i

Islamic University of Gaza

Deanery of Higher Studies

Faculty of Science

Department of Physics

Natural Pigments Extracted from Plant Leaves

as Photosensitizers for Dye-sensitized Solar Cells

الأصباغ الطبيعية المستخلصة من أوراق النباتات

في الخلايا الشمسية الصبغيةBy

Almohanad Mohammad Dawoud

B.Sc. in Physics, Islamic University of Gaza

Supervisors

Dr. Taher M. El-Agez Dr. Sofyan A. Taya Associate Professor of Physics Associate Professor of Physics

Submitted to the Faculty of Science as a Partial Fulfillment of the Master

Degree of Science (M. Sc.) in Physics

1437 – 2016

ii

iii

Dedication

I dedicate my dissertation work to my family for their endless love, support and

encouragement. A special feeling of gratitude to my loving parents, whose words of

encouragement and push for tenacity ring in my ears and to my beloved brothers and

sisters who stands by me when things look bleak. I also dedicate this dissertation to

my friends who have supported me throughout the process. I will always appreciate

all they have done.

iv

Acknowledgment

First and foremost, I thank Allah for endowing me with health, patience, and

knowledge to complete this work.

I would like to express my sincere thanks and gratitude to my supervisors Dr. Sofyan

Taya and Dr. Taher El-Agez. Despite being busy in various activities, they was always

available for his excellent guidance. As a matter of fact without there enormous

support and encouragement, it won't be possible for me to complete this master

research work.

Special acknowledgement is also given to the Islamic university of Gaza and the staff

members in Physics department for their continued support. I have to give a special

mention for the support and assistance given by Hatem El-Ghamri during experimental

work.

I also thank my entire family for all their support, from the first to the last day of my

study. So many thanks go to my parents for their unending support through my studies,

as well as initially providing me with the drive and desire to succeed in something

which is so difficult. Thank you so much.

Last, but not least, I am grateful to every individual who has helped me in one way or

another during my master study.

v

What most people know but don't realize they know is that the world is almost

entirely solar-powered already. If the sun wasn't there, we'd be a frozen ice ball at

three degrees Kelvin, and the sun powers the entire system of precipitation. The

whole ecosystem is solar-powered.

Elon Musk

vi

Abstract

The objectives of this thesis are to prepare dye sensitized solar cells (DSSCs) using

titanium dioxide (TiO2) based on twelve natural dyes extracted from plant leaves which

are lemon, mandarin, orange, almond, avocado, peach, pomelo, apple, fig, loquat,

orange, olive and pomegranate as sensitizers. Among the previous dyes, lemon showed

the best efficiency.

The performance of DSSCs are improved through adjustment of the solution extraction

temperature, the pre-treatments of the fluorinated tin oxide (FTO) glass substrate and

post-treatments of the TiO2 film using hydrochloric (HCl), phosphoric (H3PO4), and

nitric (HNO3) acids.

The results showed that an extracting temperature of 50 °C could be used as an optimal

value. In this case, the short current density Jsc rose from 0.77 (30 °C) to 1.52 mA/cm2

and the photoelectric conversion efficiency of the DSSC rose from 0.24% to 0.5%.

The dye sensitized solar cells extracted from lemon, orange and mandarin leaves showed

improvement in efficiency by 140%, 180% and 148% respectively as a result of the

surface pre-treatments of the glass using nitric acid.

The TiO2 semiconductor layer using a nitric acid post-treatment showed an increase in

dye sensitized solar cell efficiency by 128% and the short current density Jsc rose from

1.08 to 1.25 mA/cm2, while the open circuit voltage decreased from 0.61 to 0.58V.

Key words: Dye sensitized solar cell, natural dyes, solar cell.

vii

Abstract in Arabic

تحضير خلايا شمسية صبغية باستخدام ثاني أكسيد التيتانيوم كطبقة شبه إلىهذا البحث يهدف

، ن البرتقالالليمو النباتات وهي أوراقطبيعية مستخلصة من ةصبغ اثني عشرموصلة معتمدة على

هذه . من بين ليلبومواالتفاح ، لزيتون ا الخوخ، الأفوكادو،اللوز، التين، البشملة، ،الرمان، الكلمنتينا

أعلى كفاءة. الليمون أظهر الأوراق

ن خلالم بعض الأصباغ السابقةلقد تم تحسين أداء الخلايا الشمسية الصبغية المستخلصة من

باستخدام 2TiOوطبقة شبه الموصل FTOسطح معالجة و ضبط درجة حرارة استخلاص الصبغة

. بالإضافة إلى ذلك HNO)3(، والنيتريك )4PO3H(، الفوسفوريك ) (HClأحماض الهيدروكلوريك

الاستخلاص مع ثبات درجة الحرارة. تم تسريع عملية استخلاص الصبغة بواسطة تعديل زمن

لها كقيمة مثلى لاستخلاص الصبغةمكن استعماي سيلسيوس 50النتائج أن درجة الحرارة أظهرتو

أن الخلايا الشمسية الصبغية المستخلصة من صبغة كماساعات. 3وذلك في مدة زمنية قدرها

على %148، %180، %140كفاءة بنسبة تحسنا في ال أظهرت الكلمنتيناو الليمون والبرتقالأوراق

جة لطبقة . في حين أن المعالالنيتريكنتيجة المعالجة لسطح الزجاج باستخدام حمض الترتيب وذلك

.% 128كفاءة الخلية بنسبة في زيادة تباستخدام حمض النيتريك أظهر 2TiOشبه الموصل

الخلايا الشمسية الصبغية، أصباغ طبيعية، خلايا شمسية. كلمات مفتاحية:

viii

Table of Contents

Dedication ...................................................................................................................... iii

Acknowledgment .......................................................................................................... iv

Abstract ......................................................................................................................... vi

Abstract in Arabic ........................................................................................................ vii

List of Figures ............................................................................................................ xi

List of Tables .......................................................................................................... xiii

List of Abbreviations .................................................................................................... xiv

1 Introduction ................................................................................................................. 1

1.1 Energy resources .................................................................................................... 1

1.1.1 Non- renewable energy resources ........................................................................ 1

1.1.2 Renewable energy resources ................................................................................ 3

1.2 Energy usage.............................................................................................................. 7

1.3 A history of solar technology................................................................................. 8

1.4 Solar cells generations ............................................................................................... 9

1.5 The solar spectrum ................................................................................................... 11

1.6 Energy sources in Palestine ...................................................................................... 13

1.7 Purpose and outline of this thesis ........................................................................... 14

2 Theoretical Background…..................................................................................... 15

2.1 Historical background and state of the art.............................................................. 15

2.2 DSSCs basic materials............................................................................................. 17

2.2.1 Transparent conducting glass substrate .............................................................. 17

2.2.2 Nanocrystalline film (Photoanode) ..................................................................... 18

2.2.3 Dye sensitizers .................................................................................................... 18

2.2.4 Electrolyte and redox couples ........................................................................... 20

2.2.5 Counter electrode ................................................................................................ 21

2.3 DSSC operation ...................................................................................................... 21

2.4 Basic parameters to evaluate the performance of DSSC........................................ 24

2.4.1 Short circuit current ............................................................................................. 24

2.4.2 Open circuit voltage ............................................................................................ 25

2.4.3 Fill factor ............................................................................................................. 25

2.4.4 Power conversion efficiency ............................................................................. 25

ix

2.4.5 Series and shunt resistance ....................................................................................26

2.5 Advantages of the dye-sensitized solar cells......................................................... 27

3 Experimental work ................................................................................................... 28

3.1 Device characterization ......................................................................................... 28

3.1.1 Ultraviolet–visible spectroscopy ....................................................................... 28

3.1.2 Current-Voltage measurements .......................................................................... 29

3.1.3 Electrochemical impedance spectroscopy ......................................................... 30

3.2 Device fabrication.................................................................................................... 31

3.2.1 Materials ............................................................................................................. 31

3.2.2 Tools ................................................................................................................... 32

3.2.3 Preparation of TiO2 paste ................................................................................. 32

3.2.4 Preparation of TiO2 electrode ...................................................................... 32

3.2.5 Preparation of natural dye sensitizers .............................................................. 33

3.2.6 DSSC assembling ............................................................................................... 34

3.3 Experimental studies ............................................................................................. 34

