In-Situ Infrared Spectroscopy of Organic Electrochemical Devices

In-situ Infrared Spectroscopy of Organic Electrochemical Devices

by Parisa Shiri

B.Sc., Amirkabir University of Technology, 2014

Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science

in the Department of Chemistry Faculty of Science

Copyright in this work rests with the author. Please ensure that any reproduction or re-use is done in accordance with the relevant national copyright legislation. Approval

Name: Parisa Shiri

Degree: Master of Science Title: In-situ Infrared Spectroscopy of Organic Electrochemical Devices

Examining Committee: Chair: Dr. Hua-Zhong Yu Professor Dr. Loren Kaake Senior Supervisor Assistant Professor

Dr. Byron D. Gates Supervisor Associate Professor Dr. Gary W. Leach Supervisor Associate Professor Dr. Steven Holdcroft Internal Examiner Professor

Date Defended/Approved: April 15, 2019

ii Abstract

Organic electrochemical transistors (OECTs) offer low voltage operation and a feasible platform for flexible, large-area, and low-cost devices, especially in the context of printed electronics. However, these devices often suffer from sluggish performance as a result of ion intercalation into the bulk of the organic semiconductor. We have characterized the time dependent behaviour of OECTs based on poly(3-hexylthiophene) (P3HT) and a poly(ethylene oxide): lithium perchlorate (PEO:LiClO4) gate dielectric using in-situ infrared spectroscopy. Because charge carriers in P3HT have a characteristic absorption in the mid infrared, we can monitor the rate of device charging and discharging spectroscopically. The dependence of the charging rate on parameters such as channel length, semiconducting polymer thickness and dielectric thickness have been investigated. Our results indicate that several distinct mechanisms are at play, with the rate limiting step being determined by device geometry. Using these results, we have also examined the effect of the structure of the counter-ion on its diffusivity in the organic semiconductor once doping occurs.

Keywords: FT-IR, OECTs, electrochemical doping, ion-diffusion

iii Acknowledgements

First and for most I would like to thank my supervisor Dr. Loren Kaake. The door to his office was always open to me and his knowledge, patience and encouragement always lightened up the research path. I am very honored to be his first graduate student. Next I would like to thank my committee members Dr. Gary Leach and Dr. Byron Gates who provided more insight and guidance during this project. I am also grateful for the students and friends who helped me with data collection in our group especially Earl Dacanay and Brennan Hagen. I am fortunate for this chance to work with all the great staff in Chemistry department as well as 4DLabs facilities without whose help and contribution this work would not be possible.

Lastly, I would like to give my heartfelt thanks to my loving family for their lifetime love and support and my dearest friends Mojo and Sparrow, who are my second family, for their unconditional love and sympathetic ear when I got weary. I would also acknowledge the support of my lab mate and friend, Nastaran Yousefi, who always provided me with extra strength and motivation to get things done. In hard days she always cheered me on and her presence and friendship was a true blessing I will never forget.

iv Table of Contents

Approval ...... ii Abstract ...... iii Acknowledgements ...... iv Table of Contents ...... v List of Tables ...... vi List of Figures ...... vii List of Acronyms ...... xi

Chapter 1. Introduction ...... 1 1.1. Organic Semiconducting Materials ...... 2 1.2. Polymer Electrolytes and Ion Conduction ...... 6 1.3. Organic Thin Film Transistors ...... 7 1.3.1. Working Principle of OTFTs ...... 7 1.3.2. Transient Behaviour Models ...... 9 1.4. Technologies and Application ...... 12 1.4.1. OECT Based Applications ...... 12 1.4.2. Electrochemical Energy Storage ...... 15 1.4.3. Electrochromic Windows and Mirrors ...... 16 1.4.4. Light-emitting Electrochemical Cells ...... 17 1.5. Summary ...... 18

Chapter 2. Experimental ...... 20 2.1. ATR-FTIR Spectroscopy ...... 20 2.1.1. Fourier Transform Infrared Spectroscopy ...... 20 2.1.2. Attenuated Total Internal Reflection or ATR Technique ...... 24 2.2. FTIR Setup (Hardware) ...... 27 2.2.1. Mirror Alignment ...... 27 2.2.2. Sample Holder and Cryostat ...... 29 2.2.3. Electronic Setup ...... 29 2.3. Software and Data Analysis ...... 31 2.4. Sample Fabrication ...... 31 2.5. Characterization Techniques ...... 36 2.5.1. Atomic Force Microscopy (AFM) ...... 36 2.5.2. Profilometry ...... 37

Chapter 3. Results and Discussion ...... 39

Chapter 4. Future Work ...... 52

References ...... 54

Appendix. AFM and Profilometry Results ...... 60

v List of Tables

Table 1.1 Comparison between characteristics of organic and inorganic technologies, reproduced from Pecqueur et. al.41 ...... 14 Table 2.1 Properties of ATR crystal materials, reproduced from Fundamentals of Fourier Transform Infrared Spectroscopy by B. C. Smith55 ...... 27 Table 3.1 Different mechanisms of charging process and the corresponding dimension of the device ...... 43

vi List of Figures

Figure 1.1 Structure and energy states of (a) neutral trans-polyacetylene and (b) radical cation on trans-polyacetyne ...... 3 Figure 1.2 Absorbance spectra of polarons in RR-P3HT Reprinted with permission from Österbacka et. al. 5 Copyright 2000 The American Association for the Advancement of Science...... 3 Figure 1.3 Chemical structures of some conjugated polymers: (a) Benzodithiophene- thienothiophene (BDT-TT) based polymer where R=2-ethylhexyl, reproduced from Holliday et. al,11 (b) a dithienylbenzothiadiazole(DTBT) based polymer, reproduced from Duan et. al12, (c) a bithiophenesulfonamide (BTSA) based polymer where R can be different different alkyl chains, reproduced from Eastham et. al13, (d) a polythiophene with glycolated side chains p(g2T-TT), reproduced from Rivnay et. al.14 ...... 5 Figure 1.4 Regioregurelar and regiorandom P3AT ...... 5 Figure 1.5 Examples of anions and cations in ionic liquids commonly used as ion-gel dielectric materials. Cations: 1-butyl-3-methylimidazulium (BMIM), 1-ethyl- 3-methylimidazulium (EMIM), 1-ethyl-2,3-dimethylimidazolium (EMMIM) and 1,3-diallylimidazolium (AAIM). Anions: bis(fluorosulfonyl)imide (FSI), Tetrafluoroborate (BF4), bis(perfluoroethanesulfonyl)imide (BETI) and hexafluorophosphate (PF6)...... 7 Figure 1.6 Organic-based transistors, reproduced from Rivnay et. al.14 ...... 8 Figure 1.7 Comparison of OFETs and OECTs in terms of transconductance, adapted from Friedlein et. al.23 ...... 9 Figure 1.8 Diagram of OECT equivalent circuit according to Bernard’s model24, adapted from Friedlein et. al.23 ...... 10 Figure 1.9 Experimental source-drain current upon applying a constant gate voltage of 0.5 V (a) A square step gate voltage is applied and two different current behaviours are observed: (b) Current monotonic response due to charge transport dominating the rate and (c) spike-and-recovery behaviour since the ion charging occurs faster than charge transport. Reprinted with permission of Bernard et. al.24 Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim...... 11 Figure 1.10 (a) Schematic of the gate modified OECT device as glucose sensor. The platinum gate electrode has been modified with Pt nanoparticles (pt NPs), multiwalled carbon nanotube (MWCNT) and glucose oxidase (GOs). (b) The dependence of ∆Vgeffas a function of log[Cglucose] for CHIT/GOx/Pt (line I), MWCNT-CHIT/GOx/Pt (line II) and CHIT/GOx/Pt-NPs/Pt (line III) gate electrodes. Reprinted from Tang et. al35 Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim...... 13 Figure 1.11 (a) Schematic of the ion-gel gated OECT and (b) schematic of the artificial synapse based on OECT. (c) Synaptic spike response to gate voltage of - 1.0V with duration of 20ms. (d) OECT schematic during off state(d) and (e) on state after applying gate voltage. Reprinted with permission from Qian et. al.40 Copyright 2016 American Chemical Society...... 15

vii Figure 1.12 Schematic of an all-organic battery in (a) discharging and (b) charging states. Reprinted with permission from Muench et. al.44 Copyright 2016 American Chemical Society...... 16 Figure 1.13 Absorption spectra of ECP-Red and its thin film at neutral (left) and oxidized state (right). Reprinted with permission from Dyer et. al.46 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA...... 17 Figure 1.14 (a) From left to right: schematic of PLEC operation. First no bias is applied and then with a negative bias, in-situ electrochemical doping occurs at anode and cathode interfaces. The doped regions expand until a p-n junction is formed. (b) The schematic of the device is also shown. Reprinted with permission from Gao et. al.49 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA...... 18 Figure 2.1 Schematic of FTIR instrument ...... 21 Figure 2.2 (a) Interferogram and (b) single beam spectrum of the IR light source. .. 22 Figure 2.3 Snell’s law (a)The light travelling from medium one (silicon crystal in this study) to medium two (P3HT film in this study) with a different refractive index will have a different incidence angle, (b) at the critical angle, the incidence beam will touch the interface, and (c) total internal reflection phenomenon where the incidence will occur in the same medium which results in production of evanescent wave. Adapted from Stuart.50 ...... 25 Figure 2.4 The FTIR setup. The light (red) coming out of the interferogram is guided to the ATR crystal by a set of mirrors and then enters the detector ...... 28 Figure 2.5 Total internal reflection of red laser in a glass ATR crystal used to check system alignment ...... 28 Figure 2.6 Schematic of the sample holder in (a) 3D view and (b) top view. ATR crystal is placed on the edges of the slot. Three pairs of threaded holes are used to mount the source, drain and gate contact and one pair is used to mount the sample holder on the cryostat. (c) The sample is contacted by copper pieces once placed on the sample holder...... 30 Figure 2.7 Schematic of the in-situ FTIR instruments. A voltage is applied to the sample and the current flow is measured...... 30 Figure 2.8 schematic of the sample holder for polishing the silicon crystal to form 45° edges ...... 32 Figure 2.9 Schematic of sample preparation steps in top view (left) and side view (right). (a) ATR crystal is used as the substrate and (b) semiconducting layer is deposited on top of it. (c) The gold source and drain electrodes are deposited on the semiconducting film. (d) Dielectric layer is then spin coated on top of source and drain electrodes and finally (e) gold gate electrode is deposited on top of the dielectric...... 33 Figure 2.10 Chemical structure of all the chemicals used in this research ...... 34 Figure 2.11 Shadow mask schematic illustrating dimensions common to both masks. Mask containing 1500 µm and 3100 µm electrode spacing is shown...... 34 Figure 3.1 Side views and top view of the device. A voltage is applied to gate electrode while source and drain are grounded. The IR beam bounces through ATR crystal and provides spectroscopic data about changes in the device during charge and discharge process ...... 39

viii Figure 3.2 A sample of single beam spectrum of the electrochemical device (tP3HT=160 nm, tPEO= 300 nm and lc= 300 µm) before applying any voltage...... 40 Figure 3.3 Absorbance spectra of (a) charge and (b) discharge for different times. When charging, the peak rises due to increase in doping while when discharging, the process is reversed and the peak intensity decreases. V = -2.4 V and the sample dimensions are tP3HT=160 nm, and tPEO=300 nm and and lc = 300 µm...... 40 Figure 3.4 Integrated absorbance showing charge and discharge rate. The discharge has not occurred fully in the recorded times due to trapped charges in the channel. V = -2.4 V and the sample dimensions are tP3HT=160 nm, tPEO=300 nm and lc = 300 µm ...... 41 Figure 3.5 Absorbance spectra for different times and voltages for a sample with lc=3100 µm. (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. Polaron peak intensity grows with charging time. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆V = 0.2 V). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm- 1 at different voltages. Specification of the device are mentioned in the graph...... 42 Figure 3.6 Integrated absorbance spectra for devices with different channel lengths at -2.4V...... 43 Figure 3.7 Absorbance spectra for different times and voltages for samples with different dielectric thicknesses. (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆V = 0.2 V). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm-1 at different voltages. Specification of the device are mentioned in the graphs...... 45 Figure 3.8 Absorbance spectra for different times and voltages for samples with different P3HT thicknesses (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆V = 0.2 V). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm-1 at different voltages. Specification of the device are mentioned in the graph...... 46 Figure 3.9 Integrated absorbance over 1500-6000 cm-1 and fitted by the model. The charging curves presented here are for devices with (a) different PEO thickness and (b) devices with different P3HT thickness all at a voltage of -2.4 V...... 49

