electrolyte-based organic electronic...

Linköping Studies in Science and Technology

Licentiate Thesis No. 1330

Electrolyte-Based Organic Electronic Devices

Elias Said

LiU-TEK-LIC-2007:39

Dept. of Science and Technology

Linköping University, LiU Norrköping

SE-601 74 Norrköping

Norrköping 2007

LiU-TEK-LIC-2007:39

Printed by UniTryck, Linköping, Sweden 2007

ISBN: 978-91-85895-90-8

ISSN: 0280-7971

Abstract

The discovery of semi-conducting and conducting organic materials has opened new

possibilities for electronic devices and systems. Applications, previously unattainable for

conventional electronics, have become possible thanks to the development of conjugated

polymers. Conjugated polymers that are both ion- and electron conducting, allow for

electrochemical doping and de-doping via reversible processes as long as both forms of

conduction remain available. Doping causes rearrangement of the π-system along the polymer

backbone, and creates new states in the optical band gap, resulting in an increased electronic

conductivity and also control of the color (electrochromism). Doping can also occur by charge

injection at a metal – semiconducting polymer interface. Electrochemical electronic devices

and solid state devices based on these two types of doping are now beginning to enter the

market.

This thesis deals with organic based-devices whose working mechanism involves electrolytes.

After describing the properties of conjugated polymers, fundamentals on electrolytes (ionic

conductivity, types, electric double layer and the electric field distribution) are briefly

presented. Thereafter, a short review of the field of organic field effect transistors as well as a

description of transistors that are gated via an electrolyte will be reviewed.

Paper I present a novel technique to visualize the electric field within a two-dimensional

electrolyte by applying the electrolyte over an array of electronically isolated islands of

electrochromic polymer material on a plastic foil. By observing the color change within each

polymer island the direction and the magnitude of the electric field can be measured. This

technology has applications in electrolyte evaluation and is also applicable in bio-analytical

measurements, including electrophoresis. The focus of paper II lies on gating an organic field

effect transistor (OFET) by a polyanionic proton conductor. The large capacitance of the

electric double layer (EDL) that is formed at organic semiconductor/polyelectrolyte upon

applying a potential to the gate, results in low operation voltages and fast response. This type

of transistor that is gated via electric double layer capacitor is called EDLC-OFET. Because

an electrolyte is used as a gate insulator, the role of the ionic conductivity of the electrolyte is

considered in paper III. The effect on the electronic performance of the transistor is studied as

well by varying the humidity level.

Acknowledgements

The work presented in this licentiate thesis would not have been performed without many

help and support from some people.

Firstly, I would like to express my gratitude to my supervisor Prof. Magnus Berggren for

giving me the opportunity to work and study at one of the most outstanding research areas and

environments as well as for all his help and support. I would like also to thank my co-

supervisor Assoc. Prof. Xavier Crispin, who always had time for me when I needed help, for

his patience, support and encouragement. Thank you Xavier for all the help with this thesis.

Thanks go to my co-supervisor Docent Nathaniel D. Robinson for all guidance and support,

and for all time he has spent helping me and answering my questions.

Special thanks to the Organic Electronic research group that consists of Magnus, Xavier,

Sophie, PeO, Peter, Payman, Joakim, Fredrik, Lars, Oscar, Maria and the recently joined

members; Isak, Klas, Xiangjun, Georgios, Daniel, Yu and Hiam, for many valuable

discussions, great friendship and after-work leisure activities. I should not forget thanking the

former members of the Organic Electronic group Dr. David, Dr. Max, Docent Nate, Emilien

and Linda. Thank you Sophie for all the administrational help. I would also like to thank the

personal at ACREO for all help I got in the lab.

Thanks to a lot of people at ITN for various reason: Prof. Stan Miklavcic in guidance of my

first work at ITN, Dr. George Baravdish for giving me the opportunity and helping me

teaching, Mari Stadig-Degerman for all the fun we had in the preparation and teaching Kemi

A for Basåret, Frédéric Cortat and Martin Evaldsson for all the fun and discussions in various

subjects.

I want to thank my friends Allan Huynh, Mattias Andersson and Henry Behnam for their

friendship.

Finally, I would like to thank my dear father and mother; David and Sara, my brothers;

Maurice and Robert, and my sister with family; Lilian, Simon and little David, for their love,

joy and support.

Elias Said, Norrköping, September 2007

List of included papers

Paper I:

Visualizing the Electric Field in Electrolytes Using Electrochromism from a Conjugated

Polymer

Elias Said, Nathaniel D. Robinson, David Nilsson, Per-Olof Svensson, Magnus Berggren

Electrochemical and Solid-State Letters, 8 (2) H12-H16 (2005)

Contribution: All the experimental work except the theoretical result. Wrote the first draft and

was involved in the final editing of the paper.

Paper II:

Polymer field-effect transistor gated via a poly(styrenesulfonic acid) thin film

Elias Said, Xavier Crispin, Lars Herlogsson, Sami Elhag, Nathaniel D. Robinson and Magnus

Berggren

Applied Physics Letters, 89, 143507 (2006)

Contribution: All the experimental work. Wrote the first draft and was involved in the final

editing of the paper.

Paper III:

Role of the ionic currents in electrolyte-gated organic field effect transistors

Elias Said, Oscar Larsson, Magnus Berggren and Xavier Crispin

Manuscript

Contribution: All the experimental work. Wrote the first draft and was involved in the final

editing of the paper.

Contents

Contents

1 INTRODUCTION TO PLASTIC ELECTRONICS 1

2 POLYMERS: FROM INSULATORS TO CONDUCTORS 3

2.1 BACKGROUND 3 2.2 ELECTRICAL PROPERTIES (CONDUCTIVITY) OF CONJUGATED POLYMERS 5 2.3 TRANSPORT OF CHARGE CARRIES IN CONJUGATED POLYMERS 6 2.4 PROPERTIES OF CONJUGATED POLYMERS 8 2.4.1 ELECTROCHROMISM 8 2.4.2 ELECTROLUMINESCENCE AND PHOTOVOLTAIC EFFECT 10 2.5 EXAMPLES OF SPECIFIC CONJUGATED MATERIALS 10 2.5.1 POLY(3,4-ETHYLENEDIOXYTHIOPHENE) 10 2.5.2 POLY(3-HEXYLTHIOPHENE) 12 2.5.3 POLYANILINE 13

3 ELECTROLYTES 15

3.1 SOLID ELECTROLYTES 16 3.1.1 POLYMER ELECTROLYTES 16 3.1.2 POLYELECTROLYTES 17 3.2 ELECTRIC DOUBLE LAYER CAPACITORS 17 3.3 ELECTRIC FIELD IN ELECTROLYTES 20

4 ORGANIC FIELD EFFECT TRANSISTORS 21

4.1 WORKING PRINCIPLES 21 4.2 DEVICE STRUCTURES AND SEMICONDUCTOR MATERIALS 23 4.3 OFET CHARACTERISTICS 24 4.4 ELECTROLYTE-GATED OFETS 26 4.5 HUMIDITY EFFECT 29

5 SUMMARY OF THE INCLUDED PAPERS 31

5.1 PAPER I 31 5.2 PAPER II 31 5.3 PAPER III 32

6 REFERENCES 33

PAPER I 39

PAPER II 55

PAPER III 65

Introduction to Plastic Electronics

1 Introduction to Plastic Electronics

Since discovery of the world’s first transistor in 1947, thin film transistors made of inorganic

semiconductor materials, such as silicon, are today dominating the electronic industry.

