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MNT-301
Unit-1
Organic Semiconductors
Organic light emitting devices(OLEDs),
Self assembly of complex organic molecules
Molecular switches, Thermochromic switches
Motor molecules
Bio-mimetic components
Charge transfer complexes
Molecular connections, Contact issues
Conducting polymers
Light emitting polymers
Polymerpolymer heterostructures
Plastic FETs,
Organic Solar cellsOrganic Photodiodes,
Electronic paper, Ink jet printing
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Applications: such as large area, flexible light sources and displays, low-cost printed
integrated circuits or plastic solar cells from these materials.
Organic semiconductors are not new.
The first studies of the photoconductivity of anthracene crystals. 20th century.
Later on, triggered by the discovery of electroluminescence in the 1960s.
Molecular crystals were intensely investigated by many researchers.
These investigations could establish the basic processes involved in optical excitation and
charge carrier transport.
The 1970s the successful synthesis and controlled doping of conjugated polymers
established the second important class of organic semiconductors which was honoured
with the Nobel Prize in Chemistry in the year 2000.
Together with organic photoconductors (molecularly doped polymers) these conducting
polymers have initiated the first applications of organic materials as conductive coatings or
photoreceptors in electrophotography.
In 1980s, the undoped organic semiconductors revived due to the demonstration of an
efficient photovoltaic cell incorporating an organic heterojunction of p- and n-conducting
materials as well as the first successful fabrication of thin film transistors from conjugated
polymers and oligomers.
High-performance electroluminescent diodes from vacuum-evaporated molecular films.
Organic light-emitting devices (OLEDs) have progressed rapidly and meanwhile lead to first
commercial products incorporating OLED displays.
Other applications of organic semiconductors e.g. as logic circuits with organic field-effect
transistors (OFETs) or organic photovoltaic cells (OPVCs) are expected to follow in the
near future
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1.1.2 Types of Organic Semiconductors:
Two major classes of organic semiconductors: low molecular weight materials and polymers.
An important difference between the two classes of materials lies in the way how they are
processed to form thin films.
Whereas small molecules are usually deposited from the gas phase by sublimation or
evaporation.
polymers can only be processed from solution. e.g. by spin-coating or printing techniques.
Both have in common a
conjugated -electron system
being formed by the pz-orbitals of
sp2-hybridized C-atoms in the
molecules.
As compared to the -bonds forming the backbone of the molecules, the -bonding is
significantly weaker.
Therefore, the lowest electronic excitations of conjugated molecules are the -
*transitions with an energy gap typically between 1.5 and 3 eV leading to light absorption
or emission in the visible spectral range.
Table 1: The family of the
polyacenes. The energy gap
can be controlled by the
degree of conjugation in a
molecule. Thus chemistry
offers a wide range of
possibilities to tune the
optoelectronic properties of
organic semiconducting
materials.
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Some prototype materials that can be used for optoelectronic applications are:
Basic Properties of Organic Semiconductors
1.1.3 Basic Properties of Organic Semiconductors:
Organic molecular crystals are van der Waals bonded solids implying a considerably
weaker intermolecular bonding as compared to covalently bonded semiconductors like Si or
GaAs.
Consequences are seen in mechanical and thermodynamic properties like reduced
hardness or lower melting point.
But much weaker delocalization of electronic wavefunctions among neighbouring
molecules, which has direct implications for optical properties and charge carrier transport.
But in case of Polymers
The morphology of polymer chains can lead to improved mechanical properties.
Nevertheless, the electronic interaction between adjacent chains is usually also quite weak
in this class of materials.
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1.1.3.1 Optical Poperties:
The weak electronic delocalization, to first order the optical absorption and luminescence
spectra of organic molecular solids are very similar to the spectra in the gas phase or in
solution
In particular, intramolecular vibrations play an important role in solid state spectra and often
these vibronic modes can be resolved even at room temperature.
Due to the crystal structure
or the packing of polymer
chains a pronounced
anisotropy can be found.
Additionally disordered
organic solids usually show
a considerable spectral
broadening.
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As a consequence of this weak electronic delocalization, organic semiconductors have two
important peculiarities as compared to their inorganic counterparts.
One is the existence of well-defined spin states (singlet and triplet) like in isolated
molecules which has important consequences for the photophysics of these materials (see
Fig. 4).
However, since intersystem crossing is a
weak process, this also sets an upper
limit for the electroluminescence
quantum efficiency in OLEDs.