3.3.1 Testing twelve natural dyes ............................................................................ 34

3.3.2 Optimizing of the extracting temperature and time of lemon leaves ...... 35

3.3.3 Effect of pre-treatment of FTO glass substrates ………………………….. 35

3.3.4 Effect of surface post-treatment of TiO2 layer ………………………….... 35

4 Results and Discussions ....................................................................................... 36

4.1 Testing twelve natural dyes.............................................................................. 36

4.1.1 Absorption spectra of the dyes ……………………………………………… 36

4.1.2 J-V characterization of DSSCs …………………………………………….. 37

4.2 Optimizing of the extracting temperature and time of lemon leaves dye... 44

4.2.1 J-V characterization of DSSCs at different extracting temperature ……….44

4.2.2 J-V characterization of DSSCs at different extracting times ………………47

4.3 Effect of pre-treatment of FTO glass substrates................................................ 49

4.3.1 J-V characterization of DSSCs with the pre-treatment of FTO glass........ 50

4.4 Effect of surface post-treatment of TiO2 layer ……………………………… 55

4.4.1 J-V Characterization of DSSCs with post-treatment of TiO2 electrode ……. 55

4.5 Electrochemical impedance spectroscopy (EIS) analysis …………………….. 58

4.5.1 EIS of the DSSC sensitized by lemon leaves without any treatment …….……. 58

x

4.5.2 EIS of the DSSC sensitized with lemon leaves with pre-treated FTO ……...… 60

Conclusion ………………………………………………………………….……..... 61

References …………………………………………………………………….......... 63

xi

List of Figures

Fig. 1.1 World energy consumption by source 2014………………………..………… 7

Fig. 1.2 Solar cell generations ……………………………………………….………. 10

Fig. 1.3 Incident spectrum of sunlight ……………………………………………….. 11

Fig. 1.4 Solar irradiance spectra …………………………………………………... 12

Fig. 2.1 Overview of the working principle of DSSC …………………………….. 23

Fig. 2.2 Illustration of current-voltage characteristics of a solar cell ……………… 24

Fig. 2.3 The equivalent circuit of a solar cell ……………………………………… 26

Fig. 3.1 UV-Vis spectrophotometers used in this study …………………………….29

Fig. 3.2 Simplified layout of solar simulation system ……………………..…………29

Fig. 3.3 Schematic representation of doctor blade method………………...…………..33

Fig. 3.4 Assembled dye sensitized solar cell……………………………………….34

Fig. 4.1 UV–Vis absorption spectra of natural dyes extracted ……………..……….. 36

Fig. 4.2 J–V curves for the DSSCs sensitized by lemon, mandarin and orange ……. 37

Fig. 4.3 J–V curves for the DSSCs sensitized by peach, loquat and pomegranate ......38

Fig. 4.4 J–V curves for the DSSCs sensitized by almond, fig and pomelo ………… 38

Fig. 4.5 J–V curves for the DSSCs sensitized by olive, apple and avocado ……..…. 39

Fig. 4.6 P–V curves of the DSSCs using lemon, mandarin and orange ………….… 39

Fig. 4.7 P versus V curves of the DSSCs using peach, loquat and pomegranate ……40

Fig. 4.8 P versus V curves of the DSSCs using almond, fig and pomelo ………… 40

Fig. 4.9 P versus V curves of the DSSCs using olive, apple and avocado ……………41

Fig. 4.10 The basic molecular structure of Chlorophyll ……………………....……. 44

Fig. 4.11 J–V curves for the DSSCs sensitized at different extracting temperature… 45

Fig. 4.12 P-V curves of the DSSCs sensitized at different extracting temperatures... 46

Fig. 4.13 DSSC efficiency versus the extracting temperature of lemon leaves…….. 46

Fig. 4.14 J–V curves for the DSSCs sensitized at different extracting times ………48

Fig. 4.15 P–V curves of the DSSCs sensitized at different extracting times ……........48

Fig. 4.16 DSSC efficiency (η) versus the extracting time of lemon leaves ……...…49

Fig. 4.17 J–V curves for the DSSCs with the pre-treatment of FTO using lemon …50

Fig. 4.18 P–V curves of the DSSCs with the pre-treatment of FTO using lemon dye..51

Fig. 4.19 J–V curves of the DSSCs with the pre-treatment of FTO using mandarin..51

Fig. 4.20 P–V curves of the DSSCs with the pre-treatment of FTO using mandarin..52

xii

Fig. 4.21 J–V curves for the DSSCs with the pre-treatment of FTO using orange…. 52

Fig. 4.22 P–V curves of the DSSCs with the pre-treatment of FTO using orange…. 53

Fig. 4.23 J–V curves of DSSCs with post-treatment of TiO2 electrode ……………...56

Fig. 4.24 P–V curves of the DSSCs with post-treatment of TiO2 electrode ……...... 56

Fig. 4.25 Nyquist plots of the DSSC sensitized by lemon leaves without treatment.. 59

Fig. 4.26 The equivalent circuit for the DSSCs without treatment ………………59

Fig. 4.27 Nyquist plots of the DSSC sensitized by lemon leaves with nitric acid pre-

treatment of FTO glass substrates ………………………..………………………… 60

xiii

List of Tables

Table 3.1 Photos of the dyes sources extracted from the leaves of plants .…..…31

Table 4.1 Photovoltaic parameters of the DSSCs sensitized by 12 kinds of natural dyes

extracted from plant leaves ………………………………………………….………41

Table 4.2 Solar cell parameters of the DSSCs sensitized with lemon leaves dye

extracted at different temperatures …………………………………………..………47

Table 4.3 Photoelectrochemical parameters of the DSSCs sensitized with lemon

leaves dye extracted at various times at 60º C ……………………………….………49

Table 4.4 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using lemon leaves dye ………….…… 53

Table 4.5 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using mandarin leaves dye ……………54

Table 4.6 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using orange leaves dye ………… 54

Table 4.7 Photovoltaic parameters of the DSSCs with post-treatment of TiO2

electrode by HCL and HNO3 acids ……………………………………………….. 57

Table 4.8 EIS results from data-fitting of Nyquist plots to the equivalent circuit model

in Fig. 4.26 ………………………………………………………….……………… 59

Table 4.9 EIS results from data-fitting of Nyquist plots to the equivalent circuit model

in Fig. 4.27 ………………………………………………………………..............…60

xiv

List of Abbreviations

A Absorbance

a.u. Arbitrary units

AM 1.5 G Air mass 1.5 global spectrum

AM0 Air mass zero

a-Si Amorphous silicon

CO2 Carbon dioxide

CIGS Copper indium gallium selenide

CdTe Cadmium telluride

RCT Charge transfer resistance

oC Celsius degree

I-V Current voltage

J-V Current density – Voltage

N3 cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid)

ruthenium(II) - C26H16O8N6S2Ru

N719 cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylato)

ruthenium(II) bis(tetrabutylammonium) - C58H86O8N8S2Ru

DSSC Dye Sensitized Solar Cell

S* Excited energy state of the sensitizer

eV Electron volt

Eg Energy bandgap

EIS Electrochemical impedance spectroscopy

e- Electron

FF Fill factor

FRA Frequency response analyzer

FTO Fluorine-doped tin oxide

GW Gigawatts

S Ground energy state of the sensitizer

HOMO Highest occupied molecular orbital

xv

HCl Hydrochloric acid

IR Infrared

I- Iodide

I3- Tri-iodide

ITO Indium-doped tin oxide

IUG Islamic university of Gaza

Jsc Short circuit current density

oK Kelvin degree

LOMO Lowest unoccupied molecular orbital

MW Megawatts

Pm Maximum power

Vm Maximum voltage

nm Nanometer

HNO3 Nitric acid

S+ Oxidized state of the sensitizer

SnO2 Tin oxide

PV Photovoltaics

Pin Power of incident light

Pt Platinum

H3PO4 Phosphoric acid

R Resistance

Ru Ruthenium

RS Series resistance

Rsh Shunt resistance

xvi

Isc Short circuit current

T Temperature

TW Terawatts

TiO2 Titanium dioxide

TCO Transparent conducting oxide

UV Ultraviolet

UV-Vis Ultraviolet-Visible

Z' Real impedance

Z" Imaginary impedance

ZC Capacitance impedance

Zim Imaginary impedance

ZR Resistance impedance

Zre Real impedance

α Absorption coefficient

η Solar energy to electric power conversion efficiency

λ Wavelength of the light

λmax Maximum wavelength

1

Chapter 1

Introduction

This chapter reviews renewable and non-renewable energy resources as well as the

global usage of each resource. The chapter also includes a history of solar cell

technology, solar cell generations and solar spectrum. Finally, the purpose and outline

of this thesis are introduced.

1.1 Energy resources

Energy may be our most important resource, and all societies require energy services

to meet basic human needs. Abundant energy makes it easy to be sustainable because

it can be used to produce essential resources such as lighting, food, communication

and potable water where they are scarce. Today the greatest attention in the world is

devoted to energy resources while many areas of the world are experiencing energy

shortages. Human energy consumption has grown steadily throughout human history.

Early humans had modest energy requirements, mostly food and fuel for fires to cook

and keep warm. Thus, the continuous increase in energy consumption nowadays has

led to take advantage of currently available energy resources. An energy resource is

something that can produce heat or electricity. Primary energy sources take many

forms, including nuclear energy, fossil energy (non-renewable) like oil, coal and

natural gas and renewable sources like wind, solar, geothermal and hydropower. These

primary sources are converted to electricity, a secondary energy source, which flows

through power lines and other transmission infrastructure to your home and business.