Nowadays the processing methods for fabricating modern devices become increasingly more

complex resulting in higher production cost. Because the size of transistors becomes very

small, the device density, on for example processor chips, is increased giving rise to cost

reduction (per transistor). However, the transistor size is now approaching critical dimensions

that give rise to short channel effect in transistor channel (< 100 nm) or electron tunneling for

very narrow channel (< 30 Å). Beside the high cost that limits the range of applications in

daily life applications, their poor mechanical flexibility also limits their use on applications

requiring flexible substrates.

When organic semiconductors and conductors became available, new outlooks for electronic

systems have been opened. Optoelectronic devices that were previously thought to be

unsuitable in some applications, such as food industry, are now brought closer to nearly every

product. For instance they can be produced on plastic substrates and applied on packages.

Compared to silicon based components, organic based electronic devices as a promising

future despite their lower performance as a semiconductor because of several unique

properties of organic materials. First, many of them can be processed in solution and do not

require high purity levels. This makes possible to produce components using conventional

printing techniques, which allows large area, high volume and low-cost applications. In

comparison, building a silicon chip usually takes weeks of work and requires ultra clean

working environment. Organic devices can be faster produced under less carefully controlled

conditions. Second, organic devices can be made and integrated together with other

applications simultaneously. For example an organic sensor device that is woven in textile.

Third, they are flexible and light. Because the devices are made by thin films, they become

lightweight. It will be a weight reduction when an electronic news papers replace the

conventional news papers, because of less materials are used in production (up to 80% paper

reduction according to plastic logic, www.plasticlogic.com). Thanks to these features, the

organic electronic industry is expected to become as big as the silicon industry and will have a

large impact on our daily life.

1

Introduction to Plastic Electronics

Figure 1. A mobile device rollable polymer display for reading personal information,

newspapers and books (By Polymervision, see www.polymervision.com).

One of the earliest applications of conductive polymer was to produce antistatic coatings,

lightweight batteries and materials for circuit boards. Today, few of organic (opto)electronic

devices start to move from R&D and labs into the market; such that in the next couple of

years low cost and even disposable plastic electronic devices will emerge. Already displays

that use organic light-emitting diodes (OLED) are found on the market. Another example of

polymer displays that entered the market is shown in figure 1. Unlike conventional displays,

polymer display can be rollable, which enables for instance mobile devices to incorporate a

display that is larger than the device itself. It should be noted that plastic electronics never

will match the switching speed and miniaturization of conventional electronics but they will

complete and go to places that conventional semiconducting industry cannot reach.

This thesis deals with organic based-devices whose working mechanism involves electrolytes.

After describing the properties of conjugated polymers in chapter 2, fundamentals on

electrolytes (ionic conductivity, types, electric double layer and the electric field distribution)

are briefly presented in chapter 3. Chapter 4 gives a short review of the field of organic field

effect transistors as well as a description of transistors that are gated via an electrolyte.

2

http://www.polymervision.com)/

Polymers: From Insulators to Conductors

2 Polymers: From Insulators to Conductors

2.1 Background

Polymers that are assigned as conventional polymers, even called plastics, have been

considered under long time as insulators. At the beginning, polymers served in clothes, tools,

plastic bags, wire shielding and many others applications1. A huge interest in polymer science

started when important discoveries concerning modification and creation of semi-synthetic

polymeric materials as rubber were made from natural polymers. This gave later rise to the

development of a large number of fully-synthetic polymers, such as polystyrene and

poly(methyl methacrylate). Polymers are macromolecules that are formed by linking together

monomers in a chain through chemical reaction known as polymerization2. The formed

polymer chains can have linear, branched or a network structure.

The main element in many polymers is the carbon atom C. The valence electron configuration

for a C atom is 2s22p2 or 2s22px12py

1. With this configuration, the C atom can form two bonds

and not four with other atoms. Forming four bonds require that one 2s electron is excited to an

orbital with higher energy leading to the configuration 2s12px12py

1pz1. This process is called

promotion3. The unpaired electrons in separate orbitals of the promoted C atom can be paired

with other four electrons in orbitals that are provided by four other atoms. Notice that

excitation of the 2s electron to a higher orbital requires an investment of energy. But for C

atom case the energy is more than recovered when forming four bonds instead for two with

other atoms. This model was historically introduced like this and still is used when one

describes the C atom. However, it should be noted that this just is a model and doesn’t

represent the reality. The C atoms in polyethylene chain, which consists of a monomeric

repeat unit -(CH2 – CH2)n- are sp3 hybridized and each C atom binds to other four adjacent

atoms by sigma (σ) bond (figure 2.1a). The wavefunction of the σ-electrons are formed by the

large overlap between the sp3 hybrid atomic orbitals, which in turns form wide σ-bands, but

also large bandgap between the σ-band and the σ*-band. For polyethylene, the optical

bandgap is on the order of 8 eV4. The high bandgap makes conventional polymers electrical

insulators.

3


σ orbital π orbital

a)

b)

Figure 2.1. A schematic representation of the structure and atomic orbital involved in the

electronic structure of a) polyethylene, b) trans-polyacetylene. In polyethylene each carbon

atom forms four σ bonds with other atoms (2 carbon and 2 hydrogen atoms), while three σ

bonds and one π bond are formed in case of polyacetylene resulting in alternating single and

double bonds.

There exists another class of polymers with quite different properties, called conjugated

polymers. The simplest conjugated polymer that has been extensively studied is trans-

polyacetylene (figure 2.1b). Unlike conventional polymers formed entirely by σ bonds, in

conjugated polymers, each C atom forms three σ bonds with other atoms in the sp2 hybridized

state. The remaining electrons in the pz orbital of adjacent carbon atoms overlap with each

other to form delocalized π-orbitals forming π-bands. Since there is one electron per pz orbital

and there is the same numbers of π-orbitals than there are pz orbitals involved, conjugated

polymers with equal bond lengths would have half-filled band like metals, but this is not the

case. The reason is the Peierls instability that states that a one-dimensional metal is unstable

and will undergo structure distortion opening a band gap and becoming a semiconductor. The

structure distortion results in alternating single and double bonds. Therefore there is a filled

valence band and a conduction band. The top of the valence band is called the highest

occupied molecular orbital, HOMO; and the bottom of the conduction band is called the

lowest unoccupied molecular orbital, LUMO. The energy difference between these two states

defines the energy bandgap Eg, which is in the range of visible light and near infrared (4 eV to

1 eV), similar to inorganic semiconductors. In other word, pure conjugated polymers are

semiconductors.

4


2.2 Electrical properties (conductivity) of conjugated polymers

To increase the conductivity of the conjugated polymers, doping of these materials is

necessary. Doping can be obtained chemically via a redox molecule or electrochemically by

charge transfer with an electrode. The first high conducting polyacetylene was achieved via

chemical doping when a film of polymer was exposed to iodine vapor. Figure 2.2 illustrates

the range of conductivities for polyacetylene5 and other conjugated polymers compared with

other common materials. In electrochemical doping, the doping level is determined by the

applied voltage between the conducting polymer and the counter electrode. The doping charge

is supplied by the electrode to the polymer; while oppositely charged ions migrate from the

electrolyte to balance the electronic doping charge. This resembles an electrochemical cell.

