Download Controlling ion transport in organic devices Xiaodong Wang (⊚ᲃḻ)

Controlling ion transport
in organic devices
Xiaodong Wang (⊚ᲃḻ)
Norrköping
2013
Controlling ion transport in organic devices
Xiaodong Wang
During the course of the research underlying this thesis, Xiaodong Wang was
enrolled in the graduate school Agora Materiae, a multidisciplinary doctoral
program within material science at Linköping University, Sweden.
Linköping Studies in Science and Technology. Dissertation, No.1536
Copyright ©, 2013, Xiaodong Wang, unless otherwise noted
Printed by LiU-Tryck, Linköping, Sweden, 2013
ISBN: 978-91-7519-547-6
ISSN: 0345-7524
In memory of my uncle, Shengwu Wang
Abstract
Organic electronics and printed electronics have been attracting more and more
research interest in the past decades. Polymers constitute an important class of
materials within the field organic electronics due to their unique physical and
chemical properties. One great benefit of the polymers is their solution
processability, which provides us the possibility to utilize conventional printing
techniques to fabricate devices on flexible substrates.
This thesis focuses on utilizing and controlling the ion transport in
polyelectrolytes in electronic devices for different applications. A polyelectrolyte
is a polymer in which the polymeric backbone includes ionic sites compensated
by counter ions.
Firstly, we have used a specific property of the polyelectrolyte: its electric
polarization is strongly dependent on the humidity level. The ions are screened
by water molecules; this improves the mobility and dissociation of ions. A
polyelectrolyte-based capacitor is thus ideal to sense humidity. Such a capacitor
is integrated into an LC resonant circuit possessing a specific resonant frequency.
The wirelessly detected resonant frequencies of the sensing circuit indicate the
corresponding humidity levels. With the appropriate choice of materials, the
complete sensing circuit (resistor, capacitor, capacitor-like sensor head) can be
screen-printed on an antenna manufactured using a roll-to-roll dry phase
patterning technique.
Secondly, we have modified the polarization characteristics of ions in a
polyelectrolyte layer by trapping the ions in molecular macrocycles dispersed in
a polymer overlayer. The resulting remanent polarization is read out as a
hysteresis loop in the capacitance-voltage characteristic of a capacitor. The
strategy is further implemented in an electrolyte-gated organic transistor to
control its threshold voltage by applying defined programming voltages.
Although the lifetime of the “remanent” polarization is rather short, the concept
might be further improved to fit those of memory applications.
Finally, we take use of the ionic selectivity of a polyelectrolyte to stabilize the
operation of a water-gated organic field-effect transistor. The polyanionic
membrane is added onto the semiconductor channel to prevent small anions of
the aqueous electrolyte to penetrate into the p-channel semiconductor. Moreover,
the polyelectrolyte layer protects the semiconductor and thus strongly stabilizes
the shelf lifetime of those transistors. This improved version of the water-gated
organic transistor is a candidate for developing transistor-based sensors working
in, for instance, biological media.
Populärvetenskaplig sammanfattning
Organisk elektronik och tryckt elektronik har vuxit kraftigt som
forskningsområden under de senaste decennierna. Inom organisk elektronik är
polymerer en vanligt förekommande klass av material, på grund av deras unika
fysikaliska och kemiska egenskaper. Bland annat kan polymerer processas i
lösningsform (elektroniskt bläck), vilket gör det möjligt att använda
konventionella tryckmetoder (t.ex. screentryck) för att tillverka komponenter och
kretsar på flexibla substrat.
Denna avhandlings fokus är kontroll och utnyttjande av jontransport i
polyelektrolyter för olika typer av tillämpningar. Polyelektrolyter är material
bestående av polymerkedjor som innehåller elektriskt laddade kemiska grupper,
samt motjoner till dessa.
Den första tillämpningen bygger på egenskaper hos polyelektrolyter som gör att
deras elektriska polariserbarhet är kraftigt beroende av omgivande luftfuktighet.
Vattenmolekyler som tränger in i polyelektrolyten växelverkar med jonerna och
bidrar till att öka deras rörlighet. Därför är polyelektrolytbaserade kondensatorer
ideala att använda som fuktsensorer. De kan integreras i en LC-resonanskrets för
att åstadkomma en komponent som har en resonansfrekvens som varierar
beroende på luftfuktighet. Frekvensen kan mätas trådlöst, ungefär på samma sätt
som när man detekterar stöldskyddsetiketter i butikslarm. Med genomtänkta
materialval kan en hel fuktsensor screentryckas ovanpå en förtillverkad antenn på
ett plastsubstrat.
Vidare har vi använt en speciell klass av molekyler, ”macrocyles”, för att fånga
in joner i en polyelektrolyt och på så sätt modifiera polarisationsegenskaperna.
Detta leder till en kvardröjande polarisation som kan detekteras genom elektriska
mätningar och därigenom fungerar som minne. Samma strategi är även
användbar i transistorer med polyelektrolyt som gateisolator, där
tröskelspänningen kan kontrolleras genom att förprogrammera materialet.
Livslängden hos den kvardröjande polarisationen är relativt begränsad, men
konceptet bör trots det kunna vidareutvecklas för att möjliggöra billiga
minnestillämpningar.
Slutligen har vi använt polyelektrolyternas jonselektivitet för att stabilisera en
särskild typ av organiska transistorer, där styrspänningen verkar genom ett lager
av vatten. Här används ett membran med polyelektrolytegenskaper för att skydda
halvledaren från små joner från vattnet, vilket stabiliserar de elektriska
egenskaperna och även bidrar till att öka livslängden hos lagrade transistorer.
Denna förbättrade transistor är en intressant kandidat for utveckling av sensorer
som kan användas i biologiska system.
Acknowledgements
Without the support and help from many people around me, it would not been
possible to finish this doctoral thesis. I would like to express my sincere gratitude
to:
Magnus Berggren, my supervisor, for offering me the opportunity to work in
the Organic Electronics group and all your encouragements, inspirations and
support. ‘Always be positive and optimistic.’
Xavier Crispin, my co-supervisor, for introducing me to polymers, transistors
and chemistry; for your encouragements, help and guidance in my experiments.
Isak Engquist, the project leader, for all the collaborations and support in the
projects and your help and guidance in my experiments.
Sophie Lindesvik, our administrator, for all the help in administrations.
Lars Gustavsson, our lab engineer, for knowing everything in the lab and all the
support and help.
All the co-authors of the included paper, especially, Oscar Larsson and Ari
Laiho, for all the valuable scientific discussions and help in the experiments.
The entire Organic Electronics (Orgel) group:
Daniel for sharing your knowledge in webpages and computers; Erik and Malti
for taking care of my plants and fruits; Amanda, Björn, Henrik, Hiam, Jesper,
Jun, Loïg, Negar, Olga and Zia for organizing great activities in Norrköping;
Klas and Kristin for interesting topics and fun conversations during ‘fika’; Hui,
Jiang and Yu for all scientific discussions in Chinese and all your help; Anders
and Lars for answers to tricky processing issues; Elina and Simone for being my
new ‘neighbors’.
Brains and Bricks, research center in Linköping University for high-technology
constructions, for financing and support my research work; special thanks to
Hans Olson from O&P consulting AB and Lars Gutwasser from PEAB AB for
hosting and coordinating the projects.
All people from Acreo; especially Duncan Platt for teaching me the knowledge
in RF electronics; Mats Sandberg, Jessica Åhlin, Marie Nilsson, Lars-Olov
Hennerdal and Staffan Nordlinder for discussing and solving problems in
printing.
Jinlan Gao and Johan Sidén from Mid Sweden University for great
collaborations in the Fuktsensor project.
Mats Thunell at Invisense AB for supporting the Fuktsensor project.
In the end, I would like to thank Jia Tan, my love, for her endless love and
support; My father and mother, Shengxian Wang and Meibao Xu, for always
being there and taking care of everything.
List of included papers
Paper I:
Proton motion in a polyelectrolyte: A probe for wireless humidity sensors
Oscar Larsson, Xiaodong Wang, Magnus Berggren and Xavier Crispin
Sensors and Actuators. B, Chemical, 2010, 143(2), 482-486
Contribution: Half of the experimental work and characterization of the
measurements.
Paper II:
An all-printed wireless humidity sensor label
Xiaodong Wang, Oscar Larsson, Duncan Platt, Staffan Nordlinder, Isak Engquist,
Magnus Berggren and Xavier Crispin
Sensors and Actuators. B, Chemical, 2012, 166-167, 556-561
Contribution: Design of device and all experimental work. Wrote the first draft
and was involved in final editing of the manuscript.
Paper III:
Printed low loss capacitors for use in a wireless humidity sensor label
Xiaodong Wang, Duncan Platt, Xavier Crispin, Isak Engquist and Magnus
Berggren
Manuscript
Contribution: All experimental work. Wrote the first draft and was involved in
final editing of the manuscript.
Paper IV:
Remanent polarization in a cryptand-polyanion bilayer implemented in an
organic field effect transistor
Xiaodong Wang, Ari Laiho, Magnus Berggren and Xavier Crispin
Applied Physics Letters, 2012, 100(2), 023305
Contribution: All experimental work. Wrote the first draft and was involved in
final editing of the manuscript.
Paper V:
Improving the stability of water-gated organic transistors for sensing
applications
Xiaodong Wang, Xavier Crispin and Magnus Berggren
Manuscript
Contribution: All experimental work. Wrote the first draft and was involved in
final editing of the manuscript.
Related work not included in this thesis
Moisture sensor
European Patent: EP2275806, 2011
United States Patent: 8236164, 2012
Table of Contents
1.
2.
Introduction..................................................................................... 1
1.1.
Organic electronics and printed electronics ............................... 1
1.2.
Motivations and goals of this thesis ........................................... 2
Materials .......................................................................................... 5
2.1.
Semiconducting polymers .......................................................... 5
2.2.
Charge carriers and transport in conjugated polymers ............... 8
2.3.
Insulating polymers .................................................................. 10
2.4.
Polyelectrolytes and ion exchange membranes ....................... 15
2.4.1.
Polyelectrolytes ................................................................ 15
2.4.2.
Electric double layers at a metal/electrolyte interface........ 17
2.4.3.
Nafion ............................................................................... 17
2.5.
3.
Crown ethers ........................................................................... 19
Electronic and ionic devices ........................................................ 21
3.1.
Passive components: resistors, capacitors and inductors ........ 21
3.2. Frequency-dependent impedance in an alternating current
(AC) circuit ......................................................................................... 22
4.
5.
3.3.
Mutual inductance and reflected impedance ............................ 23
3.4.
Resonance in an LC circuit ...................................................... 25
3.5.
Quality factor in RLC resonant circuits ..................................... 29
3.6.
Passive sensor labels .............................................................. 30
3.7.
Transistors ............................................................................... 32
Rheology and surface wettability ................................................ 38
4.1.
Rheology ................................................................................. 38
4.2.
Surface wettability ................................................................... 40
Device manufacture and characterization .................................. 43
5.1.
Printing .................................................................................... 43
5.1.1.
5.2.
Dry phase patterning ............................................................... 47
5.3.
Spin coating ............................................................................. 48
5.4.
Photolithography...................................................................... 49
5.5.
Impedance spectroscopy ......................................................... 50
5.5.1.
5.6.
6.
Screen printing ................................................................. 44
Randles Model.................................................................. 52
Humidity .................................................................................. 53
Conclusions and outlook ............................................................. 55
References ........................................................................................... 58
Background
1. Introduction
1.1. Organic electronics and printed electronics
Natural polymers, such as rubber, had been used for many centuries before
recognized as polymers. Hermann Staudinger, a German chemist, firstly
proposed the hypothesis of polymers in 1920. Polymers are macromolecules
which are made up of covalently bonded elementary units, called monomers.
This hypothesis was gradually accepted during 1920s. Staudinger was awarded
the Nobel Prize in Chemistry in 1953, "for his discoveries in the field of
macromolecular chemistry" [1]. Today polymers can be found almost
everywhere in our daily life. Because of their unique mechanical and thermal
properties and since they are inexpensive, polymers have replaced many other
materials like metals, woods, ceramics, textiles, etc. Moreover, in many
electronic products, polymers are extensively used as the electrically insulating
material. This view, i.e. polymers are electrical insulators, was not changed until
1970s. In 1976, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa
demonstrated that the conductivity of polyacetylene can be enhanced by several
orders of magnitude by exposing it to iodine vapors. They were jointly awarded
the Nobel Prize in Chemistry in 2000, "for the discovery and development of
conductive polymers" [2]. Conductive polymers became the cornerstone for a
new type of electronics: organic electronics. Among the electronic devices
developed, one can mention polymer-based sensors [3, 4], organic light emitting
diodes [5, 6], organic photovoltaic devices [7, 8], organic thin-film transistors [911], organic thermoelectric generators [12, 13], etc. A vast array of applications
can be targeted thanks to the versatility of the organic electronic polymers.
