SYNTHESIS AND CHARACTERIZATION OF CARBAZOLE BASED …

SYNTHESIS AND CHARACTERIZATION OF CARBAZOLE BASED HOLE TRANSPORTING

MATERIALS FOR APPLICATION IN PHOTOVOLTAIC CELLS

FINAL TECHNICAL REPORT

BACK TO LAB PROGRAMME Project Reference No: 01-27/WSD-BLS/2016/CSTE dtd 31/10/2016

WOMEN SCIENTIST DIVISION

KERALA STATE COUNCIL FOR SCIENCE TECHNOLOGY AND

ENVIRONMENT

Project Period: 01/02/2017 to 31/01/2020

Women Scientist: Scientist Mentor:

Saritha C. Dr.K.R.Haridas

Souparnika Professor

Muthathy East School of Chemical Sciences

Kanayi P.O. Kannur University

Kannur-670307 Edat P.O.- 670327

KANNUR UNIVERSITY SCHOOL OF CHEMICAL SCIENCES

Swami Anantha Theertha Campus Edat-670 327, Payyanur, Kannur, Kerala

MARCH 2021

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AUTHORIZATION

The work entitled “Synthesis and characterization of carbazole based hole-

transporting materials for application in photovoltaic cells” by Saritha C. was carried out

under the “Back to lab programme” of women scientist division, Kerala State Council for

Science Technology and Environment, Govt. of Kerala. The work was carried out at

School of Chemical Sciences, Payyanur campus, Kannur University, Kerala with Dr.K R

Haridas as Scientist Mentor. The project was initiated vide sanction order no.01-27/WSD-

BLS/2016/CSTE dtd 31.10.2016. The project was completed on 31.01.2020.

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ACKNOWLEDGEMENTS

It gives me great pleasure in expressing my gratitude to all those people

who have helped me and had their contributions in this endeavor. I would

like to express my heartfelt gratitude and appreciation to my Guide, Dr. K.R.

Haridas, Professor, School of Chemical Sciences, Swami Anantha Theertha

Campus, Kannur University for his valuable guidance, reasonable advice,

academic freedom, encouragement and motivation throughout my research

work.

I am deeply indebted to Dr. Baiju K.V, Assistant Professor and

present Head and Dr. S. Sudheesh, Associate Professor and former Head,

School of Chemical Sciences, Kannur University for their constant supports,

encouragement and help rendered in the course of my work.

I express my sincere thanks to Dr. K.R. Gopidas, Dr. Suraj Soman, Dr.

K.N. Narayanan Unni (NIIST), Dr. M.K. Jayaraj and Mr. Kurias (CUSAT)

for their suggestions and help during photovoltaic measurements. I am

fortunate to receive help from some research institutes to carry out chemical

analysis, sophisticated instrumental facility, IIT Madras, IISC Bangalore

and Mangalore University. My indebtedness to scientists and technicians of

these institutes is inexplicable. I express my special thanks to Mrs. Swapna M.V. and Mr. Rajeesh P.

for their constant interest in my work and voluntary help at the hour of

needed. I thank Ms. Raghi K.R for her help for carrying out the

computational studies.

I would also like to express thanks to my friends and colleagues at

school of Chemical Sciences, Kannur University for their help during my

research period.

I am grateful to the Teaching and non-teaching staff of the

Department and also other departments in Payyanur Campus. I sincerely

4

acknowledge Mr. Sabu Vincent, Junior Librarian, for all the help provided

for my research.

I am grateful to Kerala state Council for Science Technology and

Environment (KSCSTE) - Women Scientist Division (WSD) for providing

financial support in the form of Back to Lab Research Fellowship

Programme. I also thank Dr. KR Lekha, Head, Women Scientist Division,

KSCSTE for her valuable advices and encouragement throughout my research

work. I would like to express my sincere thanks to Dr. Manoj A.G.

Namboothiri, Associate Professor, IISER, Trivandrum for his valuable

suggestions and motivation throughout my research tenure.

I am also grateful to Kannur University for providing financial

assistance in the preliminary stage of my Ph.D. programme.

On the personal front, I would like to take this opportunity to express

my deepest gratitude to my dear husband, Mr.Sajeev K, for his co-operative

and moral support. My immense love and gratitude to my dear daughter,

Niharika for her love and support during this period. I express my immense

gratitude to my parents for always being their support to me emotionally and

spiritually. I thank my sisters and family members for their constant

encouragement and support.

I thank the Almighty God for giving me the strength, ability and

opportunity to undertake this research work and whose blessings have

enabled me to accomplish my research work successfully.

Saritha C.

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CONTENTS

Sl. No. Title Page No.

Abstract 6

1 Introduction and Review of Literature 7

2 Objectives of the study 26

3 Materials and Methods 27

4 Results and Discussion 61

5 Summary 88

6 Achievements 91

7 Scope of future work 93

8 Bibliography 94

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ABSTRACT

At present the fast growing and developing modern world is in an urgent need for

the latest technologies to fulfill the energy requirements from various fields, finally which

focuses on renewable energy resources as the conventional power production techniques

are not competent to overcome the present crisis. Among renewable energy resources,

solar energy becomes the most prominent because of its large number of advantages over

others. Now-a-days various methods and techniques are applied to convert solar energy

into electrical energy. Researches are mainly focusing to develop a technology, which

overcomes the disadvantages like high rate of expensiveness, short life span, lack of eco

friendliness etc., faced by most of the existing solar energy conversion techniques. The

new technology to resolve all the above mentioned disadvantages is nothing but the Dye

Sensitized Solar Cell (DSSC).

DSSCs are very promising because of its large potential to convert solar energy to

electrical energy at low cost and it lacks leakage or sealing problems that exist in liquid

electrolyte dye sensitized solar cell. In DSSC, electron injection takes place in ultra-fast

speed from a photo excited dye into the conduction band of TiO2, followed by the

subsequent dye regeneration and hole transport to the counter electrode which are the

processes that govern the efficient electrical power generation.

The present work mainly focuses on the synthesis of novel hole transporting

materials based on carbazole. Carbazole based material constitute a well-known class of

hole conducting material. The charge carrier mobility and photo conductive properties of

these materials have been undergone for study by various groups. More and more

investigations have been going on about hole-transporting materials based on the

carbazole moiety for the last decade. Various reasons for this is that very interesting

features such as low cost of the starting material [9H-carbazole], good chemical and

environmental stability provided by the fully aromatic unit, easy substitution of the

nitrogen atom with a wide range of functional groups permitting a better solubility and a

fine tuning of the electronic and optical properties.

The power conversion efficiencies obtained are different for different HTMs.

Maximum efficiency of 0.93 % is obtained. The low efficiency of DSSC may be due to

the poor pore filling of HTM and also due to the possible degradation of natural sensitizer.

The other reason is the use of natural dye as sensitizer instead of ruthenium dyes and silver

counter electrode instead of gold. The transformation of non-conventional energy, even to

its curtailed efficiency is amelioration to mankind.

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1. Introduction and Review of Literature

Nowadays the necessity for the use of energy is tremendously increasing beyond

our imagination. The increasing energy demand invites a lot of pressure on the

conventional energy sources such as oil, gas, coal etc. The extracted from the fossil under

the earth is in diminishing tendency and it is polluted the atmosphere as a whole. So an

alternative energy sources which provides energy in a sustainable manner are needed. The

need for sustainable energy should be due to the limited fossil fuels[1].

At present the world is in huge demand of higher level technologies for providing

inexhaustible energy. One of the main challenges is how to procure the hike in global

energy demand without immolating our environment in future[2], [3]. The level of solar

energy striking on the earth in an hour is much more than the overall energy utilized on the

earth in one year. The solar energy gives a clean ample energy and one the best energy

source for the future environment. The solar cells are the devices which transform light

energy into electrical power.

Fossil fuels like coal, petrol, diesel, kerosene and natural gas are treated as

conventional energy sources. When biological materials were degraded fossil fuels are

produced which takes long period of time. Energy demand and consumption of fossil fuels

are both sides of a coin. While energy demand shows an increased rate of growth, the rate

of consumption of fossil fuels also shows high rate which is beyond the rate of formation

of fossil fuels.

Combustion of fossil fuels results in the formation of carbon dioxide which is

normally emitted in the atmosphere. This process is one of the prominent reasons for

greenhouse effect. Due to this effect the average global temperature of the earth is

increasing. The main driving forces to promote renewable energy resources are the

scarcity of fossil fuel supply and global warming-serious threat to the earth - which is the

aftermath of greenhouse effect. Considering the above mentioned threats we can

unequivocally say that it is nothing but the solar power is the ultimate solution.

Sun’s energy becomes the ultimate choice as a clean energy source, which is

abundant and give security for the future development and growth. The main advantages

of solar energy includes clean energy source, everlasting renewable source of energy,

environment friendly and very large source of energy.

8

Photovoltaic

In 1839 Becquerel, a French scientist introduced a new principle to transform

energy from sunlight to electrical power which is known as photovoltaic effect. The

energy in sunlight is scientifically termed as photons or packets of energy. According to

the variations in wavelength of light these photons carry various amounts of energy. Solar

cell is one of the most modern and important way to conquer the increasing demand in

energy. Solar power conversion devices are considered as inevitable future energy

candidate because of the unlimited supply of light energy.

Solar cell is an electronic device which transforms solar power directly into

electrical power through the photovoltaic effect. When the light with certain wavelengths

falls on the device, the semiconducting materials are absorbed the photons in the light,

subsequently electrical charge carriers, electrons and holes are produced. These carriers

concentrate to the junction where a strong electric field exists. As the presence of this

strong electric field electrons and holes are separated and produce an electric power in the

external load.

In the light of increased awareness by the public in environmental and energy

issues, renewable energy sources are attracted much more attention. Solar energy is

expected to play a crucial role as a future energy source while considering all other

abundant and nonpolluting renewable energy sources. The main goal of solar cell research

is to improve the solar energy conversion efficiency at cheaper rate to provide a cost

effective sustainable energy source.

In 1950s conventional solar cells were invented and later it was commercialized in

1960s for use in space programmes. From there the solar cell research field attained steady

and steep growth in the efficiencies and reliability of these cells. All over the world,

especially in developed countries rate of solar power is much more than the rate of

electrical power generated in other methods. Hence all the scientist’s eyes are concentrated

in the field to develop cost effective types of solar cells.

Every solar cell compulsorily needs a light absorbing substance within the cell

structure to absorb photons and generate electrons through the process of photovoltaic

effect. The materials used in solar cells possess the feature of absorbing solar light reached

in surface of the earth in certain wavelengths; some of them have the capacity to catch the

light beyond the reach of earth’s atmosphere. While technologies are seeking for better

efficiencies, the bitter experience is that some of the cells shows higher efficiencies but on

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the other hand they are not cost effective and some type of cells possess cost effectiveness

but unfortunately they have poor efficiency. So the research is aimed to resolve the

imbalance between power conversion efficiency and cost effectiveness.

Photovoltaic devices are devices that transform the incident photon energy of the

solar radiation into electrical power. The photovoltaic effect was first experimentally

demonstrated by French physicist A. E. Becquerel. Albert Einstein explained the

mechanism of photovoltaic effect. In the field of photovoltaics, dye sensitized solar cell is

considered to be a promising candidate as a low cost renewable energy source. Photo

voltaic is an area of technology and research which related to the practical application of

photo voltaic cells in generation of electricity from light.

Types of photovoltaic cells

Photovoltaic cell technologies can be classified based on the type of absorbing

material used, manufacturing technique employed and the type of junction formed.

Classification of Solar Cells

Crystalline silicon solar cells-monocrystalline silicon and polycrystalline silicon

solar cells

Thin film solar cells-Copper Indium Gallium Diselenide (CIGS), Cadmium

Telluride, Amorphous silicon etc come under this type.

Emerging technologies- Thin film silicon, Dye sensitized solar cells, Polymer based

organic solar cells, perovskite cells

Dye Sensitized solar cells

Scientists Gratzel and O.Regan after their long term research, in 1991 discovered a

new type of photovoltaic technology known as Dye Sensitized Solar Cells. These cells are

worked on the basis of Nature’s principles of photosynthesis. A typical DSSC consists a

thin layer of nanoporous titanium dioxide covered with a sensitizer dye which is

10

responsible for the absorption of sunlight. Dye has the property similar to chlorophyll in

green plants. The main advantages of DSSCs are the facile and cost effective

manufacturing processes and simple fabrication techniques[23].

Dye-sensitized solar cells will play a vital role in the upcoming photovoltaic

technologies for the development of renewable and cost effective energy[24]. There are

two types of DSSCs based on the material used as redox mediator. They are with liquid

electrolyte or solid hole conductor [10, 11]. Nanocrystalline wide band gap

semiconductor oxide normally TiO2 deposited on a transparent conducting oxide (FTO)

glass substrate is one of the major parts of these cells[25], [26]. The oxide surface is linked

with a molecular sensitizer through an anchoring group. The sensitizer dye absorbs light

and it gets excited[27], [28]. During the time of excitation, it injects an electron into the

conduction band of the semiconductor. By the use of a liquid electrolytic redox system

photo excited dye can be regenerated traditionally.

Liquid electrolyte based DSCs have reached conversion efficiencies over 12%.

However, these liquid based DSCs may follows potential leakage and corrosion issues

[29][12,13] practical advantages have been obtained by substituting the liquid electrolyte

with an organic solid hole transporting material (HTM) [14,15]. Now a day solid state

DSC attained conversion efficiency over 9%. [16]. DSCs with liquid electrolyte shows

better power conversion efficiency than that of solid state hole conductors.

The feature of illuminated organic dyes to generate electricity at oxide electrodes

in electro chemical cells was revealed in 1960s. Schematic representation of DSC is

described in Figure. In the view of such experiments electric power generation via the dye

sensitization solar cell principle was depicted.

Solid state dye sensitized solar cells (DSSC)

Let us replace inorganic semiconductor photo voltaic devices with Solid state dye

sensitized solar cells as they are cost effective. Recently, DSSCs offer practical advantages

compared to electrolyte junction cell hence they capture much more attention. In DSSCs

traditional liquid electrolyte is replaced with inorganic p-type semiconductors such as CuI

and CuSCN or organic hole conductors which helps to solve the issues of sealing, leakage

and dye desorption[30]. While comparing with the conventional p-n junction solar cells,

merit on the side of DSSC is that only majority carriers are actively participated in the

photo electric conversion processes. In the solid state DSSC the charge transport is

electronic and no ions are migrating inside the electrolyte whereas in liquid electrolyte

ionic transportation occurs.

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A typical solid state DSSC consists of various layers, optically transparent

electrode, blocking layer of compact titania, the dye absorbed on the nanoporous layer,

solid organic or inorganic p-type layer (hole transport layer: HTM) and a gold or silver

counter electrode. In solid state DSSC the processes that determines the efficient

generation of electric power includes, the electron injection from a photo exited dye into

the conduction band of titania and followed by dye regeneration and subsequent hole

transport to the counter electrode. In DSSC the interfacial processes determines the overall

performance of the device whereas in typical inorganic silicon solar cell, bulk properties

such as crystallinity [18] and chemical purity decide the efficiency .

Basic Structure of DSSC

A typical DSSC comprises of TCO coated glass substrate normally fluorine doped

tin oxide (FTO) with nanoporous titania layer covered with the dye solution in the middle

of the cell. Then the liquid electrolyte and the counter electrode which is normally

fabricated with platinum. While considering solid state DSSC, a solid hole conductor was

used in the place of liquid electrolyte. This brings the use of inexpensive and easily

available better quality materials in these devices. The structure of DSSC can be depicted

as dye sensitized hetero junction of extremely large contact area between two inter

penetrating, individual continuous networks of TiO2 and hole conductors. Figure

represents the Schematic representation of dye sensitized solid state solar cell contains

hole transporting material.

Initial charge separation occurs in an interfacial process with very high efficiency

when Light is absorbed directly at interface by a mono layer of adsorbed dye. After the

initial separation, electrons are confined in the TiO2 and holes in the electrolyte both are

at discrete phases. As a result of the mesostructured nature of the inter penetrating

networks, the formation of substantial potential barrier at the interface to suppress the

subsequent charge recombination is difficult. In these kind of solar cells, close proximity

of electrons and holes throughout the networks and the extremely large interface lead to

the interfacial recombination which results in major energy loss.

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Structure of Solid State DSSC

Operation principle of Dye Sensitized Solar Cells

The dye sensitized solar cell comprises of a transparent conducting oxide coated

glass electrode, nanoporous titania(nc-TiO2) deposited on the TCO coated glass electrode,

dye molecules deposited on the surface of the nc-TiO2, hole transporting material and a

catalyst coated counter electrode. At the time of illumination, the cell generates voltage

and current through an external load connected to the electrodes. The dye molecules in the

DSSC is responsible for the light absorption. Then at the semiconductor HTM interface

the charge separation by electron injection from the dye to the TiO2 develops. If a single

layer of dye molecules are present then the incoming light absorption was very less[23]. In

order to increase the optical thickness of the layer, dye molecules are arranged simply on

top of each other. Here only the dye molecule in direct contact with the semiconductor

electrode surface is responsible for charge separation and hence to the current generation.

To overcome this problem a nanoporous TiO2 electrode with large internal surface area

was used by Gratzel research group and this allow large amount of dye to be percolated.

In the DSSC the sensitizer molecules which is adsorbed on the nanoporous TiO2

working electrode surface is responsible for the absorption of photons then it will be

promoted to their excited state(S*) from the ground state(S). Then the excited dye

molecules gets oxidized (S+) by the injection of electrons to the conduction band of TiO2

working electrode. The electrons from the conduction band of TiO2 transferred through the

compact layer of TiO2 and then to the conducting electrode, finally through an external

load to the counter electrode. At the counter electrode the electron was transferred to the

HTM and it gets oxidized. Whenever the oxidized dye molecules gets reduced by the

HTM the internal circuit will be completed[31]. The various steps involved in the photo to

electricity conversion are described below.

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Operation principle of DSSC

Materials for the Dye Sensitized Solar Cells

Substrates

The performance of the device depends mainly on the transparent conducting

oxide. The TCO layer performs as a current collector and functions as a support for the

semiconductor layer. The other side of glass helps electrons to move through surface

treatment. If the glass gets high transmittance and low surface resistance, the movement of

electrons become free and provides easy passage of light. The low electrical resistivity

enhances the electron transfer process and the energy loss reduces. Generally for

photovoltaic device fabrication both organic and inorganic materials are used for

transparent conducting coatings. Inorganic films were developed using a layer of

transparent conducting oxide (TCO)[32] such as indium doped tin oxide (ITO), flurine

doped tin oxide(FTO) and aluminium doped zinc oxide. Organic films were made up of

graphene, carbon nanotubes networks and various polymer derivatives. The most widely

used TCO material for efficient photovoltaic application is a glass substrate coated with

ITO or FTO. But the main problem with ITO is the decrease in conductivity during the

calcination process in the fabrication of DSSCs. This is because of that at high temperature

there is a trend to reduce in oxygen vacancies which causes in the decrease of electric

14

carriers. Indium is very expensive and toxic. Therefore FTO become the most efficient and

preferred transparent conducting oxide used for the DSSC fabrication. The main

advantages of FTO over ITO are better stability at high temperature, cost effective and its

high transmittance in visible wave region.

