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BRIEF VIEW OF PHTHALOCYANIN FOR SOLAR CELL
HARVESTING
INTRODUCTION:
The accidental discovery of phthalolocyanine happened more than
seventy years ago and a metal-free phthalolocyanine was found for the first
time in 1970 as by product during the preparation of 2 - cyanobenzamide.
Linstead was the first to investigate the structure of phthalolocyanine and
describe the name from the Greek word naptha (rock oil) and cyanine (blue).
Phthalocyanine (Pc) is a large, planer molecule that has attracted
considerable interest as an organic semiconductor. The basic molecule is
metal - free Phthalocyanine, C32H18N8. By removing the two centeral
hydrogen atoms and replacing it with a metal atom. , it is possible to
synthesize various mettalo Phthalocyanines with the chemical formula
C32H16N8M where M is a bivalent metal atom. Indeed, it is possible to create
Phthalocyanine with an aggregate of atoms (e.g. TiO) as long as it has an
oxidization state of - 2.
The chain of events which eventually led to the elucidation of
structure Phyrhalolocyanine began in 1982 at Grangemouth plant of Scottish
Dyes Lt,. Four chemists (Dandridge, Drescher, Dunwirth and Thomas) at
Scottish Dyes Lt,. noticed the formation of an insoluble blue coloured
material during the manufacture of phthalimide from phthalic anhydride and
ammonia in the glass lined iron vessel The coloured product was analyzed
and was found to be iron Phythalolocyanine, by Linstead at Imperial
College. He proposed a structure, which was further confirmed by Robertson
using X - ray diffraction and was patented in 1928. In the year 1933,
Professor Linstead of Imperial College of Science and Technology used the
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term Phythalolocyanine for the blue product. The word Phythalolocyanine
was derived from Greek, "naptha" means rock oil and "cyanine" means dark
blue.
Four - isoindole units join together through a conjugated system [Fig.
1a] to give the structure of Phythalolocyanine molecule. This planar
tetradenate molecule is found to form number of complexes with metals and
metalloids by replacing the two hydrogen atoms situated at the center of the
Phythalolocyanine molecule [Fig.lb]. The structure similarity of
Phythalolocyanine molecule with biologically important molecules like
chlorophyll [Fig. 2a] and hemin [Fig. 2b] forced the host of scientists to
focus their attention on the physio-chemical properties associated with
Phythalolocyanine class of compound. During 1930's metal
Phythalolocyanines have emerged as major colorants in the market.
Fig 1(a) : Structure of metal free Phthalocyanin with two numbering systems
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Fig 1(b) The structure of metal Phthalocyanin
Phythalolocyanines are an attractive group of materials for use in solar
cells as they have smaller band gap than most other organic semiconductors;
These molecules are pigments and can absorb photons in the optical range
with strong absorption peaks in the 600 – 800 nm range ( orange through
near infra red). The color of the molecule depends somewhat on the central
metal atom.
CuPC Molecules
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If the central atom is small, for instance Cu, Ni, or Fe, the planar
nature of the molecule is unaffected and they form close packed layers.
Phythalolocyanines containing larger metal atoms like lead or with
combinations of atoms like TiO have a three dimensional structure and don't
stack in dense layers. The regular shape of the Phythalolocyanine molecule
allows a high packing density.
Phythalolocyanine is chemically and thermally stable. This allows
thermal evaporation of thin films under high vacuum. As pigments, this class
of materials is not soluble in organic or most inorganic solvents making it
difficult to use spin, coating or similar techniques to form uniform layers of
molecules.
The Chemistry of Formation of Phthalolocyanine
The chemistry of formation of phthalolocyanine macromolecule
reveals the union of four isoindole units arranged around a metal atom in
single reaction system. In the synthesis of copper Phthalolocyanine starting
from phthalic anhydride, urea, copper (II) Chloride and a catalyst involves
the formation of phthalmide in the first step which is subsequently converted
into mono and diiminophthamide. In all these steps, there occurs the addition
of nitrogen atoms to the maleic anhydride residue or phthalmic acid or acid
or phthalic anhydride. The required nitrogen atoms are obtained either from
urea, urea polymer or decomposed products of urea, because of reaction
temperature at which phthalolocyanine is formed, the urea molecule
decomposes and urea polymers are formed.
The probable intermediates formed prior to the formation of
phthalolocyanine derivatives are
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Phthalic anhydride Phthalamide
Monoimino phthalimide Diimino phthalimide
The mono or diimiophthalamide thus formed undergo self
condensation to form an adduct, as the adduct (I) reacts with metal chloride
to form.
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(II)
The adducts (I) and (II) undergo condensation to form
phythalolocyanine molecule with liberation of ammonia.
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From the literature it has been confirmed that the phythalolocyanine
molecule is formed through intermediate products of phthalamide,
monoimino and diiminophthalamide.
