, 5].

Typically light emitting diodes are separated into two categories: inorganic (hereby

referred to as LEDs) and organic (referred to as OLEDs). OLEDS may be further

categorized as either small molecule (SMOLED) or conjugated polymer (PLED) devices.

Devices based on inorganic materials are generally comprised of compound

semiconductors such GaAs, GaP, AlGaAs, InGaP, GaAsP, SiC, ZnSe or InAlGaN.

However, the technologies used to deposit these materials are similar to that utilized to

fabricate silicon integrated circuits thereby making them relatively expensive [6]. On the

other hand, while SMOLED technology may also take advantage of precision deposition

technologies such as those requiring vacuum, depositing polymer materials for PLEDs is

quite cheap as the material can be deposited from solution by spin coating, sometimes in

ambient laboratory conditions.

In any OLED, emission of light requires electrical charge (electrons and holes) to

be injected into the organic thin films, for the electrical charge to be transported in the

material with minimum energy loss, and for the electrons and holes to recombine and

emit light. The focus of this research is the charge transport properties, namely the hole

mobility ([th), of a novel polymer material: poly(3,4-propylenedioxythiophene-

diethylhexyloxy)-cyano-p-phenylene vinylene substituted with dodecyloxy chains on the









phenylene ring, referred to as PProH:CNP(MEH) or PProH for simplicity. Charge

transport was measured as the current versus applied voltage as a function of processing

and time. Spin-coating was investigated as an easy deposition method which allows the

sample film thickness to be varied. The current transport is also correlated with changes

in the film thickness.

This work describes relatively simple process for gaining insight to the charge

transport properties of new polymer materials. By measuring and reporting hole

transport properties of new materials, this work provides knowledge that may be used in

the future to improve OLED based devices and their respective efficiencies.

In this thesis, a review of the literature in Chapter 2 provides a description of

polymer based device physics, and electrical properties. The experimental procedures

section, Chapter 3, includes a review of the processing methods and a description of

characterization tools that were used to determine the hole-mobility from electrical

properties. Chapter 4 contains the experimental results and comparison of electrical

properties of devices utilizing films of PProh:CNP(MEH). Finally, Chapter 5 provides a

summary and conclusions from the experimental results of this work as well as

recommendations for future work.














CHAPTER 2
LITERATURE REVIEW

2.1 Introduction

The purpose of the study described in this thesis is to measure carrier transport

properties in conducting, conjugated polymers. This was accomplished by comparing

experimental data from current-voltage (I-V) measurements with the space-charge

limited current (SCLC) or trap-limited current (TLC) models to extract the hole mobility

(~4). The results from this work are useful for several applications, including those related

to organic light emitting diode (OLED) applications with respect to charge balancing for

efficient electron-hole recombination and photon emission. This chapter reviews

background information which includes a brief history of the OLEDs, device architecture

and operation, conducting polymer physics, and the modeling of carrier transport

mobility.

2.2 Organic Light Emitting Diodes

2.2.1 History

Electroluminescence (EL) is the non-thermal generation of light resulting from the

application of an electric field, and is accomplished by recombination of charge carriers

of contrary sign (electrons and holes) that are injected into a semiconductor in the

presence of an external circuit [6]. EL was demonstrated in organic materials in 1963.

EL has been reported as first being observed from inorganic ZnS phosphor powder by

Destriau et al. in 1936 [7]. Recently however, some have credited the earlier works of

Oleg Losev with his published reports on light emission from zinc oxide and silicon









carbide crystal rectifier diodes in 1927 [8]. In the 1960s, the General Electric Company

introduced the first commercially available LED devices based on inorganic compound

semiconducting materials [6]. The development of organic EL devices was initially

hampered by the high voltages--on the order of 100V or above--required to achieve major

light output by injection of charge into organic crystals such as anthracene. However

research in the field was stimulated by the findings of Tang and VanSlyke in 1987. In

their work, the researchers from the Eastman Kodak Company demonstrated a novel thin-

film device structure utilizing a two-layer architecture made from an aromatic diamine

emissive layer and an organic small molecule carrier transport layer composed of 8-

hydroxyquinoline aluminum (Alq3). Their device was driven to significant brightness by

a dc voltage as low as 2.5V [9].

The first account of EL from a semiconducting, conjugated polymer was reported

in 1990 by Burroughes et al. and was based on an emitting layer of poly(p-phenylene

vinylene) (PPV) [10]. Burroughes' OLED adopted the thin film layer device architecture

which has become quite common for use in experiments (Figure 2-1). In this basic

device architecture, indium tin oxide (ITO), which is sputtered onto a glass substrate,

serves as the anode. In many cases, the polymer layers are spin coated over the ITO and

a metal is thermally deposited over the polymers and serves as the cathode. The anode

and cathode are then connected to an external circuit and forward biased with the positive

voltage on the ITO anode. Additionally, each of the films thicknesses are between 10nm

to a few hundred nanometers.

PPV was initially shown to emit in the green-yellow part of the spectrum, but

several different polymer compositions of varying bandgaps have since exhibited









emissions with wavelengths scattered throughout the visible part of the spectrum [12, 13,

14]. Also, variations in device architecture such as multi-layered or stacked devices that

incorporate enhanced carrier transport (or barrier) layers have been shown to increase

light output efficiency and lower turn on voltages [15, 16]. More recently, at a meeting

of the Materials Research society, it was reported that white OLEDs with an efficiency of

57 lumens per watt of power (lm/W), were produced in Japan [17]. Comparison of both

conventional, inorganic and white-organic LEDs shows increase in efficiency over the

years indicates the (Figure 2-2). It is noted that on average, fluorescent bulbs produce

about 60-100 Im/W while incandescent bulbs produce 17 Im/W [18].

In this review of the literature, methods used to calculate hole mobility are

presented along with some of the strengths and weaknesses of each method. First though,

a brief summary of carrier recombination and light production in OLEDs, including a

discussion of device physics, and device architecture is presented. A review of the

electronic structure of conjugated polymers is also presented and provides a basis for the

discussion of carrier transport in this special group of semiconducting, organic materials.

2.2.2 Organic/Polymer Semiconductor Physics

As their name suggests, some organic materials exhibit semiconducting behavior.

The basis for this behavior is related closely to the formation of a double covalent bond

between two carbon atoms. Double bond formation can be explained by the occurrence

of specific bond angles as predicted by the valence-shell electron pair repulsion theory

(VSEPR theory) as well as by the energies and locations of electrons as predicted by the

Pauli Exclusion Principle and Molecular Orbital Theory. In the simple case of an

ethylene molecule (Figure 2-3), the double bond that forms between carbon atoms

requires that four electrons are shared between the two carbon nuclei.









Additionally, two of the electrons, one each from the carbons, are present in sp2

hybridized orbitals (Figure 2-4) and overlap end to end to form a sigma bonding

molecular orbital (o-bonding MO) [19]. The two remaining electrons (again, one each

from the carbon atoms) remain available for further bonding. However, as stated by the

Pauli Exclusion Principle, these electrons cannot exist in the same quantum state (e.g.,

the same orbital or the same space and around the same nucleus) as the other two sp2

electrons. Therefore, the remaining electron exists in unhybridized p-orbitals.

In each p-orbital, two regions of high charge density are located on opposite ends

of the central nucleus and in the case of the preceding double bond, are positioned

perpendicular to the o-bond. For the second bond to form, the p-orbitals from each

carbon overlap which results in formation of the pi-bond (7t-bond) [20]. Constructive

interference during the p-orbital overlap creates a i7-bonding molecular orbital (MO),

while that resulting from destructive interference is the 7t*-antibonding MO. Each

essentially forms half of the total 7t-bond (Fig. 2-5).

As the backbone of organic molecules becomes longer, such as those consisting of

four or more carbons, they can sometimes form conjugated systems of alternating single

and double bonds. Conjugated systems may consist of a benzene ring, or a system of a

few (<100) linked unit molecules (-mers) called oligomers, or longer chains (-1000-

10,000) of -mers called polymers. The electrons associated with each carbon atom in

these systems first fill the available orbitals (i.e., the closest to the atom outward to the

valence 7t-orbitals), with the last pair of electrons occupying what is known as the highest

occupied molecular orbital (HOMO). The next molecular orbital beyond the HOMO is

known as the lowest unoccupied molecular orbital (LUMO). Interestingly, the 7t-bonds









that form the HOMOs and LUMOs of each carbon can actually overlap above the single

bonds and in effect, the electrons in the double bonds are delocalized over the whole

macromolecule [21]. The delocalized electrons that are weakly bound (known as the K7-

electron cloud) can be ionized relatively easily and the electron vacancy (hole) or surplus

electron can travel along the molecule with relative ease [22]. The HOMO and LUMO

therefore act similarly to the valence and conducting bands found in inorganic

semiconductors with a band gap separating the two energy levels.

Polymers of various chain lengths result in varying band gaps. This is easily

predicted by a simple substitution of a single system of repeat units combined to a chain

of length 'L', into the free electron orbital model [23].

Consider the solution to the time-independent Schrodinger's equation for a free

electron in a one dimensional potential well of infinite depth and width L as given by

Equation 2-1 [24].

Yn(x) = A, sin knx + Bn cos kx (2-1)

The wave function in this well known "particle in a box" model are required to be

continuous at the boundaries (i.e., x=0 at the origin and x=L at the width of the potential

well), therefore the solution requires that for Yn = 0 at point x = 0, and Bn must also be

zero. Furthermore, for Yn = 0 at x = L, the sin(knL) value must also equal zero.

Therefore, the proper solution is summarized with Equation 2-2,


Yn(x) = An sin knx = A sin- (n = 1, 2, 3, ...) (2-2)
L

From this solution, it can easily be seen that,


k = (n = 1, 2, 3,...) (2-3)
L









where kn, known as the k-value has been related to a set of energy levels (energy eigen

values, En) defined as

/2mEn (2-4)
k,=,, (2-4)

In effect, the boundaries of the potential well have defined a discrete set of allowed k-

values and therefore, a discrete set of energy eigen values as given in Equation 2-5

h2kn2 n2h2
E,= = L (n= 1, 2, 3, ...) (2-5)
2m 8mL2

where h is Planck's constant, m is the electron mass and n defines the energy level, i.e., is

a set of quantum numbers [21, 23, 24, 25].

For a polymer of length L made up of N repeat units and separated by a distance d

(i.e, L in Equation 2-5 approaches 'Nd' for long chains), we see that the energy values

given by Equation 2-5 can be modified as given by Equation 2-6:

n2h2
En = (n = 1, 2, 3,...). (2-6)
8m(Nd)2

The energies of the HOMO and LUMO levels of the chain are assumed to be defined by

the 7n electrons from the N p-orbitals. Recalling that each molecular orbital is filled by

two electrons, and that each orbital is separated by one energy level, the HOMO has the

energy given by Equation 2-7 and that of the LUMO is given by Equation 2-8



f 2
8m(Nd)2 (2-7)




E(LUMO) =2 (2-8)
8m(Nd)2









Now, given that the energy necessary to promote an electron from the HOMO to

the LUMO is called the band gap, or Eg, this value is easily defined as the difference

between ELUMO and EHOMO as:

X2 2
2 p2 (N+1)h2 (2-9)
Eg = AE = ELUMO EHOMO = ) (2
8m(Nd)2 8m(Nd)2 8m(Nd)2


The limit, reached by large chains (large N), is given in Equation 2-10.

h2
Eg 8 = md2 (2-10)
(8md2)N

From Equation 2-10, it can be seen that as the chain length increases (i.e., as N

increases), the band-gap (Eg) decreases. However, because the electron density given by

the alternating double bonds in conjugated polymer systems is not equally distributed, the

inter-carbon lengths (the spacing between the carbon-carbon double bounds versus

carbon-carbon single bonds) are not equal. Experimentally it has been shown that this

result forces a limit below which the band gap of a given conjugated system will not

decrease, even with added length of additional mer-repeat-units [26, 27]. The length at

which no change in band-gap is attained is known as the saturation length.

There have been several successful attempts to engineer the conjugation lengths of

the active conjugated systems for use in OLEDs thereby altering the band gap and hence,

precisely tuning the emission wavelength of the devices [28, 29]. One of the notable

challenges in tuning a material for a particular low band gap by increasing the chain

length has been to achieve conjugation lengths that gives both the desired band gap and

polymer solubility [28]. As was stated previously, one of the advantages of polymer

OLEDs is in the fact that they are easily processible from solution.









2.2.3 Device Operation

The fundamental purpose of electroluminescent polymer devices is to convert

electrical power into light. The processes involved in the production of light within an

OLED are summarized simplistically in Fig. 2-6.

First, charges of opposite sign (i.e., positive holes and negative electrons) are

injected from opposing electrodes by application of an external voltage (forward bias).

The carriers then travel through additional layers which either promote or inhibit their

motion. Finally, when the carriers of opposite sign travel close enough to attract one

another, either in a special recombination layer or at the interface between the hole and

electron transport layers, they recombine forming a singlet exciton which can then decay

radiatively and results in emission of a photon [11].

One of the indicators used to describe how efficiently a device produces light is the

external quantum efficiency, lext, which is the ratio of the number of photons emitted by

the device (into the viewing direction) to the number of electrons injected. The reason

viewing direction is important in this definition is because many of the photons that are

produced emerge from the sides of the device or can be re-absorbed within the various

organic layers. Therefore, rlext is several times lower than internal quantum efficiency,

rwin, which incorporates all photons produced over all angles and with negligible

reabsorption [31].

Though the above summary of OLED device physics is presented in a very general

form, it can be seen that one of the greatest challenges in this field is to increase the

percentage of electron and hole pairs (excitons) that recombine radiatively at an interface

or within an emitting layer in order to increase the quantum efficiency. To meet this

challenge, one must control the number of carriers entering the various layers of the









device and also exploit the rate at which the carriers are able to move through each layer.

Knowledge of parameters such as the band gap as well as the carrier mobilities becomes

important to properly tune the location and rate at which recombination occurs and

thereby increases the quantum efficiency [32].

To gain an appreciation of the device physics as it relates to the generic device

architecture mentioned above, one must study the energy diagram of an OLED. A typical

structure and band diagram (Figure 2-7). In the figure, EA is the electron affinity, IP is

the ionization potential, HTL is the hole transport layer, LUMO is the lowest unoccupied

molecular orbital, HOMO is the highest occupied molecular orbital, EML is the emitting

layer, (Pa is the anode work function, (pc is the cathode work function and Ev is the vacuum

potential [33]. The LUMO and EA are equivalent to the conduction band in inorganic

semiconductors and will therefore be referred to simply as the LUMO in reference to

organic semiconductors. Additionally, the HOMO and IP are equivalent to the valence

band and will all be referred to as the HOMO.

When a voltage is applied to the electrodes, such that the anode is biased positively,

the electronic bands are bent in a manner similar to Figure 2-8. Ideally, this bending

narrows the otherwise high energy barriers (pb) that are present at the electrode/polymer

interface. Hence, hole injection from the anode Fermi level, EFa, and electron injection

from the cathode Fermi level, EFc, are promoted to the organic layer's HOMO and

LUMO, respectively. This injection is represented by the curved arrows which indicate

carriers tunneling through the energy barrier, (b. Furthermore, the use of a high work

function anode pa, such as indium-tin-oxide (ITO), and a low work function cathode pc,

such as calcium, helps ensure that carrier current is not injection limited (i.e., the (b's are









low enough so that the rate of charge carrier injection is large enough to not limit the

amount of current being conducted).

If the bias is instead applied with the opposite polarity (reverse bias), carriers are

not able to surmount the potential barrier that is present at either of the electrode/polymer

interfaces. In this case, with a negative potential placed on ITO (which has a large work

function value) for example, the electrons would need to be injected into the polymer

from ITO instead of Ca. This would result in the electrons being blocked from injection

into the LUMO of the polymer due to the large pb. This phenomenon is commonly

referred to as rectifying behavior and is characteristic of diodes [35].

In some cases it may be useful to reduce the number of carriers in a device. To

measure a transport property of one particular type of carrier in a semiconducting

polymer, for example mobility, the number of charges of the other carrier type being

injected into the material must be limited. In other words, to make a "single carrier

device", one must lower the carrier injection efficiency of a particular contact by

selecting appropriate materials for the anode or cathode.

For example, to measure hole mobility in regioregular poly(3-hexyl-thiophene) (or

RR-P3HT) diodes, a group led by Michael McGehee at Stanford University constructed

"hole-only" devices. The devices employed aluminum cathodes with a work function of

(,=4.2eV to have large mismatched energies with the LUMO of RR-P3HT at 5eV [36].

With this configuration, the electron injection energy barrier pb is equal to 0.8eV and

therefore negligible electron injection.

In another example (Figure. 2-9), four materials and their respective work

functions are shown as cathodes in a three layer device with MEH-PPV selected as the









semiconducting polymer (LUMO =2.8eV, HOMO = 4.9eV). Indium, with a work

function of about 4.2eV offers the lowest electron injection energy barrier of (b = 1.4eV

while gold, with a work function of about 5.2eV, offers the largest barrier with (pb

=2.4eV.

Comparing the various cathodes introduced in Figure. 2-9 with the resulting I-V

curves of Figure 2-10, one can see that although the energy barriers for electron injection

may vary between 1.4eV and 2.4eV, the overall current in the device remains the about

same at a given voltage, indicating that electron injection from the cathode is not

controlling current in the device.

These data are evidence that the device current is dominated by holes. In fact,

holes are often cited as the major charge carrier in OLEDs because hole-mobility is

generally higher than that of electrons in conjugated polymer materials [38].

Alternatively, an electron-only device can be assembled by mismatching the band-

offset between an anode's work function, (a, and the organic semiconductor's HOMO,

while maintaining a low barrier between the cathode and LUMO.

