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2013-01-01

Synthetic Asphaltene as A Novel Dye in DyeSensitized Solar Cells DSSCsHassan SharifUniversity of Texas at El Paso, hsharif@utep.edu

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Recommended CitationSharif, Hassan, "Synthetic Asphaltene as A Novel Dye in Dye Sensitized Solar Cells DSSCs" (2013). Open Access Theses & Dissertations.1732.https://digitalcommons.utep.edu/open_etd/1732

SYNTHETIC ASPHALTENE AS A NOVEL DYE

IN

DYE SENSITIZED SOLAR CELLS DCCs

HASSAN MOHAMED SHARIF

Environmental Science and Engineering Doctoral Program

Benjamin C. Flores, Ph.D. Dean of the Graduate School

APPROVED:

Russell Chianelli, Ph.D., Chair

Barry Benedict, Ph.D.

Narayan, Mahesh, Ph.D.

Juan Noveron, Ph.D.

Copyright ©

by

Hassan Sharif

2013

Dedication

I dedicate my work to the soul my Father Mohamed Abubaker Al-Sharif ask Allah to

forgive him and my mother Salema Muftah El-Fortass ask Allah to give her long and healthy

life. Both they gave me the birth, raised me, loved me and supported me. I devote this work to

them for their love and support, their love of me to be in high level of education, the huge

sacrifices they made to guarantee I get to this stage of life, and for everything they have done to

me. I also dedicate this work to my beloved wife for her support and help during my study.

اهداء

،ادعو هللا ان يتقبله بواسع رحمتهمحمد ابوبكر الشريف الي روح والدي المرحوم باذن هللا

سالمة مفتاح الفرطاسوالي والدتي اطال هللا في عمرھا وامدھا بالصحة والعافية

الي ھذه المرحلة . لقد قدموا الكثير من حتى اصلاھدي لھم ھذا العمل لما قدموه من تضحيات

ما وصلت له اليوموراحتھم من اجل توفير جميع السبل واالمكانيات حتى تمكنت من الوصول الي قوتھم

كما ال انسى ان اھذي ھذا العمل الي زوجتي الغالية التي قدمت لي العون والمساعدة والجلھم كلھم

ملاھدي ھذا الع

أ

SYNTHETIC ASPHALTENE AS A NOVEL DYE

IN

DYE SENSITIZED SOLAR CELLS DSSCs

by

HASSAN MOHAMED SHARIF, M.Sc.

DISSERTATION

Presented to the Faculty of the Graduate School of

The University of Texas at El Paso

in Partial Fulfillment

of the Requirements

for the Degree of

DOCTOR OF PHILOSPHY

Environmental Science and Engineering Doctoral Program

THE UNIVERSITY OF TEXAS AT EL PASO

August 2013

v

Acknowledgements

Finally, I reached this point, I spent a lot of time and I did a lot of job just to reach this

point of my life. No one can imagine how I am happy when I was writing these words. In 2008, I

arrived here and I started my trip to complete my study, to get my PhD. I started this trip which

going too fast, I cannot believe that I stayed here for five years, but today when I started writing

my thesis, I am feeling that I arrived to the most important moment in my life. I started my study

here and I know there is no choice only to finish and get the success. It is big challenge, all there

in my country are waiting for me to complete my study and get the certificate. Five years, this is

the most important period of my life.

Today when I reached to this stage of my life, I have to give my deep thanks to who set

beside me and give me the support, those who helped me to win the race and make the success

become true. Special thanks to my advisor Prof. Russell R. Chianelli for his significant guidance

during my Ph.D. study in the Department of Environmental Science and Engineering. Prof,

Russell R. Chianelli was the main performer in developing my research interest and directed me

to a meaningful understanding of research methodology in science and technology. He also

taught me science and he taught me how to invent new ideas from nothing and how to approach

and achieve the goals. He taught me how to solve problems. Chianelli has taught me the most

important thing in my life; he taught me how I depend on myself. This work would not have

been come possible and easier without his careful supervision. His inspiration, enthusiasm,

suggestions and encouragements have helped me to improve my research skills, expand my

scientific imagination and defeat difficult scientific challenges.

I greatly thankful to all the members of my dissertation committee, Professor Barry

Benedict, Professor Narayan, Mahesh, and Professor Juan Noveron, first for their accepting me

as one of their students in their committee, and for their advises which helped me during my

work and also for devoting their time and expertise to review my dissertation.

vi

Special gratitude to my beloved wife Khadija Aldali and also to my siblings: Amany,

Ahmed, Fatima and my little boy Mohamed for their emotional support during my stay in El

Paso and for their love. Special thanks also go to all my family members in Libya to my brothers

and sisters for their continued support, supplications, generosity, love, and encouragement.

Nonetheless, my work and time in UTEP would have been far poorer without the

academic and moral support from; Dr. Rajab Abujnah, whom was gave me the help along my

stay in El Paso and he was the man who helped me to come here to El Paso and get the

acceptance to study at UTEP; thank you for all the Libyan students at UTEP live here was so

excited and I did not feel boring during stay in El Paso because of you, I wish you all the best on

your study and hope to meet you again in the future. I am very thankful to the Ministry of Higher

Education and Scientific Research in Libya, Canadian Bureau for International Education CBIE

for the financial support.

Last but not the least, and the most important, I would like to express my heart full thanks

to both persons who were the reason to be here in this life, they are very special, they suffered a

lot during their life just to give me a good life and they got tired to give me rest, they stayed

awake all nights to let me sleep, those people they paid a lot to teach me, and let me grew up to

reach this point. They are my parents, my father Mohamed Abubaker AL Sharif who passed

away in 2000, I ask Allah bless him, how it was great moment if he still alive and he saw me

today, and the greatest woman in the world, my mother Salema Muftah Elfortas, she is

supporting me all the time I am here and away from her sight. She supporting me by her

continuous prayers and supplication may Allah protect her and give her long life and good

health.

vii

Abstract

This dissertation approves the possibility of the first time use of synthetic asphaltene as

new dye in dye sensitized solar cells. In which the conversion efficiency of synthetic asphaltene

DSSC has improved up to 2.0 %. Synthetic asphaltene sample was prepared from the starting

material as in literature and applied for various solar cell parameters of TiO2 based dye-

sensitized solar cell DSSC. DSSCs were fabricated using the synthetic asphaltene as dye.

Different variables and parameters were carried out to test the photovoltaic performance of the

cells, these parameters include open-circuit voltage, short-circuit current; fill factor. The first

result was obtained with using synthetic asphaltene as dye was 0.4%. The maximum energy

conversion with using synthetic asphaltene as dye after adjusting some variables and adding

some additives was reached 2.0%. The benefit to study this material will help us to find new dye

and also to understand more the characterization of crude oil asphaltene which leads us to

improve its efficiency in dye sensitized solar cells.

viii

Table of Contents

Acknowledgements……………………………………………………………………………v

Abstract………………………………………………………………………………………vii

Table of Contents……………………………………………………………………………viii

List of Tables…………………………………………………………………………………xi

List of Figures………………………………………………………………………………..xii

List of Illustrations…………………………………………………………………………..xiv

Chapter 1: Background and Motivation

1.1Introduction……………………………………………………………………………1

1.1.1 The Energy Challenge…………………………………………………….………4

1.1.1 The Energy Challenge…………………………………………………………….4

1.2 Environmental Issues………………………………………………………………….5

1.3 The Sun………………………………………………………………………………..6

1.3.1 Solar Energy…………………………………………………………………….7

1.3.2 Solar Energy from Sun to Earth………………………………………………...7

1.3.3 Solar Energy Basics……………………………………………………………..8

1.3.4 Why Solar Energy?.............................................................................................10

1.3.5 Conversion of Sunlight into Electricity………..………………………………11

1.3.6 Sun as Energy Source………………………………………………………….12

1.4 Solar Energy Captured by Earth……………………………………………………..13

1.4.1 Solar Constant……………………………………………………….…………13

1.4.2 Solar Radiation Striking the Earth Surface………………………………….…15

1.4.3 World Distribution of Solar Radiation……………… ……………….………16

1.4.4 How Much Solar Energy is Available?..............................................................18

1.5 Motivation……………………………………………………………………………19

1.6 Dissertation Outline……………………………………………………...…………..21

ix

Chapter 2: Asphaltene and Synthetic Asphaltene

2.1Asphaltene………………………………………………………………………...23

2.1.1 Introduction………………………………………………………...............23

2.1.2 Elemental Composition…………………………………………………….24

2.1.3 Structure of Asphaltene…………………………………………………….26

2.1.4 Asphaltene Molecular Weight……………………………………………...28

2.1.5 Asphaltene Size and Shape………………………………………………....29

2.2 Synthetic Asphaltene……………………………………………………………..30

Chapter 3: Photovoltaics.

3.1 Photovoltaic Effect……………………………………………………….….…38

3.2 Application of Photovoltaic Technology…………………………………….…39

3.3 Environmental Effects from Operation of PV Systems………………….….….41

3.4 Organic Photovoltaic……………………………………………………….…..42

3.5 Mechanisms of Organic Photovoltaic Effect……………………………….….43

3.6 Photovoltaic Cell Performance………………………………………………...45

Chapter 4: Dye Sensitized Solar Cells DSSCs

4.1 Introduction……………………………………………………………………48

4.2 History and Development……………………………………………………...49

4.3 Brief Description of DSSC…………………………………………………….51

4.4 Requirements of Efficient Dye in DSSC………………………………………53

4.5 Structure of Dye Sensitized Solar Cell………………………………………...54

4.6 Characteristic Parameters of DSSCs…………………………………………..64

4.7 Operating Principle of the Dye-Sensitized Solar Cell…………………………67

4.8 Applications of DSSC…………………………………………………………71

Chapter 5: Materials and Methods

5.1 Introduction…………………………………………………………………....72

5.2 Building the DSSC………………………………………………………….…79

5.3 The Electrolyte………………………………………...………….…………...84

5.4 Assembling of the Synthetic Asphaltene Cells ……..………………….…......84

x

Chapter 6: Results and Discussion

6.1 Introduction…………………………………………………………….…...….86

6.2 The First Use of Synthetic Asphaltene as Dye in DSSC……………………....86

6.3 Effects of Different Layers of the Paste……………………………….…….…87

6.4 UV-Ozone Treatment of the Photo-electrode…………………………….…....88 6.5 Effect of Some Additives on the Cell Performance Efficiency…………..….…89

6.6 Purified Synthetic Asphaltene………………………………………….……....91

6.7 Effect of Chenodeoxycholic Acid on the Photovoltaic Performance….….…..92

Chapter 7: Conclusion and Future Work

7.1 Conclusion…………………………………………………………….…...…93

7.2 Future Work………………………………………………………………….94

References ……………………………………………………………………..…...95 Abbreviatios ………………………………………………………………………...107 Vita……………………………………………………………………………............110

xi

List of Tables

Table 2.1: Comparison of some chemical properties of Synthetic Asphaltene and HVAR. …....37

Table 6.1: The first result of Synthetic Asphaltene dye in dye sensitized solar cell………….…86

Table 6.2The new results of new Synthetic Asphaltene sample…………………………….......87

Table 6.3: The effect of different paste layers on the performance of Synthetic Asphaltene

cell……………………………………………………………………………………………..…88

Table 6.4: The effect of UV-O treatment on the Synthetic Asphaltene cell performance……....89

Table 6.5: The results of mixing Synthetic Asphaltene with Porphyrin………………………....90

Table 6.6: The photovoltaic performance efficiency of Synthetic Asphaltene dye mixed with

crude oil Asphaltene…………………………………………………………………………..…90

Table 6.7: The performance efficiency of purified Synthetic Asphaltene dye and unpurified

Synthetic Asphaltene dye…………………………………………………………………...……91

Table 6.8: The performance efficiency of Synthetic Asphaltene dye with additives of

Chenodeoxycholic Acid …………………………………………………………………………92

xii

List of Figures

Figure 1.1: World marketed energy consumption, 1990-2035………………………………...…1

Figure 1.2: World energy consumption by fuel sources 1990-2035……………………………...2

Figure1.3: World net electricity generation by fuel type, 2008-2035………………………….....4

Figure 1.4: Brightness vs. wavelength for various temperatures………………………………….9

Figure: 1.5: ASTM G173-03 Reference Solar Spectra on Earth………………………………...13

Figure1.6: Annual global average insolation from July 1983- June 2005……………………....17

Figure: 2.1: Idealized molecule structure of asphaltene……………………………….………...27

Figure 2.2: Chemical Structure of Cholesterol…………………………………………………..31

Figure 2.3: (a) IR spectrum of Athabasca asphaltene…

(b) IR spectrum of Synthetic Asphaltene prepared from cholesterol and sulfur…..31

Figure 2.4: (a) 1 H NMR spectrum of Athabasca asphaltene.

(b) 1 H NMR spectrum of Synthetic Asphaltene prepared from cholesterol and

sulfur…………………………………………………………………………..32

Figure 2.5: (a) UV spectrum of Athabasca asphaltene

(b) UV spectrum of Synthetic Asphaltene prepared from cholesterol and sulfur….32

Figure 2.6: Structure of discotic liquid crystal where R=undecyl, heptyl, hexyl, undecanoyl…..33

Figure 2.7: Diagram of pyrolysis of TOCP followed by self-assembly of asphaltene from a sea

of free radicals……………………………………………………………………..35

Figure 2.8: Schematic diagram of aliphatic and polyaromatic thermal products upon the heat

treatment of TOCP at 360-4200C involving radical coupling and aromatic condensation……...35

Figure 2.9: Infrared spectra of the TOCP precursor, Synthetic Asphaltene and HAVR (Heavy

Arab Vacuum Reside) asphaltene showing the significant similarity between the synthetic and

the HAVR asphaltene……………………………………………………………………………36

Figure 3.1: Some of the early investigated organic molecules. Top: TPP and anthracene. Bottom:

phtalocyanine and Chl-a………………………………………………………………………....42

xiii

Figure 3.2: Difference between organic and inorganic semiconductor………………………….43

Figure 3.3: Current-Voltage (IV) curve………………………………………………………….47

Figure4.1: Cross section view of the design of a dye sensitized solar cell under illumination

conditions (left), and energy levels of the different components of the cell that represent the

energetics of operation of such devices (right)…………………………………………………..52

Figure4.2: Schematic of the structure of the dye sensitized solar cell…………………………...54

Figure 4.3: On the left: scanning electron microscope picture of the fractal TiO2 film used in the

first embodiment of the DSSC in 1988. On the right: scanning electron micrograph of a TiO2

anatase colloid film………………………………………………………………………………57

Figure 4.4: Chemical structure of the two of ruthenium complexes N3 and Black dye…………59

Figure 4.5: I-V characteristics of a typical solar cell…………………………………………….65

Figure 4.6: I–V curves of an organic PV cell under dark (left) and illuminated (right)

conditions………………………………………………………………………………………...66

Figure4.7: Principle of operation and energy level scheme of the dye-sensitized nanocrystalline

solar cell………………………………………………………………………………………….68

Figure 4.8: Electron transfer processes in a DSC during light-electric power conversion

energy…………………………………………………………………………………………….69

Figure 4.9: Schematic illustration of kinetics in the DSSC……………………………………...70

Figure 5.1a: Analysis of H1 NMR peaks of TOCP……………………………………………...73

Figure 5.1b: Analysis of H1 NMR peaks of TOCP……………………………………………...74

Figure 5.2: IR Spectra of the Precursor TOCP…………………………………………………..75

Figure 5.3: UV-Vis absorption of Synthetic Asphaltene………………………………………...77

Figure 5.4: IR Spectra of different Synthetic Asphaltene samples………………………………78

Figure 5.5: Scanning electron microscope (SEM) of a TiO2 Paste……………………………...81

Figure 5.6: Elemental analysis of TiO2 paste……………………………………………………82

xiv

List of Illustrations

Illustration1.1: Image of the Sun from NASA……………………………………………………6 Illustration: 1.2: Calculation of Solar constant………………………………………………….14 Illustration: 1.3: the path length of the solar radiation through the Earth’s atmosphere in units of Air Mass AM…………………………………………………………………………………….16 Illustration 2.1: the appearance of asphaltenes extracted from Mars-P crude oil using of different solvents (a) n pentane (n-C5) and (b) n-heptane (n-C7)…………………………………………23 Illustration 2.2: Relaxed simulation of the TOCP precursor molecule………………………….34 Illustration 4.1: Tri-iodide/iodide redox couple………………………………………………….64 Illustration 5.1: Showing a) the precursor material, b) synthetic asphaltene after synthesized……………………………………………………………………………………….76 Illustration 5.2: Filter paper after Synthetic Asphaltene filtered………………………………...79 Illustration 5.3: Preparation procedure of P-25 paste……………………………………………80

Illustration 5.4: The Thickness of TiO2 paste…………………………………………………...81

Illustration 5.5: the fabrication steps of new cell ……………………………………………..…83

Illustration 5.6: Schematic diagram showing completed synthetic asphaltene DSSC…………..85

1

Chapter1: Background and Motivation

1.1 Introduction

During the last few decades there have been major efforts to address the enormous

growth of both the worldwide energy demand and the emissions of greenhouse gases

(GHG) that are due to burning of fossil fuels and other energy consumption.