Doping is reversible (if not kinetically blocked via one step in the mechanism). The

electrochemical doping of the π-polymer is illustrated by the following formulas:

(π-polymer)n + [M+X-]solution [(π-polymer)+yX-y]n + M electrode (p-type doping)

(π-polymer)n + [M]electrode [(M+)y(π-polymer)-y]n + [M+X-]solution (n-type doping)

Figure 2.2. Electrical conductivities of conjugated polymers compared with other common

materials.

As mentioned previously, electrochemically doping and undoping must involve counterion to

stabilize the doped state. Doping can also occur without involving any ions by charge

injection at a metal – semiconducting polymer interface. At the interface the polymer can be

5


oxidized upon hole injection into HOMO or reduced when electrons are injected into empty

bands. The induced charge carriers will lead to increase the electrical conductivity at the

polymer interface. However, this induced conductivity (or the doping) is maintained as long

as the carriers are injected (e.g. by applying the bias voltage). Upon removal the carrier

injection, the doped polymer at the interface returns to its original state. While in case of

electrochemical doping, the doping level is permanent until carriers are intentionally removed

by undoping.

2.3 Transport of charge carries in conjugated polymers

Generally, charge carriers in conventional semiconductors are created by addition or removal

of electrons. These charge carriers (electrons and holes) are delocalized in the crystal

structure. While in conjugated polymers the nature of charge carriers is different. Electrons or

holes will not be generated at HOMO or LUMO upon addition or removal of electrons from

the polymer, instead a defect, which is associated to a molecular distortion in the polymer

chain, is created. Unlike conventional semiconductors, this defect is localized in the electronic

structure of conjugated polymers. These defects include the quasi-particles solitons, polarons

and bipolarons. Solitons as charge bearing species are characteristics for polymers with a

degenerate ground state system formed by two geometries of polymer units with the same

energy. The polymer with a degenerate system is trans-polyacetylene. Solitons originate from

odd number of carbon atoms in trans-polyacetylene chains. The soliton is an unpaired electron

located at the border line between two phases with different bond length alternations. This

unpaired electron is lying in an energy level in the middle of the band gap of the polymers

(figure 2.3b). The unpaired electron in the state leads to a neutral soliton with spin ½.

Unoccupied or doubly occupied of the state the soliton is charged and spinless. The soliton is

mobile along the polymer backbone. When doping levels become high enough, the charged

solitons start to interact with each other forming a soliton bands (figure 2.3c). These bands

will eventually merge with the edges of HOMO and LUMO to create metallic conductivity6,7.

6


Figure 2.3. a) The energy diagram of a neutral polymer chain. The difference between high

occupied molecular orbital (HOMO) and low unoccupied molecular orbital (LUMO) defines

the bandgap Eg. b) The oxidation and reduction of the neutral soliton (left) results in positive

(center) and negative (right) soliton. c) High redox-doping forms soliton band.

Most of conjugated polymer systems are non-degenerate ground state systems. In these

polymers the interchange of the single and double bonds produces higher energy geometric

configuration. Addition/removal of an electron on a neutral segment causes distortion and

formation of a localized defect that moves together with the charge. This combination of an

additional charge coupled to local lattice distortion is called a polaron8. Depending on the sign

of the charge remove, one speaks about positive polarons or negative polarons. The polaron

has a localized electronic state in the bandgap. Upon further addition of charges to the

polymer chain, two charges might couple together, despite electrostatic repulsion, thanks to

lattice distortion to create bipolarons. At high enough doping level a bipolaron band is formed

and eventually merges with the HOMO and LUMO bands respectively to produce partially

filled bands and metallic like conductivity. Unlike polarons, bipolarons are always charged

and have zero spin.

7


Figure 2.4. The energy diagram of a polymer with a non-degenerate ground state system.

Upon doping new localized states are created. a) The oxidation and reduction of the neutral

polaron (left) results in positive (center) and negative (right) polaron. b) When two polarons

bind together a bipolaron will be formed. Neutral bipolaron does not exist. c) High redox-

doping forms bipolaron bands.

Conduction processes in conjugated polymers is often related to the mobility and the density

of charge carriers. For band like transport, charge carriers are in delocalized states resulting in

a metallic like conductivity. This kind of transport is typical for metals. In conducting

polymers however, the transport is dominated by variable range hopping or tunneling

processes9,10. The charge carriers are transported through hopping between localized states in

the band gap. Hopping to energetically states require energy, which is provided by the

phonons. Therefore the hopping conductivity increases with the temperature, opposite to band

like conductivity decreases and is finite at zero temperature. Carriers with a little energy are

limited to hop to close levels. In case that transport mechanism is assisted by tunneling, there

are less conducting regions presented between highly conducting regions. It should also be

mentioned that charges are transport via interchain hops between π-orbitals of adjacent chains.

The transfer of the charge can be affected in this case of the intermediate doping ions. Upon

high doping, the charge transport of the conjugated polymers is based on metallic grains

surrounded by a media with localized states in the band gab11. In the grains the transport is

band like because the polymer chains are densely packed that giving rise to strong interaction

between the chains, whereas the charge transport will be limited to hopping or tunneling

mechanism between the grains.

2.4 Properties of conjugated polymers

2.4.1 Electrochromism

Electrochemical oxidation or reduction (i.e. p-doping/de-doping) of conjugated polymers

causes not only conductivity change but also electrochromism12-14, the change of visible light

absorption with electrochemical state. Electrochromism can easily be demonstrated in an

electrochemical cell with two polymer electrodes connected via an electrolyte, a typical

display element structure (figure 2.5). Upon application of an appropriate potential between

the electrodes, the negatively biased polymer electrode is reduced while the other oxidizes,

8


causing a color change in one or both of the electrodes. This doping/de-doping redox switch is

fully reversible and can be repeated many times15.

Figure 2.5. Schematic representation of an electrochemical cell based on positively doped

polymer electrodes. When a voltage is applied the positively biased polymer electrode is

oxidized while the other reduces. At the oxidized polymer electrode, the concentration of

positive (bi)polarons is increased, therefore anions A- (or cations C-) migrate into (out) the

polymer film in order to maintain the electroneutrality. On the contrary, the reduced electrode

undergoes a decrease of positive polaron and the migration of oppositely charged ions

occurs from the electrolyte. This type of cell is typically used as a display element.

Doping of the conjugated polymers causes rearrangement of the π-system along the polymer

backbone, and creates new states in the optical bandgap (polaron and bipolaron are formed

inside the bandgap), resulting in an increased electronic conductivity and also control of the

color change. This feature makes conjugated polymers useful in many applications as

displays16, smart windows17 and determining the direction and the magnitude of the electric

field in electrolytes18. The color change between doped and undoped forms of the polymer

depends on the magnitude of the bandgap of the undoped polymer. Thin films of polymers

with bandgaps greater than 3 eV are colorless and transparent in undoped form, but colored in

the doped state. While for bandgaps in order to or less than 1.5 eV, polymer films are colored

in undoped state and transparent in doped state, since the polaronic states create optical

absorption in the infra-red region. Polymers with intermediate bandgaps, as polypyrrole, will

switch between distinct colors. In case of polypyrrole, the color change is from yellow-green

(insulating) to blue-violet (conductive) upon reversible oxidation8.

