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University of Groningen
Molecular organic semiconductors for electronic devicesJurchescu, Oana Diana
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1Introduction
1.1 Organic conductors - general aspects
Organic semiconductors are a fascinating class of materials, with a wide range
of properties [1–7]. The interest in organic electronics is motivated by increasing
demands of supplementing Si-based electronics with materials that offer easier
processing methods completed with functionalization by chemical manipulation.
Typical deposition techniques for organics are readily available or not very difficult
to implement (spin-coating, drop-casting, direct printing via stamps or ink-jets).
Their compatibility with light-weight, mechanically flexible plastic substrates,
completed by new, innovative fabrication routes, make them possible candidates
for future electronic devices. Moreover, due to different molecular tailoring pos-
sibilities via chemical synthesis, organic materials present an infinite variety in
functionality. The design of the molecular structures can be engineered to enhance
particular properties (solubility in different solvents, color of light emission, crystal
packing). In particular, addition of polar groups in polymers (e.g. polyvinylidene
fluoride and its copolymers with trifluoroethylene and tetrafluoroethylene) leads
to rich systems for investigation of ferroelectricity [8]. By incorporating them in
transistors (ferroelectric field-effect transistors - FeFETs), combined functionality
can be achieved for non-volatile memory devices that are compatible with flex-
ible plastic substrates [9]. Modification of the chemical end-groups also allows
fabrication of large transistor arrays for organic sensors. Different methods are
developed to increase the sensitivity and selectivity of organic sensing transistors
to different chemical or biological species. Applications include environmental
issues (e.g. air pollution), pH indicators [10], food freshness, toxic compounds,
1
2 Chapter 1. Introduction
stress and pressure indication in clothes [11].
Rapid progress is being made in the industrial development of the organic
electronic devices. Organic devices can be implemented on large scale application
area when their operation offer high performance, reliability, stability, long life
time, good control and reproducibility. Impressive steps have been accomplished
towards incorporation of organic conductors in devices, as constituents of plastic
electronic components, by continued innovation in materials, processing methods
and device design. For example, Philips and Polymer Vision already presented
their prototype rollable display based on pentacene FETs that will be used for fab-
rication of mobile phones [12]. OLED1 technology has already been incorporated
in commercial applications such as small displays for mobile phones, car radios,
and digital cameras [13]. It was suggested that if ”the field continues to progress
at its current, rapid pace, electronics based on organic thin-films materials will
soon become a mainstay of our technological existence” (S. Forrest [14]).
In the academic community, the interest in electrical properties of organic ma-
terials has grown enormously in the last years [16–26]. Different materials and
structures are actively being investigated. On the one hand, the research is mo-
tivated by new applications for organic (plastic) electronics that can overcome
limitations of inorganic materials. On the other hand, the effort is driven by
the intellectual challenges that the investigations of new generation of functional
materials offer. This implies addressing new questions, concepts and models that
allow the manipulation of chemical, structural, physical properties and device
performance. The field is moving very fast; at the moment researchers are able to
control the material properties at the molecular level. They can impose different
molecular packing during growth, add side groups that enhance particular prop-
erties, as a result of the understanding of the various microscopic contributions to
the charge transport. Past investigations were hindered by material instabilities
(to environmental conditions and/or processing methods) and structural defects
that prevented the measurement and understanding of the intrinsic properties of
the organic materials. In the last years, by careful design of new methods that
enable the exploitation of the fascinating properties of carbon-based semiconduc-
tors, good control of the structural properties and charge density distribution in
organic materials is achieved (both by electrostatic and chemical doping). The
degradation is avoided as a consequence of a better understanding of the material
behavior in different conditions (temperature, moisture, light) and interactions
during processing.
1OLED - organic light emitting diodes
1.2. Charge transport in organic semiconductors 3
1.2 Charge transport in organic semiconductors
Organic semiconductors are wide-band-gap and small bandwidth semiconductors.
The HOMO2-LUMO3 gap is in the range of 1-4 eV [27]. With such a large gap,
it might be expected that the organic materials are insulators (an electron would
have to acquire a large thermal energy to make the jump from the valence band
to the conduction band). There are some effective methods that can generate
charge carriers in the organic semiconductors:
⋄ injection of carriers from metallic electrodes;
⋄ optical excitation (creation of electron-hole pairs);
⋄ electrostatic or chemical doping.
Different organic molecular structures are of current interest for the contin-
ued search for novel and useful properties. Carbon nanotubes (CNTs) are one-
dimensional, cylindrical systems that are attractive for nanometer-sized electron-
ics because they combine the high electrical and thermal conductivity with good
mechanical strength and flexibility [28]. The physical properties of nanotubes
are imposed by their diameter, length, and chirality, or twist. For example, they
can be both metallic and semiconducting, depending on the chirality [29]. Nan-
otubes are attractive also for the field emission properties [30] because they are
long and very thin, conductive, with high mechanical strength, and have a low
work-function. As the CNTs are generally accepted as being the bridge between
the nano- and microscopic length scales, a step further was done in coupling them
to biological systems aiming the fabrication of biosensors [31, 32]. More recently,
superconductivity has been observed in nanotubes [33].
When the carbon sheet is not rolled into a tube, it forms a graphene sheet.
Graphene is a candidate for nanometer-scale electronic devices that combine the
properties of carbon nanotubes with the compatibility with established micro-
electronics manufacturing techniques. Low scattering yields mobilities up to 104
cm2/Vs [34]. This value gives ultra-fast-switching transistors for electronics. Also,
the quantum Hall effect was recently reported in graphene [35].
The most common 3D structure in the fullerene structural family is C60,
the spherical buckyball (Fig. 1.1(d)). Because of its particular electronic struc-
ture, C60 exhibits semiconducting properties, but it reveals one amazing prop-
erty after another upon doping [36]. It can be tuned to a metallic or insulating
2HOMO - highest occupied molecular orbital3LUMO - lowest unoccupied molecular orbital
4 Chapter 1. Introduction
state by transferring charge on the molecule through doping. At low temper-
ature this metallic state often exhibits superconductivity, with transition tem-
peratures only exceeded by the high-temperature superconducting copper ox-
ides [37]. This is not an unique type of organic superconductor. Several organic
superconductors have been identified, such as the quasi-one-dimensional Bech-
gaard salts ((TMTSF)2X4 and (TMTTF)2X
5, with X= PF6−, FSO3
−, ClO4−,
etc.), and quasi-two-dimensional salts derived from the donor group BEDT-TTF6
(Fig. 1.1(a)) [38]. Organic superconductors are charge transfer compounds, un-
conventional superconductors, in which the interchain transfer integral can be
changed with pressure, temperature, and composition (the nature of the anion
X controls the chemical pressure), leading to a delicate balance of interactions
that gives rise to a rich variety of physical phenomena. These include charge or-
dering, spin-Peierls ground state, antiferromagnetism, spin density waves (SDW),
metallicity and superconductivity [38].