Even though the phthalocyanines are obtained by condensing the four
molecules of phthalonitrile along with other required reactants under
appropriate conditions formation of phthalanitrile as an intermediate is
doubtful when phthalic anhydride is used as a starting material, because the
formation of phthalanitrile is associated with the opening of the maleic
residue only for it to reclose again. The opening of the ring may also involve
replacement of the α - carbon atom of the maleic residue with a carbon atom
of the urea molecule. However, it has been shown that the carbon atom from
urea molecule doesn't enter the phythalolocyanine molecule by the
replacement of α - carbon atom of the maleic acid residue.
Phthalocyanine is an intensely coloured macrocyclic compound that is
widely used in dyeing. Phthalocyanines form coordination complexes with
most elements of the periodic table. These complexes are also intensely
colored and also are used as dyes.
Properties
Phthalocyanine, abbreviated H2Pc, has several unusual properties. It
and its complexes have low solubility in virtually all solvents. One litre of
warm (40 °C) benzene dissolves less than a milligram of H2Pc and CuPc.
These compounds dissolve in sulfuric acid owing to the protonation of the
nitrogen centres that link the pyrrole rings. They are also highly stable
thermally and typically resist melting. CuPc sublimes at >500°C under one
atmosphere of nitrogen. Relevant to their main application, phthalocyanines
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strongly absorb light in the red portion of the optical spectrum (about 600 to
700 nm), thus these dyes are characteristically blue or greenish.
Phthalocyanines are structurally related to other macrocyclic
pigments, especially the porphyrins. Both feature four pyrrole-like subunits
linked to form a 16-membered ring. The pyrrole-like rings within H2Pc are
closely related to isoindole. Both porphyrins and phthalocyanines function as
planar tetradentate dianionic ligands that bind metals through four inwardly
projecting nitrogen centers. Such complexes are formally derivatives of Pc2,
the conjugate base of H2Pc.
Many derivatives of the parent phthalocyanine are known. In addition
to the ring-substituted derivatives, there also exist subphthalocyanines,
superphthalocyanine, and hemiporphyrazine.
An unidentified blue compound, which we now know was metal-free
phthalocyanine, was described in 1907. In 1927, Swiss researchers
accidentally synthesized copper phthalocyanine, copper naphthalocyanine,
and copper octamethylphthalocyanine in an attempted conversion of o-
dibromobenzene into phthalonitrile. They remarked on the enormous
stability of these complexes but did not further characterize these blue
complexes. The same blue product was further investigated at Scottish Dyes,
Ltd., Grangemouth, Scotland (later ICI). ColorantHistory.org hosts an old
documentary about the discovery of the pigment.
Synthesis
Phthalocyanine forms upon heating phthalic acid derivatives that
contain nitrogen functional groups. Classical precursors are phthalonitrile
and diiminoisoindole. In the presence of urea, the heating of phthalanhydride
is a useful route to H2Pc. These reactions are more efficient in the presence
of metal salts. Other precursors include, o-cyanobenzamide, and
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phthalimide. Several of these starting materials are shown in the figure
below (right side). Using such methods, approximately 57000 tons of various
phthalocyanines were produced in 1985.
Halogenated and sulfonated derivatives of copper phthalocyanines are
commercially important. These compounds are prepared by treating CuPc
with chlorine or oleum.
Applications
Approximately 25% of all artificial organic pigments are
phthalocyanine derivatives. Copper phthalocyanine (CuPc) dyes are
produced by introducing solubilizing groups, such as one or more sulfonic
acid functions. These dyes find extensive use in various areas of textile
dyeing (Direct dyes for cotton), for spin dyeing and in the paper industry.
Direct blue 86 is the sodium salt of CPC-sulfonic acid whereas direct blue
199 is the quaternary ammonium salt of the CPC-sulfonic acid. The
quaternary ammonium salts of these sulfonic acids used as solvent dyes
because of their solubility in organic solvents, e.g. Solvent Blue 38 and
Solvent Blue 48. The dye derived from cobalt phthalocyanine and an amine
is Phthalogen Dye IBN. 1,3-Diiminoisoindolene, the intermediate formed
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during phthalocyanine manufacture, used in combination with a copper salt
affords the dye GK 161.
All major artists' pigment manufacturers produce variants of copper
phthalocyanine, designated color index PB15 (blue) and color indexes PG7
and PG36 (green).
Phthalocyanine is also commonly used as a dye in the manufacture of
high-speed CD-R media. Most brands of CD-R use this dye except Taiyo
Yuden and Verbatim DataLife (which use cyanine and azo dyes,
respectively).
Niche applications
Metal phthalocyanines have long been examined as catalysts for redox
reactions. Areas of interest are the oxygen reduction reaction and the
sweetening of gas streams by removal of hydrogen sulfide.
Phthalocyanine compounds have been investigated as donor materials
in molecular electronics, e.g. organic field-effect transistors.