9


2.4.2 Electroluminescence and photovoltaic effect

Electrochromism described in previous section occurs when the polymer film is

chemically/electrochemically doped in present of counter ions. In a typical polymer light

emitting diode, the conjugated polymer is not doped and sandwiched between two metal

electrodes. The hole injection at anode together with electron injection at the cathode result in

generation of positive and negative quasi-particles (polarons) in the polymer film. Each

particle is transported, thanks to the applied electric field present across the polymer film until

they meet and form an excited but neutral species called exciton. The relaxation of the exciton

to the electronic ground state takes place via light emission (i.e. radiative), called

electroluminescence; or via non-radiative decay. It is only a small amount of the excitons (~

25%) that contributes to light emission10. The wavelength of the emitted light can be tuned

within the visible region depending on which polymer is used.

Exciton can also be generated when a photon with an energy that is matched to the bandgap of

the polymer is absorbed. Recombination of the excitons will give rise to fluorescence. For

photovoltaic applications, the exciton (electron-hole pair) must be separated in a hole and an

electron and the recombination of the exciton is undesirable. Therefore the diffusion length of

the exciton should be long such that the exciton has a chance to meet an active centre (such as

a volume heterojunction) or an internal field, which gives rise to hole and electron

separation10. These charges will be then diffuse towards the electrodes. For organic solar

cells, efficiencies of 4-5% have been reached19.

2.5 Examples of specific conjugated materials

2.5.1 Poly(3,4-ethylenedioxythiophene)

Poly(3,4-ethylenedioxythiophene), abbreviated as PEDOT, is one of the most successful

conducting polymers, which was developed by the scientists at the Bayer AG research

laboratories in Germany20. PEDOT is synthesized by chemical or electrochemical

polymerization of EDOT monomers in an aqueous solution comprising the the electrolyte.

The positive doping charges carried by PEDOT is balanced by the counter anion poly(styrene

sulfonic) (PSS-). The resulting polymer blend PEDOT-PSS has been intensively investigated

and used in different research applications (figure 2.6). In the work presented in paper 1, thin

10


film of PEDOT:PSS, 200 nm thick (named OrgaconTM foil21) on plastic substrate has been

used.

Figure 2.6. The chemical structure of poly(3,4-ethylenedioxythiophene), PEDOT (lower part

of the figure) with poly(styrene sulfonic) acid, PSS-, as counterion (upper part). The positive

polaron responsible for the charge transport in PEDOT:PSS is also sketched.

Neutral PEDOT has a bandgap of 1.6 – 1.7 eV that makes it deep blue in color. The bandgap

can be controlled by using various oxidative agents giving rise to neutral polymers with colors

ranging over entire rainbow of colors20. Thanks to the low oxidation potential, films of neutral

PEDOT are not stable and oxidize rapidly in air. Therefore handling under an inert

atmosphere is required. In the doped and conducting (oxidized) state, PEDOT:PSS films with

conductivities between 1 and 10 S/cm are highly stable and have high transparency in the

visible region (strong near infrared absorption)22. The reduction formula of the doped PEDOT

is represented by the half reaction

−+−+−+ +→++ PSSMPEDOTeMPSSPEDOT :: 0

where +M denotes a positively charged ion and an electron. Reducing to the neutral state

results in a decrease in the number of charge carriers in PEDOT, giving rise to a decrease in

conductivity

−e

23 and a shift of the optical absorption spectrum from the near infrared region into

the visible region. The positive charge in doped PEDOT is delocalized over several monomer

11


units20. Thus, the representation PEDOT+ in the above equation is intended to represent a

group of consecutive monomer units.

The electrical conductivity of PEDOT can be enhanced through morphology change. For

example, conductivity of PEDOT:PSS has been increased three orders of magnitude when a

secondary dopant diethylene glycol (DEG) added to the PEDOT:PSS emulsion24. The origin

of the high conductivity is attributed to the phase separation of the excess PSS from the

PEDOT:PSS regions, which results in better pathways for conduction and an interconnected

three dimensional conducting network.

PEDOT:PSS was initially developed for antistatic applications in the photographic industry21,

but it took no long time to find the ability of this conducting polymer in other applications.

Thanks to its high stability and conductivity, easy processability and even the electrochromic

properties, PEDOT:PSS is a one of the promising candidate for developments in the area of

the cheap and flexible electronic systems. It can now be found as the electrode or conductor

material in electrochemical transistors25, organic field effect transistors26, organic light

emitting diodes27, electrochemical displays16, smart windows17, batteries and capacitors28,29.

2.5.2 Poly(3-hexylthiophene)

Polythiophene and its derivative, the poly(3-hexylthiophene) (P3HT) is one of the most

studied semiconducting polymer thanks to its good solubility and processability30. It is stable

both in doped and undoped state. The 3-hexyl substituent in a thiophene ring can be

incorporated into a polymer chain with two different regio-regularities31, shown in figure

2.7a: head to tail (HT) and head to head (HH). A regio-random P3HT consists of both HH and

HT 3-hexylthiophene in a random pattern, while a regio-regular P3HT (figure 2.7b) has only

one kind of 3-hexylthiophene, either HH or HT. Regio-regular P3HT has great potentials as

organic semiconductors in organic electronics because its strong tendency to self assemble

into crystallites with ordered structures upon casting into thin films, giving rise to carrier

mobilities in order of 0.1 cm2 V-1s-1 32. While the regio-random P3HT has a twisted chain

conformation with poor packing and low crystallinity, resulted in mobilities of only 10-4 cm2

V-1s-1 31 and higher optical band gap than its regio-regular structure has a lower bandgap. The

12


transport within the crystallites of the regio-regular P3HT is highly anisotropic. Depending on

processing conditions and regio-regularity of the polymer, the polymer chains can be ordered

in two different orientations, parallel and normal to the substrate32. The mobility has been

found to be three orders of magnitude higher when π-stacking direction is parallel to the

substrate, which reflecting fast and efficient interchain transport of carriers in two

dimensional conjugated polymer chains. The morphology and charge transport of the regio-

regular P3HT has also been shown to be affected by the molecular weight (MW)33. Low MW

films have a higher degree of crystallinity, while the high MW films have the higher mobility.

In addition, the choice of solvents will strongly influence the quality of the film and the

mobility30.

Figure 2.7. a) Two different regioregularities that can be incorporated into a polymer chain. b)

The chemical representation of regio-regular poly(3 hexylthiophene) (P3HT).

2.5.3 Polyaniline

Polyaniline, PANI, is one of the most studied conjugated polymers34,35. Thin film of PANI

exhibits multiple colors depending on the redox state (figure 2.8b). The electrical and

electrochromic properties of PANI do not only depend on the oxidation state but also on the

protonation state as shown in figure 2.8a. Hence conductivity can be tune simply by changing

the pH value of the electrolyte. It can exist as salts or bases in three separate oxidation states.

The neutralized thin film of PANI is transparent yellow and insulating (leucoemeraldine).

Upon oxidation in acidic medium, the film turns green and becomes conducting (emeraldine).

Further oxidation will give rise to blue color in the film (pernigraniline). When the conductive

emeraldine PANI, which is protonated, comes in contact with a base solution, it deprotonates

and becomes blue emeraldine base (figure 2.8c), which is an insulator. The protonation and

deprotonation processes is reversible. To get back the conductive PANI, the film should be

immersed in acid solution such as hydrochloric acid. Recently, it was found that the resistivity

13


of PANI film decreased when the temperature was lowered down to 5K indicating the

metallic behaviour of the film36. It is useful as a stable and low-cost conductor37,

electrochromic display window14, electrodes in organic transistors38, ion and pH sensors39 and

battery materials40.