Indeed, with chemical synthesis, there is a multitude of possibilities to
functionalize an electronic polymer; such that their physical (optical, electrical)
and chemical properties can be tailor-made for specific applications and features
[14].
Besides some similarities in their electrical and optical properties with metals and
inorganic semiconductors, the conductive and semiconducting polymers possess
both the mechanical properties of plastics and their unique processing advantages
[15, 16]. Electronic devices can be fabricated from solutions and without high
temperature treatments. As a consequence, organic electronics can be
manufactured at a relatively much lower cost than traditional electronics.
1
Organic electronics is now one of the corner stones of the so-called printed
electronics technology. This terminology underlines the process used to
manufacture electronic devices: printing methods including screen printing,
flexography printing, gravure printing, offset printing and inkjet printing, etc.
The inks for printed electronics are typically made from solution-based materials,
especially conductive polymers, organic semiconductors, inorganic particles with
organic vehicles (organic binders), etc. Compared to conventional electronic
fabrication methods, using printing techniques to manufacture electronics enables
a simple and high-volume production process at very low cost. This makes
printed electronics a promising candidate in many cost-sensitive applications [17].
1.2. Motivations and goals of this thesis
Moisture is one of the largest concerns for the construction and building industry
today. In Sweden alone, the estimated annual cost for combating moisture-related
problems in houses and buildings exceeds 300 million Euros. A significant part
of this cost arises because those leakage problems are detected too late, when
mould damage has already occurred. Further, the present cumulated value of
these escalating problems is estimated to exceed 10 billion Euros [18]. Hence,
monitoring moisture variations in construction materials and buildings is
necessary and highly desirable. Today, accurate humidity measurements inside
walls and floors in buildings necessitate physical damages on the building. For
instance, a hole is drilled in the wall, and the humidity sensor is then inserted
through the small opening to reach inside the wall. The sensor is then connected
to the detecting electronics by cables. Wireless RFID sensors are another option
[19, 20] if they are positioned in the wall during its construction. Reading can be
realized wirelessly at any time without damaging the wall. The drawback is the
high cost and complicated manufacturing processes for the RFID technology,
which prevents distributing such RFID sensors on every construction material.
In this thesis, one of my goals has been to combine wireless reading and low
manufacturing cost for remote humidity sensors. Our approach is to use screen
printing technology and low-cost organic materials. The sensor label is passive
and can be readout wirelessly. Thus, it is aiming for usage-friendly and low-cost
applications in the construction industry and elsewhere.
2
Paper I, II and III are the results of an application-orientated research project
initiated by Brains & Bricks, a research center for high-technology constructions
and subsequently supported by the Swedish Research Council FORMAS.
After utilizing the humidity-controlled migration of ions in polyelectrolytes, the
second aim of this thesis is to control and manipulate the ion transport in order to
provide new functions in organic electronic devices. One such function is to use
the polarization of ions in organic electronics for memory applications. So far,
the most successful approach to achieve memories, in the community of printed
organic electronics, is to use ferroelectric polymers [21]. The ferroelectric
polymer chains can align their dipoles in a collective manner and possess a
remanent polarization. An electric field opposite to the polarization direction of
these dipoles of the ferroelectric layer does not switch the dipoles. This is
however only true for an electric field below a certain threshold [22, 23].
Ferroelectric capacitors can be introduced in 2D matrices including many
ferroelectric elements [22, 23]. Such memory elements are typically difficult to
address in a large array. This issue can be solved by introducing the memory
function of the capacitor in a transistor structure [24, 25]. One of the drawbacks
of such transistors is the high operating voltage since it is a challenge to maintain
proper ferroelectric properties also for very thin ferroelectric layers. New
concepts have been explored and demonstrated to reduce the operating voltage in
transistors down to <1 V by using a polyelectrolyte layer as highly polarizable
insulator [9]. The polarization of the polyelectrolyte involves the rearrangement
of ions in electric double layers of high capacitance. In Paper IV, we investigated
the possibility to trap ions from a polyelectrolyte layer to a trapping layer. Our
goal was to generate a remanent polarization within the insulating layer, similar
to the remnant polarization found in ferroelectrics and investigate the potential
for capacitive and transistor memories in organic electronics.
In addition to combining a memory effect with an electrolyte-gated organic
transistor, a water-insoluble polyanion is also used and investigated in order to
improve the shelf lifetime and operation lifetime of a water-gated organic
transistor. A few liquid-gated organic transistors have been demonstrated for
biological sensing applications. Most of them are operating in the
electrochemical mode [26-28]. Although a water-gated organic field-effect
transistor is also an attractive candidate for biological sensors [29, 30], the
stability of the water-gated organic field-effect transistor is rather poor. The main
problem is that the water-gated organic field-effect transistor suffers dramatically
3
from degradation in ambient conditions [31]. For example, photodegradation
usually occurs in organic semiconductors when they are directly exposed to air
and light [32]. Moreover, when an electrolyte is in contact with the
semiconducting channel of the transistor, small ions from the electrolyte start to
penetrate into the semiconductor through weak Van der Waals bonds between
organic semiconductor chains [33]. In Paper V, we attempted to improve the
stability of water-gated organic transistors by including an ion exchange
membrane layer that prevents the fast photodegradation and keeps ions in the
liquid electrolyte from reaching massively the organic semiconductor channel.
4
2. Materials
2.1. Semiconducting polymers
Atoms, the basic units of matter, consist of positively charged nuclei surrounded
by a cloud of negatively charged electrons. The electrons occupy atomic orbitals
where they are most likely to be found in space. These atomic orbitals represent
the only allowed states for electrons owing to quantum mechanics. Each atomic
orbital can only accommodate two electrons of different spin states (spin-up Ĺ
and spin-down Ļ).The atomic orbitals are arranged in shells around the nucleus
due to their different energy levels, characterized by the principle quantum
number n = 1, 2…. A shell comprises n2 individual orbitals, which are denoted as
s, p, d, f…. Since organic semiconductors are based on light and abundant
elements such as carbon, oxygen, sulfur, nitrogen, s and p orbitals are the two
most important orbitals. The real part of the wavefunction of the orbital is
illustrated in Figure 2.1.
Figure 2.1 Illustrations of the atomic (a) s orbital and (b) p orbital.
When two atoms A and B approach each other, the electrons of atom A in the
outermost shell, which are called valence electrons, start to sense the attraction
from the nucleus of atom B, such that their wavefunctions are modified and
extend over the two nuclei. Those wavefunctions are called molecular orbitals. In
a first approximation, the molecular orbital can be represented as the
superposition, the linear combination, of the two atomic orbitals (LCAO). Since
the combination of the atomic orbitals is either constructive (higher electron
density between two nuclei) or destructive (lower electron density between two
nuclei), the original atomic orbitals split into two molecular orbitals, a bonding
5
one and an anti-bonding one, shown in Figure 2.2. The bonding orbital has a
lower energy level than the two originally individual atomic orbitals. It is
therefore more energy-favorable for the valence electrons to arrange themselves
into the bonding orbital to form a stable molecule. However, the anti-bonding
orbital has a higher energy level which destabilizes the molecule. If the formed
molecular bonding is cylindrically symmetric around the internuclear axis, the
bond is termed a bond. But a bond has no cylindrical symmetry around the
internuclear axis and has two lobes of electron density separated by a nodal plane.
and
are denoted as the corresponding anti-bonding molecular orbitals.
Figure 2.2 Two 1s atomic orbitals overlap and form a bonding (ı)
molecular orbital and an anti-bonding (ı*) molecular orbital.
Carbon is the most important element in organic materials. A carbon atom has six
electrons filling in its ground-state 1 2 2 2 .The configuration suggests
that only two bonds can be formed by a carbon atom, not four. To explain the
existence of methane (CH4) which has four equivalent C-H bonds, a notional
promotion is required to excite a 2 electron to a vacant 2 orbital resulting in a
configuration of 1 2 2 2 2
with four unpaired valence electrons.
Moreover, the hybridization of orbitals is explained for identically covalent
and
stand for three different
bonds found in carbon compounds. ,
types of hybrid orbital in which the 2 orbital interferes with one, two or three of
2 orbitals respectively. For example, the carbon atom in methane forms four
6
bonds with neighboring hydrogen atoms where the angle between
identical
these hybrid bonds is 109.47° [34].
A polymer consists of a large number of constitutional repeating units which are
linked by covalent bonds. The constitutional repeating units are called monomers.
Both the nature of the monomer and the configuration (stereostructure) of the
polymer influence the properties of a polymer. In a sense, the simplest carbonbased polymer is polyethylene, which comprises only carbon and hydrogen
atoms. Each carbon atom in polyethylene has four
hybrid orbitals. Two of
hybrid orbitals form bonds with two neighboring carbon atoms and
these
the remaining two are bonded to two hydrogen atoms. Since the bonds are
strong and localized, polyethylene is electrically insulating. Unlike polyethylene,
semiconducting polymers, also known as conjugated polymers, are composed of
alternating single and double carbon-carbon bonds. Trans-polyacetylene is a
typical conjugated polymer, in which each carbon atom possesses three
orbitals forming bonds in the backbone and one 2 orbital forming bonds
perpendicular to the plane of the backbone. The bonds are relatively weak and
they can overlap along the backbone. Consequently, the electrons in 2 orbitals
are delocalized to a great extent along the polymer chain. Those electrons can be
removed or displaced without breaking the polymer chain since its structure is
maintained by the bonds. As mentioned in the LCAO model, 2 orbitals start
anti-bonds when the number of carbon atom
to split up as bonds and
increases in the polymer chain. For a very long polymer chain, the energy
difference among these splitting (or ) energy levels is considerably small
which can be treated as continuous bands in analogy to band structures in
inorganic semiconductors. There are two important energy levels in the and
bands, i.e. the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO). For example, in trans-polyacetylene, the
HOMO level locates at the top of the band (the band is filled) and the LUMO
level is at the bottom of the
band (the
band is empty). Assuming that all
the 2 orbitals between adjacent carbon atoms equally overlap, i.e. all carboncarbon bonds have the same length, the energy difference between HOMO and
LUMO vanishes and the polymer should behave as a one-dimensional metal.
However, a one-dimensional metal is energetically unstable as explained in
Peierls’ theorem [35]. The stabilization effect known as Peierls’ distortion is
visualized as a decrease of the polymer chain symmetry through the alternation
of the carbon-carbon bonds, one with single bond character (long) and the
7
adjacent with double bond character (short). The structural distortion can
stabilize the polymer since the occupied level at the top of the valence band
becomes stabilized. This leads to a certain band gap ( ) between HOMO and
LUMO. The band gap is typically found in the range from 1.5 eV to 3 eV [36].
Figure 2.3 shows the evolution of the energy levels when the size of the
conjugated molecule increases leading to the band diagram in transpolyacetylene.
Figure 2.3 Energy levels and electronic bands in trans-polyacetylene.
2.2. Charge carriers and transport in conjugated polymers
Intrinsic (undoped) conjugated polymers have low electrical conductivities,
usually in the range from 10-10 to 10-5 S/cm due to their large band gap between
HOMO and LUMO, which results in few free charge carriers. The electrical
conductivity can be enhanced by several orders of magnitude by introducing
more charge carriers in the polymer through either chemical doping [37] or
electrochemical doping [38].
Unlike charge carriers (electrons or holes) in inorganic semiconductors, charge
carriers in most organic semiconductors (or conducting polymers) are polarons.
Polarons are treated as introduced charges accompanied by a delocalized
distortion of the electronic and geometric configuration over just a few
monomers in the polymer chain. Polarons are in the quinoid form, a state that is
8
less stable than the aromatic form. As a result, a polaron is characterized by
localized energy levels in the middle of the band gap. Two polarons can combine
and form one bipolaron which sometimes is more energetically stable. Polarons,
bipolarons in polythiophene and their associated band diagrams are shown in
Figure 2.4.
Figure 2.4 The polaron and bipolaron in polythiophene: (a) neutral state;
(b) positive polaron; (c) positive bipolaron; (d) the band diagrams for
polarons and bipolarons.