2.3 Charge Carrier Transport

2.3.1 Mobility

Charge carrier mobility ([t) is an important parameter to characterize the carrier

transport and resulting performance of polymers that are used as electroluminescent

materials in light emitting devices [39]. At low electric fields, the drift velocity (v) is

proportional to the electric field strength (E), and the carrier mobility is the

proportionality constant between these two values [40]. Therefore, carrier mobility is

defined as the carrier drift velocity per unit applied electric field:










i v = cm21 (2-10)
E V-s

Polymer semiconductors are known to have low charge carrier mobilities in

comparison to carrier mobility in inorganic materials. For example, the hole mobilities of

a wide range of organic semiconductors for use in optoelectronic applications have been

reported in the range of 10-7 to 10 cm2/V-s while that of p-type gallium nitride, which is

used in inorganic LEDs, has been reported at 400 cm2/V-s [41, 42]. The low values for

polymers are attributed mainly to disorder of the polymer chains in conjunction with

trapping due to the presence of impurities [43, 44]. Extrinsic variables such as

temperature and applied electric field strength are also known to affect mobility [45].

High carrier mobility results in faster response as well as to reduction of operating

voltage in LEDs [44]. However, in a stacked structure a low mobility in one layer

followed by a high mobility in the next layer can cause charges to accumulate and

effectively form a parallel plate capacitor [46]. Knowing the mobilities of charge in the

various layers can assist in designing an optimized device where charge is accumulated at

the desired locationss.

Other conducting organic devices that depend on carrier mobility include organic

field-effect transistors (OFETs). OFETs may be integrated into a number of products

ranging from RFID tags to active-matrix displays. Similarly, a disadvantage of FETs that

contain organic materials is that the active layer charge mobility, as compared to their

inorganic equivalents, is relatively low [47]. However, OFETs may be processed with

greater ease and have superior mechanical (flexural) properties. These advantages

generally result in lower cost and a broader range of applications.









It is important to note the relationship between mobility and conductivity of a

material. For the case of n free charge carriers per unit volume, each with a charge q, the

charge density is nq. With the electric current density (J) defined as the charge density

times drift velocity we have:

J= nqv. (2-11)

From Ohm's Law, the current density is defined as conductivity, C, times the electric

field:

J= E (2-12)

Equating Equations 2-11 and 2-12 results in:

GE = nqv (2.13)

Now, substituting for electric-field from Equation 2-10 above:

S= nqt (2.14)

i.e., conductivity can be expressed in terms of the mobility [25]. It is important to note

that a large drift mobility does not automatically equate to a high conductivity because

c also depends on the concentration of charge carriers.

Over the years, several experimental methods have been used to measure the

mobility of charge carriers in semiconductors [36, 40, 41, 48, 49, 50]. Generally, these

methods vary based on the way external fields affect charge carrier transport. For

example, Hall mobilities rely on the effects on charge carriers of both electric and

magnetic fields. The time-of-flight method relies on a pulsed external light source, such

as a laser, to generate carriers in combination with an applied electric field. Finally, the

space charge limited current method (SCL or SCLC) relies solely on an applied electric-

field to inject and transport the carriers [48]. Each of these methods is discussed below.









2.3.2 Hall Effect Method

An electric field (Ex) applied to the x-axis of a sample along with a magnetic field

(Bz) applied perpendicularly along the z-axis, gives rise to a traverse electric field (in the

y-direction) which exerts force on the charge carriers. As carriers are forced in the y-

direction, the induced electric field Ey (the Hall field), which is derived from the induced

potential (Vy) and thickness of the sample in that direction (W), balances the force

induced by the magnetic field [40]. From the Hall Effect, it can be shown that:


E, -V RH xB (2-15)


where R is the Hall coefficient and is equal to (1/nq) and Jx is the current density of the

charges flowing in response to Ex. RH can be determined from the values of Ey, Jx and Bz

using Equation 2-15. If one type of carrier dominates, the carrier concentration and type

can be determined from the Hall coefficient. The carrier mobility can be determined

using Equation 2-16 [25].

RH = [ (2.16)

In 1958, a method was developed that exploits the Hall Effect, but is modified for

measurement of flat samples of arbitrary shape [51]. This method, known as the Van der

Pauw technique, has been commonly used for measuring transport properties of many

inorganic semiconductors such as p-GaN, n-InN and n-InGaN having mobilities of 46, 2

and 107 cm2V- s-1, respectively [52]. However, the Van der Pauw technique is not

commonly used to measure the carrier mobility of polymers. Though organic samples

can be prepared to meet the requirements for applying this technique, the current density,

Jx, is often too low to induce a measureable Hall voltage. Essentially, the carrier mobility

in conjugated polymers is usually too low to be measured by the Hall Effect [53]. The









Hall voltage, which is typically in the range of a few meV even for high mobility

materials, is too low in polymers, resulting in a signal-to-noise ratio that is very low.

Mobilities in polymers have been reported utilizing the Van der Pauw

technique/Hall Effect. For example, the Hall mobility of AsF doped (17%) polyacetylene

was reported to be -2x10-2 cm2V- s- [54]. In another case, the Hall technique was used

to measure mobility in poly(4,4-dipentoxy-2,2'bithiophene). Results were comparable to

amorphous inorganic semiconductors with values of 10-1 to 102 cm2V-s- [43].

2.3.3 Time of Flight

The time-of-flight technique is the most widely used method to measure mobilities

in organic conductors [49]. In this method, the organic material is placed between two

electrodes, one of which is transparent and does not inject carriers. A sheet of charge

carriers is photogenerated by external pulsed laser or lamp illumination through the

transparent electrode and the carriers move through the sandwiched sample to the other

electrode under the influence of an applied electric field [55].

To determine the hole mobility, a thin charge layer may be generated near the

anode (Figure 2-12. In this case, the light pulses must not be absorbed in the cathode or

by the sample. Photogeneration must take place at the generation layer and electrons

collected at the nearby anode, while holes migrate across the complete thickness of the

sample layer [56].

A transient photocurrent profile is recorded and a clear inflection point is

determined from the current-decay pattern [57]. The inflection point, is the intersection

of two tangents on the log current-log time plot (Figure 2-13), and is equal to the carrier









transient time, Ttr, of the fastest carriers [44]. In other words, Ttr is the time required for

the photogenerated carriers to reach the opposite transparent electrode.

The charge carrier mobilities are then calculated by manipulating Equation 2-11

into:


P (2-17)


where d is the sample thickness and V is the applied bias.

Utilizing the time of flight method, Kaneto et al. has reported hole mobility of

regioregular poly(3-hexylthiophene) (MW 29 kg/mol) of [lh = 4x10-4 cm2/V-s for drop

cast films having a thickness of 7[tm [41].

TOF requires thick films so that the absorption depth of the optical excitation is

small compared to the sample film thickness. TOF experiments on poly-phenylene-

vinylene (PPV) for example, have been reported with film thickness of 1-10[m. To

attain such thickness, the deposition by spin-coating requires slow spin rates. Therefore

it is preferable to form thick films by drop-casting. Unfortunately, preparation by this

method can influence mobility because the material structure of drop-cast films differs

from that of spin-cast films [45]. Differences in mobility measured by TOF between

drop cast and spin coated films have been reported for regiorandom-poly(3-

hexylthiophene). These mobilities were [t= 10-7 cm2/V-s for drop cast films having a

thickness of 5400nm, and t=10-5cm2/V-s for the spin deposited films having thickness of

200nm [41].

Determining the TOF-mobility of films with a distribution of mobilities dispersivee

transport) is also complicated since the arrival of carriers at the electrodes is spread out









and usually the fastest carriers make up the bulk of the measurement [53].

2.3.4 Space Charge Limited Current

Charge transport through a thin organic polymer film is said to be either an

electrode limited process or a bulk limited process [39]. In an electrode limited process,

carrier injection is "limited" by the energy barrier between the electrode's work function

and the polymer's HOMO or LUMO levels, depending on whether the injected carrier is

a hole or an electron, respectively. In a bulk limited process, carrier transport is limited

by the drift velocity through the material, i.e., the mobility of the charge carrier.

When the relationship between current (I) and voltage (V) is linear, it is indicative

of an ohmic system. This is the case for transport in metals where the I-V relationship is

described by Ohms' law, V = IR, or alternatively with respect to current density (J),

V
J = qcn (2-18)
d

In this modified version of Ohm's law, q is the carrier's charge, n the charge carrier

density, u the carrier mobility, Vthe applied voltage, and d the thickness of the sample.

This is also typically the case at low voltages when injecting carriers from a metal contact

into a conjugated semiconducting polymer. However, a point exists where this condition

breaks down and non-linear I-V behavior, characteristic of all organic LEDs and known

as space charge limited current (SCLC), is observed [58]. At higher voltages, more

carriers are being injected across the barrier, and at a faster rate than they can travel

through the polymer and exit at the other contact. This results in charge accumulation

near the injecting electrode which redistributes the electric field intensity, controls

transport through the polymer bulk and determines the I-V dependence [59]. In practice,









when at least one of the interfacial barriers is lower than -0.3eV, the current in an organic

LED will be SCLC [60].

In cases where these conditions apply plus no traps are present in the polymer film

and the mobility is independent of the field, Equation 2-19 describes trap free space

charge limited current.

9 V2
J -co- r d3 (2-19)
8 d

where So and Er are the permittivity of free space and relative permittivity, respectively

[61, 62]. Conversely, due to their intrinsically disordered nature, the carrier mobility in

most conjugated polymers is field dependent [36, 63]. Equation 2-19 has therefore been

modified as

9 F2
J = 9 o,0e897 (2-20)
8 d

Utilizing either Equations 2-19 or 2-20, it is possible to extract the charge carrier mobility

by fitting the experimental J-V data [39]. To prevent charge recombination, single carrier

devices as previously described must be constructed. This allows for measurement of the

current and resulting mobility for either electrons or holes. Unfortunately, since the work

function of one of the electrodes has to be adjusted to make the device either a hole-only

or electron-only diode, the electron and hole mobilities cannot be measured with the same

device [53].

Hole mobility was measured for poly(3-hexylthiophene) in the space charge

regime. It was found that the lower molecular weight samples had mobilities that were

field-dependent. Fig. 2-14 shows a plot of J0.5 vs V for low molecular weight poly(3-









hexylthiophene) and film thicknesses of 96nm and 192nm. The inset shows non-linear J

vs Vbias characteristic, indicative of space charge limited current.

Equation 2-19, shown as the dark straight lines in Figure 2-14, does not fit the data

well, especially for the 96nm sample. However the data overlaps the dashed line, which

is Equation 2-20, reasonably well. This indicates that the hole mobility in fact is field-

dependent in low MW samples of poly(3-hexylthiopene). The resulting hole mobilities

varied from 1.33+ 0.41 x10-5 cm2/V-s (MW 2.89 kg/mol), 1.13 0.37 x10-4 cm2/V-s (MW

9.72 kg/mol) and 3.3 + 0.73 x105 cm2/V-s (MW 31.1 kg/mol) and are in agreement with

the time of flight measurements of Kaneto et al. as reported above [36].

To date, several other I-V experiments have been performed on various polymers to

determine carrier mobility from the space charge limited current model [39, 64, 65]. The

present study will use this SCLC method to determine the hole mobilities of poly(3,4-

propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylene-vinylene substituted with

dodecyloxy chains on the phenylene ring.

2.3.4 Trap Charge Limited Current

For trap-free films, the space charge model predicts that I aV2. Yet in an early

assessments of space charge in inorganic materials, Smith and Rose observed a high

power dependence (I aVm with m > 2) for insulating CdS crystals and attributed this to

the filling of traps followed by an excess of charges after all the traps are filled, resulting

in an increased number of carriers [66]. While they recognized that the presence of traps

can determine the relationship between current and voltage (i.e., the power law

dependence), they did not apply this model to organic materials. More recently, the

presence of traps in anthracene has been reported with a very high power exponent of









IcV12 (at T = 200C) [67].

Organic materials such as conjugated polymers may have traps present as a result

of structural disorder, impurities, geminate pairs or self-trapping [68]. Traps affect the

charge transport properties by removing carriers that contribute to the charge transport.

When traps have a continuous exponential energy distribution, the dependence of current

on voltage for this I-V region is called trapped-charge limited. It is in this region that as

the applied forward bias is increased, traps below the quasi Fermi energy level for a

specific carrier are filled. During the trap-filling the trap-free space charge equation

(Equation 2-19) must be modified to reflect the fact that not all carriers are available for

transport. The relation between J and V for trapped-charge-limited currents (TCLC) is

defined by Equation 2-21:



lm 2m + l Nt(mn +1) d2 m+1
\ m+lz )Nt1 +l? d2m+l (2-21)


where N is the density of states in the conduction band or the valence band, Nt is the total

trap density, m = Tc/T where Tc is the characteristic temperature of the traps, T the

absolute temperature, and the balance of the terms are defined above. At high current, all

the traps are filled and they no longer affect charge carrier transport. The current at this

point behaves similar to that of a trap-free space-charge limited current [65].

















Figure 2-1. Structure of polymer LED (OLED) [11]


u
E
1
i-

(B
t
C
a-


0,1 tI I I
1970 1975 1980 1985 1990 1995 200D 2005
Year
Figure 2-2. Progress in LED efficiency [17]. Red line is conventional LEDs.




H H

C=C

H H
Figure 2-3. Ethylene molecule Lewis Structure



porbitals







Figure 2-4. Ethylene molecule depicting s-oribitals, sigma bonds and p-orbitals [19].

















H H

H KH
Val [ i, I, y.!i
Figure. 2-5. Depiction -bonds in ethylene molecule [20].

Figure. 2-5. Depiction it-bonds in ethylene molecule [20].


Electron
Tram p. ir
Layer

Hole
Tramp' In
Layer


r2-





Figure 2-6.


Light
Summary of carrier transport and recombination in OLEDs [30].


Figure 2-7. Energy band diagram of typical OLED [33].
































Figure 2-8. Band diagram of an organic electroluminescent layer (OEL) under forward
bias [34].


M.1V





049v


- h
I Al
- ACu

??ZRW Au


42eV
4-3eV
4.7eV

S.eV


rrO 4.7eV

Figure 2-9. Three layer device depicting four possible cathode materials [37].






1.2







0.6 -



0.4 -*- 42
--I 4.V
4--Al 4;3eV
a Ag 4heV
0.2 --- -u 4.7ev
-: Au 52tV


a 5iilO 1x10 1-5xtOe 2x10d
Field (V/m)

Fig. 2-10. I-V for ITO/MEH-PPV device with various anodes [37].



























Figure 2-11. Basic setup to measure carrier concentration using the Hall effect [40].









Semi-transparent I Aluminium
ITO cathode anode

Pulsed optical Ii --- Rhodamine 6G
source I I charge
generation
Sample l I ayer
material


Fig. 2-12. Time of Flight Experimental Setup [56]












180V Tr

10-4 loov ..






.c- Bias voltage


106
o




-ITOIHT-PHT(casl)iAI+ -- hv

10.6 10-5 10-4
Time (s)
Fig. 2-13. Time of Flight photocurrent profile at various applied bias voltages [41].





Low MW
0.4 hO= 1.33 x 105cm2N. 0.02

S= 2.4 x (mV)112 0.01
0.3
E G 012345
96 nm V'M
0.2


0.1 192 nm


0.0
0 1 2 3 4 5 6
Vappi Vbi -r (V)

Figure 2-14. J0.5 vs. V plot for low MW (29.9 kg/mol) poly(3-hexylthiophene).














CHAPTER 3
EXPERIMENTAL METHODS

3.1 Introduction

This chapter describes the various tools, techniques and procedures which were

used to prepare samples and collect the experimental data reported and discussed in this

thesis. First, the method used to prepare the indium tin oxide (ITO) coated glass

substrates is described. The next section describes device architecture and how devices

utilizing a high work function anode and cathode were used to ensure single carrier

transport. The use of a simple two point probe setup to measure current (I) versus

voltage (V) applied to the devices is then described. Next, the use of profilometry and

atomic force microscopy (AFM) to characterize film structure is summarized. Last, an

overview of the software utilized for data analysis and computation of carrier mobility

(|th with units cm2/V-s) is presented.

3.2 Device Preparation

3.2.1 Substrate Cleaning and Etching

Glass slides coated with 150-200 nm films of ITO were purchased from Delta

Technologies, Ltd.. The slides measured 1 x 1 inch-square were reported to have a sheet

resistance (Rs) of 8-12 Q per square CB40-IN Corning 1737 ITO One Surface). To begin

the process of device fabrication, the ITO was patterned by a room temperature acid

vapor etch. The acid used was a mixture of three parts hydrochloric acid (HC1) to 1 part

nitric acid (HNO3) by volume, commonly referred to as aqua-regia. This step makes it

possible to later deposit metal contacts on the device in such a way as to make eight









devices on a single substrate. To remove the ITO in select areas, a mask made of simple

scotch-tape was utilized to expose specific areas of the slide to the aqua-regia vapor for

approximately 10 minutes. After etching, the areas were tested with an ohmmeter to

verify that resistance was over 1 Mega-ohm, indicating that the ITO had been removed.

Figure 3-1 summarizes this process and shows the ITO pattern remaining on the slide

after the aqua-regia etch.

Because of the contamination that occurs when samples are handled, packaged,

exposed to air, and then ITO etched, the samples were subsequently cleaned to remove

dust, organic contaminants and adhesive residue from the scotch-tape mask. First the

substrates were placed in a beaker filled with a "detergent" solution of 20mg of sodium

dodecyl sulfate dissolved in 500mL of de-ionized water. The beaker was then placed in a

sonicator and the substrates were cleaned for approximately 15 minutes. Next, the

detergent solution was emptied and the beaker was then filled only with deionized water

and the samples were again cleaned in the sonicator. This process was repeated three

more times using solutions of methanol, acetone and finally, 2-isopropyl alcohol. Used

solvents were distilled for reuse in this process. The cleaned substrates were placed in

clean slide cases and then in vacuum sealed desiccators to limit exposure to moisture and

environmental contaminants.

3.2.2 ITO Surface Treatment

After the samples had been cleaned and stored as described above, they were

inserted into a HARRICK PDC-32G Plasma Cleaner and exposed to an oxygen plasma

for 20 minutes just prior to polymer coating. During the oxygen-plasma treatment, the

ITO undergoes physico-chemical and electronic property modifications, including a

smoother surface, higher surface energy with high polarity, and an increased work









function [69]. A smooth and high-polarity ITO surface promotes better adhesion of a

polymer film and reduces the interfacial tension between the polymer and substrate. The

increased work function, which is attributed to removal of organic surface residues

during oxygen ion bombardment, results in increased hole injection due to the decreased

energy barrier between the conduction band of ITO and the HOMO level of the polymer

[70].