Semiconductor material

The working electrode in a DSSC consists of a nano structured semiconductor

material, attached to a transparent conducting substrate. Various n-type oxides such as

TiO2, ZnO and SnO2 can be used as the semiconductor material in DSSC. The most

extensively utilized wide band gap semiconductor material in DSSC is TiO2. It has wide

applications in optical as well as electronic materials. This is because TiO2 have

remarkable physical, chemical and optical characteristics. Use of this semiconductor

material results in optimum performance of the device. TiO2 is inexpensive, nontoxic, and

plentiful in nature which is a raw material for paint production, sun screen and food[33],

[34]. The electrode composed of interconnected nanoparticles in the size 15 – 30 nm. They

form a transparent porous electrode, with a typical thickness of 1-15 μm. Screen printing

and doctor blading are the deposition methods used for the film preparation.

Titania exists in three phases such as anatase, rutile and brookite. The crystal

structures of titania are shown in figure. Among these forms rutile is the most stable phase

but the rate of electron transfer is very slow which results in low current in DSSC[35]–

[37]. So anatase phase of titania is extensively used as the photoanode material which

results in a power conversion efficiency of about 12% due to high surface area and greater

electron transfer rate. The TiO2 semiconductor has three functions in the DSSC: it

provides the surface for dye adsorption, it functions electron accepter for the excited dye

and it acts as electron conductor. ZnO is considered as an alternative to TiO2 which has

similar band structure and relatively high electron transport. However in the presence of

sensitizers with acidic groups ZnO is not stable. Thus in our work the working electrode

used is nanoporous TiO2.

The properties of the colloidal titania solutions used for deposition greatly

influences the properties of the nanostructured TiO2 photoelectrode. For the preparation of

colloidal TiO2 is based on the procedure developed by Kalyanasundaram and Gratzel[38].

Usually titanium(IV) alkoxides are used in high performance photovoltaic cells.

Preparation of nanostructured titania electrodes

The thin films of nanoporous titania as photoelectrode was prepared by doctor

blade technique or screen printing technique. Normally the titania electrodes were

15

deposited as a thin film on the TCO coated glass substrate from a paste containing TiO2

nanoparticles of desired size. The paste was prepared by using commercially available P25

powder. The titania paste was dispersed by grinding in a mortar with water, stabilizers like

acetyl acetone and surfactant Triton X-100.Then spread the paste on the TCO coated

substrate by doctor blading, sintered at 450oC in order to get transparent film of titania.

Whereas in screen printing technique the paste was printed on the TCO coted substrate

followed by sintering at 450 oC. The thickness of the film rely on the paste composition,

repetition of printing and the screen mesh size.

Sensitizer Dyes

One of the main problem associated with the DSSC fabrication was the high cost

for electricity generation due to its high material price. This arises because of the high cost

of the dye material. The commonly used dyes in laboratory are high cost ruthenium dye or

N719 dye developed by Gratezel and coworkers.There are some materials used to convert

the colour of substances like cloths or hair, at the same time absorbs light and produce

electrons is known as nothing but the sensitizer dyes. A coloured substance has the

peculiarity of absorption of every light except the colour of its own. Usually black dye is

opted because it absorbs all the colours of visible light. It is needed that the dye to be

bound to the TiO2 surface via physisorption or chemisorptions for better electron injection.

For the surface attachments of the sensitizers to the semiconductors it is better to use

groups such as carboxylate, phosphonate or sulphonate. The amount of sensitizer adsorbed

on the surface of the TiO2 layer depends on the morphology and porosity of the

nanoporous titania.

Since the dye molecules are absolutely small in nature, the layer of dye molecules

should be made fairly thick what it is actually in size to capture a reasonable amount of

the incoming light. To resolve this issue, generally a nano material is used as a scaffold to

hold large number of the dye molecule in a 3D matrix, which increases the number of

molecules for any given surface area of the cell. At present in the existing design,

semiconductor material provides scaffolding which performs double duty. The sensitizers

used in DSSC should satisfy some basic properties. The LUMO (Lowest unoccupied

molecular orbital) of the sensitizer molecule should match the conduction band edge of the

semiconductor oxide to reduce the energetic potential losses during the electron transfer

process. The HOMO (highest occupied molecular orbital) of the dye molecule should be

sufficiently low to accept electron from the hole transporting materials.

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In DSSC, sensitizer dyes deposited on the semiconductor electrode surface is

responsible for the absorption of incident photon. Various features of the dye molecules

attached to the semiconductor material determines the process of conversion of light to

energy by the DSSC. Generally the metal oxide films are dipped in the dye solution for 12

to 24 hours in order to attain maximum adsorption of the dye molecule on the surface of

nanoporous TiO2. The sensitizer dye adsorbed on the semiconductor material should have

high stability at the semiconductor-electrolyte interface in the working environment to

exposure to natural daylight[39].

Ruthenium complexes

Gratzel and coworkers made solar cells by using metallo-organic ruthenium

complexes along with nanostructured titania electrodes. The most promising type of

ruthenium complexes as sensitizer dyes have the general formula ML2(X)2, Where M

stands for ruthenium or osmium, L stands for 2,2’-bipyridyl-4,4’-dicarboxylic acid and X

for halide, cyanide or water[40]. Among these the N3 dye (cis-RuL2(NCS)2) is considered

as the best choice for dye sensitized solar cells and it shows superior performance.

Recently, the black dye, (tri(cyanato)-2,2’,2’’-terpyridyl-4,4’,4’’-

tricarboxylate)rythenium(II)) has overcome the performance of N3 dye and achieve the

record cell efficiency of 10.4%[39].

Structure of N3 dye and Black dye

The most important characteristics of ruthenium dyes are their extraordinary

stability when adsorbed on the TiO2 surface. Ruthenium dye forms a charge transfer

complex with Ti3+ surface states. This will induce a π-backbonding and results in

stabilization of ruthenium complex. In order to improve the device performance and the

stability of DSSCs a large number of ruthenium based sensitizer dyes have been

17

investigated [41]–[44]. Recently the dyes, K19 and K77 have shown an excellent

photovoltaic performance and have higher molar extinction coefficients compared to N3,

N719 and Black dye[45], [46]. This is due to more extended conjugation of these

molecules.

Structure of K19, K77 and N719 Dye

Organic dyes

A large number of metal free organic dyes have developed over the last few years

trying to challenge the ruthenium complexes[47]–[51]. When compared to conventional

ruthenium based dyes, organic dyes have various advantages. They have higher molar

extinction coefficients and are easily synthesized using low cost starting materials and can

be easily modified. The photovoltaic performance of DSSCs with organic dyes as

sensitizers has improved remarkably during the last years and the current achieved power

conversion efficiency is comparable to the conventional ruthenium complexes. Organic

dyes are very cheap compared to ruthenium complexes, since noble metals are not

included in organic dyes.

In 2000 Sayama et al reported a merocyanine dye (Mb(18)-N) with an efficiency of

4.2%[52]. Organic sensitizers based on coumarin[53]–[55], triphenylamine[56], [57][97]

18

and indoline[58], [59] have been investigated and have achieved efficiencies in the range

of 3-8%[53].

Figure : Structure of organic sensitizers

The most important things relating to the development of new sensitizer dyes are

the photo conversion efficiency and the stability of the chromophores. Perylene based

compounds represent highly photo stable sensitizers and are used as sensitizers in DSSC

but the efficiency was low[60]–[63] . This type of dyes have high absorption coefficients

in the visible region. The design, synthesis and development of novel organic sensitizers

with large absorption coefficients and with absorption in the near IR region are essential to

improve the performance of the DSSC with organic photosensitizers.

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Structure of perylene based dyes

In order to develop a low-cost and environment friendly alternative to conventional

ruthenium complexes natural dyes extracted from plants were used as

photosensitizers[64]–[66] . Anthocyanin dyes extracted from blackberries with a photo

conversion efficiency of about 0.6% in nanocrystalline titania solar cell were reported by

Cherepy et al[67]–[69].Various natural dyes like cyanin[70]–[72], carotene [73] ,

tannin[74] and chlorophyll[75][126] have been utilized as sensitizers in DSSCs. Calogero

and Marco reported the use of red Sicilian orange juice dye as sensitizer and a power

conversion efficiency of 0.66% was attained[76]. Wongcharee et al reported a conversion

efficiency of 0.70% by using rosella as sensitizer in DSSC. Roy et al employed Rose

Bengal dye as sensitizer in DSSC and the achieved efficiency is about 2.09%[77]. Later

Wang et al employed modified coumarin derivatives as sensitizers in DSSC and reported

an efficiency of 7.6%[55][127]. In order to increase the efficiency of DSSC optimization

of the structure of natural dyes would be carried out[78].

Hole Transporting Materials

The most widely used electrolyte material in dye sensitized solar cell is

Iodine/Iodide based electrolytes[42]. These electrolytes have various disadvantages like

leakage problems and fast photochemical degradation [79]. This results in the short life

time of the cell. In order to overcome these problems solid state electrolytes that is, hole

transporting materials have been developed[80]. The solid state materials usually have

poor transparency which results in the low efficiency of the device.

The main functions of the hole transporting materials include regeneration of the

oxidized dye molecule after electron injection to the semiconductor and the transfer of

positive charge to the counter electrode. The hole transporting material should have higher

redox potential than the redox potential of the oxidized species then only the regeneration

20

of the oxidized dye occurs. There should be a large potential difference between the two

processes. In order to achieve the maximum device voltage under illumination the redox

potential of the hole conductor should be low, since the photovoltage of the dye sensitized

solar cell depends on the redox potential of the hole conductor. The main requirement for

a hole transporting material is that, the highest occupied molecular orbital (HOMO) and

the lowest unoccupied molecular orbital (LUMO) levels of the molecule should be

comparable with the HOMO of the sensitizer molecule and the conduction band of titania.

If the recombination reaction is much slower than the injection of the excited

electron into the semiconductor, the excited state of the sensitizer will be quenched even if

the hole conductor is not capable to do it. Another important criteria for the hole conductor

is that it seldom absorb light in the visible range as it compete with the dye molecule. The

hole transporting material deposition on the substrates should be done carefully without

damaging the sensitized semiconductor. By the use of materials with high glass melting

temperature can also ensure the stability of the deposited films, the hole transporting

materials should be in an amorphous state rather than crystalline to ensure a better pore

filling. Pore filling will be reduced dramatically whenever the hole conductor

crystallisation takes place inside the pores. It is inevitable to penetrate the hole

transporting material to the pores of the nanocrystalline titania network. This limits the

size of the molecules of the compound, as the pores have sizes in the nanometre scale. To

minimize the internal resistance of the device the contact with the counter electrode should

have an ohmic behavior. The conductor should have the ability to transport current

without diffusion losses or high resistances. Generally the conductivity was low for many

organic semiconductors.

The development of a new generation of hole transporting materials with cheaper

cost, facile synthesis and enhanced charge carrier mobility is of better consideration.

Because of the interesting photochemical properties of carbazole based derivatives, they

captured a significant attention by the researchers[81]. Recent interest towards carbazole

derivatives is due to its better charge transport function, which can be explored in the

molecular design of novel types of HTMS in DSSCs. Besides this, the flexibility of the

carbazole reactive sites which can be substituted with a wide range of functional groups is

another advantage that permits fine tuning of its optical and electrical properties [82].

Carbazole is one of the most important heterotricyclic aromatic organic compound.

It consist of pyrrole ring fused with two benzene rings. The derivatives based on

Carbazole have created great attention because of their amusing photochemical properties.

21

Recently the carbazole derivatives have been focused more by its excellent charge

transport functions, which can be utilized for the molecular designs of advanced forms of

HTMs in DSSCs. The other captivating advantage or quality of carbazole reactive sites is

its flexibility which can be replaced with a wide range of functional groups, which

allowing a fine tuning of its electrical and optical properties. The carbazole derivatives

also possess an excellent hole transport properties and thermal stability[83]. The

combination of triphenyl amine derivatives and carbazole derivatives is expected to offer

an advanced morphological and thermal stabilities as well as its better hole transport

properties[82] . Basic work in our laboratory include synthesis of hole transporting

materials for the fabrication of solar cells which are sensitized with natural dyes.

Molecules contain a π-rich heterocyclic or aromatic ring system functionalized

with one or more electron donating substituents exhibits good hole transporting

properties[84]. The most commonly encountered substituents are amino and alkoxyl

groups, which contain single bonded hetero atoms possessing sharable lone pairs[85]. The

most widely used hole transport molecules are aromatic amines[86], [87]. The Carbazole

based derivatives which are attracted more by its fascinating photochemical properties,

good environmental and chemical stability added by the total aromatic unit, smooth

substitution of nitrogen atom with a vast range of functional groups granting a good

solubility and a better tuning of the optical and electronic properties[88], [89] .

Carbazole based material constitutes a very well established category of hole

conducting material. The photo conductive properties and charge carrier mobility of these

materials have been studied by various groups. Carbazole based hole-transporting

materials have been one of the main subject of an accentuating number of research and

investigations over the last decade[90], [91].

Triphenylamine cored star-burst materials are largely investigated for their smooth

and easy modification, superior ability of hole-transporting and the propeller molecular

conformations[92]–[94]. These were also used as the core part to build up an opto-

electronic functional based star-burst material. The triarylamine based compounds are

getting an increasing importance as a hole transporting material in diverse electro optical

applications such as light emitting, photovoltaic devices and photo refractive systems[95].

The triarylamine moiety confirms the need of reversible and easy oxidation and

accordingly constitutes the building block of many hole transporting compounds[96]. The

different derivatives of triarylamine have been applied successfully in the solid state dye

sensitized TiO2 solar cells[80] .The materials which are needed for DSSC fabrication are

22

micro and nano layers of nanocrystalline TiO2 as semiconductor, glass substrate coated

with Fluorine doped tin oxide, metal as electrodes, organic compounds as the hole

transporting dyes and layers. The capability of the cells depends on the materials used,

thickness and the mode of coating of all the layers[97].

Structure of various hole transporting materials used in DSSC

The use of spiro-OMeTAD (2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-

9,9’-spiro-biflurene) as hole transporting material in solid state dye sensitized solar cell

was found most successful with a power conversion efficiency of around 6%.Spiro-

OMeTAD has various advantages over the standard hole transporting material N,N’-

diphenyl-N,N’-bis(3-methyl phenyl)-1,1’-biphenyl-4,4’-diamine (TPD) such as high glass

transition temperature and formation of stable amorphous films[98], [99]. The formation

of amorphous layers results in enhanced pore filling. The use of TPD films in DSSC may

cause crystallization after two months whereas the spiro-OMeTAD layer will remain in

the amorphous state for years[100]. Due to these reasons spiro-OMeTAD was successfully

employed as the HTM in solid state dye sensitized solar cells [80].

23

Counter electrode

Platinum is used exclusively as the most efficient counter electrode in DSSC using

I-/I3-. The deposition of the counter electrode was usually done by sputtering a platinum

layer of 200nm thickness or by the pyrolysis of the chloroplatinic acid solution on the

FTO coated substrate[101]. The main advantages of using platinized FTO as the counter

electrode include the electro catalytic activity of the platinum that improves the reduction

of I3- by enhancing the electron exchange [102] and it facilitating the light reflection due to

the mirror effect of the platinized FTO. But the platinum counter electrode has several

disadvantages which limits its application such as limited material availability, high cost

and corrosion of platinum in I3- containing electrolyte to form platinum iodide that affects

the long term stability of the electrode[103]. This leads to the development of platinum

free counter electrode with high catalytic activity.

Kay and Gratzel developed DSSC with porous carbon counter electrode which is

made from a mixture of carbon black and graphite with a power conversion efficiency of

6.7% [104]. Many other carbon materials like graphite, carbon nanotubes and activated

carbon[105]–[107] have utilized as catalysts on FTO glass for the counter electrode. Later

conducting polymers based on poly(3,4-ethylenedioxythiophene) were utilized as catalysts

on FTO glass as the counter electrode[108], [109].When compared to platinum counter

electrode the cost of carbon materials and conducting polymers are very less. In order to

improve the efficiency of the device the problems of less energy conversion efficiency and

poor adhesion on to FTO substrate must be eliminated. DSSC using carbon black counter

electrode with efficiency of about 9% was successfully fabricated [110]. Metals like gold,

silver and aluminium were widely used as counter electrode in solid state dye sensitized

solar cell. The development of DSSC with low cost and large scale production was

achieved by the replacement of noble metal electrodes like gold and silver [111].The

24

deposition of metals like silver and gold as counter electrode can be done by using vacuum

thermal deposition method.

The development of photovoltaic cell to the market

Silicon based inorganic solar cells are ruling the solar cell market with a typical

power conversion efficiency of upto 30% [112]. For inorganic solar cells the production

cost is high and resulting in high energy consumption. Since organic solar cells are in

initial stage of its development they have not yet entered into the commercial market

because of its low power conversion efficiency. Third generation solar cell refers to solar

cells that are fabricated with technologies and photovoltaic devices that operate differently

in theory to conventional semiconductor photovoltaic cells. So the search for the

substitutes now reached into the fields of organic molecules and polymers; they have

several advantages over inorganic materials. Physical properties of organic materials like

band gap, valance and conduction energies, charge transport, solubility and morphological

properties are chemically tunable. Small quantities are needed for device preparation.

Therefore organic photo voltaic solar cells play an important role in the search for low cost

modules for the production of domestic electricity. DSSCs have attracted much attention

because of their environmental friendliness and low production cost.

In 1991 Gratzel et al introduced a modern class of solar cells namely DSSCs,

which opened a new way of hope in the field of solar power generation to the researchers.

DSSCs have welcomed by them because of the prominent peculiarities like eco

friendliness and cost effectiveness. Whenever DSSC achieve high efficiency with cost

effectiveness without any doubt we can say it will conquer the market by sweeping out all

other existing type of solar cells.

Literature review

The glorified history of photovoltaics later known as ‘photovlotaic effect’ started in

1839 when the discovery made by French Physicist Becquerel in 1839[113], which is

defined as the production or change of electrical potential between two electrodes

separated by a suitable electrolyte or other substance upon light radiation. The higher the

rate of development of world the higher its demand for energy. When energy consumption

increases demand for energy also increases which causes CO2 emission and pollution

ultimate result is climate changes including global warming. Because of the increased

energy consumption and limited quantity of conventional fossil fuel resources like oil, coal

and gas, these supplies will be completely emptied within the next generations. Therefore

25

it is very much essential to replace the fossil fuels with cost effective, eco-friendly and

efficient energy resources; they are nothing but renewable energy sources especially solar

energy.

After the development of the first silicon solar cell in 1954, the solar cells were

used primarily in space applications until about the mid-70s [114]. Later solar cells were

used in various fields like customer electronics products, small scale remote residential

power systems, as well as in communication and signalling. However, grid connected

photovoltaic systems were entered into the market in the second half of previous decade

by implementing intensive roof programs in Japan, Germany and US.

Conventional solar cells convert light energy into electrical energy through the

photovoltaic effect on semiconductor materials like crystalline silicon[115]. Even though

technology of harnessing solar energy is more effective, it faces some drawbacks like high

rate of manufacturing costs, scarcity of sufficient amounts of Silicon, module assembly

costs, rigidity of panels etc. By using cost- effective raw materials and processes and by

applying new technologies we can overcome these issues. Research to develop organic

based solar cells started in the 1970s. Organic solar cells are a potentially cost - effective

option for utilising solar energy. The first organic based DSSC was developed by Hagen et

al [116]. The technology is still in its development process, and it will take an important

place in the energy sector as its ease of use, diversity of applications and comparatively

simple manufacturing process[117], [118].

The most renowned and well discussed unconventional photovoltaic system is the

dye-sensitized nano structured solar cell developed by Professor Gratzel in 1991 [119]. At

the moment this unique photoelectrochemical solar cell based on a TiO2 nanoparticle

photo electrode sensitized with a light harvesting metallo-organic dye, is on the verge of

commercialization offering an interesting alternative for the existing silicon based solar

cells as well as for the thin film solar cells. At the same time the research activity as well

as the interest in industrial point of view around the technology is growing fast. Therefore

research to develop latest generation of HTMs [120] with low cost, facile synthesis and

high charge carrier mobility is going on with high priority[121].