1.1.1 Non-renewable energy resources

Energy sources are classified as nonrenewable if they cannot be replenished in a short

period. A non-renewable resource is a resource that does not renew itself at a sufficient

rate for sustainable economic extraction in meaningful human time-frames. Most non-

renewable energy sources are fossil fuels, which converted to usable energy by thermal

reactions.

There are three major non-renewable energy resource:

2

Oil

Oil is available in abundance in most of the middle east countries including Saudi

Arabia, Kuwait, Iran, Iraq and UAE while some limited oil wells are present in North

America and Canada. Most of the countries still have their huge dependency on these

countries for their oil requirements. Oil was made out of dead plants and animals that

had lived millions of years ago. When plants and animals died they were covered with

thick layer of mud and sand which created huge pressure and temperature. Wide usage

of oil and oil related products has resulted in massive air pollution. It is a major source

of fuel that is used in vehicles. Due to the process of combustion, harmful gases like

carbon dioxide are released when oil is burnt. Oil is transported to other nations using

pipelines or ships. Leakage in ships leads to oil spill which affects animals and plants

that live inside or around the sea.

Natural gas

Natural gas consists primarily of methane plus minor amounts of other light

hydrocarbons. It is associated with oil because it forms from similar material through

similar processes. Methane that escapes to the atmosphere is considered a pollutant

and is a potent greenhouse gas. Natural gas is considered a “clean” fossil fuel because

burning it emits less CO2 and sulfur per unit energy than other fossil fuels, so it causes

the least environmental damage than coal or oil. It is the fuel most commonly used for

heating. One downside of natural gas is that it takes up more volume than liquid oil

and gasoline, making it more expensive to transport. Currently natural gas is

transported under pressure and is highly flammable, which may result in dangerous

explosions and fires.

Coal

Coal is the most abundant form of fossil fuel available on earth. They were formed by

the decay of old plants and animals several centuries ago. Coal is mostly found below

the earth and is major source of fuel for electricity generation as of today. Most power

stations on earth require huge reserves of coal to produce electricity continuously

without break. Because coal takes millions of years to form, it is considered a non-

renewable resource, meaning that coal use is unsustainable. Known reserves of coal

3

far exceed those of other fossil fuels, and may be our best bet for an energy source of

the future. Still, burning of coal produces large amounts of waste products, like SO2

and soot, that pollute the atmosphere. This problem needs to be overcome before we

can further exploit this source of energy. Coal is the dirtiest source of energy. It has

the largest carbon footprint, meaning it emits more CO2 per unit energy over its life

cycle than any other energy source. Coal burning releases more CO2 worldwide than

any other human activity. Carbon dioxide is one the gas responsible for global

warming. Coal burning also releases toxic metals like mercury, sulphurous and nitrous

oxides that contribute to acid rain, and particulates and ozone that contribute to ground-

level air pollution.

1.1.2 Renewable energy resources

Renewable energy is generated from sources that are derived from, and quickly

replenished by, the natural movements and mechanisms of the Earth. For decades now,

the perception of dwindling fossil fuels has motivated government, economists, and

scientists to find an alternative energy source that is not only endless in supply, but

also inexpensive and comparable in terms of storage density and capacity to fossil

fuels. Therefore, the largest challenge for our global society is to find ways to replace

the vanishing fossil fuel supplies by renewable resources, and at the same time, avoid

negative effects from the current energy system on climate, environment, and health.

For development to be sustainable, delivery of energy services needs to be secure and

have low environmental impacts.

There are five major renewable energy resource:

Wind energy

Winds develop when solar radiation reaches the Earth’s highly varied surface

unevenly, creating temperature, density, and pressure differences. Tropical regions

have a net gain of heat due to solar radiation, while polar regions are subject to a net

loss. This means that the Earth’s atmosphere has to circulate to transport heat from the

tropics towards the poles. The Earth’s rotation further contributes to semipermanent,

planetary-scale circulation patterns in the atmosphere. Topographical features and

local temperature gradients also alter wind energy distribution. A region’s mean wind

4

speed and its frequency distribution have to be taken into account to calculate the

amount of electricity that can be produced by wind turbines. Wind resources can be

exploited in areas where wind power density is at least 400 watts per square m at 30

m above the ground (or 500 watts per square m at 50 m). Moreover, technical advances

are expected to open new areas to development. The following assessment includes

regions where the average annual wind power density exceeds 250–300 watts per

square m at 50 m. Wind power results in zero emissions, but can only be effectively

utilized in a limited number of locations. Startup costs are large and require large areas

of land to establish such a “wind farm”.

Hydropower

Hydropower is power derived from the energy of falling water or fast running water,

which may be harnessed for useful purposes. Water, when it is falling by the force of

gravity, can be used to turn turbines and generators that produce electricity.

Hydroelectricity, which depends on the natural evaporation of water by solar energy,

is by far the largest renewable resource used for electricity generation. Because rainfall

varies by region and even country, hydro energy is not evenly accessible. Although

the effective cost is zero but the manufacturing and building a dam and installation of

the turbines is very costly due to which many countries do not employ this alternative

source of energy. If the initial cost had been less; then it would have used more

commonly. Its construction requires a lot of human capital and labor. Its maintenance

is also very costly. Although hydroelectricity is generally considered a clean energy

source, it is not totally devoid of greenhouse gas emissions, ecosystem load, or adverse

socioeconomic impacts. For comparable electricity outputs, greenhouse gas emissions

associated with hydropower are one or two orders of magnitude lower than those from

fossil-generated electricity. Ecosystem impacts usually occur downstream and range

from changes in fish biodiversity and in the sediment load of the river to coastal erosion

and pollution.

Geothermal energy

Geothermal energy is generally defined as heat stored within the Earth. The Earth’s

temperature increases by about 3 degrees Celsius for every 100 metres in depth, though

5

this value is highly variable. Heat originates from the Earth’s molten interior and from

the decay of radioactive materials. Four types of geothermal energy are usually

distinguished:

- Hydrothermal—hot water or steam at moderate depths (100–4,500 m).

- Geopressed—hot-water aquifers containing dissolved methane under high

pressure at depths of 3–6 km.

- Hot dry rock—abnormally hot geologic formations with little or no water.

- Magma—molten rock at temperatures of 700–1,200 degrees Celsius.

Today only hydrothermal resources are used on a commercial. Environmental aspects

of geothermal energy use relate primarily to gas admixtures to the geothermal fluids

such as carbon dioxide, nitrogen, hydrogen sulphides or ammonia and heavy metals

such as mercury.

Biomass energy

Biomass can be classified as plant biomass (woody, non-woody, processed waste, or

processed fuel) or animal biomass. Most woody biomass is supplied by forestry

plantations, natural forests, and natural woodlands. Non-woody biomass and

processed waste are products or by-products of agroindustrial activities. Animal

manure can be used as cooking fuel or as feedstock for biogas generation. Municipal

solid waste is also considered a biomass resource. For biomass to become a major fuel,

energy crops and plantations will have to become a significant land-use category. Land

requirements will depend on energy crop yields, water availability, and the efficiency

of biomass conversion to usable fuels. Assuming a 45 percent conversion efficiency

to electricity and yields of 15 oven dry tonnes a hectare per year, 2 square km of

plantation would be needed per megawatt of electricity. Biomass is often perceived as

a fuel of the past because of its low efficiency, high pollution, and associations with

poverty. Biomass is the fuel most closely associated with energy-related health

problems in developing countries. Exposure to particulates from biomass or coal

burning causes respiratory infections in children, and carbon monoxide is implicated

in problems in pregnancy. In addition, large-scale production of biomass can have

considerable negative impacts on soil fertility, water and agrochemical use, leaching

of nutrients, and biodiversity and landscape. The collection and transport of biomass

6

will increase vehicle and infrastructure use and air-borne emissions. The biomass

challenge is not availability but sustainable management, conversion, and delivery to

the market in the form of modern and affordable energy services.