3.2.3 Addition of Hole Transporting Layer

After the samples have been exposed to the oxygen plasma, the next step is to

deposit the polymer layers. A hole transporting layer (HTL) consisting of Poly(3, 4-

ethylenedioxythiophene )-poly(styrenesulfonate), otherwise known as PEDOT-PSS

(Bayer Baytron P VP Al 4083), was spincast over the ITO plus the etched areas of the

substrate (Figure 3-1). Subsequently, the active polymer film was also deposited by spin

coating because of its ease and reliability in creating uniform thin films [4]. For spin

casting, the samples were placed on the sample-holder of a Chemat Technology, Inc.

model KW-4A spin coater. The spin coater spin rate was set manually with the

revolutions per minute (RPM) displayed on an LCD. While the sample was at rest,

400[tL of PEDOT-PSS in an as prepared aqueous solution (Bayer Baytron P VP Al

4083) filtered through a 0.2[tm nylon filter, was added using a micropipette. Once the

PEDOT-PSS covered the entire surface of the sample, spinning was initiated at a rate of

3000 RPM. During the spin coating process, most of the polymer solution was removed

from the substrate by centripetal force. After a short time (about 15 seconds), a thin

liquid film is left on the surface and the solvent evaporates which results in an increase in

the viscosity of the film [71] thereby resulting in the final film thicknesses of 40 nm. In









the present study, once the PEDOT:PSS was spin coated onto the substrate, the samples

were placed in a vacuum oven and baked at 1500C for 4 hours to completely remove all

solvent (water) from the films. When the films were dry, the slides were placed in an

argon atmosphere of an isolated glove box. For each run, which included preparation of

several devices, one slide with only the PEDOT-PSS layer was set aside to be used as a

reference in determining the layer thickness by profilometry.

3.2.4 Addition of Active Layer

Research has shown that device characteristics are strongly influenced by the

presence of moisture [72]. Therefore, a glovebox (Figure 3-2) manufactured by MBraun

GmbH and filled with dry argon was utilized to complete the device fabrication by

adding the active layer and top cathode contact.

Spin coating of the active polymer, gold contact deposition and current-voltage

measurements are all performed within the glove box to prevent exposing the devices to

moist air. Though the glove box ambient is pure argon (99.9% from Praxair), oxygen and

water concentrations can increase when samples are transferred from the outside

environment into the glove box. All experiments were performed with less than 5ppm

oxygen and 5ppm water. Solutions ranging between 8 to 28 mg/mL of poly(3,4-

propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with

dodecyloxy chains on the phenylene ring ("PProH") in di-chlorobenzene or toluene

solvent were prepared in small glass vials. Similarly, 20 mg/mL solutions of poly(3-

hexyl-thiophene) ("P3HT") in dichlorobenzene and of poly(3,4-

propylenedioxythiophene) ("PProDOT") in toluene were prepared. Prior to being placed

in the glove box, the polymer solutions were placed in a glove-box load-lock. In order to

reduce the amount of oxygen and moisture from the contents entering the glove-box, the









load-lock is exposed to a double decontamination cycle where the load-lock contents are

exposed to a vacuum of approximately 25 in. Hg refilled with argon, exposed once

again to the same vacuum, and finally equilibrated with the glove-box atmosphere.

A volume of 300[tL of the active polymer solution was then added over the

PEDOT-PSS layer in an even fashion using a micro-pipette. Spin rates ranging from

700rpm to 1200rpm were used to produce a range of PProH, P3HT and PProDOT film

thicknesses. After spin coating the polymer layer over the PEDOT-PSS, the samples

were prepared for metal top electrode deposition. Once again, devices were placed in a

vacuum oven and baked at 1500C for 4 hours to completely remove all solvent (toluene

or dichlorobenzene) from the films.

3.2.5 Vapor Deposition of Gold Electrodes

A stainless steel shadow mask, patterned as in Table 3-2 to allow for eight different

connected dot electrodes per 25 x 25 mm2 slide, was placed against the polymer film.

The slides were then placed in a vacuum thermal evaporator system which was pumped

down to 10-6 Torr using a turbomolecular pump backed by a oil -sealed roughing pump.

Three or four pieces of Au-shot ~1 mm in diameter were placed in a tungsten boat that

was clamped between two post connected to electrical feed-throughs. Au evaporation

was monitored by the MBraun integrated thin film deposition controller with a deposition

rate for gold inputted at 2.5 A/s. Deposition stopped automatically when the film

thickness reached 100nm. After deposition, the samples were allowed to cool for 1.5

hours. The thermal evaporator was then backfilled to latm with argon from the glove

box. Figure 3-3 summarizes the device fabrication including cathode deposition.









3.3 Current-Voltage Measurements

3.3.1 Keithley Source Meter

Current-Voltage (I-V) data were collected to determine the hole mobility ([t) in the

active polymer layer using a two point probe technique to apply a bias voltage between

the anode and cathode electrodes. Two probes, one which is biased positively and

the other biased negatively during measurement, were attached to a Keithley Series 2400

source-meter. The voltage step and rate, starting voltage and end voltage were specified

utilizing the LabTrace software package from Keithley. The positive probe was

contacted to the ITO surface (exposed by rubbing as described below) and the negative

probe contacted the vapor-deposited gold electrode.

3.3.2 Sample Holder

The custom sample holder (Figure 3-4) was used to hold each sample for I-V

measurements. The holder has 12 pins that are static on one side and the Keithley probes

are connected to these pins by alligator clips (Figure 3-2).

The other end of each pin fits through a hole in the holder and makes contact with

the sample with spring loaded, gold contacts. Eight of the pins which are located around

the center of the holder are designed to be compressed against the eight gold cathode

electrodes of the sample (Figure 3-3, step #5). The remaining four pins located at the

corners of the holder are compressed against the ITO (anode) surface of the samples,

where the active polymer and PEDOT:PSS layers were removed by rubbing each corer

with a cotton swab saturated in dichlorobenzene. In this process, single strokes were used

to avoid redeposition of material on subsequent strokes. While the active polymer was

removed by dissolution in the dichlorobenzene, the PEDOT-PSS was removed by

mechanical force. Removal of the polymer layers was verified visually by a change in









the reflective properties of the corner areas versus the remainder of the sample which was

still covered with polymer. For further verification of removal, the resistance between

the pins contacting the ITO was measured. If the resistance between two pins was 100Q

+/-20Q, then it was concluded that the pins were touching the ITO. If resistance was

much higher than this value, the sample was removed from the holder and another swab

with solvent was used to remove the polymer layers. This process was repeated until the

resistance between all pins contacting the exposed ITO surface was sufficiently low.

3.4 Structural Characterization

3.4.1 Profilometry

To measure the thickness of the active conjugated polymer layer, a cotton swab was

used, as described above, to remove the active polymer and PEDOT-PSS layers again to

expose the ITO. In this case, material was removed along narrow paths running parallel

to two edges of the samples, from one end to the other. Visual inspection was used to

verify that the polymer layers had been removed. Each path where the polymer was

removed consisted of a trough surrounded by layers of polymer. A Tencor Alphastep

200 surface profilometer was used to measure the depth of the troughs and therefore the

total thickness (dtot) of both polymer layers. The same material removal procedure was

used to produce troughs on the samples with only a PEDOT-PSS layer, one of which was

prepared for each batch of samples as reported above. The thickness of the PEDOT-PSS

layer (dpEDOT) was also measured using the profilometer. The difference between dtot and

dPEDOT is then equal to that of the active polymer (dp).

3.4.2 Atomic Force Microscopy

Atomic force microscopy (AFM) images were taken with a VEECO Dimension

3100 AFM at the University of Florida in the Major Analytical Instrumentation Center









(MAIC). The same samples that were prepared for measurement with the profilometer

were also measured in the AFM for comparison. Images were taken in the tapping mode

and the thickness of the polymer layers was determined. The root-mean-square surface

roughness (RMS roughness) was also determined.

3.5 Experimental Procedures

3.5.1 Film Thickness Variation

By varying the rate at which films were spin-deposited between 700 to 1000

revolutions per minute, the thickness of the active conjugated polymer films were varied.

Other parameters such as ITO surface-treatment time, spin speed of the PEDOT-PSS

layer, and volume of polymer initially added to the substrate were kept constant. The

hole mobility was measured as a function of the active film thickness.

3.5.2 Increased Temperature Exposure

Current-Voltage data were collected from as prepared samples. The samples were

then placed on a hot plate, while still inside of the glove-box, and heated in argon to

approximately 100C for 4 hours. The temperature of the hot place surface was verified

with a thermocouple. I-V data was again collected for the samples after they were

allowed to cool for 30 minutes.














Step Top View Side View


A clean 25 x 25 mm
1. square of ITO on a glass
substrate is used for
etching.


12 mm
ITO is etched. The light 5 mm
areas indicate area
where ITO was
removed using aqua-
regia acid vapor.


Figure 3-1. Summary of ITO patterning procedure.


Figure 3-2. Braun Glove Box used for sample preparation.











Step


Top View


PEDOT-PSS is spin coated
3. onto the etched ITO surface
and then baked and vacuum
dried.


The active polymer is spin
4. coated on top of the
PEDOT:PSS layer in the
dry box.


The metal electrodes are
5. vapor deposited on top of
active polymer in the dry
box.


Side View








i


Figure 3-3. Continuation of device preparation, showing (3) PEDOT-PSS hole transport
layer, (4) PProH active polymer layer, and (5) Au metal contact depositions.


Figure 3-4. Sample Holder (a) with pins (b) and measurement probes (c) attached.














CHAPTER 4
EXPERIMENTAL RESULTS

4.1 Background

In this experiment, current-voltage (I-V) data were collected and compared with the

space-charge limited current model (as presented in section 2.20) to extract the hole

mobility of the conjugated polymers. Current-voltage (I-V) data are taken for devices

with active polymer layers of poly(3,4-propylenedioxythiophene-diethylhexyloxy)-

cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring

("PProH"), poly(3-hexyl-thiophene) ("P3HT"), or poly(3,4-propylenedioxythiophene)

("PProDOT") films (Table 4-1 for the chemical structure of these materials). To extract

hole mobilities from I-V data, three values are required: applied voltage (Vbias), the

resulting current density (J), and the polymer film thickness (d). Although current and

voltage are collected with an automated Keithley controller, control and measurement of

the film thicknesses provided the biggest challenge in the experiment. Even after the

rigorous cleaning steps discussed in Chapter 3, deposition of smooth and uniform

polymer films was hindered by particulates on the sample surfaces or the effects of

different solvents during spin coating. For example, inconsistent surface wetting by the

various polymer solutions on the glass/ITO/PEDOT-PSS samples sometimes resulted in

non-uniform active layer thicknesses on the samples. This will be discussed further as

the experimental results, such as surface characteristics of the various deposited polymer

films and the calculated hole mobility, are reported below.









4.2 Results

4.2.1 Physical Characterization

4.2.1.1 Film preparation for thickness measurements

Polymer films were spin coated onto a glass substrate coated with a transparent

conducting indium tin oxide (ITO) and a PEDOT-PSS layer. Film thicknesses for a

given solution were varied by changing spin rates between about 700 to 1000 rpm for a

constant 30 seconds. To prepare the samples for thickness measurements, portions of the

films were removed by scratching straight lines across the samples with a sharp edge.

This process removed polymer from the surface of the glass/ITO substrate making small

channels down to the surface of the ITO. As depicted in Figure 4.1, control samples on

which only PEDOT-PSS was deposited over ozone plasma treated ITO-on-glass

substrates were measured. Once other films were deposited over the PEDOT-PSS, it

becomes difficult to measure the PEDOT-PSS thickness so samples with only PEDOT-

PSS were first measured to determine its film thickness. The dark horizontal lines in

Figures 4-1 and 4-2 represent the channels formed by scratching the films with the edge

of a razor blade, and the channels are represented as breaks in the upper film layer.

With only PEDOT-PSS deposited on the glass/ITO substrate, measurement across

the channels with the AFM tip provided the thickness of the PEDOT-PSS layer (i.e.,

dDOT). Because the spin rates and spin times were held constant for PEDOT-PSS

deposition, it was assumed that the thickness of the PEDOT-PSS films for all samples

was constant. Approximately 300mL of an as-received aqueous solution of PEDOT-PSS

(Bayer Baytron P VP al 4083) [73] was filtered using a .2[tm filter and then spun at 3500

rpm for 30 seconds resulting in films with 40 +10 nm thickness.









After determining the PEDOT-PSS thickness and as depicted in Fig. 4.2, scratches

were also formed across samples having both PEDOT-PSS as well as additional active

polymer films.

The channels on these samples provided a "depth" over which to measure the total

thickness (i.e., dTOT) of all films deposited over the ITO. By substracting the known

PEDOT film thickness from the total thickness of all films measured by AFM, the

polymer film thickness (d) was determined by simple calculation utilizing Equation 4-1.

dTOT dDOT = d (4-1)

The above method determined the film thickness used in the space charge and

trapped charge limited current models. However it is based on the two assumptions that

the PEDOT-PSS thickness was constant across all samples, and that none of the ITO film

was removed by scratching.

4.2.1.2 Atomic force microscopy

Polymer film thicknesses were first measured by profilometry, however there were

small scratches on the samples after this procedure, raising concerns that the metal tip of

the profilometer caused damage to the delicate polymer surface. Therefore, AFM was

adopted as the appropriate measuring technique for this experiment. The AFM allowed

for better thickness measurement precision, as well as a better and more quantitative

understanding of film topography by visual representation of the surfaces and

quantification of the root mean square (RMS) surface roughness.

AFM measurements were taken only after the I-V measurements were completed.

In order to measure the film surface, the samples were removed from the inert gas glove

box and thereby exposed to laboratory air. Once exposed to air and humidity, no further

electrical measurements were collected. Although no further electrical measurements









were made after samples were removed from the glove box, samples were nonetheless

stored in clean, single-slide plastic containers. The containers were further stored in

desiccators and kept under vacuum. This storage technique was used as a precaution to

minimize surface contamination during physical transport of the samples between labs

and testing areas.

To characterize the film surfaces, the lxl cm samples were cleaved into smaller

-5x5 mm sections so as to fit properly in the AFM sample holder. For these

measurements, cleaved samples were inspected by eye and selected based on proximity

of the cleaved portion to the gold cathode areas. By selecting these proximity areas, it

was assumed that measurements were representative of the film surface directly below

the gold contacts where the carrier transport of interest occurs during I-V measurements.

The result of a typical roughness analysis based on the AFM data with the RMS surface

roughness of the area is highlighted (Figure 4-3).

To determine the thickness, the AFM was operated in the section analysis mode.

Various points along the scratched channels were measured by the AFM to determine the

thickness. Fig. 4-4 shows a typical analysis of the total film thickness (i.e., dTOT).

In Figure 4-4, the vertical distance between two points located at the bottom of a

channel (e.g., the left side red-arrow in the figures) and the surface of the polymer film at

a location near the channel (e.g., right side red-arrow in the figures) was measured as

62nm (i.e., dTOT = 62nm). It must be noted however, that because the film surface varied,

appearing as a series of non-uniform peaks and troughs on the AFM output, the film

thickness measurement is based on the assumption that the two points chosen were

representative of average film thickness. By utilizing the above thickness Equation and









subtracting the thickness for PEDOT of 40nm, the polymer film thickness for this

particular sample was easily calculated as: 62nm-40nm = 22nm.

Table 4.2 summarizes the film thickness and RMS surface roughness values for

all films used in this experiment. Also included in the table are the solvents in which the

polymers were dissolved, the concentrations of solutions based on weight of polymer to

the volume of solvent, and the spin rates used to deposit each of the films.

4.2.2 Electrical Characterization

4.2.2.1 Current-Voltage (I-V) Measurements

Poly(3,4-ethylene-dioxythiophene) doped with poly(styrenesulfonate) (also known

as PEDOT-PSS, PEDOT or PDOT) has been utilized as a hole injecting material in

OLEDs for some time [74]. While having a high work-function of 5.1eV, PEDOT-PSS

provides reduced hole injection barriers (about 0.2eV) into the HOMO energy level of

numerous conjugated polymers, thus serving as a pseudo-ohmic contact to ITO anodes.

In addition to these properties, PEDOT-PSS is commonly referred to as an organic metal

since it exhibits ohmic transport similar to inorganic metals (Figure. 4-5).

"Hole-only", or more realistically "hole-dominated" devices, as described in

Chapter 2, were prepared by evaporating gold electrodes onto the masked surface of two

layer polymer films consisting of an active layer over the PEDOT-PSS layer. The

samples were then placed in the sample holder (Figure 3-2) to measure I-V characteristics

at room temperature. Voltage was ramped between 0 to 5 Volts (V), stepped by 0.05V,

to provide several data points for curve-fitting. As depicted in Fig. 4-6, I-V data from a

PProH device show exponential behavior similar to that of other conjugated polymers

reported in the literature [36, 64, 75].









The I-V data for PProH films depended on the thickness as predicted by the space

charge Equation, which shows that current is inversely proportional to thickness cubed

(i.e., J a d-3). This dependence is qualitatively consistent with the PProH data presented

in Fig. 4-7 in that the current is dramatically larger for the thinner layers. Note that at <

0.5V, the films maintain similar current density values, but the thinner 21nm film

exhibited much higher currents at higher voltages. Additionally, the 88nm film exhibits

the lowest currents for all voltages over the entire voltage range.

Since the space charge model shows that current-density is proportional to the

square of applied voltage (i.e., J a V2), a plot of log J vs log V should result in a line

having a slope of 2. Figure 4-8 shows shows such plots of the data presented in Figure

4-7.

A linear regression analysis of the data (Fig. 4-8) reveals that the slopes are best

fitted by values of 2.1, 1.7 and 1.9 for 21, 44 and 88nm thick films, respectively. An

average slope of 2 is indicative of space charge limited current, in contrast to materials

which exhibit trap-limited-currents that show different slopes (i.e., I caVm with m > 2 ).