Related compounds
Relationship of the phthalocyanine with the porphyrin macrocycle
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SYNTHESIS OF PHTHALOCYANIN - 8
General Synthesis Routes:
Up to now over seventy different metallic and non - metallic cations
have incorporated in the central cavity of phthalocyanine moiety, thereby
enabling the control of the oxidation
Scheme 1: synthesis routes for MPc. Reagents and conditions
i. Phthalonitrile, metal dry solvent temperature 180-190oC
ii. O- Cyanobenzomide , metal dry solvent boiling point 300 oC
iii. Solvent, room temperature
iv. 1.3 diminniosoindoline, solvent boiling point
v. Phthalic anhydride, metal salt solvent , urea, catalyst 200 oC
vi. Solvent boiling point
The majority of MPc's can be prepared by the high temperature
cyclotetramerization of phthalonitriles in the presence of corresponding
metal or metal salt or by latter insertion of the metal into PcH2. On account
of the insolubility of unsubstituted phthalocyanine in common organic
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solvents, soluble impurities can be removed by extracting with hot organic
solvents or boiling with acids or bases. More soluble substituted
phthalocyanine can be purified by common methods used for organic
compounds, usually by chromatography re-crystalization and extraction.
Recently, substituted phthalocyanine are prepared III high yield under
microwave heating in the presence of suitable solvent.
Industrially, phthalocyanine were produced by using inexpensive
materials like phthalic anhydride and urea which is more useful and cheaper
than phthalonitrile route (Scheme 1) to produce higher - volume with lower
cost application as shown scheme 2.
Scheme 2: Industrial preparation of phthalocyanines
Electrochemical Synthesis of phthalocyanines
Yang and coworker have reported the electrosynthesis of metal free
and metallophthalocyanines through the electroreduction of phthalonitrile in
high yields in polar protic and aprotic solvents. Generally, the yields of the
electrosynthesis are affected by several factors such as solvent, the reaction
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temperature, concentration of phthalonitrile the intensity of the current and
the amount of charge, which was used during the procedure.
Soluble phthalocyanines
Due to strong interaction between rings un substituted MPc's are
practically insoluble in common organic solvents, such as DMSO, and DMF,
but insoluble in water and most of the organic solvents, like alcohol, ether,
carbon tetrachloride and benzene. The introduction of voluminvus
hydrophobic substituent’s into the periphery of the macrocycle enhances the
solubility in various solvents. Another approach employed to enhanced the
solubility is to the introduct substituent’s at the central metal atom
decreasing the aggregation effect. By the introduction of selected
substituent’s, the physical and electrical properties of phthalocyanines can be
modified and tailored, resulting in the broadening of their applications.
The best investigated soluble substituted phthalocyanines are the tetra-
and octasubstituted ones.
Solar Cells
Cells are described as photovoltaic cells when the light source is not
necessarily sunlight. These are used for detecting light or other
electromagnetic radiation near the visible range, for example infrared
detectors, or measurement of light intensity.
History of solar cells
The term "photovoltaic" comes from the Greek meaning "light", and
"voltaic", meaning electric, from the name of the Italian physicist Volta,
after whom a unit of electro-motive force, the volt, is named. The term
"photo-voltaic" has been in use in English since 1849.
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The photovoltaic effect was first recognized in 1839 by French
physicist A. E. Becquerel. However, it was not until 1883 that the first solar
cell was built, by Charles Fritts, who coated the semiconductor selenium
with an extremely thin layer of gold to form the junctions. The device was
only around 1% efficient. In 1888 Russian physicist Aleksandr Stoletov built
the first photoelectric cell (based on the outer photoelectric effect discovered
by Heinrich Hertz earlier in 1887). Albert Einstein explained the
photoelectric effect in 1905 for which he received the Nobel prize in Physics
in 1921. Russell Ohl patented the modern junction semiconductor solar cell
in 1946, which was discovered while working on the series of advances that
would lead to the transistor. The photovoltaic cell was developed in 1954 at
Bell Laboratories. The highly efficient solar cell was first developed by
Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a
diffused silicon p-n junction. In the past four decades, remarkable progress
has been made, with Megawatt solar power generating plants having now
been built.
A solar cell made from a monocrystalline silicon wafer
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A monocrystalline solar cell
A solar cell (also called photovoltaic cell) is a solid state device that
converts the energy of sunlight directly into electricity by the photovoltaic
effect. Assemblies of cells are used to make solar modules, also known as
solar panels. The energy generated from these solar modules, referred to as
solar power, is an example of solar energy.
Photovoltaics is the field of technology and research related to the
practical application of photovoltaic cells in producing electricity from light,
though it is often used specifically to refer to the generation of electricity
from sunlight.
Cells are described as photovoltaic cells when the light source is not
necesssarily sunlight. These are used for detecting light or other
electromagnetic radiation near the visible range, for example infrared
detectors), or measurement of light intensity.
Efficiency
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The efficiency of a solar cell may be broken down into reflectance
efficiency, thermodynamic efficiency, charge carrier separation efficiency
and conductive efficiency. The overall efficiency is the product of each of
these individual efficiencies.
Due to the difficulty in measuring these parameters directly, other
parameters are measured instead: thermodynamic efficiency, quantum
efficiency, VOC ratio, and fill factor. Reflectance losses are a portion of the
quantum efficiency under "external quantum efficiency". Recombination
losses make up a portion of the quantum efficiency, VOC ratio, and fill
factor. Resistive losses are predominantly categorized under fill factor, but
also make up minor portions of the quantum efficiency, VOC ratio.