In the IEO2011, world marketed energy consumption grows by 53 percent from 2008 to

2035. Total world energy use rises from 505 quadrillion British thermal units (Btu) in 2008 to

619 quadrillion Btu1 in 2020 and 770 quadrillion Btu in 2035 (Figure 1.1). Much of the growth in

energy consumption occurs in countries outside the Organization for Economic Cooperation and

Development (non-OECD nations) where demand is driven by strong long-term economic

growth. Energy use in non-OECD nations increases by 85 percent in the Reference case, as

compared with an increase of 18 percent for the OECD economies.

Figure 1.1: World marketed energy consumption, 1990-2035 (1)

11 quadrillion -BTU of electricity per year = 33.5 x 106 kW per year x (365 days per year x 24 hours per day) = 293.5 x 109

KW-hrs = 293,500 GW-hrs

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In the IEO2011, World consumption of petroleum and other liquids grows from 85.7

million barrels per day in 2008 to 97.6 million barrels per Day in 2020 and 112.2 million barrels

per day in 2035. Natural gas consumption rises by 52 percent, from 111 trillion cubic feet in

2008 to 169 trillion cubic feet in 2035. Because of absence the national policies and international

agreements that could help to reduce greenhouse gas emissions, world coal consumption is

expected to increase from 139 quadrillion Btu in 2008 to 209 quadrillion Btu in 2035, at an

average annual rate of 1.5 percent.

As mentioned in the IEO2011, Electricity is the world's fastest-growing form of end-

use energy consumption. Net electricity generation increases by 2.3 percent per year from

2007 to 2035, while total world energy demand grows by 1.4 percent per year. The

growth in electricity generation is as result of rising standards of living, increase demand for

home purposes and the increase of commercial and residential services. Coal still remains

to provide the largest part of world electricity generation. It contributed for 42 percent of

total electrical generation in 2007, and its share is largely constant through 2035 (Figure 1.2).

The combustion of coal, however, adds a significant amount of carbon dioxide to the

atmosphere per unit of heat energy, (2) more than does the combustion of other fossil fuels.

In contrast, liquids, natural gas, and nuclear power all lose market share of world

generation over the course of the projection period, displaced by the strong growth

projected for renewable sources of generation. To satisfy the growing demand, a sharp

increase in the utilization of coal over the next 20 years is predicated, as well as an

extreme increase in the development of renewable energy sources as viable solutions to the

demand.(1)

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5

1.2 Environmental Issues

The increasing energy demand and shortage of fossil fuel supply is the issue of global

warming and climate change, caused by burning of fossil fuels. Intergovernmental Panel on

Climate Change (IPCC) report Climate Change 2007 (4) identified long-lived green-house

gases (GHGs) as the main factor affecting global warming and the radiative forcing of the

climate system, and pointed out that global GHGs emissions by human related activities have

increased 70% between 1970 and 2004. Between 1970 and 2004, global emissions of CO2,

CH4, N2O, HFCs, PFCs and SF6, weighted by their global warming potential (GWP),

have increased by 70% (24% between 1990 and 2004). The emissions of these gases have

increased at different rates; CO2 emissions have grown between 1970 and 2004 by about 80%

(28% between 1990 and 2004) and represented 77% of total anthropogenic GHG emissions

in 2004. The largest growth in global GHG emissions between 1970 and 2004 has come

from the energy supply sector (an increase of 145%) and combustion of Fossil fuel (4).

Some of the impacts of the ongoing climate change has been already observed, these

include more frequent occurrence of storms and flooding, alterations in disturbances of

forests due to fires and pests, excess heat-related mortality in Europe, changes in infectious

diseases vectors, earlier greening of vegetation in the spring, and others. Greater negative

impacts, such as decreases in the biodiversity and productivity of crops, increase in risk for

coastal erosion due to sea-level rise, and an increase in malnutrition are projected if the

current GHG emissions trends (and as a result, the trend in globally increasing temperature)

continue in the next 50-100 years4. Of all the available technologies producing renewable

energy, sun energy is a hot topic in current research.

The following chapter will give introduction about the sun as the main source of

energy. Later it will give a discussion on the quantity, availability and distribution of solar

6

energy worldwide. This chapter will end then with my motivation and the design of the

dissertation.

1.3 The Sun

The sun is a star, a hot ball of glowing gases at the heart of our solar system. Its effect

extends far beyond the orbits of distant Neptune and Pluto. There would be no life on Earth

Without the sun's energy and heat. The Sun is by far the largest object in the solar system. It

contains more than 99.8% of the total mass of the Solar System. The Sun is about 70% hydrogen

and 28% helium by mass and about 2% metals. (8)

Illustration1.1: Image of the Sun from NASA

7

1.3.1 Solar Energy

Solar energy is the energy produced directly by the sun and collected elsewhere, usually

the Earth. The sun generates its energy through a thermonuclear process that converts about

650,000,000 tons (5) of hydrogen to helium every second. The process creates heat and

electromagnetic radiation. The heat remains in the sun and is instrumental in maintaining the

thermonuclear reaction. The electromagnetic radiation (including visible light, infra-red light,

and ultra-violet radiation) flows out into space in all directions. (6)

1.3.2 Solar Energy from Sun to Earth

The sun is located at the center of the solar system and provides the energy to all

planets; Earth obtains its main energy in the form of light and heat from the Sun.

The enormous amount of energy continuously emitted by the sun is dispersed into

outer space in all directions. Only a small fraction of this energy is intercepted by the earth and

other solar planets.

The solar energy reaching the periphery of the earth's atmosphere is considered to be

constant for all practical purposes, and is known as the solar constant. Because of the difficulty

in achieving accurate measurements, the exact value of the solar constant is not known with

certainty but is believed to be between 1,353 and 1,395 W/m2. The solar constant value is

estimated on the basis of the solar radiation received on a unit area exposed perpendicularly to

the rays of the sun at an average distance between the sun and the earth.

In passing through outer space, which is characterized by vacuum, the different types of solar

energy remain intact and are not modified until the radiation reaches the top of the earth's atmosphere.

In outer space, therefore, one would expect to encounter the types of radiation listed in Table 1, which

are: gamma ray, X-ray, ultraviolet, and infrared radiations. (7)

8

1.3.3 Solar Energy Basics

At its core, solar energy is actually nuclear energy. In the inner 25% of the Sun, hydrogen

is fusing into helium at a rate of about 7 x 1011 kg of hydrogen every second. If this sounds like

a lot, it is because it is: this is equivalent to the amount of mass that can be carried by 10 million

railroad cars. There is no need to fear, though, that we are going to run out of fuel anytime soon,

as the Sun has enough hydrogen in the core to continue at this rate for another 5 billion years.

This energy production, coupled with gravitational compression, keeps the Sun’s center near a

sweltering 16 million K, which is about 29 million 0F. Heat from the core is first primarily

radiated, and then primarily convected to the Sun’s surface, where it maintains at a temperature

of 5800 K. (8)

The amount of energy that is emitted by the Sun, and the amount of solar energy that we

receive here on Earth, is dependent upon this surface temperature. A change of 1% in the

temperature of the Sun (58 K) can result in a change of 4% in the amount of energy per unit area

that we receive here. While this might not sound like a lot, it is more than enough to plunge us

into brutal ice age or hellish global warming.

The type of radiation coming from the Sun also depends on temperature. The Sun is

emitting electromagnetic radiation in wide variety of wavelengths. However, most of the

radiation is being sent out in the visible spectrum due to its surface temperature. Wien’s Law

states that the wavelength at which the most energy will be radiated depends inversely upon the

temperature of an object. Thus, as an object gets hotter, the peak radiation will come from

shorter wavelengths, and vice-versa. Figure 2 shows a theoretical plot of the energy emitted by

three perfect blackbody radiators of different temperature. An object that has a temperature of

4000 K has its peak energy being radiated at about 750 nanometers, which is in the near infrared.

An objec

about 50

will appe

waveleng

Sun’s tem

W

described

waveleng

reduced.

lessened

must spr

ct that has a

0 nanometer

ear to the hu

gths. The fir

mperature of

Figu

While our S

d above. It r

gths. Howev

Between th

by spreadin

read to fill a

surface temp

rs, which is

uman eye is

rst object wil

f 5800 K) wi

ure 1.4: Brig

un is not a

radiates 1.6

ver, by the t

he Sun’s an

ng and absor

all available

perature of 6

in the green

s determined

ll appear a v

ill appear a b

ghtness vs. w

a perfect bla

x 107 watt

time that it h

nd the Earth

rption. Ligh

space. Whi

9

6000 K, thou

n region of

d by just how

very dim red

bright white

wavelength f

ackbody rad

s of power

has reached

h’s surfaces,

t travelling

ile the total

ugh, has its p

the visible s

w much ene

d; while the s

that has a hi

for various te

diator, its ou

per square m

the Earth’s

, the energy

from a sphe

amount of

peak energy

spectrum. H

ergy is in ea

second (whi

int of yellow

emperatures

utput is fai

meter from

s surface, thi

y density of

erical object

energy of t

being radiat

How these ob

ach of the v

ch is close t

w.

rly close to

its surface

is value is v

f the radiati

t such as the

the radiation

ted at

bjects

isible

to our

o that

at all

vastly

ion is

e Sun

n will

10

remain the same, the amount of energy crossing any square meter of space will be reduced by the

square of the distance between the object and the area in question. The Sun is about 150 million

kilometers from the Earth; so the energy density per unit time of the sunlight reaching the upper

atmosphere of the Earth is only 1340 W/m2. (9)

1.3.4 Why Solar Energy?

Although most of the world's current electricity supply is generated from fossil fuels such

as coal, oil and natural gas, these traditional energy sources face a number of challenges

including rising prices, security concerns over dependence on imports from a limited number of

countries which have significant fossil fuel supplies, and growing environmental concerns over

the climate change risks associated with power generation using fossil fuels. As a result of these

and other challenges facing traditional energy sources, governments, businesses and consumers

are increasingly supporting the development of alternative energy sources and new technologies

for electricity generation. Renewable energy sources such as solar, biomass, geothermal,

hydroelectric and wind power generation have emerged as potential alternatives which address

some of these concerns. As opposed to fossil fuels, which draw on finite resources that may

eventually become too expensive to retrieve, renewable energy sources are generally unlimited

in availability. Solar power generation has emerged as one of the most rapidly growing

renewable sources of electricity.

However the burning of fossil fuels is considered as a major source of many harmful

pollutants into the atmosphere and contributes to environmental problems like global warming

and acid rain, solar energy is completely nonpolluting source. While big area of land must be

destroyed to feed a fossil fuel energy plant its required fuel, the only small land area is needed

for a solar energy plant. This ability to decentralize solar energy is something that fossil fuel

11

burning cannot match. As the primary element of construction of solar panels, silicon, is the

second most common element on the planet, there is very little environmental disturbance caused

by the creation of solar panels. In fact, solar energy only causes environmental disturbance if it is

centralized and produced on a huge scale. Among the renewable resources, only in solar power is

only the energy source able of supplying more energy than is used. (6)

1.3.5 Conversion of Sunlight into Electricity

Solar energy can be converted directly into electrical power in photovoltaic (PV) cells,

generally called solar cells. The sun has a surface temperature of about 6,000°C, and its hot gases

at this temperature emit light that has a spectrum ranging from the ultraviolet, through the

visible, into the infrared.

According to quantum theory, light can behave either as waves or as particles, depending

on the specific interaction of light with matter; this is called the wave-particle duality of light. In

the particle description, light consists of packets of energy called photons. Sunlight contains

photons with energies that reflect the sun’s surface temperature; in energy units of electron volts

(eV), the solar photons range in energy (hν) from about 3.5 eV (ultraviolet region) to 0.5 eV

(infrared region). The energy of the visible region ranges from 3.0 eV (violet) to 1.8 eV (red); the

peak power of the sun happens in the yellow region of the visible region, at about 2.5 eV. At

high noon on a cloudless day, the surface of the Earth receives 1,000 watts of solar power per

square meter (1 kW/m2). (3)

12

1.3.6 Sun as Energy Source

The sun is an infinite source of energy mostly composed of gases. Sun is like one

enormous thermonuclear reactor. Sun emits energy into the space in the form of

electromagnetic radiation.

Fusion is the energy source of the sun and stars. In fusion, two light nuclei (such as

hydrogen) combine into one new nucleus (such as helium) and release enormous energy in the

process. On earth, fusion has the potential to be an abundant and attractive source of energy for

the future. However, creating fusion conditions on earth and tapping that energy in practical

forms continues to challenge the world’s scientists, although tremendous progress has been

made. (10)

The current stellar classification, has given the sun a spectral class label of G2V. G2

indicates its surface temperature, and V signifies that the Sun, is a main sequence star, and

therefore generates its energy by nuclear fusion (11). The exact nuclear process is not

well known, but the most likely is one by which hydrogen, the Sun's most abundant fuel,

is converted to helium by nuclear fusion. It is believed that the sun convert more than

700 million metric tons of hydrogen to about 695, million metric tons of helium each

second. In this process of nuclear fusion, huge amount of energy in the form of

electromagnetic rays are released. Most of this electromagnetic radiation emitted from the

sun's surface received by Earth surface lies in the visible band centered at 500 nm (1 nm = 10-9

meters) Figure (1.5), although the sun also emits significant energy in the ultraviolet and

infrared bands, and small amounts of energy in the radio, microwave, X-ray and gamma ray

bands.

Most of

infrared

electrom

and elec

1.4 Sola

1.4.1 Sol

T

luminos

increasi

sphere i

square o

the irra

f the energy

d rays, whi

magnetic rad

ctrons. (11)

Figur

ar Energy C

lar Constan

The sun em

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ingly larger

is the same,

of the radius

adiative ene

emitted by

ch we feel

diation. The

re: 1.5 ASTM

Captured by

nt

mits some am

sun). As th

spherical ar

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the sun is vi

as heat. V

e sun also em

M G173-03 R

y Earth

mount of tota

hat energy i

reas. Since

y per unit are

sphere) from

oss-sectiona

13

isible light a

Visible light

mits particle

Reference So

al energy fro

s radiated a

the total am

ea decreases

m the sun. It

l unit area

and a related

t and infrar

e radiation,

olar Spectra

om its surfac

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mount of en

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t is often mo

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ce every sec

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Sun provides

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This quant

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d the "Sola

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ation: 1.2 Ca

the amount

l to the incid

Sun. The so

ate. Reliable

0-year recor

satellite dat

Wm_2. Of the

(40.7 mm),

.4 mm). (13)

14

ar constant"

e area (ASE)

alculation of

t of solar r

dent radiatio

olar constant

e measureme

d has been

ta suggests

e radiant ene

about 40%

)

abbreviated

) of the "big

f Solar consta

radiation re

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t is an impo

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obtained ba

a solar con

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sphere".

ant

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r constant c

ased on ove

nstant of 13

from the Su

ble region (0

nd is simply

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for the studi

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mately

), and

15

Due to the Earth's elliptical orbit, the actual distance between Earth and Sun

varies from a minimum of 147.097.000 km to a maximum of 152.086.000 km (14). For

computing the value of the solar constant, the astronomical unit AU is used.3The

astronomical unit is the principal unit of measurement or average earth-sun distance is used.

The big sphere centered at the Sun with a radius of DSE (see illustration 1.1) has the total

surface area of ASE = 4(DSE)2 = (2.81 1023 m-2). The energy

output of the sun, 3.87 1026 W, is uniformly distributed on this sphere surrounds the

sun see illustration (1.1). Therefore the solar constant is equal so = (3.839 1026 W)/ 2.81

1023 m-2 =1377 W m-2.

1.4.2 Solar Radiation Striking the Earth Surface

Solar radiation entering the Earth’s atmosphere is absorbed and scattered by atmospheric

gases, aerosols, clouds, and the Earth’s surface (13). When the Sun is directly overhead the

insolation, that is the incident energy arriving on a surface on the ground perpendicular to the

Sun's rays, is typically 1000 Watts per square meter (15).