A second important difference originates
from the fact that optical excitations
(excitons) are usually localized on one
molecule and therefore have a
considerable binding energy of typically
0.5 to 1 eV.
Thus in a photovoltaic cells this binding energy has to be overcome before a pair of independent positive and negative charge
carriers is generated (see Fig. 5).
1.1.3.2 Charge Carrier Transport
Transport of electrons or holes in an organic molecular solid is based on ionic molecular
states.
(In order to create a hole, an electron has to be removed to form a radical cation M+ out of
a neutral molecule M. This defect electron can then move from one molecule to the next.
In the same way, electron transport involves negatively charged radical ions M-.
(Qualitatively, the same arguments hold for polymers, however, in this case charged states
are usually termed positive or negative polarons.)
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From this picture one can clearly see that due to the already mentioned exciton binding
energy the optical gap between the ground state and the first excited singlet state is
considerably less than the single particle gap to create an uncorrelated electron-hole pair.
In going from molecular crystals to disordered organic solids one also has to consider locally
varying polarization energies due to different molecular environments which lead to a
Gaussian density of states for the distribution of transport sites as shown in Fig. 7.
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Depending on the degree of order the charge carrier transport mechanism in organic
semiconductors can fall between two extreme cases: (Carrier transport)
Band and Hopping transport.
Band transport is typically observed in highly purified molecular crystals at not too high
temperatures.
However, electronic delocalization is weak the bandwidth is only small as compared to
inorganic semiconductors
At room temperature mobilities in molecular crystals reach only values in the range 1 to 10
cm2/Vs
The band transport is the temperature dependence given by
T-n
with n = 1, 2 ,3
Disordered materials (Amorphous), for example polymers are based on hopping transport.
Hopping transport have much lower mobility values (103 cm2/Vs) in many cases is much less.
The mobility is depends on the applied electric field:
The mobility strongly depends on the degree of order and purity in organic
semiconductors and therefore to a great deal on the preparation and growth
conditions.
At macroscopic level, the current through a material is given by the charge carrier density
(n) and the carrier drift velocity (v), mobility () and the electric field (F)
Charge Current Density:
charge carrier density (n), the intrinsic carrier density in a semiconductor with an energy
gap (Eg) and an effective density of states N0
Taking typical values for an organic semiconductor with Eg= 2.5 eV and N0=1021 cm3
leads to a hypothetical carrier density of ni=1 cm3 at room temperature.
(Nevertheless the corresponding value for Si (Eg= 1.12 eV and N0=1019 cm3) is with
ni=1010 cm3 many orders of magnitude higher)
In order to overcome the limitations posed by the low intrinsic carrier density, different
means to increase the carrier density in organic semiconductors can be applied:
1. (electro-)chemical doping,
2. carrier injection from contacts,
3. photo-generation of carriers, and
4. field-effect doping.
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Self Assembly in Polymers
Several amphiphilic substances also form ordered layers on solid substrates.
Requirement for such behavior is: an adsorption of these molecules onto the surface, anintramolecular mobility and intermolecular stabilizing interactions.
Typical examples are the n-aliphatic tail-groups with medium chain length (8-30 carbon
atoms) connected to a hydrophilic polar or easily polarizable head group that can react
with the substrate surface, and with end-groups that do not react with the substrate.
In the case of head-groups bound on the surface in a high density, the end-groups
stabilize each other by van der Waals interactions. So a cooperative effect of layer
stabilization can be observed, which results in two-dimensional highly ordered monolayers.
These arrangements are denoted as self-assembled monolayers (SAM).
The self-assembly of amphiphilic ionic/hydrophobic block copolymers generally takesplace in dilute solution.
Block copolymers with both strongly and weakly dissociating (pH-sensitive) ionic
blocks are considered.
We focus mostly on structural and morphological transitions that occur in self-
assembled aggregates as a response to varied environmental conditions (ionic
strength and pH in the solution).
The assembly of amphiphilic (macro) molecules in aqueous environments is a generic
mechanism of self-organization.
means, the spontaneous formation of self assembled structures of phospholipids and
biomacromolecules, (it is the outcome of a delicate balance between attractive andrepulsive forces, among which hydrophobic attraction, hydrogen bonding, metal-
coordination forces, and steric or electrostatic repulsion play dominant roles.)
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Charge transfer complexes
According to electrical properties, materials can be divided into four-types:
insulator, semiconductor, conductor and superconductor.
In general, a material with a conductivity less than 10-7 S/cm is regarded as an insulator.