In polymers, the electrical properties are dominated by disorder that localize
the charges at low temperature. Still, these materials present fascinating proper-
ties that make them attractive for organic electronic devices [1,2,6]. We presented
in Section 1.1 the potential of polymers, that were already incorporated in com-
mercial devices, as components of the OLEDs. Their semiconducting properties
are also used in FETs and solar cells [16–18]. In OLEDs and solar cells an exciton
(excited electron - hole pair) is generated in the organic material. In OLEDs
light is generated through radiative recombination of electrons and holes and it
is emitted through the transparent electrode. Two distinct processes, equally im-
portant, govern the operation of the device: charge transport and recombination.
The color of the emitted light is tuned by the band gap of the active material that
is used. In solar cells light is absorbed through the transparent electrode and the
photons absorbed in the semiconductor create mobile excited electron-hole pairs.
These excitons subsequently undergo dissociation yielding free electrons and holes
that are collected at the contacts. The efficiency of the solar cell is given by the
photon absorption efficiency, exciton dissociation efficiency and charge collection
efficiency. Organic photovoltaic devices are limited by exciton lifetime, and low
charge carrier mobilities; only a fraction of the light-induced excitons contribute
to the current generation. Tang et al. demonstrated that the efficiency can be in-
creased considerably if a heterojunction of a electron donor and electron acceptor
material is manufactured [39]. To enhance the quantum separation efficiency, a
4TMTSF stands for tetramethyltetraselenafulvalene5TMTTF stands for tetramethyltetrathiafulvalene6BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene
1.2. Charge transport in organic semiconductors 5
Figure 1.1: Chemical formulae of organic molecules with different functionalities.
6 Chapter 1. Introduction
network of internal heterojunctions, with a large contact area between donor and
acceptor species (referred to as ”bulk heterojunctions”) was proposed [40].
Despite the fast developments in the field of organic electronics and dramatic
improvement in knowledge and manipulation of the charge injection and trans-
port in organic semiconductors, a reliable relation between microscopic properties
and their effect on the physical properties is still under development. There is
significant work done in understanding the mechanism of conduction in organic
semiconductors. Correct correlations between morphology, molecular packing and
the resulting electronic properties are essential, in order to elucidate fundamental
questions regarding charge transport in these materials. Small molecule con-
ductors allow these type of studies. Because of a higher degree of order than
solution-processed polymers, they can be structurally characterized straightfor-
ward using diffraction, and microscopy techniques. Complementary to this, the
theoretical modelling can be easier implemented because of the lower complexity
of these systems.
In organic molecular solids, the intermolecular forces are weak (van der Waals
and electrostatic type). A detailed description of these issues will be developed
in Section 1.4. The bonding energies are considerably lower than in covalent and
ionic inorganic semiconductors [27,41]. For this reason, the mechanism of charge
transport is fundamentally different. In organic conductors, the charge carriers
interact strongly with the lattice environment leading to polarization effects and
tendency of charge carrier localization. The weak van der Waals interactions result
in a small electronic bandwidth, strong electron-lattice interaction, and polaron
formation. For example, calculations performed on organic molecular crystals
that are free of defects, yield bandwidths in the order of 0.1-0.5 eV [42–47]. This
is more that one order of magnitude lower than the bandwidth in silicon (∼ 10
eV, [48]). The small bandwidths is reflected in low charge carrier mobilities (10−5-
10 cm2/Vs [16–26] for organic semiconductors, compared to 50-500 cm2/Vs in
silicon [49]), and strong interactions between free charge carriers and the lattice.
This interaction facilitates the localization of the charge carriers and narrowing
of the bandwidths even further, and thus are expected to crucially affect the
transport properties. Considerable effort is involved in describing the polaron
dynamics, lifetime, binding energies, and diffusion in the lattice [50, 51].
The mobility of the charge carriers reflects the drift velocity of the charges in
the lattice, thus it is influenced by all interactions that it encounters:
µ =vd
E(1.1)
where µ is the drift mobility, vd is the drift velocity and E the electric field.
1.2. Charge transport in organic semiconductors 7
Scattering by defects, impurities and phonons (lattice vibrations) will decrease
the drift velocity of charges, thus the electronic mobility. The conductivity of the
material is given by charge carrier density n and charge carrier mobility µ.
σ = neµ (1.2)
where e is the elementary charge. The mobility is influenced by the scattering
events, so that [52]:
µ =eτ
m∗(1.3)
where τ is the time between two consecutive scattering events, and m∗ is the
effective mass. This provides a direct relation between the morphology of the
materials and their mobility. In polymers the conduction (µ = 10−8 − 10−4
cm2/Vs) [16, 17] is limited by disorder and hopping of charges between polymer
chains, and thus is lower than in oligomers (µ = 10−3 − 10 cm2/Vs). Even for
the class of oligomer devices, the mobility increases with the degree of structural
order. Structural defects and chemical impurities present in the crystal lattice
promote the localization of the charge carriers. They can either form new states
in the semiconductor band-gap, leading to electronic traps, or scatter the charges.
Both processes result in a decrease of the mobility.
The value of the mobility directly affects the performance of the material in
devices, as it is related with the switching speed. Dramatic improvement in con-
trol and understanding of the transport mechanism in organic materials, on the
molecular level, together with the knowledge on the influence of intrinsic and ex-
trinsic factors on a good performance, has been achieved lately (the value of the
mobility increased 5 orders of magnitude in the last 15 years, reaching the value
of amorphous silicon [49]). This is very important, as some of the possible ap-
plications, like switching devices for active-matrix flat-panel displays (AMFPDs)
based on OLED displays, active-matrix backplanes of OTFTs (organic thin film
transistors) for ”electronic paper” displays, or radiofrequency identification tags
(RFID) require mobilities greater than 1 cm2/Vs [53], values already exceeded in
devices built on organic single crystals [19]. Parallel to organic electronic devices,
organic-inorganic hybrids emerge as new applications by coupling high carrier
mobilities inorganic semiconductors with the flexibility of organic materials [54].