Figure 3.10 Rate constants kexp and kdif corresponding to ion movement in the dielectric and semiconducting film, respectively. Dependence of rate constants to voltage and thickness of device components: (a)kexp for different dielectric thicknesses and (b) kdif for different P3HT thicknesses for all voltages ... 50 Figure 3.11 Integrated absorbance spectra of different ions ...... 51 Figure 4.1 (a) Polaronic transitions related to amorphus (A) and crystalline (C) regions of the P3HT film and (b) how these transitions appear in absorbance

x List of Acronyms

AFM Atomic Force Microscopy ATR Attenuated Total Reflection BDT Benzodithiophene BNC Bayonet Neill–Concelman BMIM 1-butyl-3-methylimidazulium CHIT Chitosan CMOS Complementary Metal-Oxide-Semiconductor CPDT Cyclopentadithiophene DC Direct Current DNA Deoxyribonucleic Acid DPP Diketopyrroloyrrole EGOFET Electrolyte-Gated Organic Field Effect Transistor FFBT 5,6-difluorobenzothiadiazole FFT Fast Fourier Transfor FTIR Fourier-transform infrared

GO Graphene Oxide GPE Gel Polymer Electrolyte HOMO highest occupied molecular orbital IDT Indacenodithiophene LAC Library and Archives Canada

LiClO4 Lithium Perchlorate

LiPF6 Lithium Phosphorus Fluoride LiTFSI Lithium, Bis(trifluoromethylsulfonyl)amine LUMO Lowest Unoccupied Molecular Orbital MCT Mercury Cadmium Telluride MWCNT Multi-walled Carbon Nanotube NP Nanoparticle OAP Off-Axis Parabola OECT Organic Electrochemical Transistor OFET Organic Field-Effect Transistor OPD Optical Path Difference P3HT Poly(3-hexylthiophene-2,5-diyl)

xi PDMS Polydimethylsiloxane PEDOT Poly(3,4-ethylenedioxythiophene) PEO Poly(ethyleneoxide) PPF Paired-Pulse Facilitation PTFE Poly(tetrafluoroethylene) RR-P3HT Regioregular Poly(3-hexylthiophene-2,5-diyl) SFU Simon Fraser University SPE Solid Polymer Electrolytes STP Short-Term Plasticity TDP Thienopyrrolodione TFT Thin Film Transistor TT Thienothiophene

xii Chapter 1.

Introduction

Many state-of-the-art technologies and applications use organic semiconducting polymers. These polymers are flexible and easy to process which make them suitable for printed electronics and sensors. Electrochemical doping of these polymers is a common way of introducing charge carriers in the semiconducting channel of the device. In particular, organic electrochemical transistors (OECTs) are devices that make use of ion injection from an electrolyte into the bulk of the polymer to modulate the charge state of the polymer. However, due to coupled hole/ion diffusion, it usually comes with the cost of slower operation. There are many unanswered questions about factors that determine the dynamics of this behaviour. Understanding the fundamental mechanisms in transient behaviour of OECTs is the purpose of this study.

When a voltage is applied to the ion conductor in an organic electrochemical device, there are several aspects that can affect the device’s charging speed: device geometry such as channel length, channel and electrode thicknesses, as well as the nature of counter ions penetrating the channel. In-situ spectroscopic measurements can be used to study the device under operation and are of great importance in examining the organic semiconductor/electrolyte dielectric interface, giving insight into different aspects of device operation. Therefore, in this work the effect of several different device parameters on device performance were investigated using an in-situ FTIR technique.

In this chapter, the principles of organic semiconductors and charge carriers in them are discussed. Next, ion conducting dielectric materials are discussed. Since the configuration of the devices studied in this work resembles that of an OECT, one section is dedicated to different kinds of thin film transistors and the models used to describe their transient behaviour of OECTs. Lastly, the applications of electrochemical doping of conducting polymers both in the form of OECTs, as well as in other forms are mentioned with the idea that lessons learned from this study could eventually be applied in these contexts as well.

1 1.1. Organic Semiconducting Materials

Organic semiconducting materials refer to polymers and molecules mainly made of hydrocarbons and heterocyclic compounds.1 More specifically, conjugated polymers and aromatic compounds are structures with alternating single and double bonds with hybridized sp2 structures.

Conjugated systems are systems containing a continuous array of � orbitals that can overlap through the whole system. The conjugated structure of semiconducting polymers is essential for formation of delocalized electronic states. This conjugation and alternation of bonds provide a continuous overlap of p-orbitals to form � – �∗ hybrid orbitals that allow charge carriers (e.g. electrons, holes) to move along the bands.

However, most conjugated polymers lack intrinsic charge carriers and, therefore, charge carriers must be introduced to their system in order to make them conductive. This process is referred to as “doping” which involves partial oxidation or reduction of the host polymer. Four possible methods for achieving this include chemical, electrochemical, interfacial and photo-induced doping. Chemical doping involves charge-transfer redox chemistry: oxidation of polymer (p-type doping) or reduction of polymer (n-type doping). For example, the oxidation reaction of a p-type polymer with iodine is mentioned below:2

3 � − �� + � → � − �� + � 2

In electrochemical doping, on the other hand, applying a voltage causes the removal or addition of electrons and incorporation of anions or cations. In other words, no charge transfer occurs between the ionic species and the polymer chain and the ionic species are only used to balance the charges. Electrochemical doping is the type of doping under study in this work.

Most conjugated polymers are p-type due to their electron-rich systems.3 However, a number of conjugated polymers with high electron affinity that can be used as n-type semiconductors have also been reported.4

As a result of doping, radical cations and anions are formed on the conjugated polymer. These species, which can act as charge carriers, are called “polarons”. An example of a positively charged polaron in trans-polyacetylene can be found in (figure

2 1.1). Removing a charge from the polymer interferes with the natural bond alteration pattern, resulting in the formation of a pair of states, located in the band gap.3

a) conduction band

Valance band +e- b) Conduction band b)

!# Valance band

Figure 1.1 Structure and energy states of (a) neutral trans-polyacetylene and (b) radical cation on trans-polyacetyne

Below the HOMO-LUMO gap, polarons have two electronic transitions: �, � which occur in mid-IR and near IR range, respectively. Figure1.2 shows the absorption spectra of these transitions in a regioregular poly(3-hexylthiophene-2,5-diyl) or P3HT 5 doped with I2. Therefore, IR measurements are a way to observe the existence of polarons in organic semiconductors, which is the most used technique in this study.

!! " " !! ##

Conductivity in metals and semiconductors is proportional to the product of mobility and carrier concentration. Therefore, when it comes to quantifying charge transport, the main characteristic parameter is carrier mobility. Mobility describes how fast a charge carrier can move if placed in an electric field. If no voltage is applied, the charge transport occurs by diffusion:

< � >= �� Equation 1 where < � > is the mean-square displacement of charges, D is diffusion coefficient, t is time and n is a number equal to 2, 4 and 6 for 1D, 2D and 3D systems, respectively.6 Charge mobility � and diffusion coefficient are related to each other by the

Einsten-Smoluchowski equation:

� = Equation 2 where � is the electron charge and � is the Boltzmann constant. Now if an external voltage is applied, the drift of charges occurs, induced by the electric field. In this case the mobility is:

� = Equation 3 where � is the amplitude of electric field and � is the velocity of charges. The difference between diffusion and drift is that the former occurs due to concentration ingredient but the latter is caused due to an electric field. In the surface of an organic film where the field is applied, drift dominates the charge transport while further away from the surface and in the bulk, transport is mainly dominated by diffusion,6 which is the case in OECTs.

There is a wide variety of organic semiconducting polymers available for different applications. Figure 1.3 shows some examples of these polymers. Regioregularity is a term used to describe the percentage of head-to-tail arrangements along the polymer chain. If we take a poly(3-alkylthiophene) chain as an example, a RR-P3AT contains only head-to-tail connections while regiorandom P3AT is a mixture of head-to-tail, tail-to-tail and head-to-head connections (Figure 1.4).7 In this work, regioregular P3HT is studied (Figure1.3) because it is a highly studied system. Regioregularity strongly impacts polymer nanomorphology and electron transport properties. RR-P3HT forms thin films with

4 nanocrystalline lamellae perpendicular to the substrate with in-plane � − � stacking interactions.8 This well-defined lamellar structure of RR-P3HT is the reason for its reported high mobility as the regular arrangement of side chains improves the �-stacking of the backbone8. The mobility of P3HT with 70% head-to-tail arrangement is on the order of 10 �� which increases to a much higher value of 0.1 �� for P3HT with 99% regioregularity.8 Therefore, RR-P3HT has been a model system to study charge transport and doping in polymers and remains as an active layer choice in studying sensors and transistors.9,10

a) b)

c) d)

Figure 1.3 Chemical structures of some conjugated polymers: (a) Benzodithiophene-thienothiophene (BDT-TT) based polymer where R=2-ethylhexyl, reproduced from Holliday et. al,11 (b) a dithienylbenzothiadiazole(DTBT) based polymer, reproduced from Duan et. al12, (c) a bithiophenesulfonamide (BTSA) based polymer where R can be different different alkyl chains, reproduced from Eastham et. al13, (d) a polythiophene with glycolated side chains p(g2T-TT), reproduced from Rivnay et. al.14

Figure 1.4 Regioregurelar and regiorandom P3AT

5 1.2. Polymer Electrolytes and Ion Conduction

Poly(ethylene oxide) (PEO) as a polymer electrolytes were first studied in 1973 by Fenton and coworkers15 who found that PEO can form a complex with alkali metal salts and conduct ions.15 Generally, polymer electrolytes are categorized in two groups: solid polymer electrolytes (SPEs) and gel polymer electrolyte (GPEs). Solid polymer electrolytes consist of a polymer matrix that can dissolve metal ions (frequently lithium salts) and result in ionic conductivity. Polyethers such as PEO are good candidates for SPEs since they are electrochemically stable. PEO with flexible ethylene oxide segments act as donors for Li+ cations.16 Ion conduction occurs by segmental motion of polymer chain. These motions produce free volume and lithium ions can migrate from one coordination site to another and therefore, move along the chain.16 It has to be mentioned that ion conductivity occurs primarily in amorphous regions of the polymer, crystallinity decreases the ion conductivity.

Alternatively, GPEs are a mixture of ionic liquids in polymers. Ionic liquids are salts with melting points below 100℃ that are used in electrolyte materials. They are well-known for their high thermal stability and ion conductivity.17 These liquids are highly polar, usually consisting of organic cations containing nitrogen and inorganic anions18 (Figure 1.5).