The carbazole based derivatives, which are attracted more due to its fascinating

photochemical properties, good environmental and chemical stability added by the total

aromatic unit, smooth substitution of nitrogen atom with a vast range of functional groups

26

granting a good solubility and a better tuning of the optical and electronic properties

[81]–[83]. Carbazole based material constitutes a very well established category of hole

conducting material. The photo conductive properties and charge carrier mobility of these

materials have been studied by various groups. Carbazole based hole-transporting

materials have been one of the main subject of an accentuating number of research and

investigations over the last decade[89]–[91].Triphenylamine cored star-burst materials are

largely investigated for their smooth and easy modification, superior ability of hole-

transporting and the propeller molecular conformations[122]. These were also used as the

core part to build up an opto-electronic functional based star-burst material. The

triarylamine based compounds are getting an increasing importance as a hole transporting

material in diverse electro optical applications such as light emitting, photovoltaic devices

and photo refractive systems. The triarylamine moiety conforms the need of reversible and

easy oxidation and accordingly constitutes the building block of many hole transporting

compounds[95], [123]. The different derivatives of triarylamine have been applied

successfully in the solid state dye sensitized TiO2 solar cells[87], [118], [124], [125] .

Dye sensitized hybrid solar cells with nanoporous TiO2 layer and different organic

hole transporting polymers based on triphenyl amine was investigated[126]. In 2004 the

preparation steps of solid state dye sensitized nanocrystalline TiO2 solar cells were

optimized with respect to the blocking TiO2 layer which is one of the essential layers in

such multilayer solar cells[30]. Researches are very enthusiastic and active in the field of

organic solar cells all over the world. Around 25% of the total research fund in countries

like USA, Japan and Germany are used in solar cell research field.

Steady and continuous experiments and observatios are going on in our country

also to achieve high efficient solid state cells. In laboratories, we have already developed

solid state cells using natural dye. In our laboratory Siji Mathew et al synthesized naphthyl

amine based HTM[127] and fabricated DSSC with red sandal dye as sensitizer[97].

2. Objectives of the study

The present work mainly concentrated and focused on the synthesis of various

carbazole based hole transporting materials by Ullmann coupling. Solid state dye

sensitized solar cell was fabricated by using the synthesized novel hole transporting

materials and a natural dye extracted from red sandal wood as sensitizer. The main

objectives of the present work are summarized below.

27

i) To synthesize organic hole transporting materials based on carbazole using multi

step organic synthesis.

ii) To characterize the synthesized compounds using sophisticated instrumental

techniques such as UV-Visible, FT-IR, 1H-NMR and 13C-NMR spectroscopy.

iii) To study the thermal and electronic parameters of the synthesized compounds

using differential scanning calorimetry (DSC) and cyclic voltammetry (CV).

iv) To fabricate organic solar cell devices using the above synthesized compounds as

hole transporting materials and the natural dye extracted from red sandal wood

as sensitizer.

v) To optimize the results using the synthesized compound.

vi) To synthesize derivatives of the parent compound according to need to improve the

efficiency of the device.

3. Materials and Methods

Solvents

Solvents used for synthetic works such as acetone, benzene, cyclohexane, 1,2-

dichlorobenzene and tetrahydrofuran(Merck, India) were purified according to the

standard procedure given by Vogel and Armaarego[128], [129].Solvents such as diethyl

ether, ethyl alcohol, ethyl acetate, hexane, methyl alcohol, etc were used for the working

up of reactions and for chromatography were of commercial grade and distilled twice

before use.

Spectroscopic grade chloroform, carbon tetra chloride, dimethyl formamide, ethyl

alcohol, methyl alcohol (Merck, India) was used for UV-Visible and Fluorescence

spectrometric analysis. Deuterated chloroform (CDCl3. 99.8 % containing 0.03 v/v of

tetramethyl silane(TMS), Deuterated dimethyl sulphoxide (DMSO-d6, containing 0.03 v/v

of TMS), trifluro acetic acid(TFA)(Aldrich) were used as solvents for recording nuclear

magnetic resonance (NMR) spectra of samples. Potassium bromide (KBr)(Merck, India)

was used for fourier transform infrared(FT-IR) spectroscopy measurements. In cyclic

voltammetry (CV) glassy carbon was used as the electrode and acetonitrile (ACN)

containing 0.1M of supporting electrolyte, tetrabutyl ammonium hexafluro phosphate

(TBAPF6) were used as solvents.

Reagents

Carbazole: Merck (India) Limited and is used as such without any further

purification.Mp:243-246 oC

28

N-bromosuccinimide -C4H4BrNO2 : Loba Chemie, Mp:183oC, Light yellow solid,

recrystallized from boiling water, dried with CaCl2 and used.

Triphenylamine: Merck (India) Limited and is used as such without any further

purification, Mp: 127 oC

Tetraphenylethylene: Sigma Aldrich and is used as such without any further purification,

Mp: 226 oC

Benzidine -C12H12N2: Merck (India) Limited and is used as such without any further

purification, Mp: 122-125 oC

Sodium : Merck (India) Limited, Mp : 97 oC

Aniline : Merck (India) Limited and is used as such without any further purification.

Acetic acid: Merck (India) Limited and is dried with CaH2 then distilled at reduced

pressure and used.

Bromine: Merck (India) Limited.

Sodium nitrite- NaNO2: Merck (India) Limited and is used as such without any further

purification.

Decolourising carbon : Sigma Aldrich

1-Propanol: Merck (India) Limited and is used as such without any further purification. 1-

Butanol: Merck (India) Limited and is used as such without any further purification.

Copper (I) chloride-CuCl: Merck (India) Limited and is used as such without any further

purification,

Electrolytic copper-Cu: Merck (India) Limited and is used as such without any further

purification.

Potassium carbonate- K2CO3: Merck (India) Limited and is used as such without any

further purification. Mp 891 oC.

18-Crown-6 ether: Merck (India) Limited and is used as such without any further

purification, Mp: 38-41 oC

Potassium Iodide- KI: Merck (India) Limited and is used as such without any further

purification. Mp: 681 oC

Dimethyl formamide-C3H7NO: Merck (India) Limited and is used as such without any

further purification.

Instrumentation

Infrared (IR) spectra were recorded on a Shimadzu FT-IR 8400 S spectrometer,

where the percentage of transmittance versus wave number (in reciprocal of centimeters)

29

was plotted. Solid samples were recorded as potassium bromide disc whereas liquid

samples as neat or solvent spectra, using spectroscopic grade solvent.

Ultraviolet-Visible (UV-Visible) spectra were recorded on a Schimadzu 1700 UV-

Visible spectrophotometer using a closed type quartz tube of 1.0cm square dimensions.

Spectroscopic grade ethanol, methanol and chloroform were used as solvent for UV-

Visible spectra.

Proton(1H) and carbon-13(13C) nuclear magnetic resonance(NMR) spectra were

recorded on a NMR-JEOL GSX-400 spectrometer. The proton spectra were recorded

using broad band inverse probe where the inner coil is for protons and outer coil for’X’

nuclei. Phase coherent solvent suppression was employed in some of the cases where the

solvent signal is very strong compared to the sample signals. All the carbon-13 spectra

were recorded in the dual (13C/1H) probe where the inner coil is for C-13 and the outer coil

for protons. The decoupling of the proton was done by employing waltz-16 sequence. The

spectral parameters like number of scans, time domain data points etc were adjusted

depending on the nature of the samples and the relaxation parameters like T1 and T2 were

taken into account for obtaining the required information. The chemical shifts were

reported in ppm unit with tetra methyl silane as internal standard. DMSO and CDCl3 were

used as solvents.

Electrochemical stability of the compounds were measured using Cyclic

Voltammetry (CV).CV measurements were carried out on a Autolab at a glassy carbon

electrode using millimolar solutions in acetonitrile (ACN) containing 0.1 M of supporting

electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), in a three electrode cell

and potentiostat assembly at room temperature. The potentials were measured against

platinum as reference electrode and each measurement were carried out with an internal

standard, ferrocene/ ferrocenium redox system.

Thermal characterization of the samples were determined using Differential

Scanning Calorimetry (DSC).DSC studies were performed with a TA Q10 model under

nitrogen atmosphere. About 3-5 mg of the samples were taken in an aluminium pan. The

samples were scanned from -50oC to 250 oC at the rate of 10 oC/minute. The instrument

was calibrated with indium standard before measurements. The compounds were analysed

for heating and cooling thermograms (cyclic) in nitrogen atmosphere from -50 oC to 250

oC at the rate of 10oC/minute. Glass transition temperature (Tg), melting point (Tm) and

crystallization temperature (Tc) were measured using DSC.

30

Preparation of intermediate compounds

1,3,5-Tribromobenzene (A)

Dissolve 10g (0.11 mol) (11.8mL) aniline in 40g of glacial acetic acid (44.6mL)

and stir well with mechanical stirrer while running in slowly a solution of 52.8g (17mL,

0.33mol) of Br2 in 34mL glacial acetic acid. The beaker should be cooled in ice during the

addition (exothermic). The final product should be coloured yellow by addition of a little

more bromine. Pour into excess of water, filter wash with water ether dried. Dissolve 10g

(0.03 mol) of 2,4,6-Tribromoaniline by heating on a water bath with 60mL of rectified

spirit and 15mL of benzene in a 200mL two necked flask filtered with a condenser. The

second neck closed with a stopper. 3.5mL of conc H2SO4 added to hot solution via side

neck, add gently swirl the liquid. Replace the stopper and heat on water bath until the clear

solution boils. Remove the flask from water bath and add 3.5g of powdered NaNO2 in two

approximate equal portions via side neck. After each addition replace the stopper and

shake the flask vigorously, add the second portion of NaNO2. Heat the flask on boiling

water bath as long as gas is evolved. Shake well from time to time allow the flask in ice

bath. A mixture of tribromobenzene and sodium sulphate crystallizes out. Filter, wash

with small quantity of ethanol, then with water. Dissolve 7.5g of crude tribromobenzene in

a boiling mixture of 120mL of glacial acetic acid and 30ml of water, boil the solution with

2.5g of decolourising carbon and filtered. Allow the solution to cool. Collect the crystal

and wash with small quantity of rectified spirit and dried. The 1,3,5-tribromobenzene thus

obtained was characterized by FT-IR and NMR spectroscopic methods. The melting point

was found to be 122 oC. Scheme 1gives the synthetic route for the synthesis of 1,3,5-

tribromobenzene.

Scheme 1: Synthetic route for 1,3,5-tribromobenzene(A)

3,6-Dibromocarbazole (B)

Carbazole 1.67g (0.01mol) was dissolved in 15mL DMF at 00C with stirring

followed by the addition of a solution of NBS 3.63g (0.02mol) in 10mL of DMF. The

resulting mixture was stirred at room temp for 2 hr. and the solution then poured into

100mL of water, filtered and washed with water. The crude product was recrystallized

31

from ethanol. Yield: 64%, Appearance: White crystalline solid, Melting point:

2040C.Scheme 2 gives the synthetic route for the synthesis of 3,6-dibromocarbazole.

The completion of the reaction is monitored by TLC.

Scheme 2: Synthetic route for 3,6-dibromocarbazole(B)

3,6-Dialkoxycarbazole (C-F)

Into a three necked flask fitted with a condenser, N2 inlet, addition funnel and

magnetic stirrer, was added 25mL of dry alcohol. The entire solution was cooled to 00C

with ice water before sodium 2.3g (0.1mol) was added gradually. The ice bath was

removed and the mixture was stirred until all the sodium had reacted. To this sodium

alkoxide solution was added DMF 12.5mL, CuCl, 3.8g (0.03mol), 3, 6-dibromocarbazole

1.65g (0.005mol) and another 12.5mL of DMF. The resulting mixture was heated to reflux

for 12 hrs. under nitrogen. The precipitate was filtered while hot and the filtrate was

diluted with 50mL of water and extracted with chloroform (30mL x 3). The combined

organic layers were neutralized with 5% HCl followed by washing with water and brine,

drying by passing through sodium sulphate and concentrating in vacuum. The residue was

recrystallized from methanol. Methanol, Ethanol, 1-Propanol,1-Butanol and 1-Pentanol

were used as alcohol to get corresponding 3,6-dialkoxycarbazoles. Scheme 3 gives the

synthetic route for the synthesis of 3,6-dialkoxycarbazoles. Table 1 gives the yield and

physical data of 3,6-Dialkoxycarbazoles(C-F)

Scheme 3: Synthetic route for 3,6-dialkoxycarbazole(C-F)

32

Table 1 : Yield and physical data of 3,6-Dialkoxycarbazoles(C-F)

Compound Melting

point (C)

Yield

(%) Appearances

C 104C 72 White solid

D 165C 66 Light Brown solid

E 175C 65 Brown Solid

F 182C 63 Brown Solid

Tris (4-bromophenyl)amine (G)

Triphenylamine 2.45g (0.01mol) was dissolved in 20mL DMF at 00C with stirring

followed by the addition of a solution of NBS 5.34g (0.03mol) in 10mL of DMF. The

resulting mixture was stirred at room temp for 6 hr. and the solution then poured into


from methanol.Yield:62%, Appearance: Pale yellow solid, Melting point: 1410C. Scheme

4 gives the synthetic route for the synthesis of tris (4-bromophenyl)amine.

Scheme 4: Synthetic route for tris (4-bromophenyl)amine(G)

Tetra (4-bromophenyl) ethylene (H)

Tetraphenylethylene 3.32g (0.01mol) was dissolved in 20mL DMF at 00C with

stirring followed by the addition of a solution of NBS 7.12g (0.04mol) in 20mL of DMF.

The resulting mixture was stirred at room temp for 6 hr. and the solution then poured into


from methanol.Yield:65%, Appearance: Pale yellow solid, Melting point: 1520C

Scheme 5 gives the synthetic route for the synthesis of tetra (4-bromophenyl)ethylene.

33

Scheme 5: Synthetic route for tetra (4-bromophenyl)ethylene(H)

4,4’-Diiodobiphenyl (I)

Benzidine 15.6g (0.085 mol) was dissolved in a solvent mixture of 5mL of

conc.HCl and 5mL of water at 00C with stirring and diazotized by the addition of a

solution of 1.2g sodium nitrite in 40mL of water. Then add potassium iodide solution (4g

in 100mL water) drop wise to the resulting solution with shaking. The crude 4,4’-

diiodobiphenyl was separated out. Filtered, washed and dried to get the product. It was

recrystallized from alcohol. Yield:72%, Appearance: Pale yellow solid, Melting point:

620C.Scheme 6 gives the synthetic route for the synthesis of 4,4’-diiodobiphenyl.

Scheme 6: Synthetic route for 4,4’-diiodobiphenyl (H)

Synthesis of Hole Transporting Materials (HTM)

Synthesis of hole transporting compounds 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-

yl)benzene(series I)

3,6-Dialkoxycarbazoles (C-F) 0.0075 mol, 1,3,5-tribromobenzene 0.8 g (0.0025 mol),

K2CO3 2.65 g(0.02 mol), Copper powder1.8 g(0.03 mol) and 18-crown-6 200 mg(0.00075

mol) were heated together in 50mL of orthodichlorobenzene under nitrogen atmosphere

and the resulting mixture was refluxed for 48hours at 1700C. Completion of the reaction

was checked by TLC. The inorganic compounds were removed by filtration. The solvent

was removed under vacuum. Then ethylacetate (150 mL) and water (100 mL) were added

to the mixture. The organic phase was separated, washed with water (100 mL×2) and brine

solution (100 mL), dried over anhydrous sodium bisulphite, filtered and the solvent was

removed in vacuum. The product was purified with column chromatography using ethyl

34

acetate: hexane as eluent to obtain brown solid which was recrystallized from acetone.

Scheme 7 gives the synthetic route for the synthesis of Series I compounds. Table 2 gives

the yield and physical data of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-yl)benzene (Series I )

Scheme 7: Synthetic route for the synthesis of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-

yl)benzene (Series I).

35

Table 2: Yield and physical data of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-yl)benzene

(Series I )

Compound Melting

point (C)

Yield

(%)

Reaction

time (hrs) Appearances

SC 011 149 58 48 Light Brown solid

SC 012 173 61 48 Brown solid

SC 013 184 56 48 Brown Solid

SC 014 189 53 48 Dark Brown Solid

1. SC 011-1,3,5-tris(3,6-dimethoxy-9H-carbazol-9-yl)benzene

2. SC 012- 1,3,5-tris(3,6-diethoxy-9H-Carbazol-9-yl)benzene

3. SC 013- 1,3,5-tris(3,6-dipropoxy-9H-Carbazol-9-yl)benzene

4. SC 014- 1,3,5-tris(3,6-dibutoxy-9H-Carbazol-9-yl)benzene

Synthesis of hole transporting compounds Tris (4-(3, 6-dialkoxy-9H-Carbazol-9 yl)

phenyl) amine (series II)

3,6-Dialoxycarbazoles(C-F)(0.003mol),tris(4-bromophenyl) amine 0.4820g

(0.001mol), K2CO3 2.65g(0.02mol), Copper powder 1.8g (0.03mol) and 18-crown-6

200mg (0.00075mol) were heated together in 50mL of orthodichlorobenzene under

nitrogen atmosphere and the resulting mixture was refluxed for 36hrs at 1700C.

Completion of the reaction was checked by TLC. The inorganic compounds were removed

by filtration. The solvent was removed under vacuum. Then ethylacetate (150 mL) and

water (100 mL) were added to the mixture. The organic phase was separated, washed with

water (100 mL×2) and brine solution (100 mL), dried over anhydrous sodium bisulphite,

filtered and the solvent was removed in vacuum. The product was purified with column

chromatography using ethyl acetate: hexane as eluent to obtain brown solid which was

recrystallized from acetone. Scheme 8 gives the synthetic route for the synthesis of Series

II compounds.

36

Scheme 8: Synthetic route for the synthesis of Tris (4-(3, 6-dialkoxy-9H-Carbazol-9 yl)

phenyl) amine (series II)

Table 3 : Yield and physical data of tris(4-(3,6-dialkoxy-9H-Carbazol-9

yl)phenyl)amine(Series II)

Compound Melting

point (C)

Yield

(%)

Reaction



SC 022 186 62 36 Dark brown solid


SC 024 198 60 36 Light brown Solid

37

1. SC 021- tris(4-(3,6-dimethoxy-9H-Carbazol-9

yl)phenyl)amine

2. SC 022- tris(4-(3,6-diethoxy-9H-carbazol-9-yl)phenyl)amine

3. SC 023- tris(4-(3,6-dipropoxy-9H-Carbazol-9

yl)phenyl)amine

4. SC 024- tris(4-(3,6-dibutoxy-9H-Carbazol-9 yl)phenyl)amine

Synthesis of hole transporting compounds 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-carbazol-

9-yl)phenyl)ethene(series III)

3,6-Dialkoxycarbazoles(C-F) (0.004 mol), tetra (4-bromo phenyl) ethylene 0.6476

g (0.001 mol), K2CO3 2.65 g (0.02 mol), Copper powder 1.8 g (0.03 mol) and 18-crown-6

200 mg (0.00075 mol) were heated together in 50 ml of orthodichlorobenzene under

nitrogen atmosphere and the resulting mixture was refluxed for 48 hours at 1700C.

