Brief View Of Phthalocyanin For Solar Cell Harvesting

Brief view of Phthalocyanin for solar cell harvesting

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

Dept. of Chemistry, Sahyadri Science College, Shimoga 1


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



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



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



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.



(II)

The adducts (I) and (II) undergo condensation to form

phythalolocyanine molecule with liberation of ammonia.



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



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



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



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



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.



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



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



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.



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



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



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



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.


Direct path for charge carriers to electrode

Large interfacial area due to phase reoperation in the polymer blends

but precotation needed


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



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



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



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



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.



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.



BIBLIOGRAPHY

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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

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phycobilisomes,” Advanced Materials 21, 1-5 (2009).

M. Bora, K. Celebi, J. Zuniga, C. Watson, K.M. Milaninia, and M.A.

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biosensor applications,” Optics Express, 17, 329-336. (2009).



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



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



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).



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).


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Brief View Of Phthalocyanin For Solar Cell Harvesting

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