The important difference between SPE and GPE is their polarization response time. In SPEs, the response is limited to less than 100Hz at room temperature while GPEs exhibit a much higher response time (up to 10 kHz) as well as higher capacitance.19 Using GPEs as the gate dielectric material in transistors can have the benefit of high polarization response times (∼1 ms) and large specific capacitance (> 10 ��/��).20,21

Ionic conductivity of polymer electrolytes is temperature dependant. One of the main mechanisms that describes this dependence is Vogel–Tamman–Fulcher (VTF) equation:

� = �exp (− ) Equation 4 () where A is a pre-factor, � is activation energy and � is temperature. � is usually considered 50°� below the glass transition temperature of the polymer. The VTF equation has been used to fit polymer conductivity data for many decades.22

Figure 1.5 Examples of anions and cations in ionic liquids commonly used as ion-gel dielectric materials. Cations: 1-butyl-3-methylimidazulium (BMIM), 1-ethyl-3-methylimidazulium (EMIM), 1-ethyl-2,3- dimethylimidazolium (EMMIM) and 1,3-diallylimidazolium (AAIM). Anions: bis(fluorosulfonyl)imide (FSI), Tetrafluoroborate (BF4), bis(perfluoroethanesulfonyl)imide (BETI) and hexafluorophosphate (PF6).

1.3. Organic Thin Film Transistors

One of the main applications of conjugated polymers is in Organic Thin Film Transistors or (OTFTs). In the next section, different types of OTFTs such as Organic Field Effect Transistors or (OFETs), Electrolyte-gated Organic Field Effect Transistors or (EGOFETs) and Organic Electrochemical transistors or (OECTs) are described and compared with each other. A more detailed explanation is provided about how OECTs operate as well as the accepted models to describe their behaviour and applications.

1.3.1. Working Principle of OTFTs

OTFTs have received a lot of attention during the past decades due to applications in flexible and printed electronics and biosensing. All these transistors consist of electrodes (source, drain and gate), a polymer semiconducting channel and an insulator

7 layer. In a way, it is similar to a capacitor where one plate is the gate electrode and the other a conducting channel (organic semiconductor) between the source and drain electrodes. The gate electrode works as a switch for the current flow between the source and drain by changing the number of charge carriers in the channel.14

To understand the principle of how OECTs work, which is the focus of this study, it is helpful to describe two other common OTFT types: OFETs and EGOFETs (see figure 1.6). In OFETs, the dielectric insulates the semiconducting channel from gate electrode and charging occurs via field-effect doping of the channel. Upon applying a voltage, opposite charges accumulate in the channel at the semiconductor-dielectric interface. The charging can be thought of in terms of a metal-insulator-semiconductor capacitor, where the number of charges can be calculated by � = �(� − �) where C is the capacitance of the dielectric, � is the gate voltage and � is the threshold voltage. The threshold voltage is the gate voltage at which current begins to flow. a) b) c)

electrolyte Gate electrode

dielectric + + + + + + channel + + + + + + + +

FET Electrolyte-gated FET OECT Figure 1.6 Organic-based transistors, reproduced from Rivnay et. al.14

To maximize the number of charges, one can maximize the capacitance by decreasing the thickness of the dielectric layer. In the extreme case, this distance can be reduced to the molecular scale in an electrochemical double layer. In this case, the capacitance is calculated from the width of the double layer formed in the channel- dielectric interface. The dielectric in this case is an electrolyte and the transistor is called electrolyte gated OFET or EGOFET.

Both OFETS and EGOFETs are only able to dope the interfacial area of the channel and the bulk of it remains unused. In OECTs, the electrolyte contains mobile ions that can enter the channel and dope not only the surface, but the entire film. In this case, the capacitance increases by orders of magnitudes as it is volumetric capacitance.14 Therefore, OECTs have larger numbers of charge carriers and larger source-drain

8 currents. However, this comes with the cost of coupled ion-hole diffusion, which makes the behavior of OECTs more complicated and slower.

When it comes to comparing transistor performances, a parameter called “transconductance” is evaluated. Transconductance is the ratio of the current change at the output in response to change in voltage input:

∆ � = Equation 5 ∆

Figure 1.7 compares the transconductance of OECTs with OFETs for different frequencies. The values for gm for OECTs are much higher than OFETs, usually on the order of millisiemens. On the other hand, OFETs can perform at much higher frequencies, on the order of megahertz.

%! OECTs ) ! ( m g $! OFETs

&"# '"# Frequency ("#) Figure 1.7 Comparison of OFETs and OECTs in terms of transconductance, adapted from Friedlein et. al.23

1.3.2. Transient Behaviour Models

The most well-known quantitative model for OECT behaviour is presented by Bernard and Malliaras.24 They propose a model consisting of an electronic and ionic circuit (Figure 1.8). The electronic circuit is related to the channel and described by Ohm’s law while the ionic circuit is related to the transport of ions in the dielectric material, described with a capacitor and transistor in series. Based on their model, depending on the drain 23,24 voltage regime, the steady-state channel current � can be calculated as:

9 �� 1 − � > � − � � = Equation 6 () −�� < � − �

where � is the hole mobility, � is the volumetric capacitance of the channel and

�, � and � are drain, gate and threshold voltages and �, � and � are channel width, thickness and length, respectively. Their model was able to successfully fit the experimental data of organic transistors.

Gate VG

RS VD ! VS Electrolyte ",$%& CCH Source Drain Organic Semiconductor

Substrate

Figure 1.8 Diagram of OECT equivalent circuit according to Bernard’s model24, adapted from Friedlein et. al.23

Bernard’s model is also applicable to the transient behavior of OECTs. In order to model time dependent drain current, one assumes a constant source and drain voltage and varies gate voltage with time. In this case, the drain current � only depends on the response of the ionic circuit. The drain current is the weighted sum of displacement current

� caused by displacement of ions and channel current � caused by drift of charges in channel. The displacement current can be calculated with the following formula:23

� � = � × � Equation 7 , where �, is the electrolyte voltage, which is determined by the time response of ionic

RC circuit. The channel current � can be found by the steady-state model (equation 6).

Substituting �, for � for a square step voltage, Bernard’s model calculates the drain current � � as:

10 � � = � � + ∆� 1 − � �� (− ) Equation 8

where � � shows steady-state source-drain current at gate voltage � and ∆� = �(� = 0) − � � ; � is the charge transport time constant equal to . � is the ionic time constant defined as � = �� and � is the weighing factor.

From equation 8, it is apparent that the ratio plays an important role in the transient behaviour of the transistor. Since τ is determined by resistance and capacitance of the ionic / double layer, using the Gouy-Chapman model for an ionic double layer shows τ~�/� where � is the electrolyte thickness and and � is the ionic concentration. Therefore,

~ �� /�� and any change in thickness of dielectric or semiconductor, as well as drain voltage can cause a change in the transient behavior of the device.24

Depending on which occurs faster between ionic charging and electronic transport, the drain response can fall in two different regimes. If ion charging is limiting, the drain response will be monotonic (Figure 1.9b) while if the electronic transport is limiting the charging rate, the drain shows a spike-and-recover behavior (Figure 1.9c).

Figure 1.9 Experimental source-drain current upon applying a constant gate voltage of 0.5 V (a) A square step gate voltage is applied and two different current behaviours are observed: (b) Current monotonic response due to charge transport dominating the rate and (c) spike- and-recovery behaviour since the ion charging occurs faster than 24 charge transport. Reprinted with permission of Bernard et. al. Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Many models have attempted to find the weighing factor �. Friedlein25 et al. simply use � = which leads to � � = � � − � � and � � = −� � − � � , showing that source and drain currents have different amplitudes. However, their observation was

11 26 only valid for � ≪ (� − �). Feria et al. defined the � factor as the ratio between drain and gate current. In this way, the � factor can be directly calculated if the gate current is measured and known.

This model allows inferring of an unknown voltage input for a known current response, which is a common challenge in biosensor applications27. Another proposal to find � was to assume of the length of the channel contributes to drain current while the rest of the channel (1 − ) contributes to source current28. When integrating over the length of the channel, the voltage-dependent value of � is obtained. This model showed agreement with experimental data.

1.4. Technologies and Application

There are numerous applications that make use of electrochemical doping of conjugated polymers. A group of these applications are based on OECTs such as electronic circuits,29-32 sensors33-35 and neuromorphic engineering.36-41 Examples of other applications involving ion transport in organic semiconductors include energy-storage, light emitting electrochemical cells and electrochromic windows.

1.4.1. OECT Based Applications

Many technologies have been developed that make use of OECTs including three main areas of biochemical sensors, electronic circuits and neuromorphic engineering. They are briefly discussed below.

Due to their high transconductance and low voltage operation, OECTs have been integrated into electronic circuits for many promising applications such as smart electronic labeling and skin patches. Hamedi29 et. al. reported the fabrication of electrochemical transistors by coating monofilaments with the semiconducting polymer PEDOT. They constructed an inverter and multiplexer based on these transistors which is a demonstration of integrating OECT circuits in 3D woven fibers.29

Nilsson and coworkers also reported building a circuit for a single-stage logical invertor, oscillator, NAND gates, and NOR gates.30 Rothländer et al. have also fabricated NAND gates based on PEDOT:PSS structured by nanoimprint lithography.31 A more

12 recent work32 reports building functioning flip-flops and 2-bit shift registers based on an integrated system of 36 OECTs in the circuit.

Bioelectronics are one of the growing fields that make use of electrochemical doping of organic semiconductor for biological applications such as detecting biological analytes33and drug delivery34. Sensors based on OECT have several characteristics that make them good candidates for biological sensing applications. For example, their operation voltage is low in the range of 1V which guarantees hydrolysis would not occur. They are highly stable in aqueous electrolytes for long periods of time. These highly sensitive sensors benefit from biocompatibility, good selectivity and can be incorporated in complex and flexible systems. Such sensors have been used for DNA, enzyme, ion and cell biosensing applications. a) b)

Figure 1.10 (a) Schematic of the gate modified OECT device as glucose sensor. The platinum gate electrode has been modified with Pt nanoparticles (pt NPs), multiwalled carbon nanotube (MWCNT) and glucose oxidase �� (GOs). (b) The dependence of ∆�� as a function of log[Cglucose] for CHIT/GOx/Pt (line I), MWCNT-CHIT/GOx/Pt (line II) and CHIT/GOx/Pt- 35 NPs/Pt (line III) gate electrodes. Reprinted from Tang et. al Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

As an example, Tang35 et al. have used an OECT as highly sensitive glucose sensor. The gate electrode was modified with chitosan (CHIT) and multi-walled carbon nanotubes (MWCNT) to increase the sensitivity. They could reach the high detection limit of 20�� and 5 �� for MWCNT-CHIT/GOx/Pt and CHIT/GOx/Pt-NPs/Pt electrodes, respectively.

13 In 1990, The term “neuromorphic” was coined by Carver Mead as “artificial neural systems whose architecture and design principles are based on those of biological nervous systems”.36,37 It involves studying biological systems to apply the same information-processing logic to computer systems. Biological systems have a better ability to process ill-conditioned data in comparison to digital computers, use far less power and occupy less physical volume.36 As an example, a mammalian brain has about 1010 neurons with 105 synaptic connections.38 In order to simulate only one synapse with complementary metal-oxide-semiconductor (CMOS) technology, at least 10 transistors are required39, which means a huge system of parallel transistors are required to mimic the brain. Also, brain and other biological systems adapt signal processing to changes in surroundings and based on previous signal history40. Table 1.1 41 summarizes the different properties of conventional computers and biological systems in comparison to organic ion/electron technology that shows why using the latter is suitable for neuromorphic engineering.

Table 1.1 Comparison between characteristics of organic and inorganic technologies, reproduced from Pecqueur et. al.41 CMOS Technology Organic-Ion/Electronic Biology Technology Operational dry (sealed-packaged) air/water stable wet Environment Information Carrier electrons/holes electrons/holes/ions/chemicals ions/chemicals Mobility (cm2/(Vs)) 450 (hole, silicon) 10−6 to 10 (electronic) 10−3 (ionic) 1400 (electron, silicon) 10−4 to 10-3 (ionic) Capacity (mF/cm2) 1 (high-k oxide with 500 (130nm PEDOT film with 1 (cell’s thickness of few nm) note: volumetric capacitance) note: membrane) capacity decreases with capacity increases with thickness thickness Fabrication top-down lithography top-down lithography bottom- bottom-up up, self-assembling, eletropolymerization As an example of such systems, Qian et al.40 have reported fabrication of an ion- gel gated P3HT-based OECT that mimics the synapse response exhibiting the synaptic features such as paired-pulse facilitation (PPF), short-term plasticity (STP), self-tuning, the spike logic operation, spatiotemporal dentritic integration, and modulation.