A material with conductivity larger than 103 S/cm is called as a metal.
The conductivity of a semiconductor is in a range of 10-4 10 S/cm depending upon doping
degree.
Organic polymers usually are described by (sigma) bonds and bonds. The - bonds
are fixed and immobile due to forming the covalent bonds between the carbon atoms. On
the other hand, the -electrons in a conjugated polymers are relatively localised, unlike the
electrons.
Conductive polyacelene (PA) doped with iodine. is a new field of conducting polymers,
which is also called as synthetic metals.
Principle:
a polymer has to imitate a metal, which means that electrons in polymers need to be free
to move and not bound to the atoms.
an oxidation or reduction process is often accompanied with adding or withdrawing of
electrons, suggesting an electron can be removed from a material through oxidation or
introduced into a material through reduction.
In 1977, Alan G. MacDiarmidthey accidentally discovered that insulating conjugated PA
could become conductor with a conductivity of 103 S/cm by iodine doping.
Materials:
scientists thought that PA (Poly Acetylene) could be regarded as an excellent candidate of
polymers to be imitating a metal.
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Since discovery of conductive PA by iodine
doping [1], other -conjugated polymers, such
as polypyrrole (PPy), polyani line (PANI),
polythiophenes(PTH), poly(p-
phenylene)(PPP), poly(p-
phenylenevinylene)(PPV), and poly(2,5-
thienylenevinylene)(PTV) have been reported
as conducting polymers.
Usually the ground states of conjugated polymers are divided into degenerate and non-
degenerate.
The prototype of degenerate polymers is trans-polyacetylene, which has alternating C-C
and C=C bonds as shown. The total energy curve of trans-polyacetylene has two equal
minima, where the alternating C-C and C=C bonds are reversed.
On the other hand, a non-degenerate polymer has no two identical structures in the ground
state. Most conjugated polymers, such as PPy and PANI belong to non-degenerate.
The band gaps of conjugated polymers are estimated to be typically in the range between 1
and 3 eV from their electronic absorption spectra. These observations are consistent with
their insulator or semiconductor electrical properties
Molecular Switches
The principle of nano switch is the molecular movements that are linked to electronic
processes, and on the other hand, electronic transfers lead to, at least temporarily, changes
in the chemical structure.
Nanoswitching processes are not limited to tunneling effects or single molecular processes.
Because, the nanomorphology of complex materials provides possibilities for nanoelectronic
switching.
Example: The use of the spontaneous spatial organization of domains in block-polymers
with a sequence of electron-conductive sections to control the electrical conductivity.
Mixtures of pentadecylphenol with polystyrene-poly-p-vinylpyridine block-copolymer that has
been protonated with methylsulfonic acid exhibits thermally controlled electrical conductivity.
This effect is caused by molecular reorientation processes from a lamellar domain structure
with character istic dimensions of 35nm and 5nm at 100oC over a non-lamellared block
structure (with increased conductivity) into a matrix structure with integrated columns with
distances of about 28nm at 150oC
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Motor molecules
In general terms, a motor may be defined as a device that consumes energy in one form
and converts it into motion or mechanical work.
Molecular motors are biological molecular machines that are the essential agents of
movement in living organisms.
Many protein based molecular motors harness the chemical free energy released by the
hydrolysis () of ATP in order to perform mechanical work.
(ATP) adenosine-5-triphosphate
In terms of energetic efficiency, this type of motor can be superior to currently available
man-made motors.
One important difference between molecular motors and macroscopic motors is that
molecular motors operate in the thermal bath, an environment in which the fluctuations
due to thermal noise are significant.
A molecular motor is a protein that uses the energy of hydrolysis of a small molecule such
as a nucleoside triphosphate (NTP) to complete an enzymatic cycle during the course of
which the protein performs directional motion.
Molecular motors are therefore unusual machines that accomplish what man-made devices
are unable to do: the direct and isothermal conversion of chemical energy into mechanical
energy, without the need to rely on an intermediate energy carrier, heat or electricity.
Synthetic Molecular Motor:
Synthetic molecular motors are molecular machines capable of rotation under energy input.
The term "molecular motor" has traditionally referred to a naturally occurring protein that
induces motion (via protein dynamics), some groups also use the term when referring to
non-biological, non-peptide synthetic motors.
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bio-mimetic components
Conducting Polymers
Conjugated polymers are intrinsic semiconductors whose conductivity increases through doping.
This doping can be achieved by either chemical or electrical methods.