Completion of the reaction was checked by TLC. The inorganic compounds were removed

by filtration. The solvent was removed under vacuum. Then ethylacetate (150 mL) and

water (100 mL) were added to the mixture. The organic phase was separated, washed with

water (100 mL×2) and brine solution (100 mL), dried over anhydrous sodium bisulphite,

filtered and the solvent was removed in vacuum. The product was purified with column



III compounds. Table 4 gives the yield and physical data of 1,1,2,2-tetrakis(4-(3,6-

dialkoxy-9H-carbazol-9yl)phenyl)ethene (Series III)

38

Scheme 9:Synthetic route for the synthesis of 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-

carbazol-9-yl)phenyl)ethene(series III)

Table 4:Yield and physical data of 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-carbazol-

9yl)phenyl)ethene (Series III)

Compound Melting

point (C)

Yield

(%)

Reaction






1. SC 031-1,1,2,2-tetrakis(4-(3,6-dimethoxy-9H-carbazol-

9yl)phenyl)ethene

2. SC 032- 1,1,2,2-tetrakis(4-(3,6-diethoxy-9H-carbazol-

9yl)phenyl)ethene

3. SC 033- 1,1,2,2-tetrakis(4-(3,6-dipropoxy-9H-carbazol-

9yl)phenyl)ethene

4. SC 034-1,1,2,2-tetrakis(4-(3,6-dibutoxy-9H-carbazol-

9yl)phenyl)ethene

39

Synthesis of hole transporting compounds 4,4’-bis(3,6-dialkoxy 9H-carbazol-9 yl)1,1’-

biphenyl (series IV)

3,6-Dialoxycarbazoles(C-F) (0.002 mol), 4,4’-diiodobiphenyl 0.406 g (0.001 mol),

K2CO3 2.65 g (0.02 mol), Copper powder 1.8 g (0.03 mol) and 18-crown-6 200 mg

(0.00075mol) were heated together in 50 mL of orthodichlorobenzene under nitrogen

atmosphere and the resulting mixture was refluxed for 24 hours at 1700C. Completion of

the reaction was checked by TLC. The inorganic compounds were removed by filtration.

The solvent was removed under vacuum. Then ethylacetate (150 mL) and water (100 mL)

were added to the mixture. The organic phase was separated, washed with water (100

mL×2) and brine solution (100 mL), dried over anhydrous sodium bisulphite, filtered and

the solvent was removed in vacuum. The product was purified with column



IV compounds. Table 5 gives the yield and physical data of 4,4'-bis(3,6-dialkoxy 9H-

carbazol-9 yl)1,1'-biphenyl (Series IV)

Scheme 10: Synthetic route for the synthesis of 4,4’-bis(3,6-dialkoxy 9H-carbazol-9

yl)1,1’-biphenyl (series IV)

40

Table 5: Yield and physical data of 4,4'-bis(3,6-dialkoxy 9H-carbazol-9 yl)1,1'-biphenyl

(Series IV)

Compound Melting

point (C)

Yield

(%)

Reaction






1. SC 041- 4,4'-bis(3,6-dimethoxy 9H-carbazol-9 yl)1,1'-

biphenyl

2. SC 042- 4,4'-bis(3,6-diethoxy 9H-carbazol-9 yl)1,1'-

biphenyl

3. SC 043- 4,4'-bis(3,6-dipropoxy 9H-carbazol-9 yl)1,1'-

biphenyl

4. SC 044-4,4'-bis(3,6-dibutoxy 9H-carbazol-9 yl)1,1'-

biphenyl

Characterization

Synthetic strategy

The compounds were synthesized by multistep organic reactions. Ullmann

coupling reactions are used for the synthesis of hole transporting materials[130].

Spectroscopic characterization

The compounds synthesized were characterized by using various instrumental

techniques such as UV-Visible, FT-IR, 1H-NMR, 13C-NMR spectroscopy. From the UV-

Visible spectrum of the compounds the wavelength of maximum absorption (λmax) and

intensity of the absorption are obtained. For taking UV-Visible spectra the compounds

were dissolved in suitable spectroscopic grade solvents. The organic compounds absorb

light energy in the ultraviolet and visible region and the electrons are excited to the higher

energy states from the ground state[131]. The FT-IR spectra of the compounds were

obtained by placing the sample in double beam infrared spectrometer and by measuring

the relative intensity of transmitted light. The samples were mixed with KBr and the

41

spectrums are obtained. The characteristic infrared absorption frequency of various

functional groups are also determined[132].The Nuclear magnetic resonance spectroscopic

technique is based on the magnetic properties of certain nuclei. From the NMR spectrum

the information regarding the number and type of chemical moieties present in a molecule

are obtained. Based on the local chemical environment, different protons in a molecule

resonate at different frequencies. The NMR spectra of the compounds were recorded in a

solution of deuterated chloroform. 1H-NMR spectroscopy determines the number and type

of protons present in a molecule which allows the identification of structure of the

compound.13C-NMR spectroscopy provides information about carbon atoms present in a

molecule and it is an important tool in the structure elucidation of compounds in organic

chemistry[133].

1,3,5-Tribromobenzene (A)

UV-Visible spectra (Ethanol,nm): 224, 210

FT- IR Spectra (KBr, cm-1): 3071, 1326, 723

1H-NMR (CDCl3, ppm): 7.89

13C-NMR (CDCl3, ppm):123.4, 133.2

In UV-Visible spectra the λmax of 1,3,5-tribromobenzene is observed at 224 nm.

FT-IR spectra gave characteristic peaks at 3071 cm-1 indicates the aromatic –CH

stretching, 1326 cm-1 indicates the aromatic C=C stretching and the peak at 723 cm-1 is

due to C-Br stretch.

In 1H-NMR spectrum, peaks are observed at 7.89 ppm which confirms the

presence of three aromatic protons.

In 13C-NMR spectrum aromatic carbons absorb in the range 123.4-133.2 ppm.

3,6-Dibromocarbazole (B)

UV-Vis (Ethanol,nm): 300, 267.

FT- IR (KBr, cm-1): 3406, 3068,

1471, 1284, 570

1H-NMR (CDCl3, ppm): 7.44-8.24

13C-NMR (CDCl3, ppm): 109.7,

42

122.6, 124.3, 126.2, 135.4

In UV-Visible spectra the λmax of 3,6-Dibromo carbazole is observed at 300 nm.

The parent compound carbazole shows λmax at 293 nm. The increase in λmax may be due

to substitution of Br with lone pair of electron.

FT-IR spectra gave characteristic peaks at 3406 cm-1 indicates -NH stretching

frequency, 3068 cm-1 indicates the aromatic C-H stretching, 1471 cm-1 indicates the

aromatic C=C stretching, 1284 cm-1 indicates the C-N stretching and 570 cm-1 indicates

the C-Br stretching.

In 1H-NMR spectrum, peaks are observed at 7.44-8.24 ppm which confirms the

presence of aromatic protons.

In 13C-NMR spectrum aromatic carbons absorb in the range 109.7-135.4ppm.

3,6-Dimethoxycarbazole (C)

UV-Vis (Ethanol, nm) : 302,

296, 266

FT-IR (KBr,cm-1): 3409,3070,

2879, 1326, 1053

1H-NMR (CDCl3, ppm): 3.8,

7.2-7.9, 11.63

13C-NMR (CDCl3, ppm):55.6,

108.2, 112.6, 131.7, 154.4

In UV-Visible spectra the λmax of 3,6-Dimethoxy carbazole is observed at 302

nm.

FT-IR spectra gave characteristic peaks at 3409 cm-1 indicates the -NH stretching

frequency, 3070 cm-1 indicates the aromatic C-H stretching, 2879 cm-1 indicates the

methyl C-H stretching, 1326 cm-1 indicates the aromatic C=C stretching and the peak at

1053 cm-1 is due to C-O stretch.

In 1H-NMR spectrum, the peaks at 3.8 ppm is due to the methoxy proton and the

peaks at 7.2-7.9 ppm confirm the presence of aromatic protons. The peak at 11.63 ppm

indicates the presence of the –NH proton.

43

In 13C-NMR spectrum the methoxy carbon absorb at 55.6 ppm and the aromatic

carbons absorb in the range 108.2-154.4 ppm.

3,6-Diethoxycarbazole (D)

UV-Vis (Ethanol, nm) : 304, 276

FT-IR (KBr, cm-1): 3418, 3065, 2882, 1337,

1058

1H-NMR (CDCl3, ppm): 1.34, 4.06,

7.20-7.72, 11.66

13C-NMR (CDCl3, ppm): 14.6, 64.3, 108.2,

111.8, 117.2, 131.6, 153.2

In UV-Visible spectra the λmax of 3,6-Diethoxy carbazole is observed at 304 nm.

There is an increase in λmax with increase in length of alkyl substituents.


frequency, the peak at 3065 cm-1 indicates the aromatic C-H stretching, the peak at 2882

cm-1 indicates the ethyl C-H stretching, the peak at 1337 cm-1 indicates the aromatic C=C

stretching and the peak at 1058 cm-1 is due to C-O stretch.

In 1H-NMR spectrum, the peaks at 1.33 ppm indicates the methyl protons, the

peaks at 4.06 ppm indicates the methylene proton and the peaks at 7.20-7.72 ppm confirms

the presence of aromatic protons. The peak at 11.66 ppm indicates the presence of the –

NH proton.

In 13C-NMR spectrum the methyl carbon absorb at 14.3 ppm, methylene carbon

absorbs at 55.6 ppm and the aromatic carbons absorb in the range 108.2-153.2 ppm.

3,6-Dipropoxycarbazole (E)

UV-Vis (Ethanol, nm):

305,300,282

FT-IR (KBr,cm-1): 3425,

3072,2877, 1360, 1126


1.74, 3.9, 7.20-7.72, 11.61

44


22.7, 69.5, 108.4, 111.6, 117.3,

131.5, 151.3, 153.7

In UV-Visible spectra the λmax of 3,6-Dipropoxycarbazole is observed at 305 nm.

The λmax of 3,6-dipropoxycarbazole is higher than that of the ethoxy derivative due to the

increase in length of alkyl substituent.


frequency, the peak at 3072 cm-1 indicates aromatic C-H stretching, the peak at 2877 cm-1

indicates the propyl C-H stretching, the peak at 1360 cm-1 indicates the aromatic C=C


In 1H-NMR spectrum, the peaks at 0.97 ppm indicates the methyl proton, the peaks

at 1.33 ppm indicates the methylene protons, the peaks at 3.9 ppm indicates the methylene

proton attached to oxygen and the peaks at 7.20-7.72 ppm confirms the presence of

aromatic protons. The peak at 11.61 ppm indicates the presence of –NH proton.

In 13C-NMR spectrum the methyl carbon absorb at 10.4 ppm, methylene carbons

absorb at 22.7 ppm and 69.5 ppm. The aromatic carbons absorb in the range 108.4-153.7

ppm.

3,6-Dibutoxycarbazole (F)


286

FT-IR (KBr,cm-1): 3435, 3068,

2874, 1343, 1209


1.47, 1.74, 4.1, 7.17-7.77, 11.62


19.1, 31.5, 68.7, 108.5, 111.6,

112.8, 131.7, 153.2

45

In UV-Visible spectra the λmax of 3,6-Dibutoxy carbazole is observed at 307 nm.

When compared to the λmax of 3,6-dipropoxycarbazole, this compound shows higher

bathochromic shift due to the increase in alkyl chain length.

FT-IR spectra gave characteristic peaks at 3435 cm-1 indicates the -NH stretching

frequency, the peak at 3068 cm-1 indicates the aromatic C-H stretching, the peak at 2874

cm-1 indicates the butyl C-H stretching, the peak at 1343 cm-1 indicates the aromatic C=C



at 1.47 ppm and 1.74 ppm indicates the methylene protons, the peaks at 4.1 ppm indicates

the methylene proton attached to oxygen and the peaks at 7.17-7.77 ppm confirms the

presence of aromatic protons. The peak at 11.61 ppm indicates the presence of –NH

proton.

In 13C-NMR spectrum the methyl carbons absorb at 14.3 ppm, methylene carbons

absorb at 19.1 ppm, 31.5 and 68.7 ppm. The aromatic carbons absorb in the range 108.5-

153.2 ppm.

Tris (4-bromophenyl)amine (G)


290

FT-IR(KBr,cm-1):

3068,2950,1403, 740


7.35

13C-NMR (CDCl3, ppm): 121.4,

132.7, 136.4, 144.6

In UV-Visible spectra the λmax of tris(4-bromophenyl)amine is observed at 315

nm.

46

FT-IR spectra gave characteristic peak at 3068 cm-1 and 2950 cm-1 indicates

aromatic -C-H stretching, the peak at 1403 cm-1 indicates the aromatic C=C stretching

and the peak at 740 cm-1 indicates C-Br stretching.




Tetra (4-bromophenyl) ethylene (H)

UV-Vis (Ethanol, nm) : 356,311

FT-IR (KBr, cm-1): 3072, 2976,

1389, 752

1H-NMR (CDCl3, ppm): 7.35-

7.62

13C-NMR (CDCl3, ppm): 122.4,

128.3, 131.7, 141.1

In UV-Visible spectra the λmax of tetra(4-bromophenyl)ethylene is observed at

356 nm.

FT-IR spectra gave characteristic peak at 3072 cm-1 and 2976 cm-1 indicates the

aromatic -C-H stretching, the peak at 1389 cm-1 indicates the aromatic C=C stretching and

the peak at 752 cm-1 indicates the C-Br stretching.



In 13C-NMR spectrum aromatic carbons absorb in the range 122.4-131.7 ppm and

the ethylene carbon absorbs at 141.1 ppm.

4,4’-Diiodobiphenyl (I)


263, 223

FT-IR (KBr,cm-1): 3067, 1380,

47

495


7.89

13C-NMR (CDCl3, ppm): 94.3,

130.1, 138.3, 139.6

In UV-Visible spectra, λmax of 4,4’-diiodobiphenyl is observed at 307 nm.

FT-IR spectra gave characteristic peaks at 3067 cm-1 indicates the aromatic –CH

stretching , the peak at 1326 cm-1 indicates the aromatic C=C stretching and the peak at

495 cm-1 is due to C-I stretch.




1,3,5-tris(3,6-dimethoxy-9H-carbazol-9-yl)benzene(SC 011)

UV-Vis (Chloroform,nm): 309, 300

FT-IR (KBr,cm-1): 3070,2893, 1431,

1376, 1263, 1218.

1H-NMR (CDCl3, ppm): 3.48, 6.60-

7.87

13CNMR(CDCl3,ppm):55.3,108.2,

112.1,118.4,121.3, 135.6,

143.3,154.1

In UV-Visible spectra the λmax of the compound is observed at 309 nm. This may

indicate that the coupling reaction make the compound to bathochromic shift, when

comparing to the individual compounds.

FT-IR spectra gave characteristic peaks at 3070 cm-1 indicates the aromatic C-H

stretching, the peak at 2893 cm-1 indicates the aliphatic C-H stretching, the peak at 1431

cm-1 indicates the aliphatic C-H bending, the peak at 1376 cm-1 indicates the aromatic

48

C=C stretching, the peak at 1263 cm-1 indicates the C-N stretching frequency and the peak

at 1218 cm-1 indicates the C-O-C stretching. The peaks due to N-H stretch and C-Br

stretch are absent, this confirms the formation of the compound.

In 1H-NMR spectrum, the peaks at 3.81 ppm indicates the methoxy proton and the

peaks at 6.60-7.87 ppm confirm the presence of aromatic protons.

In 13C-NMR spectrum the methyl carbons absorb at 55.3 ppm and the aromatic


1,3,5-tris(3,6-diethoxy-9H-Carbazol-9-yl)benzene (SC 012)

UV-Vis (Ethanol, nm) :311,303

FT-IR (KBr,cm-1): 3072, 2882,

1391, 1374, 1261, 1217


3.90, 6.60-7.96


64.1, 108.5, 111.4, 112.7, 118.4,

135.3, 153.7

In UV-Visible spectra the λmax of the compound is observed at 311 nm. It is

observed that there is an increase in λmax when comparing to the individual compounds.

FT-IR spectra gave characteristic peaks at 3072 cm-1 indicates aromatic C-H



C=C stretching, the peak at 1261 cm-1 indicates the C-N stretching and the peak at 1217

cm-1 indicates the C-O-C stretching.

In 1H-NMR spectrum, the peaks at 1.34 ppm indicates the methyl protons, the peak

at 3.90 ppm indicates the methylene proton and the peaks at 6.60-7.96 ppm confirm the




49

1,3,5-tris(3,6-dipropoxy-9H-Carbazol-9-yl)benzene (SC 013)

UV-Vis (Ethanol, nm) : 314

,309

FT-IR(KBr,cm-1):

3072,2879,1403,1371,1261,

1217,961


1.75, 4.00, 6.60-7.97

13CNMR (CDCl3, ppm): 10.4,

22.3, 69.3, 108.3, 110.2, 112.5,

118.7, 135.3, 143.6, 153.1







C=C stretching, the peak at 1261 cm-1 indicates the C-N stretching, the peak at 1217 cm-1

indicates the C-O-C stretching and the peak at 961 cm-1 indicates aromatic C=C bending.


at 1.75 ppm indicates the methylene proton and the peak at 3.90 ppm indicates the

methylene proton attached to the oxygen atom. The peaks at 6.60-7.97 ppm confirm the




ppm.

50

1,3,5-tris(3,6-dibutoxy-9H-Carbazol-9-yl)benzene (SC 014)


306

FT-IR (KBr, cm-1): 3068, 2976,

1431, 1379, 1261, 1217, 963


1.47, 1.74, 4.00, 6.60-8.02


19.3, 31.8, 68.4, 109.8, 110.2,

112.5, 132.3, 135.7, 141.4,

152.6

In UV-Visible spectra the λmax of the compound is observed at 318 nm. The

increase in λmax may be due to the increase in extent of conjugation.





indicates the C-O-C stretching and the peak at 963 cm-1 is due to aromatic C=C bending.


at 1.47 ppm and 1.74 ppm indicates the methylene protons, the peaks at 4.1 ppm indicates

the methylene proton attached to oxygen and the peaks at 6.60-8.02 ppm confirms the

presence of aromatic protons


absorb at 19.3 ppm, 31.8 ppm and 68.4 ppm. The aromatic carbons absorb in the range

109.8-152.6 ppm.

51

Tris(4-(3,6-dimethoxy-9H-Carbazol-9 yl)phenyl)amine (SC 021)

UV-Vis (Ethanol, nm : 321, 310, 301

FT-IR (KBr,cm-1): 3072, 2897, 1437,

1379, 1268, 1214

1H-NMR (CDCl3, ppm): 3.81, 6.40-

7.96


108.2,112.1, 116.4, 118.5, 124.7,

135.8, 154.4







C=C stretching, 1268 cm-1 indicates the C-N stretching frequency and the peak at 1214

cm-1 indicates the C-O-C stretching. The peaks due to N-H stretch and C-Br stretch are

absent, this confirms the formation of compound.

In 1H-NMR spectrum, the peaks at 3.81 ppm indicates the methoxy proton and the


In 13C-NMR spectrum the methyl carbons absorb at 55.8 ppm and the aromatic


Tris(4-(3,6-diethoxy-9H-carbazol-9-yl)phenyl)amine(SC 022)


309,300

FT-IR (KBr, cm-1): 3056, 2916,

2827, 1379, 1361, 1234, 1212


4.06, 6.30-7.91

52


64.6, 109.3, 111.2, 112.4, 135.6,

142.7, 153.1

In UV-Visible spectra the λmax of the compound is observed at 324 nm. There is

an increase in λmax when comparing to the individual compounds.