Most organic semiconductors behave as either p-type or n-type semiconduc-
tors (they have either holes, or electrons as majority carriers) (see Fig. 1.1 c, d).
The absence of ambipolar behavior is a severe limitation for the organic electronic
devices for the fabrication of CMOS7-like circuits. Different strategies were pro-
posed to overcome this problem. Most of them involve separate steps to fabricate
7CMOS - complementary metal oxide semiconductor
8 Chapter 1. Introduction
n-type and p-type transistors [17,55]. However, for many materials it is generally
accepted that this is not an intrinsic property, but rather a result of trapping of
one type of charge carriers [16] or due to a high energetic barrier for either elec-
tron or hole injection from the metal electrodes, which is caused by the relatively
large bandgap of organic semiconductors.
1.3 Organic electronic devices
1.3.1 Field effect transistors - operation principles
Organic semiconductors are active materials in different devices. Field effect tran-
sistors, light emitting diodes and solar cells are intensely studied. Organic mate-
rials are soft, fragile and relatively reactive, thus the conventional semiconductors
device fabrication technologies are not always compatible with these compounds.
For this reason the intrinsic electronic properties could not be reached in de-
vices for a long period of time. They were explored with different techniques
(e.g. time of flight [56]). Only lately, using innovative approaches, the fabrication
of organic electronic devices was successful and a good reproducibility between
research groups was achieved [19, 24, 25].
Figure 1.2: Structure of a field effect transistor (FET) with organic semiconductor
as active material. The source electrode is connected to the ground.
This convention is valid for all the devices presented in this thesis.
1.3. Organic electronic devices 9
The field effect transistor (FET) is a three terminal device. The three contacts
are referred to as gate (G), drain (D) and source (S, connected to ground). The
schematic picture of a a FET is drawn in Figure 1.2. The active channel forms at
the semiconductor-insulator interface. The gate insulator acts like a capacitor and
the electric field applied at the gate electrode determines the density of the charge
carriers accumulated at the interface. The current between source and drain is
modulated by the gate voltage (VG). The operation principle of a FET was
introduced by Lilienfeld [57], in 1930, and later developed by Shockley and Pearson
[58]. In 1947 Bardeen, Brattain and Shockley (Bell Laboratories) discovered the
transistor effect and fabricated the first device [59]. They were awarded the Nobel
Prize in physics in 1956 for their discovery. The first metal-oxide-semiconductor
field-effect transistor (MOSFET) was introduced in 1960 by Khang and Atalla
[60].
MOSFETs, built at the surface of inorganic semiconductors, were intensively
studied, and they are incorporated in integrated circuits [41]. Owing to similar
experimental I-V characteristics between organic field-effect transistors (OFETs)
and MOSFETs, the theory developed for MOSFETs is used as starting point
in modelling the OFET behavior. However, the electrical transport in organic
semiconductors is different than in the covalently bonded inorganic semiconduc-
tors. Some attempts to describe the operation principle in OFETs were per-
formed [61, 62].
The transistor channel is active only when the gate voltage (VG) value exceeds
the value of the threshold voltage (VT ). Below this point, the transistor is turned
off, and there is no conduction between drain and source (sub-threshold region).
In the operation of a FET, two distinct regimes can be distinguished (Fig. 1.3(a)).
In the linear regime (small drain-source VD voltages, VD ≪ VG−VT ), the current
between drain and source (ID) depends linearly on the applied voltage (Eq. 1.4).
The device acts as a gate voltage - controlled variable resistor. The value of the
drain current, ID, is given by:
ID =W
LµCi(VG − VT )VD (1.4)
where L is the channel length, W is the gate width, Ci is the capacitance per unit
area of the gate insulator. This model assumes constant velocity, electric field,
and inversion layer charge density between the source and the drain. A more
realistic approach accounts for the variation of the inversion layer charge between
source and drain, and yields the following expression for the current:
ID =W
LµCi
[
(VG − VT )VD −V 2
D
2
]
(1.5)
10 Chapter 1. Introduction
Figure 1.3: I−V curves for a FET operation. (a) Output characteristics (ID−VD)
for different applied gate voltages (VG). The curves are obtained from
simulating the I − V curves that correspond to expression 1.5. The
linear and saturation regimes are indicated. The dotted line points
the pinch-off that separates the linear region of operation on the left
from the saturation region on the right. (b) Transfer characteristics
(ID − VG) for different drain voltages. The curves correspond to ex-
perimental points for a pentacene transistor.
At higher VD voltages, the channel is not continuous, but a depletion area forms at
the drain contact. The onset of this region is called pinch-off (Fig. 1.3(a)). Beyond
this point, the operation regime is referred to as saturation regime. The drain
current is now relatively independent of the drain voltage, being only controlled
by the gate voltage, and varies quadratically with the field:
ID =W
2LµCi(VG − VT )2 (1.6)
Equations 1.4, 1.5, and 1.6 represent expressions that yield the value of the mo-
bility of the semiconductor. The mobility can also be estimated from the gate
voltage sweep (Fig. 1.3(b)), at low drain voltages VD. Here, the expression for
the transconductance (gm) is:
gm =∂ID
∂VG(1.7)
From this equation, the value of the mobility can be extracted:
µ =L
W
1
CiVD
( ∂ID
∂VG
)
VD→0(1.8)
1.3. Organic electronic devices 11
However, all these expressions assume a field independent mobility, and that all
the charges induced by the gate voltage are mobile.
Key parameters in the operation of a FET are the electronic mobility of the
charge carriers and the on/off ratio. The first determines the switching speed and
the maximum current, and the later impose the switching of the device from a
non-conducting (off ) to a conducting state (on).
Good performance organic field-effect transistors (OFETs) were fabricated at
the surface of the organic single crystals using deposition techniques that min-
imize damage at the interface [19, 24]. Fabrication of devices with competitive
characteristics remains an ambitious task for large-scale applications.