Solar energy

Solar energy is a constant renewable resource available in many areas of the world and

offers a continuous increase of achievable efficiencies. Of all the available

technologies producing renewable energy, photovoltaic energy is a hot topic in current

research. All renewable energy sources have many advantages but solar energy is the

only in this competition which can meet the global demand by providing full spectral

composition of sunlight with vast amount of clean energy. While silicon-based

technologies have been developed to harness solar energy efficiently, they are not yet

competitive with fossil fuels mainly due to the high production costs. It is an urgent

task to develop much cheaper photovoltaic devices with reasonable efficiency for

widespread application of photovoltaic technology. In this context, dye sensitized solar

cells (DSSCs) have emerged as an important alternative to conventional silicon solar

cells owing to their fascinating features such as low fabrication cost and relatively high

efficiency. At present, continuous consumption of these stored energy resources have

caused not only scarcity but also a severe impact on the environment due to

increasing CO2 levels producing problems such as those presented by greenhouse

gases and by environmental pollutants. Addressing such issues and the ever-

increasing human demand for energy has led to the need to develop new direct solar

energy conversion methods. Solar energy represents a clean, vast and a renewable

energy source and is therefore an excellent candidate for a future environmentally

friendly energy source. However, the large-scale availability of solar energy will thus

depend on a region’s geographic position, typical weather conditions, relatively high

production costs of conventional solar cells and land availability, which have limited

their widespread commercialization. We clearly must move toward a more sustainable

energy economy. So,with growing demands for energy, the necessity of stemming

increases in carbon dioxide emissions and the need to develop renewable, clean

sources of energy are becoming increasingly crucial. Solar energy holds the

possibility of addressing these energy concerns. Fabrication of dye-sensitized solar

7

cells (DSSCs) offers a potential solution to this problem, as these novel cells

can conceivably be produced more inexpensively compared to standard Si-based

cells. Recent research and technological developments of DSSCs have attracted

much attention because of their characteristics of being inexpensive and having

the capability for large-scale solar energy conversion .

1.2 Energy usage

With the explosion of the world population at the beginning of the 20th century and

its growing energy needs, humans started to exhaust non-renewable sources of energy

like oil, gas and coal extensively. In the recent world of technological

developments, energy has become one of the important requirements for life.

Fossil fuels are non-renewable energy sources and they are the basis to run our

planet these days. In 2014, the worldwide energy consumption primarily originates

from fossil fuels, e.g. 35.7% come from oil, 19.3% from coal, 25.6% from gas and

9.5% from renewable energy sources as illustrated in Fig.1.1 [1].

Fig. 1.1. World energy consumption by source 2014.

Energy from renewable sources i.e. biomass, hydropower, wind, solar, and geothermal

currently accounts for about 9.5% of global energy consumption. Worldwide Solar

Photovoltaics (PV) capacity reached 135 gigawatts in 2013, and the current solar PV

35.7%

19.3%

25.6%

9.9%

9.5%

Oil

Coal

Gas

Nuclear

Renewable

8

capacity growing to 1,721 gigawatts in 2030 and a massive 4,674 gigawatts in 2050

[2]. That would be able to generate around 16% of global electricity needs. Today,

global primary energy is consumed at a rate of about 16 TW, whereas our planet

receives about 173 × 103 TW of solar radiation, which is about 10000 times more than

the global population currently consumes. Even if only a fraction of this energy can be

harvested, the solar energy source is enormous and dwarfs all known non-renewable

sources. The current energy demand can be fully met by covering 0.15% of earth’s

surface with 10% efficiency solar cell panels [3]. Our world will need at least 10

Terawatts of additional energy from clean and low-cost energy source by 2050 to

maintain worldwide peace and prosperity [3]. In 2014, Geothermal heating currently

produces over 12.8 GW of power, while solar and wind energy produce 180 GW and

373 GW of power respectively.

Whilst this thirst for energy is clearly not diminishing, the resources from which we

gain this energy are. Oil, gas and coal are all products that are highly energy rich and

convenient to process, however come with a price. High CO2 emissions create an

imbalance in the earth’s atmosphere, the consequences of which are well known and

may prove to be cataclysmic. Whilst nuclear energy provides similar energy outputs

to traditional fossil fuels without the carbon footprint, the nuclear disaster at

Fukushima Daiichi in Japan has served as a stark reminder of the significant risks

associated with this technology.

1.3 A History of solar technology

The first solar cell concept came into existence in the 1800s, when it was

observed that the presence of sunlight was capable of generating usable electrical

energy. Alexandre Becquerel observed the photoelectric effect via a silver-coated

platinum electrode in a conductive electrolyte solution exposed to light in 1839 [4]. In

1883, Charles Fritts developed a solar cell using selenium on a thin layer of gold to

form a device giving less than 1% efficiency. Adams and Becquerel described the basis

for the first selenium solar cell, which was built in 1877 [5]. This phenomenon was

not fully understandable until the theoretical explanation of the photoelectric effect by

Einstein in 1905. He was awarded a Nobel Prize in 1921 for his theoretical explanation.

In 1918, a method for monocrystalline silicon production was discovered, which

9

enabled monocrystalline solar cells production [6]. The first silicon monocrystalline

solar cell was constructed in 1941. In 1955, a 2% efficient commercial solar cell was

created. In the following years, the use of a grid contact technique method to

reduce the cell's resistance was introduced. In 1958, the first solar powered satellite,

Vanguard I, was launched with a 0.1W, 100-cm² solar panel. After Vanguard I, many

progresses in the space field happened: in 1964 the Nimbus spacecraft was launched

with a 470 W PV array; in 1966 the Orbiting Astronomical Observatory was

launched with a 1 kW PV array. However, the cost of the photogenerated electricity

was so high that this newly born technique became an exception, only used in

extraterrestrial applications

In the 1970s, there was a dramatic growth in research funding in solar energy due to

the energy crisis. Solar cells were of significant interest during this period and various

methods of producing cells with improved efficiencies were explored. The

fundamental understanding of modern photovoltaic science was developed and rooted

mainly in that period. Moreover, some photovoltaic companies were established in the

energy market, like Solar Power Corporation in 1972, Solarex Corporation in 1973

and Solec International and Solar Technology International in 1975.

In 1991, Michael Grätzel and O’ Regan introduced a new kind of photovoltaic

device based on charge separation processes by a sensitizer as light-absorbing

material associated with a wide band gap nanocrystalline semiconductor. This new

type of solar cells has attracted worldwide attention because of their low cost and good

levels of efficiency [7].

1.4 Solar cells generations

There are many different types of solar cells such as the silicon solar cell, solid-state

solar cell, organic solar cell, thin-film solar cell, thick-film solar cell, and dye-

sensitized solar cell. These different types of solar cells are usually classified

into three main categories based on their performance and cost effectiveness as

shown in Fig. 1.2.

10

Fig. 1.2. Solar cell generations

The first generation of solar cells is based on bulk silicon photovoltaic cells. These

solar cells are fabricated on single crystal silicon wafers and demonstrate a high

efficiency. Single junction silicon devices are approaching the theoretical limiting

efficiency of 31%. Due to their high efficiency, they still dominate the solar market.

The cost of silicon wafers has restricted their wide spread use in energy generation.

The second generation solar cells are based on thin-films materials that are cheaper

to produce. However, thin film solar cell efficiency is inferior compared to first

generation silicon solar cells. Thin film solar cells have drawn a wide spread interest

from residential solar markets because of cost, flexibility, lighter weight, and ease of

integration, compared to wafer silicon cells. The most popular materials used for

second generation thin-film solar cells are amorphous silicon (a-Si), copper indium

gallium selenide (CIGS), cadmium telluride (CdTe), copper zinc tin sulfide (CZTS)

and polycrystalline silicon. CIGS and CdTe solar cells have certified efficiencies of

about 21%.

The third generation solar cells utilize new materials, architecture and solar cell

concepts. They are in general not focused on typical p-n junction based energy

conversions. The focus is to find highly efficient, and low cost alternatives to

traditional silicon solar cells. Several new solar cell technologies such as polymer-

photovoltaic

1st gen classical silicon

Mono c-Si

Poly c-Si

2nd gen Thin film

a-Si CIGS CdTe

3rd gen organic

DSSCQuantum

DotOrganic ploymer

11

based cells, quantum dots, tandem/multi-junction cells, and dye-sensitized solar

cells have been proposed in recent years.

1.5 The solar spectrum

The spectrum of the solar light that reaches the earth is influenced by absorption of

radiation in the earth’s atmosphere and therefore also by the path length of the photons

through the atmosphere. The Sun is capable of sustaining fusion reactions in its core

and constantly radiates light with a power of 4×1024 W. Its emission spectrum is close

to that of a blackbody with a temperature of 5800º K. Only a small fraction (1.74×1017

W) of this power enters the Earth’s atmosphere, where about 42% of the light is lost

due to absorption and scattering. The amount of light that strikes the Earth’s surface is

over 5000 times larger than current energy consumption.

Fig. 1.3. Incident spectrum of sunlight just above the earth’s atmosphere (AM0) and

at the earth’s surface (AM1.5G), used as a measurement standard.

12

Air mass zero (AM0) radiation is defined as the intensity of light of the sun outside

the Earth’s atmosphere. The accepted value of AM0 sunlight is 1.353 kW/m2, but is

attenuated by nearly 30 % inside the Earth’s atmosphere. AM1 radiation is the incident

radiation intensity on the Earth’s surface when the sun is directly overhead. The most

commonly used spectrum is the Air Mass 1.5 Global (AM 1.5G). The AM1.5G

spectrum corresponds to an angle between the incident solar radiation and the zenith

point of the measurements of 48.2°, and integrates to 1000 Wm-2. Once the light enters

the earth’s atmosphere, the intensity is attenuated by scattering from molecules,

aerosols and dust particles, as well as absorption by gases in the atmosphere. These

are highlighted in Fig. 1.4 where there are dips in the spectral irradiance intensity from

absorption by H2O, O3, O2 and CO2.

