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Transcript
Linköping Studies in Science and Technology
Dissertation No. 921
An Organic Electrochemical Transistor
for Printed Sensors and Logic
David Nilsson
Dept. of Science and Technology
Linköpings Universitet, SE-601 74 Norrköping, Sweden
Norrköping 2005
Illustration by Steffen Uhlig
ISBN 91-85297-26-7
ISSN 0345-7542
Printed by UniTryck, Linköping, Sweden, 2005
Abstract
Conducting polymers entered the research field in late 70´s and efforts aimed at
achieving printed electronics started a decade later. This thesis treats printable
organic electrochemical transistors (OECT). Some conjugated polymers can be
switched between a high conducting and a low conducting state in an
electrochemical cell. In this thesis, the work carried out using poly(3,4ethylenedioxythiophene) (PEDOT) as the active material in an electrochemical
transistor is reported. The electrochemical transistors, presented, can be designed
into a bi-stable and dynamic mode of operation. These transistors operates at
voltages below 2V and current on/off ratios are typically 5000, but 105 have been
reached. The transistor device can be built up from all-organic materials using
common printing techniques such as with screen-printing. The bi-stable transistor can
be combined with an electrochromic (EC) display cell to form a smart pixel circuit.
Combining several of these smart pixels yield an actively addressed cross-point
matrix display. From this an all-organic active matrix display printable on paper has
been achieved. The OECT, combined with a resistor network was successfully used
in inverter and logic circuits.
One important feature of these organic electrochemical devices is that both ions
and electrons are used as the charge (signal) carriers. This is of particular interest
and importance for chemical sensors. By combining a proton-conducting electrolyte
(Nafion®) that changes its conductivity upon exposure to humidity, a simple OECT
humidity sensor was achieved. This proves the use of this OECT as the ion-toelectron transducer.
Sammanfattning
I slutet av 70-talet upptäckte Heeger, Shirikawa och MacDiarmid att polymerer
(plaster) kan leda ström. Genom olika design på polymererna kan elektrisk
ledningsförmåga från halvledare till relativt bra ledare erhållas. I dagsläget har man
visat att ledande polymerer går att använda i tillämpningar som transistorer,
lysdioder, batterier och olika elektrokemiska komponenter. Jag har i den här
avhandlingen fokuserat på elektrokemiska transistorer. Elektrokemiska transistorer
använder sig av både joner och elektroner som laddningsbärare. I en elektrokemisk
cell kan polymeren oxideras eller reduceras via en applicerad potentialskillnad vilket
leder till att den elektriska ledningsförmågan i polymeren förändras. Detta utnyttjas i
den elektrokemiska transistorn där strömnivån förändras beroende på vilken
spänningsnivå som är applicerad. Transistorn har kopplats samman med en
elektrokemisk pixel för att erhålla en smart-pixel, som i sin tur bygger upp en display.
Transistorn har även tillsammans med ett resistornätverk använts för att bygga upp
olika logiska kretsar, bland annat inverterare, NAND-, och NOR-grindar. Dessa
logiska kretsar är huvudbyggstenarna för att bygga upp mer komplexa logiska
system. Att använda joner som signalbärare ger möjligheter att koppla transistorn till
olika biologiska system där joner fungerar som signalbärare. Elektrokemiska
transistorer kan därför fungera bra för olika sensorapplikationer. Genom att på
transistorn applicera en elektrolyt som ändrar jonledningsförmåga beroende på
luftfuktigheten har en fuktsensor realiserats. Materialen som används i den
elektrokemiska transistorn är lösbara i vanliga lösningsmedel vilket möjliggör för
vanliga tryckteknologier som tillverkningsmetod.
Acknowledgements
I would like to express my sincere gratitude to the following people:
First of all to my supervisor Magnus Berggren for giving me the opportunity to work in
the field of organic electronics and also for creating an encouraging research
environment.
Before I met Robert Forcheimer I thought that building electronic circuits was rather
boring. But after discussions with Robert my opinion has become more positive,
especially when it is possible to build circuits with a new type of electronics.
To the staff at ACREO institute, especially Tommi Remonen, Anurak Sawatdee,
Anna Malmström and Jessica Häll, for valuable discussions and laboratory support.
To Thomas Kugler for valuable discussions, presently working at EPSON, UK.
To Olle-Johnny Hagel and Lena Malmby for a lot of help in the lab in the beginning of
my graduate studies, presently working at Thin Film Electronics AB.
To all group members of organic electronics for friendship and fun discussions of
crazy ideas. It has been a lot of fun to work in a group with a lot of openness and
friendship.
Special thanks to Nathaniel Robinson, in addition to all scientific discussions you
have also helped me a lot with correcting English and not least every time my
computer made me frustrated.
To Sophie Lindesvik for help will all kinds of administrative problems.
Till mina föräldrar Mari-Anne Åkeson och Inge Nilsson för att ni alltid ställer upp,
Tack!
And finally, to Karin, Anton and the fourth family member on its way for all the love
that you are giving me.
List of included papers
Paper 1: Bi-stable and Dynamic Current Modulation in Electrochemical Organic
Transistors
David Nilsson, Miaoxiang Chen, Thomas Kugler, Tommi Remonen, Mårten
Armgarth, and Magnus Berggren
Adv. Mater. 14, 51-54, (2002).
Author’s contribution to the paper: Some writing and major part of experimental
work
Paper 2: An all-organic sensor-transistor based on a novel electrochemical
transducer concept printed electrochemical sensors on paper
David Nilsson, Thomas Kugler, Per-Olof Svensson, and Magnus Berggren
Sens. Actuators B-Chem. 86, 193-197, (2002).
Author’s contribution to the paper: Some writing and major part of experimental
work
Paper 3: Active Matrix Displays Based on All-Organic Electrochemical Smart Pixels
Printed on Paper
Peter Andersson, David Nilsson, Per-Olof Svensson, Miaoxiang Chen,
Anna Malmström, Tommi Remonen, Thomas Kugler, and Magnus Berggren
Adv. Mater. 14, 1460-1464, (2002).
Author’s contribution to the paper: Some writing and parts of experimental work
Paper 4: Electrochemical Logic Circuits
David Nilsson, Nathaniel Robinson, Magnus Berggren and Robert
Forchheimer
Adv. Mater. In Press
Author’s contribution to the paper: Some writing and all experimental work
Related published manuscripts and
patents
Manuscripts
Electric current rectification by an all-organic electrochemical device
Miaoxiang Chen, David Nilsson, Thomas Kugler, Magnus Berggren, and Tommi
Remonen, Applied Physics Letters, vol. 81, 2011-2013, (2002).
All-organic electrochemical device with bi-stable and dynamic functionality
D. Nilsson, M. Chen, P.-O. Svensson, N. Robinson, T. Kugler, M. Berggren, Proc.
SPIE vol. 5051, 468-476, (2003).
Polymer-based electrochemical devices for logic functions and paper displays
M. Berggren, D. Nilsson, M. Chen, P. Andersson, T. Kugler, A. Malmström, J. Häll, T.
Remonen, N. D. Robinson, Proc. SPIE vol. 5051, 429-436, (2003).
Organic Electrochemical Smart Pixels
P. Andersson, D. Nilsson, P.-O. Svensson, M. Chen, A. Malmström, T. Remonen, T.
Kugler, M. Berggren, Materials Research Society Symposium Proceedings, vol. 736,
D6.6 (2003).
Pinch-off and Saturation Behaviour in an Electrochemical polymer Transistor
based on PEDOT:PSS
Per-Olof Svensson, Nathaniel D. Robinson, David Nilsson, Jessica Häll and Magnus
Berggren, 3rd International IEEE Conference on Polymers and Adhesives in
Microelectronics and Photonics - POLYTRONIC, pp. 279-282, (2003).
Visualizing the Electric field in Electrolyte Using Electrochromism from a
Conjugated Polymer
Elias Said, Nathaniel D. Robinson, David Nilsson, Per-Olof Svensson,
and Magnus Berggren, Electrochemical and Solid-State Letters, vol 8, H12-H16,
(2005).
Patents
Electrochemical device
US2004211989
Kugler Thomas, Remonen Tommi, Berggren Magnus R., Chen Miaoxiang, Nilsson
David A., Forchheimer Robert and Armgarth Mårten
Electrochemical pixel device
US6642069
Kugler Thomas, Remonen Tommi, Berggren Magnus R., Nilsson David A.,
Andersson Peter K. and Armgarth Mårten
Electrochemical sensor
WO03046540
Nilsson David A., Svensson Per-Olof J. and Berggren Magnus R.