1.3.2 Fabrication of OFETs
Owing to the fragility of organic materials, processing to incorporate them in
electronic devices represents a challenge at this early stage. However, they trigger
interest to develop revolutionary methods that are simple and efficient to be used
for large scale applications. In this section we will describe recent advances in
organic electronic devices, focusing on OFETs fabricated on molecular crystals.
Different device structures were proposed to study the electrical transport at the
surface of organic crystals, in order to measure a high intrinsic mobility, that
is not diminished by disorder introduced during processing. There are many
factors involved in the good performance of the device. In spite of the better
reproducibility that is archived lately, the results reported by different groups
are still not always consistent. This can be attributed in part to the fact that
the performance of the electronic devices depends critically on the quality of the
crystals and the interfaces, as well as on the extrinsic factors (like, for example
the environmental conditions in which the experiments were performed). Organic
semiconductor quality, dielectric properties, contacts, and interface properties are
equally important.
Deposition of the organic semiconductor
The most common method used for the deposition of small molecule conductor
films is vacuum sublimation. The macroscopic electronic properties of the films
are imposed by their crystallinity [53]. Better crystallinity and larger grain size
facilitates higher mobilities. Values of 1 cm2/Vs were reached in vacuum subli-
mated pentacene TFTs, after optimization of the fabrication process [85].
We mentioned in Section 1.1 that the highest impact that the organic elec-
tronics can produce over traditional Si-based technology, is the relatively easy
12 Chapter 1. Introduction
processing techniques that their deposition requires. Polymers are attractive be-
cause they are soluble in organic solvents, thus they can be spin coated or printed
on flexible substrates, forming amorphous or polycrystalline films. Still, the high-
est mobilities are achieved in devices build with small molecules. A drawback is
that their solubility is limited and they require deposition methods like vacuum
sublimation, or physical vapor deposition. These demands are not straightforward
to accomplish. Because a high electronic mobility is not sufficient for a material
to be competitive for large scale applications, scientists develop different methods
to facilitate the compatibility with cheap and easy solution processing techniques.
Herwig et al. proposed a synthetic concept for fabrication of a soluble pentacene
precursor [63]. The precursor is converted to pentacene via thermal [63] or ir-
radiative [64] treatment, and the obtained thin film transistors (TFTs) exhibit
mobilities of 0.2 cm2/Vs. A different route to increase the solubility of oligomers
is the attachment of flexible side groups. At the molecular design, careful atten-
tion is payed to the interplay between the degree of solubility and the molecular
stacking that the side group induces. Field-effect transistors with maximum mo-
bilities of 0.01 cm2/Vs were fabricated from quaterthiophene and hexathiophene
end-substituted with 3-butoxypropyl groups [65].
The above mentioned directions represent a compromise, a balance between
performance and cost, because the mobility of materials deposited from solution
remains lower than that of the thermal evaporated material [19, 20]. Moreover,
in polycrystalline films [20, 21, 23], the mobility is lower than in single crystals
[19, 24–26]. This is partially caused by a large grain boundary resistance.
Gate dielectric materials
In field-effect transistors the conduction takes place at the surface of the semicon-
ductor, thus the performance is limited by the quality of the interface between
organic and dielectric, and only in part by the bulk properties. This is evident
from experiments that demonstrate that the gate insulator can modify the charge
density at the interface, having a crucial effect on the operation of both poly-
mer [66] and small molecule [67] devices. Particularly important are the roughness
of the semiconductor/dielectric interface, and the density of defects and impuri-
ties present in this region. Different treatments of the dielectric were proposed in
order to decrease the trap density (e.g. OTS8 treatment [68]).
General requirements for a high quality dielectric include several parameters.
The introduction of a large capacitance, that governs the magnitude of charge
8OTS denotes octadecyltrichlorosilane
1.3. Organic electronic devices 13
Figure 1.4: Different geometries of the OFETs. Source (S), drain (D), and gate
electrodes are indicated. The transport takes place at the interface
between semiconductor layer and gate insulator layer. (a) bottom-
gate geometry; (b) top-gate geometry.
induced in the channel by gate effect, can be done using high-k dielectrics or by
varying their thickness. Besides this, a good insulator in FETs should account
for a large breakdown voltage and low leakage currents, excellent thermal and
chemical stability. There are three classes of dielectric materials incorporated in
OTFTs [69]: inorganic dielectric materials (e.g. SiO2, Ta2O5, Al2O3), organic
dielectric materials (e.g. parylene, PS9, PMMA10, Fig. 1.1(e)) and self-assembled
mono-and multilayers. In FETs with top-gate configuration (Fig. 1.4(a)), the
deposition of inorganic dielectrics partially damages the surface of the organic
crystals and introduces chemical and structural disorder, due to violent process-
ing methods (sputtering, e-beam, plasma-enhanced chemical vapor deposition).
This is the reason why organic dielectrics are used only in bottom-gate devices
(Fig. 1.4(b)). The deposition of an organic dielectric requires lower temperatures
(typical deposition techniques are spin-coating, casting, and printing), are not
destructive, and yield remarkably high mobilities (8 cm2/Vs in rubrene single
crystals transistors with parylene gate dielectric [24]).
Contacts
Contact issues are amply discussed in the field of molecular electronics. Different
models are proposed, as it is believed that the charge transport is either limited by
injection problems due to poorly defined contacts, or by bulk conductivity. Baldo
9PS denotes polystyrene10PMMA denotes polymethylmethacrylate
14 Chapter 1. Introduction
et al. elaborated an interface injection model, that accounts for two distinct
injection steps [70]. During the first step, charges are injected from the contacts
into an interface region that contain a broad distribution of states induced by
interface dipoles. In the second, limiting step, the charges migrate from the
interface to the bulk region.
Two types of device geometries are used in the fabrication of the field-effect
transistors: top-contact and bottom-contact structures. In the first case, it was
observed that the deposition of contacts on the organic semiconductors inflicts
with their chemical and mechanical fragility, and introduce traps locally, at the
metal/organic interface [71]. Different deposition methods are used, depending
on the nature of the contact. Metal contacts are usually evaporated or sputtered
using shadow masks, or painted (the latter method is also applied for colloidal
graphite paste). The choice of the metal contact gives the value of the Schottky
barrier, thus the deviation from the ohmic regime, and also the type of the major-
ity carriers. Alternative contact materials, with specific deposition requirements
were proposed and successfully applied. Room temperature lamination of metal
coated elastomeric stamps (e.g. PDMS11 [19]) represents a non-destructive, high
resolution method to fabricate contacts. The polymer PANI12 and the copolymer
PEDOT:PSS13 (Fig. 1.1(b)) are currently widely used in organic devices, espe-
cially OLEDs and solar cells because of their excellent transparency in the visible
region, but also in FETs due to good electrical conductivity, and environmental
stability [72].