Polypyrole
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Synthesis:
Polymer chains may be doped chemically through oxidation - primarily when an atom with
high electron affinity such as iodine or oxygen is present.
Oxidation of the chain results in the formation of a polaron, which is a radical cation
associated with lattice distortion (Figure 1.2).
Upon further doping, an additional electron can be removed from either the polaron to form
a bipolaron, or from elsewhere on the chain to form two polarons.
Polythiophenes are a very good example of this class of compounds. In addition to offering
high conductivity upon doping, they have the added benefit of being relatively
environmentally and thermally stable.
Unsubstituted polythiophene has good thermal stability and moderate conductivity after
doping with iodine, however the resulting polymers are insoluble and not melt-processable.
The conductivity of such polymers is the result of several processes. E.g., in traditional
polymers such as polyethylenes, the valence electrons are bound in sp3 hybridized
covalent bonds.
Such "sigma-bonding electrons" have low mobility and do not contribute to the electrical
conductivity of the material.
However, in conjugated materials, the situation is completely different.
Conducting polymers have backbones of contiguous sp2 hybridized carbon centers.
One valence electron on each center resides in a pz orbital, which is orthogonal to the other
three sigma-bonds.
The electrons in these delocalized orbitals have high mobility when the material is "doped"
by oxidation, which removes some of these delocalized electrons.
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Limitations: The manufacturing costs, material inconsistencies, toxicity, poor solubility in
solvents, and inability to directly melt process.
Applications:
organic solar cells, printing electronic circuits, organic light-emitting diodes, actuators,
electrochromism, supercapacitors, chemical sensors and biosensors.
Light emitting Polymer
The discovery of electroluminescence in poly(para-phenylene vinylene) (PPV) (Burroughes
et al. 1990) has led to a re-awakened interest in conjugated polymers.
These technologies include cheap and flexible light emitting displays, photovoltaic devices,
optical switching, and field-effect transistors.
We now turn to a description of the optical properties. Figures 1 and 2 show the
characteristic linear absorption spectrum of the phenyl-based light emitting polymers.
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Polymer-polymer heterostructures
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Organic Light Emitting Diode
An OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive
electroluminescent layer is a film of organic compound which emits light in response to an
electric current.
This layer of organic semiconductor material is situated between two electrodes.
Generally, at least one of these electrodes is transparent.
Materials: Polyparaphenylenevinylene exhibits a high electrical conductivity and is used as
a material for organic light emitting diodes (OLEDs).
Construction: By integration of electrically conductive polymers such as substituted
polythiophenes in a nanoporous membrane and wiring by a metal base electrode and at
least a partially transparent membrane electrode, organic light diodes have been
constructed.
They generate photons via the field-based emission of electrons from a cathode, which are
then accelerated over a short distance onto a luminescent material.
To achieve large electrical field strength (about 0.3 V nm) with moderate voltages, the gaps
between the cathode and counter electrodes should be as small as possible.
HTL: hole transport layer.
ETL: electron transport layer
EML: emission layer.
Alq3
Current scaling with the 3rd power of the
reciprocal thickness
Instead of the displayed combination of a
triphenylamine derivative and Alq3,
polymeric OLEDs usually employ a
conductive polymer.
(PEDOT:PSS) together with luminescent
polymers like PPV or PFO derivatives.
Injection of charge carriers from contacts plays important role for the operation of organic
light-emitting devices (OLEDs).
This requires low energetic barriers at the metal-organic interfaces for both contacts to
inject equally high amounts of electrons and holes.
Relatively high electric fields being applied to OLEDs (typically 5 to 10 V across a layer
thickness of 100 nm yield F = 0.5...1MV/cm), low mobility materials Alq3
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In organic semiconductors large fraction of the excited states formed by charge carrier
recombination are triplets.
The most efficient OLEDs nowadays make use of energy transfer to so-called triplet
emitters, where the presence of heavy metals renders the transition from the triplet state to
the ground state via phosphorescence an allowed process.
Organic Photo Detector or Diode
OPDs, the device was fabricated onto an ITO coated glass substrate by OMBD at a
background pressure of 105 Pa.
The device consists of the heterostructure of copper phthalocyanine (CuPc) and N,N-bis(2,5-
di-tert-butylphenyl) 3,4,9,10-perylenedicarboximide (BPPC) as a p-type and an n-type material,
respectively.
CuPc fi lm has high sensitivity and stability in air, and it shows a strong absorption band at the
wavelength range of 550780 nm, whereas BPPC shows an absorption band in the range of
400550 nm.