This is due to the absorption of the Sun's energy by the Earth's atmosphere that dissipates

abbot 25% to 30% of the radiant energy (15). A concept which describes the effect of a clear

atmosphere on sunlight is the air mass, this equal to the relative length of the direct beam path

through the atmosphere. On a clear summer day at sea level, the radiation from the sun at zenith

corresponds to air mass 1 (abbreviated to AM1); at other times, the air mass is approximately

equal to 1/cos θ, where θ is the zenith angle. (16)

3 AU, mean distance between the earth and sun; one AU is 149,604,970 km (IAU 2009)

Illustratio

T

zenith a

spectrum

(17). It

absorbe

1.4.3 Wo

T

energy

world’s

D

one geo

on: 1.3 the pAir

The path len

angle. It incr

m is the pref

quantifies th

ed by air and

orld Distrib

The energy

in the sunlig

energy requ

Distribution

ographic loc

path length oMass AM

ngth of the so

reases from u

ferred standa

he reduction

d dust.

ution of Sol

comes from

ght reaching

uirements. (1

of Solar rad

ation to ano

of the solar

olar radiatio

unity for 0o

ard spectrum

in the powe

lar Radiatio

the sun to th

g the Earth s

18)

diation over

other depend

16

radiation thr

on through th

(zenith) to 1

m for solar ce

er of light as

on

he Earth is i

surface is eq

the world is

ding on alti

rough the E

he Earth’s at

1.5 for 48o a

ell efficiency

it passes thr

in the form o

quivalent to

s unequal, a

itude, which

arth’s atmos

tmosphere, c

and 2.0 for 6

y measureme

rough the atm

of Radiation

around 10,0

and its intens

h is combin

sphere in un

changes with

60o. The AM

ents in litera

mosphere an

n. The amoun

000 times of

sity differs f

ned with sea

nits of

h the

M1.5

ature

nd is

nt of

f the

from

ason,

latitude

of poll

geograp

of the d

T

Souther

Solar in

of its g

radiatio

has high

scattere

5.0-6.5

, and atmosp

lution. The

phic areas w

direct annual

Figure1

The most fa

rn parts of A

nsolation in

eographical

n. The equa

h atmospher

d radiation.

KWh/m2/da

pheric cond

following

with favorabl

average gro

1.6 Annual g

avorable bel

Asia. It has o

this region a

location mo

atorial belt l

ric humidity

. Sunshine

ay. The less

ditions, whic

guidelines

le solar ener

ound solar en

global averag

lt (15-35° N

over 3.000 h

averages bet

ore than 90%

located betw

y and cloudi

is estimate

favorable be

17

ch are deter

are useful

rgy condition

nergy compo

ge insolation

N) covers m

h/year of sun

tween 6.5-7.

% of the in

ween (0-15°

iness that te

ed at 2500

elt is the reg

rmined by c

for the b

ns in the wo

onent of sun

n from July

many of Nor

nshine and l

.5 KWh/m2/d

ncident solar

N), is mode

end to incre

h/year wit

gion between

cloud cover

broad identi

orld based o

nlight from 1

1983- June 2

rthern Afric

limited cloud

day (NASA

r radiation c

erately favor

ase the pro

th solar inso

n (35-45° N)

rage and de

ification of

on the collec

983-2005 (1

2005 (19)

can Nations

d coverage.

2011). Beca

comes as di

rable becaus

oportion of

olation betw

). The scatte

gree

the

ction

19).

and

The

ause

irect

se, it

the

ween

ering

18

of the solar radiation in this belt is significantly high because of the higher latitudes and lower

solar altitude. In addition, cloudiness and atmospheric pollution are important factors that tend

to reduce sharply the solar radiation intensity. Solar insolation in this region is between

4.0-5.0 KWh/m2/day. Regions beyond 45° N have less favorable conditions for the use of

direct solar radiation, because almost half of it is in the form of scattered radiation, which

is more difficult to collect for use. However this limitation does not strictly apply to the

potentials for solar UVR applications. In the Southern hemisphere there are three zones of

high radiation; in south America, again on the western coast, moving from latitude 10 S to

30 S; in South Africa, between 10 and 35 S; and in Australia, between 15 and 30 S. (19)

1.4.4 How Much Solar Energy is Available?

The amount of energy from the sun that falls on Earth's surface is huge. All the energy

stored in Earth's reserves of coal, oil, and natural gas is matched by the energy from just 20 days

of sunshine. Outside Earth's atmosphere, the sun's energy contains about 1,300 watts per square

meter. About one-third of this light is reflected back to space, and some is absorbed by the

atmosphere. By the time it reaches Earth's surface, the energy in sunlight has fallen to about

1,000 watts per square meter at noon on a cloudless day. Averaged over the entire surface of the

planet, 24 hours per day for a year, each square meter collects the approximate energy equal of

nearly one barrel of oil each year, or 4.2 kilowatt-hours of energy each day. This number differs

by location and weather patterns. Deserts, because of very dry air and little cloud cover; they

receive more than six kilowatt-hours per day per square meter. Northern climates get closer to

3.6 kilowatt-hours. Sunlight varies by season also, some areas receiving very little sunshine in

the winter. Seattle in December, for example, gets only about 0.7 kilowatt-hours per day. These

data signify the maximum available solar energy that can be captured and used, but solar

19

collectors capture only a portion of this, depending on their efficiency. For example, a one square

meter solar electric panel with an efficiency of 15 percent would generate about one kilowatt-

hour of electricity each day in Arizona. (20)

1.5 Motivation

A photovoltaic solar cell is a proven technology to capture the solar energy and

provides clean and renewable electrical energy that can reduce the world’s dependency on

fossil fuels, and reduce GHG emission, global warming and other related environmental

issues. In spite of the substantial growth over the past decades the high cost of photovoltaic

solar cells has remained a limiting factor for the implementation of the solar electricity in a

large scale. More rapid and widespread implementation of photovoltaic electricity generation

requires more advanced technological developments. Par ticularly, technological inventions

that reduce the cost of photovoltaic electricity substantially could drive a rapid expansion in the

implementation of photovoltaic technology.

Nowadays, the main photovoltaic technology is based on solid-state pn junction

devices, in which semiconductor absorbers produce electron-hole pairs and the electron-hole

pairs are separated by a build-in electrical field in the pn junction to generate electricity. The

main semiconductor absorbers used in solid-state solar cells include polycrystalline silicon,

(21) amorphous silicon, (22) cadmium telluride (CdTe), (23) and copper indium gallium

diselenide Cu (In, Ga) Se2, (24) etc. These types of solar cells have high power conversion

efficiency; however, suffer from high manufacturing and material cost.

Among the material systems currently of interest for this reason are the group of

molecular-based materials combinations often referred to as „organic‟ or „molecular‟

photovoltaic materials. These refer to conjugated molecular species, such as polymers,

20

molecules and dyes, which are capable of absorbing light and conducting charge and

thereby acting as organic semiconductors (25). Their attraction lies primarily in the

possibility of processing such materials directly from solution, and so with bulk synthesis of

the chemical materials and bulk solution processing of photovoltaic modules, the cost of the

photoactive material could fall by an order of magnitude compared even with thin film

inorganic semiconductors (26), (27). Additionally, the less challenging manufacturing

environment, compared, for example, with crystalline silicon wafer production, promises to

reduce the capital cost for production facilities and to make the technology more widely

accessible, especially in developing countries ( 28). Moreover it is also believed that if such

materials are naturally synthesized and stable the cost could be further reduced.

Dye sensitized solar cell (DSSC) is one of the low cost alternatives for the

conventional solar cells, and commercially promising because it is low-cost materials and do

not require highly technology manufacturing facilities. Currently there are three types of dyes

(29) that can produce cells with AM1.5 conversion efficiencies over 10%. Nevertheless, these

types of dyes suffer from the drawback that they are based on the rare ruthenium transition

metal.

Synthetic Asphaltene is one of these materials that primary results shows it can fulfill

the requirements of organic semiconductors; i.e. absorbing a broad range of visible and near

infra-red light. Even though tell now no studies are showing the capability of this material for

conducting charges, and there are no literatures as well as about crude oil asphaltene. Synthetic

Asphaltene has a good ability to absorb visible light. It was also found that this material can be

adsorbed easily to the surface of the nanocrystalline particles TiO2, therefore synthetic

asphaltene is an excellent candidate for use in DSSC.

21

The most important issue regarding Synthetic Asphaltene, those researchers found that it

is similar in characteristics to the crude oil asphaltenes. Therefore, the following chapter will

bring an introduction about crude oil asphaltene in a latest literature survey on asphaltene

chemical and elemental structure, molecular size and shape, and its new application in the

field of photovoltaic, also, it will give the information about Synthetic Asphaltene, it will

review the different methods were used to prepare Synthetic Asphaltene illustrated with

figures and graphs that show the similarity between both crude oil asphaltenes and synthetic

Asphaltene.

1.6 Dissertation Outline

Chapter one of this dissertation give an introduction to the main problem related to

the world energy; increases energy demand, shortage of energy supply and environmental

issues associated with combustion of fossil fuel. The Sun description, its energy source,

total quantity of the solar energy reaching the earth surface, as well as world distribution of

solar energy is also discussed. This chapter also will include brief description about

conversion of sunlight into electricity. The end of this chapter will address to the amount of

solar energy on the earth, the importance of capturing this energy.

In chapter two, this chapter divided into two sections, section one gives an

introduction to crude oil asphaltenes, also it gives reviews about chemical composition,

molecular weight and shape, physical properties of crude oil asphaltenes. The second section

of chapter two will explain Synthetic Asphaltene. In this part; preparation of Synthetic

Asphaltene from different methods will be explained and illustrated with graphs and

figures that show the similarity between Synthetic Asphaltene and crude oil asphaltenes.

22

In the chapter three, discussion the history of photovoltaic, status, its development

and trends in the photovoltaic industry are then will be given, then chapter four goes to a deep

study of dye sensitized solar cell in particular as it is the main subject of this dissertation. It

starts with a brief abstract of the device's history, the technological ground of its

development, its operation principle, the main cell components; materials used to

manufacturing it, its characterization parameters and end up with ways to improve its

performance. The last chapter, chapter five will focus on the results of experimental work and

discussion. Chapter six will illustrate the conclusion and discussion, and the last chapter,

chapter seven will summarized recommendation of this work and the future work.

2.1 Asph

2.1.1 Int

A

high m

Asphalt

sources

I

distillati

similar;

(31) As

known

of the A

the meth

Illustratio

Ch

haltenes

roduction

Asphaltene

molecular w

tene appears

and decomp

In 1837, J.B

ion of bitum

it is insolu

sphaltene ex

as C7-Asph

Asphaltene “

hod of precip

(a) n pen

on 2.1 the apsolv

hapter 2: A

is a part of c

weight bond

brown or bl

poses when t

B. Boussing

men: insolub

uble in n-alk

xtracted usin

haltenes. Th

solubility cl

pitation. (32

tane (n-C5)

ppearance ofvents (a) n pe

Asphalten

crude oils th

ded aromatic

lack in color

the temperat

gault define

le in alcoho

kanes, such a

ng n- pentan

he amount, c

ass” vary sig

2); (33); (34)

f asphaltenesentane (n-C5

23

nes and Syn

hat contain a

c hydrocarb

r, and the me

ture exceeds

ed the term

ol and solubl

as n-pentane

ne known a

chemical com

gnificantly w

; (35); and (

(b) n

s extracted fr5) and (b) n-

nthetic As

a large numb

bons compo

elting point d

s 300-400 °C

of asphalte

le in turpent

e or n-hepta

as C5-Asph

mposition, a

with the sour

(36)

n-heptane (n

from Mars-Pheptane (n-C

sphaltene

ber of struct

onents with

differs with

C. (30)

enes as the

tine. The de

ane, and sol

haltene and

and molar m

rce of the cru

n-C7).

P crude oil usC7).

tures, in spec

h hetero-ato

oil geograph

residue of

finition toda

luble in tolu

with n-hep

mass distribu

ude oil and w

sing of differ

cific

oms.

hical

f the

ay is

uene.

ptane

ution

with

rent

24

2.1.2Elemental Composition

The heteroatoms (nitrogen, oxygen, sulfur, and others) play an important role in the

formation of the intermolecular forces responsible for the physical stability of asphaltenes.

(53)

The main elemental components of petroleum asphaltene are hydrogen and carbon.

Ouchi, ( 1985) has shown that H/C ratio is a linear function of the portion of aromatic

carbons of petroleum fractions as measured by C13-NMR (carbon nuclear magnetic

resonance). (37) The H/C ratios of asphaltene averaged between 1.1:1 to 1.4:1 (38), and

(34). C13 nuclear magnetic resonance (NMR) and Carbon K-edge X-ray absorption near-edge

spectroscopy (XANES) studies shows that about 50 % of the carbon is aromatic and the rest

saturated. (39)

Speight in 1991 reported that the elemental composition of 57 different asphaltene from

8 countries. Speight found that, carbon and hydrogen contents of asphaltene do not vary

significantly; however, the proportion of hetero-elements, such as oxygen, sulfur and nitrogen

varies significantly from 0.3 to 4.9% for oxygen; from 0.3 to 10.3% for sulfur; from 0.6 to 3.3%

for nitrogen. (40) The hydrogen atoms contained in saturated groups while 40% of the carbon

contained in aromatic structures. (41)

2.1.2.1Sulfur

Sulfur is the most abundant element found in asphaltene. The average sulfur content

of asphaltene varies from 2-6 %, but it can be identified at a concentration above 10 %.

(42) Sulfur forms identification in asphaltene has been studied by the application of x-ray

absorption spectroscopy by George et al., (1990); and by XANES (X-ray Absorption near

Edge Structure) spectroscopy. (43) The data gave a clear demonstration of the existence of

25

nonvolatile sulfide and thiophene sulfur in the asphaltene with thiophene species are the

most dominant. Other forms of sulfur that found to occur in asphaltene include the alkyl–

alkyl sulfides, alkyl–aryl sulfides and aryl–aryl sulfides. (44); (45)

2.1.2.2 Nitrogen

Studies on the disposition of nitrogen in petroleum asphaltene indicated the existence

of nitrogen as various heterocyclic types. (46); (47); (48) Much of the nitrogen believed

to be in aromatic locations.(49) There is also evidence for the occurrence of carbazole

nitrogen in Asphaltene.(50) and; (51) Application of XANES spectroscopy to the

determination of nitrogen species in asphaltene, confirmed the presence of pyrrolic nitrogen

and pyridinic nitrogen being the two major types of nitrogen. (52) In fact, the technique

showed the predominance of pyrrolic nitrogen in the samples examined.

2.1.2.3 Oxygen

The presence of oxygen in asphaltene evaluated mainly through elemental analysis and

by chemical reaction. The total content of oxygen in asphaltene from different sources may

differ from 1% to 7% by weight. Oxygen is found to exist in asphaltene as in carboxylic,

phenolic and ketonic forms (46), hydroxyl and ester groups. (53)

Some evidences for the location of oxygen within the asphaltene fractions acquired

by infrared spectroscopy. Examination of dilute solutions of the asphaltene in carbon

tetrachloride shows that at low concentration (0.01% wt/wt) of asphaltene a band occurs at

3585 cm-1, which is within the range anticipated for free non-hydrogen-bonded phenolic

hydroxyl groups. In keeping with the concept of hydrogen bonding, this band becomes barely

noticeable, and the appearance of the broad absorption in the range 3200 to 3450 cm-1

26

becomes evident at concentrations above 1% by weight. (46)

2.1.2.4 Metal Content Several metals (e.g., Ni, V, Fe, Al, Na, Ca, and Mg) shown to accumulate in the

asphaltenes fraction of crude oil, typically in concentrations less than 1 % w/w. (54), (55),

(56), and (57) Vanadium and nickel, the most abundant of the trace metals, present mainly as

chelated porphyrin compounds, and they linked to catalyst poisoning during upgrading of

heavy oils (58), (59). The concentrations of other trace metals not bound in porphyrin

structures (e.g., Fe, Al, Na, Ca, and Mg) indicated to change in deposits as a function of

well depth (60), and amongst sub fractions of asphaltene. (54), (56)

2.1.3 Structure of Asphaltene

Beca use of the complexity of asphaltene molecules, finding the exact structure of

asphaltene has proven to be an overwhelming task. As discussed by Ruiz-Morales, and

Mullins, 2007 (52), the structure of asphaltene investigated by different physical methods

include infrared spectroscopy (IR), nuclear magnetic resonance (NMR), electron spin

resonance (ESR), a l s o mass spectroscopy, X-ray, a n d ultra-centrifugation, electron

microscopy, and small angle neutron scattering SANS, and small angle X-ray scattering

SAXS, quasi-elastic light scattering spectroscopy, vapor pressure osmometry (VPO) and gel

permeation chromatography (GPC).

