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Contents:
0 Introduction
I Concepts and Materials
I.1 Chemistry of Carbon
I.1.a Hybrid orbitals
I.1.b The benzene ring
I.1.c Conjugated molecules
I.2 Examples of organic semiconductors
I.3 Excitations in organic semiconductors
I.3.a Polarons and excitons
I.3.b Light emission from organic molecules
I.3.c Controlling the bandgap
I.4 Charge carrier injection
II Organic semiconductor applications and devices
II.1 Synthetic Metals
III.1.a Water- based synthetic metals
III.1.b Applications of synthetic metals
II.2 Organic field effect transistors
III.2.a Description of organic FET operation
III.2.b Requirements on OFET materials
II.3 Organic light emitting devices
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II.3.a Overview over basic phenomena
II.3.b Bipolar carrier injection
III.4.c Exciton formation
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0 Introduction
Knowledge of the following concepts and terms
from solid state physics is essential and will bepresumed:
Metal, semiconductor, Fermi level, work function,
conduction band, valence band, band gap, doping,
hole, chemical / electrochemical potential, wave
vector, quasiparticle, polaron, (Wannier)
exciton, conductivity, (positive / negative)temperature coefficient (PTC / NTC).
Please read up in a solid state physics textbook
about any of these concepts you feel you are not
familiar with.
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I.1 Chemistry of Carbon (C)
For our context, the key difference between
organic and inorganic solid matter is that
excitations in inorganic matter are delocalised
and best described by a wave vector k, while in
organic matter, excitations usually are localised
and k is not a good quantum number. To understand
organic semiconductors (and, maybe synthetic
metals?), we have to understand how something
like a bandgap can arise within a single
molecule. The key to this understanding lies in
the chemistry of carbon.
The most common carbon isotope is 12C (nucleus has
6 neutrons, 6 protons), but there is a natural
abundance of 1.2% the 13C isotope with 7 neutrons,
6 protons. This has a nuclear magnetic momentum,
which is used in NMR. In atomic carbon, the 6
electrons occupy the following orbitals, table
I.1:
Table I.1
Orbital: 1s 2s 2px 2py 2pz
No. of electrons: 2 2 1 1 0
Table I.1: Electronic configuration of carbon. 1smeans principle quantum number (QN) n=1, Orbital
QN l = 0, consequently magnetic QN 0; 2 electrons
go into 1s due to 2 spins. Briefly, this is
written as 1s22s
22p
2.
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I.1.a Hybrid orbitals
Carbon, like most chemical elements, forms
covalent bonds. The driving force for chemical
reaction is the desire to share electrons between
different atoms to complete electronic shells.Thus, usually,
Atomic Orbitals Molecular Orbitals
Carbon should form 2 bonds to add 2 electrons to
complete the vacancies in the two incomplete p
orbitals (px and py): Carbon should be divalent(form 2 single bonds).
In reality, carbon forms 4 bonds. In C (and some
other atoms), chemical bonding proceeds via
intermediate steps: Promotion and
Hybridisation:
Atomic orbitals Hybrid Orbitals MolecularOrbitals
For hybridisation, carbon promotes one 2s
electron into the empty pz orbital, we arrive at
1s22s12p3. Then, C combines (hybridizes) the
remaining 2s electron and either:
three 2p orbitals sp3 hybrids
or two 2p orbitals sp2 hybrids
or one 2p orbital sp hybrid
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sp3
hybrid orbitals have 4 fingers pointing
into space symmetrically, i.e. into the corners
of a tetraeder. The angle between the fingers
is 109.5o. In this form, C can form 4 bonds, e.g.
by sharing electrons with Hydrogens 1s shell:CH
4(methane), or with other sp
3carbon (e.g. H
3C-
CH3, ethane). The C-C bond in ethane is called a bond. bonds are very strong: Diamond consistsof carbon held together by bonds entirely.
sp2hybrid orbitals have 3 fingers in a plane,
with 120
o
to each other, plus one remaining porbital perpendicular to the plane. In this form,
C needs another sp2 hybrid C to form a molecule,
e.g. H2C=CH2: 2 of the 3 fingers of each C bond
to H, as before, the third overlaps with another
C sp2 orbital to form a bond ( bond). The
remaining p orbitals of either C overlap, as
well, to form another carbon/carbon bond, the so-
called bond. (1+1 bond: Carbon double bond)This is a weaker bond, and the respective orbital
is more delocalized, i.e. occupies relatively
large space rather far away from its original
carbon.
sp hybrid orbitals have 2 fingers along one
axis (say, x) 180
o
to each other, plus 2remaining p orbitals (along y and z axis). In
this form, C can bond e.g. with 2 H and another
sp hybrid. It forms one bond between the sp
orbitals, plus the remaining 2 p orbitals of each
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molecule overlap to form two bonds (carbon
triple bond). This is Ethene (acetylene), HCCH.
The bond lengths are: C-C 1.45 , C=C 1.33 ,
CC z
(1 = 10-10 m)
I.1.b The Benzene ring
sp2 hybrid orbitals have an angle of 120o
with
respect to each other. Hence, by -bonding 6 sp2
carbons we can form a regular hexagon. Each Cwill form 2 bonds, one with each of its
neighbours. There remains one sp2 orbital per C
to be capped, e.g. by a H. The remaining p
orbitals will again overlap to form bonds. The
resultingbenzene molecule may look like shown in
Fig. I.1:
Fig. I.1: The two possible borderline
structures of benzene.
It is not quite clear where the bonds should
be. In reality, a quantum mechanical
superposition of the two borderline states is
adopted, wherein it is impossible to assign
double bonds: The electrons are completely
Or
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delocalized to form a cloud that spans the
entire molecule.
Fig. I.2: The structure of benzene. The side of a
ring is 1.39 in length, intermediate between C-
C and C=C bond lengths.
The benzene ring is one of the most important and
versatile building blocks of organic chemistry.
Its delocalized electrons have remarkable
properties with respect to their interaction with
light, and many molecules containing benzene
rings can donate or accept electrical charges
with relative ease. Much of molecular physics,
including organic semiconductor physics, is
concerned with molecules containing benzene
rings.
I.1.c Conjugated Molecules
The benzene ring is the prototype of conjugated
molecules, that is molecules with alternating
single/double or single/triple carbon bonds. In
conjugated molecules, electrons delocalize
= 1 /2
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throughout the entire molecule and are relatively
loosely bound.
We can think of conjugated molecules being built
step-by-step by binding hybridised carbons
together (linear combination of atomic orbitals,
LCAO). In the molecular orbitals (MO)
description, we instead imagine a given, rigid
set of points at which atomic nuclei are fixed,
and fill that skeleton with electrons to arrive
at the molecule. The LCAO and MO approach
correspond to two schools of quantumchemistry/computer simulation. Both approaches,
of course, should lead to the same molecules, but
in the MO picture, the correspondence to
semiconductors is easier to see. We have to put N
electrons into the molecule to balance N positive
charges. The first electrons will cluster closely
to the atomic nuclei, resulting in almost
undisturbed atomic orbitals that is equivalentto saying that e.g. the carbon 1s electrons do
not participate in chemical bonds. But the last
few electrons will go into what we have called
delocalized orbitals. Although we can trace
orbitals to the hybridised atomic orbitals of
carbon, we have seen that in a conjugated
molecule, they may delocalize far from their
original carbon hence, for the cloud, the
MO picture is more appealing.
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Here follows a reference list of conjugated
molecules and polymers, which are of importance
in the field of organic semiconductors, together
with acronyms by which they will be referred to
throughout this text. The sheer length of this(hopelessly incomplete!) list underscores one of
the key assets of organic semiconductors: The
practically unlimited diversity of synthetic
organic chemistry allows tailoring materials with
a large property portfolio.
Historically, the organic semiconductordiscipline distinguishes between polymeric and
low molecular weight organic semiconductors.
This distinction is nowadays blurred due to the
advent of a number of hybrid materials, that
combine properties and attributes of low-
molecular weight and polymeric materials. A few
examples of these are included in the following
list, table I.2. We will refer to these materialsthroughout the text, often with their acronyms
only as given in table I.2.