The thickness of CuPc layer was set at 30 nm, which corresponds to the exciton diffusion
length in the photogenerated layer.
As an electrode, 30 nm thick Au was deposited on the BPPC layer.
The device was covered by a glass plate in Ar gas atmosphere to prevent oxidation of the
organic layers.
The active area of the device was fixed at 0.01 mm2, in order to reduce the influence of the RC
time constant.
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Organic Solar cell (Plastic Solar Cell or
Polymer Solar Cell) An organic solar cell orplastic solar cell is a type of polymer solar cell that uses organic
electronics,
It deals with conductive organic polymers or small organic molecules, for light absorption
and charge transport to produce electricity from sunlight by the photovoltaic effect.
The optical absorption coefficient of organic molecules is high, so a large amount of light
can be absorbed with a small amount of materials.
These cells are made by sandwiching a layer of organic electronic materials between two
metallic conductors
The difference of work function between the two
conductors sets up an electric field in the organic
layer.
When the organic layer absorbs light, electrons will
be excited to the Lowest Unoccupied Molecular
Orbital (LUMO) and leave holes in the Highest
Occupied Molecular Orbital (HOMO) forming
excitons.
this can be overcome by making use of a
photoinduced charge transfer between an
electron donor like PPVand the fullerene
C60 as an acceptor
Bulk heterojunction devices usually
consist of a mixture of soluble PPV (or
P3AT) and fullerene derivatives.
Alternatively, mixed layers of evaporated
small molecules like CuPc and C60 can
be used.
Materials:
Bulk heterojunction devices usually consist of a mixture of soluble PPV (or P3AT) and
fullerene derivatives.
Structure:Alternatively, mixed layers of evaporated small molecules like CuPc
(Polycarbonate) and C60 can be used.
Other than organic material, previously used materials has the high absorption coefficient
(105 cm-1) than organic semiconductors. Organic semiconductors faces some problem of
the high binding energy.
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Due to the short exciton diffusion length of typically 10 nm only, efficient OPVCs use the
so-called bulk-heterojunction concept of mixing donor and acceptor in one single layer.
In spite of the huge progress recently achieved, there are still challenges to achieve
sufficient lifetime of OPVCs under ambient conditions or the availability of low-band gapmaterials to make better use of the solar spectrum
Organic Field effect transistor or plastic FETs
An organic field-effect transistor (OFET) is a field effect transistor using an organic
semiconductor in its channel.
OFETs can be prepared either by vacuum evaporation of small molecules, by solution-
casting of polymers or small molecules, or by mechanical transfer of a peeled single-
crystalline organic layer onto a substrate. These devices have been developed to realize
low-cost, large-area electronic products and biodegradable electronics.
The most commonly used device geometry is bottom gate with top drain- and source
electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using
thermally grown Si/SiO2 oxide as gate dielectric.
Organic polymers, such as poly(methyl-methacrylate) (PMMA), can also be used as
dielectric.
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Materials:
One common feature of OFET materials is the inclusion of an aromatic or otherwise
conjugated -electron system, facilitating the delocalization of orbital wavefunctions.
Electron withdrawing groups or donating groups can be attached that facilitate hole or
electron transport. OFETs employing many aromatic and conjugated materials as the
active semiconducting layer have been reported, including small molecules such as
rubrene, tetracene, pentacene, diindenoperylene, perylenediimides,
tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly
3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly
p-phenylene vinylene (PPV).
Rubrene-based OFETs show the highest carrier mobility 2040 cm2/(Vs).
pentacene-based OFETs reported 10 times lower mobilities than rubrene.
Polycrystalline tetrathiafulvalene and its analogues result in mobilities in the range 0.1
1.4 cm2/(Vs).
The mobility exceeds 10 cm2/(Vs) in solution-grown or vapor-transport-grown single
crystalline hexamethylene-tetrathiafulvalene (HMTTF).
Design:
Three essential components of field-effect transistors are the source, the drain and the gate.
Field-effect transistors usually operate as a capacitor.
They are composed of two plates. One plate works as a conducting channel between two
ohmic contacts, which are called the source and the drain contacts.
The other plate works to control the charge induced into the channel, and it is called the gate.
The direction of the movement of the carriers in the channel is from the source to the drain.
Hence the relationship between these three components is that the gate controls the carrier
movement from the source to the drain.
When this capacitor concept is applied to the device design, various devices can be built up
based on the difference in the controller - i.e. the gate.
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e-Paper
ink jet printing