Abbreviations
CE
Counter Electrode
D
Drain
EC
Electrochemical
FET
Field Effect Transistor
G
Gate
HEC
Hydroxyl Ethyl Cellulose
LEC
Light Emitting Electrochemical Cell
LED
Light Emitting Diode
MOSFET
Metal Oxide Semiconductor Field Effect Transistor
NMP
N-methylpyrrolidone
OECT
Organic Electrochemical Transistor
OFET
Organic Field Effect Transistors
PEDOT
Poly(3,4-ethylenedioxythiophene)
PEO
Poly(ethylene oxide)
PPy
Polypyrrole
PSS
Poly(styrene sulphonate)
PVA
Poly(vinyl alcohol)
RE
Reference Electrode
Redox
Reduction-Oxidation
RH
Relative Humidity
S
Source
SMU
Source Measure Unit
SPE
Solid Polymer Electrolytes
WE
Working Electrode
Contents
1.
Introduction
1
2.
Electroactive organic materials
4
2.1
Structure
5
2.1.2
Doping of polymers
6
2.1.3
Free charge carriers in conjugated polymers
8
Specific materials
12
2.2.1
PEDOT:PSS
12
2.2.2
Electrolytes
15
Semiconductor devices
3.1
4.
4
2.1.1
2.2
3.
Conjugated polymers
The organic field-effect transistor
Electrochemical devices
19
19
22
4.1
The bi-stable configuration
22
4.2
The dynamic configuration
24
4.3
Electrochemical transistors
27
4.3.1
The four-terminal transistor
27
4.3.2
The three-terminal transistor
30
4.3.3
Chronoamperometric response
32
4.3.4
Other techniques
33
4.4
Sensors
4.4.1
Humidity sensor
35
35
4.5
Characterisation of materials
37
4.6
Circuits
38
4.6.1
The OECT smart pixel
38
4.6.2
Logic circuits
40
5.
The experimental equipment
46
6.
Outlook for the future
47
7.
References
48
Paper 1
55
Paper 2
69
Paper 3
83
Paper 4
99
Introduction
1. Introduction
The story of synthetic polymers (plastics) started in the beginning of the 20th
century. One of the first plastic materials that most of us recognize is Bakelite. In the
early years, the US government considered Bakelite as a candidate for new
weaponry and lightweight war machinery, something that steel could not match.
Bakelite was also used for domestic purposes such as in electrical insulation, and it
proved to be more insulating than any of the other materials available. In fact,
Bakelite is still used in our society. It is electrically resistant, chemically stable, heatresistant, shatter-proof and, doesn’t crack, fade, crease, or discolor from exposure to
sunlight, dampness or sea salt. The development of plastics has since then
expanded to a large industry, and now plastics affect all of our lives.
Plastics are polymers, which are large molecules that consist of many subunits,
called monomers. Polymers can also be found abundantly in nature, in the form of
cellulose, proteins and DNA, etc. In the late 70´s, Heeger, Shirikawa and
MacDiarmid1 found that it is possible to achieve conducting polymers. Previously,
polymers were considered as true insulators. From the moment of this discovery, the
story of plastics took a new direction. Today, polymers can be synthesised into a vast
array of conducting forms possessing a broad range of conductivities, all the way
from semiconductors to reasonably good conductors. Electroactive polymers have
1
Introduction
attracted considerable attention during recent decades due to their unique
functionalities and good electrical characteristics while utilised in different electronic
devices, such as field-effect transistors (FET),2-4 light emitting diodes (LED),5
electrochemical (EC) devices6-9 and polymer based batteries.10-12 In addition, since
polymers can be dissolved in common solvents, that is promising for low-cost
production of devices using printing techniques.13,14
This thesis focuses on a class of polymers that can serve as the active material
in electrochemical devices, such as in electrochromic displays,15-17 batteries,18 lightemitting
electrochemical
electrochemical transistors.
cells
6,8,9,24
(LEC),19-21
anti-corrosion
coatings22,23
and
Here, both electrons and ions act as charge
carriers, while the polymer can undergo reversible electrochemical oxidation and
reduction. By oxidation and reduction, the conjugated polymer gets doped or dedoped, which results in a change in the electronic structure of the polymer. Oxidation
introduces new electronic states and free charge carriers along the polymer chain. As
the reduction-oxidation (redox) state is switched, the colour and the conductivity of
the conjugated polymer are changed. In the past 20 years, various electrochemical
devices based on conducting polymers and conducting oxides have been developed.
One example is the electrochemical transistor. The electrochemical transistors
developed so far can be divided into two categories: the organic and the inorganic
versions. The major group is the organic types based on conducting polymers as the
active material. The main fields of application for electrochemical transistors have
been sensors and as the tool to characterise different electroactive materials. The
key function utilised in electrochemical transistors is impedance modulation due to
switching of the active material between different redox-states. As a consequence,
the active material is switched between a semi-conducting (or close to insulating) and
a conducting state.
Electrochemical transistors base their function on electrochemical modulation of
the active material, which involves transport of ions and electrons. In contrast, fieldeffect transistors base their function on the electric field effect modulation of the
number of holes, in the valence band, and electrons, in the conduction band within
the active layer. For the electrochemical transistor, the chemical change and
movement of ions results in moderate switching speeds compared to field-effect
2
Introduction
transistors. On the other hand, a transistor that rely on electrochemical switching
opens for many new possibilities for logic and analogue devices that rely on both ion
and electron conductivity.
Some of these possibilities are explored and described in this thesis.
3
Electroactive organic materials
2. Electroactive organic materials
Electroactive materials used for electrochemical devices are either organic or
inorganic. Most commonly, organic conjugated polymers or inorganic metal oxides
are used. In this thesis only organic materials in the form of conducting polymers are
treated. Switching a conjugated polymer between different reduction-oxidation
(redox)-states induces a metal-insulator transition together with a change in the
fundamental electronic structure of the conjugated polymer. This changes the optical
properties, which makes it possible to use these materials as electrochromic
materials, i.e. as switchable colorants.
2.1 Conjugated polymers
Shirikawa, Heeger and MacDiarmid first discovered conducting polymers in
1977.1 They demonstrated that it was possible to dope polyacetylene with arsenic
pentafluoride (AsF5), resulting a polymer that conducts electrical current. The
fundamental structure of a conducting polymer is built up from conjugated double
bonds, as described in the coming section.
4
Electroactive organic materials
2.1.1 Structure
The key structure of a conducting polymer is the linear chain of conjugated
units, in which double and single bonds alternate. In a polymer chain consisting of
conjugated double bonds every carbon atom is sp2 hybridised. Three of the electrons
in the sp2-hybridised orbital will take part in three σ-bonds, while the remaining
electron in the pz-orbitals form a π-bond. The pz-orbitals overlap between adjacent
carbon atoms, which leads to electron delocalisation along the backbone of the
polymer. In Figure 1 different conducting polymers are shown.
Figure 1. The molecular structure of some conjugated polymers. a) Two different
conformations of polyacetylene. b) Polyaniline. c) Poly(3,4-ethylenedioxythiophene)
(PEDOT). d) Polypyrrole (PPy). e) Polythiophene. “n” denotes that each monomer is
repeatedly forming a linear chain molecule.
Depending on the geometry of the conjugated polymers, they can be divided
into two groups: such as polymers that have a degenerate ground state and
polymers that have a non-degenerate ground state. If single and double bonds can
be interchanged without changing the ground state energy, the polymer system is
denoted as a degenerate ground system. If an interchange of single and double
bonds is associated with two states of energy levels, the system is said to have a
5
Electroactive organic materials
non-degenerated ground state. Polyacetylene can be found in two different forms,
either as trans-polyacetylene or as cis-polyacetylene. trans-polyacetylene has a
degenerate ground state, while cis-polyacetylene has a non-degenerate ground
state; see Figure 1. The quinoid and aromatic forms of cis-polyacetylene have
ground states with different energies; the quinoid form is the high-energy state due to
higher degree of overlap of atomic orbitals. Polythiophenes; see Figure 1, are an
example of conjugated polymers that have a non-degenerate ground state. It is more
common for conjugated polymers to have non-degenerate ground states.