Table I.2
a.) Low molecular weight organicsemiconductors
S
S
S
S
S
S6T
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Pentacene
Perylene
N
CH3
N
CH3
TPD
O
NN
PBD
C60
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N
O
Al
N
O
N O
Alq3
N
PtN N
N
PtOEP
N
S
2
IrO
O
btpacac
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N
N
Eu
O
O 3
ADS053RE
N
S
C7H15O
SC12H25 7O-PBT-S12
S
S
S
S
S S
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13 HHTT
N
N
O
OH
OABTo
Ru
N
N
O
OH
OABTo
N
N
S
S
N
Ru
N
N
O
OH
O
ABTo
N
O
ABTo
N
N S
S
S
N3 Black dye
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O
NO2
O2N
O2N
TNF
b.) polymeric organic semiconductors
*
*n
PPV
MEH-PPV
C
N
n CN-PPV
* *n
PPE
O
O
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** n
PPP
MeLPPP
S
R
* *n
PAT
SS *n
PTV
PTAA
N
n
H3C
H3C
C10H21
C10H21
* *
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R R
** n
PF
C8H17 C8H17
*
N N
S
*n
F8BT
C8H17 C8H17
*S
S
*n
F8T2
c.) hybrid materials
N
**n
PVK
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N
N
N N
ST 638
sQP
N N
O(CH2)n
O
O
(H2C)nO
O
O
oxTPD
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N
NDSB Dendron
(G2)
d.) synthetic metals
*
*n
PA
**n
PDA
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NH
N
N
NH
PAni
S*
*
OO
n
PEDOT
Table I.2: A selection of organic semiconductors
and synthetic metals.
Let us briefly discuss the attributes of the
listed materials immediately. This discussion
will preview many of the topics that are
introduced more systematically later in thedescription of organic semiconductor devices;
nevertheless the reader is invited to immerse
her/himself into the fascinating world of
molecular diversity at this stage.
a.) low molecular weight materials:
Hexithiophene (6T), Pentacene, Perylene, and TPD
(N,N'-bis-(m-tolyl)-N,N'diphenyl-1,1-biphenyl-
4,4'-diamine) are hole transporting and more or
less strongly fluorescent organic semiconductors
(Perylene more, 6T and TPD less). 6T is one
representative of the thiophene family of organic
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semiconductors, which are known for their fast
hole mobilities and are often used in organic
transistors. PBD (2-(biphenyl-4-yl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole) is an electron
conductor. Both TPD and PBD have been used ascarrier injection layers in multilayer device
architectures. C60 is a material with very high
electron affinity, and C60
derivatives have been
used as electron acceptors in organic
photovoltaic devices. However, electron transport
in C60 is very sensitive to even traces of oxygen,
which limits its practical potential. Alq3
(tris(8-quinolinolato)aluminum(III)) is an
organometallic Al chelate complex with efficient
green electroluminescence and remarkable
stability. Alq3 was used as the emissive material
in the first double layer organic light- emitting
device. PtOEP is a red phosphorescent porphyrine
derivative. The central Pt atom facilitates spin-
orbit coupling that allows light emission from
triplet excitons. btp2Ir(acac) (bis(2-(2'-
benzothienyl)-pyridinato-N,C-3')
iridiumacetylacetonate) is a representative of a
family of highly efficient phosphors that have
been used successfully as triplet- harvesting
emitters in efficient electrophosphorescent
devices. ADS053 RE is a trade name for the red-
emitting organolanthanideTris(dinapthoylmethane)mono (phenanthroline)-
europium(III). Organolanthanides transfer both
singlet- and triplet excitons to an excited
atomic state of the central lanthanide, resulting
in very narrow emission lines, i.e. spectrally
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pure colours (here, the 612 nm red line of
Europium). 7O-PBT-S12 and HHTT are hole-
transporting calamitic and discotic liquid
crystals, respectively. Due to the stacking of
conjugated cores in smectic (7O-PBT-S12) and somediscotic (HHTT) liquid crystalline phases, both
can display rather high charge carrier
mobilities. N3 and black dye are
organometallic dyes with broad absorption spectra
spanning the red and near infrared, which are
often used for the harvesting of solar photons in
dye- sensitized photovoltaic cells (Grtzel
cells). Trinitrofluorenone (TNF) is often used
as electron acceptor for the formation of charge
transfer complexes with conjugated molecules.
b.) polymeric organic semiconductors
Poly(para-phenylene vinylene) PPV played an
outstanding role in the development of organic
electroluminescence. MEH- PPV and Cyano- PPV (CN-
PPV) are sidechain- substituted PPVs. Sidechains
promote solubility and also can change the
bandgap, and the type of transported charge
carrier. Poly(phenylene ethynylene) PPE and
poly(para phenylene) PPP are variations on a
similar theme. Methylated ladder- type PPP
(MeLPPP) is similar to PPP, but with all backbonerings forced to be coplanar. PPVs, PPP, PPE, and
MeLPPP have been explored extensively in organic
light emitting devics. Poly(alkylated thiophene)
(PAT) and poly(thienylene vinylene) (PTV) are
less emissive, but have higher hole mobilities
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and low ionisation potentials, which qualifies
them for use in organic field effect transistors.
However, thiophene- containing materials are
generally sensitive to oxygen, which for
practical organic transistor applications can notbe excluded. Instead, variations of
poly(triarylamine) (PTAA) are being developed for
hole- transporting transistors. Polyfluorene (PF)
is a blue emitter that has recently competed
successfully with PPV as organic light emitting
material. Typically, PF is copolymerized rather
than sidechain- substituted to modify its
properties. F8BT is an electron transporter and
efficient green emitter. F8T2 is a hole
transporter that works well in transistors. PF,
F8BT, and F8T2 also display interesting liquid
crystalline phases.
c.) hybrid materials:
Poly(vinyl carbazole) PVK is historically one of
the first (the first?) polymeric organic
semiconductors. PVK clearly is a polymeric
material, with the film forming and morphological
properties typical of polymers. However, the
semiconducting carbazole units dangle laterally
from a non- conjugated backbone, and are isolated
from each other. The electronic properties of PVKare therefore very similar to those of low
molecular weight carbazole. PVK is used in
photocopiers, and was the first polymer for which
electroluminescence (EL) was reported. ST 638 is
the tradename for 4,4,4-Tris(N-(1-naphthyl)-N-
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phenyl-amino)-triphenylamine. This is a low
molecular weight material, but due to its
sterically hindred starburst architecture it
has a very high glass transition temperature and
does typically not crystallise when spincast fromsolution, but forms a glassy film, like many
polymers. The glassy morphology has considerable
advantages for device applications; a tendency to
crystallise is a major problem with hole
transporting small molecules such as TPD. The
same structural theme was employed for the design
of electron transporting starburst- type
phenylquinoxalines (not shown here). Another
structural theme that can be used to suppress
crystallisation in non- polymeric materials is
the use of spiro- links between two (or more)
para-phenylene units, here exemplified by a
spiro- linked pair of quaterphenyls (sQP). Note
the cross- shaped 3- dimensional architecture of
spiro compounds that is difficult to sketch on
paper. oxTPD is clearly a low- molecular weight
compound, but via the oxetane functions that are
attached with flexible spacers it can be
crosslinked in- situ with the help of a suitable
(photo)initiator. The result is a highly
crosslinked, inert hole transporting film with no
crystallisation tendency that has been used
successfully in multilayer devices. NDSB Dendron(G2) is a second generation, nitrogen- cored
distyryl benzene dendrimer. The core displays
visible absorption and emission, the meta- linked
dendronic sidegroups have a bandgap in the UV and
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for the purposes of charge injection, transport,
and light emission can be considered as inert.
d.) synthetic metals:
The distinction between organic semiconductors
and synthetic metals is somewhat arbitrary, as
the synthetic metals shown here are in the
undoped state, when they display semiconducting
rather than metallic or quasimetallic properties.
Metallic properties are observed only after
chemical doping. PA (poly(acetylene)) is the
classic example, the (chance) discovery thatiodine- doped PA displays metallic conductivity
was a milestone discovery that earned the 2000
chemistry Nobel prize. Poly(diacetylene) (PDA)
has a widely tunable bandgap if substituted with
suitable sidegroups. Both PA and PDA are of
historic, but no longer of practical interest.
Poly(aniline) (PAni, here shown as emeraldine
base) and poly(3,4-ethylenedioxythiophene) PEDOT
are more modern developments. These are made
metallic by acid- rather than redox doping.
Water- based PAni and PEDOT preparations are now
commercially available. PEDOT that is acid- doped
with poly(styrene sulfonic acid) (PSS)
(PEDOT/PSS) is now very popular in the OLED
community to modify (or replace) the commonlyused transparent ITO anodes.
As a general remark on the above list of
materials, it is very important to keep in mind
that materials that nominally are the same often
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show very different performance in devices.