Crystalline silicon devices are now approaching the theoretical
limiting efficiency of 29%.
Cost
The cost of a solar cell is given per unit of peak electrical power.
Manufacturing costs necessarily including the cost of energy required for
manufacture. Solar-specific feed in tariffs vary worldwide, and even state by
state within various countries. Such feed-in tariffs can be highly effective in
encouraging the development of solar power projects.
High-efficiency solar cells are of interest to decrease the cost of solar
energy. Many of the costs of a solar power plant are proportional to the area
of the plant; a higher efficiency cell may reduce area and plant cost, even if
the cells themselves are more costly. Efficiencies of bare cells, to be useful
in evaluating solar power plant economics, must be evaluated under realistic
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conditions. The basic parameters that need to be evaluated are the short
circuit current, open circuit voltage.
The chart at the right illustrates the best laboratory efficiencies
obtained for various materials and technologies, generally this is done on
very small, i.e. one square cm, cells. Commercial efficiencies are
significantly lower.
A low-cost photovoltaic cell is a thin-film cell that has a price
competitive with traditional (fossil fuels and nuclear power) energy sources.
This includes second and third generation photovoltaic cells, that is cheaper
than first generation (crystalline silicon cells, also called wafer or bulk cells).
The Solar grade silicon shortage in 2008 made thin film solar more
attractive, however with the increase in raw silicon production, many
manufacturers have decided to stop producing the far more inefficient thin
film cells in favour of expanding production on crystalline solar cells, which
places even more downward pressure on crystalline cell prices.
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Grid parity, the point at which photovoltaic electricity is equal to or
cheaper than grid power, can be reached using low cost solar cells. It is
achieved first in areas with abundant sun and high costs for electricity such
as in California and Japan. Grid parity has been reached in Hawaii and other
islands that otherwise use diesel fuel to produce electricity. George W. Bush
had set 2015 as the date for grid parity in the USA. Speaking at a conference
in 2007, General Electric's Chief Engineer predicted grid parity without
subsidies in sunny parts of the United States by around 2015.
The price of solar panels fell steadily for 40 years, until 2004 when
high subsidies in Germany drastically increased demand there and greatly
increased the price of purified silicon (which is used in computer chips as
well as solar panels). One research firm predicted that new manufacturing
capacity began coming on-line in 2008 (projected to double by 2009) which
was expected to lower prices by 70% in 2015. Other analysts warned that
capacity may be slowed by economic issues, but that demand may fall
because of lessening subsidies. Other potential bottlenecks which have been
suggested are the capacity of ingot shaping and wafer slicing industries, and
the number of specialists who coat the wafers with chemicals.
Organic photovoltaic devices have gained a broad interest in the last
few years due to their potential for large area low cost solar cells From the
first reports on molecular thin film devices more than 30 years ago, their
power conversion efficiencies have increased considerably from 0.0010/0 in
1975(1) to 1% in 1986(2) and more recently to 5.5% in 2006(3-6) the
progresses in efficiency will possible make them a competitive alternative to
inorganic solar cells in the near future .Different concepts have been
published using either small molecules, conjugated polymers, combinations
of small molecules and conjugated polymers or combinations of inorganic
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and organic materials as the active layer. "Active layer" refers here to the
layer in which the majority of the incident light is absorbed and changes are
generated. Small molecules and polymers differ in their molecular weights.
Commonly macromolecules with a molecular weight larger than 10,000 are
called polymers, whereas lighter molecules are referred to as" oligomers" or
"small molecules".
Historically, small molecules were mainly deposited by vaccum
deposition techniques, since they showed limited solubility in common
solvents. In contrast to these small molecule thin films, the preparation of
thin polymer layers doesn't require high vacuum sublimation~ steps. Large
polymer thin film areas can be deposited by several methods, such as spin-
coating, screen printing, spray coating or ink jet printing, allowing for large
area, ultra - thin, flexible and low cost devices. Currently, there is a head - to
- head race going on between solution processed and sublimed organic solar
cells, but the ease of processebility may finally tip the balance in favors of
polymers or small molecules blended with polymers. Although it should be
noted that currently there are some efforts to develop soluble oligomers to
allow for cost efficient solution processing techniques, the concept of
efficient complete small molecules based devices prepared from solution
processing has yet to be proven.
The basic working principle of organic solar cells is the disassociation
of photogenerated excitons at the interface between electron donor and
acceptor phases by a photoinduced charge - transfer process with subsequent
transport of the charge carriers in the respective phases to the electrode§"
Critical parameters for the photocurrent generation are therefore the active
layer absorption, the efficiency of the charge transfer, and the transport of
charge carriers in the materials involved.