Fig. 1.4. Solar irradiance spectra at the top of the atmosphere and at sea level (AM

1.5G) compared to the spectrum of a 5250 °C black body.

The level of change is highly dependent on the amount of atmosphere the light has to

pass through. When the sun is directly overhead, the distance the light has to travel to

reach the earth is minimum. As the sun moves away from the zenith through the sky,

13

light has to pass through a greater portion of the atmosphere, and so the intensity of

light is reduced as it reaches the earth’s surface. Assuming the sun is at an angle, θ,

from the zenith, then the distance the light has to travel through the atmosphere is the

Air Mass = 1/cos θ. Assuming the sun is overhead, the Air Mass is defined as 1, or

AM1, and increases as it passes through the sky. For standard measurement conditions,

AM1.5G has been used as an average spectrum for terrestrial applications, which is

the global irradiance from the sun at an angle of approximately 48° from the zenith.

1.6 Energy sources in Palestine

The energy sources in the Palestinian territories are very limited and it depends on the

imports of different types of fuel from the Israeli occupation government and

neighboring countries like Egypt and Jordan. An energy crisis is currently hitting the

Gaza Strip's public services hard and could lead to a severe humanitarian crisis if a

sustainable solution is not found soon. The Gaza power station was completely

dependent on external aid and had to reduce electricity production due to supply

limitations imposed by the Israeli occupation government and some economic and

political factors. This situation continued to impact on the daily life of Gaza

inhabitants. Gaza Strip is currently supplied with only 208 Megawatts (MW) of

electricity from three sources: purchases from Israel (120 MW) and Egypt (28 MW),

and production by the Gaza power plant (currently 60 MW). This supply meets only

46% of the estimated demand (452 MW) [8]. Rolling power outages currently reach

up to 12 hours per day and the electricity is expected to be available only 6 hours per

day at any time. Palestine receives a relatively high quality of solar energy all over the

year with an average solar radiation of 5.46 kWh/ m².day. The West Bank and Gaza

Strip are located at the geographic latitude of approximately 30º N, where the annual

incident solar irradiance is about 2000 kWh per m2. However, there is no use of solar

energy effectively inside Palestine. Therefore, the concerned authorities in Palestine

should adopt new policies in the field of energy, including the use of solar energy as

one of the effective solutions to the energy problem. In addition, research and

educational institutions in Palestine should focus on the areas of renewable energy,

particularly solar energy by providing the necessary support for research and studies

14

in this field and through the adoption of clear strategies to raise awareness of the solar

energy importance.

1.7 Purpose and outline of this thesis

A dye-sensitized solar cell (DSSC) is one of the third generation photovoltaic devices

that convert visible light into electric energy, which provides a technically and

economically credible alternative concept to silicon photovoltaic devices. The

performance of dye sensitized solar cells is strongly based on the dye as a

sensitizer. Natural dyes have become a viable alternative to expensive and rare

organic sensitizers because of its low cost, easy attainability, abundance in supply

of raw materials and no environment threat.

In this work various plant leaves have been tested as sensitizers. The

characterization of dye-sensitized solar cells based on natural dyes extracted from the

plant leaves will be studied. The current-voltage (I-V) characteristics of the dye-

sensitized solar cells will be employed to determine the power conversion

efficiency of the cells. The effects of changing extracting temperature and time will

be reported. The influence of acids on a fluorine-doped tin oxide (FTO) glass and TiO2

paste will be investigated and discussed.

This thesis is divided into four parts:

Chapter 1 gives an introduction to the thesis by describing the general overview

of the energy resources and briefly mention the purpose of the study as well as

the outline of the thesis.

Chapter 2 contains a thorough discussion about the operating principles of the

DSSCs, historical review of DSSC, device structure and DSSC basic materials.

Chapter 3 describes the experimental part of this work. The fabrication process

are discussed. A detailed description on the experimental procedures carried

out in this research is also provided.

Chapter 4 focuses on the results and discussions.

15

Chapter 2

Theoretical Background

In this chapter, the operation principle of dye sensitized solar cells and materials used

in the fabrication of DSSCs are presented. Historical review of DSSCs and key works

in the DSSCs field are reviewed.

2.1 Historical background and state of the art

The use of dye-sensitization in photovoltaics remained rather unsuccessful until

a breakthrough in 1991. In the Laboratory of Photonics and Interfaces in Ecole

Polytechnique Federale de Lausanne, Switzerland, Grätzel and his co-workers

developed a solar cell by the successful combination of nanostructured electrodes

and efficient charge injection dyes. This cell was hence termed the dye sensitized

nanostructured solar cell [9]. While initial indications showed great promise, two

problems, efficiency and durability, have yet to find effective solutions. Most research

into dye sensitized solar cells since their invention has focused on the various

components of the cells independently. These include organic and natural dyes,

electrolytic carriers, counter electrodes, and semi-conductors. As early as 1992,

Grätzel and others were searching for improvements in all of these. The use of

sintered mesoporous titanium dioxide (TiO2) was the breakthrough that established

DSSC technology and raised the DSSC efficiency from 1% to 7%. DSSCs have

been subjected of a large number of experimental investigations since then. An

impressive number of new sensitizers are being developed in recent years and thus can

be grouped into metal-complex based dye, natural dye, synthetic organic dye, and

polymer dye and quantum dot sensitizer.

Kumara et al. (2006) investigated synergistic sensitization by dye cocktail (shisonin

and chlorophyll) obtained from shiso leaves and successfully achieved broadening of

spectral response of dye-sensitized solar devices [10].

Yamazaki et al. (2007) assembled DSSCs by using natural carotenoids, crocetin and

crocin, as sensitizers and demonstrated that crocetin can attach effectively to the

surface of TiO2 film due to the presence of carboxylic groups in the molecule and

16

perform the better photosensitization effect than crocin that has no carboxylic group

in the molecule [11].

Furukawa et al. (2009) fabricated various DSSCs using natural dyes of red cabbage,

curcumin, red perilla and their mixtures. They found that the conversion efficiency of

the solar cells fabricated using the mixture of red cabbage and curcumin was much

larger than that of solar cells using one kind of dye. [12]

Sandquist and McHale (2011) explored the use of a blocking layer and treatment by

TiCl4 in order to optimize the performance of the betanin-based dye-sensitized solar

cells, and recorded the highest energy conversion efficiencies as high as 2.7% for a

DSSC containing a single unmodified natural dye sensitizer [13].

Oprea et al. (2012) identified betacyanins as the useful constituents of red beetroot

extract as natural sensitizers and emphasized on the role of extract purification for

better results [14].

Although a number of natural dyes have been tested for their performance in DSSCs,

most of them yielded conversion efficiencies of less than 1%. Betanin extracted from

beet roots achieved an efficiency of 2.71% [15], Xanthomonascin-A extracted from

Monascus purpureus achieved an efficiency of 2.3% [16], and isobutrin from Butea

monosperma achieved an efficiency of η =1.8% [17]. Chlorophyll derivatives yielded

conversion efficiencies over 2% [18]. Natural chlorophyll derivative chlorin-e6

exhibited energy conversion efficiencies over 4% [19].

Juices extracted from fruit were found to make good dyes for DSSCs. Blackberries

were found to be an excellent choice, as the anthocyanins bonded readily with

crystalline titanium dioxide [20]. These include berry juice, pomegranate juice and

hibiscus leaves [21]. Research on electrolyte solutions has been less intense since

Grätzel used an iodide solution in his research. While other electrolyte solutions have

been studied with varying success, use of any liquid poses problems [22]. A liquid

sealed permanently inside the cell poses manufacturing issues, and outdoor exposure

of liquid filled cells to low temperatures causes shortened life. A dry or “solid state”

cell is therefore considered far more practical, and has been the greatest focus of

research [23-25]. Some counter electrode work has also been carried out, although this

17

has been an area of secondary concern. Some of the research is dedicated to improving

the original carbon coating electrode used by Grätzel [26]. More efficient, although

probably not economically practical, have been counter electrodes produced by

various applications of platinum. A great deal of research has been dedicated to the

semi-conducting layer. Grätzel started with titanium dioxide, and this is still the

standard for dye sensitized solar cells, although other metal oxides including zinc, tin,

aluminum and others have been used with success. The dye sensitized solar cells can

achieve efficiencies over 10%, but have not yet been successfully scaled up for

production [27].

T. El-Agez et al. (2013) investigated the performance of dye-sensitized solar cells

based on natural dyes extracted from ten different plant seeds and the best performance

was for the DSSC sensitized with Eruca sativa with a solar energy conversion

efficiency of 0.725% [28].