In organic electronic devices, charge carriers need to travel not only within
individual molecules (intramolecular transport) but also in between adjacent
molecules (intermolecular transport). In amorphous and semicrystalline materials,
9
such as conjugated polymers, charge transport is usually characterized by
thermally activated hopping. However, in highly ordered organic materials,
charge transport takes place in a band-like motion as the charge carriers are
delocalized. The charge carrier mobility varies from 10-6-10-3 cm2/V·s in
disordered organic polymers [39, 40] to 10-102 cm2/V·s in highly ordered organic
single crystals [41, 42].
2.3. Insulating polymers
Unlike metals that have a ‘sea of electrons’ which allows electrons to move
freely under an electric field to conduct a current, electrons in an insulating
polymer are bounded closely to nuclei of the polymer chains through covalent
bonds and thus the polymer becomes electrically insulating. With covalent
bonds, the polymer can be semiconducting as described in section 2.1, while with
exclusively bonds, the band gap of the polymer is very large and the polymer is
thus an electrical insulator. The dielectric properties of insulating polymers are
useful in many applications, such as in capacitors, electrical encapsulation,
switches, etc.
The dielectric constant and the dielectric loss are two important parameters
characterizing insulating polymers in an alternating current (AC) circuit.
Polymers with large intrinsic electric dipole in their monomer unit are polar
polymers. Polar and non-polar polymers polarize differently under an alternating
electric field.
If electrons in a covalent bond are shared equally with two bonding atoms,
leading to a symmetrical electron distribution in the bond, such a bond is nonpolar, exemplified by the C-C bond in ethane. But due to the difference in
electronegativities (EN) of atoms, which characterize the intrinsic ability to
attract the electrons, the bonding electrons in a covalent bond tend to be attracted
more strongly by one atom than the other. The asymmetrical distribution of
electron density results in a polar bond. The most electronegative atom is fluorine
(EN=4) and the least is cesium (EN=0.7). The values of electronegativity for
some elements are summarized in Table 2.1.
10
element
electronegativity
F O N Cl Br
I
S
C H
4.0 3.5 3.0 3.0 2.8 2.5 2.5 2.5 2.1
Table 2.1 The values of electronegativity for some elements [43].
As a rule of thumb, if the electronegativities between two bonding atoms differ
less than 0.5, the bond is treated as nonpolar bond. If the difference in
electronegativities is between 0.5 and 2, the bond is polar. In an ionic bond, the
electronegativities between two atoms differ by more than 2. For instance, C-H
bond is non-polar while C-O and C-N bonds are polar.
In addition to the bond polarity, the structure of a molecule also determines if it is
polar or non-polar. The molecular polarity is the vector sum of all individual
bond polarities and lone-pair contributions in a molecule. If the center of all
positive charges in a molecule does not coincide with the center of all negative
charges in that molecule, the molecule can be considered as a dipole. The dipole
moment is a measure of net molecular polarity, which is defined as
=
×
(2.1)
where [C·m] is the dipole moment, [C] is the charge at either end of the
molecular dipole and [m] is the distance between the charges in the dipole.
For example, polytetrafluoroethylene (PTFE) is treated as a non-polar polymer.
This is due to that the dipole moment of PTFE is zero because of the symmetry in
its molecule structure, although C-F bonds in the polymer chain is largely polar.
Polyethylene (PE), polypropylene (PP), polystyrene (PS) are non-polar as well
but poly (methyl methacrylate) (PMMA), polyvinyl chloride (PVC),
polycarbonate (PC) have permanent dipole moments. Figure 2.5 shows their
chemical structures. Table 2.2 gives the values of dipole moment for some
compounds.
11
Figure 2.5 Chemical structures of some insulating polymers: (a)
polytetrafluoroethylene (PTFE); (b) polyethylene (PE); (c) polypropylene
(PP); (d) polystyrene (PS); (e) poly (methyl methacrylate) (PMMA); (f)
polyvinyl chloride (PVC); (g) polycarbonate (PC).
Compound
Dipole moment
CH3Cl
1.87
H2O
1.85
CH3OH
1.7
CH4
0
C 6H6
0
Table 2.2 The values of dipole moment for some compounds [44].
The dielectric constant ( ) or relative permittivity ( ) represents how well a
material screens an external electric field. The dielectric constant is the ratio of
the capacitance of a parallel-plate capacitor including a dielectric material versus
the capacitance of the same capacitor with vacuum in between the plate
electrodes. The values of dielectric constant for several materials are listed in
Table 2.3.
12
material
dielectric constant
vacuum
teflon
polyethylene
polystyrene
polyvinyl
water
1
2
2.3
2.6
4-8.5
81
Table 2.3 The values of dielectric constant for some materials [45].
When an alternating electric field is applied to a parallel-plate capacitor,
molecules in the dielectric material are polarized, see Figure 2.6. These
molecules attempt to align themselves to the electric field accordingly. Several
mechanisms of polarization in a dielectric material are proposed, including
electronic polarization, distortion polarization and orientation polarization [46].
Electronic polarization refers to the distortion of electron distribution with
respect to the nucleus under an applied electric field. Distortion polarization
arises from the displacement of nucleus by an applied electric field. Both
electronic and distortion polarizations contribute to an induced electrical dipole
moment. Orientation polarization relates to the reorientation of a polar molecule
with a permanent electric dipole moment.
13
Figure 2.6 Illustration of the polarization of a parallel-plate capacitor
sandwiching a dielectric material and the electric potential profile across
the capacitor.
In polar polymers, the dipoles require a certain time (>0.1 ns) to align themselves
with the applied electric field [47]. Thus the dielectric constant of a polar
polymer is usually frequency-dependent with respect to the applied electric field.
At very high frequencies (>1010 Hz) the polar polymer does not have sufficient
time to align completely with the applied electric field before the electric field
varies its direction and the dielectric constant is low. Oppositely, if the applied
electric field alternates at lower frequencies (<1010 Hz), the dipoles in the
polymer have enough time to align with the electric field before the field changes
its direction. Polar polymers have higher dielectric constant at low frequencies.
However, non-polar polymers are typically much less dependent on the
frequency of the applied electric field because the electronic and distortion
polarizations in those polymers take place instantaneously when the direction of
electric field is changed.
As the
always
angle (
caused
14
dipoles try to align themselves with an alternating electric field, they
slightly ‘lag’ behind the electric field, which is described as the phase
) (see details in section 3.2), and energy is thus absorbed by the friction
by rotational motion of dipoles. The absorbed energy in molecular
polarizations is measured by the dielectric loss. The amount of dielectric loss in a
( )).
capacitor can be characterized quantitatively by the dissipation factor (
( )=
·
(2.2)
where [°] is called the loss angle which equals 90°
; [rad/s] is the angular
frequency;
[F] is the capacitance and
[ȍ] is the equivalent series
resistance in the capacitor.
For polar polymers, the energy dissipation is low when the applied electric field
has both very high (>1010 Hz) and very low frequencies (<<1010 Hz). This
because the dipoles either do not have sufficient time or have enough time to
align themselves before the electric field changes its direction. But at
intermediate frequencies the dielectric loss is large due to “lag” of polarization.
Non-polar polymers usually show much smaller dielectric loss as compared to
polar polymers. There is no permanent dipole moment in non-polar polymers and
meanwhile the electronic and distortion polarizations are effectively in phase
with the applied electric field, which gives very little dielectric loss in general.
2.4. Polyelectrolytes and ion exchange membranes
An electrolyte, a substance that dissociates into ions and transport ions, may be a
solution, a liquid or a solid material. Moreover, an electrolyte can be strong or
weak in terms of the degree of dissociation of the electrolyte. A strong electrolyte
can be almost entirely dissociated as ions, such as salts, strong acids and strong
bases. In contrast, a weak electrolyte is only partially dissociated as ions, like
weak acids and weak bases.
2.4.1. Polyelectrolytes
A polyelectrolyte is made up of a polymeric backbone covalently bonded with
repeating electrolytic groups which can be acids, bases or salts. The electrolytic
groups are able to dissociate into charged polymer chains and oppositely charged
counter ions in a polar solvent, such as water. Polyelectrolytes can be classified
into two types, polycations and polyanions, in terms of the chemical structure of
the polyelectrolyte. Polycations comprise positively charged polymeric
backbones and negatively charged counter ions. Conversely, polyanions are
composed of negatively charged polymeric backbones and positively charged
15
counter ions. In the solid phase, the charged polymeric backbones are treated as
effectively immobile since they are relatively much larger than the counters ions.
Therefore, the ionic conductivity of solid state polyelectrolyte is represented by
transport of the atomic or molecular counter ions. This unique feature of
polyelectrolytes is exploited in an electrolyte-gated organic field-effect transistor
in order to avoid possible electrochemical doping in the semiconducting channel
[9, 48]. Due to the counter ion condensation phenomena [49] in solid
polyelectrolytes, not all counter ions are dissociated from the polymeric
backbones and can be transported freely. The activation energy of the
dissociation strongly depends on the amount of solvent contained in the solid
polyelectrolytes. Polyelectrolytes are usually hydroscopic, because of their
polarity characteristics, and thus become promising candidates for humidity
sensing [50-54]. Based on the specific electrolytic groups, three major categories
of polyelectrolytes are used in humidity sensors; they are: quatermary ammonium
salts [55-57], sulfonate salts [52, 58-60] and phosphonium salts [61-63]. Figure
2.7 shows three typical polyelectrolytes.
Figure 2.7 Chemical structures of three typical polyelectrolytes: (a)
poly(diallyldimethylammonium chloride); (b) poly (styrene sulfonate)
sodium salt; (c) poly (vinyl phosphonic acid).
When the counter ions are dissociated from the polymeric backbones, solvent
molecules form a shell around mobile ions. Then, the ions can move together
with their solvent shells. The ionic charge transport in polyelectrolytes is mainly
attributed to the diffusion along concentration gradients and migration under
electric fields. During ion transport, dissociated ions experience frictional forces,
which are proportional to the viscosity of the solvent and the size of the solvated
ion.
16
2.4.2. Electric double layers at a metal/electrolyte interface
When a potential difference occurs at the interface of a metal electrode and an
electrolyte, the interface is charged. The charges in the metal electrode appear at
the outermost surface of the electrode. These charges are compensated by
oppositely charged ions in the electrolyte in the vicinity of the interface. These
two layers of charges at the interface are called an electric double layer (EDL).
The EDL structure is usually characterized by Goüy-Chapman-Stern (CGS)
model [64]. In this model, two different layers are considered in the electrolyte
[65]. One is the Helmholtz layer which contains solvent molecules, some
specifically adsorbed species (ions or molecules) and solvated ions. In analogy to
a parallel-plate capacitor, the solvated ions are just a few angstroms away from
the oppositely charged electrode. This results in an electric double layer
capacitance (EDLC) on the order of tens of ȝF per cm2 [9, 66]. Next to the
Helmholtz layer we find the diffuse layer that extends into the bulk electrolyte.
The illustration of the EDL and its electric potential profile is given in Figure 2.8.
Figure 2.8 Illustration of Goüy-Chapman-Stern model for electric double
layer and the electric potential profile at the interface between a metal
electrode and an electrolyte.
2.4.3. Nafion
17
Perfluorinated membranes have been extensively used in fuel cells as separators
over the last 50 years because of their excellent chemical and thermal stability.
Perfluorinated membranes are polyelectrolytes in the membrane phase that are
cation permeable. Unlike other conventional ion exchange membranes [67, 68]
which are made up of cross-linked polyelectrolytes, perfluorinated membranes
are electrolytic polymers with polytetrafluoroethylene (Teflon) backbones.
Similar to Teflon, perfluorinated membranes have an excellent chemical stability
and can be kept intact in most solvents. Two important commercially available
perfluorinated membranes have been established: Nafion [69] which is
perfluorosulphonic and Flemion [70] which is perfluorocarboxylic.
Nafion is a cation-conductive material, particularly for protons. Cations in Nafion
membrane can jump from one sulfonic acid site to another, whereas anions or
electrons cannot pass through the membrane. Therefore, Nafion possesses a
unique selectivity and high ionic conductivity for small cations [71].
Figure 2.9 Chemical structure of Nafion.
Figure 2.9 shows the chemical structure of Nafion. The morphology in a Nafion
membrane is characterized by ion clustering [72]. A cluster network is suggested
within the Nafion membrane, which contains inverted spherical micelles
interconnected by short narrow channels in the fluorocarbon backbone. These
inverted micelles are formed by hydrophobic fluorocarbon backbones and inner
surfaces containing sulfonate groups, protons and water. The high hydration of
Nafion membrane results in enlarging the size of the cluster (swelling), which
18
lowers the activation energy for transporting ions through the channels and
promotes the rate of ion transport [73]. Therefore, Nafion has been used as a
humidity sensitive material in many humidity sensors [3, 74, 75] because the
variation in the ionic conductivity of Nafion can indicate the change in humidity
level.