Another interesting example is the application of OECTs in storing memory. Winther-Jensen et al.42 developed a PEDOT-based material which would mechanically collapse upon undergoing redox reactions due to changes in the conformation of the

14 polymer. This results in a stable hysteresis in conductivity as a function of voltage. This hysteresis was then used as memory function in an OECT made of this material to show short term to long term memory transition.43

Figure 1.11 (a) Schematic of the ion-gel gated OECT and (b) schematic of the artificial synapse based on OECT. (c) Synaptic spike response to gate voltage of -1.0V with duration of 20ms. (d) OECT schematic during off state(d) and (e) on state after applying gate voltage. Reprinted with permission from Qian et. al.40 Copyright 2016 American Chemical Society.

1.4.2. Electrochemical Energy Storage

In electrochemical storage, conjugated polymers are used not only as electrolyte but as electrodes. P-type polymers are used as cathode and n-type polymers as anode. When applying a sufficient current, the cathode is oxidized and anode reduced, with the compensated charges from the electrolyte44 (Figure 1.12).

For example, the redox reaction for discharge of a polymer-lithium cell is:

[��] [� ] + �� → �� + �� Cathode

�� → �� + �� anode

However, the practical capacitance in electrochemical capacitors is limited by electrochemical stability and ion transport. Ion transport can be especially problematic in

15 thicker(>100 ��) electrodes.45 Two different reported approaches that tackle this problem are by increasing the porosity in the polymer active layer and by using a gel morphology where ions can be transported in a liquid-like environment rather than in the solid-state.45

a) b)

1.4.3. Electrochromic Windows and Mirrors

Electrochromic materials are materials with tunable optical transmittance. In conjugated polymers in their neutral state, the bandgap energy is correlated to a low- energy absorbance in the mid-IR. For example, for ECP-Red, which is a copolymer of an acyclic branched alkoxy thiophene (3,4-OEtHx-Th) with a dimethoxy thiophene (3,4-OMe- Th), has a single-peak in the visible range at 525 nm at neutral state, resulting in its red color.46 When the polymer is doped, the mid-gap states and polarons are formed with a lower energy and therefore transitions occur in longer wavelengths. At intermediate levels of oxidation, the absorption spectra consist of both neutral and oxidized contributions. However, at highly oxidized levels the band gap transition no longer occurs while the low energy transitions extend to the visible range in the spectra, giving the polymer a blue color46,47(Figure 1.13). This process is reversible by reducing the polymer. The described process refers to p-type polymers. However, n-type electrochromism has been reported as well.

Figure 1.13 Absorption spectra of ECP-Red and its thin film at neutral (left) and 46 oxidized state (right). Reprinted with permission from Dyer et. al. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

Most organic electrochromic polymers have both donor and acceptor cites. These materials are synthesized by copolymerization of electron-rich and electron-poor units. Examples of common acceptor and donor units are thiophene rings and pyridine based acceptors with sp2 nitrogen, respectively.

Electrochromic behavior of polymers can be tuned to produce any desired color through several synthetic and electrochemical techniques. Electrochromic materials have several applications in windows, displays and mirrors46.

1.4.4. Light-emitting Electrochemical Cells

Light-emitting Electrochemical Cells or LECs are another category of light emitting devices such as Organic light emitting diodes (OLEDs) that can be used in display applications. Depending on the energy-gap of the organic active layer, it can be tuned to emit light of any color.48

The major difference between LECs and other common OLEDs is their use of mobile ions in the semiconductor. Figure 1-1449 shows the schematic of the device and working mechanism of a polymer based LEC. The luminescent polymer is in contact with both electrodes and upon applying a DC voltage, ions are injected in the polymer. The p-

17 doping and n-doping from both sides continues until a p-n-junction is formed, which results in a great electronic current. The use of mobile ions in PLECs, however, comes with the cost of short life-time and low efficiency when high-luminescence is desired and more research is required in this field both to provide models and better performing devices.49

Figure 1.14 (a) From left to right: schematic of PLEC operation. First no bias is applied and then with a negative bias, in-situ electrochemical doping occurs at anode and cathode interfaces. The doped regions expand until a p-n junction is formed. (b) The schematic of the device is also 49 shown. Reprinted with permission from Gao et. al. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

1.5. Summary

Due to all the applications that electrochemical doping of organic semiconductors, it is important to study the transient behaviour of OECTs. As it was mentioned earlier, Bernard’s model is the commonly used model for this purpose due to its simplicity. However, it is based on transistor measurements and leakage current can make this model complicated. Also, it is hard to disentangle the role of electrolyte and semiconducting channel based on this model. Therefore, understanding the structure- property relationship in OECTs and how it can affect device behaviour is vital.

In this work, we use an spectroelectrochemical method to study the transient behaviour of devices based on electrochemical doping. We investigate the fundamental mechanisms that operate to fill the channel with charges. More specifically we will examine

18 the role of charge carrier diffusion in the semiconducting channel, ion movement in the dielectric, and the diffusion of ion-hole pairs in the semiconductor. The spectroscopic method used has many advantages over standard transistor measurements, most notably its independence of charge transport and dielectric leakage current.

19 Chapter 2.

Experimental

This chapter includes a description about how the experiments were performed with sufficient details to make it straightforward to reproduce this experiment. Since ATR- FTIR spectroscopy is the main technique used throughout this research, FTIR and ATR concepts are discussed in depth at the outset. Then, details of the in-situ experiment are described including the optical path, sample holder and cryostat configuration as well as the electronics setup and the software used for serial communications with electrical instruments/data analysis. An explanation of how the raw data was treated to produce the graphs shown is discussed in the results section. The next section provides detailed information on how the electrochemical transistors were fabricated. The final section covers characterization techniques used in this work, such as atomic force microscopy or AFM and profilometry.

2.1. ATR-FTIR Spectroscopy

In both physics and chemistry, FTIR is an important characterization technique which is used for measuring the absorbance/transmittance of a sample over a wide spectral range. It is a relatively easy-to-perform method that can be used for gas, liquid and solid phases as well as complex materials and surface studies. The majority of this work is centered around analysis of FTIR spectroscopic data and therefore, it is important to have a good understanding of this technique. In this chapter, first I will explain the basic components of the spectrometer, including the source and detector as well as the Michelson interferometer. Next, I will explain how Fourier transforms are used in data analysis for FTIR output. Last, I will discuss methods of sample investigation using ATR.

2.1.1. Fourier Transform Infrared Spectroscopy

Infrared spectroscopy is most often used as a means of characterizing chemical compounds based on the absorption of molecular vibrations. Different functional groups result in specific features at certain frequencies. For example, the carbonyl stretch mode appears as a peak at 1710 cm-1 and is typically one of the strongest IR absorbers.

20 Figure 2.1 shows the basic components of a spectrometer. In a conventional spectrometer, first IR radiation is generated at a source. The beam then passes through a Michelson interferometer before reaching the sample where a fraction will be absorbed. Light reaching the detector is converted into a voltage signal and plotted as a function of the moving mirror position, in a graph called an interferogram.

Michelson Interferometer

fixed mirror

beamsplitter IR Figure1- FTIR schematic source moving mirror

sample

detector

Figure 2.1 Schematic of FTIR instrument

The IR light source is a resistively heated element which produces IR radiation through blackbody radiation. The intensity and shape of the spectrum depends on the temperature of the radiating object. Two common used IR sources for obtaining a beam in the mid-infrared region are Nernst and Globar.50 The Globar source is the source used in our setup. It is made of silicon carbide and can reach temperatures up to 1300 K and emit light in range of 450-6500 cm-1. Nernst glowers, on the other hand, are fabricated from a mixture of refractory oxides and can reach even higher temperatures in comparison to Globar sources. However, the IR radiation produced by a Nernst glower is below 2000 cm-1.51

Once an IR beam is generated at the source, it reaches the Michelson interferometer. The interferometer consists of two plane mirrors and a beamsplitter. One of the mirrors is stationary while the other mirror can freely move along a path normal to its plane. There is a semi reflecting optical element called the beamsplitter at the bisection of these two mirrors. When monochromatic radiation with wavelength � passes through

21 the beamsplitter, half of its incident energy will be reflected to the stationary mirror with the other half transmitted to the moving mirror. The reflection from the two mirrors then recombines at the beamsplitter. This beam is perpendicular to the beam first coming from the source and is called transmitted beam.

As the moving mirror changes position, an optical path difference or OPD occurs between the two arms of the interferometer. The OPD is measured accurately by laser. When OPD = ��, the interference is constructive whereas when OPD = (� + )� the interference is destructive50. The resulting pattern is called an interferogram, which is shown in Figure 2.2a. In the inteferogram, the x-axis represents the moving mirror position and the y-axis shows the intensity of the interference. At zero path difference, a sharp peak, known as a centerburst, is created. The interferogram has all the frequencies encoded in it which will later be Fourier transformed to produce the single beam spectrum.

a) Figure 2- Interferogramb) 12 10 8 6 4 intensity absorbsnce 2 0 0 2000 4000 6000 Mirror position wavenumber (cm-1) Figure 2.2 (a) Interferogram and (b) single beam spectrum of the IR light source.

When the transmitted beam comes out of the interferometer, it will then pass through the sample where the light absorption occurs. Note that the sample can be mounted either before or after the interferometer as the absorption will be missing from the beam in both cases. However, placing the sample after the interferometer is more convenient.

The sample can interact with the infrared beams in many ways depending on its phase and characteristics. One of the oldest and most straightforward methods is transmission where the beam passes through the sample and enters the detector, which was described above. Another method of IR spectroscopy is reflection which is used for samples difficult to be analyzed by the transmission method. This method can be categorized to internal and external reflections. The internal reflection is the technique

22 extensively used in this research and, therefore, will be thoroughly explained in the next section. In external reflection, the beam directly reflected from the sample is collected. Microsampling methods are also available where the spectroscopy is coupled with a microscope and can be performed on very small samples.50

The detector is a photoconductive element that transduces the light into electrical signal, which are varying voltages. When quick response and a wide spectral range is required, the most commonly used detector is made of Mercury Cadmium Telluride or MCT (HgCdTe) which is a semiconducting material which becomes conductive when illuminated with IR light. The band gap in MCT is very small to make it sensitive to low energy radiation. Therefore, it is very sensitive to temperature and needs to be cooled down by liquid nitrogen. Improper cooling can lead to very noisy signals.52

Spectroscopists are interested in the spectrum in the frequency domain. The Fourier Transform is the mathematical method by which the interferogram is converted into the single beam spectrum. The numerical algorithm used to perform the Fourier transform on a discreet data set is called Fast Fourier Transform or FFT.50

In order to report the data in terms of absorption, a subtraction method is usually used where the single beam of background, usually referred to as blank, is subtracted from the single beam containing the data of interest. It is assumed that the sample does not scatter any light and the reflection from background is same to that of the sample.

� = log � − log (�) Equation 9 where A is the absorbance and � and � are the intensity in the background and spectrum, respectively. The concentration dependence of the absorbance can be determined by Beer Lambert’s Law:

� = �� Equation 10 where � is molar absorptivity, � is the pathlength and � is the concentration.

In this research, we are interested in the spectral changes that occur in response to the application of voltage to our device. Thus, the sample without applied voltage is used as the blank for our measurements.