Figure 2.8. a) The redox and protonation states of PANI. b) An image showing the multi

electrochromism properties of green protonated emeraldine film when it became oxidized

(pernigraniline) and reduced (leucoemeraldine). Notice that the PANI film was in contact with

a gelled electrolyte that in turn was connected to the outer electrodes. c) Deprotonation of

emeraldine film when immersing it in a solution with different pH. The contrast of the blue

color is strong at high pH levels.

14

Electrolytes

3 Electrolytes

The chemical compound that dissociate in ions in a solvent is called an electrolyte. Due to the

electrostatic interaction between solvent and the ions, the ions are surrounded by solvent

molecules. This process is commonly known as solvation. Electrolytes behave as electrically

conductive medium thanks to the solvated ions. Electrolytes can be found in liquid phase, but

also as molten electrolytes and solid electrolytes. Depending on the concentration of the salt,

the ionic conductivity might or might not vary drastically. Depending on the solute

dissociation that forms free ions, electrolytes are usually divided into two classes, strong and

weak electrolytes. In strong electrolyte (e.g., salts or strong acids), all or high amount of the

solute is dissociated and the ionic conductivity is quasi constant with the electrolyte

concentration. If the solute is partially dissociated in aqueous solution, the ionic conductivity

depends strongly on the solute concentration and this is called a weak electrolyte (e.g., weak

acids and bases). In addition, the choice of the solvent should be considered in

electrochemical applications. Each solvent has a defined potential window before it becomes

decomposed. For example water has narrow potential window (~3V) than acetonitrile

(~6V)41, which makes it not suitable for electrochemical measurements that require high

potentials.

The ionic conductivity, ĸ, of an electrolyte is given by

m)(inyResistivit1

Ω=κ , measured in S m-1

The ionic conductivity depends on the number of ions present and is usually expressed in

terms of the molar conductivity, defined via the relationship between the conductivity and the

concentration,

cκ

==ionConcentrattyConductivityconductiviMolar

The conductivity of the electrolytes can easily be measured using impedance spectroscopy

techniques42,43. For solid electrolytes, four probes techniques can also be used in measuring

the conductivity, which comes directly from

VLj Δ= /κ

15

Electrolytes

where j is the current density (current divided by the thickness t and width w of the

electrolyte), ΔV is the potential measured between two of the four fingers, and L is the

distance between the measured fingers. The ionic conductivity of solid electrolytes is usually

several orders of magnitude lower than the ionic conductivity of liquid electrolytes, because it

is limited by both low mobility and low concentration of ions. The ions in solid electrolytes

are introduced in a non-ionic matrix that conducts easily ions.

3.1 Solid electrolytes

Most solid electrolytes are polymer-based. The advantages of using solid electrolytes in

practical applications are many. The chemical and electrochemical stability and the flexibility

of the material makes it mechanically robust; thin films are easily processed on large areas.

Solid electrolytes are divided into two general categories: polymer electrolyte and

polyelectrolytes.

3.1.1 Polymer electrolytes

Polymer electrolytes are composed of a salt dispersed in a neutral polymer matrix (that is not

it-self an electrolyte)44. The salt is dissociated in ions screened by the polymer matrix. The ion

motion is coupled to local motion of polymer chain and transition between ion coordinating

sites44. The most studied polymer electrolytes are poly(ethylen oxide) (PEO) (figure 3.1a) that

consists of the repeating units of ether groups (-CH2CH2O-)n43,44 and its low molecular weight

polymer poly(ethylene glycol) (PEG)45,46. PEO is still one of the useful candidates in

designing new types of batteries47. PEO exhibits low ionic conductivity (10-9 - 10-8 S cm-1) at

room temperature because of its tendency to crystallize. Depending on the molecular weight,

the melting point of PEO varies from 60 oC for low weight to 66 oC. Thanks to the high

ability to dissolve high concentration of salts such as lithium and sodium salts, the crystalline

phase is suppressed enhancing the amorphous phase that results in higher ionic conductivity48.

PEO is soluble in water and a number of common organic solvents.

16

Electrolytes

Figure 3.1. a) The ether group repeating unit of poly(ethylene oxide) (PEO). b) Poly(styrene

sulfonate) (PSS-) with counter ion H+ (left) and Na+ (right).

3.1.2 Polyelectrolytes

Polyelectrolytes are polymers that bear ionized units49. The small mobile counter ions in

polyelectrolyte aqueous solutions dissociate making the immobile polymer chains charged.

The conductivity of aqueous polyelectrolyte solutions is in order of 0.1 µS cm-1 42. The

polyelectrolytes presented in Fig. 3.1.b are strong polyelectrolytes in solution. However, in

solid phase, because of the lack of screening solvent, the electrostatic potential attraction

between the immobile ions and the charged polymer chain is large and some mobile ions

remain bound to the polymer chains. This is known as a counter ion condensation

phenomena42,49 that divides the counter ions into bound and free counter ions. Bound counter

ions will stay in the vicinity of the charged polymer, while the free will dissociate from the

polymer chain and interacts through a screened Coulomb potential42. Poly(styrene sulfonate)

(PSS-) either with protons as counter ions or sodium (Na+) is displayed in figure 3.1b. Another

well-known proton conductor is NafionR. NafionR has been used as material for sensing

humidity in an electrochemical transistor50. When a nafionR film is exposed to water, it

becomes hydrolyzed allowing for effective proton transport across the film. Gelled PSS:Na

has been prepared as mixed together with other hygroscopic material to achieve higher ionic

conductivity in amorphous or semi-solid phase for using in electrochemical devices16,50,51.

The PSS:Na based electrolyte conductivity is in order of 10 µS cm-1 18.

3.2 Electric double layer capacitors

An interesting feature of electrolytes is its ability to form electric double layer capacitors

(EDLCs). Electrolytes are electron insulators but ionic conductors. Upon contact with two

oppositely charged ion-blocking electrodes sandwiching a common electrolyte (figure 3.2a),

the anions migrate towards the anode, while the cations migrate towards the cathode (figure

17

Electrolytes

3.2b). At the electrolyte/electrode interfaces, electrical double layers are then formed (figure

3.2c). The formed double layer can be seen as two-plate capacitor separated by a distance of

few angstroms, typically the thickness of the first solvation shell around the ions. Because of

this small distance, the capacitance of this model plate capacitor is large41. The resulting

EDLCs’ capacitance can reach high values, up to 500 µF cm-2 52. These capacitances are

almost independent on the thickness of the electrolytes, since the capacitors are formed in the

vicinity of the electrode surfaces. Thanks to charge separation that is formed within

microseconds EDLCs respond quickly to an applied electric field41. EDLCs can be

characterized using impedance spectroscopy. The effective capacitance for a capacitor with

poly(vinyl phosphonic acrylic acid) P(VPA-AA) electrolyte is of the order of 10 µF cm-2 at

100 Hz25.

Figure 3.2. The principle of the electric double layer capacitor (EDLC). a) A common

electrolyte is sandwiched between two ion blocking electrodes. At time t = 0 with no potential

difference between the electrodes, the ions are distributed uniformly in the electrolyte

medium. a) Upon applying a voltage at t = t1 anions (A-) start to move towards the positively

charge electrode, while the cations (C+) moves to the negatively charge electrode. c) After a

while at t2 > t1, the ions start to pack near the electrode interfaces giving rise to formation of

the electric double layer (EDL). The thickness of the double layer is in order of molecule size.