Since the discovery of the highly conductive polyacetylene, a tremendous
number of different conjugated polymers have been synthesised. Polyaniline; see
Figure 1, was the first conjugated polymer developed with stable “metallic”-like
properties. Polyaniline can be processed into its highly conductive form. In order to
achieve a highly conductive polymer, doping is needed. In another class of
conjugated polymers, an interesting electrochemical feature has been explored. As
polypyrrole is switched between different redox states, corresponding to doping and
de-doping, the molecular geometry causes the volume of the film to change. This
effect is utilised in microactuators.25
2.1.2 Doping of polymers
In their neutral state, conjugated polymers span a conductivity range from close
to an insulator to semiconductors. Upon doping, their conductivity can be increased
by several orders of magnitude. In Figure 2 conductivity of doped and non-doped
polyacetylene and PEDOT is shown. In crystalline semiconductors like silicon and
germanium, only a very small percentage (ppm) of dopants, donors or acceptors, is
needed to increase the conductivity several orders of magnitude. Here the dopant
replaces an atom along the lattice structure and binds covalently to neighbouring
atoms in the matrix. For conducting polymers, doping occurs in a different manner.
First the doping fraction required is much higher. If every monomer unit is considered
to be a possible doping site, then several percent of the monomer units must be
doped in order to achieve a highly conducting polymer. In the case of doping
conjugated polymers, the dopant forms an ionic complex with the polymer chain.
6
Electroactive organic materials
Figure 2. Conductivity levels of polyacetylene and PEDOT. In comparison, conductivity of
some other materials is given, from very good insulators to metallic conductors.
Doping of polymers can be achieved by several methods; chemical doping,
electrochemical doping, photo-doping and charge-injection doping.26 The most
commonly used methods are the chemical and the electrochemical doping
approaches. Conjugated polymers can be both p-doped and n-doped. The oxidation
of the polymer chain is denoted to as p-doping. The process involved in p-doping is
equivalent to withdrawing electrons from the π-system of the polymer backbone. This
results in a positively charged unit in the conjugated system:
P + yA- ⇄ Py+A-y +ye-
(1)
Where P denotes the polymer chain, A denotes the charge-compensating
counterion, e- the electron and y is the number of counter ions. If the chain is instead
reduced compared to its neutral state it is n-doped. For n-doping, electrons are
introduced into the π-system of the polymer backbone to form a negatively charged
unit in the conjugated system.
P + ye- + yA+ ⇄ Py-A+y
(2)
7
Electroactive organic materials
So far, p-doped conjugated polymers are mainly used in devices and
investigations since conjugated polymers that are easily n-doped are normally
unstable in ambient atmosphere since they react quickly with oxygen in air.
During electrochemical switching, electroneutrality must be maintained within
the film. This is achieved by combining the solid polymer with an electrolyte. The
electroneutrality is maintained by ions, counterions, which either enter or escape the
conjugated polymer film. Fully doped conducting polymer can therefore be treated as
a salt complex. In some cases, the counterion species can be immobile itself, an
example is poly(styrene sulphonate) (PSS). For the case of PSS, cations move into
and out of the polymer film in order to compensate for electronic charges during
switching. An important point here is that conducting polymer conducts both
electrons and ions.27 This is one of the key properties utilised in electrochemical
devices.
2.1.3
Free charge carriers in conjugated polymers
In order to reach electrical conduction in a conjugated polymer, some form of
electronic species must carry the charge. Charge carriers can be created through
oxidative or reductive doping, as described above. These charge carriers are
transported along the π-bonded polymer chain. The charge carriers can either be
solitons, polarons or bipolarons, which are not real physical particles, but rather
quasi-particles. When combining two chain segments of trans-polyacetylene, with
different bond order, a defect in the form of an unpaired electron is created. The
unpaired electron will end up at a new energy level inside the band gap. This defect
is called a neutral soliton. The state of a soliton can carry from zero to two electrons.
Thus, solitons can either be neutral, positive or negatively charged. Charged solitons
have no spin, while the neutral soliton has spin but no charge. The three classes of
solitons are shown in Figure 3.
8
Electroactive organic materials
Figure 3. Three classes of solitons in trans-polyacetylene. ● denotes an electron not
participating in a bond, ⊕ denotes a hole and – denotes a negative charge.
For non-degenerate polymers with a preferred bond order, the charge carriers
are called polarons or bipolarons; see Figure 4. By oxidising the polymer, an electron
is removed and the associated positive polaron occupies an energy level in the band
gap. By withdrawing an electron from a polymer with a non-degenerate ground state,
a cation-radical pair is formed; see Figure 4. In between the cation and the radical, a
change in the polymer structure is created. This confined change in bond order and
the associated charge is called a polaron. In Figure 4 this is presented for PEDOT
and the deformation from the aromatic form into the quinoid form upon creating a
polaron is shown. The quinoid structure is a higher energy state compared to the
aromatic form. In contrast to solitons, polarons must overcome an energy activation
barrier related to the aromatic-quinoid transformation while moving. A polaron
occupies up to approximately five monomer units along the polymer chain. If two
electrons are withdrawn from the conjugated polymer, a positive bipolaron, with two
9
Electroactive organic materials
positive charges, is created. If the polymer is then oxidised even further, bipolaron
energy bands are generated in the band gap.
Figure 4. Generation of positive polaron and bipolaron in PEDOT. Energy levels of the
neutral polymer, a polaron, a bipolaron and a polymer with bipolaron energy band are
described above. ● denotes an electron not participating in a bond and ⊕ denotes a hole.
The conductivity in conjugated materials increases considerably upon doping.
Doped conjugated polymers exhibit good conductivity for two reasons; the number of
free charge carriers is increased and the charge carrier mobility is increased due the
formation of new electronic bands.26 The conductivity σ is defined as
σ = neµ
(3)
10
Electroactive organic materials
where n is concentration of charge carriers, e the charge of an electron and µ is the
charge carrier mobility. However, if the number of charge carriers is increased
beyond a certain level, the conductivity starts to decrease due to interactions
between the charge carriers. Highest conductivity is associated with “mixed valence”
states of fractional charges per repeat unit of the polymer. High conductivity should
occur when the polymer contains both charged sites and non-charged sites to which
the charged sites can move. Measurements on polythiophenes, polypyrroles and
polyaniline suggest that a finite potential window of high conductivity is a general
feature of conjugated polymers.8 A typical example of this is polyaniline,8 which starts
to show a decrease in conductivity when the polymer chain is oxidised to a level of
more than 0.5 electrons per repeat unit. Polyaniline shows the greatest conductivity
when oxidised to a level of 0.5 electrons per repeat unit.
Doping the polymer causes changes beyond the electronic conductivity. The
optical properties are also controlled since doping introduces new states in the
energy band gap,28 causing the absorption to shift towards lower energies; see
Figure 5. Conjugated polymers have a band gap typically in the region of 1.7-3eV,
i.e. they absorb visible wavelengths of light. This strong absorption in the visible
region in the neutral form is switched towards infrared absorption by doping. This
phenomena is denoted as electrochromism. An un-doped polymer absorbs at
wavelengths matching electron transitions across the band gap. The band gap of the
polymer is dependent on the structure of the polymer, which determines the
conjugation length. A long conjugation length, i.e. large degree of electron
delocalisation, yields a small band gap. By introducing different side groups along the
polymer, the band gap can be tuned. Adding electron donating or withdrawing side
groups allows the overall electron affinity of the polymer can be controlled. Adding
more or less bulky side groups can control the conjugation length along the polymer.
In addition to these factors, inter-polymer overlap of orbitals may contribute to the
resulting electronic band. At low doping levels different low energy features become
visible in the absorption spectra, caused by the polarons created. By further doping
to bipolarons (causing bipolaron bands) the absorption at low energy is increased
while the peaks widen. At high doping levels, the wide absorption at low energies is
11
Electroactive organic materials
often called the free carrier tail since it originates from absorption by the free charge
carriers.
Figure 5. Schematic description of the allowed optical transitions and their associated optical
spectra. a→d increasing of doping level.
Some conjugated polymers have a band gap greater than 3 eV. These
materials are transparent in the non-doped state (absorption occurs in the ultravioletregion), while they have a deep colour in the doped state. The doping/dedoping
behaviour of conjugated polymers can be studied and characterised by using various
spectroscopy techniques and measuring the conductivity.
2.2 Specific materials
2.2.1 PEDOT:PSS
One of the most studied and characterised conjugated polymers is the p-doped
poly(3,4-ethylenedioxythiophene) (PEDOT).29 The structure of PEDOT is shown in
Figure 4. PEDOT can be found in numerous applications such as the electrode or
conductor in FETs,2,3 LEDs,5,40 memories,30 EC-transistors,6,9 LECs,19-21 smart
12
Electroactive organic materials
windows,17 EC-displays,15-17 batteries,10-12 super capacitors,12,31 anti-static coatings32
and corrosion protections;22,23 see Figure 6.