Device performance can be very sensitive to low
levels of impurities and/or chemical defects, and
often, different chemical routes that lead to the
same material introduce different levels andtypes of defects and/or impurities. Experience
shows that the Gilch route is superior to Wittig,
Horner, and Stille coupling for high quality
poly(arylenevinylene)s, while Suzuki coupling is
to be preferred over Yamamoto or Kovacic coupling
for poly(arylene)s. The highest regioregularity
in poly(alkylthiophene)s are achieved with the
help of Rieke Zinc. Companies or research groups
that are able to provide conjugated materials in
the quality required for practical devices are
few and far between. Even if chemistry is
ideal, the same material can still display very
different properties when prepared in different
ways. The solvent and casting method used for
solution processing, or deposition rate, type and
temperature of substrate for evaporated films,
thermal treatment cycles, and the presence or
absence of even trace amounts of oxygen and/or
water can have a decisive impact on the resulting
device.
I.3 Excitations in organic semiconductors
In any semiconductor application, the material
will not be in its ground state. To transport
charge, and/or emit light, the semiconductor
needs to sustain excitations, and in the case of
charge transport, these excitations also need to
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illustrates that in the format of a chemical
reaction:
Eq. I.1: DIp
D+
+ e
A + e Ea A
Ip
and Ea
are conceptually closely related to
electrochemical Redox potentials. The main
difference is that Ip and Ea are defined with
respect to electrons in vacuum, while Redox
potentials are normalised with respect toelectrons in a reference electrode.
Experimentally it is much more common to measure
a molecules Redox potentials with a technique
called cyclic voltammetry (CV).
Note that in a metal, there is only one frontier
orbital, namely the Fermi level EF
. The energy
required to remove an electron from a metal is
called work function . As the frontier orbitalin a metal is the Fermi level, the nave
expectation is = - EF. This expectation,
however, is not met. The work function is a
surface property, while the Fermi level is a bulk
property. For a more detailed discussion, see
[T4, Ch. 18]. As a practical consequence, when
two metals are in contact, their Fermi levels
will equilibrate, but the work functions will
not. Instead, the work function will be that of
the metal that constitutes the particular surface
through which the electron leaves the metal.
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As a practically important example for an organic
radical ion, a neutral and a positively charged
polythiophene segment are sketched in Fig. I.3.
Note how the missing electron (hole) leads to a
redistribution of the -bonds and hence, to
different bond lengths, bond angles, and nuclear
positions.
Fig. I.3: A polythiophene segment and the derived
radical cation. See [T1, Ch. 14] for moreinformation.
Apart from polarons, the most important
excitation in an organic semiconductor is known
as exciton. This can be visualized as an electron
that is removed from the HOMO, but is placed into
the LUMO instead of being removed entirely. Atypical way of lifting an electron from the
HOMO into the LUMO is via the absorption of a
photon. Note that the exciton is electrically
neutral. Alternatively, an exciton can result
S S
S
S
S
SS
SS
S
S
S S
S
S
S S
S
Holeinjection
(oxidation)
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from the combination of a hole- and an electron
polaron.
After exciton formation, the electronsredistribute into the excited * orbitals, which
are also known as antibonding orbitals, as they
destabilise the molecule. The strong bonds are
crucial in keeping the molecule intact
nevertheless: The presence of a - bonded
backbone makes the difference between
photophysics and photochemistry. Again, theexcitation leads to a related structural
relaxation of the surrounding molecular geometry.
Fig. I.4 shows the geometric relaxation and
redistribution of electron density in an excited
phenylene- vinylene segment:
Excitation
Fig.I.4: The transition from an aromatic,
bonding phenylene- vinylene system to a
quinoidal, antibonding * system on optical or
electrical excitation.
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The size of the exciton is about 3 repeat
units, or 10 nm, and the exciton has clearly
intramolecular, one- dimensional character. This
makes organic excitons Frenkel excitons, while in
inorganic semiconductors, excitons typically are
more delocalised Wannier excitons. Due to the
mutual attraction of electron and hole in the
exciton, and structural relaxation of the
molecule, the energy difference between the
excitonic state and the ground state is lower
than the difference between Ip
and Ea
suggests
(which in turn is lower than the difference
between HOMO and LUMO). This energy difference is
known as exciton binding energy Eb. E
bis larger
in Frenkel than in Wannier excitons; typical
organic (Frenkel) exciton binding energies are in
the range 0.2 to 0.5 eV. Note how considerable
ambiguity arises in the term bandgap when
applied to organic semiconductors, which can meaneither the energy difference between LUMO and
HOMO, or Ip- E
a, or (I
p E
a) E
b.
Fig. I.5 gives an alternative, more schematic
representation of the electronic ground state,
radical ions (here called polarons), and excitons
in organic semiconductors.
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Fig. I.5: Level diagrams for excitations in
organic semiconductors
Fig. I.5 shows two different types of exciton,
singlet and triplet excitons. The different types
of excitons result from the fact that electrons
as well as holes posses a spin. The quasiparticle
exciton has an overall wavefunction that
contains a spatial and a spin part, and the spin
part of the wavefunction results from a
combination of the respective electron- and hole
spins in a way that is consistent with the basic
LUMO
HOMOGround state
Holepolaron
Electronpolaron
Tripletexciton
Singletexciton
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rules of quantum mechanics, in particular the
Pauli principle. There is three ways in which
hole and electron spin can combine so that the
resulting overall spin part of the wavefunction
is symmetric under particle exchange, and hastotal spin S = 1 - namely >,>, and 1/2(>
+ >). Excitons with that property are called
triplet excitons. One combination of spins,
namely 1/2(> - >), results in a spin part
of the wavefunction that is antisymmetric under
particle exchange, and total spin S = 0. This
combination is called a singlet exciton. Theproperties of excitons are summarised in table
I.3:
Table I.3
Spin state ket S Sz
symmetric(+)/
antisymmetric(
-)
> 1 1 +> 1 -1 +
1/2(> +>) 1 0 +1/2(> ->) 0 0 -Table I.3: The possible spin combinations (spin
state kets) of electron / hole pairs (excitons),
the resulting total spin, spin z- component, and
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symmetry property under particle exchange.
Excitons with a spin ket symmetric under particle
exchange are triplet excitons, excitons with spin
ket antisymmetric under particle exchange are
singlet excitons.
I.3.b Light emission from organic molecules
In most simplistic terms, the absorption of a
photon that has generated an exciton on an
organic molecule can in some cases be reversed
(ignoring some intermediate steps to be discussed
below): The excited electron drops back from theLUMO into the HOMO, emitting a photon in the
process. This phenomenon is known as
fluorescence. Since excitons can also be
generated electrically by the combination of
polarons; this paves the way to organic
electroluminescence (EL). First, we will discuss
fluorescence in some detail, using a framework
based on ground- state and excited state
molecular orbitals. Then, we will return to
discuss a striking difference between
fluorescence and EL, which is best understood in
the more schematic picture used in Fig. I.5.
Fluorescence in the molecular orbitals picture
The following diagram Fig. I.6 is used to discuss
fluorescence in the MO picture. The two curves in
the potential energy / distance diagram describe
the ground state and first excited state of a
chemical bond, with the equilibrium distance
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being the bond length. Bond length is longer for
the excited state due to the presence of
antibonding * orbitals. The horizontal lines
represent the vibrational states of the bonds.
Nuclear distances oscillate around the
equilibrium bond length within the limits of the
intersection of the horizontal line with the
potential curve. Vibrational levels are quantised
with a vibronic spacing in the order 0.1 eV
1100 K; consequently, at room temperature, almost
all bonds are in the lowest vibronic level of the
ground state. From there, they may be lifted intothe first excited state by absorption of a
photon.
Fig. I.6: Energy diagram for molecules in ground-
and excited states, showing potential energy E
vs. nuclear distance. See e.g. [T6, Ch. 17]
E
r
S0
S1
R0 R1
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The absorption process is governed by the Franck-
Condon principle: Electronic transitions are much
faster than nuclear rearrangement. Hence,
transitions always occur vertically in the
diagram, from ground state equilibrium position
to the turning point of a vibrational mode of the
excited state that is close to the ground state
equilibrium distance. Transitions from the 0
vibronic level of the ground state into the 0 / 1
/ 2 / vibronic level of the first excited state
are known as 0-0 / 0-1 / 0-2 / transitions. The
relative intensity of these is controlled by the
overlap integral between the electronic ground
state / vibronic ground state (0 vibronic state)
wavefunction and excited state / vibronic state
0, 1, 2, wavefunctions. The overlap integral can
be separated into an electronic and a vibronic
integral; the square of the vibronic integral is
known as the Franck-Condon factor of thetransition. A detailed discussion is in [T6, Ch.