2.5. Crown ethers
Molecular recognition is the most fundamental concept in host-guest chemistry.
The importance of molecular recognition was realized as early as during the
nineteenth century when Pasteur observed that molds or yeasts can recognize and
utilize only one of the chiral isomers of tartaric acid. Later, a ‘lock and key’
mechanism [76] was proposed by Emil Fischer, who pointed out the structure
fitting between the recognizing molecule and the recognized molecule. In 1987,
Lehn, Pedersen and Cram were together awarded the Nobel Prize in Chemistry
for their tremendous contributions in host-guest chemistry (also known as
supramolecular chemistry), "for their development and use of molecules with
structure-specific interactions of high selectivity" [77].
Crown ethers were firstly discovered as artificial host molecules. Pedersen
accidentally synthesized the dibenzo 18-crown 6-ether during his experiments
and found its affinity to some specific metal ion [78]. He proposed that a
complex structure is formed where the metal ion is trapped in a cavity created by
the crown ether. The cyclic ether host binds the metal ion guest like ‘wearing’ a
crown. Therefore, such cyclic compounds are named as crown ethers. The name
of crown ether is expressed as m-crown-n where the “m” indicates the total
number of atoms in the ring and the “n” is the number of oxygen atoms in the
cyclic structure. Because of the high electronegativity of oxygen atoms in the
cyclic crown ether, ion-dipole interactions between ions and the inner cavity of
the crown ether contribute to the molecule recognition. Matching the size of ions
and the size of the cavity in the crown ether is very critical in order to achieve an
efficient binding. In other words, crown ethers with a specific cavity size can
only accommodate cations of similar size. In Table 2.4, various crown ethers and
their binding constants to metallic cations are summarized. The larger binding
constant indicates the higher binding capability between the host crown ether and
its guest cation.
19
15-Crown-5
18-Crown-6
21-Crown-7
(0.15-0.22 nm) (0.26-0.32 nm) (0.34-0.43 nm)
Na+ (0.194 nm)
+
K (0.266 nm)
+
Cs (0.334 nm)
3.32
4.28
2.12
3.5
5.67
4.3
2.74
4.5
5.01
Table 2.4 The values of the binding constant in methanol (log Ka) for
three different crown ethers [79]. (The numbers in the brackets represent
the diameter of a cation and the size of the cavity in a crown ether.)
Monocyclic host-like crown ethers, also called coronands, have relatively
flexible rings in their structure. The structural freedom of crown ethers leads to a
possible sandwich-type complexation. Two smaller crown ethers surround one
larger ion. This is typically a disadvantage of simple crown ethers to form highly
selective bindings. More rigid cryptands are thus synthesized. Cryptand is an
oligocyclic host. The arm in the middle across the cycle makes cryptands less
flexible than crown ethers and defines a three-dimensional binding cavity. So,
cryptands can thus only strictly bind size-matched guest ions and possess an
enhanced binding selectivity as compared to simple crown ethers. Figure 2.10
shows the chemical structures of 18-crown-6 ether and cryptand 2.2.2.
Figure 2.10 Chemical structures of (a) 18-crown-6 ether and (b) cryptand
2.2.2.
20
3. Electronic and ionic devices
3.1. Passive components: resistors, capacitors and inductors
A resistor is a piece of conducting material with two terminals. The electrical
resistance is defined as the ratio of the potential drop across a resistor versus the
current passing through it; this is Ohm’s law.
=
where [ȍ] is the resistance,
the current.
(3.1)
[V] is the voltage across the resistor and [A] is
The resistance of a material depends on both resistivity and geometric factors of
that material.
=
(3.2)
where [ȍ·m] is the electrical resistivity of material, [m] is the length of the
material and [m2] is the cross-sectional area of the material. The electrical
conductivity, ı [S/m], is the reciprocal of resistivity. The symbol and the
illustration of a resistor are given in Figure 3.1 (a).
A capacitor is a device capable of storing charges. Capacitance is the ratio of the
stored charges in a capacitor versus the voltage across it.
=
(3.3)
where [F] is the capacitance, [C] is the stored charges in the capacitor and
[V] is the voltage across the capacitor.
In its simplest form, the capacitor consists of a pair of parallel conducting plates
separated by an insulating layer which is commonly represented by a dielectric
material. The capacitance of a parallel-plate capacitor is expressed as
=
(3.4)
where
is the relative permittivity (also known as dielectric constant), is the
vacuum permittivity which equals 8.854×10-12 [F/m], [m2] is the area of each
and is
plate and [m] is the distance between two plates. The product of
also referred to as the permittivity of the dielectric material. Both the symbol of a
21
capacitor and the illustration of a parallel-plate capacitor are given in Figure 3.1
(b).
An inductor is a coil of a conducting wire wrapped either around an insulator or a
ferromagnetic material. When the current in the coil is changed, the magnetic
flux density surrounding the coil is varied correspondingly. Since the coil itself is
cut by the changing flux, Lenz’s law states that the polarity of the self-induced
voltage opposes the attempt to change the current flowing through it. The
inductance is defined as the ratio between the induced voltage and the change in
current running through the coil versus time.
= ( )
where
[H] is the inductance,
[V] is the induced voltage and
(3.5)
[A/s] is the
rate of change of current.
Both the symbol of an inductor and the illustration of a 6-turn inductor are given
in Figure 3.1 (c).
Figure 3.1 Symbols and illustrations of (a) a resistor; (b) a parallel-plate
capacitor; (c) an inductor.
3.2. Frequency-dependent impedance in an alternating current
(AC) circuit
An alternating current (AC) circuit is defined as an electrical circuit in which the
current periodically reverses its direction, or the voltage periodically reverses its
polarity, with a specific frequency ( ).
22
Unlike in a direct current (DC) circuit, the impedances of the passive components
in an AC circuit are complex numbers, designated as ( ). The is the angular
frequency that equals 2 .
The general form for the complex impedance is
( )
( )
( )=
=
( )+
( )
(3.6)
( )
where ( ) is the complex voltage and ( ) is the complex current. The
( ) are the real part and the imaginary part of the complex impedance
and
respectively. represents the imaginary number.
The phase angle is defined as
( )
( )
( ) = tan
(3.7)
For an ideal resistor, capacitor and inductor, the impedance can be expressed as
=
=
=
= 0°
(3.8)
=
(3.9)
90°
= 90°
(3.10)
3.3. Mutual inductance and reflected impedance
When two coils, a primary coil and a secondary coil, get close to each other and
one of the coils is sourced by an AC current, their magnetic fields interact and
magnetic coupling occurs. To quantitatively characterize the electromagnetic
coupling between these two coils, a mutual inductance is introduced [80].
The mutual inductance is defined as
=
1
(3.11)
[H]
where [H] is the mutual inductance, is the coupling coefficient and
[H] stand for the inductance of the primary and secondary coils
and
respectively.
If there is no leakage flux which escapes between the primary and secondary
coils, the coupling coefficient equals one.
23
As the result of mutual inductance coupling between the primary coil ( ) and
the secondary coil ( ), two equivalent voltage sources, ,
and , , are
introduced on both sides in the equivalent circuit, as given in Figure 3.2 (a).
,
=
(3.12)
=
(3.13)
,
where
and
is the current on the primary and secondary sides respectively.
By applying Kirchhoff’s voltage law, the equivalent circuit on the primary side is
described as
+
=
,
(3.14)
And on the secondary side is described as
+
=
(3.15)
,
Substituting Eq.3.12 and 3.13 into Eq.3.14 and 3.15 respectively,
=
(3.16)
=(
(3.17)
)
Inserting Eq.3.17 into Eq.3.16,
=
(
=
)
(
+(
)
)
is the total impedance on the primary side and
If
the secondary side, Eq.3.19 can be simplified as
=
+
(
)
=
+
(3.18)
(3.19)
is the total impedance on
(3.20)
where the reflected impedance is defined as
=
(
)
(3.21)
In the equivalent circuit, the reflected impedance effectively replaces the induced
voltage source on the primary side, as shown in Figure 3.2 (b).
From Eq.3.20, any changes of impedance on the secondary side cause impedance
variations on the primary side. The ‘impedance reflection’ phenomenon makes
24
passive wireless detection possible, which is utilized in work reported in the
Paper I, II and III.
Figure 3.2 Electrical equivalent circuits of (a) electromagnetic coupling
between a primary coil and a secondary coil and (b) reflected impedance
on the primary side.
3.4. Resonance in an LC circuit
If an ideal coil (an inductor) is connected with an ideal capacitor, electrical
resonance can be obtained. Such a circuit is called an electrical resonant circuit.
The resonant circuit can be as simple as just one inductor and one capacitor that
are connected together. Therefore, such a circuit is termed as an inductorcapacitor (LC) resonant circuit. Figure 3.3 displays ideal LC resonant circuits in
series and parallel configurations.
25
Figure 3.3 Electrical schematics of (a) a series LC circuit and (b) a
parallel LC circuit.
The total impedance in the series LC resonant circuit is expressed as
=
+
=
(3.22)
where [ȍ] is the total impedance of the series LC resonant circuit,
[ȍ] is the reactance of the capacitor.
the reactance of the inductor and
[ȍ] is
The total impedance in the parallel LC resonant circuit is expressed as
=
=
=
=
(3.23)
[ȍ] is the total impedance of the parallel LC resonant circuit, [S] is
where
[S] is the susceptance
the total admittance of the parallel LC resonant circuit.
of the inductor and [S] is the susceptance of the capacitor.
Electrical resonance is defined as when the applied voltage and the resulting
current are in phase [81]. Therefore, at resonance, the complex impedance of a
circuit consists only the real resistive part and the imaginary part is zero, which
means that the inductive reactance (susceptance) and the capacitive reactance
(susceptance) cancel each other in a series (parallel) circuit.
Hence, the resonant frequency of a series (parallel) LC circuit is given by
=
where
(3.24)
[Hz] is the resonant frequency.
Figures 3.4 (a) and (b) show the reactance versus angular frequency in a series
LC circuit and the susceptance versus angular frequency in a parallel LC circuit,
respectively. At resonant frequency , the total reactance (susceptance) is zero.
26
As the frequency varies, the total impedance reaches its minimum or maximum
value at resonance for a series or parallel LC circuit respectively, as shown in
Figure 3.4 (c). The frequency at the peak (valley) of the curve is referred as the
resonant frequency.
27
28
Figure 3.4 (a) The reactance versus angular frequency in a series LC
circuit, (b) the susceptance versus angular frequency in a parallel LC
circuit and (c) the total impedance versus frequency.
3.5. Quality factor in RLC resonant circuits
Electrical resonance is a very useful phenomenon that is utilized in many
electronic filters. Selecting desired frequencies and rejecting other unwanted
frequencies can thus be achieved by including LC resonant circuits in various
systems. The frequency selectivity depends on the quality factor (Q factor) of the
LC resonant circuit. A good selectivity requires a high quality factor.
The quality factor is defined as
=2 ×
(3.25)
From this definition, the Q factor can be considered as a measure of the
efficiency of a circuit.
The Q factor of an inductor is expressed as
=
where
(3.26)
[ȍ] is the equivalent series resistance (ESR) of the inductor.
Similarly, for a capacitor, the Q factor is given by
=
where
(3.27)
[ȍ] is the equivalent series resistance (ESR) of the capacitor.
In a series RLC circuit (see Figure 3.5 (a)), in which R is equivalent to the total
resistive loss, the Q factor at resonance is given by
=
where
=
=
=
(3.28)
[rad/s] is the resonant angular frequency.
Similarly, in a parallel RLC circuit (see Figure 3.5 (b)), the Q factor at resonance
equals
=
=
=
(3.29)
29
Figure 3.5 (c) shows the comparison of voltage versus frequency measured on
the resistor R in a series RLC circuit with different Q factors. Clearly, a high Q
factor results in a sharp voltage versus frequency curve, whereas a low Q factor
leads to a flat curve.
Figure 3.5 Electrical schematics of (a) a series RLC circuit and (b) a
parallel RLC circuit. (c) The curves of voltage versus frequency measured
on the resistor R in a series RLC circuit with different Q factors.
3.6. Passive sensor labels
Wireless passive sensors are desired in many applications since they provide
contactless readout, remote query capability and do not require an internal power
30
supply. The working principle of sensor labels, as the ones reported in the paper
II and III, is based on the variation of the resonant frequency of a LC resonant
circuit in response to the environmental parameter such as humidity. The LC
resonant sensor label consists of a planar inductor (antenna), tuning capacitors
and the sensors. The sensor label is wirelessly powered and interrogated by a
detecting antenna through the electromagnetic coupling between the sensor label
and the reader. The resonant frequency of the sensor label is determined by the
voltage spectrum from the reader circuit.