Even though, the complete structure of asphaltene not yet completely discovered,

some common structures were established. Asphaltenes believed to consist of condensed

aromatic rings that carry alkyl and salicylic systems with hetero elements (i.e., nitrogen,

oxygen and sulfur) scattered throughout in various, including hetro-cyclic locations.(61), (52)

The aro

correspo

of rings

fluores

of aspha

enormou

a small

A

propose

three as

chemica

are shap

fingers r

omatic carb

onding H/C

s in a single

cence depo

altenes and

us, some wi

ring system

At the risk

ed. These th

sphaltene str

al speciation

ped like you

representing

bon content

atomic ratio

e fused ring

olarization te

related comp

th nitrogen,

m, an occas

Figure: 2.1

k of oversim

hree structure

ructures con

n, and aroma

ur hand with

g the periphe

of asphalt

o of 1.0-1.2

g system aro

echnique app

pounds. It fo

others with

sional molec

Idealized m

mplifying, t

es showed in

nsistent with

atic rings sh

h the palm r

ral alkane su

27

tenes is in

. The NMR

ound 7. (62

plied to surv

found that, th

sulfur, some

cule with a

molecule stru

three idealiz

n figure 2.1

h overall m

howed with

representing

ubstituent. (6

the range

R results sho

2) Groenzin

vey the mole

he variability

e with a big

metal, a po

ucture of asp

zed asphalte

. Idealized m

molecular siz

h darker lines

the core aro

64), (65)

of 40 to

ow that the a

and Mullin

ecular size o

y in asphalte

g ring syste

orphyrin, etc

phaltene

ene structur

molecular st

ze, aromatic

s. (63) Aspha

omatic ring

60 %, wit

average num

ns, (2000), (

of a broad ra

ene molecule

em, others w

c.

res have b

tructures for

c ring syste

altene molec

system and

th a

mber

(63),

ange

es is

with

been

r the

ems,

cules

d the

28

2.1.4 Asphaltene Molecular Weight

The molar mass of asphaltene is a topic of continued controversy over the last

decades beca use of its tendency to aggregate or associate even at low concentration. The

tendency of asphaltenes to aggregates in toluene at concentrations as low as 50 mg/liter. (66)

Though, with the development of more advanced techniques the discussion on

molecular mass of asphaltene seems to be over. In the past, the molar mass of asphaltenes

was measured by different methods such as Vapor Pressure Osmometry (VPO), mass

spectrometry (MS), size exclusion chromatography (SEC), and scattering phenomena such

as small angle X-ray (SAXS), and small angle neutron scattering (SANS). Molecular

mass determined by light scattering methods (SAXS, SANS) and fluorescence depolarization

technique differ by as much as a factor of 10 or more. It understood that many of

these techniques measured molar mass of aggregated asphaltene or micelles than a single

asphaltene molecule. Moreover, the molar mass of asphaltene depends on the technique

and experimental condition (time, concentration and temperature) employed for the

measurement. Size exclusion chromatography yielded average molecular weights as high

as 10,000 amu. (66) In different research, results of laser desorption mass spectroscopic

yielded asphaltene molecular weights of 400 amu with a range of roughly 200-600 amu.

Fluorescence depolarization measurements indicated the molecular weights of 750 amu with

a range of roughly 500-1000 amu. (63)

Some difficulties on using MS to measure the molar mass of asphaltene because of

that some ionization methods may cause aggregation, fail to ionize some components, or

break high- molecular-weight species into smaller pieces. Nevertheless, two recently

developed MS methods look to correct those difficulties. Both methods indicate molecular-

29

weight of asphaltene between 600-1100 amu. The first is two-steps laser desorption ionization

MS (L2MS), used by Pomerantz, et al (2008) and coworkers. (68) In this technique, an

infrared CO2 laser desorbs species from the surface by heating them enough to pop them off as

neutral molecules but not enough to break them apart. The researchers found a molecular

weight distribution that peaks around 600 amu and extends to more than 1,000 amu. The

second new method is laser-induced acoustic desorption (LIAD), utilized by Shea and

coworkers. (69) LIAD gets around this problem by evaporating asphaltene as neutral

molecules rather than ions. Shea 2007 and coworkers deposit samples on titanium foil and fire

a laser at the opposite side. The laser energy converted to a sound wave that travels through the

foil and pushes molecules off the other side. The molecules then subjected to electron

ionization, which non-selectively ionizes all organic compounds; they found a molecular

weight of asphaltene ranges from 400 to 1,100 a.m.u. the study of asphaltene samples from

around the world shows that the molecular weight distributions vary depending on

geographic origin. For example, Brazilian asphaltene samples have a lower molecular

weight range than North American asphaltene. (70)

2.1.5 Asphaltene Size and Shape

The asphaltene particle size and shape widely differ in literature (69) indicated that

the diameter of disk-like asphaltene entities in tetrahydrofuran for safanya asphaltene

fractions is about 13 nm. In 2004, Rajagopal and Silva (71) measured the spherical particle

size of asphaltene dissolved in toluene by light scattering method, and they projected it to be

23 nm even though at very low concentration of about 1 ppm. The freeze-fracture-

transmission electron-microscopy (FFTEM) technique used to study the asphaltene particle

diameter in toluene medium, and it was found to be in the range of 7-9 nm. (71) Another study

30

carried out by Chianelli et al. (2007) (64) by using WAXS wide angle x-ray scattering and

SAXS on Venezuelan and Mexican Asphaltene, show the presence of the asphaltene

particles within sizes in the range of 3–5 nm. Therefore, the review of the literatures indicates

that the asphaltene particle diameter varies between 3 and 23 nm. The variation in data is

due to the difference in solvents, techniques, and concentration range used. The variety of

shape of asphaltene includes thin-disk, spherical, fractal-like, oblate ellpsoid, prolate and

discoid. (72) (78)

Results from analytical methods such as time-resolved fluorescence depolarization

(TRFD) (73); Nuclear magnetic diffusion (74); Fluorescence correlation spectroscopy (FCS)

(75); DC conductivity (76, 77) show that the size of asphaltene molecules is ~ 1.5 nm.

2.2 Synthetic Asphaltene

Unlike crude oil asphaltenes, Synthetic Asphaltene does not get more attention from

researchers. However, Jones, Ignasiak, and Straus, (1978) (79) started working on preparing

Synthetic Asphaltene. In their study, they found the possibility of producing Synthetic

Asphaltene with different ways. They first used reaction of cholesterol with sulfur, also

reaction of maltenes with sulfur, and finally they found that synthetic asphaltene prepared in

the lab has similar properties to those of natural asphaltene. The analysis results of the

Synthetic Asphaltene showed that the 1H and 13C NMR and IR spectra were qualitatively

similar to those of the Athabasca asphaltene as shown in the Figures 2,3,and 4 below. From

this study, the researchers confirmed that asphaltene could be formed in nature by the reaction

of smaller molecules either of biological origin or already present in petroleum with sulfur at

moderate temperatures leading to a polymer with carbon-sulfur bridges. The Synthetic

Asphaltene consists of small units held together by sulfur bridges.

Figure (2

2.3) (a) IR s

(b) IR s

Figure

spectrum of

spectrum of

2.2 Chemic

Athabasca a

Synthetic A

31

cal Structure

asphaltene.

Asphaltene pr

of Choleste

repared from

erol

m cholesteroll and sulfur.

32

Figure (2.4) (a) 1 H NMR spectrum of Athabasca asphaltene.

(b) 1 H NMR spectrum of Synthetic Asphaltene prepared from cholesterol and sulfur.

Figure (2.5) (a) UV spectrum of Athabasca asphaltene

(b) UV spectrum of Synthetic Asphaltene prepared from cholesterol and sulfur

a

b

T

they var

polyarom

asphalten

formation

Synthetic

They als

liquid cry

show sim

The follo

compund

Figure (2

The chemical

ried in thei

matic discoti

ne. This wo

n and the ph

c Asphaltene

so explained

ystal-like mu

milar physica

owing figure

ds and also s

2.6) Structure

l compositio

ir derivativ

ic molecule

ork will he

hysical char

e can be pr

d the synthe

ultikylated a

al properties

shows the g

hows molec

e of discotic

ons of aspha

e sources.

es can be u

elp research

racteristics o

roduced by

etic formatio

aromatics. Th

to real asph

general form

cular simulat

c liquid cryst

33

altenes comp

Russell et

used in a t

hers deeply

of Synthetic

pyrolysis TO

on of aspha

he thermolys

altenes.

mula of highly

tion of the TO

tal where R=

plex during o

al. 2007

thermal sim

understand

Asphaltene

OCP (tetra

altene partic

sis products

y oriented d

OCP.

=undecyl, he

occurring in

(64) found

mulation mo

d the proces

s. Chianelli

octyl carbo

les by therm

of the disco

discotic liquid

eptyl, hexyl,

n nature, and

d that simp

odel of synt

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et al. found

xylate peryl

malizing dis

tic liquid cry

d crystaline

undecanoyl

d also

lified

thetic

altene

d that

lene).

scotic

ystals

Illustrattion (2.2) Relaxed simu

34

ulation of thee TOCP preccursor moleccule.

Figure (2

Figure(2

2.7) Diagramfree rad

.8)Schematictreatmencondens

m of pyrolysidicals.

c diagram ont of TOCsation.

is of TOCP f

of aliphatic CP at 360-4

35

follwed by s

and polyaro4200C invo

elf-assembly

omatic thermolving radic

y of asphalte

mal productcal couplin

ene from a se

ts upon theng and arom

ea of

heat matic

W

ester of T

below w

thermally

fragment

below sh

Residuum

position a

Figure (2

When the tem

TOCP becom

when temper

y converted

ts. Finally, C

hows the IR s

m), and it is

and intensity

2.9). InfraredArab Vsyntheti

mperature in

mes the wea

rature becom

into a sea o

Chianelli et a

spectrum of

clear that a

y of the IR sp

d spectra of Vacuum Resiic and the HA

ncreased dur

akest region

mes above

of free radic

al. successfu

TOCP, Syn

absorption b

pectrum of H

f the TOCP pide) asphaltAVR asphal

36

ring the ther

for thermal

4200C in t

cals containi

ully prepared

thetic Aspha

ands of Syn

HVAR. (64)

precursor, Sene showingltene.

rmal proces

l cleavage. T

the absence

ing aromatic

d Synthetic A

altene, and H

nthetic Asph

)

Synthetic Asg the signif

s above 350

This is clear

e of air, TO

c cores and

Asphaltene, a

HVAR (Hea

haltene match

sphaltene andficant simila

00C, the alip

r in Figure (

OCP compl

long-chain

and Figure (

vy Arab Vac

h well with

d HAVR (Harity betwee

phatic

(2.10)

letely

alkyl

(2.10)

cuum

band

Heavy n the

37

The comparison in chemical peoperties between Synthetic Asphaltene and HVAR illustrated in

the table below

Table (2.1) Comparison of some chemical properties of Synthetic Asphaltene and HVAR

Chemical properties of synthertic asphaltene and HVAR

H/C %C Aromatic

%C

Aliphatic MW

Synthetci asphaltene 1.06 58 42 2,750

HVAR asphaltene 1.10 33-45 55-65 1.000-10,000

38

Chapter 3: Photovoltaics

3.1 Photovoltaic Effect

3.1.1 Introduction

The photovoltaic effect represents the generation of a potential difference at the junction

of two different materials in response to visible or other forms of radiation. The whole area of

solar energy conversion into electricity is named “photovoltaics”. (88)

Photovoltaic comes from the words photo meaning “light” and volt, a measurement of

electricity. Sometimes photovoltaic cells called PV cells or solar cells for short. (80). In another

definition, Photovoltaics means “light-electricity”, as “photo” is a stem from the Greek word

“phõs” meaning light and "Volt” is an abbreviation of Alessandro Volta who was a pioneer in

the study of electricity. Since a layman often does not know the meaning of the word

photovoltaics, a popular and common term to refer to PV solar energy is solar electricity. (88)

The photovoltaic process combined of four stages includes light absorption, charge

generation, charge transport, and charge collection. (81) Light absorption occurs when the

material has a semiconducting property that responds to incident waves. The absorption

characteristic is based on the band gap of the semiconducting material, and its intrinsic extinction

coefficient. Then charge generation. When the incident photon hits electrons at the ground state,

inorganic semiconductors generate free carriers. Though, in organic semiconductors, excited

electrons slightly relax and then form an exciton, a bounded electron and hole pair. To make an

efficient organic photovoltaic cell, effective dissociation of excitons is an important issue as the

binding energy of the exciton is large. (82)

39

3.1.2 History

In 1839 Edmund Becquerel found that some materials could generate a small amount of

electric current when exposed to light. This phenomenon is called the photovoltaic effect. It is

the essential process in which a solar cell changes sunlight into electricity. In 1905, the nature of

light and the photoelectric effect was explained by Albert Einstein based on photovoltaic

technology. In the 1970s, during the energy crises, photovoltaic technology got credit as a source

of power for non-space applications. In the 1940s and 1950s, steps were taken for the

commercialization of photovoltaics when the Czochralski process developed for producing

highly pure crystaline silicon. In 1954, at Bell Laboratories using the Czochralski process,

scientists developed the first crystalline silicon photovoltaic cell with a conversion efficiency of

4%. (83)

In the 1990s and 2000s, interest in photovoltaics expanded. In large part, this interest was

prompted by the widespread deregulation of the energy supply and environmental topics related

to global warming, which has increased a desire to secure alternative energy sources. (84)

3.2 Application of Photovoltaic Technology

Throughout the research and development stage of photovoltaic technology for space

applications, researchers never considered this technology practical for mass power production on

Earth. Oil and coal fossil fuels are extremely cheap compared with electricity production costs of

silicon solar cells. (85)

In the 1970s two oil crises facilitated research on photovoltaic technology that improved

the performance of PV cells. Research focused on developing device physics and process

technology. (86) As a result, establishment of the biggest photovoltaic companies took place

40

between1970 and 1979. In 1970, Solar Power Corporation was founded. Three years later, in

1973, Solarex Corporation established. At Delaware University a solar photovoltaic-thermal

hybrid system, one of the first photovoltaic systems for domestic application, was developed. A

silicon solar cell of US$30 per W was produced. In 1974, the Japanese Sunshine project

commenced. A year later, in 1975, Solec International and Solar Technology International was

established (87).

Many important events in the field of photovoltaic occurred in 1980-1990. Over 1 MW

was generated per year with photovoltaic modules by ARCO Solar. BP purchased Lucas

Energy Systems and entered the solar industry business. Saudi Arabia installed solar powered

seawater desalination systems with 10.8 kW peak power in Jeddah. Volkswagen tested

photovoltaic systems placed on vehicle roofs with 160 W peak powers for vehicle start up. In

California in 1984, 1 MW photovoltaic power plant started to work in Sacramento. The same

year ARCO Solar introduced the first amorphous modules. In 1985, researchers of the

University of New South Wales in Australia built a solar cell with more than 20%

efficiency. BP built a power plant in Sydney and shortly after another one in Madrid. In

1986, the first commercial thin film photovoltaic module was presented by ARCO Solar. In

1989, BP obtained a patent in thin film technology for a solar cells production.

The total commercial production of solar cells in 2004 was 1GWp and was 1250

MWp. Crystalline silicon based solar cells contributed up to 95% of the total production and

was the dominant solar cell technology. The a-Si:H technology contributed with 4% of the

total production in 2004. The only non-silicon technology that was delivering commercial

modules was the CIS and CdTe technology with almost 1% of the total production. In 2004,

most of the solar modules were manufactured in Japan (47%). The USA contributed with

41

11%, and Europe with 27%, the rest of the solar cells was produced in countries such as

China, India and Australia. (88)

3.3 Environmental Effects from Operation of PV Systems

Normally there are no effects of PV systems operation on the environment. They do not

make noise, solid waste, or other pollutants that could damage the environment. Thus, PV

systems and the electricity they generate make an important contribution to the protection of the

environment. PV systems do not generate emissions during operation and all of their life cycle

emissions of carbon dioxide are indirect ones. For example, in industrial countries each kWh that

is generated by PV plants avoids an output of the greenhouse gas carbon dioxide. In Germany,

each kWh generated by PV systems avoids nearly 650 grams of carbon dioxide. Compared with

fossil fuel technologies, photovoltaic technology has various environmental advantages in

generating electricity. For example, carbon dioxide emissions and other pollutants become

negligible by applying photovoltaic technology. The most major environmental problem

associated with photovoltaic systems results from the use of toxic chemicals like cadmium

sulfide and gallium arsenide in the manufacturing process.(87) Because these chemicals are very

toxic and stay in the environment for centuries, the disposal of unused cells could cause

significant problems to the environment. Though, the most useful cells with low cost, mass

production, and high efficiency are those manufactured by using silicon. This material produces

the cells with less expense and is environmentally friendly and safer than the cells produced by

heavy metal. (89)

3.4 Orga

T

revealed

30 year

organic

commer

increase

as meth

(93).