23 2.1.2. Attenuated Total Internal Reflection or ATR Technique

In this research, the sample of interest is an organic transistor with a thickness on the order of microns (i.e. a thin film). In addition, the sample contains opaque metal electrodes, making a regular transistor measurement extremely difficult, if not impossible. To allow measurements to be collected on operating devices we used the technique of multiple attenuated total internal reflection or ATR. In this method, a material transparent in the mid-IR is used both as the sample substrate and the ATR crystal. Light is directed into the crystal and reflecting multiple times. At each reflection, the IR light interacts with the sample where it can be absorbed. Eventually, the light leaves the crystal and is directed to the detector.

To understand total internal reflection, it is typical to begin by discussing Snell’s law, which describes refraction of light as it passes between two non-absorbing materials. A beam incident on a surface can be described in terms of the angle it makes with the surface normal (Figure 2.3a). The angle the beam takes after entering the second medium changes due to the difference in refractive indices of the materials in a manner described by Snell’s Law:53

� sin � = � sin � Equation 11 where θ1 is the incident beam, θ2 is the transmitted beam, �and � are the indices of refraction for the two different materials.

If �>�, as the incident angle increases at some point, the refraction angle becomes 90° (��90° = 1) and the refraction beam is completely reflected from the interface of the two media (Figure 2.3b). This angle is called critical angle and can be calculated by equation 12. In other words, this is the minimum angle required for total internal reflection to occur.

� = �� Equation 12

If the incident angle further increases beyond the critical angle, the transmitted beam will be totally reflected back to the first medium which is called Total Internal Reflection (Figure 2.3c). In this case, ��>1 and is not real quantity but an imaginary number. Although no net energy is transferred to the second medium under conditions of

24 ! < ! a) " % b) !" = !% c) !" > !%

!#=90° Evanescent wave medium two

medium one

!" !# � !" !"

� total internal reflection

Figure 2.3 Snell’s law (a)The light travelling from medium one (silicon crystal in this study) to medium two (P3HT film in this study) with a different refractive index will have a different incidence angle, (b) at the critical angle, the incidence beam will touch the interface, and (c) total internal reflection phenomenon where the incidence will occur in the same medium which results in production of evanescent wave. Adapted from Stuart.50

total internal reflection, an optical disturbance occurs in it which is called "evanescent wave". The evanescent wave is an electromagnetic field which decays exponentially. If we assume the plane of incidence is an x-z plane, we can define the incident wave as the electric field in those dimensions:54

� = � �� (��1 + �� 1) Equation 13 where � is the wave vector and � is the incident angle. The cosine term can be substituted by the mathematical relation between sine and cosine (equation 14) yielding the wave function expressed only in terms of the sine function.

cos �2 = 1 − (sin(�1)) Equation 14

And then substituting the sine term with Snell’s Law, the electric field in the z direction is given by equation 15:

� = � �� [−�� sin(� ) − 1] Equation 15 1

25 In ATR experiments, it is important to know how far the evanescent wave can penetrate into the sample. The amplitude of the evanescent wave decays as a function of 54 distance from the interface. This distance is defined as depth of penetration �:

� = Equation 16 ( ) where � is the wavelength of the light. Therefore, the depth of penetration is a function of the refractive index of the ATR crystal and angle of incidence as well as the wavelength of the light.

It is clear that � depends on the refractive index of the ATR object used. Table 2.1 55 shows common materials used as ATR crystals and their refractive indices. All these materials have large refractive indices in order to make total internal reflection possible. In addition to the crystal’s refractive index, properties such as its useful wavenumber range, pH sensitivity (i.e. the pH range of the medium where the crystal would remain stable without undergoing oxidation or reduction), durability, and toughness are important considerations as well. The chemical stability determines the procedures for cleaning ATR crystals. For example, crystals like silicon are stable in strong acidic solutions such as piranha, making them easy to clean. The durability of ATR crystals is important because they must be handled during sample fabrication. Another concern when choosing ATR material is their transparency window. The crystal itself should not have any absorbance in the IR range of interest. In this research, silicon was used as an ATR crystal as it has a relatively good resistance to mechanical shock, is less expensive and it is transparent in the frequency range where the polaron absorption of interest occurs (1500-6000cm-1).

FTIR Setup (Hardware)

A Newport 80350SP01 FTIR instrument equipped with a light source and detector is used for this research. In addition, optical elements are required to collimate and change the direction of the beam so that it can be passed into the thin ATR crystal and sent to the detector. Figure 2.4 shows the instrument setup. All optical components were purchased from ThorLabs. In particular, three off-axis parabolic mirrors and a plane mirror are used in the setup. An off-axis parabola (OAP) mirror can focus a collimated beam, and the off- axis design can spatially separate the focal point from the rest of the beam path. the surf-

26 Table 2.1 Properties of ATR crystal materials, reproduced from Fundamentals of Fourier Transform Infrared Spectroscopy by B. C. Smith55 Crystal Refractive Wavenumber Color pH Range Comments index Range(s) cm-1 KRS-5 2.36 20,000–250 Red 5-8 Soft, highly toxic, rarely used at 10.6 ��* today ZnSe 2.42 15,000–600 Yellow 5-9 brittle, attacked by strong acids at 10.6 �� and bases Si 3.67 8900–660 Grey 1-12 at 0.82 �� Ge 4 5500–600 Grey 1-14 Shallow DP, durable at 0.35 �� Diamond 2.37 30,000–2200, Clear 1-14 Tough, durable, absorbs at 10 �� 2000–400 in mid-infrared, $$$ ZnS56 2.2 17,000-950 Clear 5-9 Good thermal shock resistance at 9.8 �� *Refractive indices are reported from https://refractiveindex.info, accessed by April, 2019 ace of the mirrors is gold coated which offers higher reflectance and more stability. The first parabolic mirror is placed after the interferometer and directs the light to the second OAP mirror. These two mirrors have different focal lengths so as to reduce the beam diameter. Reducing the beam diameter is useful in sending the maximum amount of collimated light into the ATR crystal. Following the pair of OAPs, a plane mirror is used to direct the beam towards the cryostat. The cryostat has zinc selenide windows which are transparent to IR and allow the beam to enter the ATR crystal. Once out of the crystal and cryostat, it is directed towards the final OAP which focuses the beam into the detector.

2.1.3. Mirror Alignment

Since the IR beam is not visible to the naked eye, the system was aligned using a 625 nm diode laser. In this case, a glass ATR crystal is used because silicon absorbs visible light. To make alignment more straightforward, protocol for mirror adjustment is described in Figure 2.4.

The first OAP mirror is used to send the beam across the table at 90 degrees. Since the light will enter the ATR crystal at 90 degrees, it is more convenient to direct the beam at 90 degrees. The second OAP mirror is placed on the path of the beam the way that the focal points of the two mirrors overlap. This is in order to shrink the beam so that it can travel through the tiny edge of the crystal. In other words, two focal points are

27 matching to collimate the light so that it can enter the ATR crystal. The second OAP mirror is used to turn the beam at another 90 degrees. At this point, if the previous steps are done correctly, the laser beam must be collimated to a one small point that can be detected on a piece of paper at even further distances away from the table. Next, a flat mirror is used to direct the beam to the cryostat window where it passes through the crystal. If the room is dark enough, the ATR bounces of the laser beam in the glass ATR crystal are visible to see (Figure 2.5).

IR Michelson Source Interferometer

OAP Mirror OAP Mirror

Cryostat Plane Mirror

ATR Crystal

ZnSe Windows

Detector

Figure 2.4 The FTIR setup. The light (red) coming out of the interferogram is guided to the ATR crystal by a set of mirrors and then enters the detector

Figure 2.5 Total internal reflection of red laser in a glass ATR crystal used to check system alignment

28 The beam exiting the cryostat window will then go to the detector. If the laser source and glass ATR crystal are replaced by an IR light source and a silicon ATR crystal, a good interferogram signal will be obtained. Another factor to check if the alignment is correct, but the interferogram signal is still too low is the FTIR instrument. Usually there are two screws on the instrument to adjust the alignment of the optics inside the device.

2.1.4. Sample Holder and Cryostat

Once the sample is fabricated, it should be mounted on the sample holder shown in Figure 2.6. The sample holder is made of copper and wrapped in Kapton tape to ensure there is no electrical contact with the sample. The sample is held in the sample holder and electrically addressed by three different parts, each covered with copper tape. The gate contact is made from a thin piece of sheet metal, which provides mechanical compliance such that the gate contact gently sits on top of the gate electrode. The source and drain contacts are Delrin bars that can be attached to the sample holder by means of machine screws. Contact is made to the source and drain electrodes by a piece of copper tape covering the Delrin bars such that the copper tape comes into electrical contact when the assembly is screwed onto the sample holder. The sample holder itself is attached to the end of the cryostat (Janis ST-100 optical cryostat). A cryostat is a chamber like device that can be cooled down by using a cryogenic fluid such as liquid nitrogen or liquid helium. However, it also can be used as a vacuum chamber, allowing spectroscopic measurements to be collected in the absence of water and oxygen exposure when connected to the vacuum pump. The pump used in this research was Pfeiffer HiCube80 Eco which could pump the cryostat down to pressures as low as 5×10-5 mbar.

2.1.5. Electronic Setup

To apply voltage to the device and record the resulting current, an electronic setup consisting of a function generator, an amplifier and a digital ammeter were used (see Figure 2.7).

An arbitrary function generator (Tektronix AFG3022C) was used to generate the voltage signal associated with the gate voltage that was applied to the sample. The source and drain electrodes were tied together and grounded. The arbitrary function generator

29 a) b)

Copper contacts

Figure 2.6 Schematic of the sample holder in (a) 3D view and (b) top view. ATR crystal is placed on the edges of the slot. Three pairs of threaded holes are used to mount the source, drain and gate contact and one pair is used to mount the sample holder on the cryostat. (c) The sample is contacted by copper pieces once placed on the sample holder.

Function Generator

Cryostat

Amplifier

Ammeter

Figure 2.7 Schematic of the in-situ FTIR instruments. A voltage is applied to the sample and the current flow is measured. enables any voltage function to be applied and can provide up to 25 MHz frequencies. It also provides a DC offset voltage adjustable between -5 V to +5 V. However, the primary limitation of the function generator is that it provides only a small amount of current. For this reason, the output of the function generator was sent to a KEPCO bipolar power

30 amplifier. This device accepts the input from the function generator and amplifies the voltage by -4x up to ±10 � and ±20 �. The maximum frequency of the power amplifier output is 10 kHz. To read the current, a digital ammeter (Keysight 34465A) was connected in between the source/drain electrodes and ground.

2.2. Software and Data Analysis

All data analysis was performed using Igor Pro. In the program, a data set is referred to as a wave. Waves are up to four-dimensional arrays that can carry not only numbers, but also characters (text), or date-and-time entries. This makes data analysis and graphing various data against each other easier. Igor Pro also provides the user with strong curve fitting features. It is also programmable via a built-in programming environment. In this research, Igor Pro version 6.37 was used to analyze spectroscopic data as well as current measurements. To decrease the amount of time needed to load and process the data, all the analysis steps such as loading waves, integration, normalization and curve fitting are carried out using a program written previously by Dr. Kaake.

2.3. Sample Fabrication

In order to study the time dependent behavior of organic electrochemical transistors using an in-situ infrared spectroscopy method, devices were fabricated on a substrate that also serves as an infrared waveguide, hereafter referred to as an ATR crystal. The ATR crystals were created from double side polished silicon wafers with diameter of 100mm and a thickness of 1mm. These were purchased from IWS. First, the wafer was cut into 1cm x 3cm pieces using a dicing saw in 4D LABS. Then, the crystals were polished into a trapezoidal shape with 45o angles. This was done by holding onto the crystal with a metal sample holder designed with a 45 degree angle as shown in Figure 2.8. The angled edges were created by removing material using a polishing wheel. In addition to wet sandpaper, aluminum oxide Lapping films with various particle sizes (30,20,10,3 and 1�m) were purchased from Allied High Tech Products and used to polish the edges to a good optical quality. To clean any residues off from the substrates, they were then put in a piranha solution (3:1 volumetric ratio of sulfuric acid and hydrogen

31 peroxide) for at least 30 minutes. The crystals were then thoroughly rinsed with water and Crystal holder for polishing isopropyl alcohol and dried with nitrogen.