Wireless sensing is achieved through the phenomenon of reflected impedance
described in section 3.3. The reflected impedance obtained on the reader side is
always inversely proportional to the total impedance of the sensor label.
Therefore, any changes of the resonant frequency in the sensor label introduce a
corresponding shift of the voltage spectrum on the reader side. Figure 3.6 shows
the typical resonant curves of the printed humidity sensor label measured on the
reader side when the surrounding humidity varies. The resonant frequencies at
different humidity levels (wet, moderate humidity and dry) are read out from the
bottom of the valley respectively, which are expressed by the data values given in
the inset in Figure 3.6. The resonant frequency of the sensor label drops when the
humidity level increases, because of the increasing in the capacitance of the
sensor head.
31
Figure 3.6 The typical responding curves measured on the reader side at
different humidity levels. (Inset: resonant frequency versus Relative
Humidity.)
In addition to humidity sensing [82-84], LC resonant sensors are also found in
temperature sensing [85], biological monitoring [86-88], pressure detecting [8991], etc.
3.7. Transistors
Nowadays, transistors can be found in nearly all electronic products. As of 2012,
Xilinx has launched its Virtex-7 processors in which 6.8 billion transistors are
integrated in a single chip [92]. The first field-effect transistor was invented at
AT&T Bell Labs in 1959 [93]. In 1986, the first field-effect transistor, based on
an organic semiconductor, was reported by Koezuka et al [94, 95].
A field-effect transistor is essentially a resistor channel in combination with a
metal-insulator-semiconductor (MIS) capacitor. A thin-film transistor is
composed of a stack of a gate electrode, an insulating layer and a semiconductor
layer in direct contact with a source electrode and a drain electrode. The
separation between the source and drain electrodes defines the channel region
with a length L and a width W. Figure 3.7 illustrates the principle structure of a
thin-film transistor.
Figure 3.7 Illustration of a thin-film transistor with a channel length L and
width W.
32
If the source electrode is grounded ( = 0 V) which is common in transistor
( ) are designated as gate-source voltage (current)
configurations, ( ) and
is referred to as the threshold
and drain-source voltage (current) respectively.
voltage which is the minimum voltage needed at the gate to cause charge carrier
accumulation along the channel; thus the transistor is turned on. The threshold
voltage is attributed to both the flat-band voltage between the gate metal and the
semiconductor and filling of the charge traps at the insulator-semiconductor
interface [96].
Since the gate-insulator-semiconductor is usually considered as a capacitor
structure, the capacitance per unit area, , is defined as
=
(3.30)
[8.854×10-12 F/m] is the vacuum permittivity;
where
permittivity and [m] is the thickness of insulating layer.
is the relative
When the applied gate voltage ( ) exceeds the threshold voltage, either holes or
electrons are accumulated at the interface between the insulator and the
semiconductor. Thanks to this charge accumulation, a conductive path (channel)
is formed between the source and drain electrodes. Since the conducting channel
is confined at the first monolayer right next to the insulator-semiconductor
interface, by varying the gate voltage results in a significant change of
conductance along the entire channel. Hence, the drain-source current is
modulated by the gate voltage.
If a potential is applied to the drain electrode and the source electrode is
grounded, a potential gradient is established between the drain and source
electrodes. This potential gradient can be expressed by a function of its position ,
which varies between 0 and L. The charge density at any position along the
channel can be written as
( )=
where ( ) and
respectively.
( )
0<
<
(3.31)
( ) is the charge density and potential at position
Further, the drain current at position
( )=
,
in the channel is derived from
( ) ( )
(3.32)
33
where
[cm2V/s] is the charge mobility and ( ) is the electric field at position
which equals
( )
.
Substituting Eq.3.31 into Eq.3.32 yields
( )
( )
( )=
(3.33)
By integrating, we can obtain
( )
( )
=
( )
=
(3.34)
(3.35)
Three different operational regimes of transistors can thus be distinguished.
When
(
), Eq.3.35 is simplified into
=
,
(3.36)
is too small to affect the distribution of charge carrier along the channel; the
resistance throughout the entire channel is approximately the same. The drain
current is then proportional to the drain-source voltage as shown in Figure 3.8 (a).
This is referred to as the linear regime of the transistor.
If the drain-source voltage increases continuously until
=(
) , the
charge carrier density becomes zero right at the drain electrode (There is no
charge carrier accumulation at that point in the channel.). This phenomenon
causes the channel to pinch off (see Figure 3.8 (b)).
As
exceeds (
) , the pinch-off point moves towards the source
electrode and a thin depletion region is thus formed between the pinch-off point
and the drain electrode. As the result of this pinch-off point movement, the
channel length is effectively reduced to L’ (see Figure 3.8 (c)). In the depletion
region, the current turns space charge limited. For a long-channel transistor (The
channel length is much larger than the thickness of insulating layer.), the drain
current remains essentially constant when
>(
), given by
,
=
The transistor is then said to be operating in the saturation regime.
34
(3.37)
Figure 3.8 The current-voltage characteristics and corresponding
illustrations of the channel of a field-effect transistor in (a) the linear
regime; (b) the pinch-off point; (c) the saturation regime.
There are two important current versus voltage characteristics for a field-effect
transistor; the output characteristic ( versus
for different ; Figure 3.9 (a))
at a constant ; Figure 3.9 (b)).
and the transfer characteristic ( versus
35
Figure 3.9 Two typical characteristics of a field-effect transistor: (a) output
curve (Id versus Vd for different Vg) and (b) transfer curve (Id versus Vg at a
constant Vd).
The transfer characteristic reflects the drain current modulation by the gatesource voltage at a constant drain-source voltage. This is the most fundamental
and critical behavior of a transistor, and is utilized in many applications. The
on/off current ratio at a specific drain-source voltage is defined as the quotient of
the highest current (the “on” current) over the lowest current (the “off” current).
By linearly fitting the square root of the drain current curve (the red doted curve
in Figure 3.9 (b)) provides an estimate of the threshold voltage ( ) by the
36
intersection of the fitting line at the gate-source voltage axis. Moreover, from the
transfer characteristic, the mobility at saturation regime can be calculated by
=
where
,
,
(3.38)
is the slope of the linear fitting line.
37
4. Rheology and surface wettability
4.1. Rheology
Rheology is the science of deformation and flow of materials when a force (shear
stress) is applied to them. Rheology is fundamental and critical to understand and
predict the behavior of ink flow in printing processes.
Assuming that a fluid is sandwiched between two parallel plates (see Figure 4.1),
where the upper one moves at a constant speed and the lower one is stationary,
then the fluid flows following the direction of moving plate. The upper part of
the flowing fluid, which is close to the moving plate, flows faster than the lower
part. The gradient of velocity in the flowing fluid is called shear rate.
=
(4.1)
where [s-1] is the shear rate; [m/s] is the velocity of the moving plate and
[m] is the distance between the two plates.
Figure 4.1 Illustration of velocity gradient (as the black horizontal arrows)
in a fluid between a moving plate and a stationary plate.
Moreover, the viscosity of a fluid is defined as
=
(4.2)
where [Pa·s or poise (P); 1 P = 0.1 Pa·s] is the viscosity; [Pa] is the shear
stress; [s-1] is the shear rate.
38
A Newtonian liquid, such as water, displays a constant viscosity when shear rate
or shear stress varies. The viscosity of a non-Newtonian liquid, however, changes
disproportionately with shear rate and shear stress. Non-Newtonian liquids can
be further divided into plastic, pseudoplastic and dilatant liquids. Four different
types of flow behavior are given in Figure 4.2 (a). Most printing pastes are
considered to be plastic or pseudoplastic liquids.
Figure 4.2 Plots of (a) shear stress versus shear rate for newtonian,
pseudoplastic, plastic and dilatant fluids; (b) viscosity versus shear rate
for thixotropic and rheopexic fluids.
39
In a plastic liquid, the applied force must overcome a threshold stress before the
fluid starts to flow. This specific minimum stress is known as the yield stress. As
long as the applied shear stress exceeds the yield stress, the viscosity of the
plastic fluid decreases as the shear rate increases. Once the shear stress is
removed, the viscosity builds up quickly until the fluid stops flowing. This yieldstress phenomenon is very important for printing inks because a proper yield
stress provides a needed flow-out and good leveling without excessive bleeding
during printing. The bleeding means that the ink flows beyond the printing
boundaries defined by the printing template.
A pseudoplastic liquid behaves similar to a plastic liquid except that it has no
yield stress. The viscosity drops as long as the shear stress is applied. In contrast
to plastic liquids, a dilatant liquid shows an increasing in viscosity as the shear
rate increases. Dilatant liquid characteristics are rare in printing inks.
Thixotropy characterizes the time-dependency of viscosity during shearing in
pseudoplastic liquids. The viscosity reduces continuously even if the shear stress
is constant, i.e. shear thinning. A thixotropic liquid has a pronounced delay in
viscosity after the shear stress is removed. A hysteresis loop in viscosity versus
shear rate curve is observed, as shown in Figure 4.2 (b). Oppositely, rheopexic
characteristics describe the time-dependency of dilatant liquid. Thus, shear
thickening occurs in rheopexic materials.
4.2. Surface wettability
Surface wettability plays an important role in printing processes after that the ink
has been deposited onto the substrate. Simply, wettability can be considered as
the phenomenon that dictates how much a liquid will spread out on a surface, i.e.
wet the surface. The concept of wettability is valid for any combination of a
liquid material and a solid surface, but in many cases the liquid is represented by
water and planar surfaces are typically considered. In Figure 4.3 (a), the
wettability between the water droplets and the solid surface can be classified into
two general types, hydrophobic (left in Figure 4.3 (a)) and hydrophilic (right).
40
Figure 4.3 Illustrations of (a) the interactions of water droplets with
different surfaces, ranging from hydrophobic (very little wetting) to
hydrophilic (high wetting) and (b) the contact angle ș between a droplet of
liquid and a solid surface.
The wettability between the liquid droplet and the substrate is attributed to
their surface tensions. In the bulk liquid, a liquid molecule is equally
surrounded and interacts with its neighboring molecules in all directions,
resulting in a zero net force. However, the molecules at the surface cannot
interact with the bulk liquid molecules on all sides and thus the liquid either
will tend to keep its surface area small, or to increase it. A sphere has the
smallest surface-to-volume ratio. Droplets of liquids are therefore spherical.
The work needed to generate a surface is proportional to the surface area. The
coefficient of proportionality is defined as surface tension which is
=
(4.3)
where [J/m2] is the surface tension;
[J] is the work (energy required) to
change the surface area; [m2] is the surface area.
The wettability can be represented by the contact angle at the interface and the
intersection of the liquid droplet, the surrounding gas and the solid surface (see
Figure 4.3 (b)).
At equilibrium, the forces at the interface are characterized by Young’s equation.
=
+
cos
(4.4)
41
and
are the interfacial tensions between the solid and the vapor,
where ,
the solid and the liquid, and the liquid and the vapor, respectively.
42
5. Device manufacture and characterization
5.1. Printing
Printing is defined as the process of transferring inks into patterns onto substrates.
Today’s dominant printing technology can be classified into four main types,
flexography, gravure, offset and screen printing, which refers to the difference in
printing plates (image carriers) used in the manufacturing process (see Figure
5.1). The printing plates can be either flat for sheet-to-sheet printing or
cylindrical for roll-to-roll printing processes.
Flexography printing employs a soft rubber cylinder on which raised printing
elements are patterned. Inks adhere to these raised parts and are then transferred
onto the substrate by pressure.
Gravure printing works in an opposite manner to flexography printing. The
printing patterns on the gravure cylinder are recessed. Inks are flooded onto the
entire cylinder’s surface and the excessive ink is subsequently removed by a
doctor blade. Thus, only the recesses of the cylinder are filled with the ink. By
pressing the inked gravure cylinder onto the substrate, the substrate takes up the
ink from these recesses.
Offset printing exploits a printing cylinder constituted by printing and nonprinting elements. Unlike flexography printing, the surface of the printing
cylinder in offset printing is effectively flat. The printing and non-printing
elements on the printing cylinder are usually made from different materials with
different surface properties, such as metals and polymer coatings. Due to the
large differences in the surface properties (wettability), inks can only wet and
remain in certain patterns along the printing cylinder. Offset printing is an
indirect process. A blanket rubber cylinder is also utilized, as an intermediate
carrier, which transfers the ink from the printing cylinder to the substrate.