For example, if the log I vs. log V data were trap limited, a slope of three or higher would

be expected [76]. This is due to a significant number of carriers injected from the

electrodes being held at traps distributed in energy between the LUMO and HOMO

energy levels of the polymer [77].

Plots log I versus log V for samples of ProDOT films are presented in Figure 4-9.

The data for these films are consistent with trapped charge limited current (TCLC), rather

than the SCLC behavior of PProH films. Trap limited current is indicated by the fact that

I cV3 as linear regression results in a slope of 3.7 and 3.1 for the 25 and 47nm films.









It is not understood why ProDOT films exhibit TCLC while PProH films exhibit

SCLC. However, in some material/solvent combinations there may be impurities

remaining after synthesis of organic semiconductor materials and/or layers that could trap

charges. Alternatively, the traps may be the result of structural disorder of the polymer

films [78]. Structural disorder, resulting in defects, introduces localized energy levels

between the HOMO and LUMO of the organic material which may be present as discrete

energy levels, or distributed over a band of energy with a constant density of states or an

exponential distribution such as a Gaussian. One other source of traps occurs when a

charge carrier causes deformation of the organic molecule. In this case, the deformation

acts as a quasi-particle called a polaron which not only has a lower mobility than a free

carrier but also forms its own trap state in the polymer ("self trapping") [79]. Because

some of the device preparation and testing steps were outside the glove box, there also

remains the possibility that films and polymer solutions were contaminated by external

impurities during processing. Further investigation is needed to understand why ProDOT

exhibits trap limited while PProH exhibits space charge limited current transport.

To address the possibility that structural disorder and/or retained solvent was the

source of traps the devices were heated so as to allow the polymers to rearrange. While

still in the glove-box, the devices were placed on a hot plate and heated ("baked") to

approximately 100C for -4 hours. Upon removal from the hot-plate, the samples were

allowed to cool for 30 minutes at which time I-V measurements were again performed.

Increased currents were observed for the 25nm and 47nm films (Figure 4-10).

However, current-voltage measurements were again taken after an additional 15

hours of relaxation in room temperature argon ambient and the current decreased to









nearly the same values as before the baking (Figure 4-11).

These results suggest that the traps are not related to defects or solvents that are

healed or reduced at -100C heat treatments., Further experiments are needed.

4.2.2.2 Hole Mobility Analysis

Data for the hole mobility of P3HT and PProH films were analyzed based on the

space charge model (Equation 2-19), solving for fitting parameters representing constants

in the formula. The fitting parameters were calculated by iterating the field-dependent

mobility equation (Equation 2-20) for the zero-field mobility, [t(0), and the field-

dependence factor, y, as presented in equation. The results are presented in Table 4.3 for

the fitting parameters where the goodness of fit parameter, R2, approaching a value of one

represents an excellent fit to the space charge model.

Data fitting for P3HT yields [t(o) values averaging 6.95 + 1.2 x 10-6 (cm2/V-s) with

no thickness dependence, while that for PPrOH yields [t(o) values averaging 1.6 + 0.4 x

10-6 (cm2/V-s). The hole mobilities for various conjugated polymers as reported in the

literature are listed in Table 4-4.

Based on the data in Table 4.4, the hole mobilities reported for PProH are similar

to those reported for other conjugated polymers such as PPV [63, 80, 81]. However, the

hole mobilities reported for P3HT are about two orders of magnitude smaller than those

reported by Goh and Kline using a similar technique [36]. The reason is unknown. It

was not possible to fit the TCLC model to the ProDOT I-V data to extract the hole

mobility.











Table 4-1. Chemical Structure of sample polymers
Polymer Name Structure
P3HT:

RR-poly(3-hexyl-thiophene)



PProDOT:

poly(3,4-propylenedioxythiophene)



PProH:
poly(3,4-propylenedioxythiophene- o --H R, Me
diethylhexyloxy)-cyano-p-phenylene / 'H
vinylene(substituted with dodecyloxy 0 o R2=
chains on the phenylene ring) N R1


OR; CN


PENT
ITO)


Figure 4-1. Depiction of channel formation for measurement of PEDOT-PSS film
thickness












POINE FU


I GAss


PEIT



! Io


S4
Figure 4-2. Depiction of channel formation for measurement of total film thickness


Peak Surface Area Summit Zero Crossina


stoDband Execute Cursor


Peak On


Sum.dsp.on


Zero Cross. Ot1


Box Cursor


Figure 4-3. Typical AFM surface roughness analysis output with the RMS surface
roughness in the red box


Z range 24.160 nm
Mean 0.000007 nm
Raw mean 0.000007 nm
Rms (Rq) 3.069 nm
mean roughness (Ra) 2.363 nm
max -elgt (Pmax) 24.160 nm
max peak ht (RP) 10.305 nm
max depth (Rv) -10.305 nm
Surface area
surface area diff
Box x dimension 3.000 pm
Box y dimension 3.000 Pm




































Cursor: moving


Figure 4-4. Depiction of film thickness measurements. The red arrows appear to the
right and left of a channel wall.produced by scratching




Table 4.2. Thickness and RMS roughness data for all conjugated polymer films
Sample Concentration Spin Rate Thickness RMS
Material ID Solvent (mg/mL) (rpm) (nm) (nm)
PEDOT P DI-Water as received 3500 40 N/A

Dev 3 di-chloroBenzene 20 1000 143 3.47
P3HT
Dev 4 di-chloroBenzene 20 800 148 4.39

A5 Toluene 8 10 1000 25 0.85
ProDOT
A6 Toluene -8 10 900 47 3.17


6 di-chloroBenzene 8 1000 22 3.07
PProH B3 Toluene 28 900 44 5.14
B4 Toluene 28 700 88 3.73


zoom: Z:I


Cen line: Orr


urrset: urr

















0 16
4000 rpm PEDOT-PSS ({-30 nr
1 R = 70.9 Ohms
0 12

0.10

0 08

S0.06 o ala

0064
0 04 .

0.02

0.00

-0 02
0 2 4 6
V


Figure 4-5. I-V data for PEDOT:PSS.










0.8


0.7


0.6


0.5
E

C 0.4
E

0 0.3


0.2


0.1


0


0 1 2 3

Vbias (V)


4 5 6


Figure 4-6. Typical J-V data for a PProH polymer device. Note the exponential character

typical of polymer films.


r poih
ir'li


m}














PProH


--- 21nm
--- 44nm
-- 88nm


0 1 2 3 4 5 6

Vbias (V)


Figure 4-7. Dependence of J-V data on thickness for PPrOH films








52




PProH


21nm
y= 2.1x-2.9 R2= 0.9993


44nm
y= 1.7x-2.7


R= 1


0




1-
-0-5 -

-1 -

-1.5

-2 -

J -2.5
-3 -




-4 -



-4.5

-5


0 0.1 0.2 0.3 0.4
Iog(V)


0.5 0.6 0.7 0.8


Fig. 4-8. Linear regression of log I vs log V data for PProH films. The slope = 2
suggesting that the data are described by the space charge limited current
(SCLC) model


88nm ..
y = 1 9x-31 R2 = 09999


S--21nm
--44nm
- 88nm















ProDOT


U'


-0.5

-1

-1.5

-2

2 -2.5

-3

-3.5

-4


*25nm
* 47nm


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

log (V)


Figure. 4-9. Linear regression of log I vs log V data for ProDOT. The slope > 3
indicates TCLC


--25nm PreBake
--25nm Post-Bake


Vbias (V)


Figure 4-10. Increased J were observed upon heating the 25nm PProDOT samples


25nm
y=3.7x-4.3 R2 =0.999


47nm
y=3.1x-3.9 R2=0.9985


















N 0.2
E

EL 0.15


0.1


0.05



0 1 2 3 4 5 6

Vbias (V)


--25nm PreBake
--25nm Fbst-Bake
-- 25nm Relaxed


Figure 4-11. Effect of baking and relaxation on current density.







Table 4.3. Fitting parameters obtained by iterating the field-dependent mobility equation
Sample Thickness
Material ID (nm) g(o) (cm2/ V s) 1 (m/V)1/2 R2
PT Dev 3 143 7.8 x 10-6 +/- 2.5 2.0 x 104+/- 0.5 .9991
P3HT
Dev 4 148 6.1 x 10-6 +/- 1.1 1.2 x 104/- 0.6 .9997

6 21 1.4 x 10-6 +/- 0.2 2.6 x 10^5 +/- 1.0 .9999
PProH B3 44 3.3 x 10-6 +/- 0.5 8.6 x 10^6 +/- 6.0 .99996
B4 88 9.4 x 10 +/- 0.2 2.6 x 104 +/- 0.2 .99984






55

Table 4.4. Hole mobilities of various polymers
Material Mobility (cm2/V s) Method Ref #
P3HT 4 x 10-4 Time of Flight [41]
AsF5 doped
polyacetylene 2 x 10-2 Van der Pauw [54]
phenyl-amino subst.
PPV 104 10-3 Time of Flight [44]
P3HT 1.3 x 10-5 J vs V (SCLC) [36]
PPV 0.5 x 10-6 J vs V (SCLC) [64]
poly(phenylene)
Derivative -10-6 J vs V (SCLC) [65]
PPV (Spin cast) 0.8 x 10-6 Time of Flight [80]
poly (9,9-
dioctylfluorene) 4.5 x 10-2 Time of Flight [82]














CHAPTER 5
CONCLUSION

The electrical properties of thin films of poly(3-hexyl-thiophene) ("P3HT"),

poly(3,4-propylenedioxythiophene) ("PProDOT") and poly(3,4-propylenedioxythiophene-

diethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the

phenylene ring ("PProH") have been studied at various film thicknesses (<150nm). The

focus was the use of current-voltage (I-V) data to determine if the transported current was

space charge limited (SCLC) or trapped charge limited (TCLC). If the SCLC model

applied, the hole mobilities was extracted from the data.

For PProH and P3HT, hole transport was described by the SCLC model with hole

mobilities of 1.6 + 0.4 x 10-6 and 6.95 + 1.2 x 10-6 (cm2/V-s), respectively. While the

mobilities for P3HT are approximately two orders of magnitude lower than those

previously reported in the literature, it is speculated that the larger thickness of the films

(-143nm and -148nm) may have contributed to lower mobilities, as well as a

dependence of mobility upon the field strength for lower-molecular weight films [36].

In contrast, the I-V data from PProDOT fit the TCLC model in which J ac V" with

values of m between 3 and 4. Heat treatment of the spin cast films to temperatures of

100C did not significantly change the I-V data and dependence of J upon V', suggesting

that the traps were stable to these temperatures.

Future Work. While the method for characterizing the electrical properties of

conjugated polymers can be used to measure the mobilities of materials exhibiting space

charge limited currents (PProH and P3HT), it is difficult to measure the mobility for









trapped charge limited current, as for PProDOT. Knowledge of the trap distribution and

density are required, that could be provided by a technique known as thermally

stimulated currents (TSC) which has been used to measure trap levels and total density in

poly(p-phenylenevinylene) ("PPV") [83]. While devices are cooled down from room

temperature to as low as 10K, they are exposed to a strong forward bias, which fills all

of the traps [84]. For a specific trap, there is an associated transport energy, or escape

energy (i.e., the level from which a trapped carrier is most likely to be released) that is

dependent on temperature. A trap state at lower temperatures may therefore become a

transport state at room temperature [85]. Upon removing the electric field at the low

temperature, the samples are allowed to reach an equilibrium thermal state at a higher

temperature by thermal release of carriers from the traps into the bands. The thermally

stimulated current rises as the temperature is increased and the results may be interpreted

by correlating distinct current maxima with a distinct trap energy level by I a exp(-Et/kT)

for T< Tmax [86]. This technique has been shown to provide good results for devices with

conjugated polymers as the active layer [87].

Additionally, the method of measuring hole mobility was hindered by assumptions

made in calculating the film thickness (d) which contributes to the space charge model as

J a d-3. Although spin coating is quite often used as a reliable method for producing thin

films, results are not always consistent as they depend on solution viscosity which

depends on the material and solvent used. While a substrate cleaning procedure was

followed and all solutions were filtered, the spin-cast polymer films often showed signs

of non-uniform thickness, including holes, streaks and even particulates. Also, one of the

challenges in this study was to repeat previous processing parameters in order to measure









repeatability of the I-V data. Specifically with PPrOH, the total amount of conjugated

polymer was quite limited, making it difficult to a wide processing parameter field.

Further investigation into optimizing device fabrication procedures for these materials

would lead to more in depth-studies of the effects of film thickness on I-V measurements.

This study focused on measuring hole-mobilities, which are generally higher than

electron-mobilities in conjugated polymer organic semiconductors. However, knowledge

of the electron-transport properties must be acquired to further understand and optimize

the properties of double carrier devices such as organic light emitting diodes. For

example, to investigate the transport of electrons in PPV without the drawbacks of highly

reactive Ca and Ba low-work function electrodes, Mandoc et al. constructed electron-

only devices by vapor depositing alternative low work function, hole blocking materials

as the electrodes. Utilizing a sandwich configuration with aluminum (Al) as the bottom

electrode, ytterbium (Yb) as the top electrode and the active material between the

electrodes, Mandoc was able to suppress hole injection at fields up to 108 V/m and

measured an exponential distribution of electron-traps in PPV [87].















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63


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BIOGRAPHICAL SKETCH

Bryan Wilson was born to Harold J. Wilson and Isabel H. Wilson in Panama City,

Republic of Panama where he was preceded by his older sister, Natalie and older brother,

Jonathan. From ages 1-7, Bryan resided with his family in the Panama Canal Zone

neighborhood of Diablo. Upon relocating to Key Largo, Florida in 1985 and subsequently

to Miami, Florida in 1986, Bryan attended school at Cutler Ridge Elementary, Cutler

Ridge Middle School, graduating from Miami Southridge Senior High School. Bryan

attended the undergraduate program at the University of Florida (UF), graduating in

December of 2001 with a Bachelors of Science Degree in Chemical Engineering. For the

three years following graduation from UF, Bryan worked as an engineer for A&N

Corporation in Williston, Florida. At A&N, Bryan learned about the semiconductor

industry and became interested in electronic materials. It was in the Fall of 2004 that he

began his graduate studies in the Materials Science Department, again at the University

of Florida where he gained a deeper interest in semiconducting polymers. Upon

graduation with his Masters Degree, Bryan will continue his role as a Patent Examiner at

the US Patent and Trademark Office in Alexandria, Virginia where he started his career

in September of 2006.





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CHARGE CARRIER TRANSPORT IN CONJUGATED POLYMERS By BRYAN E. WILSON A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

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2007 Bryan Wilson

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To my mother, Isabel. I work hard because she's worked harder.

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iv ACKNOWLEDGMENTS First, I thank m y beautiful and wonderful Fiance, Johanna Talcott, for organizing everything from my references to my work ar ea. Without her, I'd still be digging through a pile of papers looking for the perfect referen ce. Also, I thank her for all those late night dinners while I spent hours isolated in the office working on this project. She has never given up hope that I would ever finish my thesis and without that, I may have already quit. My parents of course deserv e a lot of credit as well. Always trying to keep me focused on the bright side of things, my mother has provided continued support through the many challenges I personally faced while trying to complete this work. My father has also provided support and Im forever thankful to him for being there for me and always willing to talk things over. Many thanks to Dr. Reynolds and his research group in the Department of Chemistry at the University of Florida for pr oviding all of the materials and some of the equipment to perform my research. My interest in the field of electronic materials has its roots in my work at A&N Corporation in Williston, Florida. A&N provided the flexibility for me to attend graduate school while keeping my job as an R&D e ngineer. Special thanks to my former supervisor, Vern McCoy and to the Vaudreu il family for opening the door to so many opportunities.