Figure: 3

T

PCS. Th

to prep

complex

photosy

anic Photovo

The first or

d in the beg

rs later exac

materials

rcial potenti

ed research

hylene blue,

3.1. Some of phtalocy

The first act

his class of

are, highly

xes with

ynthetic proc

oltaic

rganic comp

ginning of th

ctly in the l

as photor

ial to produ

into photoc

that had s

f the early invyanine and C

tual organic

compounds

colored, ha

a number

cesses, the

pound in wh

he 20th cent

late 1950s a

eceptors in

uce image m

conductivity

semiconducti

vestigated orChl-a.

photovoltai

remained a

ave good se

of metal

study of de

42

hich photoc

tury by Poc

and beginni

n imaging

making mach

y and relate

ing properti

rganic molec

c cell invest

among the m

emiconducti

ions. Bec

evices fabric

onductivity

chettino (90)

ing of the

systems w

hines beside

ed subjects.

ies were dis

cules. Top: T

tigations we

most investig

ing properti

cause of th

cated from m

observed w

) and Volm

1960s the

was acclaim

es scientific

Many comm

scovered in

TPP and ant

ere tested on

gated becaus

ies and the

he analogy

many biolo

was Anthrac

er (91). Exa

possible use

med (92).

interest led

mon dyes, s

the early 19

thracene. Bo

n porphyrins

se they are e

y readily f

y with nat

gical molec

cene,

actly

e of

The

d to

such

960s

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cules

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3.5 Mech

T

localizati

photovol

spontane

separated

features a

devices u

roughly i

junctions

achieved

proposal

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The main dif

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used to ach

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.

Figure

orophylls an

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Organic Ph

fference bet

excited state

sion that, fi

er an electro

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n determinin

hieve photo

lar and poly

ganic/inorgan

efficiency a

(3.2) Differe

nd other porp

e organic pho

otovoltaic E

tween organ

es (See Figu

first, the exc

on–hole pair

wly through

ng organic p

conversion

ymer organic

nic hybrid c

among all the

ence betwee

43

phyrins, and

otovoltaic ef

Effect

nic and inor

ure 3.2). Th

cited state c

at room tem

h the materi

photovoltaic

in organic

c solar cells

cells as dye

e above men

en organic an

d the structu

ffect in orga

rganic semic

his has the

created by

mperature an

ial towards

device desig

materials.

(95), flat-lay

e-sensitized

ntioned type

nd inorganic

urally related

anic photocel

conductors i

important c

photon abso

nd, second, t

the electro

gn, and creat

These cells

yer systems

solar cells

s and will be

semiconduc

d phtalocyan

lls.

is the degre

consequence

orption doe

the charges,

odes (94). T

te many diff

s can be div

and bulk he

(96). The

e the topic o

ctor

nines

ees of

es for

s not

once

These

ferent

vided

etero-

latter

of this

44

Upper panel, in a crystalline inorganic semiconductor, the photo generated exciton

delocalized over many atoms, and readily dissociates into electron–hole pairs. So a photon

absorbed anywhere in an optically thick device generates separate charges. Lower panel, in an

organic semiconductor, the photo generated exciton localized on a single molecule and

dissociates into separate charges only when the exciton created close to an interface. Therefore,

charge pairs generated only in regions that lie within an exciton diffusion length of an interface.

Even though, exciton dissociation can occur efficiently at the bonding between metal

electrode and an organic semiconductor, the effective charge-separation efficiency at such an

interface is limited because of the high likelihood of recombination of the separated charges at

the metallic electrode (94). A main requirement of almost all organic photovoltaic devices is an

interface between two materials of different electronic properties, the purpose of which is to

cause exciton dissociation (97). After photon absorption is created, the exciton in a molecular

material can make several things it can diffuse to other molecules segments of the same material

by a series of hops, and can decay by emission of a photon. It may also decay into a more long-

lived and lower energy triplet state. It can transfer on to a different type of molecule. It can

dissociate via transfer of an electron or hole to another molecule or a metal electrode (97).

Exciton dissociation can occur when the exciton reaches an interface with the other material and

when it is most energetically favorable for the charges to separate. In the case of dye-sensitized

cell, this normally requires that a suitable level for the (HOMO) highest occupied molecular

orbital and the (LUMO) the lowest unoccupied molecular orbital of the photo-sensitizer

matching the iodine redox potential and the conduction band edge level (Ecb) of the TiO2

electrode. The HOMO level must be more positive than the iodine redox potential to accept

45

electrons, and the LUMO level must be more negative than the Ecb of the TiO2 electrode to

inject electrons (98).

To drive photocurrent, the separated charges should both be able to find pathways

towards the electrodes and continuous phases of donor material between the charge separation

interface and hole-collecting electrode and of acceptor material between the interface and

electron-collecting electrode needed. For charge separation to occur, the exciton should reach

the interface before it decays by another process. Radiative exciton decay typically occurs in

hundreds of picoseconds. The distance that an exciton can diffuse in a molecular semiconductor

on this timescale, called the exciton diffusion length, is typically a few nanometers. Therefore,

efficient light harvesting can occur only within approximately one exciton diffusion length of a

donor–acceptor interface (99)

3.6 Photovoltaic Cell Performance

In general, the percentage of the solar energy shining on a PV device that converted into

electrical energy or electricity is defined as the conversion efficiency of a photovoltaic (PV) cell.

Improving the conversion efficiency of PV devices is an important aim of study and helps make

PV technologies low cost with more traditional sources of energy. (80)

In general, a photovoltaic cell is a device that converts solar energy to electrical energy.

Electrical power production under illumination can be attained by the photovoltaic cell.

Photovoltaic cell has the ability to generate voltage over an external load, and current through the

load. This is illustrated by the current-voltage (IV) curve of the cell at the certain temperature

and illumination (Figure 3.3).

When the cell is short circuited under illumination, the maximum current and the short

circuit current (ISC) generated while no current can flow, and the voltage at its maximum called

46

the open circuit voltage (VOC) under open circuit conditions. The point in the IV-curve

producing maximum a product of current and voltage is called the maximum power point (MPP).

Fill factor (FF) is another important characteristic of the solar cell performance. The ratio

between the maximum power generated and the product of the short-circuit photocurrent density

( ) and the open-circuit voltage (Voc) is known as fill factor. The higher value of fill factor

allows for better quality of the device.

.

. 1

Using the fill factor, the maximum power output of the solar cell can be written as

. . 2

While the physical principles of different types of photovoltaic cells different, the current-

voltage curve of good performance cells are the same and can be characterized with each other in

terms of FF, VOC, and ISC. The energy conversion efficiency of the solar cell is defined as the

power produced by the cell (PMAX) divided by the power incident on the representative area of

the cell (Plight):

100% .100 3

The temperature of the cell is an essential factor that affects the efficiency of the solar cell, and

which is even more essential on the quality of the illumination, i.e. the total light intensity and

the spectral distribution of the intensity.

F

cells test

condition

that of A

power of

Wp at the

or this reaso

ts between

n used for te

AM1.5 globa

f the solar ce

ese condition

Figu

on, a standar

different lab

esting of terr

al standard s

ell is the no

ns. (88)

ure (3.3) Cur

d measurem

boratories. T

restrial solar

solar spectru

minal powe

47

rrent-Voltag

ment conditio

The light in

cells. The s

um and temp

r of the cell

ge (IV) curve

on was impro

ntensity is

spectral distr

perature of

l, or module

e

oved to comp

1000 W/m2

ribution of th

the cell is 2

, and report

pare similar

2 at the stan

he light sour

25°C. The o

ed in peak w

solar

ndard

rce is

output

watts,

48

Chapter 4: Dye Sensitized Solar Cells DSSCs

4.1 Introduction

The depletion of petroleum resources in this century and raising awareness of

environmental change caused by the combustion of fossil fuels and the exploration of renewable

energy sources, such as solar energy are more important issues for all world countries.

Furthermore, photovoltaic devices have become a favorable replacement energy source in the

last century with an expected increase in significant contribution to overall energy production

over the coming years. Silicon-based technologies also developed as new techniques of solar

energy, but they are not yet competitive with fossil fuels because of the high production costs.

The photovoltaic field now challenges the development of new nanocrystalline and conducting

polymer films based devices, with a very low-cost production and attractive features such as

transparency, flexibility, etc. that could facilitate the market entry. Amongst all of them, dye

sensitized solar cells (DSSC) are devices that appear to reach moderate efficiencies, and thus are

feasible competitors to conventional cells. (100, 101)

Dye Sensitized Solar Cell DSSC is one of the most promising types of solar cell that has

attracted much attention in the scientific community as it already belongs to the third generation

of photovoltaic (PVs). Dye-sensitized solar cell (DSSC) for its basic operation does not depend

on the principle of a p-n junction as in common solid state solar cells that are based on crystalline

silicon. (102). The DSSC can be classified as a photo electrochemical (PEC) solar cell because

of its utilization of photons, charges, and electrolyte for its basic operation (103). Meanwhile,

O’Regan and Grätzel invention by 1991, (100) DSSC attracted widespread academic and

industrial interest because it has advantages that previously did not exist in the traditional

photovoltaic cells. Based on the reports in the literature, DSSCs are easily fabricated at a low-

49

cost, are environment friendly and have high-energy conversion efficiency (104). One more

advantage of DSSCs is that changes in temperature do not affect the performance, while

temperature changes can affect the performance of conventional silicon solar cells (105)

4.2 History and Development

The basis of inventing the DSSC is associated with three main concepts in

technological developments photovoltaic, sensitization and nano-technology. The historical

events of the first one have already been addressed. The idea of modifying the band gap of

semiconductors to light of a wavelength longer than its corresponding band gap

(sensitization) was discovered accidentally and connected to the beginning of photography

dated back in 1873.

It was an exciting connection between photography and photo electrochemistry,

both of which depend on photo-induced charge separation at a liquid solid interface.

Silver halides crystals first used in photography had band gaps ranging from 2.7 to 3.2 eV

and insensitive to wavelengths longer than 460 nm. To make it photosensitive to longer

wavelengths associated dyes with the halide semiconductor grains and expand the photo

response to longer wavelengths, (106) that was the first film, able to deliver the image of a

sight practically into black and white picture. Moser shortly afterward used the same

analogy to make the first sensitization of a photo- electrode, (107) and was confirmed by

Rigollot in 1893 (108). However, it took scientists more than 80 years to understand that their

operating mechanism is by injection of electrons from photo-excited dye molecules into the

conduction band of the n-type semiconductor substrates (109) which dated from the 1960s. In

the following years, the idea improved that the dye could perform most efficiently if

chemicals adsorbed on the surface of the semiconductor.

50

Nanotechnology is a concept recognized long ago, and humans without knowing

employed nanotechnology, in making steel, paintings and in strengthening rubber. In the

early 1980s, nanotechnology got its biggest boost with two main developments, the birth of

cluster science and the innovation of the scanning tunneling microscope (STM). In 1985, this

development allowed for the discovery of fullerenes and the structural assignment of carbon

nano-tubes a few years later. In the field of semiconductors, the synthesis, electrical and

chemical properties of semiconductor nano-crystals were studied. This led to a fast

increasing number of semiconductor nano-particles of quantum dots and photovoltaic.

The use of dye-sensitization processes in photovoltaics stayed ineffective until the

early 1980s when it combined with the nanotechnology revolution in the Laboratory of

Photonics and Interfaces in the EPFL in Switzerland. I n 1 9 9 1 , Grätzel and his co-

workers established a solar cell with energy conversion efficiency above 7 %. They

successes by a combining nano-structured electrodes and efficient charge injection dyes (100),

and achieved 10% in 1993 (110). This solar cell is called the dye sensitized nano-structured

solar cell or the Grätzel cell due to the name of its inventor.

DSSCs differ from conventional semiconductor devices. They separate the function of

light absorption from charge carrier transport. Dye sensitizer solar cells absorb the incident

sunlight and exploit the light energy to induce electron transfer reaction. Therefore, DSSCs have

some advantages compared to Si based photovoltaics:

(1) DSS are not sensitive to defects in semiconductors such as defects in Si.

(2) The SEI is easy to form, and it is cost effective for production.

(3) In DSCC, it is possible to understand the direct energy transfer from photons to chemical

energy. (111)

51

4.3 Brief Description of DSSC

DSSCs join the light absorption and charge separation process by the involvement of a

sensitizer as light absorbing material with a wide band gap semiconductor usually titanium

dioxide. As early as the 1970s, it was found that titanium dioxide (TiO2) could split water with a

small voltage after exposed to light (112). However, with the large band-gap for TiO2, which

makes it transparent for visible light, the conversion efficiency was low when using the sun as an

illumination source. Dye sensitization of semiconductor electrodes dates to the 1960s (113).

Since only a monolayer of adsorbed dye molecules was photoactive, light absorption was low

and limited when flat surfaces of the semiconductor electrode were employed. This problem was

solved by introducing polycrystalline TiO2 (anatase) films with a surface roughness factor of

several hundred (114, 115). The amount of adsorbed dye increased more by applying

mesoporous electrodes, presenting a huge active surface area, and cells combining such

electrodes, and a redox electrolyte based on iodide/triiodide couple produced 7% conversion

efficiencies in 1991 (100). The current highest energy conversion efficiency is over 11% (116),

and an extra increase in efficiency is possible by designing proper electrodes and sensitization

dyes.

Both a scheme and an energy level diagram of a liquid electrolyte dye sensitized solar

cell are illustrated in Figure 4.1. Usually, DSSC contain one electrode made of a layer of

titanium dioxide nanocrystals with an average crystal size of around 20 nm sintered together to

allow electronic conduction take place. A monolayer of a sensitizer dye, usually a ruthenium

polypyridyl complex, is attached to the surface of the nanocrystalline electrode. This mesoporous

film is deposited onto a conductive, transparent substrate, indium tin oxide (ITO) or fluorinated

SnO2 (FTO), and dipped with a redox electrolyte, essentially containing I-/I3- ion couples. This

electrolyt

Light is

conductio

oxidized

the redox

electron m

Figure4.1

te is also p

then collect

on band of

dye transpo

x electrolyte

migration th

1. Cross secconditiorepresen

put in contac

ted by the d

the nanocry

orts the elect

e, where the

hrough the ex

ction view oons (left), annt the energe

ct with a co

dye producin

ystalline sem

tron accepto

e I3- back re

xternal load.

of the designd energy letics of opera

52

olloidal plat

ng photo-exc

miconductor,

ors species (

educed to I-

gn of a dye levels of theation of such

tinum cataly

cited electro

then into th

I3-), reduced

- and the ele

sensitized se different h devices (ri

yst coated c

ons that are

he conductin

d to the cou

ectrical circu

solar cell uncomponentsight).(101)

counter-elect

injected int

ng substrate

unter-electrod

uit complete

nder illumins of the cell

trode.

to the

. The

de by

ed by

nation l that

53

4.4 Requirements of Efficient Dye in DSSC

Ooyama, Y. and Harima, Y. (2009) have done an extensive literature review on the main

requirements for efficient organic molecule (dye) used so far for DSSCs. Their conclusion is

summarized as follows:

The organic molecule (dye) should have as a minimum one anchoring group like

COOH, –SO3H, –PO3H2, –OH for adsorption onto the TiO2 surface to provide a

strongly bound dye and good electron communication between them.

To achieve efficient electron injection from the excited dye to the CB of the TiO2,

the energy level of the lowest unoccupied molecular orbital (LUMO) of the dye

must be higher (more negative) than the conduction band (CB) of the TiO2

electrode. On the other hand, to achieve efficient regeneration of the oxidized

state by electron transfer from I3–/I– redox couple in the electrolyte, the energy

level of the highest occupied molecular orbital (HOMO) of the dye must be lower

(more positive) than the I3 –/I– redox potential

The organic molecule should have high molar absorption coefficients over the

wide region of sunlight extending into near infra-red, to provide a large

photocurrent to give the high light harvesting efficiency.

To achieve a stable DSSC, the organic molecule should have chemical stability in

its photoexcited state and the redox reactions throughout the reaction cycle.