Figure 6. Different applications where PEDOT can be found.
Some reasons for the popularity of PEDOT is its excellent chemical stability33
and its high conductivity. Depending on the counterion, PEDOT can exhibit
conductivities ranging from 1 to 300 S/cm.34 When poly(styrene sulphonic acid)
(PSSH) is used as the counterion, the conductivity typically reaches 10 S/cm in solid
state thin films.35 The structure of PSSH is described later. PEDOT:PSS is a
commercially available conducting polymer blend.36 One key factor for the
conductivity of the film is the morphology. By mixing PEDOT:PSS with sorbitol, Nmethylpyrrolidone (NMP) or isopropanol the morphology of the resulting film is
controlled in a favourable manner. Such treatment gives an increase in the
conductivity.37 Ouyang et al.38 have shown that solvent with more than two polar
groups, partically ethylene glycol, increases the conductivity even more. They
suggest that increased interchain interaction and conformational change is the
mechanism behind this. The morphology of a PEDOT:PSS film is a phase
13
Electroactive organic materials
segregation with grains consisting of conductive PEDOT:PSS surrounded by a
boundary of insulating PSS. The thickness of the grain boundary is about 40Å.39 The
grains show metallic-like electron conductivity while the grain boundary layer consists
of PSSH that is an ion conductor having low electronic conductivity. Therefore,
electronic current in a PEDOT:PSS film is easily transported within the grains while
the main obstacle is to transport electric current between the grains. In its doped
form, PEDOT shows excellent properties as a transparent charge injection layer in
light emitting diodes40 as well as the conducting electrodes/contacts in field effect
transistors.14 In the papers of this thesis we report electrochemical transistors utilising
PEDOT:PSS as the active material.
The PEDOT:PSS film, in its pristine state, is a mixture of doped (oxidised) and
undoped (neutral) PEDOT units. In an electrochemical cell, PEDOT:PSS can be
switched reversibly between the conducting form (PEDOT+) and the semi-conducting
state (PEDOT0), according to the reaction below:
PEDOT+:PSS-+M++e- ⇄PEDOT0+M+:PSS-
(4)
Here M+ represents the cation and e- is an electron. The arrow to the right indicates
the reduction of PEDOT while arrow to left indicates the oxidation process. PEDOT
shows a relatively high conductivity in its “neutral” state and conductivity
measurements of PEDOT near the neutral state has been performed by Johansson
et al..41 Conjugated polymers in general have conductivity in the range of 10-6 – 10-10
Scm-1 in their neutral state, whereas PEDOT has a conductivity of 10-5 Scm-1 in the
“neutral” state.41 Since PEDOT has such a high conductivity in the neutral state it is
possible to use it as its own electrode in electrochemical devices.
PEDOT is a low band gap polymer; it absorbs strongly in the red part of the
visible spectrum, resulting in a deep blue colour; see Figure 7. Upon doping PEDOT,
the optical absorption shifts to longer wavelengths, which results in nearly
transparent film.42
14
Electroactive organic materials
Figure 7. The optical absorption of PEDOT:PSS, at the reduced and oxidised state.
The optical and electrical properties described above made substrates, such as
OrgaconTM 350 EL43 produced by AGFA, a logic choice for the manufacture of the
devices described later in this thesis.
2.2.2 Electrolytes
In the electrochemical devices treated here, electrolytes are patterned in some
form. This allows for confined transport of ions within the device. The electrolytes
used can either be liquid electrolytes or solidified polymer electrolytes.
Liquid electrolytes
In electrochemical characterisation of conducting polymers, the samples are
normally immersed into a liquid electrolyte. Depending on which salt is preferred in
the experiment different solvents can be used. One important characteristic for the
solvent is that it should not itself be redox active at the potentials used for analysing
the sample. Acetonitrile is a commonly used solvent, which has a broader
electrochemical spectrum than water. Acetonitrile can withstand higher potentials
than water before it decomposes. The electrolytes used in this thesis are water
based and both liquid and solidified electrolytes have been used.
Solidified polymer electrolytes
There is a wide range of solid polymer electrolytes (SPE), such as Nafion®,44
poly (styrene sulphonic)acid (PSSH), poly (ethylene oxide) (PEO)45 and poly (vinyl
alcohol) (PVA).46,47 A more detailed description of Nafion® and PSSH will follow
below. Nafion® and PSSH are similar in the sense that both polymers have a
15
Electroactive organic materials
negatively charged immobile sulphonic acid group and a positively charged mobile
counter ion. The structures of Nafion® and PSSH are shown in Figure 8. In this figure
the cation is represented as a proton, but the proton can easily be exchanged with
other cations simply by exposing the polymer electrolyte to appropriate aqueous
electrolyte solution containing the desired cation. Ionic charge transport in SPE
occurs through cations that jump from one sulphonic acid group to another. These
ions are normally solvated, and the membranes themselves are strongly hydrated.
Figure 8. Molecular structure of Nafion® and PSSH. The protons H+ can be exchanged with
other cations. − SO3− is the sulphonic acid group.
The most common application area for Nafion® is as the proton conductor in
fuel cells. Therefore, extensive chemical and electrochemical studies have been
carried out on Nafion®.48 Nafion® has an internal structure that consists essentially of
inverted micelles joined through canals; see Figure 9. The inner surface of these
micelles is covered with − SO3− groups, which are hydrophilic in nature compared to
the fluorocarbon backbone. For this type of membrane, it has been shown that the
interaction between the mobile cations and the fixed anions is highly sensitive to the
degree of hydration of the membrane. Ionic transport is controlled by the narrow
channels between micells. These channels swell when the water content in the
membranes is increased. In the case of low water content, there is a high-energy
barrier, because the cations interact with each other in the narrow channel between
the micells. The cations need to overcome this barrier in order to allow for ionic
16
Electroactive organic materials
transport between neighbouring micells. Within a micell the uniformly lower energy
barrier must be overcome by the cation in order for them to move between sites. In
general, when Nafion® is highly hydrated, cations can easily move from one micelle
to another, whereas anions are hindered due to repulsion from the − SO3− groups.
Increasing the water content ([H2O]/[H+]) from 1.2 (34%RH) to 18 (100%RH),
increases the ionic conductivity from 1.4×10-4 Scm-1 to 6.6×10-2 Scm-1.49
Figure 9. Schematic illustration of the micells and the cation (⊕) transport in a NAFION
membrane. a) Low water content. b) High water content.50
In order to achieve electrolytes with increased conductivity, hygroscopic
materials can be mixed together with the SPE. The hygroscopic materials increase
the amount of water in the electrolyte when the electrolyte is in an open humidity
environment, which enables the ions to move faster. Two examples used in this
work, are a mixture of hydroxy ethyl cellulose (HEC), sodium citrate, glycerol and
water, and a mixture of Na:PSS, D-sorbitol, glycerol and water. The functionality of
the different ingredients is shown in Table 1.
17
Electroactive organic materials
Table 1. Ingredients and their functionality within the SPE.
Ingredient
Functionality
HEC
Scaffold
Sodium citrate Ion conductor
Glycerol
Hygroscopic and softener
PSS:Na
Scaffold and ionic conductor
D-sorbitol
Hygroscopic
The PSS based electrolyte has a conductivity in the order of 10 µS cm-1 and the
HEC based has a conductivity in the order of 100 µS cm-1 when the surrounding
environment has a relative humidity of 40%. A “solid-state” electrolyte based on
poly(vinyl alcohol)/H3PO4 applied on an electrochemical transistor has been
demonstrated by Chao et al..46 They demonstrated that the device response is
essentially the same with aqueous or “solid-state” electrolyte. It should be noted that,
poly(vinyl alcohol)/H3PO4 requires water to be present to reach a state of high ion
conductivity.
18
Semiconductor devices
3. Semiconductor devices
In everyday life, we utilise semiconductor devices in a wide range of
applications. Field-effect transistors (FET), diodes and light-emitting diodes (LED) are
key building blocks in these systems. Semiconducting devices have primarily been
manufactured with silicon as the active material. Organic semiconducting materials
have the advantage that they can be processed on flexible organic substrates with
low-cost techniques such as spin-coating, spray-coating and common printing
techniques. Today, organic electronic devices such as LED in displays for mobile
phones and electric razors have entered the market. Circuits made entirely from
organic materials have also been demonstrated.2-4
3.1 The organic field-effect transistor
Field-effect transistors (FET) based on polymers are constructed using thin film
technologies. A typical architecture for a FET is shown in Figure 10. Organic fieldeffect transistors (OFET) can be manufactured entirely of organic materials.