1.3.3 Outlook
Reviewing the recent advances in materials, methods and emerging applications,
two very important issues can be mentioned. Firstly, organic semiconductor re-
search is an exciting and interdisciplinary area of current research activity that
raised the interest of theorists, chemists, physicists, and device scientists and is
developing extremely fast. Secondly, organic materials impose a new way of think-
ing because they exhibit a wide range of electrical properties, being tunable from
insulator (in gate dielectrics) to semiconductor (as active layers in devices), to
metal, and even superconductor. A scheme with the wide range of functionalities
that the organic materials offer can be found in Figure 1.1(a-e). These diverse
properties are spectacular and give the opportunity of building ”all-organic de-
vices”.
11PDMS denotes polydimethylsiloxane12PANI denotes polyaniline13PEDOT:PSS denotes polyethylene(3,4-dioxythiophene)/polystyrene sulfonate
1.4. Organic molecular crystals 15
1.4 Organic molecular crystals
Characteristic of organic semiconductors are the intramolecular bonds. The
alternation of single (σ) and double (π) bonds, referred to as conjugation, is typ-
ical for organic conductors. The Carbon atoms involved in this type of bonding
are sp2 hybridized. The three hybrid sp2 orbitals form the σ bonds, with the
σ-electrons being highly localized. The remaining nonhybridized pz orbitals of
adjacent Carbons form the π- orbitals perpendicular to the σ bonds and delocal-
ized over the molecule. The σ bonds are very strong (they are positioned lower
in energy). The molecular orbitals that are filled (π-bonding orbitals) form the
valence states. The filled orbital with the highest energy is called the Highest
Occupied Molecular Orbital. The vacant orbitals (π∗-antibonding orbitals) form
the conduction band, with LUMO being the Lowest Unoccupied Molecular Or-
bital. The π-electrons are responsible for an important part of the intermolecular
conduction in these materials. Besides this, the molecular packing in the solid,
imposed by the physical interactions, determine the physical properties, e.g. the
electronic transport.
The intermolecular forces are weak, and result from the cooperation and
competition between π − σ and π − π interactions. The interaction energy be-
tween molecules originates from several contributions. The electrostatic inter-
action between permanent multipoles (usually quadrupoles), and the dispersion
(van der Waals) forces between induced dipoles represent the dominant force.
The interaction between the permanent multipoles and the induced multipoles,
called induction, is generally treated as a second order term. In a simple pictorial
model, the van der Waals forces between molecules originate from the instan-
taneous polarization of the neutral molecules into dipoles [27]. The temporary
generated dipoles promote the formation of new induced dipoles in neighboring
molecules and spread in the lattice. For interplanar separation that characterize
organic molecular crystals (d > 3.4 A [32]), the van der Waals forces are always
attractive. The quadrupolar interactions emerge from the coupling between the
permanent quadrupolar moments of the molecules [73, 74], and can be attractive
or repulsive, depending on the relative orientation of the quadrupoles.
The crystal structure is determined by an interplay between van der Waals
forces and quadrupolar interactions. The two competing interactions promote
different molecular stacking that minimize the energy. The van der Waals inter-
actions are optimized for ”face-to-face” orientation of the molecules, that result
in the maximum π− overlap. On the other hand, the presence of quadrupolar
moments favor the ”edge-to-face” stacking, in which the hydrogen on one ring
16 Chapter 1. Introduction
Figure 1.5: Crystal structure of pentacene single crystal. Left panel: view along
the [100] axis (the layered structure can be seen). Right panel: the
herringbone arrangement within the layer, view along the long axis of
the molecule. The unit cell orientation is also drawn.
encounter the π− network of the adjacent molecule. The most common crystal
packing that results from the afore mentioned mechanisms is the herringbone-
arrangement (Fig. 1.5). Oligomers often crystallize in a layered fashion, and
the molecular arrangement within the layers is imposed by the van der Waals
and quadrupolar interactions. Gavezzotti et al. distinguished four possible her-
ringbone modes in which polynuclear aromatic hydrocarbons pack: herringbone
structure (naphthalene, anthracene, pentacene ), sandwich herringbone structure
(pyrene, perylene), γ structure (benzopyrene, coronene), β structure (trybenzopy-
rene, tetrabenzoperylene) [75]. Cofacial π-stacking in molecular crystals is hardly
ever found. Anthony et al. synthesized these types of structure by substitution
of different groups to pentacene backbone [76]. They showed that different type
and amount of π-overlap can be controlled by the nature, size and position of the
substituent, leading to various stacking motifs. Although seldom encountered,
cofacial configurations are particularly attractive because they accommodate the
largest electronic splitting in the HOMO and LUMO levels, promoting the high-
est theoretical predicted charge carrier mobilities [77]. In the local picture (as
opposed to the band picture) of the charge transport in organic conjugated ma-
terials, the efficiency of this process reflects how easy the charges are transferred
1.4. Organic molecular crystals 17
between neighboring sites, and, as expected, is very sensitive to orientation of the
molecules with respect to each other. The electronic coupling between adjacent
molecules, quantified by the transfer integral t, is modulated by the molecular
arrangement and directly associated with the electronic mobility [45, 46, 77]. In
the framework of these calculations, the amplitude of the electronic coupling is
influenced by the intermolecular separation distance, the molecular overlap, the
length of the molecule, and, in the case of herringbone structures, the rotation
of molecular planes [77]. Additionally, the relevance of thermal motions in the
modulation of the electronic coupling between molecules in an an organic solid
was demonstrated [45].