Screen printing utilizes a frame with fine meshes as a printing plate. The meshes
are usually made from nylon or polyester. Printing patterns are defined by the
opening areas of the mesh. Non-printing areas are blocked by specific polymer
coatings. Inks are pressed through the openings of the mesh by a squeegee and
then transferred onto the substrate. In this thesis, flat-bed screen printing has been
used as the major method to fabricate the sensor labels reported in paper II and
III. More details about screen printing are given in the section 5.1.1.
43
Figure 5.1 The printing plates used in (a) flexography printing; (b) gravure
printing; (c) offset printing; (d) screen printing. (The blue parts represent
inks.)
Besides these four conventional printing technologies for high-volume
production, ink jet printing and electrophotography printing are also adopted for
low-volume print work, typically also in applications which require that the
printing image is easily changed for every new copy.
5.1.1. Screen printing
Screen printing is a versatile and well developed printing technology. Its main
components are the frame with the screen mesh and the stencil (see Figure 5.2
(a)). The printing can be accomplished on various substrates, such as paper,
plastics, textiles, and more. Due to its flexibility and simplicity, screen printing
has become one of the important processes for producing electronic devices,
exemplified by electrodes [97-100], solar cells [101-103], sensors [104-108], etc.
Screen printing is a push-through printing method. Since ordinary stencil printing
(a printing method that patterns are transferred through the openings of a stencil
made by a piece of thin metal or plastic foil) has a general disadvantage which is
that all parts of the stencil must be connected together, interconnection bridges
must be used for linking inner parts to neighboring parts. By combining screen
fabrics with stencils, this disadvantage is eliminated in the screen printing
technology. Since the screen mesh carries the stencil, additional interconnection
bridges are no longer needed.
44
In a flat-bed screen printing frame, the stencil is glued onto the screen mesh
which is attached to the frame. Woods or metals, e.g. aluminum, are often used
as the frame scaffold. Fabrics for producing the screen mesh must adhere to the
stencil material properly and must also be compatible with ink solvents and
cleaning agents. Moreover, the screen mesh must possess sufficient abrasionresistance to the stress created from the squeegee. The two most important
parameters of a screen mesh are the thread thickness, which is the diameter (ȝm)
of the fabric thread, and the thread counts, which correspond to the number of
threads per cm. The mesh width and the open screen area as a percentage of total
surface area are consequently determined as
=
(5.1)
(%) =
where [ȝm] is the mesh width;
thread thickness.
× 100%
[ȝm] is the thread spacing;
(5.2)
[ȝm] is the
45
Figure 5.2 Screen printing frame: (a) photo of a flat-bed screen printing
frame (inset: the enlarged image of the pattern of the stencil) and (b)
illustration of the screen mesh geometry.
Screen tension is another important factor in order to achieve a good quality
printing. The nature of screen fabric, the adhesion method between the screen
mesh and the frame, the shear force and shear time applied by the squeegee
during the printing determine the lifetime of a screen printing frame.
Dramatically diminished screen tension must be avoided, because uneven
tensions on the screen mesh give distortions in the resulting printed images.
46
The stencil used for a screen printing frame is commonly made by photosensitive emulsions. The emulsion is coated uniformly on both the stencil side
and the squeegee side. The printing patterns are then transferred from a
photomask to the stencil by UV exposure, causing curing of the emulsion where
it is exposed to the UV light. The uncured parts on the stencil are subsequently
washed off by chemical agents. In addition, there are several other methods to
produce stencils [109].
A printing substrate is firstly placed on a flat printing table and fixed by, for
instance, vacuum or a special adhesive. The screen printing frame is usually fixed
at a certain distance (typically a few millimeters) away and above the substrate
by a frame holder. The printing process begins with spreading the ink evenly on
the screen printing mesh by the squeegee using a gentle pressure. During the ink
spreading, the screen printing mesh and the substrate should not contact each
other. The ink is then subsequently pressed through the screen printing stencil by
the printing squeegee with a larger pressure and then transferred onto the actual
surface to be printed due to the surface wetting. Finally, the screen printing mesh
separates from the substrate upon which the ink levels out by itself and forms a
uniform ink layer. The screen printing mesh only comes into contact with the
substrate during the pressing process, caused by the printing squeegee, and
withdraws immediately with the help of the screen tension. In addition to flat-bed
screen printing, there is also a rotary screen printing technology which enables
screen printing on a web of textile or paper continuously without any seams,
using a screen printing cylinder.
Figure 5.3 Illustration of the printing process in screen printing.
5.2. Dry phase patterning
47
Dry phase patterning [110] (DPP) is a low-cost manufacturing technique for
patterning a conducting layer which is laminated on a flexible carrier layer, e.g. a
PET foil or paper. During the processing, the substrate that comprises the
conducting layer and the carrier layer passes through a nip which is defined by a
patterned cylinder (cliché roll) and a milling cutter cylinder. The milling cutter
cylinder contains sharp milling ridges on its cylindrical surface. By forcing the
substrate against the envelope of the patterned cylinder’s surface, corresponding
ridges and valleys are established perpendicular to the cylindrical surface.
Consequently, the ridged parts of the conducting layer are cut away by the
milling cutter cylinder and repeating patterns of the conducting material are
produced on the substrate. The DPP process combines patterning (analogous to
the development in a conventional photolithography) and machining (analogous
to the etching in a conventional photolithography) of the conducting layer in a
single step and is essentially entirely chemical-free. Therefore, the DPP process
is compatible to high speed roll-to-roll printing for high-volume production and
also possesses a great environmental benefit compared to conventional wetetching processes which in addition require several steps to finish a pattern on a
substrate. The residual materials from the DPP process are in a dry phase and are
therefore easily recyclable.
The DPP process has been proven to work efficiently with several substrates for
producing resistors, inductors and antennas, such as for PEDOT/PET,
aluminum/PET, copper/PET substrates, and more [111].
The antennas for the sensor labels in Paper II and III were produced by the DPP
process.
5.3. Spin coating
Spin coating is a method that is widely used in microfabrication processing to
deposit a thin film with a thickness from several tens of nanometers to a few
hundreds of micrometers. A small amount of solution, typically a solid material
dissolved in a volatile solvent, is deposited on the center of the substrate which is
fixed on a vacuum chuck. By rotating the substrate at a certain speed, the solution
spreads out on the surface of the substrate. Due to the subsequent evaporation of
the solvent, a thin material layer is formed on the substrate. The spin coating
process is illustrated in Figure 5.4. Spinning speed, spinning time, the
48
evaporation rate of the solvent and the concentration of the solution together
determine the thickness of the spin-coated film.
Figure 5.4 Illustration of a spin coating process.
5.4. Photolithography
Photolithography is a patterning process used in microfabrication. The steps in
photolithography are illustrated in Figure 5.5.
Figure 5.5 Illustration of a photolithography process.
Photoresist, such as an UV-sensitive material, is firstly deposited on a substrate
by spin coating. The photoresist layer subsequently is exposed with the UV light
through a photomask. For a positive photoresist which is used in the work of this
thesis, the exposed parts become more soluble than the unexposed areas and can
be washed away in a specific developer. After developing, the same geometrical
patterns as in the photomask have been transferred onto the substrate. In contrast,
for a negative photoresist, the exposed photoresist is cross-linked by the UV light
and remains on the substrate after developing. Consequently, the inverse
geometrical patterns compared to the photomask are transferred onto the
substrate. The unprotected parts of the upmost layer on the substrate are removed
49
by wet or dry etching with chemical agents. After the etching process, the
remaining photoresist is no longer needed and is then cleaned away by
photoresist strippers or by using plasma etching.
5.5. Impedance spectroscopy
Impedance measurements over a broad frequency range at different potentials are
very helpful in order to understand an electrochemical process, particularly for
the processes residing at an electrode/electrolyte interface. Impedance
spectroscopy is a powerful technique from which the electrode/electrolyte
interface can be modeled using various electrical equivalent circuits. Impedance
parameters are usually measured by applying a small AC voltage at a specific
frequency to a sample cell, which comprises two parallel metal plates with the
sample material sandwiched in between. The responding current is recorded and
the phase shift between the voltage and the current is characterized by the phase
angle. By changing the frequency, the impedance parameters are measured as a
function of frequency. For an ideal resistor, capacitor and inductor, their
impedance values are expressed as complex numbers (see details in section 3.2).
Two important charts, Bode plot and Nyquist plot, are mainly adopted to
characterize the results of impedance measurements.
Bode plot shows the measured impedance versus frequency, whereas Nyquist
plot reflects the measured impedance as a vector in the
vs
coordinate
diagram.
Since inductive components are rarely found in an electrochemical process, the
fitting of impedance values into a corresponding electrical equivalent circuit
model employs usually resistive and capacitive components only. In Figure 5.6,
two basic electrical circuits are given in the Nyquist plots.
50
Figure 5.6 Nyquist plots of (a) a series RC circuit and (b) a parallel RC
circuit.
For a capacitor connected with a resistor in series, the total impedance of the
circuit is expressed as
=
where
,
0 and
+
=
(5.3)
0,
51
Thus, in the Nyquist plot, the series circuit is a straight line perpendicular to the
axis. The intersection between the straight line and the
axis indicates the
value of the resistor.
For a capacitor connected with a resistor in parallel, the total admittance of the
parallel circuit is
= +
(5.4)
After rationalizing the Eq.5.4, the total impedance of the parallel circuit is
+
=
(5.5)
Thus,
=
(5.6)
=
(5.7)
=
(5.8)
Inserting Eq.5.7 into Eq.5.8 yields
(
) =(
)
(5.9)
Eq.5.9 can be also expressed as
+(
) =
(5.10)
which is a function of circle.
Therefore, the parallel circuit is displayed as a semicircle in the Nyquist plot. The
diameter of the semicircle is .
5.5.1. Randles Model
Randles model is the simplest electrical equivalent circuit for characterizing an
electrode/electrolyte interface. In Randles model, both the electrical equivalent
circuit of the electrode/electrolyte interface and its Nyquist plot are shown in
Figure 5.7. The equivalent circuit is composed of impedance elements, which
includes the electric double layer capacitance ( ), the charge transfer resistance
( ), the Warburg impedance ( ) and the electrolyte bulk resistance ( ).
52
The Warburg impedance is described as
=
where
(5.11)
is a positive constant.
The Warburg impedance represents the diffusion process in the electrolyte [112]
and appears as a straight line with a slope of 45° at low frequencies in the
Nyquist plot.
Figure 5.7 The electrical equivalent circuit of an electrode/electrolyte
interface in Randles model and its corresponding Nyquist plot.
5.6. Humidity
Humidity indicates the amount of water vapor in air or a gas. The measurements
of humidity can be absolute or relative. Dew/Frost point (D/F PT) and Parts Per
Million (PPM) are absolute measurements, whereas the Relative Humidity (RH)
is a relative measurement.
D/F PT represents the dew-point temperature at which the water vapor condenses
into liquid water and the frost-point temperature at which water vapor condenses
to ice.
PPM is the amount of water vapor in a gas by volume (PPMv) or weight (PPMw).
53
RH is defined as the ratio of the partial pressure of water vapor in a gas to the
saturated vapor pressure of water in the gas at a given temperature. RH is
expressed in percentage and most commonly used in daily life.
54
6. Conclusions and outlook
The thesis focuses on exploring ionic migration and polarization in
polyelectrolytes for different applications, in humidity sensors for construction
industry, memory applications and in to improve the stability on water-gated
organic transistors for sensing in aqueous media. Paper I proposes a novel
concept for wireless humidity sensors that utilizes ionic relaxation in the
polyelectrolyte as the sensing probe. The proposed sensor in Paper I is a
completely passive device and is powered by an external RF interrogator through
electromagnetic coupling. Therefore, the device aims for monitoring humidity
levels and alerting moisture problems in the situations where conventionally
wired humidity sensors are difficult to apply to, such as inside walls or beneath
floors, during logistic transportations or perhaps also for storage of goods, etc.
Nowadays the high cost of RFID sensors still hinders their use in many largescale applications, which are usually cost-sensitive. Paper II demonstrates the
feasibility of producing the proposed humidity sensor label in a fully printed
manufacturing process. Combining dry phase patterning with screen printing
processes enables very low-cost production of the passive sensor labels. The fully
printed humidity sensor label in Paper II constitutes a significant step towards
low-cost and large-volume manufacturing, e.g. a fully roll-to-roll printing
production process.