FT-IR spectra gave characteristic peaks at 3072 cm-1 and 2916 cm-1 indicates the

aromatic C-H stretching, the peak at 2827 cm-1 indicates the aliphatic C-H stretching, the

peak at 1379 cm-1 indicates the aromatic C=C stretching, the peak at 1361 cm-1 indicates

the aliphatic C-H bending, the peak at 1234 cm-1 indicates the C-N stretching and the

peak at 1212 cm-1 indicates the C-O-C stretching .






Tris(4-(3,6-dipropoxy-9H-Carbazol-9 yl)phenyl)amine (SC 023)


312, 301

FT-IR (KBr,cm-1): 3062, 2875,

1407, 1369, 1265, 1217, 961


1.76, 3.99, 6.47-7.93


22.6, 69.2, 108.7, 111.5, 112.1,

116.8, 118.5, 131.3, 135.4,

153.2




53







at 1.76 ppm indicates the methylene proton and the peak at 3.99 ppm indicates the





ppm.

Tris(4-(3,6-dibutoxy-9H-Carbazol-9 yl)phenyl)amine (SC 024)


314, 304

FT-IR (KBr, cm-1): 3072, 2974,

1433, 1379, 1263, 1217, 963


1.44, 1.76, 4.00, 6.60-7.99


19.1, 31.7, 68.6, 108.7, 112.4,

116.8, 117.5, 123.4, 131.7,

153.6


increase in λmax may be due to the increase in chain length of alkyl substituents.






54


at 1.44 ppm and 1.76 ppm indicates the methylene protons, the peaks at 4.00 ppm

indicates the methylene proton attached to oxygen and the peaks at 6.60-8.02 ppm

confirms the presence of aromatic protons



108.7-153.6 ppm.

1,1,2,2-tetrakis(4-(3,6-dimethoxy-9H-carbazol-9yl)phenyl)ethene (SC 031)

UV-Vis (Ethanol, nm): 394, 385, 362

FT-IR (KBr,cm-1): 3070, 2893, 1431,

1376, 1263, 1218

1H-NMR (CDCl3, ppm): 3.83, 6.41-

7.98


108.3, 112.4, 118.6, 129.3, 135.7,

136.2, 141.2, 154.6



comparing to the individual compounds. The increase in λmax may be due to the increase

in extend of conjugation.




C=C stretching, the peak at 1263 cm-1 indicates the C-N stretching frequency and the peak

at 1218 cm-1 indicates the C-O-C stretching. The peaks due to N-H stretch and C-Br

stretch are absent, this confirms the formation of compound.

In 1H-NMR spectrum, the peaks at 3.83 ppm indicates the methoxy proton and the peaks

at 6.41-7.98 ppm confirm the presence of aromatic protons.

55

In 13C-NMR spectrum the methyl carbons absorb at 55.8 ppm and the aromatic carbons

absorb in the range 108.3-154.6 ppm. The peak at 141.2 ppm is due to the ethylene carbon.

1,1,2,2-tetrakis(4-(3,6-diethoxy-9H-carbazol-9yl)phenyl)ethene

(SC 032)

UV-Vis (Ethanol, nm) :

397,388, 369

FT-IR (KBr,cm-1): 3056, 2916,

2819, 1369, 1234, 1212


4.02, 6.50-7.94


64.8, 108.1, 111.3, 112.6, 118.6,

127.2, 129.7, 135.4, 136.3,

141.2, 156.3




FT-IR spectra gave characteristic peaks at 3056 cm-1 and 2916 cm-1 indicates



the C-N stretching and the peak at 1212 cm-1 indicates the C-O-C stretching .





absorbs at 64.8 ppm and the aromatic carbons absorb in the range 108.1-156.3 ppm. The

peak at 141.2 ppm is due to the ethylene carbon.

56

1,1,2,2-tetrakis(4-(3,6-dipropoxy-9H-carbazol-9yl)phenyl)ethene

(SC 033)

UV-Vis(Ethanol,nm)

:401,391,372

FT-IR (KBr,cm-1): 3067, 2878,

1404, 1371, 1267, 1219, 969


1.75, 3.99, 6.60-7.99


22.6, 69.3, 108.3, 111.2, 112.8,

118.6, 127.7, 135.3, 136.4,

141.6, 153.6

In UV-Visible spectra the λmax of the compound is observed at 401 nm. This is

observed that there is an increase in λmax, when comparing to the individual compounds.


stretching, the peak at 2878 cm-1 indicated the aliphatic C-H stretching, the peak at 1404





at 1.75 ppm indicates the methylene proton and the peaks at 3.99 ppm indicates the





ppm. The ethylene carbon absorbs at 141.6 ppm.

57

4.3.21. 1,1,2,2-tetrakis(4-(3,6-dibutoxy-9H-carbazol-9yl)phenyl)ethene

(SC 034)


396, 368

FT-IR (KBr, cm-1): 3068, 2976,

1431, 1379, 1261, 1217, 964


1.47, 1.76, 4.02, 6.60-7.96


19.0, 31.8, 68.4, 111.2, 112.3,

127.5, 129.7, 135.2, 136.7,

141.3, 154.8


increase in λmax may due to the increase in the length of alkyl substituent.


stretching, the peaks at 2976 cm-1 indicates the aliphatic C-H stretching, the peak at 1431







confirms the presence of aromatic protons



111.2-154.8 ppm. The ethylene carbon absorbs at 141.3 ppm.

58

4,4'-bis(3,6-dimethoxy 9H-carbazol-9 yl)1,1'-biphenyl (SC 041)


316, 301

FT-IR (KBr,cm-1): 3086, 2993,

2855, 1582, 1423, 1279, 1211


6.61-7.95


108.6, 112.4, 114.7, 118.8,

128.6, 135.2, 154.8.







the aliphatic C-H bending, the peak at 1279 cm-1 indicates the C-N stretching frequency

and the peak at 1211 cm-1 indicates the C-O-C stretching . The peaks due to N-H stretch

and C-I stretch are absent, this confirms the formation of the compound.

In 1H-NMR spectrum, the peak at 3.84 ppm indicates the methoxy proton and the


In 13C-NMR spectrum the methyl carbons absorb at 55.8 ppm and the aromatic carbons

absorb in the range 108.6-154.8 ppm.

4,4'-bis(3,6-diethoxy 9H-carbazol-9 yl)1,1'-biphenyl (SC 042)

UV-Vis (Ethanol, nm) : 331, 321,

303

FT-IR (KBr,cm-1): 3072, 2987, 2885,

1596, 1389, 1272, 1217

1H-NMR (CDCl3, ppm): 1.34, 3.76,

59

6.61-7.91

13C-NMR (CDCl3, ppm): 14.8, 64.3,

108.5, 112.6, 114.8, 118.3, 135.6,

137.7, 153.8


increase in λmax may be due to the increase in the length of alkyl substituent.




the aliphatic C-H bending, the peak at 1272 cm-1 indicates the C-N stretching and the peak

at 1217 cm-1 indicates the C-O-C stretching .






4,4'-bis(3,6-dipropoxy 9H-carbazol-9 yl)1,1'-biphenyl (SC 043)


316

FT-IR (KBr,cm-1): 3075, 2989,

2865, 1592, 1419, 1274, 1215


1.74, 3.96, 6.63-7.99


22.6, 69.4, 108.3, 111.5, 112.6,

114.2, 118.7, 128.1, 137.7,

153.5

60





aromatic C-H stretching, the peaks at 2865 cm-1 indicates the aliphatic C-H stretching, the

peaks at 1592 cm-1 indicates the aromatic C=C stretching, the peaks at 1419 cm-1 indicates

the aliphatic C-H bending, the peaks at 1274 cm-1 indicates the C-N stretching and the

peak at 1215 cm-1 indicates the C-O-C stretching.

In 1H-NMR spectrum, the peaks at 0.99 ppm indicates the methyl protons, the

peaks at 1.74 ppm indicates the methylene proton and the peaks at 3.96 ppm indicates the

methylene proton attached to the oxygen atom. The peaks at 6.63-7.99 ppm confirms the




ppm.

4,4'-bis(3,6-dibutoxy 9H-carbazol-9 yl)1,1'-biphenyl (SC 044)


319

FT-IR (KBr,cm-1): 3079, 2992,

2865, 1592, 1355, 1272, 1217


1.49, 1.76, 3.98, 6.64-7.89


19.2, 31.7, 68.5, 108.8, 111.9,

112.6, 114.4, 118.3, 135.7,

153.5

In UV-Visible spectra the λmax of the compound is observed at 336 nm. It is

observed that there is an increase in λmax with increase in the length of alkyl substituent.

61



peak at 1592 cm-1 indicates the aromatic C=C stretching, the peak at 1355 cm-1 indicated

the aliphatic C-H bending, the peak at 1272 cm-1 indicates the C-N stretching and the peak

at 1217 cm-1 indicates the C-O-C stretching.




confirms the presence of aromatic protons.



108.8-153.5 ppm.

4. Results and Discussion

Thermal and Electrochemical Studies

The hole transporting materials used in photovoltaic cells should possess

noticeable thermal and electrochemical stability. The thermal and electrochemical

properties of the synthesized hole transporting materials are discussed in this chapter. The

glass transition temperatures (Tg), phase transition temperatures and melting points of the

HTMs were measured using Differential Scanning Calorimetry (DSC). The

electrochemical stability of the synthesized compounds were studied using Cyclic

Voltammetry (CV).From CV the Oxidation potential and reduction potential of the

compounds were obtained. The optical band gap were calculated from the UV-Visible

spectroscopy. The HOMO-LUMO values, band gap, electron affinity and ionization

potential of HTMs were determined from these values. The optimized geometry and

HOMO-LUMO of the synthesized compounds were estimated from Density Functional

Theory (DFT). From the DFT results the HOMO-LUMO values and the band gap were

obtained.

Thermal properties

Differential Scanning Calorimetry measures the heat absorbed or released by a

material as a function of temperature or time. By using DSC the glass forming properties

and phase transition of compounds can be determined. This allows the determination of

thermal characteristics such as glass transition temperature (Tg), crystallization

62

temperature (Tc) and melting temperature(Tm). In DSC measurements the compounds

were analyzed for cyclic heating and cooling thermograms in an inert atmosphere from -

50oC to 300oC at a heating rate of 10 oC/minute. The synthesized hole transporting

materials differ only in their substituents at the outer carbazole rings. Due to these changes

in the alkyl chain length of the substituents, we expect only slight variation in thermal

properties. This results in changes in the solubility and density of compounds. The DSC

curves of representative compounds in each series is given in figure. The values of glass

transition temperature (Tg) and the melting temperature (Tm) are listed in the Table 6.

DSC curve of the compound SC 011 DSC curve of the compound SC 012


63



Table 6: Tg and Tm values of compounds

Compounds Tg( oC) Tm(

oC)

SC 011 92 149

SC 012 89 173

SC 021 95 174

SC 022 127 186

SC 031 53 249

SC 032 71 282

SC 041 75 114

SC 042 85 135

From the thermal data it is observed that the synthesized compounds are thermally

stable and satisfy all the required properties. The moderate Tg values makes the

compounds to exhibit glassy nature at low temperature. This makes the compounds to

have high penetrating power, which is favourable for the HTM to intermix with the

semiconductor TiO2 in all its layers[134]. The compounds exist in solid phase even after a

long time exposure of the cell due to its moderate melting point. The compounds are

thermally stable in device structure once fabricated up to high temperature normally above

250oC. The DSC results reveal that the compounds are amorphous material with good film

64

forming properties. From the results it is clear that the synthesized compounds are good

candidates as hole transporting material in photovoltaic devices.

Electrochemical properties

Cyclic voltammetry is one of the most accurate method used to estimate the

electrochemical parameters of the compounds. The oxidation potential and the reduction

potential of the compounds can be investigated by cyclic voltammetry and from this the

HOMO and LUMO energy levels of the compounds can be calculated. The measurements

were carried out at a glassy carbon electrode in acetonitrile (ACN) containing 0.1M of

supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6) using a three

electrode cell and potentiostat assembly at room temperature. The potentials were

measured by using platinum as reference electrode. Each analysis was carried out using an

internal standard ferrocene/ferrocenium redox system [135].

For the design and fabrication of organic photovoltaic cell, the HOMO and LUMO

of the organic compounds are considered as basic parameters. The essential criteria for a

compound to be used in optoelectronic device is that it’s HOMO and LUMO values

should be comparable with that of the semiconductor[136], [137]. The HOMO and LUMO

values of the compounds were calculated from the anodic oxidation potential by using

HOMO of ferrocene. The ionization energy of ferrocene is taken as 4.8 eV. Then the

energy of the molecular orbital is the negative value. Which can be calculated by using the

relation, EHOMO= - [Eox-E1/2(ferrocene) +4.8]. E1/2(ferrocene) is equal to 0.304 eV which can used

in the equation for the calculation of EHOMO. Cyclic voltammetry can be used as one of the

most efficient method to determine the HOMO-LUMO of donor and acceptor[138], [139].

The cyclic voltammetry curve of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-

yl)benzene (series I) compounds are shown in figure and the values of electrochemical

parameters were described in table 7.

65

Cyclic voltammetric curve of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-yl)benzene (series I)

compounds

Table 7: Oxidation potential and HOMO values of 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-

9-yl)benzene (series I) compounds

Compounds Eox (volt)

(from CV)

EHOMO= -[Eox-

E1/2(ferrocene)+4.8]

(eV)

SC 011 0.75 -5.25

SC 012 0.87 -5.37

SC 013 0.77 -5.27

SC 014 0.78 -5.28

The value of oxidation potential (Eox) of the series I compounds ranging from

0.75-0.87 volt. The HOMO values of the compounds ranging from -5.37 to -5.25 eV. The

lowest value for HOMO is observed for the compound SC 012.

The cyclic voltammetry curve of tris (4-(3, 6-dialkoxy-9H-Carbazol-9 yl) phenyl) amine

(series II) compounds are shown in figure and the values of electrochemical parameters

were described in table 8.

66

Cyclic voltammetric curve of tris (4-(3, 6-dialkoxy-9H-Carbazol-9 yl) phenyl) amine

(series II) compounds

Table 8: Oxidation potential and HOMO values of tris (4-(3, 6-dialkoxy-9H-Carbazol-9

yl) phenyl) amine (series II) compounds

Compounds Eox (volt) (from

CV)

EHOMO= -[Eox-E1/2(ferrocene)+4.8]

(eV)

SC 021 0.88 -5.38

SC 022 0.90 -5.40

SC 023 0.78 -5.28

SC 024 0.47 -4.97

The value of oxidation potential (Eox) of the series II compounds ranging from

0.47-0.90 volt. The lowest value of oxidation potential is observed for the compound SC

024.The HOMO values of the compounds ranging from -5.40 to -4.97 eV. The lowest

value for HOMO is observed for the compound SC 022.

67

The cyclic voltammetry curve of 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-carbazol-9-

yl)phenyl)ethene(series III) compounds are shown in figure and the values of

electrochemical parameters were described in table 9.

Cyclic voltammetric curve of 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-carbazol-9-

yl)phenyl)ethene(series III) compounds

Table 9: Oxidation potential and HOMO values of 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-

carbazol-9-yl)phenyl)ethene(series III) compounds

Compounds Eox (volt) (from

CV)

EHOMO= -[Eox-E1/2(ferrocene)+4.8]

(eV)

SC 031 0.94 -5.44

SC 032 0.45 -4.95

SC 033 0.50 -4.99

SC 034 0.91 -5.41

68

The value of oxidation potential (Eox) of the series III compounds ranging from




The cyclic voltammetry curve of 4,4’-bis(3,6-dialkoxy 9H-carbazol-9 yl)1,1’-

biphenyl (series IV) compounds are shown in figure and the values of electrochemical

parameters were described in table 10.

Cyclic voltammetric curve of 4,4’-bis(3,6-dialkoxy 9H-carbazol-9 yl)1,1’-biphenyl (series

IV) compounds

Table 10: Oxidation potential and HOMO values of 4,4’-bis(3,6-dialkoxy 9H-carbazol-9

yl)1,1’-biphenyl (series IV) compounds

Compounds Eox (volt)

(from CV)

EHOMO= -[Eox-

E1/2(ferrocene)+4.8]

(eV)

SC 041 0.60 -5.10

SC 042 0.52 -5.02

SC 043 0.69 -5.19

SC 044 0.62 -5.12

69

The value of oxidation potential (Eox) of the series IV compounds ranging from




The electrochemical studies of the synthesized four series of compounds were

carried out by using cyclic voltammetry. The electrochemical parameters of the

synthesized compounds were described in tables 5.2 to 5.5. The CV curves of the

compounds were depicted in figures 5.9 to 5.12.

For organic compounds, the energy levels of the electronic state correspond to the

energy carried by UV or visible radiation. At the resonance condition the molecules can

absorb a definite quantity of energy from the electromagnetic radiation and this results in

the promotion of electron from the low energy molecular orbital to the higher energy

molecular orbital. These transitions can be determined by using UV-Visible spectroscopy.

The longest wavelength of absorption, λonset is used to calculate the optical band gap, Eg

by using the relation,

Eg=1242/ λonset

The values of optical band gap for all the four series of synthesized compounds

were calculated and given in table 11.

Table 11: Optical band gap of series I compounds

Compound λonset (nm) Optical bandgap(Eg)

SC 011 309 4.02

SC 012 311 4.00

SC 013 314 3.95

SC 014 318 3.91

The optical band gap of series I compounds are calculated from absorption studies.

The values of band gap ranges from 3.91 to 4.02 eV. The highest value for band gap is

observed for the compound SC 011.

70

Table 12 : Optical band gap of series II compounds


SC 021 321 3.87

SC 022 324 3.83

SC 023 329 3.78

SC 024 332 3.74

The optical band gap of series II compounds are calculated from absorption

studies. The values of band gap ranges from 3.74 to 3.87 eV. The compound SC 021 in

series II shows highest value of 3.87 for band gap.

Table13 : Optical band gap of series III compounds


SC 031 394 3.15

SC 032 397 3.13

SC 033 401 3.01

SC 034 403 3.08

The optical band gap of series III compounds are calculated from absorption

studies. The values of band gap ranges from 3.01 to 3.15 eV. In series III compounds SC

031 shows highest value for band gap.

Table 14: Optical band gap of series IV compounds


SC 041 328 3.79

SC 042 331 3.75

SC 043 334 3.72

SC 044 336 3.70

The optical band gap of series IV compounds are calculated from absorption

studies. The values of band gap ranges from 3.70 to 3.79 eV. For series IV compounds SC

041 shows highest value of 3.79 for band gap.

By combining the cyclic voltammetry and absorption studies HOMO-LUMO

values and band gap were calculated. From the CV measurements oxidation potential

(Eox) is calculated and then HOMO value is obtained by using the relation, EHOMO= -

71

[Eox-E1/2(ferrocene) +4.8]. The measurements were carried out with reference to ferrocene.

The optical band gap (Eg) was calculated from absorption studies. The difference between

EHOMO and Eg will give the ELUMO. The values of EHOMO, ELUMO and Eg of all the

synthesized compounds were calculated.

The HOMO-LUMO values and the band gap of all the four series of compounds

were given in the table 15.

Table 15: HOMO-LUMO values and band gap of series I compounds

Compounds Eox

(volt)

(from

CV)

EHOMO

(eV)

λonset

(nm)

Eg(Optical

bandgap)

(eV)

ELUMO

(eV)

SC 011 0.75 -5.25 309 4.02 -1.23

SC 012 0.87 -5.37 311 4.00 -1.37

SC 013 0.77 -5.27 314 3.95 -1.32

SC 014 0.78 -5.28 318 3.91 -1.37

The difference between EHOMO and optical band gap (Eg) gives the value of ELUMO.

The values of ELUMO of series I compounds were calculated and the values ranging from -

1.37 to -1.23.