17]. If the equilibrium bond lengths in the
ground- and first excited state were equal, the
0-0 transition would have unit Franck-Condon
factor, and all higher vibronic transitions would
have Franck-Condon factor zero, i.e. they would
be forbidden. In reality, bond lengths for
excited states are longer than for the groundstate, and higher vibronic transitions become
allowed, the so- called vibronic satellites.
Usually, however, the 0-0 transition will be the
most intense.
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In the excited state, there will be a rapid
(order 10-12
s), radiationless relaxation of the
excited state into its lowest vibronic level,
this is known as internal conversion. From there,
the photon may be re- emitted after typically 1
to 10 ns with a transition to the lowest vibronic
level of the ground state (0-0 transition), the
first vibrational state (0-1 transition), second
vibrational state (0-2 transition), etc. Vibronicspacing is similar in ground- and excited state,
therefore these often appear like mirror images.
Fig. I.7 shows absorption- and fluorescence
spectra of perylene as an example. Perylene is a
molecule with pronounced vibronic satellites.
Less rigid molecules, e.g. para- phenylenes,
often do not show resolved vibronic sidebands,
but a single, broadened peak.
Fig. I.7: Absorption and emission spectra of
perylene dissolved in cyclohexane. Both spectra
are normalized to unit maximum peak. Note that
when spectra are represented against a wavelength
scale, absorption spectra appear on the left at
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
300 350 400 450 500 550 600norma
lise
d
absor
bance
(o
)
norm
ali
sedfl
uor
es
cen
ce
(x
)
Wavelength [nm]
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shorter wavelengths (higher energies), emission
spectra on the right (lower energies). When
represented against an energy or wavenumber
scale, the situation is reversed.
In Fig I.7, the 0-0 absorption and 0-0
fluorescence peak at (virtually) the same
wavelength, as it is expected from the previous
discussion. In many cases, however, 0-0 emission
is shifted by a few nm (order 5 to 10) to longer
wavelength (lower energy). This phenomenon is
known as Stokes shift. Stokes shift results fromthe interaction of molecules with their
environment; we had so far considered isolated
molecules. No details here.
Either the onset of absorption, or the
intersection of absorption and emission spectra
in a normalised plot like Fig. I.7, is often
called the optical bandgap. In the absence of
Stokes shift, of course the intersection of
absorption- and emission spectra occurs at the
absorption / emission maximum.
Perylene has a fluorescence quantum yield FL
=
0.94 in cyclohexane solution, and many modern
conjugated polymers exceed 50 % quantum yield
even in the solid state. However, some conjugated
polymers display rather low fluorescence quantum
yields.
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In organic light emitting devices, light is
generated not by the absorption of a photon, but
by the combination of an electron- and a hole
polaron: An electron in the LUMO and an electron
vacancy (hole) in the HOMO can combine to form anexciton. The fluorescence emitted from an
electrically generated exciton is called
electroluminescence (EL). However, there is a
fundamental difference between the formation of
excitons by absorption of light, and by
combination of polarons.
When light is absorbed, only singlet excitons are
formed, and all of them are in principle capable
of fluorescence. The respective transition is
denoted as S0S
1transition (singlet ground state
to singlet 1st
excited state). Since the photon
carries a unit h of orbital angular momentum L,
S0 and S1 have orbital angular momentum quantum
numbers l differing by 1 (one). Inversely, the
S0S1 fluorescence transition does fulfil the
selection rule l = 1. The photons orbital
angular momentum provides the required angular
momentum for the S0S1 transition; conversely,
the S0S
1transition provides the orbital angular
momentum needed for photon emission. Such a
transition is called dipole allowed.
When excitons are generated electrically, naively
only 1 in 4 excitons will be a singlet exciton
(S1state). 3 out of 4 will be triplet excitons,
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corresponding to the T0
state. This is simply
because there is 3 ways to combine 2 spins into a
triplet state and only one way to combine them as
a singlet. Both S0and T
0have the same angular
momentum quantum number l, hence l = 0 for theS
0T
0transition: The triplet- to- singlet
transition is dipole forbidden. Triplets can not
emit fluorescent light, because the necessary
angular momentum h can not be supplied from the
orbital angular momentum difference between
excited and ground state. Obviously, it is bad
news for organic electroluminescence that
apparently only 1 in 4 electrically generated
excitons can emit light.
However, T0
does carry angular momentum in its
spin: Remember triplet excitons have total spin S
= 1, singlets 0. In some molecules, triplet spin
angular momentum can be transferred to a photons
orbital angular momentum. The conversion of spin
into orbital angular momentum is facilitated by
the interaction of the magnetic fields that both
electron spin and orbital angular momentum
generate. These fields interact with each other,
and interaction is proportional to the product of
orbital and spin angular momentum, L.S. This
interaction is therefore known as spin- orbit orLS coupling. The emission of light from T
0 via LS
coupling is known as phosphorescence. To have
strong LS coupling, we need to incorporate atoms
into our molecules that have filled atomic shells
with high orbital quantum number l. This will
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generally be heavy atoms (i.e., heavier than
carbon). Phosphorus may work the clue is in the
name but modern phosphors used in organic EL
are heavy metal organometallic complexes using
e.g. Iridium.
I.3.c Controlling the bandgap
In the previous chapter, we have seen how the
characteristic vibronic structure of organic
spectra arises. However, the overall location of
absorption- and emission of a semiconductor are
controlled by the size of its bandgap. Theperceived colour of light, in turn, is controlled
by the location of the emission band within the
visible spectrum. In particular for full- colour
displays, as well as for photovoltaics, the
control of the bandgap is a key requirement.
Synthetic chemistry is extremely versatile in
manipulating molecular architecture in a way that
allows bandgap tuning throughout the visible
spectrum. Bandgaps reduced or increased by
molecular engineering are often referred to as
redshifted andblueshifted, respectively. We will
discuss the two key approaches to bandgap
control: The steric approach and the
electronic approach. In the latter approach, it
is in fact possible to manipulate Ip or Ea only,leaving the respective other unchanged. It thus
becomes possible to design hole- or electron
transporting materials.
The steric control of bandgap
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When orbital overlap is improved by forcing
rings into a coplanar arrangement by chemical
bonds, bandgap will be reduced. Example:
Oligomers of para-phenylene (PPP), fluorene
(PFO), and methylated ladder-type poly(para-
phenylene) (mLPPP):
Table I.4
Number of
Benzene
Rings
Oligo-PP
[eV]
Oligofluore
ne
[eV]
Oligo-mLPP
[eV]
3 4.44 3.704 4.25
5 4.15 3.18
6 4.03 3.56
7 3.00
8 3.43
910 3.35
Table I.4: Absorption maxima for oligo(para-
phenylene)s, Oligofluorenes, and oligo(ladder-
para- phenylene)s. Data compiled from [J Grimme,
M Kreyenschmidt, F Uckert, K Mllen, U Scherf,
Adv. Mater.7, 292 (1995)] and [D Klaerner, R DMiller, Macromolecules31, 2007 (1998)].
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It is evident that at a given number of rings,
the more planarized the backbone, the lower the
bandgap.
The electronic control of the bandgap.
By introducing either electron- withdrawing or
electron-donating chemical groups into a
conjugated molecule, electron affinity and
ionization potential will be affected, hence, the
bandgap will change. Such groups can be
introduced in two ways, namely as sidechains or
in the mainchain. We will discuss the followingexamples: Alkoxy- sidechains attached to PPV
rings (MEH- PPV), CN- groups attached to alkoxy-
PPV vinylene bonds (CN- PPV, a case somewhat
intermediate between sidechain- and mainchain
modification) and fluorene copolymers (mainchain
modification).
MEH- PPV is an example for alkoxy- substituted
PPVs. Sidechains make it soluble in organic
solvents such as THF or chloroform. Sidechains
also somewhat isolate chain backbones from each
other in the solid film, which increases quantum
yield over unsubstituted PPV. An interesting
molecular engineering motif that was introduced
in the design of MEH- PPV is the use of a
branched sidechain. The branch constitutes a
chiral centre, MEH- PPV chains thus contain a
racemic mixture of right- and left handed
monomers. However, backbone conjugation is not
affected by the branch in the sidechain. This
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gives MEH- PPV the photophysical properties of a
homopolymer, but crystallisation is suppressed as
in a random copolymer. As crystallisation often
is detrimental to fluorescence efficiency, this
was a welcome step forward in the development ofconjugated polymers. The alkoxy- linkage of MEH-
PPVs sidechains to the phenyl backbone ring also
changes the electronic structure of the backbone.