The structure of the electric double layer is currently described by Gouy-Chapman-Stern

(GCS) model41. The thicknesses of the double layer at the electrode and the electrolyte sides

are not the same, because of the charges at both sides are confined differently. However, it is

dominated by the electrolyte side. The interaction between solvated ions and the charged

18

Electrolytes

electrode is mostly electrostatic, while specific adsorption of ions may take place too. At the

electrolyte side double layer consists of a compact (Helmholtz) layer of ions next to the

electrode surface followed by a diffuse layer that extends into electrolyte bulk (figure 3.3).

Ions at different position in diffuse layer do not have the same energy due to the variation of

the electrostatic potential. Therefore the thickness of the diffuse layer will be varied when the

potential profile across the layer is modified. As the electrode potential becomes higher, the

diffuse layer becomes more compact. Also, the thickness is dependent on the ionic

concentration of the electrolyte. Higher ionic concentration gives rise to more compact diffuse

layer, i.e. the charges in electrolyte become more tightly compressed against the compact

layer. According to this model, the double layer equivalently is made of two capacitors in

series with capacitances representing both compact and diffuse layer. The potential profile

(solid curve) through the electrode side of the double layer according to GCS model in shown

as the inset in figure 3.3. The potential profile in the compact layer, shown in the figure 3.3, is

linear because of the assumption of constant capacitance of this layer (according to Helmholtz

model)41.

Figure 3.3. The arrangement of ions in the electric double layer (EDL) according to Gouy-

Shapman-Stern (GCS) model. The double layer is formed by a compact layer of ions next to

the electrode followed by a diffuse layer extending into bulk solution. The capacitance of the

EDL (Cd) is equivalently the total capacitance of both compact (CH) and diffuse layer (CD).

When CH >> CD the Cd is approximately equal to CD, while at very high CD the Cd is almost

the same as CH. The solid curve (as inset) shows the variation of the electrostatic potential

with distance from the electrode/electrolyte interface into the bulk. As in the GCS model, the

more concentrated the electrolyte is, the thinner is the diffuse layer.

19

Electrolytes

3.3 Electric field in electrolytes

The electric field in electrolytes has an important role on the performance of electrolyte-based

devices. For instance, the stability, selectivity and efficiency of polymer electrolyte fuel

cells53,54, batteries55 and electrochemical sensors50 depend on the electric field inside the

electrolyte. However, the field is difficult to be measured in real devices. Commonly, the field

is determined by measuring the potential along the electrolyte by electrical probes56 that are

coupled to external conducting lines. The difference in potential between the probes is then

taken to calculate the electric field. This measurement method usually requires considerable

hardware both for contacting the electrolyte and measuring the potential at each contact. The

resolution of the measurement is limited to the electrical connection i.e. the size of the probe

that can be manufactured.

20

Organic Field Effect Transistors

4 Organic Field Effect Transistors

Organic field-effect transistors (OFETs) have since the discovery in 198757 had a march of

progress. Nowadays OFETs have started to find applications that other conventional inorganic

transistors can not compete with. They serve as the main component in low cost and flexible

electronic circuits58. Their performance is now compared with that of amorphous silicon (a-

Si:H) transistors, which is widely used in large area applications such as active matrix liquid

crystal displays (LCDs). They offer a promising platform for many new opportunities e.g. for

flexible display back planes59,60 and integration of logic circuitry into low-cost electronic

products61. Prototypes that OFETs already have been demonstrated in such as radio frequency

identification tags (RFID)62-65 are now coming close to enter into the market.

4.1 Working principles

An OFET essentially consists of four different components: an electrical conducting material,

an insulating material, an organic semiconducting material and a carrier substrate (figure

4.1a). The organic semiconducting film that connects two electrodes, the source and drain

with a gap of length L and width W (channel dimension), is separated from a third electrode,

the gate by a thin film dielectric insulator. It can be seen as a two-plate capacitor placed

between two electrodes. The conductivity of one plate that lies between the electrodes is

altered by the voltage applied to the second plate of the capacitor. With no potential

difference between the source and drain (VD), applying a voltage to the gate (VG) charges are

injected from the grounded source electrode in the semiconductor and spread to charge the

capacitor at the insulator/semiconductor interface resulting in a doped conductive channel.

The density of the charges dependents on the gate voltage.

The operation mode of OFET is similar of conventional FET66 as illustrated in figures 4.1b-d.

The channel conductance is not only controlled by the potential on the gate, but also by VD.

Upon applying VG before VD the concentration of charge carriers in the channel is uniform. A

minimum voltage on the gate is often required to turn the channel on. This voltage is called

the threshold voltage (VT). When small VD is applied, the drain current (ID) follows Ohm’s

law, i.e. the channel conductance is constant. This is the linear regime (figure 4.1b). As VD

21


increases and reaches a value close to VG, the electric potential is small in the channel region

close to the drain. This results in a reduction of the thickness of the channel next to the drain.

When VD reaches a value equals the difference between VG and VT, the channel is pinched

off, i.e. the channel thickness is reduced to zero. In order for the current to flow across the

reduced channel region, charge carriers are now swept to the drain by the high electric field in

the reduced region. The current-voltage curve starts to bend downward (figure 4.1c). Further

increasing of VD does not give rise to current increase, but it broadens the reduced region.

Because the potential at the pinch-off point remains constant and equals to VD = VG - VT, the

potential drop between the source and the point approximately is the same. This will result in

saturation of the drain current (ID,sat) (figure 4.1d).

Figure 4.1. A schematic structure and illustrations of the operating regimes of an organic field

effect transistor (OFET) with corresponding current-voltage characteristics. a) With no

applied voltages (VD and VG) at drain and gate electrodes. b) The linear regime. c) The

channel is pinched off and starts of the saturation regime. d) The saturation regime. The level

of the saturation current is altered by varying the gate voltage.

22


4.2 Device structures and semiconductor materials

The OFET can be made in different device structures depending on the physical nature of

both the semiconductor and the insulator. Even if the same components are used, the behavior

of the device also depends on the transistor geometry67. The most commonly found OFET’s

structures (in addition to the structure shown in figure 4.1) are illustrated in figure 4.2. The

differences of these structures lie on how carriers are injected in relation to the gate. For

instance, the charges are directly injected into the channel at semiconductor/dielectric

interface in a bottom contact/bottom gate configuration (figure 4.2a), while they first have to

travel through the undoped semiconductor before they reach the channel in other structure

(figure 4.2c). For all structures, plastic substrates such as polyethylene and polyester can be

used to make the transistor mechanically flexible. Even electrodes can be made from organic-

based conductors such as PEDOT:PSS and polyaniline, which commonly are solution

processible.

Figure 4.2. Common device structures of organic field effect transistors. a) Bottom contact,

bottom gate. b) Top contact, top gate. c) Top contact, bottom gate. The active area at

semiconductor/insulator interface is also indicated depending on the device structure.