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v Last, but definitely not least, Id like to extend my sin cere appreciation to Dr. Paul H. Holloway, my graduate research advisor. From the first moment I approached him about my interest in the graduate program at the Department of Materials Science and Engineering to the day before my thesis submission deadline, he has provided many hours of guidance and an infinite level of pa tience. When things got tough and I was ready to throw in the towel, Dr. Holloway alwa ys found ways to keep me in the game and always provided me with new ways of looki ng at my data and understanding my work. Additionally, this work was funded in part by the U.S. Army Research Laboratory under contract W911-NF-04-200023 with addi tional sponsorship provided by the Air Force Office of Scientific Research.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS................................................................................................. iv LIST OF TABLES........................................................................................................... viii LIST OF FIGURES........................................................................................................... ix ABSTRACT..................................................................................................................... xiii CHAP TER 1 INTRODUCTION AND MOTIVATION.................................................................... 1 2 LITERATURE REVIEW.............................................................................................4 2.1 Introduction......................................................................................................... 4 2.2 Organic Lig ht Emitting Diodes........................................................................... 4 2.2.1 History 4 2.2.2 Organic/Polymer Semiconductor Physics............................................... 6 2.2.3 Device Operation.................................................................................. 11 2.3 Charge Carrier Transport..................................................................................14 2.3.1 Mobility................................................................................................. 14 2.3.2 Hall Effect Method................................................................. 17 2.3.3 Time of Flight......................................................................... 18 2.3.4 Space Charge Limited Current............................................... 20 2.3.4 Trap Charge Limited Current................................................. 22 3 EXPERIMENTAL METHODS.................................................................................29 3.1 Introduction....................................................................................................... 29 3.2 Device Preparation............................................................................................29 3.2.1 Substrate Cleaning and Etching............................................................ 29 3.2.2 ITO Surface Treatment......................................................................... 30 3.2.3 Addition of Hole Transporting Layer................................................... 31 3.2.4 Addition of Active Layer......................................................................32 3.2.5 Vapor Deposition of Gold Electrodes...................................................33 3.3 Current-Voltage Measurements........................................................................ 34 3.3.1 Keithley Source Meter.......................................................................... 34 3.3.2 Sample Holder.......................................................................................34

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vii 3.4 Structural Characterization................................................................................ 35 3.4.1 Profilometry.......................................................................................... 35 3.4.2 Atomic Force Microscopy..................................................................... 35 3.5 Experimental Procedures.................................................................................. 36 3.5.1 Film Thickness Variation...................................................................... 36 3.5.2 Increased Temperature Exposure.......................................................... 36 4 EXPERIMENTAL RESULTS................................................................................... 39 4.1 Background.......................................................................................................39 4.2 Results............................................................................................................... 40 4.2.1 Physical Characterization......................................................................40 4.2.1.1 Film preparation for thickness measurements........................ 40 4.2.1.2 Atomic force microscopy....................................................... 41 4.2.2 Electrical Characterization.................................................................... 43 4.2.2.1 Current-Voltage (I-V) Measurements..................................... 43 4.2.2.2 Hole Mobility Analysis........................................................... 46 5 CONCLUSION........................................................................................................... 56 LIST OF REFERENCES...................................................................................................59 BIOGRAPHICAL SKETCH.............................................................................................64

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viii LIST OF TABLES Table page 4-1 Chemical Structure of sample polymers..................................................................47 4.1 Thickness and RMS roughness data for all conjugated polym er films.................... 49 4.2 Fitting parameters obtained by iterating the field -dependent mobility equation..... 54 4.3 Hole mobilities of various polymers........................................................................ 55

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ix LIST OF FIGURES Figure page 2-1 Structure of polymer LED (OLED).........................................................................24 2-2 Progress in LED efficiency...................................................................................... 24 2-3 Ethylene molecule Lewis Structure.......................................................................... 24 2-4 Ethylene molecule depicting s-or ibitals, sigm a bonds and p-orbitals...................... 24 2-5 Depiction -bonds in ethylene m olecule.................................................................. 25 2-6 Summary of carrier transport and recom bination in OLEDs................................... 25 2-7 Energy band diagra m of typical OLED.................................................................... 25 2-8 Band diagram of an organic electrolumi nescent layer (OEL) under forward bias. 26 2-9 Three layer device depicting four possible cathode m aterials................................. 26 2-10 I-V for ITO/MEH-PPV device with various anodes................................................ 26 2-11 Basic setup to measure carrier co ncentration using the Hall effect. ........................ 27 2-12 Time of Flight Experimental Setup.......................................................................... 27 2-13 Time of Flight photocurrent profil e at various applied bias voltages. ..................... 28 2-14 J0.5 vs. V plot for low MW (29.9 kg/ mol) poly(3-hexylthiophene)..........................28 3-1 Summary of ITO patterning procedure....................................................................37 3-2 Braun Glove Box used for sample preparation........................................................ 37 3-3 Continuation of device preparation.......................................................................... 38 3-4 Sample Holder.......................................................................................................... 38 4-1 Depiction of channel formation for m easurement of PEDOT-PSS film thickness.. 47 4-2 Depiction of channel formation fo r m easurement of total film thickness................ 48

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x 4-3 Typical AFM surface roughness analys is ou tput with the RMS surface roughness in the red box........................................................................................... 48 4-4 Depiction of film thickness measurements The red arrows appear to the right and left of a channel wall.produced by scratching ................................................... 49 4-5 I-V data for PEDOT:PSS......................................................................................... 50 4-6 Typical J-V data for a PProH polymer device. Note the e xponential character typical of polym er films........................................................................................... 50 4-7 Dependence of I-V data on thickness for PPrOH film s........................................... 51 4-8 Linear regression of log I vs log V data for PProH fil ms........................................ 52 4-9 Linear regression of log I vs log V data for ProDOT. .............................................. 53 4-10 Increased J were observed upon h eating the 25nm PProDOT sam ples................... 53 4-11 Effect of baking and re laxation on current density. ................................................. 54

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xi LIST OF ABBREVIATIONS Eg bandgap q carrier charge carrier drift velocity Ttr carrier transit time conductivity J current density Jx current density in x direction Diracs constant d distance between repeat units in polymer E electric field m electron mass E energy change between bands En energy eigen values EHOMO energy of highest occupied molecular orbital ELUMO energy of lowest unoccupied molecular orbital field effect factor d film thickness RH hall coefficient Ey hall field W hall film thickness Vy induced potential

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xii kn k-value, as related to energy Eigen values Bz magnetic field in z-direction mobility N number of orbitals, number of repeat units o permittivity of free space h Plancks constant L polymer chain length or infinite well length n quantized states, or number of free carriers r relative permittivity Tc trap characteristic temperature Nt trap density V volts wave function

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xiii Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science CHARGE CARRIER TRANSPORT IN CONJUGATED POLYMERS By Bryan E. Wilson December 2007 Chair: Paul H. Holloway Major: Materials Science and Engineering Current-Voltage measurements and charge transport propertie s of poly(3-hexylthiophene) (P3HT), poly(3,4-propylenedioxythiophene) (PProDOT) and poly(3,4propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring (PProH) films have been studied. The zerofield hole mobility (h) was determined from currentvoltage data by iterating curve fitting parameters in the space charge limited current model which was derived from Childs Law, also known as the Mott-Gurney Law. To measure hole mobility, hole only devices were constructed with indium-tin -oxide (ITO) anodes and gold cathodes (very large electron injection barrier ) on a glass substrate. A hol e transport layer of Poly(3,4ethylene-dioxythiophene):poly(st yrenesulfonate) (PEDOT) was spin coated between the ITO and sample polymer film in order to reduce the energy barrier for injection of holes. The effects of spin coating speed on film thickne ss, and subsequently on the electronic properties of the mate rials was also investigated.

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xiv Device preparation in a glove box using an argon ambient with oxygen and water concentrations of <5 ppm was found to be critical for reprod ucible electrical data. Spin coating speeds of 700 1000 RPM for 30 second s resulted in thin films ranging between 10-90 nm as measured by atomic force micros copy (AFM). Hole transport in films of PProH was space charge limited for voltages in the range of 0-5V, with mobilities of 1.6x10-6 cm2/V-s. In contrast, hole transport in films of PProDOT was trap limited. The origins of the traps were speculated to be residual impurities and/or structural deformations.

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1 CHAPTER 1 INTRODUCTION AND MOTIVATION The incre ased use of organic and po lymer light emitting diodes (OLEDs and PLEDs) in the solid state lighting and displa y industries is the motivating force for the research presented in this work. Currentl y, conversion of energy fr om fossil and nuclear fuels to electricity provides most of the en ergy required to artificially illuminate living and working environments. In a recent publ ication, the United States Energy Information Administration showed that th e energy used to meet domes tic residential and commercial lighting requirements in 2005 was 4.2 quadri llion BTUs [1]. However, because the currently available sources of light are ine fficient, only about 30% of this total energy was used to actually produce light with the re st being wasted as heat [2]. The limited quantities and environmental impacts associated with the use of fossil fuel and nuclear energy points to decreasing the amount of wasted energy with the introduction of new lighting technologies utilizing effi cient, emissive materials. Coincidentally, the development of new em issive materials and new technologies in the display industry is also being investigated. Technologi es such as the Cathode Ray Tube (CRT), Liquid Crystal Displays (LCDs) and Plasma Display Panels (PDPs) currently dominate the display market [3]. However, several factors have limited these technologies. For example, t hough the technology is currently the cheapest available, the bulkiness and weight of CRTs have excluded them from the popular flat panel market [3]. Also, viewing angle restrictions caused by the birefringence property of liquid

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2 crystals in LCDs has caused the need for development of compensating technologies [3]. Finally, the high energy requireme nt of PDPs has limited this technology to static (i.e., non-portable) applications. The advantages of OLEDs are that they are easy to process, are characterized by low operating voltages a nd exhibit wide viewi ng angles and high contrast ratios. Furthermore, the mechanical properties of polymer films open the door to flexible display applications [4, 5]. Typically light emitting diodes are separated into two categories: inorganic (hereby referred to as LEDs) and organic (referre d to as OLEDs). OLEDS may be further categorized as either small molecule (SMO LED) or conjugated polymer (PLED) devices. Devices based on inorganic materials are generally comprised of compound semiconductors such GaAs, GaP, AlGaAs, In GaP, GaAsP, SiC, ZnSe or InAlGaN. However, the technologies used to deposit thes e materials are similar to that utilized to fabricate silicon integrated circuits thereby making them relatively expensive [6]. On the other hand, while SMOLED technology may also take advantage of precision deposition technologies such as those requiring vacuum, depositing polymer materials for PLEDs is quite cheap as the material can be deposited from solution by spin coating, sometimes in ambient laboratory conditions. In any OLED, emission of light requires elec trical charge (electrons and holes) to be injected into the organic thin films, for th e electrical charge to be transported in the material with minimum energy loss, and for the electrons and holes to recombine and emit light. The focus of this research is the charge transport properties, namely the hole mobility (h), of a novel polymer material: poly(3,4-propylenedioxythiophenediethylhexyloxy)-cyano-p-phenylene vinylene substituted with dodecyloxy chains on the

PAGE 17

3 phenylene ring, referred to as PProH:CNP(MEH) or PProH for simplicity. Charge transport was measured as the current versus applied voltage as a function of processing and time. Spin-coating was investigated as an easy deposition method which allows the sample film thickness to be varied. The curr ent transport is also correlated with changes in the film thickness. This work describes relatively simple pr ocess for gaining insight to the charge transport properties of new polymer mate rials. By measuring and reporting hole transport properties of new materials, this wo rk provides knowledge th at may be used in the future to improve OLED based devices and their respective efficiencies. In this thesis, a review of the literatu re in Chapter 2 provides a description of polymer based device physics, and electrical properties. The expe rimental procedures section, Chapter 3, includes a review of th e processing methods and a description of characterization tools that were used to determine the hole-mobility from electrical properties. Chapter 4 contai ns the experimental results and comparison of electrical properties of devices utilizing films of PPr oh:CNP(MEH). Finally, Chapter 5 provides a summary and conclusions from the experime ntal results of this work as well as recommendations for future work.

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4 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction The purpose of the study described in this th esis is to measure carrier transport properties in conducting, conjugated polymer s. This was accomplished by comparing experimental data from current-voltage (I-V) measurements with the space-charge limited current (SCLC) or trap-limited current (TLC) models to extract the hole mobility ( ). The results from this work are useful for several applications including those related to organic light emitting diode (OLED) applicati ons with respect to charge balancing for efficient electron-hole recombination a nd photon emission. This chapter reviews background information which includes a brief history of the OLEDs, device architecture and operation, conducting polymer physics, a nd the modeling of carrier transport mobility. 2.2 Organic Light Emitting Diodes 2.2.1 History Electrolum inescence (EL) is the non-thermal generation of light resulting from the application of an electric field, and is acco mplished by recombination of charge carriers of contrary sign (electrons and holes) that are injected into a semiconductor in the presence of an external circuit [6]. EL was demonstrated in organic materials in 1963. EL has been reported as first being observ ed from inorganic ZnS phosphor powder by Destriau et al. in 1936 [7]. Recently however some have credited the earlier works of Oleg Losev with his published reports on li ght emission from zinc oxide and silicon

PAGE 19

5 carbide crystal rectifier diode s in 1927 [8]. In the 1960s, the General Electric Company introduced the first commercially available LED devices based on inorganic compound semiconducting materials [6]. The development of organic EL devices was initially hampered by the high voltages--on the order of 100V or above--required to achieve major light output by injection of ch arge into organic crystals such as anthracene. However research in the field was stimulated by the findings of Tang and VanSlyke in 1987. In their work, the researchers from the Eastma n Kodak Company demonstrated a novel thinfilm device structure utilizing a two-layer architecture made from an aromatic diamine emissive layer and an organic small molecu le carrier transport layer composed of 8hydroxyquinoline aluminum (Alq3). Their device was driven to significant brightness by a dc voltage as low as 2.5V [9]. The first account of EL from a semic onducting, conjugated polymer was reported in 1990 by Burroughes et al. and was based on an emitting layer of poly(p-phenylene vinylene) (PPV) [10]. Burroughe s OLED adopted the thin f ilm layer device architecture which has become quite common for use in e xperiments (Figure 2-1). In this basic device architecture, indium tin oxide (ITO), which is sputtered onto a glass substrate, serves as the anode. In many cases, the polymer layers are spin coat ed over the ITO and a metal is thermally deposited over the polymers and serves as the cathode. The anode and cathode are then connected to an external circuit and forward biased with the positive voltage on the ITO anode. Additionally, each of the films thicknesses are between 10nm to a few hundred nanometers. PPV was initially shown to emit in the green-yellow part of the spectrum, but several different polymer compositions of varying bandgaps have since exhibited

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6 emissions with wavelengths scattered throughou t the visible part of the spectrum [12, 13, 14]. Also, variations in device architecture su ch as multi-layered or stacked devices that incorporate enhanced carrier transport (or ba rrier) layers have been shown to increase light output efficiency and lower turn on vo ltages [15, 16]. More recently, at a meeting of the Materials Research society, it was reported that white OLEDs with an efficiency of 57 lumens per watt of power (lm/W), were pr oduced in Japan [17]. Comparison of both conventional, inorganic and white-organic LED s shows increase in efficiency over the years indicates the (Figure 2-2). It is not ed that on average, fluorescent bulbs produce about 60-100 lm/W while incandescen t bulbs produce 17 lm/W [18]. In this review of the literature, methods used to calculate hole mobility are presented along with some of the strengths and weaknesses of each method. First though, a brief summary of carrier recombination and light production in OLEDs, including a discussion of device physics, and device archit ecture is presented. A review of the electronic structure of conjugate d polymers is also presente d and provides a basis for the discussion of carrier transport in this special group of se miconducting, organic materials. 2.2.2 Organic/Polymer Semiconductor Physics As their nam e suggests, some organic materials exhibit semiconducting behavior. The basis for this behavior is related clos ely to the formation of a double covalent bond between two carbon atoms. Double bond forma tion can be explained by the occurrence of specific bond angles as predicted by the valence-shell electron pa ir repulsion theory (VSEPR theory) as well as by the energies a nd locations of electrons as predicted by the Pauli Exclusion Principle and Molecular Orbi tal Theory. In the simple case of an ethylene molecule (Figure 2-3), the doubl e bond that forms between carbon atoms requires that four electrons are shar ed between the two carbon nuclei.

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7 Additionally, two of the electrons, one each from the carbons, are present in sp2 hybridized orbitals (Figure 2-4) and overlap end to e nd to form a sigma bonding molecular orbital ( -bonding MO) [19]. The two remaining electrons (again, one each from the carbon atoms) remain available for further bonding. However, as stated by the Pauli Exclusion Principle, these electrons ca nnot exist in the same quantum state (e.g., the same orbital or the same space and around the same nucleus) as the other two sp2 electrons. Therefore, the remaining elect ron exists in unhybridized p-orbitals. In each p-orbital, two regions of high ch arge density are located on opposite ends of the central nucleus and in the case of the preceding double bond, are positioned perpendicular to the -bond. For the second bond to form, the p-orbitals from each carbon overlap which results in formation of the pi-bond ( -bond) [20]. Constructive interference during the p-or bital overlap creates a -bonding molecular orbital (MO), while that resulting from destructive interference is the *-antibonding MO. Each essentially forms half of the total -bond (Fig. 2-5). As the backbone of organic molecules beco mes longer, such as those consisting of four or more carbons, they can sometimes fo rm conjugated systems of alternating single and double bonds. Conjugated systems may consist of a benzene ring, or a system of a few (<100) linked unit molecules (-mers) ca lled oligomers, or longer chains ( 100010,000) of mers called polymers. The electrons associated with each carbon atom in these systems first fill the available orbitals (i.e., the closest to the atom outward to the valence -orbitals), with the last pa ir of electrons occupying what is known as the highest occupied molecular orbital (HOMO). The next molecular orbital beyond the HOMO is known as the lowest unoccupied molecular orbital (LUMO). Interestingly, the -bonds

PAGE 22

8 that form the HOMOs and LUMOs of each car bon can actually overlap above the single bonds and in effect, the electrons in the double bonds are delocalized over the whole macromolecule [21]. The delocalized elec trons that are weakly bound (known as the electron cloud) can be ionized relatively easily and the electr on vacancy (hole) or surplus electron can travel along the molecule with relative ease [22]. The HOMO and LUMO therefore act similarly to the valen ce and conducting bands found in inorganic semiconductors with a band gap separating the two energy levels. Polymers of various chain lengths result in varying band gaps. This is easily predicted by a simple substitution of a single sy stem of repeat units combined to a chain of length L, into the free electron orbital model [23]. Consider the solution to the time-inde pendent Schrdingers equation for a free electron in a one dimensional potential well of infinite depth a nd width L as given by Equation 2-1 [24]. xkBxkAxnnnn ncos sin)( (2-1) The wave function in this well known parti cle in a box model are required to be continuous at the boundaries (i.e ., x=0 at the origin and x=L at the width of the potential well), therefore the solution requires that for n = 0 at point x = 0, and Bn must also be zero. Furthermore, for n = 0 at x = L, the sin(knL) value must also equal zero. Therefore, the proper solution is summarized with Equation 2-2, L xn AxkAxnnn n sin sin)( (n = 1, 2, 3, ) (2-2) From this solution, it can easily be seen that, L n kn (n = 1, 2, 3, ) (2-3)

PAGE 23

9 where kn, known as the k-value has been related to a set of energy levels (energy eigen values, En) defined as 22 n nmE k (2-4) In effect, the boundaries of the potential well have defined a discrete set of allowed kvalues and therefore, a discrete set of en ergy eigen values as given in Equation 2-5 2 222282mL hn m k En n (n = 1, 2, 3, ) (2-5) where h is Plancks constant, m is the electr on mass and n defines the energy level, i.e., is a set of quantum numbers [21, 23, 24, 25]. For a polymer of length L made up of N repeat units and separated by a distance d (i.e, L in Equation 2-5 approaches Nd for long chains), we see that the energy values given by Equation 2-5 can be modifi ed as given by Equation 2-6: 2 22)(8 Ndm hn En (n = 1, 2, 3, ). (2-6) The energies of the HOMO and LUMO levels of the chain are assumed to be defined by the electrons from the N p-orbitals. Recalli ng that each molecular orbital is filled by two electrons, and that each orbital is separated by one energy level, the HOMO has the energy given by Equation 2-7 and that of the LUMO is given by Equation 2-8 2 2 2 )()(8 2 Ndm h N EHOMO (2-7) 2 2 2 )()(8 1 2 Ndm h N ELUMO (2-8)

PAGE 24

10 Now, given that the energy necessary to promote an electron from the HOMO to the LUMO is called the band gap, or Eg, this value is easily defined as the difference between ELUMO and EHOMO as: 2 2 2 2 2 2 2 2 LUMO)(8 1 )(8 2 )(8 1 2 EgNdm hN Ndm h N Ndm h N (2-9) The limit, reached by large chains (large N), is given in Equation 2-10. Eg = Nmd h )8(2 2 (2-10) From Equation 2-10, it can be seen that as the chain length increases (i.e., as N increases), the band-gap (Eg) decreases. However, because the electron density given by the alternating double bonds in conjugated polymer systems is not equally distributed, the inter-carbon lengths (the spacing between the car bon-carbon double bounds versus carbon-carbon single bonds) are not equal. Experimentally it has been shown that this result forces a limit below which the band gap of a given conjugated system will not decrease, even with added length of additional mer-repeat-units [26, 27]. The length at which no change in band-gap is attained is known as the sa turation length. There have been several successful attempts to engineer the conjugation lengths of the active conjugated systems for use in OLED s thereby altering the band gap and hence, precisely tuning the emission wavelength of the devices [28, 29]. One of the notable challenges in tuning a material for a partic ular low band gap by increasing the chain length has been to achieve c onjugation lengths that gives both the desired band gap and polymer solubility [28]. As was stated pr eviously, one of the advantages of polymer OLEDs is in the fact that they ar e easily processible from solution.