Alkoxy links have a tendency to donate electrons
to the backbone, which changes the shape and
location of the HOMO. As a result, emission is
redshifted compared to PPV, from green to orange.
The case of CN- PPV is somewhat intermediate
between sidechain- and mainchain modification. In
addition to the alkoxy- sidechains as in MEH-
PPV, highly electron- withdrawing cyano groups
are attached to the vinylene bonds. This leaves
the conjugated backbone highly electron
deficient, thus considerably increasing theelectron affinity (by about 0.6 eV [R1]). Due to
their high electron affinity, CN- PPVs are n-
type semiconductors. CN- PPVs typically are red
emitters.
Another approach to bandgap control is
copolymerization of different conjugated unitsinto the polymer backbone. Copolymers between
alkane- and alkoxy substituted PPV- type polymers
are discussed in [R1]. Here, we will focus on
copolymers of fluorene. Polyfluorenes display a
blue bandgap that is almost indifferent to the
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type of sidechain attached. Note that sidechains
are attached pairwise at a position which itself
is not part of the conjugated backbone, and these
pairs are widely spaced, so that there is neither
steric nor electronic impact of the sidechains onthe backbone properties. However, strictly
alternating copolymers of fluorenes with
comonomers with different electronic properties
have been prepared, such as F8BT (alternating
copolymer between dioctyl fluorene with
benzothiadiazole) and F8T2 (alternating copolymer
of dioctyl fluorene with two thiophenes). For
both F8BT and F8T2, the resulting bandgap is
reduced, and they both emit in the green- yellow
region. The reduction of the bandgap has
different reasons: In the case of F8BT, the
benzothiadiazole comonomer has a higher electron
affinity Ea than fluorene, thus leading to a
polymer with higher Ea. In the case of F8T2, the
two thiophene groups have a lower ionization
potential Ip than fluorene, thus leading to a
polymer with lower Ip. Copolymerization thus does
not only allow control of the bandgap, but of
both Ip and Ea in a predictable manner, and a
large number of fluorene copolymers have been
synthesized and studied. For a review, see [R13].
Both polymers have found interesting
applications: Some of the most efficient organicEL devices have been built from blends of a
minority amount of F8BT as electron injecting /
transporting material in hole injecting /
transporting polyfluorene host material. F8T2, on
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the other hand, is an excellent material for p-
type OFETs.
I.4 Charge carrier injection
The injection of charge carriers is an issue of
immense practical importance for semiconductor
devices: Transistors require the injection of one
type of carrier from an electrode, and rather
fast transport of that carrier. Light emitting
devices require the injection of carriers of both
types from different electrodes. Photovoltaic
devices need to separate excitons and transportthe resulting carriers to opposite electrodes. It
is thus paramount that we discuss the factors
controlling carrier injection and transport.
Carrier injection from a metal electrode into a
semiconductor is controlled by the work function
of the metal relative to the electron affinityEa of the semiconductor for electron injection,
and relative to the ionisation potential Ip of
the semiconductor for hole injection. A level
diagram as in Fig. I.13 is often used to
illustrate carrier injection e.g. into an organic
light emitting device(OLED). In a level diagram,
electrons minimise their energy by moving
downwards to lower energy; holes minimise their
energy by moving uphill because that implies an
electron going downwards.
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Energy [eV]
ITOPPV
Al
Ca
Ip4.7
5.2
2.52.8
4.2
Ea
Vacuumlevel
0V a c u u mlevel
Cathode
th
Location
te
Location
ITO
Fig. I.13: Energy levels for a PPV layer
sandwiched between unlike electrodes. Indium tin
oxide (ITO) is a transparent metallic material
that is commonly used as anode for OLEDs. Left:
No bias voltage applied. Right: A voltage is
applied in forward bias.
Let us clearly state the assumptions that have to
be made when Fig. I.13 is used to discuss carrier
injection. Firstly, we assume the near complete
absence of dopant- induced charge carriers in the
semiconductor. In electrical engineering, undoped
materials are often called insulators rather
than semiconductors even when they have a small
bandgap. The absence of dopant- induced charge
carriers implies that bands remain straight at
all times - bands may tilt, but they will not
bend (show curvature). Thus, Fig. I.13 describes
ametal- insulator junction. Secondly, we assume
that the levels drawn in Fig. I.13 metal work
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functions, ionisation potential, and electron
affinity are the same at the metal- organic
semiconductor interface as they are in the
isolated metal, or bulk semiconductor. Note that
due to the molecular nature of organicsemiconductors, there are no surface dangling
bonds which do usually distort bulk energy
levels in inorganic semiconductors. However, in
metal- organic semiconductor contacts, interface
dipole moments often are present (sometimes,
deliberately engineered) which will modify
barriers. Also, the work function of a metal
coated with an organic semiconductor can be
different from that of the same metal with
respect to vacuum. An example is the gold-
pentacene interface, which displays a
surprisingly large energy barrier. These issues
are discussed e.g. in [N Koch, A Elschner, J
Schwartz, A Kahn, Appl. Phys. Lett. 82, 2281
(2003)]. In the presence of such effects, levels
in Fig. I.13 have to be re- drawn at a different
energy.
After these cautional remarks, let us inspect the
salient features of Fig. I.13, that illustrates
the level scheme at the example of PPV sandwiched
between ITO- and Al (or Ca) electrodes. On the
left, we see that a hole would have to move
downwards by 0.5 eV on injection from ITO to PPV.
However, holes will voluntarily move uphill
rather than downwards. The necessary step into
the wrong (energetic) direction is called an
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injection barrier, here of 0.5 eV for holes from
ITO into PPV. Electron injection from Ca or Al
into PPV needs to overcome a barrier of 0.3 or
1.7 eV for electron, respectively. On the other
hand, for electron injection from ITO, therewould be a large barrier of 2.2 eV. Thus, the use
of electrodes made from unlike metals defines a
forward and reverse bias for the OLED. To
minimise barriers, a high workfunction anode and
a low workfunction cathode are required. Table
I.7 lists the work functions of some metals, as
well as the highly doped semiconductor ITO, and
the synthetic metal PEDOT/PSS.
Table I.7
Metal Work function [eV]Cs 1.81
Ca 2.8
Mg 3.64
Al 4.25
Ag 4.3
Au 4.7
Cu 4.4
ITO 4.7
PEDOT/PSS 5.05.2
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Table I.7: The work function of a number of
metals, most data from [T4, Table 18.1, Ch. 18].
High work function metals are known as noble
metals. Low work function metals are highly
reactive and usually need to be protected fromambient atmosphere to avoid oxidation.
The right- hand part of Fig. I.13 shows the same
device (assuming a Ca cathode) under a forward
bias. Carriers can now overcome injection
barriers by tunnelling from the electrodes so far
into the device that energy at that location ofthe tilted band is as low or lower than in the
electrode. Also, carriers can be injected
thermally. For barriers of 0.5 eV as shown here,
the current density in a device in forward bias
will usually be controlled by the injection of
carriers across the barrier (injection limited
current), rather than by the transport of
carriers across a device. The tunnelling currentdensity j(Vbias) is described by the equation of
Fowler and Nordheim (Fowler- Nordheim (F-N)
tunnelling):
(eq.I.4.1) jFN =C
V(Vbias
d)2exp[B
dV3/2
Vbias]
with B = 8(2m*)/(2.96eh) with an effective
electron mass m*. For a detailed discussion, see
e.g. [T1, Ch. 12]. Note that jFN
depends on
applied voltage Vbias and film thickness d only in
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the combination Vbias/d, i.e. jFN scales with the
applied field E. Thermally activated or
thermionic injection is described by the
Richardson- Schottky (R-S) equation:
(eq. I.4.2) jRS = AT2exp[(V Vm (E)) /kT]
with A = 4emkb2/h
3, and Vm(E) describing the
field- dependend lowering of the injection
barrier by the attraction of the injected carrier
to its mirror charge; Vm(E) = (eE/40).
Generally, R-S injection will dominate at low
fields, and F-N tunnelling at large fields that
are practically relevant for organic devices.
Experimental j(V) results on MEH- PPV diodes can
qualitatively be described by the F-N eq.n [I D
Parker, J. Appl. Phys.75, 1656 (1993)], however,the absolute current density is several orders of
magnitude lower than described by the F-N eq.n.
This is due to a large backflow of carriers from
the semiconductor back into the metal.