Dye aggregation on the TiO2 surface should be avoided. This aggregation leads to

reduction in electron-injection yield from the dyes to the CB of the TiO2 due to intermolecular

energy transfer causing low conversion efficiency of the DSSC. (117)

4.5 Stru

The

The tra

nanostruc

The tran

and trans

4.5.1 Con

T

conduct

electrom

photocu

FTO) d

extensiv

indium,

ucture of Dy

e main parts

ansparent a

ctured layer

sparent cond

sparent thin f

Figure

nducting Su

The most wi

ting oxide

magnetic rad

urrent. The m

due to its th

vely as it has

and the cos

ye Sensitized

s of the dye

and counte

r, the sensiti

ducting elec

film such as

e4.2. Schema

ubstrate

idely used su

(TCO). Sui

diation as w

most widely

hermal stabi

s higher con

stly layer dep

d Solar Cell

sensitized s

er conducti

zer, and the

ctrode TCO

fluorine dop

atic of the str

ubstrates for

itable TCO

well as high

used TCOs

lity and low

nductivity. H

position requ

54

l

solar cell are

ing electro

e electrolyte

and counter

ped tin dioxi

ructure of th

r DSSCs are

must have

electrical co

in DSSC is

w cost. Indi

However, bec

uiring vacuu

e illustrated

odes, wide

are the ma

r-electrode a

ide (SnO2).

he dye sensit

made of coa

e high tran

onductivity

fluorine dop

ium tin oxid

cause of high

um, as well

schematical

bandgap

ain four elem

are coated w

(118)

tized solar ce

ated glass w

nsparency fo

to collect a

ped tin diox

de (In2 O3,

h cost and li

as its low st

lly in Figure

semicondu

ments of the

with a condu

ell.

with a transpa

or most of

all the gener

xide (SnO2:

Sn or ITO

imited suppl

tability at hi

e 4.2.

ucting

e cell.

uctive

arent

the

rated

F or

O) is

ly of

gher

55

temperatures alternatives are required. Carbon nanotube conductive coatings are a potential

substitute (119). Aluminum-doped zinc oxide (AZO) is also widely studied because it is a cheap,

nontoxic and available material. (120)

4.5.2 The Counter Electrode

Conducting glass electrodes have poor reduction ability for I3-; therefore a catalyst is

needed in the counter electrode to overcome the high activation energy of the electron

transfer. Platinum is the most commonly used material for this purpose. It acts as a catalyst

in the redox reaction at the counter electrode, thus preventing this process from becoming

rate limiting in the light energy harvesting system. However, platinum has another problem

apart from its high price, which is the non- confirmed possibility of corrosion by the iodide

solution, which leads to the formation of PtI4 (121). Since platinum is very expensive, other

cheaper alternatives may take its place like various forms of carbon (122). In this regard, it

found that functionalized graphene sheets with oxygen-containing sites perform comparably to

platinum in DSSC (123). Gold, although expensive, is another viable alternative that is

constantly used in solid state DSSCs. (124)

ZnO with poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) can

be an effective counter electrode in DSSC with maximum power conversion efficiency of

8.17%. (125)

56

4.5.3The Working Electrode

4.5.3.1Semiconductors

Semiconductors that have narrow band gaps and absorb visible light efficiently are

subject to photo-corrosion and are not suitable for DSSC. Metals oxides such as titanium

or niobium have a wide band gap; absorption edges towards the ultraviolet, and consequently

is insensitive to the visible spectrum. For this reason, the sensitizer adsorbed onto the surface

of the semiconductor increases the absorption spectrum range and the light harvesting

efficiency (126).

TiO2 has long been the preferred semiconductor in DSSCs, in spite of some promising

properties offered by other metal oxides such as ZnO (28), SnO2 and Nb2O5 (127). Titanium

dioxide TiO2 became the semiconductor of choice as it has many advantages for sensitized

photoelectrochemistry and photochemistry; it is a low cost, available, non-toxic and

biocompatible material (128). Anatase is the most favored form among the three common

crystalline polymorphs of TiO2 (Rutile, Anatase and Brookite) (129) because it has a high

bandgap energy (3.2 eV) it becomes invisible to most of the solar spectrum as it absorbs

only below 388 nm, and it reduces the recombination rate of photo-injected electrons. Rutile

has also been used but it has a smaller bandgap (3.0 eV) and is less effective since photon

excitation within the bandgap generates holes that act as oxidants making it less chemically

stable (130).

57

Figure: 4.3 On the left: scanning electron microscope picture of the fractal TiO2 film used in the first embodiment of the DSSC in 1988. On the right: scanning electron micrograph of a TiO2 anatase colloid film. (105)

The TiO2 film morphology plays an essential role in the DSSCs performance. In order

to obtain maximum area for dye adsorption using the minimum quantity of TiO2, the

semiconductor layer should have nanostructure mesoscopic morphology important for a

high surface area. A monolayer of dye on a flat surface absorbs little light because it holds

an area that is much larger than its optical cross section. In the first laboratory embodiment

of the DSSC which dates back to 1988 (131), the photo-anode was a titanium sheet covered

with a high surface area “fractal” TiO2 film that had a roughness factor of about 150.

The overall conversion efficiency in full sun light by that DSSC had conversion efficiency

in between 1% and 2%. Years later, in 1991 Grätzel reported a breakthrough, with

efficiency of around 7% achieved by the innovative use of a nano-scopic TiO2 particle

layer that produces a junction of huge contact area. The semiconductor’s surface was thus

enlarged over 1000 times allowing for efficient harvesting of sunlight by the adsorbed

monolayer of sensitizer.

Typical semiconductor film thicknesses are 5-20 μm, with TiO2 mass of about 1- 4 mg

cm-2, film porosity of 50-65%, and an average pore size of 15 nm and particle diameters of 15-

20 nm. Semiconductor can be deposited onto the surface of the glass substrate; three

58

deposition techniques commonly used to deposit the semiconductor onto the glass substrate

are screen-printing doctor blading, and spin coating (132).

The thickness of the nanostructured TiO2 layer is the most important factors that affect

the cell's efficiency; it should be less than 20 μm to ensure the diffusion length of the

photoelectrons is greater than the nanocrystalline TiO2 layer. The most commonly

nanocrystalline semiconductor oxide electrode used in the DSSC is TiO2. It used as an

electron acceptor and to support a molecular or quantum dot QD sensitizer (30). Another wide

bandgap semiconductor oxide that is becoming common is the zinc oxide ZnO2. ZnO2

possesses a bandgap of 3.37 eV and a large excitation binding energy of 60 meV. Kim et al.

reported that the nanorods array electrode showed stable photovoltaic properties and exhibited

much higher energy conversion efficiency (133)

4.5.4 The Photo-Sensitizer (Dye)

Dye molecules (sensitizers) are the key component of a DSSC to have increased

efficiency through their abilities to absorb visible light photons Despite the fact that the

highest efficiency of the dye sensitized solar cells was achieved from a collaborative effect

of numerous physical-chemical nanoscale properties, the main topic is the theory of dye

sensitization of large band-gap semiconductor electrodes. In the dye sensitized solar cells, this

was accomplished by coating the internal surfaces of porous TiO2 electrodes with special

dye molecules tuned to absorb the incoming photons (129). The dye is the light absorber and

the photoreceptor sensitizing the semiconductor and so some conditions must be fulfilled.

Without a doubt, a dye that absorbs nearly all the sunlight radiation incident on earth, like a

black-body absorber, is highly desirable.

59

In addition, dye must carry groups to attach the dye to the surface of semiconductor

such as carboxylate (134, 135) or phosphonate (136, 137) groups, because these are the most

employed ones. Other groups, like boronic acid (138), salicylate (139), silanes (140), amides

(141), ethers (142) or hydroxamic acid groups (143) can also be employed to attach photo-and

redox-active molecules to metal oxide surfaces. The anchoring group is an essential factor

considered in the fabrication of efficient sensitizers because it may affect both the stability of

the linkage and the electronic coupling between the dye and the semiconductor (137). Upon

excitation, it should inject electrons into the solid with a quantum yield of unity, which is best

completed when the electronic coupling of the donor levels of the dye well matched with the

acceptor levels of the semiconductor. The dye a l s o should be stable enough to carry on

about 108 turnover cycles corresponding to about 20 years of exposure to natural light (144).

In order to achieve that, one has to make sure that the electron injection and the recovery of

the oxidized form by the redox couple are fast enough to suppress side reactions like

degradation (e.g., loss of ligand), desorption or aggregation. To date, polypyridyl complexes

of ruthenium (II) called N3 and Black dye found the best photovoltaic performance in

terms of conversion yield and long-term stability. (S e e figure 4.4)

Figure: 4.4 Chemical structure of the two of ruthenium complexes N3 and Black dye (105)

60

Despite the high performances of these two dyes, other non-metallic alternatives are

currently being pursued (145). Because of its scarcity, ruthenium is a very expensive metal and

there is a necessity for alternatives. The most promising alternatives are natural organic dyes,

which are heap, less stable and less efficient. However, they have a great potential for this

application due to their high absorption coefficients compared to ruthenium sensitizers (146).

Organic dyes consisting of natural pigments and synthetic organic dyes have a donor-

acceptor structure called push-pull design, which improves the absorption in red and infrared

region by increasing short circuit current density. Natural pigments, like chlorophyll, carotene,

and anthocyanin, are available in plant leaves, flowers, and fruits and fulfill these

requirements. Experimental results showing that natural-dye sensitized solar cells achieved the

efficiency of 7.1% and high stability. (147)

Co-sensitization, the use of two dyes combination that complements each other in

their spectral features is another strategy to get a broad optical absorption expanding

throughout the visible and near IR region. This strategy has the advantage of enhancing

photo-absorption in that the optical effects of the two sensitizers have been found to be

additive. In particular, there was no negative interference between the co-adsorbed

chromophores, making way for testing a multitude of other dye combinations (148).

To increase improvement of the performance of DSSC, different kinds of additives

have been added into dye solutions. For example, Kay and Grätzel found that the addition of

cholic acid (CA) with porphyrin derivatives improved both the photocurrent and photovoltage

of the solar cell. (149) Additionally, other co-adsorbents such as hexadecylmalonic acid

(HDMA) (150), deoxychoic acid (DCA) (151), and chenodeoxychoic acid (CDCA) (152) were

also used in the dye solution when fabricating sensitized TiO2 films into devices, among

61

which CDCA is the most popular co-adsorbent.

4.5.5 Electrolyte

The electrolyte is an important part of all DSSCs. It is responsible for inner charge

carrier between electrodes. It is the hole-transport material. It regenerates the dye at the

photoelectrode with the charge collected at the counter electrode. For stable operation of the

solar cell and maximal power output, the oxidized dye must be reduced back to the ground state

as rapidly as possible by a suitable electron donor. Since the maximum photovoltage (Voc)

obtainable corresponds to the difference between the redox potential of the electrolyte and Fermi

level of the electron in the TiO2 (105), it is preferable to choose a couple whose potential is as

close to the redox potential of the sensitizer as possible. In addition, the choice of the mediator

should be such that there is enough driving force for the dye reduction step to have an optimal

rate. It is also important for stable performance of the solar cell that the redox couple is fully

reversible, absorption in the visible region, and stable in the oxidized and reduced forms (153).

Furthermore, counter cations of iodides such as Na+, Li+, and R4N+ influence the cell

performance essentially because of their adsorption on nanostructured electrode (TiO2) or ion

conductivity. It found that using tert-butylpyridine in redoxing electrolyte increases cell

performance (154). Promising results obtained by using Br -/Br3 - redox couple DSSCs. When

the redox couple changed from I- /I3 - to Br -/Br3-, the Voc and Isc for the Eosin Y-based DSSC

increased (155). The redoxing electrolyte was selected so that the reduction of I3 - ions by

injection of electrons is quick and efficient. This arrives from the fact that the dependence of

both hole transport and collection efficiency on the dye-cation reduction and I -/I3 - redox

efficiency at counter electrodes are to be taken into consideration (156). In addition, as

evaporation becomes the main reason for limiting cell stability, liquid electrolyte prevents

62

fabrication of multi-cell modules since module constructing requires cells be connected

electrically until separated chemically (157, 158). Therefore, an important limitation of the dye

sensitized solar cells filled with liquid state redoxing electrolyte is the leak of the electrolyte,

causing a reduction of the cell’s lifespan, besides the associated technological problems

associated to device sealing up, and hence long-term stability (159). Many research groups study

the use of ionic liquids, polymer, and hole conductor electrolytes to substitute the need of

organic solvents in liquid electrolytes.

The ideal ionic liquids for DSSC should have good chemical and thermal stability,

negligible vapor pressure, non-flammability, high ionic conductivity and a wide

electrochemical window (160). Molten salts based on imidazolium iodides revealed very

attractive stability features (161). Despite their high viscosity, linear photocurrent response up

to full solar light intensities observed. The best results obtained with 1, 3-dialkylimidazolium

iodide compounds (162).

The solid-state DSSC is an alternative that offers itself to deal with the sealing problem

through substituting the volatile redox electrolyte with a solid p-type semiconductor

interpenetrating the nanocrystalline TiO2 structure, which would permit the charge

neutralization of dye molecules after electron injection by its hole transport properties. The

main difficulty is improving the interface between the sensitized semiconductor and the

electrolyte; it is extremely difficult to achieve an intimate contact, without voids, among

particles because of the roughness of the former and the impossibility of high-temperature

depositions of the latter. The best successfully working organic charge transfer material is

spiro-MeOTAD. It was first presented by Grätzel et al. in 1998 (163) and presently achieves

conversion efficiency above 4% (164).

63

Solid-state electrolytes overcome the disadvantage of using liquid electrolytes

however; lower conductivity and poor interface contact for solid-state electrolytes cause lower

conversion efficiency for DSSCs. Quasi-solid-state electrolytes have similar liquid

electrolyte’s ionic conductivity and interface contact property, and solid-state electrolyte’s

long-term stability. It is believed that this is the most available kind of electrolytes for

fabricating high photoelectric performance and long-term stability of DSSCs in practical

applications.

4.5.5.1 Organic Solvent

The most of chemical compounds can be used if they have low volatility, low viscosity

they easily penetrate the porous semiconductor, resistance to decomposition, good redox couple

stability, low toxicity, and low cost.

Several organic solvents such as methoxypropionitrile, (165) butyronitrile (166)

and/or methoxyacetonitrile (167) tested among others; however, acetonitrile is the most used

solvent, particularly when one wishes to maximize cell efficiency.

4.5.5.2 Redox Couple

To date, the best DSSC performance was obtained using the tri-iodide /iodide (I3-/I-) redox

couple in an organic matrix, commonly acetonitrile. The good result of this redox mediator is based

on the following kinetics:

1. The photo-oxidized dye injects an electron into the conduction band of the semiconductor

much faster than electron recombination with I3-.

2. The oxidized dye preferably reacts with I- then recombines with the injected electron.

3. The two electrons process of I- 1 regeneration from I3- occurs quickly enough at the catalyst-

coated counter electrode to be productive.

64

4. The overall result of the above processes leads to coherent diffusion of I3- towards the

counter electrode and I- diffusion in the opposite direction.

2e- + I3- ⥬ 3I-

Illustration: 4.1 Tri-iodide/iodide redox couple

4.6 Characteristic Parameters of DSSCs

4.6.1 Open Circuit Voltage

The dye sensitized solar cell can be regarded as a battery in a simple electric circuit.

In the dark, the cell does nothing. At that point system is at equilibrium, the Fermi

energy of the TiO2 electrode (corresponding to the free energy of electrons in this film

after thermalization) equilibrates with the midpoint potential of the redox couple, resulting in

zero output voltage. Under these conditions, the TiO2 Fermi level lies deep within the band

gap of the semiconductor, and the film is effectively insulating, with a negligible electron

density in the TiO2 conduction band. As well at the counter electrode, there will be high

concentrations of oxidized and reduced redox couple present in the electrolyte and no

significant change in chemical potential of the electrolyte, which remains effectively fixed at

its resting value. When light hits the working electrode, photo-excitation of the dye injects

electron into the TiO2 film which leads to a dramatic increase in electron density, raising

the TiO2 Fermi level towards the conduction band edge, and allowing the film to become

conducting. This happen in parallel with hole injection into the redox electrolyte (168) which

causes the circuit to switch on and develop a voltage, or electromotive force called the open

circuit voltage (Voc) (169).

T

differen

the redo

4.6.2 Sho

T

solar ce

circuit c

T

cell. Co

drawn f

4.6.3 The

T

to the pr

The magnitu

nce between

ox couple in

ort Circuit C

The short-circ

ell is zero (i.

current show

The short-cir

onsequently,

from the sola

e Fill Factor

The fill facto

roduct of the

ude of open

the Fermi le

the electrol

Current

cuit current

e., when the

wn on the IV

Figure .4.5

cuit current

the short-ci

ar cell.

r

or for DSSC

e open circu

n-circuit pho

evel of the s

lyte (170).

is the curren

e solar cell i

curve.