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PHTHALOCYANIN FOR SOLAR CELL HARVESTING
Organic semiconductors differ from classical crystalline inorganic
semiconductors (e.g. silicon) in many fundamental aspects:
First of all, the mobilities of organic semiconductors are several orders
of magnitude less than those found in crystalline inorganic semiconductors
Transport processes in organic semiconductors are best described by
hopping transport in contrast to the band transport in most crystalline
inorganic semiconductors. Even the highest reported hole mobilites (µ)
for organic semiconductors reach currently only about 15 cm2 V-1S-1 for
single crystals of small molecules and 0.6 cm2 V-1S-1 for liquid crystalline
polymers (silicon: µe = 450 cm2 V-1S-1). Highest Electron mobilities (µe) for
organic materials are typically lower, hovering around 0.1 cm2 V-1S-1reaching
higher values only in particular TFT structures using highly crystalline small
molecules (silicon: (µe) 1400 cm2 V-1S-1). The mobility values for
amorphous organic materials as used most commonly in organic solar cells
are even several magnitudes lower. These low mobilities limit the feasible
thickness of the organic layer in solar cells to a few hundred nanometers.
Fortunately, organic semiconductors are very strong absorbers in the UV -
VIS regime. Thus only ca. 100 nm thick organic layers are needed for
effective absorption.
Second the exciton binding energy in organic semiconductors is much
higher as e.g. in silicon. Upon absorption of a photon of sufficient energy by
the organic semiconductor, an electron is promoted into the lowest
unoccupied molecular orbital (LUMO), leaving behind a hole in the highest
occupied molecular orbital (HOMO). However, due to electrostatic
interactions, this electron- hole pair forms a tightly bound state which is
called singlet exciton. The exact binding energy of this excitation is still
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under debate but it is expected to be in a range of 200 - 500 me V. Hence,
the exciton binding energy for organic semiconductors is roughly one order
of magnitude larger than for inorganic semiconductors like silicon, where
photoexcitons typically lead directly to free carriers at room temperature.
The thermal energy at room temperature (~25 me V) is not sufficient to
efficiently generate free charge carriers in organic materials by exciton
disassociation even at typical internal electric fields (~1 06 - 107 V 1M).
For example, in the widely used poly(2-methoxy-5(2'-ethyl-hexyloxy)-p-
phenylene vinylene) (MEH-PPV) experiments revealed that only 10% of the
excitations disassociate into free carriers in a pure layer, while the raminig
excitations decay via radiative or non - radiative recombination pathways.
Thus, the energy efficiencies of single -layer polymer devices remain
typically below 0.1 %
The most important discovery on the route to high efficiency organic
solar cells was the finding that solar cells containing e hetero - junction
between hole and electron accepting organic materials exhibited
performances far superior to single component devices. Using the hetero -
junction approach photogenerated excitations (bound electron - hole pairs) in
the polymer layer can be efficiently disassociated into free carriers at the
interface, whereas is single component devices most excitations recombine
after a short time. The charge separation occurs at the interface between
donor and acceptor molecules, mediated by a large potential drop. After
photo - excitation of an electron from the HOMO to the LUMO the electron
can jump from the LUMO of the donor ( the material with the higher
LUMO) to the LUMO of the acceptor and the electron if the potential
difference between the ionization potential of the donor and the
electron affinity of the acceptor is larger than the excitation binding energy
(see Fig. 1). However, this process, which is called photo - induced charge
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transfer, can lead to free charges only if the hole remains on the donor due to
its higher HOMO level. In contrast, if the HOMO of the acceptor is higher ,
the excitation transfers itself completely to the material of lower band gap
accompanied by energy loss.
Fig 1: The interface between two different semiconductors polymers (D =
donor, A = acceptor) can either facilitate charge transfer by splitting the
excitation or energy transfer, where the whole excitation is transferred from
the donor to the acceptor.
For efficient dissociation at the hetero - junction, the donor and
acceptor materials have to be in close proximity. The optimum length - scale
is in the range of the excitation diffusion length, typically a few tens of
nanometers. On the other hand, the thickness of the active layer should be
comparable to the penetration length of the incident light, which for organic
semiconductors is typically 80 - 200 nm.
The hetero - junction can be realized in several ways (Fig. 2) . The
most straight forward approach is the preparation of a bi - layer by subliming
or by spin -coating a second layer on the top of the first, resulting in a more
or less diffused bi - layer structure if polymers are used and both materials
are soluble in the same solvents, laminating techniques can be used. This is
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bi - layer geometry guarantees directional photo - induced charge transfer
across the interface. Since both types of charge carriers travel to their
respective electrodes in pure n - type or p - type layers, the chances for
recombination losses are significantly reduced.
However the interfacial area and thus the excitation disassociation
efficiently IS limited. Higher interfacial areas and thus improved excitation
disassociation efficiencies can be achieved if layers containing both the
electron donor and electron acceptor in a mixture are prepared. These so
called bulk hetero-junction can be deposited either by co-sublimation of
small molecules or by spin - coating mixtures of polymers.
B1 – Layer Hetero-junction Bulk Hetero-junction
Light Light
Fig. 2 Two approaches to hetero - junction solar cells.