In the case of V 0, i.e. in the case of a
work function that is matched to the respectivesemiconductor level (5.2 eV for hole injection
into PPV), the metal- semiconductor contact is
termed ohmic. For ohmic contacts the F-N and R-S
equations do no longer apply. Instead, current
density will be controlled by the transport of
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charge carriers across the film. Carrier
transport is characterised by charge carrier
mobility , which plays a crucial role fororganic transistors. Practically, a contact can
be considered ohmic whenever carrier transport is
a more restrictive limit on current density than
carrier injection.
Having an ohmic contact means the problem of
injection is solved ohmic contacts are
desirable. Considerable effort has therefore been
devoted to increase the workfunction of thetransparent ITO anode by a variety of
physicochemical treatment cycles [J S Kim, M
Granstrom, R H Friend, N Johansson, W R Salaneck,
R Daik, W J Feast, F Cacialli, J. Appl. Phys. 84,
6859 (1998)]. Recently, it has become common to
coat ITO with a thin film of the high work
function synthetic metal PEDOT/PSS [L
Groenendaal, F Jonas, D Freitag, H Pielartzik, J
R Reynolds, Adv. Mater.12, 481 (2000)] ( = 5.2
eV). As cathodes, low workfunction materials such
as Ca are commonly used. These require protection
from ambient atmosphere, otherwise they would
rapidly degrade. This can be provided by
encapsulation, or by capping with a more stable
metal such as Al. Note that in such an electrode,Fermi levels of Al and Ca will equilibrate, but
the work function will remain that of the metal
through which the electron attempts to leave,
which is Ca when we consider electron injection
into the organic semiconductor.
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II.1 Synthetic Metals
According to a general theorem proven by Peierls,
there cannot be a one- dimensional metal (Peierls
transition). This seems to exclude the
possibility of metallic polymers.
However, inorganic semiconductors can be made
quasimetallic if a high level of doping is
introduced. In 1977, Heeger, MacDiarmid,
Shirakawa, and coworkers discovered a very
similar semiconductor (quasi)metal transition
in the organic semiconductor trans- polyacetylene
(PA) [H Shirakawa, E J Louis, A G MacDiarmid, C K
Chiang, A J Heeger, Chem. Comm. No. 13, 578
(1977)]. Pure polyacetylene is a semiconductor
with a bandgap in the visible. However, when PA
was exposed to halogen vapours, these doped the
semiconductor and conductivity increased
significantly. Iodine vapours were mosteffective, increasing conductivity by several
orders of magnitude up to 38 S/cm. Heeger,
MacDiarmid, and Shirakawa were awarded the 2000
Chemistry Nobel Prize for their discovery.
Besides PA, examples for synthetic metals are
polyaniline (PANi), polypyrrole (PPy), and
poly(3,4-ethylene dioxythiophene) (PEDOT or
PDOT).
Doping in synthetic metals is somewhat different
from doping in inorganic semiconductors, were
heteroatoms of different chemical valencies are
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introduced into a host crystal lattice. In
synthetic metals, doping is by a chemical
reaction between semiconducting polymer and
dopant. Both redox- and acid/base reactions
between semiconductor and dopant have been usedsuccessfully; typical examples are PA oxidation
with chloride or iodine, and acid doping of PAni
with camphorsulfonic acid, PPy with phosphoric
acid, and PEDOT with poly(styrene sulfonic acid)
(PSS). Quite high dopant concentrations of (1 to
50)% are typically used. Iodine oxidises
polyacetylene, thus removing electrons from the
HOMO. This opens up mobile vacancies (holes) in
the previously completely full HOMO (valence
band). Conductivity will depend on doping level;
for the highest doping levels PA can be as
conductive as Platinum (105 S/cm at room
temperature). Generally, synthetic metals become
more metal- like in their behaviour as dopant
concentration is increased. Above a criticaldopant level, most synthetic metals remain
conductive in the limit T 0, and display NTC
behaviour at room temperature. Fig. II.1
summarises the behaviour of a number of synthetic
metals. Multiple entries for the same material
correspond to samples with different dopant
levels.
(Fig. 1 from A B Kaiser, Adv. Mater. 13, 927
(2001))
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Fig. II.1: Room- temperature conductivity and
temperature behaviour of conductivity for several
synthetic metals at different temperatures. SWCN
= single- wall carbon nanotubes. Full squares:
NTC at RT and > 0 at T 0 (proper metal);
hollow squares: NTC at RT, but 0 at T 0;
full circles: PTC at RT but > 0 at T 0;
hollow circles: PTC at RT and 0 at T 0
(proper semiconductor). From A B Kaiser, Adv.
Mater. 13, 927 (2001).
To add to the ambiguity in classifying synthetic
metals as proper metals or not, highly doped PA
and PAni show non- zero conductivity in the limit
of zero temperature and NTC behaviour around room
temperature, but PTC behaviour at low
temperatures, with a conductivity maximum in the
region (100250) K. For a detailed discussion,
see [R18].
However, the focus of synthetic metals research
never was to resolve ambiguities in
classification, but the desire to arrive at
practical materials that can be used e.g. as
antistatic coatings, or electrodes. Like many
other mainchain- conjugated polymers, PA isintractable and insoluble. The main obstacle
towards practical applications of synthetic
metals is to prepare highly conductive organic
materials in a versatile formulation.
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II.2.a Water- based synthetic metals
Nowadays, mainly PANi and PEDOT are used as
synthetic metals. This is because both of them
are available as aqueous dispersions for
spincoating or ink jet printing. This makes them
easy to process into coatings, or for multilayer
film applications in conjunction with organic
semiconductors: Soluble organic semiconductors
are usually processed from organic solvents, in
which the water- based synthetic metals are not
soluble. The advent of water- based synthetic
metals can be seen as a second breakthrough in
addition to the initial discovery of metallic
polyacetylene. Soluble synthetic metals have
considerably larger potential for applications
than insoluble polyacetylene.
PANi is a very complex material, with several
Redox states. For a discussion, see [T2, Ch.7.9].
Here, we will discuss PEDOT, which typically is
acid- doped with poly(styrene sulphonic acid)
(PSS) to form the highly conductive PEDOT/PSS
complex. PEDOT in that case is synthesized from
EDOT monomer in an aqueous medium that already
contains PSS; in this way PEDOT/PSS become truly
inseparable:
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O O
S
Na 2S 2O 8
SO 3H
n
H2O
S
OO
S
OO
S
OO
S
OO
S
OO
SC*
OO
*
n
C
SO3H SO
3H SO
3- SO3HSO3-
**n
SO3H
++
Fig. II.2: The synthesis of PEDOT from EDOT in
aqueous medium in the presence of poly(styrene
sulphonic acid) (PSS) and Na2S2O8. Note how PSS
acts as proton donor (i.e., acid), and PEDOT as
proton acceptor (base).
Na2S2O8 acts as oxidizing agent for the couplingof EDOT monomers. Note how the chemical bonding
pattern of EDOT/PEDOT changes under acid doping:
A C=C bond opens up and the C bonds to an H+
donated by the acid. As a result, there is:
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- A net positive charge on the PEDOT chain that
will strongly attract the negative charge left on
the acid. Since this happens at many points along
the chain, PEDOT and PSS become closely
intertwined and float as a fine dispersion orcolloid in the aqueous solution, rather than
precipitate. They can not separate, and will not
dissolve in organic solvents.
- An unpaired electron remains on the main
chain that is highly mobile along the chain.
PEDOT/PSS aqueous dispersion is commercially
available under the tradename Baytron P. From
Baytron P, thin, highly transparent, conductive
surface coatings can be prepared by spincasting
or dip- coating onto almost any surface. The
resulting conductivity is in the order 1 to 10
S/cm. A typical sheet resistance for spincastPEDOT/PSS coatings is 1 M/square.
II.2.b Applications of synthetic metals
PEDOT/PSS was originally developed as antistatic
coating for photographic films. Large- scale
processing of photographic film (development)
requires the winding of films over reels in the
dark. This leads to static charging, and
discharge sparks may expose the film. A PEDOT/PSS
coating allows charges to disperse and thus
prevents the build- up of high voltages. For this
applications, rather low conductivities are
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sufficient. Today, > 108m2of photographic film
is coated with PEDOT/PSS every year.
It should be obvious, however, that an elegant
and versatile off- the- shelve product like
water- based PEDOT/PSS will find a number of
other applications, in particular as it is sold
in high quality at a moderate price. Antistatic
coating of plastic films for packaging
microelectronics components is one of them. Also,
PEDOT/PSS has been used as counter- electrode for
anodised capacitors [F Larmat, J R Reynolds,Synth. Met.79, 229 (1996)].