Rigid holder (front) Rigid holder (back)

45°

ATR Lapping film

Figure 2.8 schematic of the sample holder for polishing the silicon crystal to form 45° edges

The next step in device fabrication is to deposit a thin film of organic semiconductor by spin coating. Spin coating is an easy, effective, inexpensive and reproducible technique that is commonly used to deposit uniform thin films.57 In this method, a few drops of a solution containing the desired material are placed on the sample, the sample is then rotated at a rate < 5000 rpm. During this time, the majority of the fluid is flung off the sides of the substrate, the remaining solution evaporates and a thin film of the desired material remains. It should be noted that all sample fabrication steps including spin coating were performed in the glovebox under nitrogen atmosphere where water and oxygen contents were monitored and kept below 1ppm.

The organic semiconducting polymer (P3HT) was purchased from SigmaAldrich and was dissolved in 1,2-dichlorobenze (SigmaAldrich). Conjugated polymers especially P3HT tend to have high solubility in dichlorobenzene58 and other high-boiling point solvents. The structure of the polymer is shown in Figure 2.10. The solution was filtered through 0.2�� poly(tetrafluoroethylene) (PTFE) Iso-Disk syringe tip filter in order to improve the film quality and then spin coated on ATR crystal. The speed of rotation was initially 500 rpm for 20 s and then was increased to 1600 rpm for 100 s. Film thickness of P3HT was controlled by using solutions of varying concentrations with 5mg/ml, 10mg/ml, 20 mg/ml and 40 mg/ml resulting in films with thicknesses of 20±1.08 nm, 40±4.48 nm, 100±3.39 nm and 200±2.47 nm, respectively. The thickness of the films was verified by Atomic Force Microscopy (AFM).

32 Side View Top View

a) Si Substrate

P3HT Film b)

c) Gold Electrodes

PEO Film d)

e) Gold Gate Electrodes

Figure 2.9 Schematic of sample preparation steps in top view (left) and side view (right). (a) ATR crystal is used as the substrate and (b) semiconducting layer is deposited on top of it. (c) The gold source and drain electrodes are deposited on the semiconducting film. (d) Dielectric layer is then spin coated on top of source and drain electrodes and finally (e) gold gate electrode is deposited on top of the dielectric.

The next step in device fabrication is to deposit gold source/drain electrodes on top of the semiconductor film via thermal evaporation through a shadow mask. Gold was the chosen material for electrodes because it is easy to deposit, tends to eliminate contact resistance, and gold is highly air stable59.

The shadow mask used to achieve the desired pattern was formed from 1095 Blue Spring Steel with a thickness of 100 �m using a laser cutting tool. Two different shadow masks were used, each containing two different electrode spacing. In all shadow masks and in the center where both sets of electrodes are present, the channel length is 1.5 cm. The channel width is 1500 µm and 3100 µm in one shadow mask and 300 µm and 700 µm in the other. Other dimensions of the shadow mask are shown in Figure 2.11. The thickness of the gold deposited is 50 nm.

The next step in device fabrication is to deposit a dielectric layer whose function is to insulate the source and drain electrodes from the gate electrode. In an electrochemical transistor, the dielectric contains ions that can move in response to an applied voltage.

Figure 2.10 Chemical structure of all the chemicals used in this research

5.5cm

3.175cm 5.2cm

Figure 2.11 Shadow mask schematic illustrating dimensions common to both masks. Mask containing 1500 �� and 3100 �� electrode spacing is shown.

34 In this study, high molecular weight polyethylene oxide PEO (400,000 g/mol) was purchased from Sigma Aldrich) and was used to dissolve the lithium salt. Ether-based polymers are found to dissolve and transport a number of inorganic salts60. Three different lithium salts were studied including lithium perchlorate (LiClO4), lithium

Bis(trifluoromethylsulfonyl)amine (LiTFSI) and lithium phosphorus fluoride (LiPF6). The structure of the ions is shown in Figure 2.10. The amount of lithium salt added to the solution was chosen to obtain a 16:1 ether-O/Li+ mole fraction. This ratio was chosen because it has the greatest ionic conductivity at room temperature61. Solutions were formed using acetonitrile at a concentration of between 30-60mg/ml. As with P3HT, solution concentration was varied to create PEO films of differing thickness with solutions of 5, 10, 20, 30 and 40 mg/ml producing films of 20±1.08, 40±0.48 , 100±3.39 , 160±0.87 and 200±2.47 nm, respectively as measured by profilometry.

In order to facilitate electrical contact to the source/drain electrodes and clearly determine the active region of the device, a portion of the source/drain electrodes covered during spin coating. The cover should be adhesive enough to remain on the sample during spin coating, should not peel off any previous layer when removed, and should be thin enough not to disrupt the spin coating process. To accomplish this, a thin sheet of poly(dimethylsiloxane) (or PDMS) was used. To make the PDMS sheet, Sylgard base and curing agent were mixed with a ratio of 10:1. The mixture then was placed under vacuum for ten minutes to eliminate air bubbles. Next, the mixture was poured in a petri dish with a glass slide glued at the bottom if it. The mixture should form a thin layer on the glass slide to produce a thin layer of PDMS. The petri dish was placed in the oven at 150 ℃ for an hour. After that, the thin layer of PDMS on the glass slide was cut into appropriate pieces to cover sides of the sample (~1 cm x 1 cm pieces). The pieces were then carefully placed on the sample before spin coating the dielectric, using two pairs of tweezers. During the deposition, the dielectric solution was observed to flow over the thin PDMS film as desired. Once the spin coating is complete, to remove the PDMS sheets, one has to exercise care not to remove the dielectric layer. To prevent this, a cotton swab was wetted by PEO solution in acetonitrile and gently rubbed on PDMS sheets. The solvent dissolves the PEO film on the PDMS sheets and the PDMS pieces can be easily removed from the sample without damaging the dielectric film on the active area.

35 The final step in device fabrication is the deposition of a gold gate electrode via vacuum sublimation with the same thickness of 50 nm as used for the source/drain electrodes.

2.4. Characterization Techniques

2.4.1. Atomic Force Microscopy (AFM)

Atomic Force Microscopy (AFM) is an important high resolution imaging technique frequently used for characterization of materials. Figure 2.11 shows different parts of a typical AFM. It consists of a small cantilever mounted on a support, with a sharp tip on the free end of the cantilever. The cantilever is usually made of silicon with a tip radius of curvature on the order of tens of nanometers. The instrument works by detecting the bending of the cantilever due to the interaction between the AFM tip and the sample surface.62 When the tip is away from the surface, the dominant forces are attractive van der waals and capillary forces, which depends on the material’s properties of the sample. When the tip is in contact with the sample, repulsive forces due to elastic properties of the sample act on the tip. Imaging is typically done in the repulsive regime with the force on the cantilever being determined by focusing a laser on the back of the cantilever. The deflection of the cantilever is inferred from the deflection of the beam as detected by a four-quadrant photodiode. The photodiode output is connected to a feedback system which changes the height of the stage to maintain a constant deflection. This style of imaging is commonly referred to as contact mode.63

photodetector laser

cantilever

tip sample

stage figure2.11 Schematic of AFM. The tip of the cantilever probes the surface and any deflection is measured by the change in the laser position

36 The AFM can operate in different modes including contact mode, non-contact mode and tapping mode. In contact mode or static, the cantilever is in contact with the sample. In other words, the signal obtained from the deflection of the cantilever is a map of the distribution of the forces in the sample. This method can be harsh to the sample as the repulsive forces between the surface of the tip and sample can be strong. In non- contact or tapping mode, very small vibrations are induced in the cantilever63. The vibration frequency, amplitude and phase are modulated by the forces between the cantilever tip and the surface.62

In this study, AFM measurements were performed on P3HT films to determine their thicknesses. First, a P3HT film was deposited onto a Si substrate under the exact same spin coating conditions described above. Then, a very narrow scratch was made on the film using the tip of sharp tweezers. The scratch was imaged to measure the difference in height between the surface of P3HT and the Si substrate. This height difference is the film thickness. During imaging, one has to be careful with several parameters such as the width of the scratch, as well as the scan size and the tip velocity to make sure the tip does not break during the measurement. In addition, one must account for film texture and roughness as the AFM has difficulty measuring steep walls or overhangs. These considerations require that the scratch one chooses to image must be very narrow and clean. Moreover, the cantilever tip is sensitive to soft surfaces and the can break or become contaminated by the surface. On softer samples like PEO, a different technique is used to measure the thickness and roughness.

2.4.2. Profilometry

A profilometer is an instrument for measuring the surface profile and roughness by measuring distances between the high and low points of a surface in nanometres. There are two different types of profilometers, non-contact or optical and contact or stylus.

Stylus profilometers employ a sharp tip (stylus) to detect surface topographical features. Here the stylus is allowed to move along the plane of the surface, generating a 3-dimensional representation. In other words, the surface variations in vertical stylus displacement is measured as a function of position by using optical or electromechanical methods64. The stylus is made of diamond and the applied force on it can is important because if too high or too low it can cause surface damage or poor measurement

37 accuracy, respectively. It is slower than non-contact techniques. The resolution depends on the stylus tip size and shape65. The vertical displacement of the stylus is then amplified by a transducer and is converted for image analysis by the provided computer software. In this study, Bruker Dektak XT profilometer was used to measure the thickness of dielectric layer for different samples.

The non-contact method provides the same information as the contact methods but it is faster and there is no possibility of damaging the surface as there are no physical contacts. This method involves interferometry to accurately measure distances. The apparatus consists of a light source that is either monochromatic or broad-band as well as a beamsplitter, a combination of lenses and mirrors for focusing the beam and a sensor to convert light intensity to digital form.66

38 Chapter 3.

Results and Discussion

The devices for this study are made as mentioned in the experimental section. Figure 3.1 shows the top and side view of the device. The IR beam passes through the ATR crystal while the device is charging and discharging and can provide information about the transient behaviour. Charging is when device is under an applied voltage and discharge is when the voltage is turned off (V=0). The FTIR scans are recorded in logarithmic time fashion in order to provide more data in beginning of charge and discharge process, where the transition occurs quickly.

Side View V

PEO:LiClO 4 P3HT

Top View PEO P3HT

Figure 3.1 Side views and top view of the device. A voltage is applied to gate electrode while source and drain are grounded. The IR beam bounces through ATR crystal and provides spectroscopic data about changes in the device during charge and discharge process

First, interferograms are loaded to the software and treated with the FFT to create single beam spectra (Figure 3.2).

Figures 3.3a and 3.3b show a few sample absorbance spectra of the devices while charging and discharging, respectively. The absorbance is reported as the difference in absorbance at various times versus that before applying voltage. The broad peak between 1500 and 6000 cm-1 is referred to as a polaron peak. The polaron peak increases as a function of time in charging and decreases with time during device discharging. This is due to changes in the number of charge carriers in the channel.