Table 16 : HOMO-LUMO values and band gap of series II compounds

Compounds Eox

(volt)

(from

CV)

EHOMO

(eV)

λonset

(nm)

Eg(Optical

bandgap)

(eV)

ELUMO

(eV)

SC 021 0.88 -5.38 321 3.87 -1.51

SC 022 0.90 -5.40 324 3.83 -1.57

SC 023 0.78 -5.28 329 3.78 -1.50

SC 024 0.47 -4.97 332 3.74 -1.23

72

ELUMO of series II compounds were calculated from the difference between EHOMO

and Eg. The values for ELUMO of the compounds ranging from -1.57 to -1.23.

Table 17 : HOMO-LUMO values and band gap of series III compounds

Compounds Eox

(volt)

(from

CV)

HOMO

(eV)

λonset

(nm)

Eg(Optical

bandgap)

(eV)

LUMO

(eV)

SC 031 0.94 -5.44 394 3.15 -2.29

SC 032 0.45 -4.95 397 3.13 -1.82

SC 033 0.50 -4.99 401 3.10 -1.90

SC 034 0.91 -5.41 403 3.08 -2.33

The value of ELUMO of series III compounds were calculated from the difference

between EHOMO and Eg. ELUMO values of the compounds varies from -2.23 to -1.83.

Table 18: HOMO-LUMO values and band gap of series IV compounds

Compounds Eox

(volt)

(from

CV)

HOMO

(eV)

λonset

(nm)

Eg(Optical

bandgap)

(eV)

LUMO

(eV)

SC 041 0.60 -5.10 328 3.79 -1.31

SC 042 0.52 -5.02 331 3.75 -1.27

SC 043 0.69 -5.19 334 3.72 -1.47

SC 044 0.62 -5.12 336 3.70 -1.42

The difference between EHOMO and optical band gap (Eg) gives the value of ELUMO.

The values of ELUMO of series IV compounds were calculated and the values ranging from

-1.47 to -1.27.

The electrochemical properties of all the synthesized compounds were studied. The

variation of substituents at the carbazole ring would influence the electrochemical

properties of the compounds which results in slight variation in the value of

73

electrochemical parameters like EHOMO, ELUMO and band gap. The electrochemical

properties are in good agreement with that of the reported hole transporting materials[140]

. From the results it is observed that all the four series of compounds have excellent hole

transporting properties.

Fabrication of Dye Sensitized Solar Cell

Environment friendly, low manufacturing cost, undisturbed power supply and low

maintenance cost are the most important factors for the popularization of solar cells. The

structure of DSSC, materials used for fabrication, various steps involved in the fabrication,

photovoltaic device characterization and performance studies of DSSC are also explained

in this chapter.

Structure of dye sensitized solar cells

The way of preparing the DSSC in laboratory is by using a transparent conducting

oxide (TCO) coated glass electrode coated with porous Nano crystalline titania (nc-TiO2),

dye molecules attached to the surface of nc-TiO2, hole transporting material and a counter

electrode[141], [142]. We are mainly concentrated on the fabrication of solid state dye

sensitized solar cells. The simplest representation of the dye sensitized solar cell which we

fabricated is shown in figure.

The DSSC comprises of four components: photoelectrode with thin layer of

nanostructured semiconductor (usually Titania) attached to the conducting substrate

(fluorine doped tin oxide-FTO), a monolayer of dye adsorbed on the surface of

semiconductor, a thin layer of hole transporting material (HTM) penetrated into the

semiconductor and a counter electrode (silver)[143], [144].

Fabrication of solid state dye sensitized solar cell

Each step in the device fabrication is briefly given below.

i. Non-structured fluorine doped tin oxide (F-SnO2) glass

ii. Structured Fluorine doped tin oxide glass using Zn powder and HCl

iii. Deposition of compact TiO2, blocking layer on F-SnO2 by spin coating method.

iv. Deposition of nanoporous TiO2 film by doctor blading method.

v. Dye sensitization of nanoporous TiO2 film by dipping in dye solution.

vi. Deposition of organic HTM by drop coating method

vii. Deposition of silver, the counter electrode on HTM by thermal evaporation

method

74

Materials for the fabrication of Solid State DSSC

i) Anode Material: Fluorine doped transparent conducting oxide films deposited on

glass substrate with resistance 10 µΩ, thickness 1 mm and size 25x25 mm were

purchased from Sigma Aldrich.

ii) Blocking Layer: Blocking layer of compact titania was prepared by dissolving 1

mL titanium isopropoxide (purchased from Sigma Aldrich) in 20 mL n-Butanol

and the solution was sonicated for 10 minutes. To this solution 1 mL conc.HCl was

added followed by 5 minutes sonication. Then 2 mL distilled water was added drop

wise and again sonicated for 15 minutes which results in the formation of a sol.

iii) Semiconductor: Transparent nanocrystalline titania (purchased from Sigma

Aldrich) was used for the preparation of nanoporous layer. The titania paste is

prepared by grinding the nanopowder with ethanol and Triton X 100.

iv) Sensitizer Dye: The natural dye obtained from Red sandal wood was used as the

sensitizer. Red sandal wood was purchased commercially and powdered. The

powder mixed with ethanol and then steam distilled. The ethanol extract contain

Santalin A as the main component. The resulting product was purified by column

chromatography and characterized by spectroscopic methods.

v) Hole transporting material: carbazole based hole transporting materials are used for

the fabrication of Solid state dye sensitized solar cells. The compounds were

synthesized according to the procedure described in chapter III and

characterization of the synthesized HTMs were described in chapter IV. The

thermal and electrochemical stability of the HTMs were discussed in chapter V.

vi) Counter electrode: Silver is used as the counter electrode. Thermal evaporation

method was used for coating silver.

Red sandal dye

The sensitizer dye used for the fabrication is Red sandalwood, commonly known

as ‘rakta chandan’ (botanical name:‘Pterocarpus santalinius’, C14H14O7)[144] . Red

sandalwood contain various maroonish red colouring components, viz. santalin A, santalin

B and deoxysantalin. Among these the main component is considered as santalin A. The

Red sandal wood was purchased commercially and made fine powder. It is steam distilled

75

using ethanol. Ethanol extract contains Santalin A as the main component. The product is

purified by chromatographic techniques and characterized using spectroscopic techniques.

Santalin A is chemically,2,10-dihydroxy-6-(4-hydroxy-3-methoxy-benzyl)-5-(4-hydroxy-

2-methoxy-phenyl)-1,3-dimethoxy-benzo[a]xanthen-9-one.

Structure of Santalin A

Spectroscopic characterization of Santalin A

UV-Visible Spectra (ethanol, nm): 506.5, 474.5, 310.5

FT-IR Spectra (KBr, cm-1): 3400 cm-1 ( –OH stretch),

3070 cm1 (Aromatic –C-H stretch), 1739 cm-1 (-C=O stretch), 1050 cm-1 (-C-O stretch).

1H-NMR Spectra (CDCl3, ppm):3.73(12H,s,methoxy), 3.92(2H,s,methylene), 4.78(4H,s,

Hydroxyl), 6.11-7.20(10H, m,Aromatic).

1 3C-NMR (CDCl3, ppm): 34.2 (aliphatic carbon); 56.3(methoxy carbon); 101.8, 105.2,

107.1, 108.4,114.8, 115.4, 116.1, 117.2, 119.4, 121.8, 124.7, 125.0, 129.8, 132.5, 134.2,

136.6, 139.5, 140.4, 146.7, 149.7, 151, 157.2, 161.3, 162.3, 177.4, 181.2 (aromatic

carbons).

Different layer thickness of DSSC

76

Methods for the fabrication of Solid State DSSC

Spin coating

Spin coating is the most preferred method for the application of thin, uniform films

to flat substrates. The substrate is placed on the middle of the spinning disk with vacuum

pump in order to keep the sample in place. An exess amount of sample is placed on the

substrate. The substrate is then rotated at very high speed in order to spread the solution by

centrifugal force. The rotation is continued for some time, with solution being spun off the

edges of the substrate, until the chosen film thickness is achieved. The thickness of the

film is mainly controlled by the spinning speed and the viscosity of the solution. The

solvent is usually volatile, evaporated to get uniform film of compact titania. The main

advantage of spin coating is the repeatability.

The various stages in the spin coating process are,

i) Acceleration of the substrate up to its final, desired, rotation speed.

ii) Deposition of the coating solution onto the substrate by using a nozzle and pouring

the solution or by spraying it on to the substrate surface. Slight excess of the

coating solution is usually applied than the amount required.

iii) Spinning of the substrate at a uniform rate. This is affected by the fluid viscous

forces and the nature of solvent evaporation.

For our studies the required thickness of blocking layer is 20-30 nm. To obtain this

thickness we use a speed of 2000 rpm for 30 sec.

Doctor blading

Doctor blading technique is one of the most important technique used to deposit

the nanocrystalline TiO2 layer on the top of the blocking layer without disturbing the

latter. Doctor blading pastes were prepared by diluting the screen printable paste with

ethanol making them more fluid. The substrate were fixed on the glass support. Then it is

masked using a scotch tape, except the place were doctor blading has to be done and it

also determines the thickness of the nanoporous titania layer. The titania paste was

deposited onto the substrate and spread out using a doctor blade. The process was

continued several times to obtain uniform thickness and to avoid trapping of air bubbles.

After the deposition the layers were allowed to set in a dust free container for 30 minutes.

77

Dip coating

Dip coating or immersion coating technique is one of the simplest techniques used

for uniform coating. In this process the substrate to be coated immersed in a liquid and

then withdrawn with a well-defined withdrawal speed under controlled temperature and

atmospheric conditions and allowing it to drain. The thickness of coating was mainly

determined by the withdrawal speed, the viscosity of the liquid and by the solid content.

This technique is usually used in porous substrates. We use the dip coating technique for

the deposition of sensitizer dye on the semiconductor. The dye has to be penetrated onto

the semiconductor which has been coated on the substrate by doctor blading.

Drop coating

Drop coating is the suitable method for application of thin, uniform films to flat

substrates. The substance was first dissolved in high polar volatile solvent and a certain

amount of solution is placed on the substrate using micro pipette and allowed it to spread.

The solvent evaporates from the solution forming a thin layer. In our studies we have used

drop coating for the deposition of HTM in extra pure THF as solvent on semiconductor

and dye coated substrate.

Vacuum Thermal Evaporation

Vacuum thermal evaporation technique is used for the deposition of high melting

metals on various layer coated substrate. This technique consists of heating the material to

be deposited until its evaporation. The material vapour finally condenses in form of thin

film on the cold substrate surface and on the vacuum chamber walls. To avoid reaction

between the vapour and the atmosphere, low pressures are used (about 10-6 or 10-5Torr). At

these low pressures, the mean free path of vapour atoms is the same order of the vacuum

chamber dimensions, so these particles travel in straight lines from the evaporation source

towards the substrate.

Instruments for DSSC fabrication

Sonication was done by using an Ultrasonics 1.5L 50 of Lobalife Co. Sintering of the

substrate was carried out on a Spinot hot plate. The blocking layer of compact TiO2 on

TCO plate was coated using spin coating machine. The spin coating was carried out with

programmable spin coater SCU 2008C, Apex instruments co. India. The substrate was

spun upto a speed of 2000 rpm for 30 sec with an acceleration of 200 rpm/sec. A silver

film of 30 nm thickness was used as the contact material. The counter electrode was

78

thermally evaporated using an Indo version Box coater model BK 350 evaporator at a

pressure of 2x10-5 mbar. The rate of deposition was around 0.1 nm/sec for the first 3.0 nm

and 0.3 nm/sec until 2.0 nm before the desired thickness was obtained and then 0.1 nm/sec

until the end.

The current-voltage (I-V) characteristics was monitored and measured by using a

Keithley 276 source measurement unit. I-V measurements were generated using self-

written Lab VIEW program. The light source used was a solar simulator equipped with

Xenon lamp, AM 1.5, Solar light Company, Model 16S-300,USA. The light intensity was

measured using a Pyranometer, Solar light Company, PMA2144, USA. The infrared and

ultraviolet portion of the spectrum was eliminated by using a filter. The incident light

intensity was tuned by using neutral density filters, Eastman Kodak Company, USA.

Device assembly

Transparent conducting oxide (TCO) substrates

The choice of TCO substrate is the fundamental for the fabrication of DSSC. TCO

is a wide band gap n-type semiconductor consisting of high concentration of free

electrons. The most common types are Indium doped Tin Oxide (ITO) and Flurine doped

Tin Oxide (FTO). FTO is preferred due to the good electrical conductivity, better thermal

stability, high transparency and low material costs. The electrical properties of ITO are

degraded in presence of oxygen at high temperature [145]. FTO was much more stable at

high temperatures[146] .

The TCO, which works as current collector should permit the challenging

processing conditions for the device preparation without altering its physical properties.

The TCO usually remains stable up to temperatures somewhat above the optimized

deposition temperature. Some types of TCO show a rise in resistivity after heating at high

temperature and for a long time.

Structuring of Fluorine doped Tin Oxide glass

The structuring of the FTO glass was done by using chemical etching method. The

procedure for the structuring is given below,

i) The FTO coated substrate was cleaned by sonicating with distilled water for 30

minutes to remove the surface oxygen and other greasy substances and dried in air.

79

ii) One by third area (from one side) of the FTO substrate for the back contact was

masked by using scotch tape. The FTO substrate has to be etched from the

unmasked area for the counter electrode connection. If the etching is not done

properly there will be possibility of short circuit and the cell may be damaged.

iii) Spread dry Zinc granulates (20 mg/cm2) over the dry FTO glass.

iv) Add conc. HCl drop-wise on the Zinc powder at constant interval to avoid

vigorous reaction. Excess zinc and acid were removed by washing with distilled

water. Complete removal of FTO was checked by using multimeter. Repeat the

process until complete removal of FTO.

v) Removed the scotch tape and the substrate was washed with acetone to remove the

adhesive.

vi) The structured glass was then cleaned by ultra-sonication in various solvents like

acetone, ethanol and distilled water for 10 minutes in each solvent. It is then dried

and the cleaned substrates were stored in ethanol.

Preparation of blocking layer

The blocking layer of compact titania was deposited on the structured FTO substrate

by spin coating method. Two by third portion of the substrate was masked with scotch

tape, avoiding the middle part. The FTO plate was placed in a chamber and 150 µL of

TiO2 solution is applied on the substrate. The solution is allowed to set on top of the

substrate. The substrate is spun upto a speed of 2000 rpm for 30 sec with an acceleration

of 200 rpm/sec. Remove the scotch tape and the samples were dried for 30 minutes in air

followed by annealing at 100°C for 1 hour on a hot plate to achieve complete pyrolysis of

organic matters. The substrate is allowed to cool slowly in a dessicator.

Preparation of semiconductor layer

Doctor blading technique is used to deposit nanoporous titania layer on compact

titania layered FTO glass plate. Two by third portion of the substrate was masked with

scotch tape. TiO2 paste is prepared by grinding 1 g of nanoporous TiO2 with 2 mL ethanol

and 1.5 mL of nitric acid solution of pH 3 in a mortar and pestle.1 mL of distilled water

was also added to get a white paste. Then add a drop of the surfactant, Triton X-100 to

ensure uniformity in coating. A small quantity of the paste was deposited on the substrate

and it is spread over the substrate by means of a doctor blade. After the paste was spread,

the film of semiconductor is left to dry for 5 minutes. The scotch tape was removed and

the substrate was allowed to dry for 30 minutes at room temperature under normal

80

atmospheric condition in a dust free environment. The substrate was then annealed for one

hour at 450 ºC on a hot plate to remove organic additives of the paste. Then it was allowed

to cool down to room temperature. The obtained nanoporous titania films are highly

transparent and the substrate was stored in a dessicator over silica gel.

Dye sensitization

In the fabrication of DSSC, dye sensitization was carried out by soaking the cell in

ethanolic solution of dye, concentration 10-4 molL-1 for overnight in a specially designed

vessel. The dye adsorption was found to follow Langmuir adsorption kinetics. The typical

time for dye uptake was 12 hours. After dye sensitization the substrate was washed with

ethanol to remove excess of dye. The cell was dried and then used directly for the

assembly of photovoltaic cells. The resulting sensitized film is known as photoanode or

working electrode.

Deposition of the hole transporting material

After dye sensitization of the substrate, the hole transporting material was drop

coated using a micropipette for solid state devices. The hole transporting material is

dissolved in tetrahydrofuran (5 mg in 5 mL). 100µL of HTM solution was applied to the

substrate by drop coating method and it is dried in a vacuum desiccator. THF was used for

the better penetration of the HTM on the dye adsorbed semiconductor and for the easy

evaporation of the solvent.

Deposition of the counter electrode

Silver is used as the counter electrode DSSC. The metal counter electrode

deposition was done by thermal evaporation method in UHV (5x10-6 mbar) using a

Pfeiffer evaporator and the rate of evaporation is 0.28 nm/sec. The thickness of metal

electrode deposition was monitored by a quartz crystal microbalance (QCM) The

thickness of silver counter electrode used in solid state DSSC is of 30 nm. The other part

of the cell is masked with a metal mask. The metal in the boat was heated in high vacuum

by increasing the applied current. After the metal was melted and evaporated, the atomic

beam will hit the substrate and the metallic film grows. Thicker gold layers are more

advantageous due to their higher reflectance and conductivity. The apparatus for

evaporating gold was not completely set up during the period of our experiment. So silver

is used as the counter electrode, although it seems less advantageous than gold.

81

DSSC which we fabricated

Photovoltaic device characterization

Current- Voltage measurement

The efficiency of the fabricated DSSC was measured by using a Keithly Current-

voltage (I-V) measurement unit. The I-V measurements gives the parameters such as, the

open circuit voltage (Voc), the short circuit current (Isc), the fill factor (FF) and the power

conversion efficiency (η). The standard characterization method of photovoltaic device

include the determination of DC current-voltage characteristics under white light

illumination of different intensities and the measurement of photocurrent under low

intensity monochromatic light. The power conversion efficiency of a photovoltaic cell can

be calculated by using the equation,

Power conversion efficiency,

P max is the maximum power point (maximum power output) and is the product of

maximum current and maximum voltage in the I-V curve. Pin is the input power of the

solar cell.

The open circuit voltage (Voc) is the cell voltage measured when current in the cell

is zero.

The short circuit current (Isc) is measured at the condition where the applied

potential equals to zero. Isc increases linearly with illumination intensity.

The fill factor (FF) is the ratio of the maximum power to the external short and

open circuit values. Fill factor is an important parameter which determines the power

conversion efficiency of organic photovoltaic cell.

82

The power conversion efficiency of the photovoltaic cell depend on the Voc, Isc

and the fill factor. Improvement of DSSC performance was achieved by optimization of

these parameters. These parameters also depends on the light intensity and measurement

condition. Figure represents a typical I-V curve.

Current- Voltage characteristics of DSSC based on synthesized HTM

I-V characteristics of DSSC using Series I compounds as HTM

Current-voltage (I-V) characteristics of DSSC based on series I compounds as

HTM were measured using I-V curve plotted in figure .The values of photocurrent

density(Isc), open current voltage (Voc), Fill factor (FF) and corresponding photo energy

conversion efficiency are summarized in Table 19 .