The drawback to the bulk hetero-junction structure is that a
percolating pathway from the hole and electron transporting phase to the
electrodes is needed in order that the separated charge carries can reach their
corresponding electrodes.
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Direct path for charge carriers to electrode
Large interfacial area due to phase reoperation in the polymer blends
but precotation needed
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Fig 3: Principle of charge separation in a solar cell
If the individual layer thickness (in case of a bi - layer structure) or the
phase separated domains (in case of a blend layer) are larger than the exciton
diffusion length, then most excitons will recombine (Fig. 3). If however, the
excitons are generated in close proximity to an interface, they have a chance
to be separated into free charge carriers which may diffuse or drift to the
corresponding electrodes. The overall efficiency of this process is described
by the incident photon to converted electron efficiency (IPCE). The IPCE is
calculated by the number of electrons leaving the device under short circuit
conditions per time and area di vided by the number of photons incident per
time and area:
# extracted electrons IPCE =
# incident photons Note that the IPCE is a measure of the external quantum efficiency,
meaning that losses due to reflection at the surface or transmission through
the device are included in the IPCE value. Subtracting these two loss
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channels would lead to the internal quantum efficiency, which is however,
rarely used to compare solar cells.
Solar cells are further characterized by measuring the current - voltage
I(V) curve under illumination of a light source that mimics the sun spectrum.
A typical current - voltage I(V) curve of a polymer solar cell is shown in Fig.
4. Since organic semiconductors show very low intrinsic carrier
concentration, the metal - insulator - metal (MIM) model seems to be best
suited to explain this characteristics. The characteristic points used to
characterise a solar cell are labeled in Fig. 4. In addition, for each of these
points, the energy diagram for a single layer cell with an indium tin oxide
(ITO) anode and aluminum cathode are displayed.
Figure 4: Currents (Voltage) characteristics of a typical organic diode
shown together with the metal- insulator - metal (MIM) picture for the
characteristic point: a) Short circuit condition. B) Open circuit condition, c)
Forward bias, d) Reverse bias.
a) The current delivered by a solar cell under zero bias is called short circuit
current (Isc). In this case, exciton disassociation and charge transport is
driven by the so - called built in potential. In the MIM picture, this
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Brief view of Phthalocyanin for solar cell harvesting
potential is equal to the difference in work function (Wr) of the hole - and
electron - collecting electrodes.
For polymer solar cells the transparent ITO electrodes is often chosen
(Wr, ITO = 4.7 eV) in combination with low work function material
(Wr,ITO = 2.87 eV, Wr ,AI = 2.24 eV) as counter- electrode to achieve a
high internal fields. For example, the difference in work functions between
ITO and Ca is approximately 2 eV.
b) The voltage where the current equals zero is called open circuit voltage
(Voc) In the MIM picture this situation is described by the case were the
band is flat, since the applied voltage equals the difference in the work
function of the electrodes.
(Note: Diffusion effects are neglected in this simplified picture)
c) When V> Voc the diode is biased in the forward direction. Electrons are
now injected from the low work function electrode into LUMO and holes
from the high work function electrode into the HOMO of the organic
layer, respectively.
d) When V<O the diode is driven under a reverse biased condition the solar
cells works as a photodiode. The field is higher than in a) which often
leads a enhanced charge generation and / or collection efficiency.
The point where the electrical power P = I x V reaches maximum
value represents the condition where the solar cell can deliver its maximum
power to an external load. It is called the maximum power point. The ratio of
this maximum electrical power Pmax to the product of the short circuit current
and the open circuit voltage is termed the fill factor (FF):
Pmax
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Brief view of Phthalocyanin for solar cell harvesting
FF= Isc x Voc
Ideally, the fill factor should be unity, but losses due to transport and
recombination result in values between 0.2-0.7 for organic photovoltaic
devices. As an example, a constant slope of the 1 (V) characteristic
corresponds to FF = 0.25.
The photovoltaic power conversion efficiency () is then calculated
for an incident light power Plight:
Isc x Voc x FF ) =
Plight
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Brief view of Phthalocyanin for solar cell harvesting
CONCLUSION:
Phthalolocyanine (pc) is a large, planer molecule that has attracted
considertable interest as an organic semiconductor. The basic molecule is
metal free phyhalolocyanine C23H18N8. By removing the two central
hydrogen atoms and replacing it with a metal atom. The formation of an
insoluble blue coloured material during the manufacture of phthalimide from
phthalic anhydride and ammonia in the glass lined iron vessel. The coloured
product was analyzed and was found to be iron phythalolocyanine. The four
isoindode units join together through a conjugated system the structure of
phythalolocyanine molecule. This planer tetradenate molecule is found to
form number of complexes with metals and metalloids by replacing the two
hydrogen atoms situated at the center of the phythalolocyanine, molecule.
The structure similarity of phythalolocyanine molecule with
biologically important molecules like chloriphyll and hemin force the host of
scientists to focus their attention on the physiochemical properties associated
with phythalolocyanine class of compounds. The phythalolocyanines are an
attractive group of materials for use in solar cells as they have smaller band
gap than most other organic semiconductors. The phythalolocyanines
containing larger metal atoms like lead or with combinations of atoms like
tio have a three dimensional structure and don’t stack in dense layers. The
regular shape of the phythalolocysnine molecule allows a high packing
density.