S. Taya et al. (2014) fabricated various DSSCs using dried plant leaves and the best

performance was for the DSSC sensitized with Jasminum Grandifolium with a solar

energy conversion efficiency of 0.33% [29].

H. El-Ghamri et al. (2014) used natural dyes extracted from plant seeds in the

fabrication of dye-sensitized solar cells and found that the highest conversion

efficiency of 0.875% obtained with an allium cepa (onion) extract [30].

I. Radwan (2015) fabricated various DSSCs using natural dyes extracted from purple

carrot, carrot, beet, curcuma, kale and radish as sensitizers and found that the

conversion efficiency of the solar cells fabricated using the purple carrot was much

larger than that of solar cells using other dyes [31].

2.2 DSSCs basic materials

2.2.1 Transparent conducting glass substrate

Transparent conducting oxide (TCO) coated glass is used as the substrate for the TiO2

photo electrode. For high solar cell performance, the substrate must have low sheet

resistance and high transparency. In addition, sheet resistance should be nearly

independent of the temperature up to 500º C because sintering of the TiO2 electrode is

carried out at 450º to 500º C. Fluorine doped tin oxide SnO2 is one of the best known

18

TCO materials for DSSCs. It is low-cost and stable at high temperatures and in acidic

environments.

2.2.2 Nanocrystalline film (Photoanode)

Photo electrodes made of materials such as silicon and cadmium sulfide

decompose under irradiance in solution. In contrast, semiconductor oxide

materials, especially TiO2, have good chemical stability under visible irradiation

in solution. It has been found that TiO2 is a stable photoelectrode in photo

electrochemical systems even under extreme operating conditions. It is cheap,

readily available and non-toxic and is normally used as dye in white paint and

toothpastes. The high dielectric constant of TiO2 (€= 80 for anatase) provides

good electrostatic shielding of the injected electron from the oxidized dye

molecule attached to the TiO2, thus preventing their recombination before

reduction of the dye by the redox electrolyte. High refractive index of TiO2 (n

= 2.5 for anatase) results in efficient diffuse scattering of the light inside the

porous photoelectrode, which significantly enhances the light absorption. Dye

adsorption and microstructure of the TiO2 film are important properties when it

is used as photoelectrode for DSSCs. TiO2 occurs in three crystalline forms: rutile,

anatase and brookite. Anatase appears as pyramid-like crystals and is stable at

low temperatures whereas needle-like rutile crystals are dominantly formed in

high temperature processes. Rutile absorbs 4% of the incident light in the near

UV region. Brookite is difficult to produce and is therefore not considered in

DSSC application. The band-gap of anatase is 3.2 eV at an absorption edge of

388 nm and that of rutile is 3.0 eV at an absorption edge of 413 nm. The

common techniques employed in the preparation of TiO2 films include the doctor

blade technique, screen printing, electrophoretic deposition and tape casting

method.

2.2.3 Dye sensitizers

Dye Sensitizers must be rapidly regenerated by the mediator layer to avoid

electron recombination processes and be fairly stable, both in the ground and

excited states. The ideal sensitizer for a photovoltaic cell converting standard air

mass (AM) 1.5 sunlight to electricity, must absorb all light below a wavelength

19

of about 900 nm, which is equivalent to a semiconductor with a bandgap of 1.4

eV. An efficient photosensitizer must fulfill certain requirements such as:

An strong absorption in the visible region.

Strong chemical bonding to the semiconductor surface.

Efficient electron injection into the conduction band of the

semiconductor.

The stability of the dye should be excellent.

The dye should be cheap and easy to synthesize.

Finally, the dye should also be non-toxic and recyclable.

Energy levels of the sensitizer should match well with those of the semiconductor and

the electrolyte. The Lowest Unoccupied Molecular Orbital (LUMO) of dye needs to

be higher than the edge of the conduction band of semiconductor for efficient electron

injection, and the Highest Occupied Molecular Orbital (HOMO) of dye needs to be

aligned with the redox potential of the electrolyte, for facilitating active regeneration

of the oxidized dye. Sensitizing materials should be strongly anchored on the surface

of the semiconductor film to decrease the interface resistance and to secure stable

bonding for a long time. Electrons in the sensitizing materials need be quickly

separated from their counterpart holes and injected to the photoanodes before being

recombined. It is preferable to have a light absorption spectrum ranging from UV to

near IR region, with an absorption peak at visible region for the sensitizing material.

Good environmental stability of the sensitizing materials is also warranted for

maintaining the photovoltaic performance for long duration. The finest photovoltaic

performance in terms of both conversion yield and long term stability, has so

far been achieved with polypyridyl complexes of Ruthenium (Ru) developed by

the Grätzel group: N3, N719 and ‘black’ dyes. However, the use of these

expensive Ru metals, derived from relatively scarce natural resources corresponds

to a relatively heavy environmental burden. Hence, it is possible to use natural

dyes as alternative photosensitizers with appreciable efficiencies. Their advantages

over synthetic dyes include easy availability, abundance in supply, can be applied

without further purification, environmentally friendly and they considerably reduce

the cost of devices.

20

The efficiency of DSSC is determined mainly by the sensitizer used. The dye as a

sensitizer plays a key role in absorbing sunlight and transforming solar energy into

electrical energy. DSSCs with efficiencies of up to 15% have been designed using

ruthenium (Ru) based dyes but the limited availability and high cost of these dyes

together with their undesirable environmental impact have led to the search for cheaper

and safer dyes. There have been some interesting explorations of natural dyes in the

context of the dye-sensitized solar cell (DSSC) application using pigments obtained

from biomaterials. Several reports have emphasized on exploration into natural dyes,

such as cyanine, anthocyanin, cyanidins, chlorophyll and their derivatives,

carotenoids, betalains and many others as a cheaper, faster, low-energy requiring and

environment-friendly alternative for use in dye-sensitized solar cells. Although these

natural dyes often work poorly in DSSCs, these are expected as low cost and prepared

easily comparing to ruthenium (Ru) complex based dyes. Different types of natural

dyes showed different solar conversion efficiencies depending on the source and

chemical structure of dye and interaction between dye molecule and photo-electrode.

The advantages of natural dyes as photosensitizers are large absorption coefficients,

high light-harvesting efficiency, no resource limitations, low cost, simple preparation

techniques and no harm to the environment.

2.2.4 Electrolyte and redox couples

The electrolyte is a key component of the DSSC. It consists of a redox couple,

additives to improve the efficiency and a suitable solvent. Short circuit current density

(Jsc) and open circuit voltage (Voc) considerably depend on the electrolyte. The

electrolyte used in DSSC mostly contains I−/I3− redox ions, which mediate electrons

between the TiO2 photoelectrode and the counter electrode. The electrolyte affects the

DSSC performance mainly in three ways:

The photo voltage of the device depends on the redox couple because it sets

the electrochemical potential at the counter electrode.

The oxidized dye is reduced by the reduced part of the couple.

It affects the electrochemical potential of the TiO2 electrode through the

recombination kinetics between electrons in TiO2 and the redox species.

The electrolyte must meet a wide range of requirements:

21

The electrolyte must have long-term stability, including chemical, optical,

electrochemical, thermal and interfacial stability, which does not cause the

desorption and degradation of the dye from the oxide surface.

The electrolyte must reduce the oxidized dye rapidly. Thus, it is important

taking into account the redox potential of the dye and the electrolyte.

The electrolyte must guarantee the fast diffusion of charge carriers and

produces good interfacial contact with the porous nanocrystalline layer and the

counter electrode.

The electrolyte must not exhibit a significant absorption in the range of visible

light, otherwise it subtracts photons useful for the photosensitizer excitation.

Therefore, if the electrolyte is colored, its concentration must be optimized

taking into account not only the conventional parameters (viscosity, ionic

strength, reactivity), but also its spectral properties.

2.2.5 Counter electrode

The prerequisite of a material used as counter electrode in DSSC is that it should have

a low charge transfer resistance and high exchange current densities for the reduction

of the oxidized form of the charge mediator. The counter electrode serves to transfer

electrons arriving from the external circuit back to the redox electrolyte. It also has to

carry the photocurrent over the width of each solar cell. Hence, it must be well

conducting and exhibit a low overvoltage for reduction of the redox couple. Till now,

Pt has been the desired material for the counter electrode since it is an excellent catalyst

for I3− reduction. An interesting low cost alternative for Pt is carbon (C), because it

combines sufficient conductivity and heat resistance as well as corrosion resistance

and electrocatalytic activity for the I3− reduction.