Polyester, polyethylene and polyimide can serve as the carrying substrate and a thin
film of polymethylmethacrylate or polyimide can be used as the dielectric layer. The
19
Semiconductor devices
drain, source and gate electrodes can be made out of organic conductors, such as
conjugated polymers.51,52
Figure 10. Schematic illustration of a possible architecture for a field-effect transistor. Source,
drain and gate contacts are denoted to as S, D and G, respectively.
The basic principle of a FET is that the density of charge carriers between the
source and drain is modulated via capacitive coupling between the gate contact and
the transistor channel. The capacitor consists of the gate contact, the dielectric
medium, and the semiconducting polymer channel as the other electrode. When a
voltage VG is applied between the gate and source contact, charge carriers
accumulate at the insulator-semiconductor interface; see Figure 11. This results in a
highly conductive channel between source and drain. At low drain-source voltages
(VD) the drain-source current (ID) has a linear relationship with VD. Thus, the channel
acts as a resistor. As VD reaches VDsat, the thickness of the highly conductive channel
is reduced to zero close to one side, a result denoted as pinch off. Beyond VDsat, ID
remains essentially constant and therefore the transistor acts as a current generator.
Varying VG controls the number of accumulated charge carries at the insulatorsemiconductor interface. Therefore ID is controlled.
20
Semiconductor devices
Figure 11. Operation of a field effect transistor in accumulation mode. a) At low drain-source
voltages. As the gate is biased, charge carriers accumulate in the channel. b) Onset of
saturation, i.e. pinch off. c) Beyond saturation for different gate voltages.
21
Electrochemical devices
4. Electrochemical devices
Another class of transistors is the group of electrochemical devices. In an
electrochemical transistor, both electrons and ions represent signal carriers. The
modulation of the current between drain and source is controlled by the oxidation
state of the polymer in the channel. This results in modulation of charge carriers via a
potentially driven effect, in contrast to the field driven effect in a FET. A consequence
of using the potential, rather than the field includes low operations voltages and
devices that require no ultra-small or critical dimensions. Electrochemical devices
demonstrated, to date, both by others and ourselves, include transistors, diodes, light
emitting electrochemical cells and electrochromic devices. Before the devices will be
discussed, different types of structures that build up the devices will be discussed.
4.1 The bi-stable configuration
Switching of a conjugated polymer between different redox states can be
demonstrated with an electrochemical cell consisting of two PEDOT:PSS electrodes
connected via a common electrolyte. Since the cell consists of two PEDOT:PSS
electrodes, it will be denoted as Structure 2. A sketch of the cell is given in Figure 12.
In its pristine state, the PEDOT:PSS film is oxidised. Upon addressing the cell,
22
Electrochemical devices
electronic current in PEDOT:PSS is converted into ionic current in the electrolyte, via
electrochemistry (redox reactions) occurring at the electrodes. Representative
impedances of the cell are given in the equivalent circuit displayed in Figure 12.
Upon biasing the cell, the positively addressed electrode (anode) PEDOT will be
further oxidised, resulting in that the number of bipolarons increases. In order for this
to occur, either cations must migrate out of the film or anions have to migrate into the
PEDOT:PSS film to maintain electroneutrality. This implies that electrochemistry has
to occur at both electrodes. At the negative electrode (cathode) PEDOT+ is reduced
to PEDOT0, which decrease the number of bipolarons. The appropriate half reactions
are shown in Figure 12. Cations migrate between the film and electrolyte to maintain
electroneutrality, while PSS- is too large to leave the film.
Figure 12. a) Schematic description of the bi-stable electrochemical cell (Structure 2). M+ is
the cation, X- is the anion and e- is an electron. Oxidation occurs at the positive electrode and
reduction at the negative electrode. The darker area represents reduced PEDOT and
brighter areas oxidised PEDOT. b) Equivalent circuit of the electrochemical cell with the
impedance of the ionic transport and the redox process. The impedance of the electron
transport within the PEDOT:PSS film is omitted.
This cell is bi-stable, i.e. when the voltage is disconnected, the PEDOT in the two
PEDOT:PSS electrodes remains in their respective redox states for some time. Since
23
Electrochemical devices
the number of bipolarons decreases upon reduction, PEDOT0 in the PEDOT:PSS film
starts to absorb more light in the visible region, resulting in an opaque deep blue
colour. In the oxidised form, PEDOT+ is nearly transparent; see Figure 7. Neglecting
other process the amount of charge (Q) utilised in this redox process can be
estimated by integrating the current that passes through the cell (Icell) while the
voltage is applied.
tf
Q = ∫ I cell dt
(5)
0
4.2 The dynamic configuration
The dynamic configuration can be designed in two different ways. Either one
homogenous film of PEDOT:PSS is covered with a common electrolyte; see Figure
13, or the PEDOT:PSS film is split into three films, of which two are electrically
biased, while one is exclusively in contact with the electrolyte; see Figure 14. The
design with only one electrode will be denoted as Structure 1, and the design with
three electrodes will be referred to as Structure 3. When a potential difference is
applied between each end of Structure 1, a gradient of electrochemical potential is
distributed along the polymer film and at the interface between the polymer film and
electrolyte. This results in redox reactions within the PEDOT:PSS film in contact with
the electrolyte. Reduction of PEDOT occurs inside the film close to the negatively
addressed end, while oxidation occurs in the film close to the positively addressed
end. Redox reactions occur until the electrochemical potential is uniform (the system
reaches equilibrium). Along the stripe of PEDOT:PSS there will be a concentration
gradient of doped and undoped PEDOT, which is observed as an electrochromic
gradient. The current in the dynamic Structure 1 configuration is, after equilibrium is
established, only transported by electrons (or rather polarons). When the applied
potential is ramped, the current has a linear dependence on the applied potential for
small voltages. At a critical potential, the current reaches a constant level. This point
is called the pinch off potential and will be further described in chapter 4.3.
24
Electrochemical devices
Figure 13. a) Schematic description of the dynamic electrochemical cell (Structure 1). M+ is
the cation, X- is the anion and e- is an electron. b) Equivalent circuit of the electrochemical
cell with the impedance of the ionic transport and the redox process. The impedance of the
electron transport within the PEDOT:PSS film is denoted as “electronic”.
For the dynamic Structure 3 configuration containing, one electrode (3) in
Figure 14 with no direct electronic contact, the behaviour is somewhat different.
When a potential is applied between the outer electrodes, the electrode with the
lowest potential is reduced and the electrode with highest potential is oxidised in the
same manner as for the bi-stable (Structure 2) configuration. The current can take
two pathways through the electrolyte: ((i) or (ii) see Figure 14) either it can go straight
through the electrolyte (purely ionic) or it can enter the centre PEDOT:PSS film (3)
via electrochemistry and travel through the PEDOT:PSS film (3) as electronic current,
to be exchanged to an ionic current at the other edge. This causes induced
electrochemistry in film 3 and an electrochromic gradient is observed. The magnitude
of this induced electrochromism has been utilised to visualize the electric field within
planar electrolytes.53 The ionic current in the electrolyte and the electronic current
through 3 is balanced, and depends on the rate of electrochemistry along 3 and the
25
Electrochemical devices
electric conductivity of the PEDOT:PSS in its different redox states and the ionic
conductivity of the electrolyte. There will only be ongoing redox processes in 3 as
long as a current flows between the outer electrodes, and as 3 is not in complete
equilibrium with the electrolyte. When the polymer in 3 has come to electrochemical
equilibrium with the electrolyte, current is no longer transported through film 3 and all
current between the electrodes 1 and 2 is strictly ionic. With low conducting
electrolytes, a relatively large part of the total current will pass through the centre film
(3). When the potential between electrodes 1 and 2 is disconnected or if the
electrodes are consumed, 3 returns spontaneously to its pristine state because the
PEDOT is available as an electronic conduction path.
Figure 14. a) Schematic description of the dynamic electrochemical cell (Structure 3), where
M+ is the cation X- is the anion and e- is an electron. The outer electrodes (1 and 2) show bistable characteristics while the middle film (3) is dynamic. The darker area represents
reduced PEDOT and brighter areas oxidised PEDOT. b) Equivalent circuit of the
electrochemical cell with the impedance of the ionic transport and the redox process. The
impedance of the electron conduction within the PEDOT:PSS film is omitted.