Phythalolocyanine is chemically and thermally stable. This allows
thermal evaporation of thin films under high vacuum. Coating or similar
techniques to form uniform layers of molecules.
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Brief view of Phthalocyanin for solar cell harvesting
Phythalocyanine is an intensely coloured macrocylic compound that is
widely used in dyeing. Phthalocyanines form co-ordination complexes with
most elements of the periodic table. These complexes are also intensely
coloured and also are sued as dyes.
A solar cell made from a monocrystalline silicon wafer. Solar is a
solid state device that converts the energy of sunlight directly into electricity
by the photo voltaic effect. Assemblies of cells are used to make solar
modules also known as solar panels. The energy generated from these solar
modules referred to as solar power, is an example of solar energy. Cost of a
solar cell is given per unit of peak electrical power. Manufacturing costs
necessarily including the cost of energy required for manufacture.
A low cost photovoltaic cell is a thin film cell that has a price
competitive with traditional energy sources. This includes second and third
generation photovoltaic cells. That is cheaper than generation
phythalolocyanine is historically. Small molecules were mainly deposited by
vacuum deposition techniques. Since they showed limited solubility in
common solvents. In contrast to these small molecule thin films, the
preparation of thin polymer layers doesn’t require high vacuum sublimation.
The basic working principal of organic solar cells is the disassociation of
photogenerated exciton at the interface between electron donor and acceptor
phases by a photo-induced charge transfer process with subsequent transport
of the charge carriers in the respective phases top the electrodes.
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Brief view of Phthalocyanin for solar cell harvesting
BIBLIOGRAPHY
[1] Forrest, S.R., The limits to organic photovoltaic cell efficiency. MRS
Bulletin, 2005. 30(1): p. 28-32.
[2] Martin A. Green, K.E.Y.H.W.W., Short Communication Solar cell
efficiency tables (version 33). Progress in Photovoltaics: Research and
Applications, 2009. 17(1): p. 85-94.
[3] Peumans, P., A. Yakimov, and S.R. Forrest, Small molecular weight
organic thin-film photodetectors and solar cells. Journal of Applied
Physics, 2003. 93(7): p. 3693-3723. Chapter 27. Soft Semiconductor
Devices 27-7
[4] Sista, S., et al., Enhancement in open circuit voltage through a
cascade-type energy band structure. Applied Physics Letters, 2007.
91(22): p. 3.
[5] Kinoshita, Y., T. Hasobe, and H. Murata, Control of open-circuit
voltage in organic photovoltaic cells by inserting an ultrathin metal-
phthalocyanine layer. Applied Physics Letters, 2007. 91(8): p. 3.
Journal Articles
C.L. Mulder, L. Theogarajan, M. Currie, J.K. Mapel, M.A. Baldo, M.
Vaughn, Paul Willard, B. D. Bruce, M.W. Moss, C.E. McLain, and
J.P. Morseman, “Luminescent solar concentrators employing
phycobilisomes,” Advanced Materials 21, 1-5 (2009).
M. Bora, K. Celebi, J. Zuniga, C. Watson, K.M. Milaninia, and M.A.
Baldo, “Near field detector for integrated surface plasmon resonance
biosensor applications,” Optics Express, 17, 329-336. (2009).
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K. Celebi, P. Jadhav, K.M. Milaninia, M. Bora, and M.A. Baldo, “The
density of states in thin film copper phthalocyanine measured by
Kelvin probe force microscopy,” Applied Physics Letters, 93, 083308
(2008).
M.J. Currie, J.K. Mapel, T.D. Heidel, S.G. Goffri, and M.A. Baldo,
“High efficiency organic solar concentrators for photovoltaics,”
Science, 321, 226 (2008).
T.D. Heidel, J.K. Mapel, K. Celebi, M. Singh, and M.A. Baldo,
“Surface plasmon polariton mediated energy transfer in organic
photovoltaic devices,” Applied Physics Letters 91: 093506 (2007).
J. Chen, V. Leblanc, S.H. Kang, P.J. Benning, D. Schut, M.A. Baldo,
M. A. Schmidt, and V. Bulovic, “High Definition Digital Fabrication
of Active Organic Devices by Molecular Jet Printing,” Advanced
Functional Materials 17: 2722-2727 (2007).
T.D. Heidel, J.K. Mapel, K. Celebi, M. Singh, and M.A. Baldo,
“Analysis of surface plasmon polariton mediated energy transfer in
organic photovoltaic devices,” Proceedings of SPIE 6656: 66560I1-8
(2007).
K. Celebi, T.D. Heidel, and M.A. Baldo, “Simplified calculation of
dipole energy transport in a multilayer stack using dyadic Green’s
functions,” Optics Express 15: 1762-1772 (2007).