In Paper III, I report further development of the humidity sensor by improving
the quality factor (Q) of the printed sensor label. The Q factor is a unique and
critical characteristic parameter of the sensor label. A high enough Q factor is
required in order to achieve a reasonable detecting and readout distance [113],
e.g. 10-20 cm is needed. Besides antenna optimization, I also focused on
minimizing the loss in the printed tuning capacitor as well. For that purpose, a
non-polar insulating polymer is used as the dielectric material in the printed
capacitor instead of a polar insulating polymer. The experiment shows that the
dielectric loss in the printed capacitor is reduced from 5% (a commercially
available dielectric printing ink) to 0.1% (a modified printing solution based on
benzocyclobutene). Thanks to the low-loss printed capacitor, the Q factor in the
printed sensor label is improved consequently.
55
In the context of a future printed electronics platform, memories are certainly
crucial components. Extensive efforts are therefore devoted to develop memories
that can be easily printed and integrated into printed electronic circuits and
systems. Various materials and device systems are explored for those printed
memories. In particular, it is attractive with a printed memory device technology
that can both be programmed and read out at low voltages (integration with other
printed devices) and that is nonvolatile (No power is needed to maintain
information.). One concept, among many, for printed memories suggests utilizing
an organic field-effect transistor in which the gate-insulator-semiconductor
configuration includes a bistable memory functionality, which can be read out as
a non-volatile modulation of the drain current running along the channel. Paper
IV is related to a threshold-voltage tunable organic transistor incorporating an
ionic trapping layer included on top of an electrolyte-gated organic transistor.
This bilayer structure employs an ionic conducting layer (polyelectrolyte) and an
ionic trapping layer containing cryptands. A remanent polarization is formed in
the bilayer when the cations from the ionic conducting layer (polyanion) migrate
into the trapping sites (cryptands) in the ionic trapping layer along an electric
field. As the result of the remanent polarization, the organic transistor with the
ionic trapping layer can vary its threshold voltage by applying a programming
voltage of ±2 V (writing/erasing). We foresee that this work is potentially useful
in a memory transistor with relatively low programming voltages and could also
provide multilevel memory functionalities.
Organic field-effect transistors are explored for a vast array of sensing
applications in biological and chemical applications, in cases where the analyte
commonly is included in aqueous media. Sensing in aqueous media can be
obtained through various device configuration strategies, for instance using a
gate-aqueous media-channel configuration. Such configurations are typically
associated with large degrees of drift due to bias stress, undesired ionic doping
effects and degradations. It is thus important to develop strategies to improve
stability and also suppress any undesired stress that will potentially ruin the
stability of the sensor. Paper V reports the method to improve the stability of a
water-gated organic transistor. The first water-gated organic field-effect transistor
was recently proposed [114]. However, the operating instability of this particular
device is severe and the organic semiconductor used, P3HT, suffers from a fast
degradation in ambient environment [31, 32]. An ion exchange membrane,
Nafion, is added to cover the entire semiconducting channel. The water/Nafion56
gated organic transistor shows improved stability to air exposure over time and
exhibits also good stability characteristics in neutral (pH=7) standard buffer
solutions. Since the stable operation of the transistor in water is crucial for many
sensing applications, the development of disposable low-cost sensors based on
the water/Nafion-gated organic transistor opens for future work and
optimizations.
57
References
1.
Staudinger, H. The Nobel Prize in Chemistry. 1953 [cited 2013 May];
Available
from:
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1953/index.
html.
2.
Heeger, A.J., A.G. MacDiarmid, and H. Shirakawa. The Nobel Prize in
Chemistry.
2000
[cited 2013 May]; Available from:
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/.
3.
Nilsson, D., et al., An all-organic sensor–transistor based on a novel
electrochemical transducer concept printed electrochemical sensors on
paper. Sensors and Actuators B: Chemical, 2002. 86(2–3): p. 193-197.
4.
Nambiar, S. and J.T.W. Yeow, Conductive polymer-based sensors for
biomedical applications. Biosensors and Bioelectronics, 2011. 26(5): p.
1825-1832.
5.
Geffroy, B., P. le Roy, and C. Prat, Organic light-emitting diode (OLED)
technology: materials, devices and display technologies. Polymer
International, 2006. 55(6): p. 572-582.
6.
Gather, M.C., A. Köhnen, and K. Meerholz, White Organic LightEmitting Diodes. Advanced Materials, 2011. 23(2): p. 233-248.
7.
Cai, W., X. Gong, and Y. Cao, Polymer solar cells: Recent development
and possible routes for improvement in the performance. Solar Energy
Materials and Solar Cells, 2010. 94(2): p. 114-127.
8.
Helgesen, M., R. Sondergaard, and F.C. Krebs, Advanced materials and
processes for polymer solar cell devices. Journal of Materials Chemistry,
2010. 20(1): p. 36-60.
9.
Herlogsson, L., et al., Low-Voltage Polymer Field-Effect Transistors
Gated via a Proton Conductor. Advanced Materials, 2007. 19(1): p. 97101.
10.
Klauk, H., Organic thin-film transistors. Chemical Society Reviews,
2010. 39(7): p. 2643-2666.
11.
Herlogsson, L., et al., Low-Voltage Ring Oscillators Based on
Polyelectrolyte-Gated Polymer Thin-Film Transistors. Advanced
Materials, 2010. 22(1): p. 72-76.
58
12.
Bubnova, O., et al., Optimization of the thermoelectric figure of merit in
the conducting polymer poly(3,4-ethylenedioxythiophene). Nat Mater,
2011. 10(6): p. 429-433.
13.
Bubnova, O. and X. Crispin, Towards polymer-based organic
thermoelectric generators. Energy & Environmental Science, 2012.
5(11): p. 9345-9362.
14.
Torsi, L., et al., Side-Chain Role in Chemically Sensing Conducting
Polymer Field-Effect Transistors. The Journal of Physical Chemistry B,
2003. 107(31): p. 7589-7594.
15.
Heeger, A.J., Semiconducting and Metallic Polymers: The Fourth
Generation of Polymeric Materials. The Journal of Physical Chemistry B,
2001. 105(36): p. 8475-8491.
16.
Heeger, A.J., Semiconducting polymers: the Third Generation. Chemical
Society Reviews, 2010. 39(7): p. 2354-2371.
17.
Subramanian, V., et al. Printed electronics for low-cost electronic
systems: Technology status and application development. in Solid-State
Device Research Conference, 2008. ESSDERC 2008. 38th European.
2008.
18.
Boverket. God bebyggd miljö – förslag till nytt delmål för fukt och mögel:
Resultat om byggnaders fuktskador från projektet BETSI. 2011 [cited
2013
May];
Available
from:
http://www.boverket.se/Global/Webbokhandel/Dokument/2011/BETSIFukt-och-mogel.pdf.
19.
Oprea, A., et al., Capacitive humidity sensors on flexible RFID labels.
Sensors and Actuators B: Chemical, 2008. 132(2): p. 404-410.
20.
López, T.S., RFID and sensor integration standards: State and future
prospects. Computer Standards & Interfaces, 2011. 33(3): p. 207-213.
21.
Naber, R.C.G., et al., Organic Nonvolatile Memory Devices Based on
Ferroelectricity. Advanced Materials, 2010. 22(9): p. 933-945.
22.
Ling, Q.-D., et al., Polymer electronic memories: Materials, devices and
mechanisms. Progress in Polymer Science, 2008. 33(10): p. 917-978.
23.
Heremans, P., et al., Polymer and Organic Nonvolatile Memory Devices.
Chemistry of Materials, 2010. 23(3): p. 341-358.
24.
Konno, K., et al., An organic nonvolatile memory using space charge
polarization of a gate dielectric. Thin Solid Films, 2009. 518(2): p. 534536.
59
25.
Kim, S.-J. and J.-S. Lee, Flexible Organic Transistor Memory Devices.
Nano Letters, 2010. 10(8): p. 2884-2890.
26.
Bernards, D.A., et al., Enzymatic sensing with organic electrochemical
transistors. Journal of Materials Chemistry, 2008. 18(1): p. 116-120.
27.
Kumar, A. and J. Sinha, Electrochemical Transistors for Applications in
Chemical and Biological Sensing, in Organic Semiconductors in Sensor
Applications, D. Bernards, G. Malliaras, and R. Owens, Editors. 2008,
Springer Berlin Heidelberg. p. 245-261.
28.
Yang, S.Y., et al., Electrochemical transistors with ionic liquids for
enzymatic sensing. Chemical Communications, 2010. 46(42): p. 79727974.
29.
Kergoat, L., et al., Advances in organic transistor-based biosensors:
from organic electrochemical transistors to electrolyte-gated organic
field-effect transistors. Analytical and Bioanalytical Chemistry, 2012.
402(5): p. 1813-1826.
30.
Cramer, T., et al., Water-gated organic field effect transistors opportunities for biochemical sensing and extracellular signal
transduction. Journal of Materials Chemistry B, 2013.
31.
Scarpa, G., et al., Organic ISFET Based on Poly (3-hexylthiophene).
Sensors, 2010. 10(3): p. 2262-2273.
32.
Hintz, H., et al., Photodegradation of P3+TíA Systematic Study of
Environmental Factors. Chemistry of Materials, 2010. 23(2): p. 145-154.
33.
Yuen, J.D., et al., Electrochemical Doping in Electrolyte-Gated Polymer
Transistors. Journal of the American Chemical Society, 2007. 129(46): p.
14367-14371.
34.
McMurry, J., in Organic Chemistry. 2008, Thomson learning. inc.:
Belmont, CA, USA. p. 12-13.
35.
Peierls, R.E., Quantum Theory of Solids. 1955, Oxford UK: Oxford
University Press.
36.
Fahlman, M. and W.R. Salaneck, Surfaces and interfaces in polymerbased electronics. Surface Science, 2002. 500(1–3): p. 904-922.
37.
Chiang, C.K., et al., Electrical Conductivity in Doped Polyacetylene.
Physical Review Letters, 1977. 39(17): p. 1098-1101.
38.
Gurunathan, K., et al., Electrochemically synthesised conducting
polymeric materials for applications towards technology in electronics,
optoelectronics and energy storage devices. Materials Chemistry and
Physics, 1999. 61(3): p. 173-191.
60
39.
Assadi, A., et al., Field-effect mobility of poly(3-hexylthiophene).
Applied Physics Letters, 1988. 53(3): p. 195-197.
40.
Bao, Z., A. Dodabalapur, and A.J. Lovinger, Soluble and processable
regioregular poly(3-hexylthiophene) for thin film field-effect transistor
applications with high mobility. Applied Physics Letters, 1996. 69(26): p.
4108-4110.
41.
Podzorov, V., et al., Single-crystal organic field effect transistors with
the hole mobility ~ 8 cm[sup 2]/V s. Applied Physics Letters, 2003.
83(17): p. 3504-3506.
42.
Jurchescu, O.D., J. Baas, and T.T.M. Palstra, Effect of impurities on the
mobility of single crystal pentacene. Applied Physics Letters, 2004.
84(16): p. 3061-3063.
43.
McMurry, J., in Organic Chemistry. 2008, Thomson learning. inc.:
Belmont, CA, USA. p. 36.
44.
McMurry, J., in Organic Chemistry. 2008, Thomson learning. inc.:
Belmont, CA, USA. p. 39.
45.
Nordling, C. and J. Österman, Physics Handbook for Science and
Engineering. 8th ed. 2006, Lund, Sweden: Studentlitteratur.
46.
Atkins, P. and J.d. Paula, in Atkins' Phyical Chemistry. 2010, Oxford
University Press: Oxford, UK. p. 626-628.
47.
Bueche, F., Dielectric dispersion of polar polymers. Journal of Polymer
Science, 1961. 54(160): p. 597-602.
48.
Said, E., et al., Polymer field-effect transistor gated via a
poly(styrenesulfonic acid) thin film. Applied Physics Letters, 2006.
89(14): p. 143507.
49.
Bordi, F., C. Cametti, and R.H. Colby, Dielectric spectroscopy and
conductivity of polyelectrolyte solutions. Journal of Physics: Condensed
Matter, 2004. 16(49): p. R1423.
50.
Li, Y., et al., Humidity sensors based on polymer solid electrolytes:
investigation on the capacitive and resistive devices construction.
Sensors and Actuators B: Chemical, 2001. 77(3): p. 625-631.
51.
Lee, C.-W., et al., Polymeric humidity sensor using organic/inorganic
hybrid polyelectrolytes. Sensors and Actuators B: Chemical, 2005.
109(2): p. 315-322.
52.