I-V chara

and the ligh

ircuit curren

C is defined a

uit voltage an

65

oto-voltage d

solid under i

nt through th

s short circu

acteristics of

ht-generated

nt is conside

as the ratio o

nd short circ

developed is

illumination

he solar cell

uited). Usual

f a typical so

d current are

ered the gre

of the actual

cuit current p

s determined

n and the Ne

when the vo

lly written a

olar cell

e identical fo

atest current

maximum a

produced. Th

d by the ene

rnst potentia

oltage across

as ISC the sh

or an ideal s

t which may

attainable po

he fill factor

ergy

al of

s the

hort-

solar

y be

ower

r is a

significa

equivale

current

rest 40%

Figure: 4

4.6.4 Sol

T

(Voc) the

conversio

photocur

mWcm-2)

circuit, b

ant issue in

ent series res

produced by

% is significa

4.6 I–V curv

lar Cell Con

The photocur

e fill factor (

on efficiency

rrent density

). The relati

being JSC go

n calculating

sistance and

y the cell dis

ant for Loss.

ves of an org

nversion Eff

rrent density

(ff), and the

y (η) of the d

y-voltage ch

ion between

ot when resi

g the perfo

a high equiv

ssipated in in

.

ganic PV cell

ficiency

y calculated

power of th

dye-sensitize

aracteristics

n J and V is

stance of the

66

ormance sol

valent shunt

nternal losse

l under dark

at short circ

he incident li

ed solar cell

(IV curves

determined

e outer circu

ar cells per

t resistance h

es is less. If

(left) and ill

cuit (JSC) th

ight (Pin) ar

l. These valu

s) under AM

by changin

uit is zero (th

rformance.

have a high f

a DSSC wit

luminated (r

he open-circ

re used to de

ues can be ta

M 1.5 full su

ng the resista

hus voltage

Cells with

fill factor, so

th 60% FF t

right) condit

cuit photovo

etermine the

aken out from

unlight (Pin

ance of the

is zero) and

low

o the

then,

tions.

oltage

e total

m the

n=100

outer

d Voc

when res

the produ

an ideal c

to the m

projected

W

4.7 Oper

D

coated w

electrolyt

electrode

load conn

F

absorptio

injection

dye mole

dye mole

direct con

sistance is m

uct of J and

cell. Pmax i

maximum va

d using the fo

Where

rating Princ

Dye-sensitize

with porous n

tes containin

es. At the ill

nected to the

igure (4.7) i

on in the D

from the dy

ecules can ab

ecules on to

ntact with th

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V, and the f

s usually rep

alue that can

ollowing equ

ciple of the D

ed solar cel

nano-crystal

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umination, t

e electrodes.

is a schemat

DSSC happe

ye to the TiO

bsorb only le

op of each o

he semicond

hus photocur

fill factor sta

ported as the

n reach the

uations: (101

Dye-Sensitiz

ls (DSSCs)

lline TiO2,

ion-oxidation

the cell prod

ic illustratio

ens by dye

O2 at the se

ess than one

other increas

ductor electro

67

rrent is zero)

ates the effic

e output pow

output pow

1)

zed Solar C

consist of

dye molecu

n couple suc

duces a volta

on of the ope

molecules

emiconducto

e percent of t

ses the thick

ode surface

). The outpu

ciency of th

wer of the co

wer. The pe

Cell

transparent

ules attached

ch as I-/I3-,

age over and

erating princ

and the ch

or electrolyte

the incomin

kness of the

can only sep

ut power of t

he device com

ommercial d

erformance o

t conducting

d to the surf

and catalyst

d current thr

ciples of the

harge separa

e interface. A

ng light (100

layer, the d

parate charg

the device e

mpared to th

device and re

of DSSC ca

g glass elec

face of the T

t coated cou

rough an ext

DSSC. The

ation by ele

A single lay

). While stac

dye molecul

ges and contr

equals

hat of

elates

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ctrode

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unter-

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ribute

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Figure.4.

H

particle s

Another

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illustrat

urrent gener

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Having this c

size of 20nm

important el

alline inorgan

ents show th

me of the m

ocrystalline

Electron tran

ed in figure

ration. Grätz

ine TiO2 el

nough amou

of operationll. (105)

construction,

m, has an inte

lement of us

nic semicon

hat low mot

most effective

inorganic m

nsfer proces

e (4.8). Whe

zel’s group

ectrode stru

unt of dye co

n and energy

, a TiO2 elec

ernal surface

ing this poro

nductors is h

tility limits

e methods of

materials as th

ses in a DSC

en the dye

68

developed t

cture. It is t

ontact with th

y level sche

ctrode that i

e area greate

ous electrode

higher than

the efficienc

f improving

he electron a

C during ligh

absorbs a p

the solution

to increase t

he TiO2 elec

eme of the d

is typically 1

er than the el

e structure is

in organic p

cy of charg

charge tran

acceptors.

ht-electric p

photon an e

of this pro

the electrode

ctrode and th

dye-sensitize

10 µm thick

lectrode geo

s the fact tha

photovoltaic

e collection

nsport in org

power conver

lectron is e

oblem by us

e internal su

he electrolyt

d nanocrysta

, with an av

metrical are

at charge mo

c materials (

of organic

anic photovo

rsion energy

xcited from

ing a

urface

te.

alline

erage

ea (1).

otility

(171).

solar

oltaic

y are

m the

69

HOMO level to the LUMO level of the molecule (1). This is followed by electron injection to

the semiconductor (TiO2) conduction band (2) and the oxidized dye regenerated by the

electron captured from the redox electrolyte (5). The injected electron travels by diffusion in

the TiO2 film until it finds its way to the substrate contact where it is released to the external

electrical circuit (3). The electron is restored to the cell at the counter electrode through an

electrolyte reduction reaction and the electrical circuit of the cell finished by ionic transport of

the redox pair in the electrolyte. The main back reactions limiting the photocurrent is indicated

with red arrows; (4) radiation less relaxation of the excited state of the dye, (6) recombination

of the electrons with the oxidized dye, (7) and with the tri-iodide in the electrolyte.

Figure 4.8 Electron transfer processes in a DSC during light-electric power conversion energy

The oper

Anode:

Cathode: Cell:

F

indicate e

(60 ns), e

recombin

recombin

ms), and

rating cycle c

S

2

igure 4.9 sh

excitation of

electron inje

nation of th

nation of the

the regenera

F

can be summ

S hv S

S* S e

2S3I

I 2e(P

(Pt ) hv

hows a sche

f the dye fro

ction from t

he injected

e electron in

ation of the o

Figure4.9 Sc

marized in ch

S* Absorption

TiO2 Elect

2S I3 Re

t)3I 3

eTiO2

ematic illustr

om the HOM

the dye LUM

electron w

the TiO2 co

oxidized dye

chematic illu

70

hemical reac

n

tron injectio

egeneration

ration of kin

MO to the LU

MO level to t

with the hol

onduction ba

e by I- (10 ns

ustration of k

ction termino

on

netics in the

UMO level,

the TiO2 con

le in the d

and with a ho

s). (172)

kinetics in th

ology as:

e DSSC. Th

relaxation o

nduction ban

dye HOMO

ole (I3-) in th

he DSSC

he shown ar

of the exited

nd (50 fs -1.7

level (ns

he electrolyt

rrows

d state

7 ps),

-ms),

te (10

71

4.8 Applications of DSSC

Dye sensitized solar cells are inexpensive, environment friendly materials. Different

applications of DSSCs like power window and shingles are prospective in building integrated

photovoltaics BIPV. The Australian company Sustainable Technologies International produced

electric power producing glass tiles on a large scale for field testing, and the first building

equipped with a wall of this type. The availability of lightweight flexible dye sensitized cells or

modules are attractive for applications like light powered calculators, devices, and mobiles. Dye

sensitized solar cells can be designed as indoor colorful decorative elements. Flexible dye

sensitized solar modules releases opportunities for incorporating them with many portable

devices, baggage, gears, or outfits. In power generation, dye sensitized modules with efficiency

of 10% become the best technology to change the common crystalline Si-based modules. In

2010, Sony pronounced manufacturing of modules with 10% efficiency and later the opportunity

for commercialization of DSSC modules is possible (118).

G24 innovation created the first commercial shipment of low-light, ultra-thin, solar cell

technology called DSSC, sent to Hong Kong-based consumer electronics bag manufacturer,

Mascotte Industrial Associates for use in backpacks and bags. DSSCs are less than 1mm thick,

inexpensive, do not contain silicon or cadmium and can even operate indoors, making them ideal

for powering cell telephones, ideal for clothing and portable applications, cameras and portable

electronics. DSSCs can also be fixed into tent material to power LED lighting systems for

camping. Solar cells can also be layered onto laptops, mobile telephones, GPS units and AV

devices for additional power supply. (173)

72

Chapter 5: Materials and Methods

5.1 Introduction

Most of materials and chemicals used in this research were available in the lab.

Chemicals like solvents were purchased from VWR. Some materials used in the fabrication of

devices like FTO glass and TiO2 paste were purchased from Solaronix Co. Instruments like

FTIR, H1NMR, UV_VIS Absorption, and Solar Simulator are available in the UTEP labs.

5.1.1 Precursor Material Characterization

Before starting synthesizing and preparing the synthetic asphaltene, we started working

with the precursor material Tetra Octyl Carboxylate Perylene TOCP. First we started testing and

characterizing the precursor. Different techniques were used to analyze and characterize the

starting material. These tests were carried out to make sure that this material is similar as in

literature. First, IR spectra were tested by using FTIR spectroscopy (Spectrum 100 FT-IR

Spectrometer from PerkinElmer), and the results were Similar to those found in the literature.

The H1 NMR test also carried and the peaks were analyzed. The figures below show the results

of this work, and likewise, they show that the precursor material has similar characterizations as

in the literature.

73

Figure5.1a. Analysis of H1 NMR peaks of TOCP.

74

Figure5.1b. Analysis of H1 NMR peaks of TOCP.

75

Figure5.2: IR Spectra of the Precursor TOCP.

5.1.2 Synthesis of Synthetic Asphaltene

Synthetic Asphaltene is prepared according to the procedure in published literature (64).

The precursor material is a powder and yellow in color as shown in the picture below.

The heating process should be run in the absence of air under flow of Nitrogen gas. The

procedure can be summarized as following: 1g of starting material TOCP was poured in a

conical flask. The flask was put on the heating mantle and it has connected with Shlenk line for

Nitrogen flow. The temperature during the reaction process is monitored by inserting a

thermometer inside the conical flask. To make sure that there is no Oxygen inside the flask; the

vacuum valve was opened several times during the process. After the temperature rises to about

76

1000C, the starting material (TOCP) started melting and color changed from yellow to dark

reddish color. Continuing the heating operation and monitoring the temperature, also controlling

the flow of Nitrogen are more important. The heating process t continues until the temperature

reaches more than 3200C and not after 4200C to prevent coke formation, then the heating mantle

was switched off, Nitrogen gas flow stopped, and the flask kept to cool down to room

temperature. Then, the final product as in the picture was collected by using clean metal spatula,

weigh, and kept in clean vial for use.

(a) TOCP Precursor (b) Synthetic Asphaltene

Illustration (5.1): Showing a) the precursor material, b) synthetic asphaltene after synthesized.

5.1.2.1 Testing UV-Vis Absorption of Synthetic Asphaltene

In this experiment the model Varian Cary 5000 UV-VIS-NIR absorption spectrometer

was used. The result was shown in the graph below, and it is clear that Synthetic Asphaltene has

the ability to absorb light in the range between 350 nm – 550 nm.

77

Figure 5.3 UV-Vis absorption of Synthetic Asphaltene

Infra-red spectroscopy also checked for a Synthetic Asphaltene sample and the result was found

to be the same as that of IR spectra of crude oil Asphaltene as in the literature.

5.2.1.2 Testing of IR Spectra of Synthetic Asphaltene

The graph below illustrates the IR Spectra of TOCP, Synthetic Asphaltene and HVAR as in the

literature (64), and also the IR spectra of Athabasca Asphaltene and Synthetic Asphaltene as in

published literature (79), and I inserted between them the IR spectra of new Synthetic

Asphaltene which prepared in the lab and marked as B. From this figure, it is clear the similarity

in the peaks between all Synthetic Asphaltene samples.

78

Figure 5.4 IR Spectra of different Synthetic Asphaltene samples.

5.2.1.3 Purifying Procedure of Synthetic Asphaltene

Synthetic Asphaltene after prepared still holds some impurities because not completely

burning of starting material. These impurities can prevent absorbance of Synthetic Asphaltene

dye to the surface of TiO2. To remove these impurities, Synthetic Asphaltene sample was

purified by dissolving it in toluene for 24 hours. The solvent is then filtered by using filter paper

#40 (4.0 µm) and by using a vacuum pump and the impurities collected in the filter paper while

the clean solvent pass. The collected solvent is kept under the hood to evaporate Toluene. The

final product is collected and kept in a clean vial for use as purified Synthetic Asphaltene.

79

Illustration (5.2) Filter paper after Synthetic Asphaltene filtered.

5.2 Building the DSSC

5.2.1 Preparation of TiO2 paste (P-25)

The TiO2 paste was used in this study and prepared in the lab according to the

procedures published in literature (174). This will be referred to as P-25 paste. The

procedure can be summarized and as shown in the schematic diagram below:

6 grams of commercial TiO2 powder (P25) were purchased from Sigma Aldrich

Company mixed with 1ml of acetic acid and grinded in a mortar for 5 minutes. A total of 5ml

D.I water and 15ml of ethanol were added ml by ml with continuous grinding. The amount of

ethanol was then increased to 2.5ml for 6 times and grinding for 1 minute for each

addition. The total compound was then transferred to a beaker with the addition of 100ml

o f ethanol, and stirred with magnetic stirrer and sonication with an ultrasonic horn for 2

seconds work plus 2 second rest for 30 times. The next step is to add 20g of Terpineol with

repeating the sonication procedure. Finally, 3g of ethyl cellulose were added and the same

sonication procedure repeated. The ethanol is evaporated by using a rotary evaporator.

80

Illustration 5.3 Preparation procedure of P-25 paste

81

5.2.1.1 Characterization of P-25 Paste

Scan Electron Microscope SEM was used to determine the characterization of TiO2

paste. The illustration below shows the nanocrystalline formation of the paste and the

thickness of the TiO2 layer on FTO glass. The graph also shows the elemental analysis of the

paste.

Illustration 5.4: The Thickness of TiO2 paste.

Figure5.5 Scanning electron microscope (SEM) of a TiO2 Paste

82

Figure5.6 Elemental analysis of TiO2 paste

5.2.2 Preparation of the Photo-Electrode

5.2.2.1 Nanocrystalline TiO2 Layer

The nanocrystalline TiO2 layer in this research was prepared by applying a layer of

one or more of the above mentioned TiO2 paste (P-25) by spreading (doctor blading) on the

surface of FTO glass. A piece of glass (F-doped SnO2, FTO, (Solaronix TCO30-8 SA

Switzerland 13 Ω/cm2/&,3.3 mm thickness and 500- 1000 nm transmittance) that had been cut

to 2cm x 2cm and cleaned in a detergent solution using an ultrasonic bath for 30 minutes and

then rinsed with water and ethanol. The FTO-coated glass was covered with one layer of

parallel adhesive Scotch tape placed 1 cm apart to control the area and the thickness of the

TiO2 film (0.5 x 0.5cm) as illustrated on picture.

83

Illustration 5.5: the steps of fabrication new cell starting from A: cleaning FTO and preparing for applying paste, B, C, and D applying paste on the surface of FTO by doctor blade method, E is showing gradually heating of the paste, and F showing the cell after heated, finally G is showing the cell after was dipped in synthetic asphaltene dye for 24 hours and also area was controlled by 0.25cm2.

As shown in the picture above, the paste was applied between the tapes on the FTO-

coated glass by rolling a glass rod on the surface (also known as the doctor blade method). The

film thickness is controlled by the paste density and the adhesive tape thickness as well. Film

thickness was in one layer paste, typically 2 ± 3 µm. The TiO2 Nano crystalline electrode was

dried in the air for 2 minutes, and then gradually heated at 3250C for 5 min, at 3750C for 5 min,

at 4500C for 15 min, and at 5000C for 15 min.