J.K. Mapel, K. Celebi, M. Singh, and M.A. Baldo, “Plasmonic
excitation of organic double heterostructure solar cells,” Applied
Physics Letters 90: 121102 (2007).
B.N. Limketkai, P. Jadhav, and M.A. Baldo, “Electric field dependent
percolation model of charge carrier mobility in amorphous organic
semiconductors,” Physical Review B 75: 113203 (2007).
M. Bora, D. Schut, and M.A. Baldo, “Combinatorial detection of
volatile organic compounds using metalphthalocyanine field effect
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transistors,” Analytical Chemistry 31st (2007)
dx.doi.org/10.1021/ac061904r.
M. Segal, M. Singh, K. Rivoire, S. Difley, T. Van Voorhis, and M.A.
Baldo, “Extrafluorescent Electroluminescence in Organic Light
Emitting Devices,” Nature Materials 6: 374-378 (2007).
C.L. Mulder, K. Celebi, K.M. Milaninia, and M.A. Baldo, “Saturated
and efficient blue phosphorescent organic light emitting devices with
Lambertian angular emission,” Applied Physics Letters 90: 211109
(2007).
M.A. Baldo, M. Segal, J. Shinar and Z.G. Soos, “Comment on ‘The
Frequency Response and Origin of the Spin-1/2 Photoluminescence-
Detected Magnetic Resonance in a pi-Conjugated Polymer’ - Reply,”
Physical Review B 75: 246202 (2007).
M.K. Lee, M. Segal, Z.G. Soos, J. Shinar and M.A. Baldo, “Comment
on ‘On the Yield of Singlet Excitons in Organic Light-Emitting
Devices: A Double Modulation Photoluminescence-Detected
Magnetic Resonance Study’ - Reply,” Physical Review Letters 96:
089702 (2006). Chapter 27. Soft Semiconductor Devices 27-8 RLE
Progress Report 151
M. Segal, M.A. Baldo, M.K. Lee, J. Shinar, and Z.G. Soos, “The
Frequency Response and Origin of the Spin-1/2 Photoluminescence-
Detected Magnetic Resonance in a pi-Conjugated Polymer,” Physical
Review B 71: 245201 (2005).
M.K. Lee, M. Segal, Z.G. Soos, J. Shinar, and M.A. Baldo, “On the
Yield of Singlet Excitons in Organic Light-Emitting Devices: A
Double Modulation Photoluminescence-Detected Magnetic
Resonance Study,” Physical Review Letters 94: 137403 (2005).
P. Kiley, X. Zhao, M. Vaughn, M.A. Baldo, B.D. Bruce, and S.
Zhang, “Self-assembling Peptide Detergents Stabilize Isolated
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Photosystem I on a Dry Surface for an Extended Time” PLoS Biology
3: e230 (2005).
B.N. Limketkai and M.A. Baldo, “Charge Injection into Cathode-
Doped Amorphous Organic Semiconductors,” Physical Review B 71:
085207 (2005).
R. Das, P.J. Kiley, M. Segal, J. Norville, A.A. Yu, L. Wang, S.
Trammell, L.E. Reddick, R. Kumar, S. Zhang, F. Stellacci, N.
Lebedev, J. Schnur, B.D. Bruce and M.A. Baldo, “Solid State
Integration of Photosynthetic Protein Molecular Complexes,” Nano
Letters 4: 1079-1083 (2004).
M.A. Baldo and M. Segal, “Phosphorescence as a Probe of Exciton
Formation and Energy Transfer in Organic Light Emitting Diodes,”
Physica Status Solidi A. 201: 1205-1214 (2004).
M. Segal and M.A. Baldo, “Reverse Bias Measurements of the
Photoluminescent Efficiency of Semiconducting Organic Thin Films,”
Organic Electronics 4: 191-197 (2003).
M. Segal, M.A. Baldo, R.J. Holmes, S.R. Forrest and Z.G. Soos,
“Excitonic Singlet-Triplet Ratios in Molecular and Polymeric Organic
Materials,” Physical Review B 68: 075211 (2003).
Book Chapters
Mapel, J.K., and M.A. Baldo, “The Application of Photosynthetic
Materials and Architectures to Solar Cells,” in Nanostructured
Materials for Solar Energy Conversion, edited by T. Soga. Elsevier
B.V. pp 335-358 (2007).
M.A. Baldo and M. Segal, “Phosphorescence as a Probe of Exciton
Formation and Energy Transfer,” in Physics of Organic
Semiconductors, edited by W. Brütting. (Wiley VCH, 2005).
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Baldo, M.A., M.E. Thompson and S.R. Forrest, “Organic
Electrophosphorescence,” in Organic Electroluminescence, edited by
Z. Kafafi. Taylor and Francis, pp 267-305 (2005).
Jadhav, P., B.N. Limketkai, and M.A. Baldo, “Effective temperature
models for the electric field dependence of charge carrier mobility in
tris(8-hydroxyquinoline) aluminum,” Organic Electronics, edited by
Gregor Mellor, Springer Verlag to be published (2009).
Dept. of Chemistry, Sahyadri Science College, Shimoga 34