83

I-V curves of DSSC fabricated using SC 011, SC 012, SC 013 and SC 014 (Series I) as

HTMs

Table 19: I-V characteristics of DSSC fabricated using Series I compounds as HTM

Compound Voc(V) Isc(mA/cm2) FF (%) η (%)

SC 011 0.20 4.16 31 0.26

SC 012 0.55 2.64 22 0.32

SC 013 0.44 6.32 31 0.80

SC 014 0.22 11.61 32 0.82

The photo conversion efficiency of the series I compounds varies with the change

in substitution at the outer carbazole rings. The photoconversion efficiency obtained are

0.26, 0.32, 0.80 and 0.82 for SC 011, SC 012, SC 013 and SC 014 respectively. From the

values obtained it is observed that the efficiency increases with increase in length of length

of alkyl group. The butyl substituted derivatives gives the highest value for photo

conversion efficiency.

I-V characteristics of DSSC using Series II compounds as HTM

Current-voltage (I-V) characteristics of DSSC based on series II compounds as

HTM are measured using I-V curve plotted in figure. The values of photocurrent density

(Isc), open current voltage (Voc), Fill factor (FF) and corresponding photo energy

conversion efficiency are summarized in Table 20.

84

I-V curves of DSSC fabricated using SC 021, SC 022, SC 023 and SC 024 (Series

II) as HTMs

Table 20: I-V characteristics of DSSC fabricated using Series II compounds as HTM


SC 021 0.59 1.64 32 0.33

SC 022 0.54 2.62 27 0.39

SC 023 0.49 5.93 32 0.84

SC 024 0.45 6.4 31 0.89

The photo conversion efficiency of the series II compounds varies with the change



values obtained it is clear that as the length of alkyl group increases the efficiency

increases.

85

I-V characteristics of DSSC using Series III compounds as HTM

Current-voltage (I-V) characteristics of DSSC based on series III HTM are

measured using I-V curve plotted in figure.The values of photocurrent density(Isc), open

current voltage (Voc), Fill factor (FF) and corresponding photo energy conversion

efficiency are summarized in table 21.

I-V curves of DSSC fabricated using SC 031, SC 032,SC 033 and SC 034(Series

III) as HTMs

Table 21: I-V characteristics of DSSC fabricated using Series III compounds as HTM


SC 031 0.22 10.49 31 0.72

SC 032 0.23 11.96 30 0.79

SC 033 0.49 5.94 32 0.92

SC 034 0.47 8.22 24 0.93

86

The photo conversion efficiency of the series III compounds varies with the change



values obtained it is observed that the efficiency increases with increase in length of length

of alkyl group.

I-V characteristics of DSSC using Series IV compounds as HTM

Current-voltage (I-V) characteristics of DSSC based on series IV HTM are

measured using I-V curve plotted in figure.The values of photocurrent density (Isc), open

current voltage (Voc), Fill factor (FF) and corresponding photo energy conversion

efficiency are summarized in table 22.

I-V curves of DSSC fabricated using SC 041, SC 042,SC 043 and SC 044(Series

IV) as HTMs

87

Table 22: I-V characteristics of DSSC fabricated using Series IV compounds as

HTM


SC 041 0.49 0.59 30 0.09

SC 042 0.49 0.82 28 0.12

SC 043 0.49 1.47 23 0.17

SC 044 0.41 1.68 29 0.20

The photo conversion efficiency of the series IV compounds varies with the

change in substitution at the outer carbazole rings. The photoconversion efficiency

obtained are 0.09, 0.12, 0.17 and 0.20 for SC 041, SC 042, SC 043 and SC 044

respectively. From the values obtained it is clear that as the length of alkyl group increases

the efficiency increases. Similar to other series of compounds here also highest efficiency

is observed for butyl derivative.

While comparing the photo conversion efficiency of the DSSCs fabricated using

the various hole transporting materials synthesized such as series I, series II, series III and

series IV compounds, it is observed that series III compounds are having higher efficiency.

This is due to the fact that in series III compounds the extend of conjugation increases due

to the presence of tetrphenylethylene moiety as core which enhances the hole transporting

nature. The maximum efficiency obtained is 0.93% for the compound SC 034.

The lowest efficiency is observed for the series IV compounds where the core is

biphenyl moiety. Here the extend of conjugation is less when compared to other series of

compounds. In series IV HTMs there is only two wings of alkoxy substituted carbazole

moiety is present.

When comparing to the literature reports of the efficiency of solid state dye

sensitized solar cells these values are predominantly fair for the devices which we

fabricated[147]. Instead of the highly expensive commercial ruthenium dyes we are using

natural dye extracted from red sandal wood for the device fabrication. Better efficiency

could be reached by replacing natural dyes with commercial dyes. Then the device

become highly expensive and the environmental friendly nature of the cell may be

88

diminished. Globally when comparing to the electrolyte based devices the efficiency of

solid state DSSCs are less. This is due to the fact that in solid medium there is less

efficient hole transport, which results in relatively low hole mobilities in organic

semiconductors.

The performance of solid state dye sensitized solar cells depends on the

morphology and thickness of TiO2 film. The increase in thickness of TiO2 film follows a

general increase of Isc due to the enhanced light harvesting capability of thicker sensitized

layer[148], [149]. However the increase in TiO2 film thickness is not always beneficial for

the device since the spin coated organic hole transport layer does not diffuse properly to

the base of the titania network.

5. Summary

The solar power is presently a rapidly growing but often relatively expensive

renewable energy form. Now the commercially available solar cells are silicon based solar

cells. Due to the high cost of production of silicon based solar cells, active researches are

going on in search for the development of cost effective and environment friendly solar

cells. Dye sensitized solar cells are promising due to its low cost of production and

environmental friendliness. One of the key element in DSSC is the hole transporting

material, which is responsible for the regeneration of the oxidized sensitizer after electron

injection into the semiconductor and for the transport of positive charge to the counter

electrode. For a compound to act as HTM it should be electrochemically and thermally

stable. Carbazole based hole transporting materials find increasing applications in various

electro optical devices like organic photovoltaic cells due to their enhanced hole

transporting properties and photo physical properties.

In the present work, we have mainly focused on the synthesis of various novel hole

transporting materials (HTM) based on carbazole. Synthesis of these HTMs has done by

using Ullmann coupling reactions. The catalyst used in these type of reactions are

electrolytic copper. The main advantages of copper catalyst include the low cost and the

easy availability. Here we have synthesized four series of carbazole based hole

transporting materials by employing multi step organic synthesis. Four different moieties

are used as the core in each series. 1,3,5-tribromobenzene is used as the core moiety for

the synthesis of series I compounds. In series II compounds triphenyl amine moiety is

used as the core, tetra phenyl ethylene moiety in series III and biphenyl moiety is used as

the core for the synthesis of series IV compounds. Each series contains derivatives of

89

methoxy, ethoxy, propoxy and butoxy groups. The synthesized four series of compounds

are: 1,3,5-tris(3,6-dialkoxy-9H-Carbazol-9-yl)benzene (series I), Tris-(4-(3,6-dialkoxy-

9H-carbazol-9-yl)phenyl)amine (series II), 1,1,2,2-tetrakis(4-(3,6-dialkoxy-9H-carbazol-9-

yl)phenyl)ethane (series III) and 4,4’-bis(3,6-dialkoxy-9H-carbazol-9-yl)1,1’-biphenyl

(series IV). The structures of the synthesized compounds were confirmed by using various

spectroscopic techniques such as UV-Visible, FT-IR, 1H-NMR and 13C-NMR.

The thermal and electrochemical properties of the synthesized HTMs were studied

by using Differential Scanning Calorimetry (DSC) and Cyclic Voltammetry respectively.

The hole transporting materials used in photovoltaic cells should have noticeable thermal

and electrochemical stability. The glass transition temperatures (Tg), phase transition

temperatures and melting points of the HTMs were measured using DSC. The

electrochemical stability of the synthesized compounds was studied using Cyclic

Voltammetry (CV). From CV the Oxidation potential and reduction potential of the

compounds were obtained. The optical band gap were calculated from the UV-Visible

spectroscopy. The HOMO-LUMO values, band gap, electron affinity and ionization

potential of HTMs were determined from these values.

The main components of solid state DSSC are, transparent conducting glass

electrode coated with porous nanocrystalline TiO2, dye molecules attached to the surface

of the nanocrystalline TiO2, hole transporting material and a counter electrode. We have

fabricated the solid state dye sensitized solar cell by using the synthesized carbazole based

HTMs and a natural dye extracted from red sandal wood or rakthachandana using ethanol

as sensitizer. The fabrication of a cost effective and environment friendly DSSC can be

attained by using natural dyes instead of the commercial ruthenium dyes. The extraction

and purification of natural dyes can be done by simple methods in a cost effective manner.

The photovoltaic power conversion efficiency of the DSSCs fabricated using the

synthesized compounds as HTMs and red sandal as sensitizer was determined by using

current-voltage (I-V) characterization technique. Then the I-V curve was plotted and

photocurrent density (Isc), open-current voltage (Voc), fill factor (FF) and corresponding

photo conversion efficiency (η) were calculated. Different HTMs exhibit different photo

conversion efficiency that varies from 0.09-0.93 %. From the result it is observed that the

efficiency depends on the length and number of alkoxyl substituents present in the HTMs.

The efficiency increases with increase in alkyl chain length.

Novel hole transporting materials based on carbazole was synthesized by multi

step organic reactions. Ullmann type condensation methods are followed for the synthesis.

90

The synthesized compounds were characterized using UV-Visible, FT-IR, 1H-NMR and

13C-NMR spectroscopic techniques. The structure of the synthesized compounds were

confirmed from the spectral data. Thermal studies of the compounds were done by using

DSC. From the thermal data it is observed that the synthesized compounds are thermally

stable and satisfy all the required properties. The moderate Tg values makes the

compounds to exhibit glassy nature at low temperature. This makes the compounds to

have high penetrating power, which is favourable for the HTM to intermix with the

semiconductor TiO2 in all its layers. The compounds exist in solid phase even after a long

time exposure of the cell due to its moderate melting point. The compounds are thermally

stable in device structure once fabricated up to high temperature normally above 250oC.

The DSC results reveal that the compounds are amorphous material with good film

forming properties. From the results it is clear that the synthesized compounds are good

candidates as hole transporting material in photovoltaic devices.

The electrochemical properties of the synthesized compounds were studied by

cyclic voltammetry. The oxidation potential, reduction potential and HOMO values are

calculated from these results. By combining CV and optical band gap obtained from UV-

Visible spectroscopy the LUMO values are calculated. The optimized geometry and

HOMO-LUMO of the synthesized compounds were estimated from Density Functional

Theory (DFT).

Solid state dye sensitized solar cell was fabricated by using the various carbazole

based HTMs and the natural dye extracted from red sandal wood. The photo voltaic power

conversion efficiency was measured using I-V characterization technique. The photo

conversion efficiency varies from 0.09-0.93 %. From the results we can say that as the

length and the number of alkyl group increases the efficiency increases. Series III

compounds are having high efficiency than other series of compounds due to the presence

of highly conjugated tetraphenyl ethylene moiety as the core. The maximum obtained

efficiency is 0.93 %.

The conversion efficiency is found to be less. The low conversion efficiency is

expected and may be due to the following reasons: i) the use of natural dye as sensitizer,

the red pigment of red sandal instead of the ruthenium dye, which is commonly used for

DSSCs. The efficiency may be increased when commercial dyes were used. In such case

the cell will be expensive and the environment friendly nature of the cell may be lost. ii)

The use of back electrode. Here we have used silver as back electrode instead of gold.

These values are appreciable for solid state organic solar cell with non-commercial dyes

91

according to the literature reports (Li et al, 2013). Generally the efficiencies of solid state

DSSCs are lower than the corresponding electrolyte based cells. The higher Voc in solid

state DSSCs than that in case of electrolyte based cell may be due to the fact that the redox

potential of spiro-HTM was greater than that of I-/I3 couple.

The thickness and morphology of the titania film also depends on the performance

of solid state DSSC. The increase of Isc with the titania layer thickness can be attributed to

the light harvesting property of thicker sensitizer layer. But the titania layer thickness

increase is not beneficial for the device since the organic hole transport layer, spin coated

cannot diffuse perfectly until the base of the titania network. Another factor contributing

to the low performance of the device is the low mobility of charge carriers in the solid hole

transporting medium with increased titania layer thickness. The resulting low current

values may be due to the layer thickness of different materials and the possible

degradation of the natural dye. The performance of the device could be enhanced by

optimization of HTM layer thickness and the natural dye concentration. The use of

minimum amount of chemicals is another advantage when the natural dye is used. This

results in an introductory step to green synthesis.

DSSCs with natural dye as photosensitizer is promising due to their environmental

friendliness, simple manufacturing technique and low cost of production.The total cost

effect of the cells are comparatively less that is 30-40% to that of similar type of fabricated

cell. The transformation of non-conventional energy, even to its curtailed efficiency is an

amelioration to mankind.

6. Achievements

i. Salient findings (Technical outcome)

Novel carbazole based hole-transporting materials was synthesized by multi step

organic reactions Synthesized compounds were characterized by UV-Visible, FT-IR

and NMR Spectroscopy. Thermal and electro-chemical studies were carried out by

Differential Scanning Calorimetry and Cyclic Voltammetry.

Solid state Dye Sensitized solar Cell was fabricated using the synthesized hole

transporting material and natural dye as sensitizer.

The performance of the cell was analyzed. The conversion of non-conventional

energy even to its minimum efficiency is advancement to mankind.

92

ii. Publications/Awards/Patents etc

(A) PUBLICATIONS:

(a) List of Research publications

i. Journal Papers

a. International :

1. Saritha C, Keerthi Mohan A, Sheena K, Haridas K R (2018),

“Synthesis and Characterization of a Novel Carbazole Based Hole Transporting

Material”, The Chemist, Journal of the American Institute of Chemists.91(2), 18-

26.

2. Saritha C, Sheena K,Swapna M V, Haridas K R (2019), “Synthesis

and Characterization of a Novel Carbazole Based Hole Transporting Material

for application in Solid State Dye Sensitized Solar Cell”, Journal of Emerging

Technologies and Innovative Research(JETIR).6(4), 439-446. ISSN: 2349-5162.

b. National : Nil

ii. CONFERENCES/ SEMINARS etc..

a. International :

1. C.Saritha and K.R.Haridas (2019), Carbazole Based Novel Hole

Transporing Material Synthesis and Characterization,Proceedings of the

International Conference on Nanotechnology-2019 “Opportunities and

Challenges” St.Aloysius College, Mangalore, , January 10-11, 2019.

b.National :

1. C.Saritha and K.R.Haridas(2018), Novel Hole Transporting

Material Based on Carbazole-Synthesis and Characterization, ,Proceedings of

the National Seminar on Advanced Materials , School of Chemical

Sciences,Kannur University, March15-16 , 2018.

2. C.Saritha and K.R.Haridas(2018), Synthesis and Characterization

of a Carbazole Based Hole Transporting Material, ,National Seminar on Recent

Advances in Chemistry(RAC 2018), Govt.Brennen College, Thalassery,

November15-16 , 2018.

3. C.Saritha and K.R.Haridas(2018), Synthesis and Characterization

of a Novel Carbazole Based Hole transporting Material, Proceedings of the

National Seminar on Nascent and Sustainable Materials(NSNSM-2018), School

of Chemical Sciences,Kannur University, November 28-30 , 2018.

93

c. Regional:

1. C.Saritha and K.R Haridas (2019), Fabrication of solid state dye

sensitized solar cell with carbazole based hole transporting material,

Proceedings of 31stKerala Science Congress, Fatima Mata National College,

Kollam, February 2-3,2019.

2. C.Saritha and K.R Haridas (2020), Novel hole transporting material

based on carbazole for solid state dye sensitized solar cell, Proceedings of 32nd

Kerala Science Congress, Yuvakshethra Institute of Management

Studies,Mundur, Palakkad, January 25-27,2020.

(B) PARTICIPATION ( In Workshops/Seminars etc..)

a. International : Attended Two day Workshop on “Wet Chemical Routes to High

Efficiency Third Generation Solar Cells” at NIIST, Trivandrum, 23-24 April

2018

(C) AWARDS, RECOGNITIONS etc. (Provide details)

a. Awards : Best Poster Award in 32nd Kerala Science Congress.

7. Scope of future work

Power generation through the fabrication of photovoltaic cells reduces

transmission and distribution losses and power generation process is noiseless, nontoxic,

less maintenance and no greenhouse gases are emitted. Large scale production and

utilization of solar electrical power is widely welcomed because of it’s the huge theoretical

potential and very high practical potential. The standard silicon solar cell technology are

likely in its matured stage except in cost reduction techniques. So there is a prevailing

need for the development of new materials and concepts for the photovoltaic conversion,

with an aim to curtail the price of solar cell up to minimum level which improves its

acceptance. Major efforts have been taken place for the development of new photo

electrode material, new sensitizer and new hole transporting materials through cost

effective methodologies. DSSCs are very promising because of its large potential to

convert solar energy to electrical energy at low cost and it lacks leakage or sealing

problems that exist in liquid electrolyte dye sensitized solar cell. In DSSC, electron

injection takes place in ultra-fast speed from a photo excited dye into the conduction band

of TiO2, followed by the subsequent dye regeneration and hole transport to the counter

electrode which are the processes that govern the efficient electrical power generation.

94

The present work mainly focuses on the synthesis of novel hole transporting

materials based on carbazole. Carbazole based material constitute a well-known class of

hole conducting material. The charge carrier mobility and photo conductive properties of

these materials have been undergone for study by various groups. More and more

investigations have been going on about hole-transporting materials based on the

carbazole moiety for the last decade. Various reasons for this is that very interesting

features such as low cost of the starting material [9H-carbazole], good chemical and

environmental stability provided by the fully aromatic unit, easy substitution of the

nitrogen atom with a wide range of functional groups permitting a better solubility and a

fine tuning of the electronic and optical properties.

8. Bibliography

[1] K. Caldeira, A. K. Jain, M. I. Hoffert, Science, 299, 2052 (2003).

[2] R.A. Mulhall and J.R. Bryson, Applied Energy, 123, 327-334, (2014).

[3] P. Faria, T. Soares, Z. Vale and H. Morais, Renewable Energy, 66, 686-695, (2014).

[4] T. Surek, Journal of Crystal Growth, 2005, 275, 292.

[5] J. Nelson, Science, 293, 1059- 1060, (2001).

[6] C.J. Brabec, S. Sariciftci and J.C. Hummelen, Advanced functionalmaterials,11, 15-

26, (2001).

[7] A.M. Hermann, Solar Energy Materials and Solar Cells, 55, 83-94, (1998).

[8] A.G. Aberle, Thin Solid Films., 2009, 517, 4706–4710.

[9] K. L. Chopra, P. D. Paulson, V. Dutta, Prog. Photovolt: Res. Appl., 2004,12,69–92.

[10] S. Sharma, K.K. Jain and A. Sharma, Materials Science and Applications, 6, 1145-

1155, (2015).

[11] L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G.

Li, Y. Yang, Nat Photonics, 2012, 6, 180–185 .

[12] S. Karg, W. Riess, V. Dyakonov, M. Schwoerer, Synthetic Metals, 1993, 54, 427-

433.

[13] D.G. McGehee, M.A.Topinka, Nature Material, 2006, 5 , 675–676.

[14] J. J. M. Halls, K. Pichler, R. H. Friend, S. C. Moratti, A. B. Holmes, Appl. Phys.

Lett., 1996, 68, 3120.

[15] S. E. Shaheen, D. S. Ginley , G.E. Jabbour, MRS Bulletin, 2005,30,10-19.

[16] A. Ueda, R. Mu, M. H. Wu, Organic Photovoltaics: Mechanisms, Materials, and

Devices. Organic Photovoltaics, 2005.

95

[17] B.R. Saunders, M.L. Turner, Advances in Colloid and Interface Science, 2008, 138,

1–23.

[18] J. H. Im,; C. R. Lee, J.W. Lee, S.W. Park, N.G. Park, Nanoscale, 2011, 3, 4088-

4093.