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11 2.2.3 Device Operation The fundamental purpose of electrolumine scent polymer devices is to convert electrical power into light. The processes i nvolved in the production of light within an OLED are summarized simplistically in Fig. 2-6. First, charges of opposite sign (i.e., positive holes and negative electrons) are injected from opposing electrodes by applicatio n of an external voltage (forward bias). The carriers then travel through additional layers which eith er promote or inhibit their motion. Finally, when the carriers of oppos ite sign travel close enough to attract one another, either in a special recombination layer or at the interf ace between the hole and electron transport layers, they recombine forming a singlet exciton which can then decay radiatively and results in emission of a photon [11]. One of the indicators used to describe how efficiently a device produces light is the external quantum efficiency, ext, which is the ratio of the number of photons emitted by the device (into the viewing direction) to the number of electrons in jected. The reason viewing direction is important in this definition is because many of the photons that are produced emerge from the sides of the device or can be re-absorbed within the various organic layers. Therefore, ext is several times lower than internal quantum efficiency, in, which incorporates all photons produced over all angles and with negligible reabsorption [31]. Though the above summary of OLED device ph ysics is presented in a very general form, it can be seen that one of the greatest challenges in this field is to increase the percentage of electron and hole pairs (excitons) that recombine radiatively at an interface or within an emitting layer in order to increa se the quantum efficiency. To meet this challenge, one must control the number of car riers entering the various layers of the

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12 device and also exploit the rate at which the carriers are able to move through each layer. Knowledge of parameters such as the band gap as well as the carrier mobilities becomes important to properly tune th e location and rate at whic h recombination occurs and thereby increases the quantum efficiency [32]. To gain an appreciation of the device physics as it relates to the generic device architecture mentioned above, one must study the energy diagram of an OLED. A typical structure and band diagram (Figure 2-7). In the figure, EA is the electron affinity, IP is the ionization potential, HTL is the hole tran sport layer, LUMO is the lowest unoccupied molecular orbital, HOMO is the highest occupied molecular orbital, EML is the emitting layer, a is the anode work function, c is the cathode work function and Ev is the vacuum potential [33]. The LUMO and EA are equi valent to the conduc tion band in inorganic semiconductors and will theref ore be referred to simply as the LUMO in reference to organic semiconductors. Additionally, the HOM O and IP are equivalent to the valence band and will all be referred to as the HOMO. When a voltage is applied to the electrodes, such that the anode is biased positively, the electronic bands are bent in a manner similar to Figure 2-8. Ideally, this bending narrows the otherwise high energy barriers ( b) that are present at the electrode/polymer interface. Hence, hole injection from the anode Fermi level, EFa, and electron injection from the cathode Fermi level, EFc, are promoted to the organic layers HOMO and LUMO, respectively. This injection is repr esented by the curved arrows which indicate carriers tunneling throug h the energy barrier, b. Furthermore, the use of a high work function anode a, such as indium-tin-oxide (ITO ), and a low work function cathode c, such as calcium, helps ensure that carrie r current is not inj ection limited (i.e., the bs are

PAGE 27

13 low enough so that the rate of charge carri er injection is larg e enough to not limit the amount of current being conducted). If the bias is instead appl ied with the opposite polarity (reverse bias), carriers are not able to surmount the potential barrier that is present at e ither of the electrode/polymer interfaces. In this case, with a negative potential placed on ITO (which has a large work function value) for example, the electrons w ould need to be injected into the polymer from ITO instead of Ca. This would result in the electrons being blocked from injection into the LUMO of the polymer due to the large b. This phenomenon is commonly referred to as rectifying behavior an d is characteristic of diodes [35]. In some cases it may be useful to reduce the number of carriers in a device. To measure a transport property of one partic ular type of carrier in a semiconducting polymer, for example mobility, the number of charges of the other carrier type being injected into the material must be limited. In other words, to make a single carrier device, one must lower the carrier injection efficiency of a particular contact by selecting appropriate materials for the anode or cathode. For example, to measure hole mobility in regioregular poly(3-hexyl-thiophene) (or RR-P3HT) diodes, a group led by Michael McGeh ee at Stanford University constructed hole-only devices. The de vices employed aluminum cathode s with a work function of c=4.2eV to have large mismat ched energies with the LUMO of RR-P3HT at 5eV [36]. With this configuration, the electron injection energy barrier b is equal to 0.8eV and therefore negligible electron injection. In another example (Figure. 2-9), f our materials and their respective work functions are shown as cathodes in a three layer device with MEHPPV selected as the

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14 semiconducting polymer (LUMO =2.8eV, HOM O = 4.9eV). Indium, with a work function of about 4.2eV offers the lowest electron injection energy barrier of b = 1.4eV while gold, with a work function of about 5.2eV, offers the largest barrier with b =2.4eV. Comparing the various cathode s introduced in Figure. 29 with the resulting I-V curves of Figure 2-10, one can see that alt hough the energy barriers for electron injection may vary between 1.4eV and 2.4eV, the overa ll current in the device remains the about same at a given voltage, indi cating that electron injecti on from the cathode is not controlling current in the device. These data are evidence that the device cu rrent is dominated by holes. In fact, holes are often cited as the major charge carrier in OLEDs because hole-mobility is generally higher than that of electrons in conjugated polymer materials [38]. Alternatively, an electron-only device can be assembled by mismatching the bandoffset between an anodes work function, a, and the organic semiconductors HOMO, while maintaining a low barrier between the cathode and LUMO. 2.3 Charge Carrier Transport 2.3.1 Mobility Charge carrier mobility () is an important parameter to characterize the carrier transport and resulting performance of polymers that are used as electroluminescent materials in light emitting devices [39]. At low electric fields, the drift velocity () is proportional to the electric field strength (E), and the carrier mobility is the proportionality constant between these two valu es [40]. Therefore, carrier mobility is defined as the carrier drift velocity per unit applied electric field:

PAGE 29

15 E sV cm2 (2-10) Polymer semiconductors are known to have low charge carrier mobilities in comparison to carrier mobility in inorganic ma terials. For example, the hole mobilities of a wide range of organic semiconductors for us e in optoelectronic applications have been reported in the range of 10-7 to 10 cm2/V-s while that of p-type gallium nitride, which is used in inorganic LEDs, has been reported at 400 cm2/V-s [41, 42]. The low values for polymers are attributed mainly to disorder of the polymer chains in conjunction with trapping due to the presence of impurities [43, 44]. Extrinsic variables such as temperature and applied electric field strength are also known to affect mobility [45]. High carrier mobility results in faster response as well as to reduction of operating voltage in LEDs [44]. However, in a st acked structure a low mobility in one layer followed by a high mobility in the next laye r can cause charges to accumulate and effectively form a parallel plate capacitor [ 46]. Knowing the mobilities of charge in the various layers can assist in designing an op timized device where charge is accumulated at the desired location(s). Other conducting organic devices that depe nd on carrier mobility include organic field-effect transistors (OFETs). OFETs may be integr ated into a number of products ranging from RFID tags to active-matrix displa ys. Similarly, a disadvantage of FETs that contain organic materials is that the active layer charge mobility, as compared to their inorganic equivalents, is rela tively low [47]. However, OFETs may be processed with greater ease and have superior mechanical (flexural) properties. These advantages generally result in lower cost and a broader range of applications.

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16 It is important to note the relationshi p between mobility and conductivity of a material. For the case of n free charge carriers per unit volume, each with a charge q, the charge density is nq. With the electric current density (J) defined as the charge density times drift velocity we have: J = nq (2-11) From Ohms Law, the current dens ity is defined as conductivity,, times the electric field: J = E (2-12) Equating Equations 2-11 and 2-12 results in: E = nq Now, substituting for electric-fie ld from Equation 2-10 above: nq i.e., conductivity can be expressed in terms of the mobility [25]. It is important to note that a large drift mobility does not automati cally equate to a high conductivity because also depends on the concentration of charge carriers. Over the years, several experimental me thods have been used to measure the mobility of charge carriers in semiconductors [36, 40, 41, 48, 49, 50]. Generally, these methods vary based on the way external fields affect charge ca rrier transport. For example, Hall mobilities rely on the effects on charge carriers of both electric and magnetic fields. The time-of-f light method relies on a pulsed ex ternal light source, such as a laser, to generate carriers in combinati on with an applied electr ic field. Finally, the space charge limited current method (SCL or SCLC) relies solely on an applied electricfield to inject and transport th e carriers [48]. Each of these methods is discussed below.

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17 2.3.2 Hall Effect Method An electric field (Ex) applied to the x-axis of a sa mple along with a magnetic field (Bz) applied perpendicularly along th e z-axis, gives rise to a trav erse electric field (in the y-direction) which exerts force on the charge carriers. As carriers are forced in the ydirection, the induced electric field Ey (the Hall field), which is derived from the induced potential (Vy) and thickness of the sample in that direction (W), balances the force induced by the magnetic field [40]. From the Hall Effect, it can be shown that: zx H y yBJR W V E (2-15) where R is the Hall coefficient and is equal to (1/nq) and Jx is the current density of the charges flowing in response to Ex. RH can be determined from the values of Ey, Jx and Bz using Equation 2-15. If one type of carrier dominates, the carrier co ncentration and type can be determined from the Hall coefficient. The carrier mobility can be determined using Equation 2-16 [25]. RH In 1958, a method was develope d that exploits the Hall E ffect, but is modified for measurement of flat samples of arbitrary sh ape [51]. This method, known as the Van der Pauw technique, has been commonly used fo r measuring transport properties of many inorganic semiconductors such as p-GaN, n -InN and n-InGaN having mobilities of 46, 2 and 107 cm2V-1s-1, respectively [52]. However, the Van der Pauw technique is not commonly used to measure the carrier mob ility of polymers. Though organic samples can be prepared to meet the requirements fo r applying this technique the current density, Jx, is often too low to induce a measureable Hall voltage. Essentially, the carrier mobility in conjugated polymers is usually too low to be measured by the Hall Effect [53]. The

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18 Hall voltage, which is typically in the ra nge of a few meV even for high mobility materials, is too low in polymers, resulting in a signal-to-noise ratio that is very low. Mobilities in polymers have been reported utilizing the Van der Pauw technique/Hall Effect. For example, the Ha ll mobility of AsF doped (17%) polyacetylene was reported to be 2x10-2 cm2V-1s-1 [54]. In another case, the Hall technique was used to measure mobility in poly(4,4-dipentoxy-2,2bithiophene). Results were comparable to amorphous inorganic semiconductors with values of 10-1 to 102 cm2V-1s-1 [43]. 2.3.3 Time of Flight The time-of-flight technique is the most wi dely used method to measure mobilities in organic conductors [49]. In this method, the organic material is placed between two electrodes, one of which is transparent and does not inject carriers. A sheet of charge carriers is photogenerated by external pulsed laser or lamp illumination through the transparent electrode and the carriers move through the sa ndwiched sample to the other electrode under the influence of an applied electric field [55]. To determine the hole mobility, a thin ch arge layer may be generated near the anode (Figure 2-12. In this case, the light pulses must not be absorb ed in the cathode or by the sample. Photogeneration must take place at the generation layer and electrons collected at the nearby anode, while holes migrate across the complete thickness of the sample layer [56]. A transient photocurrent profile is reco rded and a clear inflection point is determined from the current-de cay pattern [57]. The inflection point, is the intersection of two tangents on the log current-log time plot (Figure 2-13), and is equal to the carrier

PAGE 33

19 transient time, Ttr, of the fastest carriers [44]. In other words, Ttr is the time required for the photogenerated carriers to reach the opposite transparent electrode. The charge carrier mobilities are then calculated by manipulating Equation 2-11 into: VT dtr 2 (2-17) where d is the sample thickness and V is the applied bias. Utilizing the time of flight method, Kane to et al. has reported hole mobility of regioregular poly(3-hexylthi ophene) (MW 29 kg/mol) of h = 4x10-4 cm2/V-s for drop cast films having a thickness of 7m [41]. TOF requires thick films so that the absorption depth of the optical excitation is small compared to the sample film thickness. TOF experiments on poly-phenylenevinylene (PPV) for example, have been reported with film thickness of 1-10m. To attain such thickness, the depos ition by spin-coating requires sl ow spin rates. Therefore it is preferable to form thick films by drop-casting. Unfortunately, preparation by this method can influence mobility because the ma terial structure of drop-cast films differs from that of spin-cast films [45]. Differences in mobility measured by TOF between drop cast and spin coated films have been reported for regiorandom-poly(3hexylthiophene). These mobilities were = 10-7 cm2/V-s for drop cast films having a thickness of 5400nm, and =10-5cm2/V-s for the spin deposited films having thickness of 200nm [41]. Determining the TOF-mobility of films with a distribution of mobilities (dispersive transport) is also complicated since the arriva l of carriers at the electrodes is spread out

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20 and usually the fastest carriers make up the bulk of the measurement [53]. 2.3.4 Space Charge Limited Current Charge transport through a thin organic pol ymer film is said to be either an electrode limited process or a bulk limited process [39]. In an electrode limited process, carrier injection is limited by the energy barrier between the electrodes work function and the polymers HOMO or LUMO levels, de pending on whether the injected carrier is a hole or an electron, respectively. In a bul k limited process, carrier transport is limited by the drift velocity through the material, i.e., the mobility of the charge carrier. When the relationship between current (I) a nd voltage (V) is linear, it is indicative of an ohmic system. This is the case for tran sport in metals where the I-V relationship is described by Ohms law, V = IR, or alternativ ely with respect to current density (J), J = qn d V. (2-18) In this modified version of Ohms law, q is the carriers charge, n the charge carrier density, the carrier mobility, V the applied voltage, and d the thickness of the sample. This is also typically the case at low voltage s when injecting carrier s from a metal contact into a conjugated semiconducting polymer. Ho wever, a point exists where this condition breaks down and non-linear I-V behavior, characteristic of all organic LEDs and known as space charge limited current (SCLC), is observed [58]. At higher voltages, more carriers are being injected acro ss the barrier, and at a faster rate than they can travel through the polymer and exit at the other contac t. This results in charge accumulation near the injecting electrode which redistribut es the electric field intensity, controls transport through the polymer bulk and determin es the I-V dependence [59]. In practice,

PAGE 35

21 when at least one of the interfacial barriers is lower than 0.3eV, the current in an organic LED will be SCLC [60]. In cases where these conditions apply plus no traps are present in the polymer film and the mobility is independent of the fi eld, Equation 2-19 describes trap free space charge limited current. 3 28 9d V Jro (2-19) where o and r are the permittivity of free space and relative permittivity, respectively [61, 62]. Conversely, due to their intrinsically disordered nature, the carrier mobility in most conjugated polymers is field dependent [36, 63]. E quation 2-19 has therefore been modified as 3 2 89. 08 9d V e JE ro (2-20) Utilizing either Equations 2-19 or 2-20, it is pos sible to extract the charge carrier mobility by fitting the experimental J-V data [39]. To prevent charge recombination, single carrier devices as previously described must be constr ucted. This allows for measurement of the current and resulting mobility for either elect rons or holes. Unfortunately, since the work function of one of the electrodes has to be ad justed to make the device either a hole-only or electron-only diode, the electron and hole mobili ties cannot be measured with the same device [53]. Hole mobility was measured for poly(3hexylthiophene) in the space charge regime. It was found that the lower molecula r weight samples had mobilities that were field-dependent. Fig. 2-14 shows a plot of J0.5 vs V for low molecular weight poly(3-

PAGE 36

22 hexylthiophene) and film th icknesses of 96nm and 192nm. The inset shows non-linear J vs Vbias characteristic, indicative of sp ace charge limited current. Equation 2-19, shown as the dark straight lines in Figure 2-14, does not fit the data well, especially for the 96nm sample. Howeve r the data overlaps the dashed line, which is Equation 2-20, reasonably well. This indicates that the hole mobility in fact is fielddependent in low MW samples of poly(3-hex ylthiopene). The resulting hole mobilities varied from 1.33 0.41 x10-5 cm2/V-s (MW 2.89 kg/mol), 1.13 0.37 x10-4 cm2/V-s (MW 9.72 kg/mol) and 3.3 0.73 x10-5 cm2/V-s (MW 31.1 kg/mol) and are in agreement with the time of flight measurements of Kaneto et al. as reported above [36]. To date, several other I-V experiments have been performed on various polymers to determine carrier mobility from the space char ge limited current model [39, 64, 65]. The present study will use this SCLC method to determine the hole mobilities of poly(3,4propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylene-vinylene substituted with dodecyloxy chains on the phenylene ring. 2.3.4 Trap Charge Limited Current For trap-free films, the space charge model predicts that I V2. Yet in an early assessments of space charge in inorganic materials, Smith and Rose observed a high power dependence (I Vm with m > 2) for insulating CdS cr ystals and attributed this to the filling of traps followed by an excess of ch arges after all the traps are filled, resulting in an increased number of carriers [66]. While they recognized that the presence of traps can determine the relationship between cu rrent and voltage (i.e., the power law dependence), they did not appl y this model to organic materials. More recently, the presence of traps in anthracene has been re ported with a very hi gh power exponent of

PAGE 37

23 IV12 (at T = 20oC) [67]. Organic materials such as conjugated polymers may have traps present as a result of structural disorder, impurities, geminate pa irs or self-trapping [68]. Traps affect the charge transport properties by re moving carriers that contribute to the charge transport. When traps have a continuous exponential ener gy distribution, the dependence of current on voltage for this I-V region is called trapped-charge limited. It is in this region that as the applied forward bias is increased, trap s below the quasi Fermi energy level for a specific carrier are filled. During the trap -filling the trap-free space charge equation (Equation 2-19) must be modified to reflect the fact that not all carriers are available for transport. The relation between J and V for trapped-charge-limited currents (TCLC) is defined by Equation 2-21: 12 1 1 1)1(1 12 m m m m md V mNt m m m qNJ (2-21) where N is the density of states in the conduction band or the valence band, Nt is the total trap density, m = Tc/T where Tc is the char acteristic temperature of the traps, T the absolute temperature, and the balance of the terms are defined above. At high current, all the traps are filled and they no longer affect ch arge carrier transport. The current at this point behaves similar to that of a trap-free space-charge limited current [65].