2.3 DSSC operation

A schematic diagram of a dye-sensitized solar cell is shown in Fig. 2.1. Excitation of

the sensitizer (S) such as N3 adsorbed on the TiO2 nano-particle surface by visible light

in the range of 400–800 nm results in the excitation of an electron to an excited S1

state, subsequently transferring the electron very rapidly into the conduction bands of

22

the TiO2 particles. The migrated electron diffuses among the TiO2 particles to reach a

transparent electrode, while I3− formed by an electron transfer from an iodide ion (I−)

to the one-electron oxidized dye accepts an electron at the counter electrode. The

mechanism of electric current generation, therefore, involves the diffusion of I− and

I3− in the solvent, the viscosity of which can significantly affect the response time for

photon-to-current conversion. The operating cycle can be summarized as:

The excitation of the photosensitizers (dye) from the ground state (S) to the excited

state (S*) results in the injection of an electron into the conduction band of the TiO2

electrode resulting in the oxidation of the photosensitizers.

S + photon → S∗ Absorption (1)

S∗ → S+ + e- (TiO2) Electron injection (2)

The injected electron in the conduction band of TiO2 is transported between the TiO2

nanoparticles by diffusion towards the back contact of the transparent conducting glass

and reaches the counter electrode through the external load. The oxidized dye (S+)

recaptures the conduction band electron from the iodide (I-) ion redox mediator

regenerating the ground state (S) of the sensitizer and I- is oxidized to I3-.

2S+ + 3I− → 2S + I−3 Regeneration of dye (3)

The electron travels through the outer circuit performing work, reaches the back FTO

electrode, and reduces the iodine in the electrolyte. The platinum layer on the FTO acts

as a catalyst for the reduction. Then there is the ionic diffusion in the electrolyte and

the reduction of tri-iodide at the counter electrode.

I−3 + 2e−(Pt) → 3I− (4)

e−(Pt) + hʋ→ 3I− (5)

23

Fig. 2.1. Overview of the working principle of DSSC.

The performance of the DSSC is primarily based on the four energy levels of the

components: the ground state (HOMO) and the excited state (LUMO) of the dye, the

Fermi level of the TiO2 photo electrode, which is located near the conduction band

level and the redox potential of the redox mediator(I-/ I3-) in the electrolyte solution.

The photocurrent obtained from the DSSC is determined by the energy difference

between the HOMO and LUMO of the dye corresponding to the band gap, Eg, for

inorganic semiconductor materials. The smaller the HOMO – LUMO energy gap, the

larger will be the current because of the use of the long –wavelength region in the solar

spectrum. The energy gap between the LUMO level and the conduction band of TiO2

is important and the energy level of the LUMO must be sufficiently negative with

respect to the conduction band of TiO2 in order to inject electrons effectively. Besides,

substantial electronic coupling between the LUMO and the conduction band of TiO2

also leads to effective electron injection. The HOMO level of the dye must be

sufficiently more positive than the redox potential of the I- / I3- redox mediator to accept

electrons effectively.

24

2.4 Basic parameters to evaluate the performance of DSSC

There are a number of key parameters that are typically required in the analysis of

DSSCs. The key performance parameters of the cell are obtained from current-voltage

(J-V) measurements. An example J-V curve is shown in Fig. 2.2.

Fig. 2.2 Illustration of current density-voltage characteristics of a solar cell.

2.4.1 Short circuit current density

The short-circuit current (Jsc) is the current through the solar cell per unit area when

the voltage across the solar cell is zero. The short-circuit current depends on a number

of factors:

The number of photons: (i.e., the intensity of the incident light source); Jsc from a

solar cell is directly dependant on the light intensity.

The spectrum of the incident light: For most solar cell measurements, the spectrum

is standardized to the AM1.5 spectrum. (See section 1.4).

The optical properties of the solar cell

The collection probability of the solar cell: which depends chiefly on the surface

passivation and the minority carrier lifetime in the base.

25

2.4.2 Open circuit voltage

The open-circuit voltage (Voc) is the maximum voltage available from a solar cell, and

this occurs at zero current. The open-circuit voltage corresponds to the amount of

forward bias on the solar cell due to the bias of the solar cell junction with the light

generated current. The theoretical maximum Voc of the cell is determined by the

difference between the Fermi level of the semiconductor and the redox potential of the

hole-conductor.

2.4.3 Fill factor

The fill factor, more commonly known by its abbreviation "FF", is basically a

measure of quality of the solar cell. The FF is determined by comparing the maximum

power (PMAX) to the theoretical power (PT) that would be output obtained at the open

circuit voltage and the short circuit current.

The FF is typically calculated as:

𝐹𝐹 =𝐽𝑚 × 𝑉𝑚

𝐽𝑠𝑐 × 𝑉𝑜𝑐

where Jm and Vm are the maximum current and voltage, respectively.

The fill factor (FF) is a measure of the maximum power output from a solar cell, and

it reflects the extent of electrical and electrochemical losses during cell operation. To

obtain higher fill factor, improvement of the shunt resistance and decrement of the

series resistance, with reduction of the overvoltage for diffusion and charge transfer is

required.

2.4.4 Power Conversion Efficiency

The efficiency,, is the most commonly used parameter to compare the performance

of one solar cell to another and it is defined as the ratio of maximum electrical

energy output to the energy input from the sun.

Power conversion efficiency under sunlight irradiation (e.g., AM 1.5) can be

obtained using:

𝜂 = 𝐽𝑠𝑐 × 𝑉𝑜𝑐 × 𝐹𝐹

𝑃𝑖𝑛

(6)

(7)

26

where Pin is the power per unit area of incident light. Besides the solar cell

performance itself, it depends on the incident light spectrum and intensity as well as

operating temperature.

2.4.5 Series and shunt resistance

The efficiency of solar cells, during operation, is reduced by the dissipation of power

across internal resistances. These resistances are modelled as a parallel shunt resistance

(RSH) and series resistance (RS) as shown in Fig. 2.3. The series resistance is mainly

contributed by the bulk resistance of the semiconductor and the resistance of the

contacts and interconnections. Ideally, the value of this resistance is required to be

zero. However, in practicality there is a value associated with it. A high value of the

series resistance can cause a significant reduction in the short-circuit current and hence

degrade the solar cell efficiency by reducing the fill factor. The shunt resistance is

typically due to manufacturing defects around the depletion region or leakage currents

around the edges of the cell. Low shunt resistance causes power losses in solar cells

by providing an alternate path for the current flow. This results in a significant

reduction in the open-circuit voltage (VOC) and a slight reduction in the JSC (the short

circuit current density). For an ideal cell, RSH would be infinite, which means that there

is no alternative path for current flow.

Fig. 2.3. The equivalent circuit of a solar cell.

27

2.5 Advantages of the dye-sensitized solar cells

The DSSCs are currently the most efficient third generation thin film solar cells based

on a semiconductor and photo-electrochemical system. DSSCs demonstrate many

advantages in comparison to bulk silicon p-n junction solar cells.

Friendly and cost-effective conversion of solar energy to electricity due to the

use of inexpensive, biocompatible, abundantly available and flexible materials.

Simple fabrication process and low environmental impact.

Unlike traditional solar cells, dye-sensitized cells can work effectively in low

light conditions and are less susceptible to losing energy to heat.

DSSC materials and dyes can be tuned for optimization in a variety of lighting

conditions making it suitable for indoor applications and outdoor applications.

Considering these advantages, DSSCs have the potential to be a feasible candidate for

the race of large-scale solar energy conversion systems.

28

Chapter 3

Experimental Work

In this chapter, dye-sensitized solar cell preparation techniques are discussed. The

relevant theoretical and experimental principles are briefly described. The materials

and tools applied in this study are also introduced. The chapter also includes

characterization techniques.

3.1 Device characterization

3.1.1 Ultraviolet–visible spectroscopy

Ultraviolet (UV) and visible radiation comprise only a small part of the

electromagnetic spectrum, which includes such other forms of radiation as radio,

infrared (IR), cosmic, and X rays. When radiation interacts with matter, a number of

processes can occur, including reflection, scattering and absorbance. In general, when

measuring UV-visible spectra, we want only absorbance to occur. Since light is a form

of energy, absorption of light by matter causes the energy content of the molecules (or

atoms) to increase. In general, the total potential energy of a molecule can be

represented as the sum of its electronic, vibrational and rotational energies. The

amount of energy a molecule possesses in each form is not a continuum but a series of

discrete states. In some molecules, photons of UV and visible light have enough energy

to cause transitions between different electronic energy levels. The wavelength of light

absorbed is equal to the energy required to move an electron from a lower energy level

to a higher energy level. When light passes through or is reflected from a sample, the

amount of light absorbed is the difference between the incident radiation (Io) and the

transmitted radiation (I). The amount of light absorbed is expressed as either

transmittance or absorbance. Transmittance usually is given in terms of a fraction of 1

or as a percentage and is defined as follows: T=I/Io and A=-log T. The absorption

spectra of all natural dyes were measured by using a GENESYS 10S UV–Vis

spectrophotometer with ethanol as the solvent reference. The absorbance was

measured normally in the wavelength range of 200–900 nm. The optical properties

of the films deposited on the FTO substrates were examined for their absorbance

29

at normal incident by using a V-670 and JASCO UV-Vis spectrophotometer which

were shown in Fig. 3.1.