Owing to the weak interaction forces between molecules in the solid, small
variations in the crystal packing can be present, leading to polymorphism. The
term polymorphism refers to the existence of more than one crystal structure
for a particular compound. This phenomenon is frequently displayed by organic
crystals and is driven by the growth conditions and/or subsequent treatment. In
rubrene, different polymorphs can be obtained, depending on the pressure in the
system in which the starting material is sublimed (see Chapter. 6 in this thesis
and the references therein). In quaterthiophene (α−4T) [79] and sexithiophene
(α−6T) [80,81] different molecular arrangements are induced by the source tem-
perature. The general picture is even more complex in pentacene films, where
four polymorphs were detected [78, 82]. The obtained polymorph is dictated by
the substrate type, the substrate temperature and the thickness of the film. For
pentacene single crystals, only one polymorph exists [78].
The existence of different polymorphs for some molecular crystals provides
a unique opportunity to understand the influence of the crystal packing on the
electronic properties, since these systems only differ with the orientation of the
constituent building blocks with respect to each other, the molecule being the
same [42, 46, 81]. For example, Troisi and Orlandi performed band structure
calculations on the four pentacene polymorphs and found that the mobility tensor
is highly anisotropic for three of the four considered polymorphs [83]. This result
is relevant for understanding the fundamental mechanisms of charge transport in
organic crystals.
18 Chapter 1. Introduction
1.5 Single crystals - model systems for investiga-
tion of intrinsic properties
The progress in the field of organic electronics requires a good fundamental under-
standing of factors that influence the electronic behavior. This leads to a better
control of the microscopic properties that determine the conduction of the mate-
rials used in the devices. A correct insight on the interplay between the effects of
chemical structure and molecular orientation on the transport process can only
be accomplished when single crystals are used.
Single crystals are not meant to be incorporated in applications, but to provide
a well defined structure, in which the intrinsic electronic properties of the material
can be measured [19,24–26,84]. They serve as model systems, for which structure-
properties correlations can be explored.
Although thin films (polycrystalline or amorphous) are more attractive for
organic electronic devices, here the intrinsic properties are generally masked by
grain boundaries where structural defects localize and trap the charge carriers.
The major limitation of thin-film transistors (TFTs) comes from the fact that their
performance is severely dependent on the fabrication conditions. Moreover, even
films that are grown in an identical manner can exhibit very different electrical
properties [85].
The electrical properties of organic single crystals have been successfully probed
by time-of-flight (TOF) [56] and space-charge-limited current (SCLC) methods
[26], as well as field-effect transistor (FET) experiments [19, 24, 25].
1.6 Aim of the present research
Motivated by the various properties that organic semiconductors reveal, we eval-
uate in this thesis a critical analysis of the different factors that determine the
electrical conduction in these materials. Although the progress is fast, both in
performance and reliability, there are still numerous questions that lack an answer,
or for which inconsistent explanations were given.
The questions we address in this thesis concern intrinsic and extrinsic factors
that determine the charge transport in organic conductors. In order to answer
these questions we focus our work on single crystals (the advantages of working
with single crystals are outlined in Section 1.5). We systematically study the effect
of structural defects and impurities (Chapter 2, Chapter 3), geometrical factors
(Chapter 4), molecular stacking (Chapter 6) and exposure to ambient conditions
1.7. Outline of the thesis 19
(Chapter 5).
While the organic field-effect transistor characteristics were improved in the
past years by implementation of novel processing techniques, the charge carrier
mobility remains limited. In this context, our efforts focus on the understanding
of the microscopic processes that determine the value of the mobility in molecular
crystals, in particular the generation and migration of defects. The work aims
not only at improving the performance of organic materials for possible incor-
poration in devices, but also at satisfying the pressing need for insight into the
relevant physical processes that govern the electrical conduction in these materi-
als. We show that even small defect densities are critical and act as charge traps,
consequently decreasing the mobility significantly. By systematic measurements,
we demonstrate that the origin of defects is of dual nature. The trapping sites
such as crystal defects and chemical impurities are created during crystal growth.
Additional traps are introduced during processing or exposure to ambient con-
ditions. Moreover, we propose successful methods that allow the study of high
quality materials with desired properties.
Furthermore, we incorporate the investigated materials into field-effect de-
vices. Here, the conduction channel is limited to the vicinity of the interface. We
show that the conductivity is strongly influenced by disorder and charge traps
near the interface, and propose a reliable method to overcome this limitation.
Hence, our efforts can broadly be defined as contributions in the area of fun-
damental and applied studies on the organic molecular crystals properties. By
means of correlations between electrical measurements and structural properties,
determined via X-ray diffraction, or thermodynamic response via thermogravi-
metric experiments, we wish to cover important features of the chemical, physical
and technological problems that arise in these materials.
1.7 Outline of the thesis
This thesis is organized as follows:
Chapter 2 emphasizes the importance of the control of defects and impurity
states in organic molecular crystals in order to obtain a high electronic mobil-
ity. We measure record electronic mobilities as a result of the reduction of the
number of traps by careful crystal growth and subsequent handling. The temper-
ature dependence of the mobility is consistent with the band model for electronic
transport.
In recent studies, little attention is paid to the concentration, distribution
of impurities and their consequences on electronic properties. Thus far, it has
20 Chapter 1. Introduction
been generally assumed that the impurities are evenly distributed throughout
the lattice. In Chapter 3 we will show that this is not the case for pentacene
single crystals, where the impurities are are located preferentially at the surface.
Moreover, we describe a new, reliable method, and demonstrate that in our field-
effect transistor devices the impurity states at the interface can be made inactive
by incorporating them into the dielectric gate barrier.
In Chapter 4 we give a geometrical description of the electric field distribution
in organic crystals. We report the cross-over from 1D to 2D - space charge limited
conduction in pentacene single crystals with planar contacts. Furthermore, we
establish the parameters for which the conduction is dominated by bulk charges
and show that the transition to a surface-governed transport takes place gradually.
These results incorporate corrections for the anisotropic resistivity.
Chapter 5 is dedicated to the effect of air exposure on the electronic prop-
erties of pentacene single crystals. We show experimentally that air can diffuse
reversibly in and out of the crystals. This process is reflected in the electrical
properties. We are able to distinguish two competing mechanisms that modulate
the electronic transport: the doping effect of oxygen and the trapping caused by
the presence of water vapor. By combining the gravimetric and electric measure-
ments, we can describe quantitatively these effects.
The investigations that yield the results presented in Chapters 2, 3, 4, and 5
focused on the study of pentacene single crystals, the first organic crystalline
material already incorporated in prototypes of commercial devices [20]. The mea-
surements discussed in Chapter 6 were motivated by intriguing changes observed
in the values of electronic mobility of rubrene single crystals at low temperatures.