26
Electrochemical devices
4.3 Electrochemical transistors
The three fundamental structures described above are combined in different
ways to form the EC transistors that are treated in this chapter. In my work I have
studied different types of electrochemical transistors. The transistors can be
designed as either four-terminal or three-terminal devices. These transistors are
based on PEDOT:PSS combined with an electrolyte, resulting in an all-organic
electrochemical transistor (OECT) and exhibit either a dynamic or a bi-stable
response to the gate voltage depending on how Structures 1, 2 and 3 have been
combined. The three-terminal and four-terminal transistors show different behaviour,
which can be used for different applications. First, the four-terminal device is
described.
4.3.1 The four-terminal transistor
Both bi-stable and dynamic behaviour can be achieved with the four-terminal
transistor. Schematic illustration of the devices is shown in Figure 15. Both the bistable and the dynamic transistor can be realised by patterning a PEDOT:PSS film
into T-shaped structures, in combination with adjacent gate electrode(s). Both the bistable and dynamic transistors are constructed from Structure 1 between the source
and drain. The major difference between the two devices is the gate configuration.
The bi-stable transistor uses a Structure 2 gate configuration, whereas the dynamic
transistor uses Structure 3. Electrolyte pattern is deposited onto the patterned
polymer film. Here, the channel is defined as the area of PEDOT:PSS, which is
covered with electrolyte, between the source and drain contacts. When a potential is
applied between the gate contacts, electrochemistry occurs where the electrolyte
overlaps the PEDOT:PSS as described earlier for the bi-stable and dynamic cells.
When a gate voltage (VG) with positive bias at G+ (compared to G-) is applied,
PEDOT in area AG will be oxidised and PEDOT+ in area AC becomes homogenously
reduced in the bi-stable device, as shown in Table 2. Applying a potential between
the gate contacts (G+ and G-), result in electrochemical switching of the PEDOT and
hence gives modulation of the impedance level in the channel.
27
Electrochemical devices
Figure 15. Four-terminal transistors: a) Schematic illustration of the bi-stable electrochemical
transistor. b) Schematic illustration of the dynamic electrochemical transistor. Gate
electrodes are denoted to as G+ and G-, the source and drain contacts are denoted to as S
and D respectively.
Table 2. Description of redox-state at different areas of the four-terminal transistor with an
applied gate voltage.
Applied VG
G
+
G
-
Positively biased
Negatively biased
Bi-stable transistor
Dynamic transistor
AG
AG+
Oxidised
AC
Reduced
AR
Oxidised
AG-
Reduced
AC
Oxidised
Reduced
The potential VD between S and D is applied through a separate power source,
whose potential is floating compared to the potential applied between G+ and G-. This
results in control of the source (S) to drain (D) current (ID). Typical electrochemical
transistor characteristics for the bi-stable device are shown in Figure 16a. The fourterminal transistor is a symmetric device; the same characteristics are achieved if the
potentials at the source and drain are exchanged. At low VD bias, ID shows linear
dependence on VD since the impedance along the channel (AC) is homogeneously
distributed. When VD is increased, a redistribution of the redox states in the channel
28
Electrochemical devices
occurs, resulting in increased concentration of PEDOT0 (decrease of charge carriers)
at the drain side of the channel. This results in a pinch-off effect in ID similar to that
observed in FET. With increasing VG, the total impedance within the channel
increases but the distribution within the channel remains approximately the same as
long as VD is held constant. Figure 16b shows a potential and absorbance profile
within the channel of a four terminal EC-transistor. The potential profile is almost
constant within the channel except close to the negative drain contact where the
potential drops rapidly. We have found that almost all of the resistance in the channel
is located within 100µm of the channel edge.54 This increase in impedance causes ID
to saturate at a constant value.
Figure 16. a) ID vs. VD characteristics at various gate voltages for a bi-stable four-terminal
electrochemical transistor. b) Potential () and absorption () profile within the channel of
an EC-transistor at IDsat at VD=3V VG=0V. Above the graph, an illustration of the colour
gradient within the channel is shown. The darker area represents reduced PEDOT and
brighter areas oxidised PEDOT.
For the dynamic transistor, an applied positive bias at G+ compared to G- results
in oxidation in area AG+ and reduction in area AG-, as in the bi-stable configuration
described in chapter 4.1. As long as there is a current between G+ and G-,
electrochemistry is induced in areas AC and AR (see Table 2) as in the dynamic
configuration described in chapter 4.2. Therefore if G+ is positive compared to G-,
PEDOT in area AR will be further oxidised while PEDOT in area AC becomes
reduced. AR must be greater than AC in order to reach low impedance in AC, because
29
Electrochemical devices
the PEDOT:PSS film is highly oxidised in the pristine state. The PEDOT:PSS in AR
serves as an oxidation reservoir in order to achieve enough reduction in the channel.
When the area below the electrolyte in the channel is reduced, the current between S
and D is suppressed.
The bi-stable configuration allows low-power operation since the gate is
updated directly and no continues current is needed to keep the channel reduced.
When the gate voltage is disconnected, the low conductivity state remains for some
time, due to the open circuit between the two gate contacts. For the dynamic
transistor, the conductivity of the channel returns immediately when the gate contacts
are disconnected or when the gate electrodes are consumed. The lateral dimensions
for the EC transistor are not critical for achieving low operating potentials, since
electrochemical devices are potentially driven. The operating voltages are below 2V,
and on/off ratios of 5000 are normally reached.
4.3.2 The three-terminal transistor
The three-terminal transistor has only one gate contact, which is referenced to
the source contact; see Figure 17. Just like the bi-stable four-terminal transistor, the
three-terminal transistor is a combination of Structure 1 (between the drain and the
source contacts) and Structure 2 (between the gate and the channel). The threeterminal transistor is geometrically symmetric, but not electrically symmetric while
under operation. The source is defined as ground and drain as the electrode to which
the potential (VD) is applied. When the transistor is used in circuits, it operates in the
third quadrant (see Figure 18), where the drain contact has a negative potential. The
source is defined in conventional FET as the source of the charge carriers, and drain
as the sink for the charge carriers. In our device, positive polarons are the charge
carriers. Therefore, the source is defined as ground, the drain as the negatively
biased electrode, and a positively addressed gate modulates the impedance in the
channel (AC).
30
Electrochemical devices
Figure 17. Schematic illustration of the three-terminal OECT. The gate electrode is indicated
with a G, the source and drain contacts are denoted with S and D respectively.
The characteristics of the three-terminal transistor are shown in Figure 18. In
the first quadrant (VD>0 and ID>0), the transistor behaves linearly at zero gate
voltage. When a positive gate voltage is applied, the channel is reduced, resulting in
increased impedance. As VD is increased, the channel is oxidised back to its low
impedance state as can be seen in Figure 18. In the third quadrant (VD<0 and ID<0)
the transistor shows saturation behaviour similar to the four-terminal transistor.
Figure 18. ID vs. VD characteristics, at various gate voltages for a three-terminal
electrochemical transistor. Note the similarity to I(V) characteristics for a depletion mode
MOSFET.
31
Electrochemical devices
Compared to the four-terminal transistor, the saturation of ID is achieved at
lower VD in the three-terminal transistor. The reason for this is that the four-terminal
transistor gate voltage is constant compared to the channel as VD is increased. For
the three-terminal transistor, the gate voltage relative to the channel is increased
when VD is decreased.
The area ratio between the gate and the channel has significant impact on the
transistor behaviour concerning the on/off ratio. The on/off ratio is defined as the
saturation current (IDsat) between drain and source at VG=0V divided by the current
(IDsat) at VG=1V
on
off
ratio =
I Dsat (VG = 0V )
I Dsat (VG = 1V )
(6)
In order to achieve a high on/off ratio, the gate needs to be on the order of 10 times
larger than the channel. The reason for this is that the PEDOT:PSS film has more
oxidised sites than neutral in its pristine state. The fraction of the available sites that
are oxidised is on the order of 0.8. Also, the resistance in the linear region, before
saturation occurs, is dependent on the area ratio between the gate and the channel.
Increasing the area ratio from 1 to 49 increases this resistance by a factor of 80.
When designing circuits, it is therefore important to take the area ratio into account.
4.3.3 Chronoamperometric response
There is an upper limit for the frequency at which the gate can modulate the
impedance in the channel. Many different parameters influence the modulation
frequency, including the size of the transistor channel, ionic conductivity of the
electrolyte and the overall architecture of the transistor. For a lateral device with a
channel width of around 100µm, with an electrolyte based on HEC and sodiumcitrate, a modulation frequency up to 10Hz is reached. If the channel width is reduced
to roughly 20µm, the modulation frequency increases to around 200Hz. By changing
from a lateral to a vertical design, the response times can be decreased. A
comparison of response times for a lateral and vertical OECT is shown in Figure 19.