PEDOT/PSS is now also increasingly used in
organic electronics. PEDOT/PSS displays a high
work function in the order (5 5.2) eV, which
according to I.4 qualifies it as good electrode
for hole injection into a semiconductor. It istherefor used e.g. as contact electrodes and ink-
jet printed wire in organic field effect
transistor circuits, and as anode (or anode
coating) in organic electroluminescent (EL)
devices. Also it is a useful electrode for
organic photovoltaics, were somewhat higher sheet
resistance is not a problem due to the generally
rather low current densities. We will discuss theuse of PEDOT/PSS repeatedly in the following
chapters. The excellent review [R6] is highly
recommended.
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Recently, flexible and transparent PEDOT/PSS-
coated polyester sheets with sheet resistance in
the order 1k/square have become available under
the tradename Agfa OrgaconTM. These sheets can
be patterned by an etching procedure which is
performed in a very similar way to conventional
photoresist patterning, although the chemistry is
rather different. This is the substrate onto
which future organic electronics and displays
will be prepared.
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II.2Organic field effect transistors
The transistor is the mainstay of solid state
electronics, and represents one of the seminal
inventions of the 20th century. All transistors
have three terminals. A current between two of
these three terminals can be controlled by the
input of the third terminal. Transistors can be
categorised into two families according to how
this control is exercised: Those where the
control is through a current at the third
terminal, and those where the control is through
a voltage at the third terminal.
In current- controlled transistors, the three
terminals are calledbase (B), collector (C), and
emitter (E). B controls the current between C and
E. Such transistors are realised by doping the
same semiconductor (typically, silicon) with n-
and p- type dopants in different regions, eitherin npn- or pnp configuration, and are also known
as bipolar transistors. Current- controlled
transistors are typically applied in voltage- or
current amplifiers. However, organic
semiconductors do not lend themselves for the
manufacture of bipolar transistors: Most organic
semiconductors will even without doping transport
only holes, or only electrons. An intrinsicallyp- type material can not be transformed into an
electron transporter by n- doping.
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Voltage- controlled transistors are also known as
field effect transistors (FETs). The three
terminals are called source (S), drain (D), and
gate (G). A voltage at G controls the S- D
current. The basic principle of the field effecttransistor dates back to 1930 [J E Lilienfeldt,
US Patent 1745175, 1930]. FETs are simpler in
design than bipolar transistors, require only one
type of carrier transport material (n or p), and
as we will see, not necessarily need to be doped
at all. Field effect transistors can be
manufactured in high integration densities and
are typically applied for logic gates in digital
electronics.
Due to the possible operation with only one type
of carrier, organic transistors invariably are
field effect transistors (organic FETs or OFETs;
sometimes also known as organic thin film
transistors, OTFTs), and only these shall bediscussed here. One of the major driving forces
behind organic FET (OFET) research is the idea to
make low- performance integrated circuits
completely from plastics on plastic substrates,
that are cheap enough to be discarded after
single use (disposable electronics). A target
application is e.g. an electronic pricetag on a
food wrapper that a supermarket checkout can read
remotely. Such a tag must be cheaper than the
cost of a check- out assistant pulling a barcode
across a scanner. It is now regarded as realistic
that this cost barrier can eventually be
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overcome. Also, higher added- value applications
are envisaged in combination with light emitting
organic semiconductors. Active- matrix addressed
display screens require two FETs and a capacitor
to address each pixel in a screen. Ultimately,organic semiconductor technology aims to make the
transistors as well as the light emitting
material from organics.
II.2.a Description of organic FET operation
Fig. II.3 shows the principle design of an
organic thin film transistor (OTFT) withelectrical connections suitable for basic
measurements. A voltage between source (S) and
drain (D) called drain or source- drain voltage
VDattempts to drive a drain current I
Dthrough
the semiconducting transistor channel. However, a
drain current will only flow if there are mobile
charge carriers in the channel. The channel
semiconductor in OFETs is not chemically doped;
in fact, great care has to be taken to keep
impurity levels in OFET- grade semiconductors as
low as possible to avoid unintentional doping.
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Fig.II.3: A field effect transistor in the
bottom gate, top contact architecture.
Transistors may also be built with bottom
contacts (S, D are placed directly on the
insulator), or completely the other way round
(top gate architecture).
A gate voltageVGwill pull carriers out of the
source into the semiconducting channel. However,
since the gate dielectric is an insulator, these
carriers cannot reach the gate metal. Instead,
they will accumulate in the semiconductor near
the channel / insulator interface where they
represent mobile carriers known as an
accumulation layer. The channel semiconductor
Substrate
Gate (Metal)
Gate Insulator
SemiconductorChannel
Source (Metal) Drain (Metal)
Vd
Vg
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thus gets doped by applying VG. However, other
than in chemical doping, this is quickly
reversible: Switching VG off will switch the
doping off. Hence, VG can switch the channel
conductivity. This behaviour is quantified by theso- called on / off ratio, which in some cases
can be as high as 106. Accordingly, ID at a given
VDwill change with V
G: The FET is an electronic
switch. The accumulation layer in an OFET is
generally very thin, about 5 nm or less [M A
Alam, A Dodabalapur, M R Pinto, IEEE Transactions
on Electron Devices, 44, 1332 (1997)]. This is
much thinner than the typical thickness of the
semiconducting film.
Note that an OFET is switched on by applying VD
and VG of equal polarity opposite to the sign of
the mobile carriers. Thus, OFETs can easily be
used to determine the type of carriers a
particular material sustains: If positive VG, VDswitch the transistor on, carriers are negative
(electrons), if negative VG, V
Dare required,
carriers are holes.
To characterise the gate- and source / drain
voltage dependent drain current in a field effect
transistor beyond the on / off description, twotypes of measurements may be carried out:
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The output characteristic of a FET is the family
of curves ID vs VD with VG fixed during every
single VDscan (drain sweep).
The transfer characteristic is the family of ID
vs VG curves with VD fixed during every single VG
scan (gate sweep).
In principle, both of these contain the same
physical information, as long as a sufficient
number of gate voltages (drain voltages) are
scanned for the output (transfer)
characteristics. Practically, however, output
measurements will usually scan ID vs VD at a
number of gate voltages, while transfer
characteristics often will be taken at only two
fixed VD: One VD will be chosen to be as small as
practically possible; much smaller than the
maximum VG used for the scan; while the second VDwill be chosen to be larger than the maximum VG
used in the VGscan. These two regimes are known
as the linear and saturation regime,
respectively. This terminology will become clear
with the quantitative description of OFET
behaviour.
OFET operation can be described quantitatively if
a number of assumptions are made; leading to an
ideal scenario. These assumptions are: Channel
length shall be considerably longer than device
thickness (long channel limit), source and drain
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make contact to the channel semiconductor without
contact resistance or injection barrier, the
semiconductor shall be trap- free and undoped,
and carrier mobility shall be independent of
gate- and drain voltages. Note that in reality,few if any of these assumptions will be met!
Then, the following equation applies [T1, Ch.
14.2.2.1.2]:
(eq.II.2.1.a) ID =Z
LCi(VG VT)VD for VD
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electric field and DG
the gate electric
displacement. DG has the dimensions of charge /
area, and corresponds to a surface charge density
QS of equal magnitude this is the accumulation
layer. From this consideration, we see it is theelectric displacement DG that switches the
transistor on, not the gate voltage or gate
electric field directly.
While the precise derivation of eq. II.2.1 is
somewhat technical, it can easily be understood
qualitatively if for the moment, we leave thethreshold voltage VT aside. Z/L is an obvious
geometry factor. The conductivity of the channel
will be proportional to charge / area QS= C
iV
G.
Also, for VD
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II.3. At VD = VG, the accumulation layer becomes
triangular and is on average half as strong as
for VD 0; this explains the factor 1/2 in eq.
II.2.1.b. When VD exceeds VG, the pinch- off point
moves away from the drain into the transistorchannel, and the carrier- depleted part of the
channel between pinch- off point and drain will
display very high resistance. Consequently, the
transistor displays drain current saturation for
VD VG, with ID,sat VG2. The only way to increase
ID beyond the saturation is to increase VG.
Fig. II.3: Accumulation layer (black) in a field
effect transistor. Top: VG applied, VD = 0. The
accumulation layer is fully developed. Middle: VG
applied, VD = VG. The accumulation layer is
triangular and pinches off at the drain. Bottom:
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VG
applied, VD 2VG. The accumulation layer
pinches off in the middle of the channel.