39 31.5s 3.0 1.5 5.5s 1.5s 2.5 0.5s 0s 1.0 2.0 A

1.5 Δ 0.5

Intensity 1.0

0.5 0.0 0.0 2000 4000 6000 0 2000 4000 6000 -1 -1 wavenumber (cm ) wavenumber(cm ) Figure 3.2 A sample of single beam spectrum of the electrochemical device (tP3HT=160 nm, 1.0 tPEO= 300 nm and lc= 300 �m) before applying any voltage. 0.8

0.6

0.4 A (norm) A

Δ 347.0s a) 0.2 137.5s b) 551.3s 62.0s 794.9s 1.5 31.5s 1.5 1132s 0.0 5.5s 1746.5s 1.5s 0 4000.5s 800 1200 1600 1.0 0.00s 1.0

A A

timeΔ (s) Δ 0.5 0.5

0.0 0.0

2000 4000 6000 2000 4000 6000 -1 wavenumber (cm-1) wavenumber (cm ) Figure 3.3 Absorbance spectra of (a) charge and (b) discharge for different times. When charging, the peak rises due to increase in doping while when discharging, the process is reversed and the peak intensity decreases. � = −�. � � and the sample dimensions are tP3HT=160 nm, and tPEO=300 nm and and �� = �� µ�.

In order to more simply visualize the data as function of time, the absorbance spectra were integrated over the range 1500 to 6000 cm-1. Figure 3.4 shows the integrated absorbance against time, normalized to the largest value measured. There is a quick increase right after applying voltage and reaches a plateau over time. When the discharge begins, the intensity drops. Just like the charging rate, the discharge rate is quicker in the beginning and slows down with time. However, the discharge does not reach zero in this example, possibly due to charges that have become trapped in the film during the charging process. The thicker the channel, the more trapped charges would be present in the sample. However, for thinner P3HT films the discharge would reach zero in the time period allocated for discharge in this experiment.

40 1.0 0.8 0.6 0.4

A (norm) A Δ 0.2 0.0 0 400 800 1200 1600 time (s)

Figure 3.4 Integrated absorbance showing charge and discharge rate. The discharge has not occurred fully in the recorded times due to trapped charges in the channel. � = −�. � � and the sample dimensions are tP3HT=160 nm, tPEO=300 nm and �� = �� µ�

For each sample, a total of seven runs were performed over the voltage range of −3 � to −1.8 �, with ∆� = 0.2 �. Since the intensity of absorbance depends on the IR signal coming out of the ATR crystal, in order to make these comparisons easier between different devices and independent of the light intensity, the absorbance data were divided by the maximum absorbance value and graphed as normalized absorbance.

Figure 3.5 shows the absorption data against time for charging of a sample device for different voltages. The greater the voltage is, the faster the devices charge. This trend was observed for all the devices and will be the subject of future work. Comparisons between devices with different geometry will be demonstrated with a sample voltage of � = −2.4 �.

In order to study the time-dependence of charging of the device, we consider the different steps of charging and their corresponding device parameters. These are presented in Table 3.1. Upon applying a negative voltage to the gate, the ions in dielectric are polarized with positive ions becoming concentrated near the gate electrode-dielectric interface and negative ions being concentrated near the dielectric-semiconductor interface. This ion movement rate in the dielectric acts as a resistive component and therefore depends on the dielectric thickness (tpeo), with thicker films having a slower polarization time. As ions reach the dielectric-semiconductor interface, they briefly form an electrostatic double layer as charges are injected from the counter-electrodes. The parameter related to charge injection is the channel length (lc). The charges in the organic

41 0.0s Voltage -2.4V a) 0.5s lc 3100 µm 2.0 1.5s tPEO 0.3 µm 2.5s t 0.02 m 5.5s P3HT µ 1.5 219s

A 1.0

0.5

0.0

2000 4000 6000 wavenumber (cm-1) b) time 9.5 s 2.5 lc 3100 µm -3V tPEO 0.3 µm 2.0 tP3HT 0.02µm

1.5

Δ 1.0 0.5

0.0 -1.8V

2000 4000 6000 wavenumber (cm-1) c) 1.0

0.8 Voltage -3.0V 0.6 -2.8V -2.6V 0.4 Lc 3100 m -2.4V

A (norm) A µ

Δ tPEO 0.3 µm -2.2V -2.0V 0.2 tP3HT 0.02µm -1.8V 0.0 0 10 20 30 40 50 60 70 time (s) Figure 3.5 Absorbance spectra for different times and voltages for a sample with lc=3100 µ�. (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. Polaron peak intensity grows with charging time. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆� = �. � �). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm-1 at different voltages. Specification of the device are mentioned in the graph. semiconductor can form a neutral species with the ions. These ion-charge pairs can then diffuse into the bulk of the organic semiconductor. The rate of this process should depend on the thickness of the P3HT layer (tp3ht). To see which mechanism could be the rate

42 limiting step, samples with different dimensions were prepared. To examine each mechanism’s effect, the corresponding dimension for that mechanism was chosen as variable while keeping the other parameters fixed across the devices.

Table 3.1 Different mechanisms of charging process and the corresponding dimension of the device Mechanism Corresponding dimension of the device Charge carrier diffusion Channel length Ion movement in dielectric Gate dielectric thickness Ion diffusion in channel Semiconductor thickness First, in order to study the effect of channel length on charging rate of the device, samples were made with different channel lengths. Four different lengths of 300µm, 700 µm, 1500 µm and 3100 µm were chosen for this study. Figure 3.6 shows the results for the transient region of charging for three electrode spacings at a voltage of -2.4 V. As it can be seen, all four channel lengths show very similar behavior. Small differences might be due to sample-to-sample variation, as no clear channel length dependence was observed. It is worth noting that previous reports show a channel length dependence in this system for �� > 7 ��,67 which is well above twice of the largest channel length used in this study.

1.0 0.8

Lc 0.6 300µm -2.4V 700µm 0.4 t 0.3 µm m A (norm) A PEO 1500µ

Δ tP3HT 0.02µm 3100µm 0.2 0.0 0 10 20 30 40 50 60 70 time (s) Figure 3.6 Integrated absorbance spectra for devices with different channel lengths at -2.4V.

Next, we studied the effect of gate dielectric thickness on charging behavior of the device. Four different PEO thicknesses of 0.3±0.41 µm, 1.3±0.23 µm, 5±0.37 µm and 27±0.78 µm were studied. These different thicknesses were achieved by using different concentrations of dielectric solution for spin-coating. Figure 3.7 shows absorbance spectra of the devices with different dielectric thickness against time. The charging rate of the

43 device was similar for 0.3 µm and 1.5 µm devices, decreasing for 5µm and 27 µm. The thickest layers of PEO demonstrated a device charging characteristic determined by the ion movement in PEO.

To examine the effect of ion diffusion in P3HT, the effect of P3HT film thickness on charging rate was studied. The PEO film thickness chosen for these experiments was 1.5 µm. This value was chosen over 0.3 µm because the dielectric layer must contain enough number of ions to be able to dope the entirety of the channel for all P3HT film thicknesses. First, the number of thiophene rings per volume is calculated based on the molecular weight and density of P3HT. Then, the number of PEO units per volume is calculated in same fashion and the 16:1 molar ratio is applied to find the number of anions per volume. Dividing the number of thiophene rings in p3HT and the number of negative ions available in the dielectric with the same active area, it is found that PEO should be at least 5 times thicker than P3HT to ensure there are enough ions to dope the channel completely. The P3HT film thicknesses studied were 20 nm, 40 nm, 100 nm and 200 nm. Figure 3.8 shows the transient charging behavior for devices with different P3HT film thickness. As the films are made thicker, the devices operate more slowly, with 20 nm and 40 nm devices operating at the same rate. As before, the ability to influence the rate liming step by changing the device geometry provides support for the model of charging mechanism proposed earlier.

We studied the charging rate of different devices to validate our model, using device geometry to control the rate limiting step. First, the channel length was varied and it was found that the charging is independent of channel length for channel lengths below 3100 ��. Then, devices with different dielectric thicknesses were studied and it was shown that increasing the dielectric thickness slows the charging dynamic for the thickest films studied. For thinner films, the charging rate was PEO thickness independent. Finally, the effect of P3HT film thickness was examined for four different thicknesses and it was found that in thicker channels, ion diffusion in the semiconductor is the rate limiting step. To quantify our model, we derived a set of differential equations that describe the behavior and solved them numerically, using our solution to fit the spectroscopic data.

a) voltage -2.4V 0.0s 2.0 tP3HT 0.02µm 0.5s tPEO 27µm 5.5s lc 300µm 18.0s 1.5 44.0s 219.0s

A 1.0

Δ 0.5

0.0

2000 4000 6000 -1 wavenumber (cm )

b) 0.6 time 9.5 s -3V tP3HT 0.02µm 0.4 tPEO 27µm lc 300µm

A 0.2

Δ 0.0 -1.8V -0.2

2000 3000 4000 5000 6000 -1 wavenumber (cm ) c) 1.0 0.8 0.6 tPEO 0.4 0.3µm

A (norm) A

Δ -2.4V 1.5µm 0.2 tP3HT 0.02µm 5.0µm lc 300µm 27.0µm 0.0 0 10 20 30 40 50 60 70 time(s)

Figure 3.7 Absorbance spectra for different times and voltages for samples with different dielectric thicknesses. (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆� = �. � �). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm-1 at different voltages. Specification of the device are mentioned in the graphs.

Briefly, the model describes device charging as the result of ion diffusion into the bulk of the semiconductor as done previously,21 using a time dependent boundary condi-

45 0.0s voltage -2.4V 1.5s t 0.04µm a) 2.5s P3HT 1.2 5.5s tPEO 1.5µm 18.0s lc 300µm 219.0s 0.8

Δ 0.4

0.0

2000 4000 6000 wavenumber (cm-1)

1.2 b) -3V 1.0 0.8 0.6

Δ 0.4 time 9.5 s 0.2 tP3HT 0.04µm -1.8V 0.0 tPEO 1.5µm lc 300µm

2000 4000 6000 -1 wavenumber (cm )

c) 1.0 0.8 0.6 t 0.4 P3HT

A (norm) A 20nm

Δ -2.4V 40nm 0.2 tPEOT 1.5µm 100nm lc 300µm 200nm 0.0 0 20 40 60 time(s)

Figure 3.8 Absorbance spectra for different times and voltages for samples with different P3HT thicknesses (a) Absorbance time and frequency dependence at constant voltage of -2.4 V. (b) Absorbance voltage and frequency dependence at the same time during charging process for different voltages (t=9.5 s and ∆� = �. � �). (c) Absorbance spectra integrated over frequency range of 1500 to 6000 cm-1 at different voltages. Specification of the device are mentioned in the graph. tion at the PEO-P3HT interface to describe the response of the dielectric. The model will be described in two parts. First the mixed ion-hole diffusion will be described, then the

46 inclusion of a time dependent boundary condition and the resulting numerical solution will be described. To interpret the charging curves and find the time scales of the process, we modeled the diffusion process. When P3HT is doped, the polaron’s positive charges are compensated by anions. In the beginning of charging process, the concentration of negative ions in the semiconductor/dielectric interface is greater than the bulk and these ions diffuse into the channel with time. Considering this process to be one dimensional along � axis perpendicular to the substrate, with the diffusion coefficient D independent of ion concentration:

(,) (,) = � Equation 17

where ℎ is the ion concentration and � is the distance from the dielectric/semiconductor interface. Three boundary conditions are required to solve this equation.68 First, the concentration at the dielectric/channel interface is constant or

ℎ 0, � = ℎ. Before any charging occurs, the ion concentration at any thickness of the film is also zero, or ℎ �, 0 = 0. The last boundary condition is the constant concentration of ions at � = �, which is known as the “no flux” condition and can be written as ℎ � , � = 0. Using these conditions to solve the diffusion equation leads to the following equation:21

ℎ �, � = ℎ (1 − � sin(� �)exp (−� � �)) Equation 18 where � is the diffusion rate constant in P3HT and � is the Fourier coefficient involving an integral over the initial condition of the depth dependent charge concentration and � a constant, both described below:

� = Equation 19 ()

() � = Equation 20

Modeling the resistance of ion movement in the dielectric was done by including a time dependent boundary condition at the PEO-P3HT interface, which represented the concentration of ions that could be delivered by the dielectric to the interface. In other words; the ion concentration at the interface is no longer constant but a function of time,

47 which can be written as ℎ 0, � = ℎ(1 − �� ) = �(�) where ℎ is a constant, � is the constant describing the ion movement in the dielectric, gamma is used to represent a stretched exponential function and � is the time steps. The other two boundary conditions remain the same; ℎ �, 0 = 0 and the no flux condition. In this case, we solve the equation numerically, using � to represent the time at the beginning of each iteration. At each time step, the previous boundary condition is used to update the new set of boundary conditions and to solve the equation again. The solution is given by:

ℎ �, � = ℎ(1 − sin(��)exp (−� ��)) Equation 21

where � = ℎ(�, � − ∆�) sin(��), � = and

� = 2 ℎ(� + ∆�, �) sin(�(� + ∆�) Equation 22

Figure 3.9 shows the fitted data for a sample voltage of -2.4 V for different devices. As it can be seen, the model gives a good fit. This was true for all devices fit to the model. Simpler models based on two or more exponential charging curves also provide an adequate but poorer fit. In addition, the parameters of a double exponential fitting function are not easily interpreted in terms of the model. For all of the devices, the charging curve for each voltage was fit.