The time constants τoff (switch off) and τon (switch on) were, in the lateral
32
Electrochemical devices
configuration 1,1s and 3,1s, and in this vertical configuration 0,04s and 0,72s. The
time constants are calculated based on the time to switch between 10% and 90% of
the maximum of the current level. The difference in speed is related to the fact that,
in the lateral design the channel starts to reduce closest to the gate, due to an ionic
transport limited process, when the gate is biased. The reduced area then expands
to eventually cover the whole channel. This process is rather slow compared to the
case for the vertical design where the gate is located on top of the channel. In the
vertical transistor reduction of the channel occurs laterally and is more homogenous,
resulting in a faster shut-off.
Figure 19. a) Chronoamperometric response for a lateral three-terminal OECT. b)
Chronoamperometric response for a vertical three-terminal OECT. These transistors had a
channel width of 0.5mm and the electrolyte based on NaPSS had a thickness of 100µm.
4.3.4 Other techniques
OECTs have been studied by others using the setup shown in Figure 20.55,56 In
these cases, the electroactive polymer is normally electropolymerisation onto gold
microelectrode arrays. These transistors are characterised in the following way: the
gate voltage (VG) is controlled via both a reference electrode (RE) and a counter
electrode (CE) using a potentiostat, and the drain to source voltage (VD) is applied
via a source measure unit (SMU). The gate voltage is applied relative to the
reference electrode. In order to secure a stable reference potential the potentiostat
allows no current to pass through the reference electrode. Instead all current is
33
Electrochemical devices
transported through the counter electrode. The current through the gate (IG) is
exclusively associated with the charge needed to electrochemically switch the active
material between a non-conducting and conducting state. Conductivity modulation of
the electroactive material is measured as a current change between source and drain
contacts. Typically these transistors operate in an accumulation mode.
Figure 20. Schematic illustration of the electrochemical transistor set-up for measurements in
liquid electrolytes. CE: counter electrode, RE: reference electrode, S: source and D: drain.
Compared to field-effect transistors, electrochemical transistors are not
dependent on a thin film gate dielectric or narrow lines to achieve low operating
voltages. However, to achieve faster and more sensitive electrochemical transistors,
the distance between source and drain must be decreased. 1987, Jones et al. made
an electrochemical transistor with a 50nm separation between source and drain
using shadow deposition techniques.57 By decreasing the channel length from 1.5µm
to 50-100nm, they fully switched a polyaniline-based electrochemical transistor from
off to on using a charge less than 10-9 coulomb. This is 10-2 times less charge
compared to the electrochemical transistor with the channel length of 1.5µm. This
short-channel electrochemical transistor showed power gain at frequencies
exceeding 10kHz. Variation in drain current was observed for a change of only 10-12C
at the gate.57 This demonstrates that electrochemical transistors can be both
sensitive and fast, as often desired in sensor technologies.
34
Electrochemical devices
4.4 Sensors
Chemical and biological sensors are used in many different applications.58,59
Chemical sensors can be classified as either potentiometric or amperometric. A
potentiometric sensor measures a change in potential when exposed to different
analytes. A change in current is measured in an amperometric sensor when it is
exposed to different analytes. Normally, chemical sensors have an analyte sensitive
compartment and a transducer that converts the chemical signal into an
electrochemical signal. This signal can either be an amperometric or a potentiometric
response.
Electrochemical transistors based on printable organic materials promise new
possibilities for in expensive and single-use sensors. PEDOT:PSS is a material that
shows excellent electrochemical stability and reversibility in comparison with other
conjugated polymers60 and is therefore a good candidate for an ion to electron
transducer. Electrochemical transistors have been reported to show high sensitivity
to pH,61,62 ions,63 gases64 and glucose.65,66 The electrochemical transistors have a
number of potential advantages for use in biosensors. First, these devices do not
require a potentiostat or a reference electrode to operate. The measurement of the
drain to source current can be performed with simple instrumentation at low voltages.
Second, the devices can be made very small without losing their performance.57
Third, the device can be used as a signal integrator, since every change of the redox
state of the conducting polymer requires an element (typically a cation) from the
analyte; i.e. the device acts as an integrator. Finally, the device has a memory
function since it stays in the same state when the bias voltage is disconnected.
4.4.1 Humidity sensor
Polymers have been used widely in humidity sensors.67-69 Our approach is to
combine the OECT with a humidity sensitive electrolyte. The humidity sensor
presented in paper 2 is based on the four-terminal bi-stable OECT and is briefly
described here. The OECT as a transducer converts an ionic signal to an electronic
signal, which can be read out as a change in ID. This sensor is amperometrically
sensitive to different humidity levels, resulting in different current levels through the
gate. The humidity sensor includes the proton conductor Nafion®, shown in Figure
35
Electrochemical devices
15, which acts as the electrolyte. Nafion® is known to change its internal ion
conductivity upon exposure to humidity70, as described in chapter 2.2.2. In Nafion®,
cationic
species,
specifically
protons,
account
for
the
ionic
conductivity.
Electrochemical switching of the transistor channel is dependent on these cations
and their rate of migration through the Nafion® material. As the water content in the
Nafion® film increases, the activation energy barrier for transport of cations between
the inverted micelles decreases. This results in an increase in ion conductivity of the
Nafion®.49 When the relative humidity is increased from 25% to 80%, the ion
conductivity is increased by several orders of magnitude. In the humidity sensor
based on OECT and Nafion®, this is read out as a decrease of IDS from 0.4mA to 4µA
at VDS=2V and a gate voltage of 1.2V applied for 20s; see Figure 21. In the range of
40-80% relative humidity, the drain-source output response of the device displays
almost a perfect exponential behaviour vs. relative humidity. The time required for IDS
to saturate after a step change in RH from 30 to 80% is approximately 20s; see
Figure 21b. The electrochemical transducer described here, made entirely of organic
materials, can be manufactured by printing technologies on substrates such as
plastic or polyethylene-coated fine paper. Therefore, it is a good candidate for in
expensive and single-use sensors. One drawback with using the four-terminal
transistor for sensor application is its impracticality in circuits due to the floating gate.
If it is desired to incorporate the sensor in circuits, the three-terminal device is
preferable.
36
Electrochemical devices
Figure 21. a) IDS as a function of relative humidity after the gate voltage has been applied for
15s. b) Time response when the sensor is transferred from 30-80% humidity. At (a) the gate
voltage (1.2V) is applied (30% humidity), (b) the sensor is transferred to 80% humidity and at
(c) a gate voltage of zero is applied. The measurements were done at 25°C.
4.5 Characterisation of materials
Electrochemical transistors can be used for characterisation of redox active
materials by using the set-up described in Figure 20.8,55 Measuring the potentialdependent changes of electrical conduction in materials represents one way of using
electrochemical transistors as an analytical tool. The conductivity changes are
measured by applying a small voltage, typically 10-100mV, between source and
drain and measuring the current. Ofer et al. have used this method to analyse the
conductivity versus gate potentials for polythiophenes, polypyrroles and polyaniline.8
They found that a finite window of high conductivity is typical for conducting
polymers. As the gate voltage is increased to a level outside the window, the
conductivity decreases.
Polyaniline demonstrates reversibility when oxidised to a high degree where
PEDOT becomes “overoxidised”71 at anodic overpotentials. Moreover, the potential
region of high conductivity is always associated with a region in which changes in
potential are accompanied by a charge or a discharge of the polymer, i.e. high
conductivity is found at potentials where the polymer is redox active.
37
Electrochemical devices
It is also possible to analyse how a redox active material responds to different
chemical stimuli. One example is the electrochemical transistor shown by Bélanger et
al. which turns on when two chemical criteria are met in combination.72 The polymer
used was poly(4-vinylpyridine), (4-Vpy). When pH in the electrolyte is low enough to
allow protonation of (4-Vpy) to (4-VpyH+) and an anionic redox couple, e.g. Fe(CN)63/4-
, is electrostatically bound to the polymer, the two critical criteria are met and the
transistor turns on.
4.6 Circuits
This chapter is divided into two parts describing a smart pixel circuit for displays
and logic circuits based on the OECT.
4.6.1 The OECT smart pixel
The OECT can be combined with an electrochemical display cell to form an
electrochemical smart pixel; see Figure 22. The transistor acts a switch for, in this
case, an electrochromic display element. When the transistor channel is open (low
impedance) the display cell can be updated, and as the transistor is closed (high
impedance) the display cell keeps its previously assigned state. The transistors used
so far, and presented in paper 3, employ a lateral, four-terminal configuration. Smart
pixels based on the three-terminal transistor are presently being developed.