[19] H. S.Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J.Moon, R.H

Baker, J.H. Yum, J.E. Moser, M. Grätzel, N.G. Park, Sci Reports 2012, 2, 591.

[20] D. Liu, T.L. Kelly, Nature Photonics, 2014, 8, 133-139.

[21] B. Lee, J. He, R.P. Chang, M.G. Kanatzidis, Nature, 485 (2012) 486.

[22] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Chemical

Communications, (2002) 374-375.

[23] B. O. Regan, M. Gratzel, Nature, 353, 737 (1991).

[24] B. Park, J. Moon, Procedia Computer Science, 122, 965-968, (2017).

[25] J. Desilvestro, M. Gratzel, L. Kavan, J. Moser, J. Augustynski, Journal of the

American Chemical Society, 107, 2988 (1985).

[26] N. Vlachopoulos, P. Liska, J. Augustynski, M. Grätzel, Journal of the American

Chemical Society, 110, 1216 (1988).

[27] M. P. Dare-Edwards, J. B. Goodenough, A. Hamnet, K. R. Seddon, R. D. Wright,

Faraday Discussion of the Chemical Society, 70, 285 (1980).

[28] H. Tsubomura, M. Matsumura, K. Nakatani, K. Yamamoto, K. Maeda Solar

Energy, 21, 93 (1978).

[29] D. Li, D. Qin, M. Deng, Y. Luo and Q. Meng, Energy & Environmental Science, 2,

283-291, (2009).

[30] M. Gratzel, Nature, 414, 338 (2001).

[31] D. Matthews, P. Infelta, M. Grätzel Solar Energy Materials and Solar Cells, 44, 119

(1996).

[32] A. Stadler, Materials, 2012, 5, 661-683.

[33] N.S. Allen, M. Edge, A. Ortega, G, Sandoval, C.M. Liauw, J. Stratton and R.B.

Mcintyre, Polymer Degradation and Stability, 78, 467- 478, (2002).

[34] N.S. Allen, M. Edge, A. Ortega, G. Sandoval, C.M. Liauw, J. Verran, J. Stratton

and R.B. Mcintyre. Polymer Degradation and Stability, 85, 927- 946, (2004).

[35] A. Scalafani, L. Palmisano and M. Schiavello, Journal of Physical Chemistry, 94,

829-832, (1990).

[36] J. Winkler, Titanium dioxide (European Coatings Literature), 2003.

96

[37] D. Regonini, C.R. Bowen, A. Jaroenworaluck and R. Stevens, Materials Science

and Engineering: R: Reports, 74, 377-406, (2013).

[38] K. Kalyanasundaram, M. Grätzel, Coordination Chemistry Reviews, 177, 347

(1998).

[39] A. Hagfeldt, M. Grätzel, Accounts of Chemical Research, 33, 269 (2000).

[40] G. Sauve, M.E. Cass, G. Coia, S.J. Doig, I. Lauermann, K.E. Pomykal, N.S. Lewis,

Journal of Physics Chemistry B, 104, 6821 (2000).

[41] M. Grätzel, Inorganic Chemistry, 44, 6841 (2005).

[42] C.Y. Chen, S.J. Wu, C.G. Wu, J.G. Chen, K.C. Ho, Angewandte Chemie,

International Edition, 45, 5822 (2006).

[43] K.J. Jiang, N. Masaki, J. Xia, S. Noda, S. Yanagida, Chemical Communication,

23, 2460 (2006).

[44] K. Hara, H. Sugihara, Y. Tachibana, A. Islam, M. Yanagida, K. Sayama, H.

Arakawa, Langmuir, 17, 5992. (2001).

[45] D. Kuang, C. Klein, S. Ito, J. E. Moser, R. Humphry-Baker, N. Evans, F. Duriaux,

C. Grätzel, S.M. Zakeeruddin, M. Grätzel, Advanced Material, 19, 1133 (2007).

[46] P. Wang, C. Klein, R. Humphry-Baker, S. M. Zakeeruddin, M. Grätzel, Journal of

the American Chemical Society, 127, 808 (2005).

[47] B.O. Regan and D.T. Schwartz, Chemistry of Materials, 7, 1349 (1995).

[48] K. Sayama, S. Tsukagoshi, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga, H.

Arakawa, Journal of Physical Chemistry B, 106 , 1363 (2002).

[49] K. Hara, Y. Tachibana, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H. Sugihara, H.

Arakawa, Solar Energy Materials and Solar Cells, 77, 89 (2003).

[50] P.M. Jayaweera, A.R. Kumarasinghe, K. Tennakone, Journal of Photochemistry and

Photobiology A: Chemistry, 126, 111 (1999).

[51] Q.-H. Yao, L. Shan, F.-Y. Li, D.-D. Yinb, C.-H. Huang, ‘An expanded conjugation

photosensitizer with two different adsorbing groups for solar cells’, New Journal of

Chemistry, 8, 1277 (2003).

[52] K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi, Y. Abe, H.

Sugihara, H. Arakawa, Chemical Communication, 1173 (2000).

[53] K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga, H. Arakawa, Chemical

Communication, 6, 569 ( 2001).

[54] K. Hara, T. Sato, R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H.

Sugihara, H. Arakawa, Journal of Physical Chemistry B, 107, 597 (2003).

97

[55] Z. S. Wang, Y. Cui, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, Advanced Material,

19, 1138 (2007).

[56] I. Jung, J.K. Lee, K.H. Song, K. Song, S.O. Kang, J. Ko, Journal of Organic

Chemistry, 72, 3652 ( 2007).

[57] M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen, Z. Li, Journal of Physical

Chemistry, 111, 4465( 2007).

[58] T. Horiuchi, H. Miura, K. Sumioka, S. Uchida,ournal of the American Chemical

Society, 126, 12218 (2004).

[59] T. Horiuchi, H. Miura, S. Uchida,Chemical Communication, 3036 (2003).

[60] S. Ferrere, A. Zaban, B.A.Gregg, Journal of Physical Chemistry B,101,4490(1997).

[61] S. Ferrere, B. A. Gregg, New Journal of Chemistry, 26, 1155 (2002).

[62] H. Tian, P.-H. Liu, W. Zhu, E. Gao, D.-J. Wu, S. Cai, Journal of Material

Chemistry, 10, 2708 (2000).

[63] C. Zafer, M. Kus, G. Turkmen, H. Dincalp, S. Demic, B. Kuban, Y. Teomam, S.

Ieli, Solar Energy MAterials and Solar Cells, 91, 427 (2007).

[64] P. M.Sirimanne, M. K. I. Senevirathna, E. V. A. Premalal, P. K. D. D. P. Pitigala,

V. Sivakumar, K. Tennakone, Journal of Photochemistry and photobiology A, 177,

324 (20060.

[65] S. Hao, J. Wu, Y. Huang, J. Lin, Solar Energy, 80, 209 (2006).

[66] A. S. Polo, N. Y. Murakami Iha, Solar Energy Materials and Solar Cells, 90, 1936

(2006).

[67] Q. Dai, J. Rabani, Journal of Photochemistry and Photobiology A, 148, 17 (2002).

[68] N. J. Cherepy, G. P. Smestad, M. Gratzel, J. Z. Zhang, Journal of Physical

Chemistry B, 101, 9342 (1997).

[69] M. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N.

Vlachopoulos, M. Graetzel, Journal of the American Chemical Society, 115, 6382

(1993).

[70] J. M. R. C. Fernando, G. K. R. Senadeera, Current Science, 95, 663 (2008).

[71] P. Luo, H. Niu, G. Zheng, X. Bai, M. Zhang, W. Wang, Spectrochimica, Acta Part

A, 74, 936 (2009).

[72] S. Furukawa, H. Iino, T.Iwamoto, K. Kukita, S. Yamauchi, Thin Solid Films, 518,

526 (2009).

[73] E. Yamazaki, M. Murayama, N. Nishikawa, N. Hashimoto, M. Shoyama, O. Kurita,

Solar Energy, 81, 512 (2007).

98

[74] E. Espinosa, I. Zumeta, J. L. Santana, F. Marti_nez-Luzardo, B.Gonza_lez,

S.Docteur, E.Vigil, Solar Energy Materials and Solar Cells, 85, 359(2005).

[75] K. Tennakone, G. R. R. A. Kumara, I. R. M. Kottegoda, V. P. S. Perera, P. S. R. S.

Weerasundara, Journal of Photochemistry and Photobiology. A Chemistry, 117(2),

137, 1998.

[76] G. Calogero, G. D. Marco, Solar Energy Materials and Solar Cells, 92, 1341

(2008).

[77] M. S. Roy, P. Balraju, M. Kumar, G. D. Sharma, Solar Energy Materials and Solar

Cells, 92, 909 (2008).

[78] D. Zhang, S. M. Lanier, J. A. Downing, J. L. Avent, J. Lume, J. L. McHale, Journal

of Photochemistry and Photobiology A, 195, 72 (2008).

[79] M Gratzel, Accounts of Chemical Research, 42, 1788 (2009).

[80] U.Bach , D. Lupo ,P. Comte , J.EMoser ,F. Weissortel ,J. Salbeck , H.Spreitzer ,

M.Gratzel . Nature (1998) 395 583.

[81] A.W. Schmidt, K.R. Reddy, H.J. Knolker, Chem. Rev. (2012) 112, 3193.

[82] A. Venkateswara Rao, K.R.J. Thomas, C.P. Lee, C.T. Li, ACS Appl. Mater.

Interfaces (2014) 6, 25, 28.

[83] N. Prachumrak, S. Pojanasopa, S. Namunagruk, T. Kaewin, ACS Appl. Mater.

Interfaces (2013) 5,8694.

[84] T. Ameri, G. Dennler, C. Lungenschmied, C.J. Brabec, Energy and environmental

sciences (2009) 2, 347.

[85] M.A. Green, K. Emrey, Y. Hishikawa, W. Warta, E.D. Dunlop,Research and

applications (2012) 12, 20(1).

[86] E Bellmann, S.E. Shaheen, R.H. Grubbs, S.R. Marder, B. Kippelen, N.

Peyghambarian, Chemistry of Materials, 11, 399 (1999).

[87] M. Stolka, J.F. Yanus, D.M. Pai,Journal of Physical Chemistry, 88, 4707 (1984).

[88] W. Ishikawa, H. Inada, H. Nakano, Y. Shirota, Molecular Crystals and Liquid

Crystals, 21, 431(1992).

[89] G.Yu, J. Heeger, Journal of Applied Physics (1995) 78, 4510.

[90] H. Li, K. Fu, A. Hagfeldt, M. Gratzel, S.G. Mhaisalkar, A.C. Grimsdale, Angew.

Chem. Int. Ed (2014)53, 4085.

[91] I.Y. Song, S.H. Park, J. Lim, Y.S. Kwon, T. Park, Chem. Commun (2011) 47,

10395.

99

[92] R. Fink, C. Frenz, M. Thelakkat, H.-W.Schmidt, Polymer Preprints of the

American Chemical Society, 38, 323 (1997).

[93] T.J. Boyd, Y. Geerts, J.-K. Lee, D.E. Fogg, G.G. Lavoie, R.R. Schrock, M.F.

Rubner Macromolecules, 30, 3553 (1997).

[94] Y. Yamaguchi, T. Fujiyama, H. Tanaka, M. Yokoyama, Chemistry of Materials, 2,

341 (1990).

[95] J.Ostrauskaite,Karickal.H,R,A.Leopold, D.Haarer, M.Thelakkat,

J.Mater.Chem(2002) 2,58.

[96] S. Mathew and K. R. Haridas, “Synthesis and properties of N, N, N ′-tris-(2-ethoxy-

naphthalenen-1-yl) − N, N, N ′ triphenylbenzene 1, 3, 5-triamine for dye sensitized

solar cell,” Bulletin of Materials Science, vol. 35, no. 1, pp. 123–127, Feb. 2012.

[97] Sji Mathew, K.R. Haridas, Bulletin of Material Chemistry (2012) 35,1, 133-13.

[98] R. Vogel, K. Pohl, H. Weller,Chemical Physics Letters, 174, 241 (1990).

[99] R. Suarez, P. K. Nair, Journal of Solid State Chemistry, 123, 296 (1996).

[100] C. Barbe, F. Arendse, P. Comte, Journal of American Ceramics Society, 80, 31557

(1997).

[101] G. Chmiel, A. Patrykiejew, W. Rżysko, S. Sokolowski, Physical Reviews B, 48,

14454 (1993).

[102] N. Papageorgiou, W. F. Maier, M. Gratzel, Journal of the Electrochemical Society,

144, 876 (1997).

[103] E. Olsen, G. Hagen, S. E. Lindquist, Solar Energy Materials and Solar Cells, 63,

267 (2000).

[104] A. Kay, M. Grätzel, Solar Energy Materials and Solar Cells, 44, 99 (1996).

[105] K.Suzuki, M.Yamaguchi, M.Kumagai, S.Yanagida, Chemistry Letters, 32,28(2003).

[106] H. Lindstrom, A. Holmberg, E. Magnusson, s. E. Lindquist, L. Malmqvist, A.

Hagfeldt, Nano Letters, , 97 (2001).

[107] K. Imoto, K. Takahashi, T. Yamaguchi, T. Komura, J. Nakamura, K. Murata, Solar

Energy Materials and Solar Cells, 79, 459 (2003).

[108] Y. Saito, T. Kitamura, Y. Wada, S. Yanagida, Chemistry Letters, 31, 1060 (2002).

[109] Y. Saito, W,. Kubo, T. Kitamura, Y. Wada, S. Yanagida, Journal of Photochemistry

and Photobiology A, 164, 153 (2004).

[110] T. N. Murakami, S. Ito, Q. Wang, M. K.Nazeeruddin, T. Bessho, I. Cesar, P. Liska,

R. Humphry-Baker, P. Comte, P. Pechy, M. Gratzel, Journal of Electrochemical

Society, 153, 2255 (2006).

100

[111] J. Xia, C. Yuan, S. Yanagida, ACS Applied Materials and Interfaces, 2 (7), 2136

(2010).

[112] M.A. Green, K. Emery, Y. Hishikawa, W. Warta and E.W. Dunlop, Progress in

Photovoltaics: Research and Applications, 23, 1-9, (2015).

[113] A.E. Becquerel, Comptes rendus de I’ Academie des sciences Paris (1839) 9, 561.

[114] P. Peumans, R. Forrest, Applied Physics Letters (2001) 79, 126.

[115] A.J. McEvoy and M. Gratzel, ‘Dye- sensitized nanocrystalline semiconductor

photovoltaic devices’, 2nd world conference and exhibition on photovoltaic solar

energy conversion, 6-10 July, Vienna, Austria (1998).

[116] J. Hagen, W. Schaffrath, P. Otschik, R.Fink, A. Bacher, H-W. Schmidt, D. Haarer,

Synthetic Metal, 89, 215 (1997).

[117] W. Kubo, K. Murakoshi, T. Kitamura, Y. Wada, K. Habusa, H. Shirai, S. Yanagida,

Chemistry Letters, 1241 (1998).

[118] C.C. Chen, X.Z. Li, W.H. Ma, J.C. Zhao, H. Hidaka and N. Serapone, Journal of

Physical Chemistry B, 106, 318-325, (2002).

[119] H.Nishikori ,W. Quian , M.A. El.Sayed , J Phys Chem C Lett, (2007) 111,9008.

[120] Y.Hu , H.L. Tsai , C.L. Huang , Mater Sci Eng A (2003) 344, 209.

[121] D.M. Antonelli, J,Y. Ying, Angewandte Chemie (1995) 34, 2014.

[122] N.Masayoshi, S.Yuji, U.Mitsuru, T.Kouhei, I.Musubu, T.Yoshio, Synth.Met.(2005)

115,261.

[123] C.Jager, R.Bilke, M.Heim, D.Haarer, H. Karickal, M.Thelakkat, Synth.Met.

(2001)121,1543.

[124] K.-Y. Law, Chemical Reviews, 93, 449 (1993).

[125] M. Thelakkat and H.-W. Schmidt, Advanced Material, 10, 219 (1998).

[126] A.Fujishima , T.N. Rao , D.A. Tryk D, J. Photochem Photobiol. C (2000) I,1.

[127] Siji Mathew, Karickal R.Haridas, Journal of Korean Chemical Society (2010) 546,

717-722.

[128] B. S. Furnis, A. J. Hannaford, P. W. G. Smith, A. R. Tatchell, ‘Vogel’s Text book

of practical organic chemistry’, Fifth edition, Pearson Education, India 987 (2004).

[129] W. L. F. Armaarego, D.D Perrin, ‘Purification of laboratory chemicals Fourth

edition, Butterworth Heinemann, Singapore 209 (1996).

[130] J. Pang, Y. Tao, S. Freiverg, X-P. Y, M. D’Lorio, S. Wang, Journal of Material

Chemistry, 12, 206 (2002).

[131] J. Mohan, ‘Organic spectroscopy, principle and application’, 187, 2005.

101

[132] J.R. Dyre, Prentice-Hall of India private limited, New Delhi, Chapter 3, 22, 2006.

[133] R.M. Silverstein, G.C. Bassler, T.C. Morrill, ‘Spectrometric identification of

organic compounds’, 217, 2007.

[134] C. Jager, R. Bilke, M. Heim, D. Haarer, H. Karickal, M. Thelakkat, Organic

Photovoltaic, Proceedings of SPIE International Conferences, 4108, 104 (2001).

[135] G. Gritzner, J. Kuta, Pure and Applied Chemistry, 462, 56 (1984).

[136] T. Kietzke, Advances in OptoElectronics, 2007 (2007).

[137] M. Al-Ibrahim, H.-K. Roth, M. Schroedner, A. Konkin, U. Zhokhavets, G. Gobsch,

P. Scharff, S. Sensfuss, Organic Electronics, 6 (2005) 65-77.

[138] J. Pommerehne, H. Vestweber, W. Guss, R.F. Mahrt, H. Bässler, M. Porsch, J.

Daub, Advanced Materials, 7 (1995) 551-554.

[139] R.R. Gagne, C.A. Koval, G.C. Lisensky, Inorganic Chemistry, 19 (1980) 2854-

2855.

[140] M. Thelakkat, C. Schmitz, C. Hohle, P. Strohriegl, H. W. Schimidt, U. Hofmann, S.

Schoterand, D. Haarer, Physical Chemistry and Chemical Physics, 1, 1693 (1999).

[141] J. Bisquert, J.Gracia Canadas, I.Mora Sero, E.Palomares, Journal Spin Use,

6,5215(2003).

[142] M.Gratzel ,Journal of Photochemistry and Photobiology C, 4,145(2003).

[143] K. Tennakone, G.R.R.A. Kumara, I.R.M. Kottegoda, V.P.S. Perera, P.S.R.S.

Weerasundara,Journal of Photochemistry and Photobiology A: Chemistry, 117, 137

(1998).

[144] J. Bisquert,D.Cahen, G.Hodes, S.Ruhle, A.Zaban, Journal of Physical Chemistry

B,108,8106(2004).

[145] S.Ngamsinlapasathian, T.Sreethawong, Y.Suzuki, S.Yoshikawa, Solar Energy

Material and Solar Cells, 86,269(2005).

[146] G.Bradshawa, A.J. Hughes, Thin Solid Films, 33,L5(1976).

[147] K. Peter, H.Wietasch, B.Peng, M.Thelakkat, Applied Physics A,79,65(2004).

[148] J. Nelson, Physics reviews B, 59, 15374 (1999).

[149] J. Nelson, S. A. Haque, D. R. Klug, J. R. Durrant,Physics Reviews B, 63, 205321

(2001).

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