We relate this changes with the structural transformations in the material.
References
[1] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks,
C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. -L. Bredas, M. Logdlund,
and W. R. Salaneck, Nature 397, 121 (1999)
[2] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B.
M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P.
Herwig, and D. M. de Leeuw, Nature 401, 685 (1999)
[3] A. Dodabalapur, H. E. Katz, L. Torsi, and R. C. Haddon, Science 269, 1560
(1995)
[4] B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Filas, Z. Bao, A. LaDuca, R.
Sarpeshkar, H. E. Katz, and W. Li, Nature 403, 521 (2000)
[5] C. R. Kagan, D. B. Mitzi, and C. D. Dimitrakopolous, Science 286, 945
(1999)
[6] G. Malliaras, and R. Friend, Physics Today May 2005, 53 (2005)
[7] S. Coe, W. K. Woo, M. Bawendi, and V. Bulovic, Nature 420, 800 (2002)
[8] K. Tashiro, Ferroelectric Polymers, H. S. Nalwa ed. (Marcel Dekker, New
York, 1995)
[9] R. C. G. Naber, C. Tanase, P. W. M. Blom, G. H. Gelinck, A. W. Marsman,
F. J. Touwslager, S. Setayesh, and D. M. de Leeuw, Nat. Mat. 4, 243 (2005)
21
22 References
[10] X. Guo, J. P. Small, J. E. Klare, Y. Wang, M. S. Purewal, I. W. Tam, B. H.
Hong, R. Caldwell, L. Huang, S. O’Brien, J. Yan, R. Breslow, S. J. Wind, J.
Hone, P. Kim, and C. Nuckolls, Science 311, 356 (2006)
[11] A. Bonfiglio, D. De Rossi, T. Kirstein, I. R. Locher, F. Mameli, R. Paradiso,
and G. Vozzi, IEEE Trans. on Info. Tech. in Biomed. 9, 319 (2005)
[12] http://www.research.philips.com/newscenter/archive/2004/rollabledisplay.html
[13] Components fabricated on organic materials and introduced on the market
include displays for mobile phones (Philips 639 - a clamshell with a PolyLED
external display), portable digital music players (Philips SA1784-SA179 ), car
radios, men’s shaver (Philishave 8894) and digital cameras (Kodak LS633)
[14] S. F. Forrest, Nature 428, 911 (2004)
[15] C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa,
E. J. Louis, S. C. Gau, and A. G. MacDiarmid, Phys. Rev. Lett. 39, 1098
(1977)
[16] L. L. Chua, J. Zaumseil, J. F. Chang, E. C. W. Ou, P. K. H. Ho, H. Sirring-
haus, and R. H. Friend, Nature 434, 194 (2005)
[17] E. J. Meijer, D. M. de Leeuw, S. Setayesh, E. van Veenendaal, B. H. Huisman,
P. W. M. Blom, J. C. Hummelen, U. Scherf, and T. M. Klapwijk, Nat. Mat.
2, 678 (2003)
[18] R. J. Kline, M. D. McGehee, E. N. Kadnikova, J. S. Liu, and J. M. J. Frechet,
Adv. Mat. 15, 1519 (2003)
[19] V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett, T.
Someya, M. E. Gershenson, and J. A. Rogers, Science 303, 1644 (2004)
[20] G. H. Gelinck, H. E. A. Huitema, E. van Veenendaal, E. Cantatore, L. Schri-
jnemakers, J. B. P. H. van der Putten, T. C. T. Geuns, M. Beenhakkers, J.
B. Giesbers, B. H. Huisman, E. J. Meijer, E. M. Benito, F. J. Touwslager,
A. W. Marsman, B. J. E. van Rens, and D. M. de Leeuw, Nat. Mat. 3, 106
(2004)
[21] Y. J. Zhang, J. R. Petta, S. Ambily, Y. L. Shen, D. C. Ralph, and G. G.
Malliaras, Adv. Mat. 15, 1632 (2003)
[22] D. V. Lang, X. Chi, T. Siegrist, A. M. Sergent, and A. P. Ramirez, Phys.
Rev. Lett. 93, 086802 (2004)
References 23
[23] D. J. Gundlach, T. N. Jackson, D. G. Schlom, and S. F. Nelson, Appl. Phys.
Lett. 74, 3302 (1999)
[24] V. Podzorov, S. E. Sysoev, E. Loginova, V. M. Pudalov, and M. E. Gershen-
son, Appl. Phys. Lett. 83, 3504 (2003)
[25] R. W. I. de Boer, T. M. Klapwijk, and A. F. Morpurgo, Appl. Phys. Lett.
83, 4345 (2003)
[26] O. D. Jurchescu, J. Baas, and T. T. M. Palstra, Appl. Phys. Lett. 84, 3061
(2004)
[27] M. Pope, and C. E. Swenberg, Electronic Processes in Organic Crystals and
Polymers, 2nd ed. (Oxford University Press, New York, 1999)
[28] S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley, L. J. Geerligs,
and C. Dekker, Nature 386, 474 (1997)
[29] T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H. F. Ghaemi, and T.
Thio, Nature 382, 54 (1996)
[30] N. de Jonge, Y. Lamy, K. Schoots, and T. H. Oosterkamp, Nature 420, 393
(2002)
[31] S. S. Wong, E. Joselevich, A. T. Woolley, C. Li Cheung, and C. M. Lieber,
Nature 394, 52 (1998)
[32] K. A. Williams, P. T. M. Veenhuizen, B. G. de la Torre, R. Eritja, and C.
Dekker, Nature 420, 761 (2002)
[33] Z. K. Tang, L. Zhang, N. Wang, X. X. Zhang, G. H. Wen, G. D. Li, J. N.
Wang, C. T. Chan, and P. Sheng, Science 292, 2462 (2001)
[34] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.
Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004)
[35] Y. B. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, Nature 438, 201 (2005)
[36] M. Mehring, K. F. Thier, F. Rachdi and T. de Swiet, Carbon 38, 1625 (2000)
[37] A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum,
T. T. M. Palstra, A. P. Ramirez, and A. R. Kortan, Nature 350, 600 (1991)
[38] D. Jerome, Science 252, 1509 (1991)
24 References
[39] C. W. Tang, App. Phys. Lett. 48, 183 (1986)
[40] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science 270,
1789 (1995)
[41] S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)
[42] R. C. Haddon, X. Chi, M. E. Itkis, J. E. Anthony, D. L. Eaton, T. Siegrist,
C. C. Mattheus, and T. T. M. Palstra, J. Phys. Chem. B 106, 8288 (2002)
[43] R. Silbey, J. Jortner, S. A. Rice, and M. T. Vala, Jr., J. Chem. Phys. 42,
733 (1965)
[44] G. Brocks, J. van den Brink, and A. F. Morpurgo, Phys. Rev. Lett. 93,
146405 (2004)
[45] A. Troisi, and G. Orlandi, Phys. Rev. Lett. 96, 086601 (2006)
[46] Y. C. Cheng, R. J. Silbey, D. A. da Silva Filho, J. P. Calbert, J. Cornil, and
J.-L. Bredas, J. Chem. Phys. 118, 3764 (2003)
[47] K. Hannewald, V. M. Stojanovic, J. M. T. Schellekens, P. A. Bobbert, G.
Kresse, and J. Hafner, Phys. Rev. B 69, 075211 (2004)
[48] http://www.eecs.umich.edu/∼singh
[49] J. M. Shaw, and P. F. Seidler, IBM J. Res.& Dev. 45, 3 (2001)
[50] K. Hannewald, and P. A. Bobbert, Phys. Rev. B 69, 075212 (2004)
[51] R. W. Munn, and R. Silbey, J. Chem. Phys. 83, 1843 (1985)
[52] N. W. Ashcroft, and N. D. Mermin, Solid state physics (Saunders College
Publishing, 1979)
[53] C. D. Dimitrakopoulos, and D. J. Mascaro, IBM J. Res. & Dev. 45, 11 (2001)
[54] C. R. Kagan, D. B. Mitzi, and C. D. Dimitrakopoulos, Science 286, 945
(1999)
[55] Y. Y. Lin, A. Dodabalapur, R. Sarpeshkar, Z. Bao, W. Li, K. Baldwin, V.
R. Raju, and H. E. Katz, Appl. Phys. Lett. 74, 2714 (1999)
[56] W. Warta, and N. Karl, Phys. Rev. B 32, 1172 (1985)
[57] J. E. Lilienfeld, U.S. patent 1745175 (1930)
References 25
[58] W. Shockley, and G. L. Pearson, Phys. Rev. 74, 232 (1948)
[59] W. Shockley, U.S. patent 02569347 (1951)
[60] D. Khang, and M. M. Atalla, IRE Solid-State Devices Res. Conf., Carnegie
Institute of Technology, Pittsburg, Pa. (1960)
[61] G. Horowitz, R. Hajlaoui, R. Bourguiga, and M. Hajlaoui, Synth. Met. 101,
401 (1999)
[62] G. Horowitz, and P. Delannoy, J. App. Phys. 70, 469 (1991)
[63] P. T. Herwig, and K. Mullen, Adv. Mat. 11, 480 (1999)
[64] A. Afzali, C. D. Dimitrakopoulos, and T. O. Graham, Adv. Mat. 15, 2066
(2003)
[65] H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siergist, W. Li, Y.-Y.
Lin, and A. Dodabalapur, Nature 404, 478 (2000)
[66] J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, and S. M. Khaffaf,
Adv. Func. Mat. 13, 199 (2003)
[67] A. F. Stassen, R. W. de Boer, N. N. Iosad, and A. F. Morpurgo, Appl. Phys.
Lett. 85, 3899 (2004)
[68] C. Goldmann, S. Haas, C. Krellner, K. P. Pernstich, D. J. Gundlach, and B.
Batlogg, J. App. Phys. 96, 2080 (2004)
[69] A. Facchetti, M. -H. Yoon, and T. J. Marks, Adv. Mat. 17, 1705 (2005) and
the refferences therein
[70] M. A. Baldo, and S. R. Forrest, Phys. Rev. B, 64, 085201 (2001)
[71] R. W. I. de Boer, and A. F. Morpurgo, Phys. Rev. B, 72, 073207 (2005)
[72] B. J. de Gans, P. C. Duineveld, and U. S. Schubert, Adv. Mat. 16, 203 (2004)
[73] C. A. Hunter, and J. K. M. Sanders, J. Am. Chem. Soc. 112, 5525 (1990)
[74] C. R. Newman, C. D. Frisbie, D. A. da Silva Filho, J.-L. Bredas, P. C.
Ewbank, and K. R. Mann, Chem. Mat. 16, 4436 (2004)
[75] G. R. Desiraju, and A. Gavezzotti, Acta. Cryst.B 45, 473 (1989)
[76] J. E. Anthony, D. L. Eaton, and S. R. Parkin, Org. Lett. 4, 15 (2002)
26 References
[77] J. Cornil, D. Beljonne, J.-P. Calbert, and J.-L. Bredas, Adv. Mat. 13, 1053
(2001)
[78] C. C. Mattheus, A. B. Dros, J. Baas, A. Meetsma, J. L. de Boer, and T. T.
M. Palstra, Acta. Cryst. C 57, 939 (2001)
[79] T. Siegrist, C. Kloc, R. A. Laudise, H. E. Katz, and R. C. Haddon, Adv.
Mat. 10, 379 (1998).
[80] G. Horowitz, B. Bachet, A. Yassar, P. Lang, F. Demanze, J.-L. Fave, and F.
Garnier, Chem. Mat. 7, 1337 (1995)
[81] T. Siegrist, R. M. Fleming, R. C. Haddon, R. A. Laudise, A. J. Lovinger, H.
E. Katz, P. Bridenbaugh, and D. D. Davis, J. Mater. Res. 10, 2170 (1995)
[82] C. C. Mattheus, G. A. de Wijs, R. A. de Groot, and T. T. M. Palstra, J.
Am. Chem. Soc. 125, 6323 (2003)
[83] A. Troisi, and Giorgio Orlandi, J. Phys. Chem. B 109, 1849 (2005)
[84] C. Reese, and Z. Bao, J. Mater. Chem. 16, 329 (2006)
[85] S. F. Nelson, Y.-Y. Lin, D. J. Gundlach, and T. N. Jackson, Appl. Phys.
Lett. 72, 1854 (1998)