As it was mentioned in the beginning, the ultimate goal is to separate the role of each component of the device in the charging process. According to this model, kexp is a rate constant related to polarization of ions in the dielectric and kd in equation 21 is related to diffusion of ions once they enter P3HT channel. If these rate constants are graphed against the thickness of their corresponding component in the device, the resulting graph can be valuable in determining the rate limiting step for any device geometry. The extracted values of these rate coefficients with respect to device geometry and voltages are shown in Figure 3.10. The data demonstrates the conclusions drawn earlier, further demonstrating the power of the model. Devices with the PEO films thinner than 1.5 �m show thickness independent charging dynamics for all voltages, while devices with thicker PEO films show slower dynamics, showing that the charging rate is being limited by ion polarization time in the dielectric. Similarly, devices with P3HT films smaller than 40 nm show charging dynamics independent of channel thickness for all voltages, while devices

48 with thicker P3HT films show slower dynamics, due to diffusion of ions in the channel limiting the charging rate.

The importance of developing this model, and demonstrating control over which process limits device charging is that it allows us to directly study the structure-property relationship in mixed ion-hole diffusion. These experiments were performed using different lithium salts to determine how the ion species affect the charging behaviour. The device with tP3HT=100 nm and tPEO=1.5 �m was chosen for the ion studies as the dynamic of charging is limited by ion diffusion in P3HT, as was shown earlier. Two lithium salts of

LiTFSI and LiPF6 were used. The Li:O ratio was kept 1:16 for all the salts. Figure 3.11 shows the absorption against time for devices with different lithium salts. The devices charge at the order of LiTFSI>LiPF6>LiClO4.

a) 1.0 0.8

0.6 tPEO 0.3µm 0.4 1.5µm

A (norm) A

Δ 5.0µm 0.2 -2.4V tP3HT 0.02µm 27.0µm model 0.0 lc 300µm 0 20 40 60 80 100 time(s)

1.0 b) 0.8 0.6 tP3HT 0.4 40nm data A (norm) A 100nm data

Δ 0.2 -2.4V 200nm data tPEO 1.5µm model lc 300µm 0.0 0 20 40 60 80 100 time(s)

Figure 3.9 Integrated absorbance over 1500-6000 cm-1 and fitted by the model. The charging curves presented here are for devices with (a) different PEO thickness and (b) devices with different P3HT thickness all at a voltage of -2.4 V.

49 The device with LiTFSI exhibits the fastest charging rate while the device with

LiClO4 is the slowest of the three. This trend is opposite to how the size of these species compare, with LiTFSI being the bulkiest of all and LiClO4 the smallest. Therefore, the charging rate cannot be scaled by anion size.

If the dynamics was limited by ion movement in the dielectric, the charging trend observed would match that of the mobility. A study on the mobility of these ions measured in propylene carbonate and �-butyrolactone at 25 ℃ shows the following trend:69

LiClO4 > LiPF6 > LiTFSI

1 -1.8V a) 8 -2.0V 6 -2.2V 4 -2.4V -2.6V

exp -2.8V k 2

0.1 tP3HT 20nm 8 lc 300µm 6 4 6 8 2 4 6 8 2 1 10 tPEO (µm)

b) 2 0.1

) 6

-1 4

(S -1.8V

dif 2 -2.0V k -2.2V -2.4V 0.01 tPEO 1.5µm -2.6V lc 300µm 6 -2.8V 2 3 4 5 6 7 8 9 2 100 tP3HT(nm)

50 1.0 0.8 0.6 -2.4V salt 0.4 tP3HT 0.1µm LiTFSI

A (norm) A

Δ tPEO 1.5µm LiPF6 0.2 lc 300µm LiClO4 0.0 0 10 20 30 40 50 60 70 time (s) Figure 3.11 Integrated absorbance spectra of different ions

The fact that the charging rate trend observed here does not follow the mobility trend of these ions can be possible evidence of ion diffusion in P3HT determining the rate. We propose that it is the ion solubility in the semiconducting layer that determines the dynamics of ion diffusion. The difference in the enthalpy of mixing of solvation plays a role in kinetics of ion motion.70 Also, charge trapping becomes problematic for thicker semiconducting films leading to very slow discharging rates. Increasing temperature can assist ion diffusion out of the film and therefore temperature dependent measurements are being undertaken in order to develop a quantitative model of this behavior.

51 Chapter 4.

Future Work

This study was an attempt to provide a clear picture of the rate determining steps in electrochemical doping of P3HT channels in organic electronic devices. However, there is always room for innovation and improvement to produce more accurate models. The suggested issues here discuss leakage current, wider frequency range spectrum, temperature studies and different dielectrics.

In our experiment, it would be possible to determine the number of charge carriers based on an accurate leakage-free gate current. However, our devices suffered from large leakage currents that was not negligible. When a voltage is applied to the device, the current flowing from source/drain electrodes to the channel is related to the injection of charges. However, the dielectric is not infinitely insulating and other currents can occur between the gate electrode and channel or source/drain. All these unwanted currents are referred to as leakage current. One of the problems caused by leakage current is that it remains as steady state current even in off-state and leads to higher power consumption. One possible source of leakage current can be faradaic reactions occurring at any of the electrodes such as hydrolysis. More studies are required on the mechanisms causing the leakage currents in order to enhance the stability of performance in such devices.

Another suggestion for future studies based on this work is to investigate a wider range of the spectrum. Polarons have two allowed optical transitions, A1 and A2.71Polaron excitations also causes strong IR active vibration or IRAV to appear in 1200-1400 cm-1. This is due to hybridized orbitals of the crystalline regions of P3HT.

While electrochemical doping and ion diffusion and therefore conductivity is related to amorphous regions of the semiconductor, observing this IRAV modes can lead to a more accurate study. To do so, the ATR crystal used must have transmittance in mid-IR range of the spectrum as well. Silicon crystals absorb below 1500 cm-1 making it not ideal for the mid-IR range. However, zinc selenide has an ATR spectral range of 650 to 20000 cm-1. Therefore, if a ZnSe ATR crystal is used as a substrate, the IRAV mode can be observed.

52 One of the challenges in this work, especially when testing devices with thicker channels was the trapped charges. These charges can even take several hours to completely leave the sample and, therefore, make the discharge process very slow. One way to encourage these charges

a) b)

Figure 4.1 (a) Polaronic transitions related to amorphus (A) and crystalline (C) regions of the P3HT film and (b) how these transitions appear in absorbance spectra of doped P3HT. Reprinted with permission from Kaake et. al. 71 Copyright 2010 American Chemical Society. to leave the channel is to increase the temperature. As it was mentioned earlier, the ion movement is temperature dependent and is described by VTF equation. Ion conductivity increases by increasing the temperature. Therefore, elevating the temperature can be a potential method to deal with the problem of trapped charges. However, certain aspects must be taken into consideration. For example, the temperature must be kept below the melting point of the dielectric in order to avoid damaging the device.

One other area to further expand this research is based on the choice of dielectric material. In this work, a polymer electrolyte (PEO) was studied with a variety of lithium salts such as LiClO4, LiTFSI and LiPF6. However, these systems suffer from leakage and have a relatively slow polarization response. One way to improve the performance of these dielectrics is to use these polymer electrolytes with a small amount of ionic liquid as plasticizer. This will reduce the rigidity of the polymer and therefore, improve the conductivity. Chaurasia et. al. have studied the conductivity of PEO/LiPF6 with up to 20 72 wt.% ionic liquid BMIM/PF6 added and reported an improved conductivity. Devices made of the composite dielectrics would exhibit a better performance and will be an interesting case for in-situ spectroscopy studies.

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59 Appendix.

AFM and Profilometry Results

In order to determine the thickness of P3HT for the devices, AFM images were taken. To obtain various thicknesses, solutions with different concentrations of P3HT in 1,2-dichlorobenzene were made and spin coated. Spin-coating recipe consists of two steps of 600rpm for 20s followed by 1600rpm for 100s.

Figure A1 shows the height against lateral distance that the cantilever travelled across. The height is the position of cantilever tip, set to zero when it is touching the surface of the film. Therefore, the negative height represents the scratch made on the film and the difference between these heights represents the film thickness.

The downward step observed in figure is due the cantilever tip travelling across the scratch made on the samples. The scratch should not be any larger than about 80�m in width as this is the limit of cantilever travel ability. Otherwise, it might damage the cantilever. In case of 40mg/ml P3HT solution, only one side of the scratch was imaged as the scratch was wider than the device limit. The peaks on both sides of the steps in the graphs are related to the residue of scraped P3HT.

Figure A2 shows the profilometry results for the dielectric layer. Same description that was given for AFM graphs applies to these graphs as well.

60 a) b) 120 P3HT deposited on 100 a Si substrate Image is taken as the cantilever tip 80 travels across the edge(s) of the scratch 60 40 20 height (nm) 0 -20 0 10 20 30 40 50 Scratch made by a sharp object lateral (µm)

c) d) 120 300 80 200 40 100 0 height (nm) height (nm) 0 -100 -40 0 5 10 15 20 25 0 5 10 15 20 25 30 35 lateral ( m) lateral ( m) µ µ

e) f) 1000 800 600 600 400 400 200 200 height (nm) 0 height (nm) 0 -200 -200 0 10 20 30 40 50 0 2 4 6 8 lateral (µm) lateral ( m) µ Figure A 1 (a) Schematic of the samples prepared for AFM measurements. Test results for P3HT solutions with different concentrations are as follows: (b) 5 mg/ml resulting a 20±�. �� nm film, (c) 10 mg/ml resulting a 40±�. �� nm film, (d) 20 mg/ml resulting in a 100±�. �� nm film, (e) 30 mg/mg resulting in a 160±0.87 nm film, (f) 40 mg/ml resulting in a �� ± �. �� nm film

61 a) 1.0 b) 2.0 0.8 1.5 0.6 m)

m) 1.0 µ 0.4 µ 0.5 0.2 0.0 0.0 -0.5 height ( -0.2 height ( -1.0 -0.4 -1.5 0 200 400 600 800 1000 0 200 400 600 800 1000 lateral (µm) lateral (µm)

c) 10 d) 30 20 m) 5 m) µ µ 10 0 0 -10 height ( height ( -20 -5 -30 0 200 400 600 800 1000 0 200 400 600 800 1000 lateral (µm) lateral (µm)

Figure A 2 Profilometry data of the dielectric film; (a) 30mg/ml with 1000 rpm spin-rate resulting 0.3±�. �� µm film, (b) 45 mg/ml with 1000 rpm spin- rate resulting in a 1.3±�. �� µm film, (c) 80 mg/ml with 1000 rpm spin- rate resulting in 5±�. �� µm film and (d) 60 mg/ml drop casted resulting a 27±0.78 µm film.