Figure 22. a) Schematic representation of the smart pixel consisting of three components:
the transistor (T), the display cell (D) and a resistor (R). VD and VG are biased by two
separate power sources. b) Schematic representation of the cross-point matrix design.
38
Electrochemical devices
The display cell has a vertical architecture (see Figure 23), including an opaque,
white, solidified electrolyte, leaving only the top layer of PEDOT:PSS available to
view. The bottom layer consists of the patterned transistor, patterned bottom
electrode of the display and the resistor, all defined on polyethylene-coated paper.
Figure 23. The smart pixel architecture is shown as a 3D image. The smart pixel consists of
three devices OECT (T), display (D) and resistor (R). The transistor is a lateral device while
the display is a vertical stacked device. The bottom layer consist of patterned PEDOT:PSS
on polyethylene coated paper. The spacer defines the area for the solidified electrolyte. The
top layer is a continuous film of PEDOT:PSS on polyester foil.
A thin plastic spacer with vias defines the area for the electrolyte, which are the
active areas of the transistor and the display cell. PEDOT:PSS coated on transparent
polyester foil is used as the top electrode of the display cells. When the display cell is
updated, the area of the top electrode that is in contact with the electrolyte is
reduced. The reduction of PEDOT turns it deep blue. By applying a gate voltage to
the transistor, the updating current through the display cell can be controlled.
Typically, operating voltages for both the transistor and the display cell are below 2V.
For a display cell with an area of 15×30mm2, the update through the transistors is
completed after approximately 5s at VD=2V. In order to prevent cross-talk between
the row and column conducting lines, both a resistor and an insulated line crossing
are needed. The resistors are printed with carbon paste and the line crossings
consist of a printed conductor-insulator-conductor three-layer structure, which forms
39
Electrochemical devices
a “conductor bridge” for each column. By combining several of these smart pixels, it
is possible to realise an addressable active matrix; see Figure 22b. For this set-up,
the matrix is updated row by row. A matrix of 4×10 display cells has been realised as
shown in Figure 24. Since both the transistor and the smart pixel can be created in
the same materials, only four patterning steps are required to manufacture the entire
display.
a)
b)
Figure 24. a) 4×10 active matrix display realised on polyethylene-coated paper. b)
Magnification of the letter “T” from the top figure displaying the logotype “ITN”.
4.6.2 Logic circuits
In logic circuits, transistors are used as signal switches just as FETs in modern
transistor logic. An open transistor has a low impedance state in the channel and
closed transistor has a high impedance state in the channel.
With the four-terminal transistor, it is not possible to make complex circuits
since the device has a floating gate. The three-terminal transistor, which has a
common ground, is possible to use in more complex circuits. Here, an EC-inverter,
ring oscillator, NAND and NOR gate, are described. In the circuits presented below,
transistors with a channel width of 0.5mm, channel length of 1mm and a gate to
channel area ratio of 25 are used.
40
Electrochemical devices
In logic circuits, inverters are used to convert the input signal according to the
simple function table seen in Table 3.
Table 3. Function table for the inverter.
Input
Output
1
0
0
1
The OECT operates as a depletion mode transistor, in the third quadrant, with
positive input. Therefore, a circuit shown in Figure 25a was used to achieve a true
inverter. The input is biased to the transistor gate, which leads to high input
impedance. The output is inverted at the drain side of the transistor resulting in a
negative output. The output signal is transferred to the interval between 0V and 1V
with a resistor network. This circuit requires two supply voltages, V+ and V-. We have
found that V+=3V and V-=-3V are suitable levels and R1=80kΩ, R2=60kΩ and
R3=20kΩ were found to be suitable resistor values. In the real circuit, the values of
the resistors might need to be changed since the OrgaconTM EL-350 has a resistivity
of 350Ω/□ and every conducting line represents an additional resistor. This is an
important criteria to consider in the design of circuits in polymer materials. When a 0V
input is applied, the output is determined by the resistors R1 and R2, which are
connected to ground through the transistor. Instead, if 1V is applied to the input, the
transistor switches to its high impedance state and the output is the voltage V+ and Vdivided by the resistors R1-3. The switching behaviour of the EC-inverter is shown in
Figure 25b.
41
Electrochemical devices
Figure 25. a) Circuit design for the inverter. b) Input and output signals.
One simple subsystem for demonstrating inverters is the ring oscillator. The ring
oscillator consists of an odd number of inverters coupled together in a ring as shown
in Figure 26. Ring oscillators can be used as a simple clock in digital circuits, or as a
transient characterisation tool for transistors in general.
Figure 26. Top, schematic of five inverters coupled together to form a ring oscillator. Bottom,
output voltage from one of the inverters.
42
Electrochemical devices
The logic family is completed with NAND and NOR gates. The boolean logic
represented by the NAND gate has the transfer characteristics given in Table 4. For
a NAND gate, if both inputs are high the output is low. Otherwise, the output is high.
Table 4. Function table for the NAND gate.
Input 1
Input 2
Output
0
0
1
1
0
1
0
1
1
1
1
0
By adding a transistor parallel to the transistor in the inverter (see Figure 27a),
the functionality shown in Table 4 can be achieved. In Figure 27b, the output signals
for the NAND gate are given. The notch seen at 20s appears because the two
transistors have different transient characteristics. The output changes slightly if only
one of the transistors is switched, since there is a small increase in the total
resistance over the parallel transistors.
Figure 27. a) Circuit design for the NAND gate b) In and out-signals for the NAND gate.
43
Electrochemical devices
A NOR gate has the functionality shown in Table 5. For a NOR gate, it is
enough if only one of the inputs is high in order to get a low output.
Table 5. Function table for the NOR gate.
Input 1
Input 2
Output
0
0
1
1
0
0
0
1
0
1
1
0
By adding a transistor in series with the transistor in the inverter (see Figure
28a), the NOR gate can be created. In Figure 28b the output signal from the NOR
gate is shown. A small difference in output is seen depending whether both or only
one transistor is switched.
Figure 28. a) Circuit design for the NOR gate b) In and out-signals for the NOR gate.
A push-pull amplifier based on two different conducting polymers has been
described by McCoy et al.73 They have used two different conducting polymers,
polyaniline and poly(3-phenylthiophene), which have conducting windows at different
44
Electrochemical devices
gate potentials. When polyaniline is oxidised, it becomes insulating. At the same
potential, poly(3-phenylthiophene) starts to become conducting. By using these
unusual ID-VG characteristics it was demonstrated that a push-pull amplifier is
achievable. This is an excellent example of how dedicated circuits can be
constructed out of electrochemical devices.
Circuits based on OECTs have one drawback compared to their field-effect
driven equivalents; they are slow! For some applications where low-speed is
acceptable and robust device architecture is demanded, electrochemical transistors
are excellent candidates. However, since the OECT uses both ions and electrons as
charge carriers, recording and regulation of systems using ions as signal carriers,
such as bio-systems, aqueous systems and electrochemical devices in general, is
possible. Combining this with EC-transistors, logic can be implemented, allowing
signal processing and communication with ion-based systems.
45
The experimental equipment
5. The experimental equipment
The devices presented here have been characterized in a manner similar to
traditional electronic components since the idea was to build electronic circuits with
them. The electrical characterization has therefore been done with a measurement
setup consisting of two Keithley 2400 source-meters in combination with a National
Instrument PCI data acquisition card or a HP 4155B parameter analyser. For optical
characterization a Lambda 900 optical absorption spectrometer was used. In order to
achieve a controlled atmosphere at the humidity sensor measurement a Challenge
160 environmental chamber was used.
46
Outlook for the future
6. Outlook for the future
The OECT is an interesting area of electronics since both electrons and ions
are used as charge carriers. With these devices, communication between traditional
electronics (electrons as charge carriers) and bio-systems (ions as charge carriers) is
feasible. Therefore OECT, logic can act in close proximity to the very ion-electron
transducer interface, along the Helmholtz layer of an electrolyte-electrode interface,
and in-between classical electronics and bio-systems.
Because no critical dimensions are required and the utilised materials are
soluble in common solvents, common printing techniques can be used to produce
low-voltage operating electronics on non-planar and flexible substrates. This means
that new functionality can be added onto packages, billboards, labels, etc.
Compared to traditional electronics, these devices have little or no impact on
the environment. These materials can be recycled in the same way as common
plastic and paper are recycled today, in contrast to traditional electronics, which
require special care in recycling.
47
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