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24 Figure 2-1. Structure of polymer LED (OLED) [11] Figure 2-2. Progress in LED efficiency [17]. Red line is conventional LEDs. Figure 2-3. Ethylene mo lecule Lewis Structure Figure 2-4. Ethylene molecule depicting s-oribitals, sigm a bonds and p-orbitals [19].

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25 Figure. 2-5. Depiction -bonds in ethylene molecule [20]. Figure 2-6. Summary of carrier transport and reco mbination in OLEDs [30]. Figure 2-7. Energy band diag ram of typical OLED [33].

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26 Figure 2-8. Band diagram of an organic electroluminescen t layer (OEL) under forward bias [34]. Figure 2-9. Three layer device depicting four possi ble cathode materials [37]. Fig. 2-10. I-V for ITO/MEH-PPV device with various anodes [37].

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27 Figure 2-11. Basic setup to measure carrier concentration using the Hall effect [40]. Fig. 2-12. Time of Flight Experimental Setup [56]

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28 Fig. 2-13. Time of Flight photocurrent prof ile at various applie d bias voltages [41]. Figure 2-14. J0.5 vs. V plot for low MW (29.9 kg/mol) poly(3-hexylthiophene).

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29 CHAPTER 3 EXPERIMENTAL METHODS 3.1 Introduction This chapter describes the various tools, techniques and procedures which were used to prepare samples and collect the experimental data reported and discussed in this thesis. First, the method used to prepar e the indium tin oxide (ITO) coated glass substrates is described. Th e next section describes device architecture and how devices utilizing a high work functi on anode and cathode were used to ensure single carrier transport. The use of a simple two point probe setup to measure current (I) versus voltage (V) applied to the devices is then de scribed. Next, the use of profilometry and atomic force microscopy (AFM) to characteri ze film structure is summarized. Last, an overview of the software utilized for data an alysis and computation of carrier mobility (h with units cm2/Vs) is presented. 3.2 Device Preparation 3.2.1 Substrate Cleaning and Etching Glass slides coated with 150-200 nm film s of ITO were purchased from Delta Technologies, Ltd.. The slides measured 1 x 1 inch-square were reported to have a sheet resistance (Rs) of 8-12 per square CB40-IN Corning 1737 ITO One Surface). To begin the process of device fabrica tion, the ITO was patterned by a room temperature acid vapor etch. The acid used was a mixture of three parts hydrochloric acid (HCl) to 1 part nitric acid (HNO3) by volume, commonly referred to as aqua-regia. This step makes it possible to later deposit metal contacts on th e device in such a way as to make eight

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30 devices on a single substrate. To remove the ITO in select areas, a mask made of simple scotch-tape was utilized to e xpose specific areas of the slide to the aqua-regia vapor for approximately 10 minutes. After etching, th e areas were tested with an ohmmeter to verify that resistance was over 1 Mega-ohm, indicating that the ITO had been removed. Figure 3-1 summarizes this process and s hows the ITO pattern remaining on the slide after the aqua-regia etch. Because of the contamination that occu rs when samples are handled, packaged, exposed to air, and then ITO etched, the samples were subsequently cleaned to remove dust, organic contaminants and adhesive resi due from the scotch-tape mask. First the substrates were placed in a beaker filled with a detergent solution of 20mg of sodium dodecyl sulfate dissolved in 500m L of de-ionized water. The beaker was then placed in a sonicator and the substrates were cleaned for approximately 15 minutes. Next, the detergent solution was emptied and the beaker was then fill ed only with deionized water and the samples were again cleaned in the s onicator. This process was repeated three more times using solutions of methanol, ace tone and finally, 2-isopropyl alcohol. Used solvents were distilled for reuse in this pro cess. The cleaned substrates were placed in clean slide cases and then in vacuum sealed desiccators to limit exposure to moisture and environmental contaminants. 3.2.2 ITO Surface Treatment After the samples had been cleaned and stored as describe d above, they were inserted into a HARRICK PDC-32G Plasma Cleaner and exposed to an oxygen plasma for 20 minutes just prior to polymer coati ng. During the oxygen-plasma treatment, the ITO undergoes physico-chemical and electr onic property modifications, including a smoother surface, higher surface energy with high polarity, and an increased work

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31 function [69]. A smooth and high-polarity ITO surface promotes better adhesion of a polymer film and reduces the in terfacial tension between the polymer and substrate. The increased work function, which is attributed to removal of orga nic surface residues during oxygen ion bombardment, results in increased hole injection due to the decreased energy barrier between the conduction band of ITO and the HOMO level of the polymer [70]. 3.2.3 Addition of Hole Transporting Layer After the samples have been exposed to the oxygen plasma, the next step is to deposit the polymer layers. A hole transporting layer (HTL) consisting of Poly(3, 4ethylenedioxythiophene )-poly(styrenesulfonate), ot herwise known as PEDOT-PSS (Bayer Baytron P VP A1 4083), was spincast ov er the ITO plus the etched areas of the substrate (Figure 3-1). Subsequently, the ac tive polymer film was also deposited by spin coating because of its ease and reliability in creating uniform thin films [4]. For spin casting, the samples were placed on the samp le-holder of a Chemat Technology, Inc. model KW-4A spin coater. The spin coat er spin rate was set manually with the revolutions per minute (RPM) displayed on an LCD. While the sample was at rest, 400L of PEDOT-PSS in an as prepared aque ous solution (Bayer Baytron P VP A1 4083) filtered through a 0.2m nylon filter, was added using a micropipette. Once the PEDOT-PSS covered the entire surface of the sa mple, spinning was init iated at a rate of 3000 RPM. During the spin coating process, most of the polymer solution was removed from the substrate by centripetal force. Af ter a short time (about 15 seconds), a thin liquid film is left on the surface and the solven t evaporates which results in an increase in the viscosity of the film [71] thereby resulti ng in the final film th icknesses of 40 nm. In

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32 the present study, once the PEDOT:PSS was spin coated onto the substrate, the samples were placed in a vacuum oven and baked at 150oC for 4 hours to completely remove all solvent (water) from the films. When the films were dry, the slides were placed in an argon atmosphere of an isolated glove box. For each run, which incl uded preparation of several devices, one slide with only the PEDOT-PSS layer was se t aside to be used as a reference in determining the layer thickness by profilometry. 3.2.4 Addition of Active Layer Research has shown that device character istics are strongly influenced by the presence of moisture [72]. Therefore, a glovebox (Figure 3-2) manufactured by MBraun GmbH and filled with dry argon was utili zed to complete the device fabrication by adding the active layer and top cathode contact. Spin coating of the active polymer, gold contact deposition and current-voltage measurements are all performed within the glove box to prevent exposing the devices to moist air. Though the glove box ambient is pure argon (99.9% from Praxair), oxygen and water concentrations can increase when samples are transferred from the outside environment into the glove box. All experi ments were performed with less than 5ppm oxygen and 5ppm water. Solutions ranging between 8 to 28 mg/mL of poly(3,4propylenedioxythiophene-diethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring (PPr oH) in di-chlorobenzene or toluene solvent were prepared in sm all glass vials. Similarly, 20 mg/mL solutions of poly(3hexyl-thiophene) (P3HT) in di chlorobenzene and of poly(3,4propylenedioxythiophene) (PProDOT) in toluen e were prepared. Prior to being placed in the glove box, the polymer solutions were pl aced in a glove-box load-lock. In order to reduce the amount of oxygen and moisture from the contents entering the glove-box, the

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33 load-lock is exposed to a double decontamina tion cycle where the lo ad-lock contents are exposed to a vacuum of approximately 25 in. Hg refilled with argon, exposed once again to the same vacuum, and finally equi librated with the glove-box atmosphere. A volume of 300L of the active polymer solution was then added over the PEDOT-PSS layer in an even fashion using a micro-pipette. Spin rates ranging from 700rpm to 1200rpm were used to produce a range of PProH, P3HT and PProDOT film thicknesses. After spin coating the poly mer layer over the PEDOT-PSS, the samples were prepared for metal top electrode depositi on. Once again, devices were placed in a vacuum oven and baked at 150oC for 4 hours to completely remove all solvent (toluene or dichlorobenzene) from the films. 3.2.5 Vapor Deposition of Gold Electrodes A stainless steel shadow mask, patterned as in Table 3-2 to allo w for eight different connected dot electrodes per 25 x 25 mm2 slide, was placed against the polymer film. The slides were then placed in a vacuum thermal evaporator system which was pumped down to 10-6 Torr using a turbomolecular pump backed by a oil sealed roughing pump. Three or four pieces of Au-shot 1 mm in diameter were placed in a tungsten boat that was clamped between two post connected to electrical feed-throughs. Au evaporation was monitored by the MBraun integrated thin film deposition controller with a deposition rate for gold inputted at 2.5 /s. Depos ition stopped automatically when the film thickness reached 100nm. After deposition, the samples were allowed to cool for 1.5 hours. The thermal evaporator was then back filled to 1atm with argon from the glove box. Figure 3-3 summarizes the device fabr ication including cathode deposition.

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34 3.3 Current-Voltage Measurements 3.3.1 Keithley Source Meter Current-Voltage (I-V) data were coll ected to determine the hole mobility () in the active polymer layer using a tw o point probe technique to ap ply a bias voltage between the anode and cathode electrodes. Two probe s, one which is biased positively and the other biased negatively during measurement, were attached to a Keithley Series 2400 source-meter. The voltage step and rate, st arting voltage and end voltage were specified utilizing the LabTrace software package from Keithley. The positive probe was contacted to the ITO surface (exposed by rubbing as described below) and the negative probe contacted the vapor-deposited gold electrode. 3.3.2 Sample Holder The custom sample holder (Figure 3-4) was used to hold each sample for I-V measurements. The holder has 12 pins that ar e static on one side and the Keithley probes are connected to these pins by al ligator clips (Figure 3-2). The other end of each pin fits through a hole in the holder and makes contact with the sample with spring loaded, gold contacts. Eight of the pins which are located around the center of the holder are designed to be compressed against the eight gold cathode electrodes of the sample (Figure 3-3, step #5). The remaining four pins located at the corners of the holder are compressed agains t the ITO (anode) surface of the samples, where the active polymer and PEDOT:PSS laye rs were removed by rubbing each corner with a cotton swab saturated in dichlorobenzen e. In this process, single strokes were used to avoid redeposition of material on subseque nt strokes. While the active polymer was removed by dissolution in the dichlorobenzene, the PEDOT-PSS was removed by mechanical force. Removal of the polymer layers was verified visually by a change in

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35 the reflective properties of the corner areas versus the remainder of the sample which was still covered with polymer. For further veri fication of removal, the resistance between the pins contacting the ITO was measured. If the resistance between two pins was 100 +/-20 then it was concluded that the pins we re touching the ITO. If resistance was much higher than this value, the sample was removed from the holder and another swab with solvent was used to remove the polymer layers. This process was repeated until the resistance between all pins contacting th e exposed ITO surface was sufficiently low. 3.4 Structural Characterization 3.4.1 Profilometry To measure the thickness of the active c onjugated polymer layer, a cotton swab was used, as described above, to remove the ac tive polymer and PEDOT-PSS layers again to expose the ITO. In this case, material was removed along narrow paths running parallel to two edges of the samples, from one end to the other. Visual inspection was used to verify that the polymer layers had been re moved. Each path where the polymer was removed consisted of a trough surrounded by la yers of polymer. A Tencor Alphastep 200 surface profilometer was used to measure the depth of the troughs and therefore the total thickness (dtot) of both polymer layers. The same material removal procedure was used to produce troughs on the samples with only a PEDOT-PSS layer, one of which was prepared for each batch of samples as re ported above. The thickness of the PEDOT-PSS layer (dPEDOT) was also measured using the prof ilometer. The difference between dtot and dPEDOT is then equal to that of the active polymer (dp). 3.4.2 Atomic Force Microscopy Atomic force microscopy (AFM) images were taken with a VEECO Dimension 3100 AFM at the University of Florida in th e Major Analytical In strumentation Center

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36 (MAIC). The same samples that were prepared for measurement with the profilometer were also measured in the AFM for comparis on. Images were taken in the tapping mode and the thickness of the polymer layers wa s determined. The root-mean-square surface roughness (RMS roughness) was also determined. 3.5 Experimental Procedures 3.5.1 Film Thickness Variation By varying the rate at which films were spin-deposited between 700 to 1000 revolutions per minute, the thickness of the active conjugated polymer films were varied. Other parameters such as ITO surface-trea tment time, spin speed of the PEDOT-PSS layer, and volume of polymer initially added to the substrate were kept constant. The hole mobility was measured as a function of the active film thickness. 3.5.2 Increased Temperature Exposure Current-Voltage data were co llected from as prepared samples. The samples were then placed on a hot plate, while still inside of the glove-box, and heated in argon to approximately 100oC for 4 hours. The temperature of the hot place surface was verified with a thermocouple. I-V data was again collected for the samples after they were allowed to cool for 30 minutes.

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37 Step Top View Side View 1. A clean 25 x 25 mm square of ITO on a glass substrate is used for etching. 2. ITO is etched. The light areas indicate area where ITO was removed using aquaregia acid vapor. Figure 3-1. Summary of IT O patterning procedure. Figure 3-2. Braun Glove Box us ed for sample preparation. 12 mm 5 mm 12 mm 5 mm

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38 Step Top View Side View 3. PEDOT-PSS is spin coated onto the etched ITO surface and then baked and vacuum dried. 4. The active polymer is spin coated on top of the PEDOT:PSS layer in the dry box. 5. The metal electrodes are vapor deposited on top of active polymer in the dry box. Figure 3-3. Continuation of device preparation, showing (3) PEDOT-PSS hole transport layer, (4) PProH active polymer layer, and (5) Au metal contact depositions. Figure 3-4. Sample Holder (a) with pins (b) and measurement probes (c) attached. a b c

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39 CHAPTER 4 EXPERIMENTAL RESULTS 4.1 Background In this experiment, currentvoltage (I-V) data were coll ected and compared with the space-charge limited current model (as presented in section 2.20) to extract the hole mobility of the conjugated polymers. Current-voltage (I-V) data are taken for devices with active polymer layers of poly(3, 4-propylenedioxythioph ene-diethylhexyloxy)cyano-p-phenylenevinylene s ubstituted with dodecyloxy chai ns on the phenylene ring (PProH), poly(3-hexyl-th iophene) (P3HT), or poly(3,4-propylenedioxythiophene) (PProDOT) films (Table 4-1 for the chemical structure of these materials). To extract hole mobilities from I-V data, three values are required: applied voltage (Vbias), the resulting current density (J), and the polymer film thickne ss (d). Although current and voltage are collected with an automated Keithley controller, control and measurement of the film thicknesses provided the biggest challenge in the experiment. Even after the rigorous cleaning steps discussed in Chap ter 3, deposition of smooth and uniform polymer films was hindered by particulates on the sample surfaces or the effects of different solvents during spin coating. Fo r example, inconsistent surface wetting by the various polymer solutions on the glass/ITO/PEDOT-PSS samples sometimes resulted in non-uniform active layer thicknesse s on the samples. This will be discussed further as the experimental results, such as surface char acteristics of the various deposited polymer films and the calculated hole mobility, are reported below.

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40 4.2 Results 4.2.1 Physical Characterization 4.2.1.1 Film preparation for thickn ess measurements Polymer films were spin coated onto a gl ass substrate coated with a transparent conducting indium tin oxide (ITO) and a PE DOT-PSS layer. Film thicknesses for a given solution were varied by changing spin rates between about 700 to 1000 rpm for a constant 30 seconds. To prepare the sample s for thickness measurem ents, portions of the films were removed by scratching straight lines across the samples with a sharp edge. This process removed polymer from the surface of the glass/ITO substrate making small channels down to the surface of the ITO. As depicted in Figure 4.1, control samples on which only PEDOT-PSS was deposited over ozone plasma treated ITO-on-glass substrates were measured. Once other f ilms were deposited over the PEDOT-PSS, it becomes difficult to measure the PEDOTPSS thickness so samples with only PEDOTPSS were first measured to determine its film thickness. The dark horizontal lines in Figures 4-1 and 4-2 represent the channels fo rmed by scratching the films with the edge of a razor blade, and the channels are repres ented as breaks in the upper film layer. With only PEDOT-PSS deposited on the gl ass/ITO substrate, measurement across the channels with the AFM tip provided the thickness of the PEDOT-PSS layer (i.e., dDOT). Because the spin rates and spin times were held constant for PEDOT-PSS deposition, it was assumed that the thic kness of the PEDOT-PSS films for all samples was constant. Approximately 300mL of an as-received aqueous solution of PEDOT-PSS (Bayer Baytron P VP a1 4083) [73] was filtered using a .2m filter and then spun at 3500 rpm for 30 seconds resulting in films with 40 nm thickness.