Rubinger, C.P.L., et al., Sulfonated polystyrene polymer humidity sensor:
Synthesis and characterization. Sensors and Actuators B: Chemical,
2007. 123(1): p. 42-49.
61
53.
Lv, X., et al., A resistive-type humidity sensor based on crosslinked
polyelectrolyte prepared by UV irradiation. Sensors and Actuators B:
Chemical, 2009. 135(2): p. 581-586.
54.
Sun, A., Y. Wang, and Y. Li, Stability and water-resistance of humidity
sensors using crosslinked and quaternized polyelectrolytes films. Sensors
and Actuators B: Chemical, 2010. 145(2): p. 680-684.
55.
Lee, C.-W., H.-W. Rhee, and M.-S. Gong, Humidity sensor using epoxy
resin containing quaternary ammonium salts. Sensors and Actuators B:
Chemical, 2001. 73(2–3): p. 124-129.
56.
Lee, C.-W., H.-S. Park, and M.-S. Gong, Humidity sensitive properties of
quaternary ammonium salts containing polyelectrolytes crosslinked with
2-oxazoline crosslinker. Sensors and Actuators B: Chemical, 2005.
109(2): p. 256-263.
57.
Lv, X., Y. Li, and M. Yang, Humidity sensitive properties of copolymer
of quaternary ammonium salt with polyether–salt complex. Polymers for
Advanced Technologies, 2009. 20(6): p. 509-513.
58.
Li, Y. and M.J. Yang, Bilayer thin film humidity sensors based on
sodium polystyrenesulfonate and substituted polyacetylenes. Sensors and
Actuators B: Chemical, 2002. 87(1): p. 184-189.
59.
Li, Y., M.J. Yang, and Y. She, Humidity sensors using in situ synthesized
sodium polystyrenesulfonate/ZnO nanocomposites. Talanta, 2004. 62(4):
p. 707-712.
60.
Liu, K., et al., Humidity sensitive properties of an anionic conjugated
polyelectrolyte. Sensors and Actuators B: Chemical, 2008. 129(1): p. 2429.
61.
Gong, M.-S., et al., Humidity-sensitive properties of phosphonium saltcontaining polyelectrolytes. Journal of Materials Science, 2002. 37(21):
p. 4615-4620.
62.
Gong, M.-S., et al., Humidity sensor using cross-linked polyelectrolyte
prepared from mutually reactive copolymers containing phosphonium
salt. Sensors and Actuators B: Chemical, 2002. 86(2–3): p. 160-167.
63.
Lee, C.-W. and M.-S. Gong, Resistive humidity sensor using
phosphonium salt-containing polyelectrolytes based on the mutually
cross-linkable copolymers. Macromolecular Research, 2003. 11(5): p.
322-327.
64.
Grahame, D.C., The Electrical Double Layer and the Theory of
Electrocapillarity. Chemical Reviews, 1947. 41(3): p. 441-501.
62
65.
Bard, A.J. and L.R. Faulkner, in Electrochemical Methods:
Fundamentals and Applications. 2001, John Wiley & Sons, Inc.: New
York, USA. p. 544-557.
66.
Larsson, O., et al., Insulator Polarization Mechanisms in PolyelectrolyteGated Organic Field-Effect Transistors. Advanced Functional Materials,
2009. 19(20): p. 3334-3341.
67.
Kwak, N.-S., et al., Synthesis and characteristics of a cross-linked
DMSIP-co-HDO-co-MA ion-exchange membrane for redox flow battery
applications. Journal of Membrane Science, 2013. 430(0): p. 252-262.
68.
Ye, L., et al., Synthesis and characterization of novel cross-linked
quaternized poly(vinyl alcohol) membranes based on morpholine for
anion exchange membranes. Solid State Ionics, 2013. 240(0): p. 1-9.
69.
DuPont. Nafion.
[cited 2013 May]; Available
http://www2.dupont.com/FuelCells/en_US/products/nafion.html.
from:
70.
Glass, A. Flemion.
[cited 2013 May]; Available
http://www.agc.com/english/csr/env/products/11.html.
from:
71.
Slade, S., et al., Ionic Conductivity of an Extruded Nafion 1100 EW
Series of Membranes. Journal of The Electrochemical Society, 2002.
149(12): p. A1556-A1564.
72.
Hsu, W.Y. and T.D. Gierke, Ion transport and clustering in nafion
perfluorinated membranes. Journal of Membrane Science, 1983. 13(3): p.
307-326.
73.
Eikerling, M., A.A. Kornyshev, and U. Stimming, Electrophysical
Properties of Polymer Electrolyte Membranes: A Random Network
Model. The Journal of Physical Chemistry B, 1997. 101(50): p. 1080710820.
74.
Tailoka, F., D.J. Fray, and R.V. Kumar, Humidity sensing under
aggressive conditions using nafion conductors. Ionics, 2000. 6(5-6): p.
383-388.
75.
Tailoka, F., D.J. Fray, and R.V. Kumar, Application of Nafion
electrolytes for the detection of humidity in a corrosive atmosphere.
Solid State Ionics, 2003. 161(3–4): p. 267-277.
76.
Fischer, E., Einfluss der Configuration auf die Wirkung der Enzyme.
Berichte der deutschen chemischen Gesellschaft, 1894. 27(3): p. 29852993.
63
77.
Cram, D.J., J.-M. Lehn, and C.J. Pedersen. The Nobel Prize in Chemistry
1987
[cited
2013
May];
Available
from:
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1987/.
78.
Pedersen, C.J., Cyclic polyethers and their complexes with metal salts.
Journal of the American Chemical Society, 1967. 89(26): p. 7017-7036.
79.
Ariga, K. and T. Kunitake, in Supramolecular Chemistry –Fundamentals
and Applications. 2006, Springer: Heidelberg. p. 13.
80.
Carlson, A.B., in Circuits: Engineering Concepts and Analysis of Linear
Electric Circuits. 2000, Brooks/Cole: Pacific Grove, USA. p. 362-366.
81.
Carlson, A.B., in Circuits: Engineering Concepts and Analysis of Linear
Electric Circuits. 2000, Brooks/Cole: Pacific Grove, USA. p. 274-281.
82.
Harpster, T.J., B. Stark, and K. Najafi, A passive wireless integrated
humidity sensor. Sensors and Actuators A: Physical, 2002. 95(2–3): p.
100-107.
83.
Marioli, D., E. Sardini, and M. Serpelloni, An inductive telemetric
measurement system for humidity sensing. Measurement Science and
Technology, 2008. 19(11): p. 115204.
84.
Maksimoviü, M., et al., Application of a LTCC sensor for measuring
moisture content of building materials. Construction and Building
Materials, 2012. 26(1): p. 327-333.
85.
Ong, K.G., et al., Design and application of a wireless, passive,
resonant-circuit environmental monitoring sensor. Sensors and
Actuators A: Physical, 2001. 93(1): p. 33-43.
86.
Ong, K.G., et al., Monitoring of bacteria growth using a wireless, remote
query resonant-circuit sensor: application to environmental sensing.
Biosensors and Bioelectronics, 2001. 16(4–5): p. 305-312.
87.
Riistama, J., et al., Totally passive wireless biopotential measurement
sensor by utilizing inductively coupled resonance circuits. Sensors and
Actuators A: Physical, 2010. 157(2): p. 313-321.
88.
Boutry, C.M., et al., Characterization of miniaturized RLC resonators
made of biodegradable materials for wireless implant applications.
Sensors and Actuators A: Physical, 2013. 189(0): p. 344-355.
89.
Akar, O., T. Akin, and K. Najafi, A wireless batch sealed absolute
capacitive pressure sensor. Sensors and Actuators A: Physical, 2001.
95(1): p. 29-38.
64
90.
Fonseca, M.A., et al. Flexible wireless passive pressure sensors for
biomedical applications. in Tech. Dig. Solid-State Sensor, Actuator, and
Microsystems Workshop (Hilton Head 2006). 2006.
91.
Zhai, J., T.V. How, and B. Hon, Design and modelling of a passive
wireless pressure sensor. CIRP Annals - Manufacturing Technology,
2010. 59(1): p. 187-190.
92.
XILINX. Xilinx Ships World's Highest Capacity FPGA and Shatters
Industry Record for Number of Transistors by 2X. 2011 [cited 2013
May]; Available from: http://press.xilinx.com/2011-10-25-Xilinx-ShipsWorlds-Highest-Capacity-FPGA-and-Shatters-Industry-Record-forNumber-of-Transistors-by-2X.
93.
Kahng, D., Electric Field Controlled Semiconductor Device. 1963, Bell
Telephone Laboratories: USA.
94.
Tsumura, A., H. Koezuka, and T. Ando, Macromolecular electronic
device: Field-effect transistor with a polythiophene thin film. Applied
Physics Letters, 1986. 49(18): p. 1210-1212.
95.
Koezuka, H., A. Tsumura, and T. Ando, Field-effect transistor with
polythiophene thin film. Synthetic Metals, 1987. 18(1–3): p. 699-704.
96.
Dhar, B.M., et al., Threshold voltage shifting for memory and tuning in
printed transistor circuits. Materials Science and Engineering: R:
Reports, 2011. 72(4): p. 49-80.
97.
Li, M., et al., Recent developments and applications of screen-printed
electrodes in environmental assays—A review. Analytica Chimica Acta,
2012. 734(0): p. 31-44.
98.
Kawahara, J., et al., Flexible active matrix addressed displays
manufactured by printing and coating techniques. Journal of Polymer
Science Part B: Polymer Physics, 2013. 51(4): p. 265-271.
99.
Kawahara, J., et al., Fast-switching printed organic electrochemical
transistors including electronic vias through plastic and paper substrates.
Electron Devices, IEEE Transactions on, 2013. 60(6): p. 2052-2056.
100.
Metters, J.P. and C.E. Banks, Screen Printed Electrodes Open New
Vistas in Sensing: Application to Medical Diagnosis, in Applications of
Electrochemistry in Medicine. 2013, Springer. p. 83-120.
101.
Krebs, F.C., et al., A complete process for production of flexible large
area polymer solar cells entirely using screen printing—First public
demonstration. Solar Energy Materials and Solar Cells, 2009. 93(4): p.
422-441.
65
102.
Krebs, F.C., J. Fyenbo, and M. Jørgensen, Product integration of
compact roll-to-roll processed polymer solar cell modules: methods and
manufacture using flexographic printing, slot-die coating and rotary
screen printing. Journal of Materials Chemistry, 2010. 20(41): p. 89949001.
103. Romijn, I., et al., Optimizing screen printed n-type solar cells towards 20%
efficiency. Solar Energy, 2013. 2012: p. 2011.
104.
Goldberg, H.D., et al., Screen printing: a technology for the batch
fabrication of integrated chemical-sensor arrays. Sensors and Actuators
B: Chemical, 1994. 21(3): p. 171-183.
105.
Nagata, R., et al., A glucose sensor fabricated by the screen printing
technique. Biosensors and Bioelectronics, 1995. 10(3–4): p. 261-267.
106.
Soukup, R., A. Hamacek, and J. Reboun. Organic based sensors: Novel
screen printing technique for sensing layers deposition. in Electronics
Technology (ISSE), 2012 35th International Spring Seminar on. 2012.
107.
Wei, Y., et al., Screen Printed Capacitive Free-standing Cantilever
Beams used as a Motion Detector for Wearable Sensors. Procedia
Engineering, 2012. 47(0): p. 165-169.
108.
Narakathu, B.B., et al., Novel fully screen printed flexible
electrochemical sensor for the investigation of electron transfer between
thiol functionalized viologen and gold clusters. Sensors and Actuators B:
Chemical, 2013. 176(0): p. 768-774.
109.
Kipphan, H., screen printing, in Handbook of Print Media. 2001,
Springer: Heidelberg, Germany. p. 409-422.
110.
Nordlinder, S. and F. Andersson, Methods and devices for manufacturing
of electrical components and laminated structures. 2011, Webshape AB.
111.
DPpatterning. Dry phase patterning. [cited 2013 May]; Available from:
http://www.dppatterning.com/.
112.
Park, S.-M. and J.-S. Yoo, Peer Reviewed: Electrochemical Impedance
Spectroscopy for Better Electrochemical Measurements. Analytical
Chemistry, 2003. 75(21): p. 455 A-461 A.
113.
Lee, J.-W. and B. Lee, Design of high-Q UHF radio-frequency
identification tag antennas for an increased read range. IET Microwaves,
Antennas & Propagation, 2008. 2(7): p. 711-717.
114.
Kergoat, L., et al., A Water-Gate Organic Field-Effect Transistor.
Advanced Materials, 2010. 22(23): p. 2565-2569.
66