There are two classes of materials that are used as organic semiconductors, conjugated small

molecules and polymers68. Polymers are easy processable e.g. from solutions by spin-coating,

while for small molecules like pentacene it must be vacuum sublimed. Depending on the

nature of charge carriers that are induced in the semiconductor, the channel is said to be either

p-channel or n-channel. Figure 4.3 shows examples of some molecular and polymeric p- and

n-channel semiconductors. Most of the reported organic transistors are p-channel transistors68,

thanks to its high stability under ambient conditions and under bias stress. One major problem

with n-channel transistors has been the unstability of n-doped species in the channel in

23


presence of water and dioxygen. Thus, n-channel transistors only can operate when processed

and tested them under inert conditions. A second challenge with n-channel transistors is the

electron injection into the LUMO level of the organic semiconductor from an electrode69. The

work function of the metal electrode should be low enough to promote easy electron injection

in the LUMO level of the organic semiconductor. For instance, gold electrode is suitable for

hole injections, while it is less appropriate to n-channel transport because of its high work

function, giving rise to high injection barrier for electrons at the contacts. Therefore low work

function metals such calcium are preferred needed for n-channel transistor applications.

Unfortunately, these kinds of metals oxidized easily in air. In some cases, OFET exhibits both

n- and p-channel behavior depending on the polarity of both VD and VG and the transistor is

called ambipolar69.

Figure 4.3. Chemical structures of some organic semiconductors. a) Pentacene. b) Rubrene.

c) PQT-12 (poly(3,3’’’-didodecylquaterthiophene)). d) F8T2 (poly(9,9dioctyl-fluorene-alt-

bithiophene)). e) TCNQ. Pentacene and rubrene are small conjugated molecules, while PQT-

12 and F8T2 are conjugated polymers. These are p-type materials. However, TCNQ is an n-

type semiconducting materials.

4.3 OFET characteristics

The conductivity of the channel is enhanced upon increasing the gate voltage, because OFETs

normally operate in the accumulation mode. The general trends in OFET characteristics are

usually illustrated via conventional semiconductor theory66. The relationships between the

current and voltage for the transistor’s different operating regimes has analytically been

24


derived by assuming the gradual channel approximation: the electric field across the channel

(at the semiconductor/insulator interface ) is much larger than that along the channel (between

source and drain). These are given by:

( ) ( ) DDTGilinearlinearD VVVVCLWI )2/(/, −−= μ (eq. 4.1a)

( ) ( )2, 2/ TGisaturationsaturationD VVCLWI −= μ (eq. 4.1b)

where and is the drain current of the linear and saturation regime, W and L

the channel width and length, C

linearDI , saturationDI ,

i the capacitance per unit area of the insulator layer, µlinear and

µsaturation the linear and saturation field effect mobility. Figure 4.4a shows typical output

characteristics, which is ID versus VD for different constant VG, of a p-type channel OFET.

From output characteristics the linear and saturation regimes are clearly shown.

Figure 4.4. Typical current-voltage characteristics of a p-type organic field effect transistor

(OFET). The channel length and width was 3.5µm and 200µm. a) Output characteristics. b)

Transfer characteristics in the saturation regime, indicating the threshold voltage VT

determined from the linear fit to the square root of the drain current that intersects the x-axis.

Mobility is one of the most important parameters in characterizing an OFET. It quantifies the

average drift velocity of the charge carrier per unit electric field. High mobility is desirable

because it gives higher output currents and improve the response time of the transistors upon

gate bias stress. Mobility can be affected by many factors. It varies depending on the nature of

the semiconductor film and the fabrication methods. For instance, the mobility of pentacene

and P3HT thin films is not the same; even the device configuration of both transistors is the

25


same. Pentacene, which is vacuum deposited, has higher mobility (1.5 cm2 V-1s-1)70,71 than

regio-regular P3HT (0.1 cm2 V-1s-1)32, which directly is processed from a solution. The purity

of the material and the choice of solvent influence the mobility. Annealing the film after

deposition by spin-coating has been shown to give rise systematically to higher mobility due

to the complete removal of solvent in the semiconductor films. In some case, the enhancement

of mobility by annealing can be due to morphology change accompanied with a better

molecular packing72,73. It was also shown that the mobility is both drain and gated voltage

dependent74. From the equation above, the linear and saturation mobilities can be determined:

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛∂

∂=

DiG

linearDlinear VCW

LV

I ,μ (eq. 4.2a)

½½, 2)(

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

∂∂

=iG

saturationDsaturation CW

LV

Iμ (eq. 4.2b)

Figure 4.4b shows a semilog plot of ID and square root of ID versus VG at constant VD in the

saturation regime, called transfer characteristics. The saturation mobility can easily be

calculated by extracting the gradient of the square root of ID. From transfer characteristics,

threshold voltage (VT) can be obtained by a linear fit to the square root of ID that intersects

with the VG-axis. Another important parameter that can be extracted from the transfer

characteristic is the On/Off current ratio, the ratio between ID in the on-state (at particular VG)

and ID in the off-state (at VG = 0V), ID,On/ID,Off. High On/Off current ratio is required for the

transistor to be integrated in circuits. The On/Off ratio for P3HT based transistor was obtained

as high as 106 75.

4.4 Electrolyte-gated OFETs

Most of organic semiconductor properties in OFET have been characterized using thermally

grown SiO2 as the insulator because of its ready availability. The use of these kinds of OFETs

in cheap electronics is not favorable, because of the requirement for very high operation

voltages. In order to make them applicable in electronics, the operation voltage should be

reduced without worsening the output current. The crucial parameter that governs the

operating voltage is the capacitance (per area) of the insulator Ci. From the relationship (eq

26


4.1) between current throughput, gate voltage and capacitance it is clear that to obtain low

operating voltages, but keeping the output current high, Ci should be high. The capacitance

depends on both the dielectric constant (k) and the thickness of the insulator. Various kinds of

insulators with high capacitance have been investigated76: inorganic insulating materials with

high k such as aluminum oxide (Al2O3, k = ~10) and titanium oxide (TiO2, k = ~41), and

polymeric dielectric materials such as polyvinylalcohol (PVA, k = ~10) and polyvinylphenol

(PVP, k = ~4). In addition, self-assembled monolayer (SAM), which has a thickness of

molecular monolayer (~2 nm), has been used as a gate dielectric and the capacitance reached

the order of 1µF cm-1 77.

Beside their necessity of high capacitance, the insulating materials also should fulfill demands

specific to organic electronics i.e. low-cost, compatible with flexible substrates, processible

from solutions, insoluble in the solvent used for deposition of the organic semiconductor and

should be compatible with the gate electrode materials. Electrolytes have been found to be a

great materials to gate transistors (figure 4.5a)16,25,78-83. These types of dielectrics are usually

composed of a salt distributed in a solution or matrix, e.g., polymer gel. When a gate potential

is applied, the ions in the electrolyte redistribute and migrate into the semiconductor leading

to electrochemical doping (dedoping) in the bulk of the organic semiconductor. Thus, the

channel is opened (closed). These transistors are classified as electrochemical (EC) transistors

(Figure 4.5b). They can typically operate at low drive voltages (< 2 V) and be processed using

low-cost production techniques. But however, they respond slowly. One of the successful EC

transistors has been reported by Nilsson et al16. The presented transistor was based on lateral

architecture and the electrodes consisted only of a thin film of PEDOT:PSS on plastic

substrate, while a calcium chloride based gel was the electrolyte. The area that was covered

with electrolyte between the source and drain defined the channel. This resulted in all-organic

EC transistor. Here the conductivity of the channel was decreased upon applying a gate

voltage because of the reduction of the PEDOT:PSS channel, thus the On-state is defined at

zero applied gate voltage.