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41 After determining the PEDOT-PSS thickness and as depicted in Fig. 4.2, scratches were also formed across samples having both PEDOT-PSS as well as additional active polymer films. The channels on these samples provided a depth over which to measure the total thickness (i.e., dTOT) of all films deposited over the ITO. By substracting the known PEDOT film thickness from the total thic kness of all films measured by AFM, the polymer film thickness (d) was determined by simple calculation util izing Equation 4-1. dTOT dDOT = d (4-1) The above method determined the film thickness used in the space charge and trapped charge limited current models. However it is based on the two assumptions that the PEDOT-PSS thickness was constant across all samples, and that none of the ITO film was removed by scratching. 4.2.1.2 Atomic force microscopy Polymer film thicknesses were first measured by profilometry, however there were small scratches on the samples after this procedure, raising concerns that the metal tip of the profilometer caused damage to the deli cate polymer surface. Therefore, AFM was adopted as the appropriate measuring techni que for this experiment. The AFM allowed for better thickness measurement precision, as well as a better and more quantitative understanding of film topography by visu al representation of the surfaces and quantification of the root mean square (RMS) surface roughness. AFM measurements were taken only after the I-V measurements were completed. In order to measure the film surface, the samples were removed from the inert gas glove box and thereby exposed to laboratory air. Once exposed to air and humidity, no further electrical measurements were collected. Although no further elec trical measurements

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42 were made after samples were removed from the glove box, samples were nonetheless stored in clean, single-slide plastic containe rs. The containers we re further stored in desiccators and kept under vacuum. This st orage technique was used as a precaution to minimize surface contamination during physical transport of the samples between labs and testing areas. To characterize the film surfaces, the 1x1 cm samples were cleaved into smaller ~5x5 mm sections so as to fit properly in the AFM sample holder. For these measurements, cleaved samples were inspecte d by eye and selected based on proximity of the cleaved portion to the gold cathode area s. By selecting these proximity areas, it was assumed that measurements were represen tative of the film surface directly below the gold contacts where the carrier transport of interest occurs during I-V measurements. The result of a typical roughness analysis ba sed on the AFM data with the RMS surface roughness of the area is hi ghlighted (Figure 4-3). To determine the thickness, the AFM was operated in the section analysis mode. Various points along the scratched channels we re measured by the AFM to determine the thickness. Fig. 4-4 shows a typical analysis of the total film thickness (i.e., dTOT). In Figure 4-4, the vertical distance betw een two points located at the bottom of a channel (e.g., the left side red-arrow in the figures) and the surface of the polymer film at a location near the channel (e.g ., right side red-arrow in th e figures) was measured as 62nm (i.e., dTOT = 62nm). It must be noted however, that because the film surface varied, appearing as a series of non-uniform p eaks and troughs on the AFM output, the film thickness measurement is based on the assu mption that the two points chosen were representative of average film thickness. By utilizing the above thickness Equation and

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43 subtracting the thickness for PEDOT of 40nm, the polymer film thickness for this particular sample was easily calculated as: 62nm-40nm = 22nm. Table 4.2 summarizes the film thickness and RMS surface roughness values for all films used in this experiment. Also includ ed in the table are the solvents in which the polymers were dissolved, the concentrations of solutions based on weight of polymer to the volume of solvent, and the spin rate s used to deposit each of the films. 4.2.2 Electrical Characterization 4.2.2.1 Current-Voltage (I-V) Measurements Poly(3,4-ethylene-dioxythiophene) doped with poly(styrenesulfonate) (also known as PEDOT-PSS, PEDOT or PDOT) has been uti lized as a hole inj ecting material in OLEDs for some time [74]. While having a high work-function of ~5.1eV, PEDOT-PSS provides reduced hole injection barriers (about 0.2eV) in to the HOMO energy level of numerous conjugated polymers, thus serving as a pseudo-ohmic contact to ITO anodes. In addition to these properties, PEDOT-PSS is commonly referred to as an organic metal since it exhibits ohmic transport similar to inorganic metals (Figure. 4-5). Hole-only, or more realistically hol e-dominated devices, as described in Chapter 2, were prepared by evaporating gol d electrodes onto the masked surface of two layer polymer films consisting of an ac tive layer over the PEDOT-PSS layer. The samples were then placed in the sample holder (Figure 3-2) to measure I-V characteristics at room temperature. Voltage was ramped between 0 to 5 Volts (V), stepped by 0.05V, to provide several data points for curve-fitting. As depicted in Fig. 4-6, I-V data from a PProH device show exponential behavior simila r to that of other conjugated polymers reported in the literature [36, 64, 75].

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44 The I-V data for PProH films depended on the thickness as predicted by the space charge Equation, which shows that current is inversely proportional to thickness cubed (i.e., J d-3). This dependence is qualitatively c onsistent with the PProH data presented in Fig. 4-7 in that the current is dramatically larger for the thinner la yers. Note that at < 0.5V, the films maintain similar current density values, but the thinner 21nm film exhibited much higher currents at higher volta ges. Additionally, the 88nm film exhibits the lowest currents for all voltages over the entire voltage range. Since the space charge model shows that current-density is proportional to the square of applied voltage (i.e., J V2), a plot of log J vs l og V should result in a line having a slope of 2. Figure 4-8 shows shows su ch plots of the data presented in Figure 4-7. A linear regression analysis of the data (F ig. 4-8) reveals that the slopes are best fitted by values of 2.1, 1.7 and 1.9 for 21, 44 and 88nm thick films, respectively. An average slope of 2 is indicative of space charge limited current, in contrast to materials which exhibit trap-limited-currents that show different slopes (i.e., I Vm with m > 2 ). For example, if the log I vs. log V data were trap limited, a slope of three or higher would be expected [76]. This is due to a sign ificant number of carriers injected from the electrodes being held at tr aps distributed in energy between the LUMO and HOMO energy levels of the polymer [77]. Plots log I versus log V for samples of ProDOT films are presented in Figure 4-9. The data for these films are consistent with trapped charge limited current (TCLC), rather than the SCLC behavior of PProH films. Trap limited current is indicated by the fact that I V3 as linear regression results in a slope of 3.7 and 3.1 for the 25 and 47nm films.

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45 It is not understood why ProDOT films e xhibit TCLC while PProH films exhibit SCLC. However, in some material/solvent combinations there may be impurities remaining after synthesis of organic semiconducto r materials and/or layers that could trap charges. Alternatively, the traps may be the result of structural disorder of the polymer films [78]. Structural disord er, resulting in defects, intr oduces localized energy levels between the HOMO and LUMO of the organic material which may be present as discrete energy levels, or distributed over a band of energy with a constant density of states or an exponential distribution such as a Gaussian. One other source of traps occurs when a charge carrier causes deformation of the organi c molecule. In this case, the deformation acts as a quasi-particle called a polaron which not only has a lower mobility than a free carrier but also forms its own trap state in the polymer (self tra pping) [79]. Because some of the device preparation and testing st eps were outside the gl ove box, there also remains the possibility that films and polymer solutions were contaminated by external impurities during processing. Further investigat ion is needed to understand why ProDOT exhibits trap limited while PProH exhibits space charge limited current transport. To address the possibility that structural disorder and/or retained solvent was the source of traps the devices were heated so as to allow the polymers to rearrange. While still in the glove-box, the devices were pl aced on a hot plate and heated (baked) to approximately 100oC for ~4 hours. Upon removal from the hot-plate, the samples were allowed to cool for 30 minutes at which time I-V measurements were again performed. Increased currents were observed for th e 25nm and 47nm films (Figure 4-10). However, current-voltage measurements were again taken after an additional 15 hours of relaxation in room temperature argon ambient and the cu rrent decreased to

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46 nearly the same values as before the baking (Figure 4-11). These results suggest that th e traps are not related to de fects or solvents that are healed or reduced at 100oC heat treatments., Further experiments are needed. 4.2.2.2 Hole Mobility Analysis Data for the hole mobility of P3HT and PProH films were analyzed based on the space charge model (Equation 2-19), solving fo r fitting parameters representing constants in the formula. The fitting parameters were calculated by iterating the field-dependent mobility equation (Equation 2-20) for the zero-field mobility, (0), and the fielddependence factor, as presented in equation. The re sults are presented in Table 4.3 for the fitting parameters where th e goodness of fit parameter, R2, aproaching a value of one represents an excellent fit to the space charge model. Data fitting for P3HT yields (o) values averaging 6.95 + 1.2 x 10-6 (cm2/V-s) with no thickness dependence, while that for PPrOH yields (o) values averaging 1.6 + 0.4 x 10-6 (cm2/V-s). The hole mobilities for various conjugated polymers as reported in the literature are listed in Table 4-4. Based on the data in Table 4.4, the hole mobilities reported for PProH are similar to those reported for other conjugated polymer s such as PPV [63, 80, 81]. However, the hole mobilities reported for P3HT are about tw o orders of magnitude smaller than those reported by Goh and Kline using a similar t echnique [36]. The reason is unknown. It was not possible to fit the TCLC model to th e ProDOT I-V data to extract the hole mobility.

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47 Table 4-1. Chemical Structure of sample polymers Polymer Name Structure P3HT: RR-poly(3-hexyl-thiophene) PProDOT : poly(3,4-propylenedioxythiophene) PProH: poly(3,4-propylenedioxythiophenediethylhexyloxy)-cyano-p-phenylene vinylene(substituted with dodecyloxy chains on the phenylene ring) Figure 4-1. Depiction of channel form ation for measurement of PEDOT-PSS film thickness

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48 Figure 4-2. Depiction of channel formati on for measurement of total film thickness Figure 4-3. Typical AFM surface roughness analysis output with the RMS surface roughness in the red box

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49 Figure 4-4. Depiction of film thickness meas urements. The red arrows appear to the right and left of a channel wall.produced by scratching Table 4.2. Thickness and RMS roughness da ta for all conjugated polymer films Material Sample ID Solvent Concentration (mg/mL) Spin Rate (rpm) Thickness (nm) RMS (nm) PEDOT P DI-Water as received 3500 40 N/A Dev 3 di-chloroBenzene 20 1000 143 3.47 P3HT Dev 4 di-chloroBenzene 20 800 148 4.39 A5 Toluene ~8 10 1000 25 0.85 ProDOT A6 Toluene ~8 10 900 47 3.17 6 di-chloroBenzene 8 1000 22 3.07 B3 Toluene 28 900 44 5.14 PProH B4 Toluene 28 700 88 3.73

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50 Figure 4-5. I-V data for PEDOT:PSS. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0123456Vbias (V)J (amp/cm2) Figure 4-6. Typical J-V data for a PProH pol ymer device. Note the exponential character typical of polymer films.

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51 PProH0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0123456Vbias (V)J (amp/cm2) 21nm 44nm 88nm Figure 4-7. Dependenc e of J-V data on thickness for PPrOH films

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52 Fig. 4-8. Linear regression of log I vs log V data for PProH films. The slope = 2 suggesting that the data are described by the space charge limited current (SCLC) model

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53 ProDOT 47nm y = 3.1x 3.9 R2 = 0.9985 25nm y = 3.7x 4.3 R2 = 0.999-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 00.10.20.30.40.50.60.70.8 log (V)log (I) 25nm 47nm Figure. 4-9. Linear regre ssion of log I vs log V data for ProDOT. The slope > 3 indicates TCLC 0 0.05 0.1 0.15 0.2 0.25 0.3 0123456Vbias (V)J (amps/cm2) 25nm PreBake 25nm Post-Bake Figure 4-10. Increased J were observe d upon heating the 25nm PProDOT samples

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54 0 0.05 0.1 0.15 0.2 0.25 0.3 0123456Vbias (V)J (amps/cm2) 25nm PreBake 25nm Post-Bake 25nm Relaxed Figure 4-11. Effect of baking and relaxation on current density. Table 4.3. Fitting parameters obtained by ite rating the field-dependent mobility equation Material Sample ID Thickness (nm) (o) (cm2/ V s) (m/V)1/2 R2 Dev 3 143 7.8 x 10-6 +/2.5 2.0 x 10-4 +/0.5 .9991 P3HT Dev 4 148 6.1 x 10-6 +/1.1 1.2 x 10-4 +/0.6 .9997 6 21 1.4 x 10-6 +/0.2 2.6 x 10^-5 +/1.0 .9999 B3 44 3.3 x 10-6 +/0.5 8.6 x 10^-6 +/6.0 .99996 PProH B4 88 9.4 x 10-8 +/0.2 2.6 x 10^-4 +/0.2 .99984

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55 Table 4.4. Hole mobilities of various polymers Material Mobility (cm2/V s)Method Ref # P3HT 4 x 10-4 Time of Flight [41] AsF5 doped polyacetylene 2 x 10-2 Van der Pauw [54] phenyl-amino subst. PPV 10-4 10-3 Time of Flight [44] P3HT 1.3 x 10-5 J vs V (SCLC) [36] PPV 0.5 x 10-6 J vs V (SCLC) [64] poly(phenylene) Derivative ~10-6 J vs V (SCLC) [65] PPV (Spin cast) 0.8 x 10-6 Time of Flight [80] poly (9,9dioctylfluorene) 4.5 x 10-2 Time of Flight [82]

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56 CHAPTER 5 CONCLUSION The elec trical properties of thin film s of poly(3-hexyl-thiophene) (P3HT), poly(3,4-propylenedioxythiophene) (PProDOT) and poly(3,4-propylenedioxythiophenediethylhexyloxy)-cyano-p-phenylenevinylene substituted with dodecyloxy chains on the phenylene ring (PProH) have been studied at various film thicknesses (<150nm). The focus was the use of current-vo ltage (I-V) data to determine if the transported current was space charge limited (SCLC) or trapped char ge limited (TCLC). If the SCLC model applied, the hole mobilities was extracted from the data. For PProH and P3HT, hole transport was described by the SCLC model with hole mobilities of 1.6 + 0.4 x 10-6 and 6.95 + 1.2 x 10-6 (cm2/V-s), respectively. While the mobilities for P3HT are approximately two orders of magnitude lower than those previously reported in the literature, it is spec ulated that the larger thickness of the films (143nm and 148nm) may have contributed to lower mobilities, as well as a dependence of mobility upon the field strength for lower-molecular weight films [36]. In contrast, the I-V data from PPro DOT fit the TCLC model in which J Vm with values of m between 3 and 4. Heat treatment of the spin cast films to temperatures of 100oC did not significantly change the I-V data and dependence of J upon Vm, suggesting that the traps were stable to these temperatures. Future Work. While the method for characteri zing the electrical properties of conjugated polymers can be used to measure the mobilities of materials exhibiting space charge limited currents (PProH and P3HT), it is difficult to measure the mobility for

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57 trapped charge limited current, as for PProDOT Knowledge of the trap distribution and density are required, that could be pr ovided by a technique known as thermally stimulated currents (TSC) which has been used to measure trap levels and total density in poly(p-phenylenevinylene) (PPV ) [83]. While devices ar e cooled down from room temperature to as low as 10oK, they are exposed to a strong forward bias, which fills all of the traps [84]. For a specific trap, there is an associated transport energy, or escape energy (i.e., the level from which a trapped carri er is most likely to be released) that is dependent on temperature. A trap state at lower temperatures may therefore become a transport state at room temp erature [85]. Upon removing th e electric field at the low temperature, the samples are allowed to reac h an equilibrium thermal state at a higher temperature by thermal release of carriers fr om the traps into the bands. The thermally stimulated current rises as the temperature is increased and the resu lts may be interpreted by correlating distinct current maxima with a distinct trap energy level by I exp(-Et/kT) for T< Tmax [86]. This technique has been shown to provide good results for devices with conjugated polymers as the active layer [87]. Additionally, the method of measuring hole mobility was hindered by assumptions made in calculating the film thickness (d) which contributes to the space charge model as J d-3. Although spin coating is quite often us ed as a reliable met hod for producing thin films, results are not always consistent as they depend on solution viscosity which depends on the material and solvent used. While a substrate cleaning procedure was followed and all solutions were filtered, the spin-cast polymer films often showed signs of non-uniform thickness, including holes, streak s and even particulates. Also, one of the challenges in this study was to repeat previous processing parameters in order to measure

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58 repeatability of the I-V data. Specifically with PPrOH, the total amount of conjugated polymer was quite limited, making it difficult to a wide processing parameter field. Further investigation into optimizing device fabrication procedures for these materials would lead to more in depth-studies of the effects of film thickness on I-V measurements. This study focused on measuring hole-mobilities, which are generally higher than electron-mobilities in conjugated polymer organic semiconductors. However, knowledge of the electron-transport prope rties must be acquired to further understand and optimize the properties of double carrier devices such as organic light emitting diodes. For example, to investigate the transport of elect rons in PPV without the drawbacks of highly reactive Ca and Ba low-work function elec trodes, Mandoc et al. constructed electrononly devices by vapor depositing alternative lo w work function, hole blocking materials as the electrodes. Utilizing a sandwich configuration with aluminum (Al) as the bottom electrode, ytterbium (Yb) as the top elect rode and the active ma terial between the electrodes, Mandoc was able to suppre ss hole injection at fields up to 108 V/m and measured an exponential distributi on of electron-traps in PPV [87].

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64 BIOGRAPHICAL SKETCH Bryan W ilson was born to Harold J. Wils on and Isabel H. Wilson in Panama City, Republic of Panama where he was preceded by hi s older sister, Natalie and older brother, Jonathan. From ages 1-7, Bryan resided with his family in the Panama Canal Zone neighborhood of Diablo. Upon relocating to Ke y Largo, Florida in 1985 and subsequently to Miami, Florida in 1986, Bryan attended school at Cutler Ridge Elementary, Cutler Ridge Middle School, graduating from Miami Southridge Senior High School. Bryan attended the undergraduate program at the Un iversity of Florida (UF), graduating in December of 2001 with a Bachelor s of Science Degree in Chemical Engineering. For the three years following graduation from UF, Bryan worked as an engineer for A&N Corporation in Williston, Florida. At A&N, Bryan learned about the semiconductor industry and became interested in electronic ma terials. It was in the Fall of 2004 that he began his graduate studies in the Materials Science Department, again at the University of Florida where he gained a deeper in terest in semiconducting polymers. Upon graduation with his Masters Degree, Bryan will continue his role as a Patent Examiner at the US Patent and Trademark Office in Alexa ndria, Virginia where he started his career in September of 2006.