The heating process enables the added organic polymer in the colloidal paste to be

burnt off leaving the TiO2 film with a higher surface area and more porous. It also increases

the resistivity of the film. In addition, the surface is dehydroxylated during this process,

leaving reactive Ti3+ centers available for reaction with the anchoring groups of the

84

sensitizers (dye). The temperature of electrode was cooled down up to 80°C to prevent

moisture, or it can be cooled down to room temperature, and then kept it in a closed space to

protect from pollutants. When the dye is ready, the electrode was cleaned again by using the

UV-O3 cleaning device. After treatment in a UV-O3 system (Model No. 342, Jelight Company,

Inc.) for 18 minutes, then dipped in the dye solution for 20-24 hours.

5.2.3 Preparation of the Counter Electrode

The counter electrodes were prepared by placing a drop of an H2PtCl6 solution (2mg Pt in

1mL ethanol) on clean FTO glass and heating it at 4500C for 15 min, and then keep it cool down

to room temperature. The FTO glass will look almost transparent.

5.3 The Electrolyte

A type of commercial electrolyte obtained from Solaronix Company has been used in this

research. This is Standard Iodolyte MPN-100. It consists of 100mM of tri-iodide in

methoxypropionitrile solution.

5.4 Assembling of the Synthetic Asphaltene DSSC

After the size measurement, the TiO2 film was treated with 40mm TiCl4 solution, TiO2

film was immersed into a 40mm aqueous TiCl4 aqueous solution at 700C for 30 min and rinsed

with water and ethanol and sintered at 5000C for 30 min. At 800C in the cooling, the TiO2

electrode was immersed in 0.5 mg/ml of synthetic asphaltene dye and kept at room temperature

for 20–24 hrs. to complete the sensitizer uptake.

The synthetic asphaltene dye covered TiO2 electrode and Pt-counter electrode were

assembled into a sandwich type and assembled with using a binder clamps. The clamps used to

attach the two electrodes and to provide space for the electrolyte between two electrodes.

Sealant

leakage

the hot

heated u

injected

the hole

Il

also can be

of electroly

press mach

up to join b

d into the ho

e was cover

lustration: 5

e used to ass

yte. For good

hine. Tempe

both electrod

le in the bac

ed by tape.

.6 Schematic

semble the s

d results, se

erature and p

des by using

ck of the cou

The electro

c diagram sh

85

sandwich of

alant was pu

pressure we

g the sealant

unter electro

olyte was int

howing comp

f electrodes.

ut between t

ere adjusted

t. A drop of

ode and to pr

troduced int

mpleted synth

It is can be

two electrod

and then th

f the electrol

revent leaka

to the cell b

hetic asphalte

e used to pr

des and put u

he hot press

lyte solution

age of electro

by using vac

ene DSSC

event

under

s was

n was

olyte,

cuum.

86

Chapter 6: Results and Discussion

6.1 Introduction

This chapter will introduce and discuss the results of this research. Some of results

already mentioned in the previous chapter, such as IR spectra of Precursor material and UV-Vis

absorption of Synthetic Asphaltene. In this Chapter I will focus on the showing the results of

using Synthetic Asphaltene as dye in the fabrication of DSSC.

6.2 The First Use of Synthetic Asphaltene as A dye in DSSC

Synthetic Asphaltene, after synthesized in the lab, it was used without any additives or

modifications. The sample of Synthetic Asphaltene was dissolved in Toluene as a solvent and the cell was

immersed in the solution for 24 hours. The sample was then removed from solution, cleaned and washed

with ethanol to remove any extra dye or pollutants, and then the cell was assembled and tested as

described in the previous chapter.

The following table illustrate the result of the first cell which prepared by Synthetic Asphaltene as

dye.

Table 6.1The first result of Synthetic Asphaltene dye in dye sensitized solar cell

Voc (V) Jsc (mA/cm2) FF % η (%)

Synth. Asph. 0.438 1.63 54 0.42

We can conclude from this table that the efficiency of the cell by using Synthetic

Asphaltene without any additives is about 0.4%. So, it is possible to increase the efficiency by

applying some changes and some additives.

The new sample of synthetic asphaltene was prepared by adjusting the Nitrogen flow and

also by controlling the temperature during preparation process. The new sample of synthetic

87

asphaltene was tested in the fabrication of new devices. The results in the table below showing

improvement in the performance efficiency reached up to 1.00%. So we conclude from this

experiment that the flow of Nitrogen and the temperature should be monitored carefully during

preparation of synthetic asphaltene.

Table 6.2: New results of new Synthetic Asphaltene dye

6.3 Effects of Different Layers of the Paste

One of the parameters which can affect the cell efficiency is the thickness of the paste. To

check this parameter, different layers of the paste were applied during the fabrication of the cells.

One, two, and three layers were tested during the experiment. Controlling of the number of

layers is done by the number of tape layers applied on the surface of FTO glass. For example,

one scotch tape layer means one layer of the paste. See section 5.2.2.1

The following table shows the result of this experiment, and from this table, it is clear

using one layer paste is more effective that two or three layers; it can be because the absorption

of Synthetic Asphaltene dye on the surface of TiO2 paste is higher than when using two or three

layers. It is also could be that using one layer of paste results in low aggregation.

Voc (V) Jsc ( mA/cm2) FF % η (%)

New Synthetic Asph. Sample 0.409 3.61 57 1.00

88

Table6.3 The effect of different layers of paste on the performance of Synthetic Asphaltene cell

Voc (V) Jsc ( mA/cm

2) FF % η (%)

one layer paste 0.424 4.49 65 1.35

two layers paste 0.440 3.35 57 0.93

three layers paste 0.436 2.86 56 0.83

6.4 UV-Ozone Treatment of the Photo-electrode

The main components of the TiO2 pastes utilized in this study, as well as in most DSSC

research labs, more than TiO2 particles are organic compounds used to disperse and link the

semiconductor particles to form a network film. Even though, temperature sintering processes

decompose the formation of porous TiO2 films and give high surface area. However, some

carbon atoms will deposit on the top of TiO2 particles and decrease the absorption of the dye and

the electron injection. UV–O3 treatment is the well-known technique used to remove organics on

transparent conducting oxides and it has been widely used in organic electronic devices to clean

the surface and modify the work function of ITO. Recently, UV–O3 treatment was applied to

synthesize porous nano-particulate TiO2 films at room temperature and the noticeable

improvement of photo-conversion efficiency was obtained by using this treatment. The main

effect of the UV–O3 treatment was reported to be the removal of residual organics and positive

shift in the conduction band of the nano-crystalline titanium dioxide, which promotes electron

injection from the dye. (175)

To see the effectiveness of the UV–O3 treatment on the performance of synthetic

asphaltene solar cells, two synthetic asphaltene cells were prepared as described in the previous

89

chapter and one cell was treated with UV-03 for 15 minutes and another without UV-O3

treatment. Both cells were dipped in the Synthetic Asphaltene dye for 24 hours. The cells were

tested and the result of this experiment was shown in the following table:

Table 6.4The effect of UV-O treatment on the Synthetic Asphaltene cell performance

6.5 Effects of Some Additives on the Synthetic Asphaltene Cell Performance Efficiency

6.5.1 Adding Porphyrin to Synthetic Asphaltene Dye to Increase The Efficiency of The Cell

During work with this research, Porphyrin was added to the Synthetic Asphaltene to

improve the efficiency of the cell. Porphyrin is already used as organic dye in DSSC. The new

compound is prepared in the lab by dissolving the same ratio of Synthetic Asphaltene with free

metal Porphyrin in Toluene for 24 hrs under heating; the mixture was kept under the hood to

evaporate the Toluene. The final product was collected and used in this experiment.

The table below shows the effects of mixing Porphyrin with Synthetic Asphaltene on the

conversion efficiency of synthetic Asphaltene dye:

. From the table it is clear that the new dye is increasing both current density JSc and Voc

(V), and then increasing the performance efficiency.

Voc (V) Jsc ( mA/cm2) FF % η (%)

No UV-O treatment 0.409 3.61 57 1.00

UV-O treatment 0.426 3.84 57 1.14

90

Table 6.5The results of mixing Synthetic Asphaltene with Porphyrin Voc (V) Jsc ( mA/cm

2) FF % η (%)

Synthetic Asphaltene mixed with Porphyrin and dissolved in

Toluene 0.429 6.57 54 1.85

6.5.2 Synthetic Asphaltene and Crude Oil Asphaltene

Asphaltenes material is described in chapter two. This material was used as new organic

dye in dye sensitized solar. Asphaltene has shown to be able to produce voltage, and according to

the results, asphaltene has become a good candidate in photovoltaics, due to its availability and it

is a low cost material. (176)

For this experiment, the new material was prepared by dissolving synthetic asphaltene

and crude oil asphaltene. The new material was prepared by adding Synthetic Asphaltene to

crude oil asphaltene. This Asphaltene was extracted from crude oil asphaltene and purified by

using Toluene. Real Asphaltene was dissolved with Synthetic Asphaltene in Toluene in the

ration of 1:1 and kept for 24 hours for dissolving under heating. The solvent kept under the hood

until the Toluene completely evaporated, and the solid product was collected and used for the

experiment.

Table6.6The photovoltaic performance efficiency of Synthetic Asphaltene dye mixed with crude oil Asphaltene.

Voc (V) Jsc ( mA/cm2) FF % η (%)

Real Asph. mixed with Synth Asphaltene

0.435 7.28 46 1.40

6.6 Purified Synthetic Asphaltene

91

During work with Synthetic Asphaltene as new dye in the fabrication of DSSC, some

precipitates were noticed when Synthetic Asphaltene dissolved in Toluene. These precipitates

can prevent the good absorbance of Synthetic Asphaltene dye into the surface of TiO2. So,

Synthetic Asphaltene was purified to remove any impurities. See section 5.2.1.3.

The new purified Synthetic Asphaltene sample was tested in the fabrication of DSSC and

the results is illustrated in the table (6.7).

The table below showing the results that compare between two samples of Synthetic

Asphaltene used as dye during the fabrication of cells. One of these samples represents the

Synthetic Asphaltene before purifying and another sample represents the Synthetic Asphaltene

after purifying. All factors were improved by using purified synthetic asphaltene. There is a high

increase in the current density and also fill factor, but the small increase in the voltage. The

efficiency was improved from 1.0% to 1.83%.

Table 6.7The comparing results between the cell performance efficiency of purified and unpurified Synthetic Asphaltene.

Voc (V) Jsc ( mA/cm2) FF % η (%)

Unpurified Synthetic Asphaltene 0.445 4.19 56 1.0

Purified Synthetic Asphaltene 0.455 7.05 61 1.83

92

6.7 Effect of Chenodeoxycholic Acid (CDCA) on the Photovoltaic Performance of Purified Synthetic Asphaltene Dye Sensitized Solar Cell.

The effects of chenodeoxycholic acid (CDCA) in a dye solution as a co-adsorbent on the

photovoltaic performance of dye-sensitized solar cells (DSSCs) based organic dye containing

purified synthetic asphaltene dye was studied. It was found that the coadsorption of CDCA can

eliminate and reduce the formation of dye aggregates and improve electron injection yield and

thus, Jsc. This has also led to a rise in photovoltage, which is attributed to the decrease of charge

recombination. The DSSC, based on synthetic asphaltene dye with CDCA, overall conversion

efficiency was further improved to 2.0% (Jsc = 7.00 mA/cm2, Voc = 0.506 V, FF = 0.61) upon

addition of 1:1 ratio of CDCA to the dye solution.

Table 6.8The performance efficiency of Synthetic Asphaltene dye with additives of Chenodeoxycholic Acid CDCA

Voc (V) Jsc ( mA/cm2) FF % η (%)

Purified Synthetic Asphaltene with Chenodeoxycholic Acid CDCA 0.506 7.00 0.61 2.08

93

Chapter 7: Conclusion and Future Work

7.1 Conclusion

In general, we can conclude from this study that Synthetic Asphaltene was successfully

prepared in the lab and the characterization results were similar as in the literatures. Even

primary results show that Synthetic Asphaltene has good ability to absorb sunlight, but the

results of the first use of Synthetic Asphaltene in DSSC were not satisfied. More work was

carried during the study to increase the performance efficiency from 0.4% to 2.0%. We conclude

also Synthetic Asphaltene can give good results by adding some additives, but it also can

improve the conversion efficiency just by purifying process which increase efficiency from 1.0%

up to 1.83%. Instead of additives, there is another parameters can increase the efficiency of

Synthetic Asphaltene as dye in DSSC for example, the number of layers of the paste and using

UV-O treatment. Therefore, a total overall efficiency of 3.5 % – 5.0 % could be easily attained

with good engineering design and material process.

94

7.2 Future Work

The results of this work approve that Synthetic Asphaltene is recommended as active dye

in the field of dye sensitized solar cell. Using new dye with high performance and cheap relating

to other dyes is become an important invention. This is the first step in this research and more

effort still required to improve the efficiency of this new material. Looking for new parameters in

addition to these parameters which applied in this study is also become in the high priority.

DSSC has different parameters not only the dye and work with these parameters can improve the

efficiency. Using solid electrolyte instead of liquid is highly recommended.

Higher-power conversion efficiencies can be achieved by further optimizing the device

structure. In a multi-component device like DSSCs, the overall performance of the cell depends

on the individual properties of the constituent components and processes. Optimal performance

can be achieved only when understanding the factors that control each of the components and

depends on the ability to tune to the required configuration.

95

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Abbreviations

GHG Greenhouse gasses Btu British thermal unit EIA Energy Information Administration IPCC Intergovernmental Panel on Climate Change (IPCC) OECD Organization of Economic Cooperation and Development IEO International Energy Outlook

ASTM American Society for Testing and Materials G173-03 Global Terrestrial Reference Spectra σ Stefan Boltzmann constant (5.67 × 10−8 W/m2/K4). SO Solar constant AU The astronomical unit (149,604,970 km) DSE Diameter of earth sphere ASE Surface area of earth sphere AM1.5 Air Mass at 48ᴼ zenith angle AM 0 Air Mass at 0 zenith angle UVR Ultra violet radiation AC Earth cross-sectional area TWh TeraWatt hour DSSC Dye sensitized solar cell C13-NMR Carbon 13 nuclear magnetic resonance XANES X-ray absorption near-edge spectroscopy IR Infrared spectroscopy

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FT- IR Fourier Transform InfraRed FCS Fluorescence correlation spectroscopy fs Femtoseconds ns Nanoseconds ps picoseconds ms milliseconds ESR Electron Spin resonance mass spectroscopy, VPO Vapor pressure osmometry GPC Gel permeation chromatography MS Mass spectrometry SEC Size exclusion chromatography SAXS Small angle X-ray scattering SANS Small angle neutron scattering Amu Atomic mass unit L2MS Two-step laser desorption ionization mass spectrometry LIAD laser-induced acoustic desorption FFTEM Freeze-fracture-transmission electron-microscopy WAXS Wide angle x-ray scattering CMC Critical micelle concentrations PV Photovoltaic

Chl-a Chlorophyll A STM Scanning tunneling microscope SEM Scanning electron microscope

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TCO Transparent conductive oxide ITO Indium Tin oxide FTO Fluorine doped tin dioxide LUMO Lowest unoccupied molecular orbital HOMO Highest occupied molecular orbital TBAOH Tetrabutylammonium hydroxide Voc Open circuit Voltage Isc Short circuit current IPCE Incident Photon to Current Efficiency FF Fill factor UV.VIS Ultra violet visible MPP Maximum power point PEC Photoelectrochemical

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Vita

Hassan Mohamed Sharif was born on the Tenth of April, 1966, in Alkhoms, Libya. He

holds a BSc in Environmental Studies from Sebha University college of Engineering and

Technology formally (Higher Institute of Technology) Brack Libya, since 1987, and MSc in

Environmental Science & Engineering from Delft International Institute for Infrastructural,

Hydraulic, and Environmental Engineering (IHE) Netherlands 1999. His Master’s thesis was

about the groundwater contamination by wastewater in Libya problem and solution. After his

graduation he worked for ten years in Wastewater Treatment Plant of Ras Lanuf Oil and Gas

Processing Company and then in 2002 he moved to Arabic Cement Company as Environmental

Research Consultant. In 2005, He joined the college of Science at Alemrgib University Alkhoms

Libya where he works as instructor for three years. During this period of time he taught many

courses (such as Water & wastewater treatment, Solid waste management). In addition to

teaching, he worked as a BSc research supervisor. In reward of his hard work and achievement,

he has awarded a PhD scholarship from the Ministry of High Education In Libya in 2008.

Hassan came to the United States in Aug, 2008 and in Fall 2009 joined the

Environmental Science and Engineering PhD program at the University of Texas at El Paso.

Permanent address: 5450 Suncrest Dr

El Paso, Texas, 79912

This dissertation was typed by Hassan Sharif