27


Figure 4.5. A illustration of an organic transistor gated via an electrolyte. a) When no voltage

is applied to the gate the ions in the electrolyte are distributed uniformly. b) Upon applying a

negative gate bias, ions respond to the electric field and redistribute. The migration of anions

into the semiconductor causes electrochemical doping that allowing the channel to be

opened. This is the typical behavior of an electrochemical transistor. c) If electrochemistry is

prevented because the device operates in vacuum or the anions are too big for the migration

into the semiconductor film, electrostatic charge injection instead will take place in response

to the high electric filed across the electric double layer formed at electrolyte/semiconductor

interface.

Solid polymer electrolytes have also been used in gating organic transistors79,84. For example

PEO with dispersed lithium perchlorate salt has been demonstrated as gate insulator material.

The motion of the anions in polymer electrolyte is remarkably reduced when a p-channel

transistor operates in vacuum, thus preventing bulk electrochemistry to occur. Instead,

electrostatic charge injection takes place in the semiconductor in response to the high electric

field at the interface between polymer semiconductor and polymer electrolyte (figure 4.5c)84.

Upon operation in air, current modulation resembles EC transistors. Because of bulk

electrochemistry the conductivity of the channel of these transistors in Off-state after device

operation is higher compared to Off-state before turning the transistor on25. The use of

polymer electrolytes has allowed injection of high carrier density (1014 – 1015 charges cm-2)

and the achievement of the metallic conductivities in the channel of polymer electrolyte gated

OFET79. It also enable n-channel operation85.

In order to avoid the electrochemical doping of the bulk semiconducting polymer film when

operating at ambient atmosphere, solid state polyanionic proton conducting electrolytes such

as the random copolymer of vinyl phosphonic acid and acrylic acid, P(VPA-AA) can be used

as gate material25,82. By applying a negative electric potential to the gate, protons migrate

towards the gate electrode and form an electric double layer at polyelectrolyte/electrode

28


interface. The remaining immobile polyanion chains stay close to the positively doped organic

semiconductor, allowing the formation of p-channel. Because the polyanionic chains are

immobile, they cannot penetrate the semiconductor layer, thus preventing electrochemistry in

the bulk of the channel. Unlike EC transistors that require several of seconds to switch,

OFETs gated via electric double layer capacitor (EDLC-OFET) exhibit transients in kilohertz

region. The quick formation of double layers at polyelectrolyte/electrode and

polyelectrolyte/semiconductor interfaces result in low voltage operation (< 1V) and high

output current (in µA range for L/W=7/200). Employing polyelectrolytes as gate insulator, the

thickness of the insulator and the position of gate electrode become less important since the

EDLC spontaneously forms at insulator/semiconductor interface upon applying a gate bias25.

As a result, the devices could be produced cheaply via e.g. printing techniques.

4.5 Humidity effect

The device operation of OFETs is known to be influenced by the environmental species, such

as water and oxygen86-90. These species degrade their electrical performance. Field effect

mobility of several organic semiconductors has been shown to decrease upon exposure to

moisture87,91. For polythiophenes, the effect of water present in the atmosphere is large

compared to the p-type doping effect of oxygen92. Oxygen will enhance the p-type doping

through penetration in the polythiophene layer. This will give rise to a slight increase of the

conductivity of the polymer film. The mobility in polythiophene-based transistors decreases

approximately 60% at high humidity levels, compared to the mobility in a fully dry

atmosphere. The presence of water molecules under transistor operation, which are large

compared to the number of charge carriers, interacts with the polymer chains. It was found

that the accumulation layer at ambient atmosphere with 40% relative humidity contains nearly

equal numbers of water molecules and carriers91. The water molecules stabilize the holes

giving rise to traps sites that reduce their mobility. Because of mobile ionic charges can be

induced by adsorption of water vapor in the semiconductor layer, the off-current increases

with humidity. In addition, the threshold voltage increases too87,88. The combined effects of

decreasing the on-current and increasing the off-current with humidity results in a reduction

of the on/off current ratio. Therefore encapsulation of OFET is necessary for reliable

operation.

29


The use of polymeric gate dielectrics in OFET is also affected by humidity. Here, it is

difficult to separate the humidity effect on both semiconducting layer and dielectric. Unlike

traditional OFET, transistor gated via a hygroscopic gate dielectric such as polyvinylphenol,

PVP takes advantage of ambient moisture93. The performance of the device is greatly

enhanced by moisture in the hygroscopic insulator. This is attributed to the ionic process that

occurs in the moisturized gate insulator close to the semiconductor interface. The hygroscopic

transistor’s output characteristics is lost in dry atmosphere. On the contrary, characteristics of

traditional OFETs (not gated with an electrolyte) are improved when operating in a moisture

free environment. In addition, humidity also enhances the degree of surface polarization,

which results in an increase of the drain current due to the accumulation of extra charge

carriers94. On the other hand, the conductivity in electrolytes is well known to be dependent

on the humidity. The change in response with humidity is the basic principle of some

humidity sensors16,95. The characteristics of the electrolyte gated transistors therefore changes

with humidity, i.e., it is a key parameter that governs the performance of the transistors.

30

Summary of the Included Papers

5 Summary of the Included Papers

5.1 Paper I

The paper reports on the induced electrochromism observed in the conducting polymer blend

poly(3,4-ethylenedioxythiophene)- polystyrene sulphonate (PEDOT:PSS). A patterned thin

film of PEDOT:PSS in direct contact with a thin film of gel electrolyte was used to visualize

the electric field in the electrolyte driven by an external anode and cathode. The oxidation

state, and thus the color, of the PEDOT within small electronically isolated thin-film islands

varies with the strength and direction of the electric field in said electrolyte. Both the

magnitude and direction of the electric field along the electrolyte can be estimated from the

level of induced electrochromism in each island, measured with a simple flatbed scanner. This

electric field visualization technique can be used to detect local concentration or mobility in a

2-dimensional electrolyte and to study the transport of charges in electrolytes. Because the

measurement is made optically, the electrolyte variations can be measured without direct

electrical connections. The presented technique may be relevant for areas such as

electrophoresis, electrochemical devices and for general development for electrolytes.

5.2 Paper II

The acidic polyelectrolyte, poly(styrenesulfonic acid) (PSSH) is used to gate an organic field

effect transistor. Thanks to the large capacitance of the electric double layer that is formed at

the interface between the electrolyte and organic semiconductor, the transistor operates at low

drive voltages and responds fast to the applied gate bias stress. This type of field effect

transistors gated via electric double layer capacitors is called EDLC-OFETs. Because the

polyanionic chains are almost immobile, they cannot penetrate into the organic semiconductor

film when the gate is negatively biased, thus preventing electrochemical doping in the bulk of

the semiconductor layer. The capacitance of the double layer is essentially thickness

independent. This will in turn give rise to ease in the design and manufacturing requirements

of the OFET, resulting in robust and low-cost plastic electronics.

31

Summary of the Included Papers

5.3 Paper III

Because a polyelectrolyte is used as a gate insulator in polymer transistor, the ionic

conductivity of polyelectrolyte should be considered. This ionic conductivity is known to

depend on relative humidity (RH). This paper presents the role of the ionic conductivity in the

electrolyte insulating layer and study the effect on the electronic performance of OFETs by

varying the humidity level. At low RH the drain current is almost purely electronic, while the

ionic contribution to the current becomes more and more visible upon increasing the

humidity. This will in turn give rise to enlargement the off current resulting in a decrease of

the on/off current ratio. The capacitance and the ionic resistance in the polyelectrolyte layer

are studied as well. In addition, a qualitative model that illustrates the trends of the transfer

characteristics when varying the RH is proposed.

32

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