This leaves only one important contribution toeq. II.2.1 unexplained, that is the threshold
voltage VT. The theory of VT is intricate; also,
it is often found that VTchanges in transistors
as a result of prolonged operation; this is
termed gate bias stress. Gate bias stress has
been studied e.g. by Katz et al. [H E Katz, X M
Hong, A Dodabalapur, R Sarpeshkar, J. Appl. Phys.
91, 1572 (2002)], who conclude that it may resultfrom both slowly orientating dipoles in the
insulator, or the presence of slowly mobile ions
(electret behaviour). We here will simply take VT
as an empirical constant.
Fig. II.4. shows examples for experimentally
obtained OFET output and transfercharacteristics.
[II.4.a = Fig. 3a of [H Sirringhaus, R J Wilson,
R H Friend, M Inbasekaran, W Wu, E P Woo, M
Grell, D D C Bradley, Appl. Phys. Lett.77, 406
(2000)].
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II.4.b
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60 70
|ID
|1/2
[(microAmp)
1/2]
|V G| [V]
[II.4.c = Fig. 3b of K Fujita, T Yasuda, T
Tsutsui, Appl. Phys. Lett.82, 4373 (2003)]
Fig. II.4: a.) Output characteristic of an OFET
with F8T2 as active material. From [H
Sirringhaus, R J Wilson, R H Friend, MInbasekaran, W Wu, E P Woo, M Grell, D D C
Bradley, Appl. Phys. Lett. 77, 406 (2000)]. b.)
Plot of |ID| vs |VG| with ID taken at VD = VG from
II.6.a. c.) Saturated (VD = -60V) transfer
characteristic of an OFET with pentacene as
active material. The same ID is plotted in two
ways: logI
D(left abscissa, open squares) and
ID (right abscissa, full circles). From [K
Fujita, T Yasuda, T Tsutsui, Appl. Phys. Lett.
82, 4373 (2003)].
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Charge carrier mobility can be determined from
experimental characteristics with the help of
eq.s II.2.1. A very robust method of doing so is
to plot ID vs. VG , with ID in the saturation
regime, VD > V
G. This should result in a
straight line with a slope (Z
2LCi)
1/2and intercept
VT. Such a plot can be constructed in several
ways, both from output- and transfer
characteristics. Saturation current level can be
read for different gate voltages from the
saturated (flat) part of an outputcharacteristic. Alternatively, drain and gate
voltage may be connected to the same source /
measure unit; on ramping up voltage we scan ID(V
= VG = VD), i.e. saturation current at different
gate voltages, but always taken at VG = VD. In
such a measurement, we can directly see how ID
rises parabolically with V (ID VG/D2) [D M
Taylor, H L Gomes, A E Underhill, S Edge, P I
Clemenson, J. Phys. D: Appl. Phys. 24, 2032
(1991)]. The most straightforward method,
however, is the measurement of a saturated
transfer characteristic, i.e. a gate voltage scan
with a fixed drain voltage VD that is larger in
modulus than the gate voltage at all times. In
this way, it is ensured that drain current is
always saturated. Plotting ID vs VG with VD >
VG gives direct access to and VT via slope and
intercept. Another important FET characteristic
is the on / off ratio, defined as I(VG= V, V
D=
V)/I(VG = Voff, VD = V) with an operational voltage
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V that corresponds to the voltage available in
the respective application, and Voff
the gate
voltage that minimises the drain current; usually
Voff 0. On / off ratios typically are large
numbers roughly in the region 103107, and can be
extracted most conveniently from a saturated
transfer characteristic when this is plotted with
the ID axis on a logarithmic scale.
We note that saturated mobility often differs
from mobility at small drain voltage. The
transconductance gm is defined in the linearregime of the output characteristic (VD
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We had seen that short channel length L will
result in higher drain currents, but L is even
more important for switching speed. The maximum
speed at which a transistor circuit can operate
is limited by the time it takes carriers to
cross the FET channel. That is the time the
accumulation layer takes to be emptied of charges
through the drain after VG has been switched off.
At frequencies f > f0= 1/, the output signal
(ID) drifts out of phase with the input signal
(VG), and switching amplitude decreases. isgiven by eq.II.2.4:
(eq.II.2.4) 1
= f0 VD
L2
L enters the equation squared: Obviously the
transit time of carriers through the channel will
be proportional to L. Also, the lateral electric
field that pulls carriers across the channel is
proportional to 1/L. is known as the RC time of
the transistor, and can be calculated
alternatively from the channel resistance and
capacitance, with the same result eq.II.2.4.
Assuming VD = 10 V and = 10-2 cm2/Vs, f0 is 1 kHzfor L = 100m, but 100 kHz for L = 10 m. In
contrast, inorganic computer electronics work
with GHz frequencies. It is obvious that organic
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electronics has to compete on price rather than
performance.
It should be noted with care that eq.II.2.4
represents an upper limit for f0. There may be
other capacitances than the accumulation layer in
the transistor that charge (discharge) when VG is
switched on (off). These parasitic (stray)
capacitances may for example arise from an
overlap of source / drain electrodes with the
gate electrode.
Switching speeds of organic transistor circuits
can be studied with the help of ring oscillators.
A ring oscillator consists of an odd number of
logic NOT gates, which can be made of two FETs.
The NOT operation converts LOW (HIGH) input
voltage into HIGH (LOW) output voltage. The
output of each NOT gate is fed into the input ofthe next, with the output of the last of an odd
number of NOT gates being fed back into the input
of the first NOT gate. Thus, a ring is
completed that has no self- consistent state, see
Fig. II.5. Instead, output will oscillate between
HIGH and LOW with a frequency f that is related
to f0, the switching frequency of an individual
transistor. f will be smaller than f0, as morethan one transistor has to switch, and will
increase with the number of NOT gates or stages
of the oscillator.
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Fig. II.5: A ring oscillator (schematically). If
we assume the output of the bottom NOT gate were
LOW, the output of the top right NOT gate should
be HIGH, the output of the top left NOT gate
should be LOW, leading to HIGH output of the
bottom NOT gate which is inconsistent with the
original assumption. The ring oscillator has noself- consistent state and will oscillate between
HIGH and LOW with frequency f.
When the oscillating output is displayed on an
oscilloscope, f can be extracted. Often, the
inverse of f divided by (2 x number of NOT gates
= total number of transistors) is reported as propagation delay per stage. In the absence of
stray capacitances, this equals = 1/f0. Ring
oscillators allow a dynamic measurement of
carrier mobility. In any case, f is a practically
relevant measure of circuit speed. Fast OFET ring
oscillators with f > 100 kHz have been
demonstrated by the group at Siemens [W Fix, A
Ullmann, J Ficker, W Clemens, Appl. Phys. Lett.
81, 1735 (2002)]. Ring oscillators are not purely
a diagnostic tool, but have important
applications. The group at 3M have demonstrated a
radiofrequency identification tag (rf ID tag)
NOT
NOTNOT
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that uses a pentacene- based organic ring
oscillator with a frequency of about 200 Hz [P F
Baude, D A Ender, M A Haase, T W Kelley, D V
Muyres, S D Theiss, Appl. Phys. Lett. 82, 3964
(2003)]. The rf ID tag has an antenna (LCcircuit) that absorbs an incoming radio signal
(frequency 13.6 MHz), and re- emits a signal of
the same frequency, but with an amplitude
modulation. The amplitude modulation is
facilitated by the ring oscillator periodically
shorting / re- engaging the antenna circuit.
II.2.b Requirements on OFET materials
Good OFETs should display high drain current at
low drain and gate voltages, without reliance in
optimized geometry factor; high on / off ratios,
which means drain current at VG
= 0 should be
extremely low; and fast . This implies strong
requirements on all materials used in OFETs semiconductor, insulator, and metals. In the
field of organic semiconductors, the one property
that has been addressed most is charge carrier
mobility, and that is the one we will discuss in
detail. However, let us just list a few other
material requirements:
- Source metals should make ohmic (barrier- free)
contacts to the semiconductor.
- Gate insulators should be thin without
displaying pinholes (tricky for solution
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processed organics!) or any other current
leakage, and should have a high dielectric
constant.
- organic semiconductors must be free of
unintentional dopants down to very low
concentrations, otherwise the transistor will not
switch off properly.
- organic semiconductors must be free of charge
carrier traps. Trapped carriers are no longer
mobile, which means part of the accumulation
layer DG cannot contribute to the drain current.
- The interface between insulator and
semiconductor is particularly important. Often,
inorganic oxides are used as gate insulators
(SiO2, Al
2O3, Ta
2O
5). These are a found to improve
when they are modified by a self- assembl