Fig. 3.1. UV-Vis spectrophotometers used in this study (1) GENESYS 10S (2) V-670

3.1.2 Current-voltage measurements

One of the most important characterization techniques for solar cells is current–voltage

(I–V) measurements, from which the solar cell energy conversion efficiency can be

determined. The dye sensitized solar cell characterization is performed under AM1.5G

illumination. The I-V characteristics are monitored under illumination by changing the

external load from zero (short-circuit conditions) to infinite load (open circuit

conditions). The cells were illuminated using a tungsten halogen lamp. The intensity

of the incident light was about 100 mW/cm2. The cells are contacted using a custom-

made sample holder which can be mounted repeatedly in the center of the illuminated

area during each measurement. The photocurrent voltage characteristics of the solar

cells were carried out using a NI USB6251 data acquisition card controlled by a

computer using a LabVIEW controlled source-meter as show in Fig 3.2. From these

measurements, the short circuit current density (Jsc), open circuit voltage (Voc), fill

factor (FF) and efficiency (η) of the device can be determined.

30

Fig. 3.2. Simplified layout of solar simulation system.

3.1.3 Electrochemical Impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a powerful diagnostic tool that can

be used to characterize limitations and improve the performance of fuel cells, and it

has become a major tool for investigating the properties and quality of dye-sensitized

solar cell devices. There are several advantages of this technique for the determination

of device parameters and characteristics: (i) It is a small perturbation and noninvasive

method that can be applied under any operating conditions and thicknesses of the

device, so that the dependence of parameters on voltage can be scanned, (ii) It allows

for the simultaneous determination of several characteristics, such as capacitance and

transport resistance. This allows the carrier distribution in the device to be probed at

the same time as kinetic information about carrier displacement. During an impedance

measurement, a frequency response analyzer (FRA 32) is used to impose a small

amplitude AC signal to the solar cell via the load. EIS measurements were performed

with an Autolab AUT 85276 Potentiostat Gelvanostat. All the measurements were

carried out with the NOVA software. Impedance measurements were implemented in

the frequency range 10 KHz to 10 mHz, using 20 mV AC amplitude. Impedance was

measured under illumination and under dark conditions. Under illumination, the cell

was illuminated with a range of intensities and impedance was performed at open-

circuit condition. In the dark, a bias potential was applied. From EIS, several

31

parameters can be obtained, such as charge-transfer resistance and electron

recombination resistance.

3.2 Device fabrication

3.2.1 Materials

FTO conductive glass (fluorine-doped SnO2, sheet resistance: 15Ω / sq).

Commercial P25 TiO2 nano-powder with particle size in the range of 15–20

nm.

A redox (I−/I3−) electrolyte solution composed of 2 mL of acetonitrile, 8 mL of

propylene carbonate (p-carbonate), 0.668 g of Potassium iodide, and 0.0634 g

of I2.

Platinum counter electrode (cathode) sputtered on FTO substrate.

Natural dyes extracted from plant leaves as listed in table 3.1.

Ethanol was used for extracting natural dyes from plant leaves.

Distilled water, TritonX-100 and acetyl acetone were used for preparing TiO2

paste.

Nitric acid, hydrochloric acid and phosphoric acid were used for FTO and TiO2

treating processes.

Table 3.1 Photos of the plants used for the dyes extraction.

Dye Image Dye Image

Lemon

Avocado

Mandarin

Almond

32

Orange

Peach

Apple

Pomelo

Fig

Loquat

Olive

Pomegranate

3.2.2 Tools

Equipments used in this study were a transparent glass, binder clip, scotch tape,

cutter, mortar (grinding), tissue paper, filter paper, alligator clip wire, glass

beaker, pipette drops, measuring cup, oven, stirrer rod, multimeter, cotton

swabs, tweezers, washing bottle, hot plate, glass funnel and test tubes.

3.2.3 Preparation of TiO2 paste

A TiO2 paste was prepared by blending 2g of TiO2 powder, 10 µl

acetylacetone, and 4 ml of distilled water. After that 50 µl of the surfactant

Triton X-100 were added to the paste and mixed by grinding continuously

for 30 min.

3.2.4 Preparation of TiO2 electrode

The FTO substrates were cut into pieces of dimensions 1.6 cm × 1.6 cm. The

substrates were washed with distilled water and ethanol for 15 min in an

33

ultrasonic bath. To prepare the DSSC photoanodes, doctor blade technique was

used as shown in Fig. 3.3. The prepared paste was spread on FTO conductive

glass by using a glass rod. A scotch tape of thickness 20µm was fixed on the

two sides of the cleaned FTO conductive glass sheet to restrict the thickness

and area of TiO2 film. The paste was spread to have an active area of 0.25 cm2.

After tape removal, the film was gradually sintered at 450º C for 30 min. Then

the TiO2 film was cooled to 80º C, and subsequently immersed in natural dye

sensitizer solution at room temperature for 24 h.

Fig. 3.3. Schematic representation of doctor blade method.

3.2.5 Preparation of natural dye sensitizers

Locally available fresh leaves of 12 plants as listed in Table 3.1 were collected,

washed with water and dried. The leaves were crushed into small pieces. Then

0.5g of each fresh leaves were immersed into 10 ml of ethanol. The solutions

34

were kept for 24 hour in the dark at room temperature. Then, the residual (solid)

parts were filtered out and the resulting filtrates were used as dye solutions.

3.2.6 DSSC assembling

The DSSCs with an active area of 0.25 cm2 were assembled as follows:

The sensitized films were rinsed with ethanol to remove excess dye

remaining on the surface and then air-dried at room temperature. For the

preparation of counter electrodes, platinum with a mirror finish is

sputtered on FTO glass substrates. The dye-covered TiO2 electrode and

Pt-counter electrode were assembled into a sandwich type cell using a

spacer as show in Fig 3.4. The gap between the electrodes was filled with

the electrolyte.

Fig. 3.4. Assembled dye sensitized solar cell.

3.3 Experimental studies

3.3.1 Testing twelve natural dyes

Plant leaves of lemon, avocado, mandarin, almond, peach, pomelo, apple, fig, loquat,

orange, olive and pomegranate were collected, washed and dried. All dyes were

35

prepared by immersing 0.5 g of the natural leaves in 10 ml of ethanol as a solvent for

24 h. All solutions were protected from direct light exposure. After filtration of the

solutions, natural extracts were obtained and the resulting filtrates were used as

sensitizers. After the cells were assembled, the fabricated DSSCs were connected to

the solar cell I-V measurement system and the photovoltaic parameters were studied.

3.3.2 Optimizing of the extracting temperature

Fresh lemon leaves of 0.5 g was extracted in 10 ml of ethanol solvent at different

temperatures for 3 hours. The effect of extracting temperature was studied at 30º C,

40º C, 50º C, 60º C and 75º C, using ethanol as an extracting solvent. Then the prepared

TiO2 electrodes were immersed in these dyes solution at room temperature for 24 h.

After the cells were assembled, the photovoltaic parameters were studied.

3.3.3 Effect of pre-treatment of FTO glass substrates

The FTO glass substrates were cleaned by acetone and ethanol, successively for

20 minutes. The substrates were then immersed in 0.1 M of one of the acids mentioned

before at room temperature for 5 min then washed with ethanol. Then the TiO2 paste

was spread uniformly on the FTO substrates as mentioned in chapter three sec (3.2.4).

The TiO2 electrodes were soaked in an ethanol solution of lemon, mandarin and orange

dyes at room temperature for 24 hours. After the cells were assembled, the

photovoltaic parameters were studied.

3.3.4 Effect of surface post-treatment of TiO2 layer

FTO glass plates were cleaned using an ultrasonic bath for 20 min, rinsed with water

and acetone and dried in ambient conditions. TiO2 electrodes were prepared on FTO

substrates by a doctor blade method. The TiO2 electrodes were soaked in 0.1 M HCl,

and HNO3 at room temperature for 5 min followed by washing with ethanol. Then, the

electrodes were sintered at 450oC for 30 min and cooled down to 100oC in ambient air.

After the treatments, the TiO2 electrodes were soaked in an ethanol solution of lemon

leaves dyes at room temperature for 24 hours.

36

Chapter 4

Results and Discussions

In this chapter, the results of four experimental studies are presented in details,

including determination of the best dye of twelve natural dyes extracted from plant

leaves, optimizing extracting temperature and time, pre-treatment of the FTO glass

using several acids, and post-treatment of TiO2 layer. Finally, the results of

electrochemical impedance spectroscopy are given.

4.1 Testing twelve natural dyes

In this work, twelve natural dyes extracted from plant leaves were used as sensitizers

to fabricate dye-sensitized solar cells (DSSCs). The purpose of this study was to

determine the best dyes in terms of efficiency to be used in subsequent studies.

4.1.1 Absorption spectra of the dyes

The absorption spectra of the dye solutions extracted from the leaves of lemon,

mandarin and orange in ethanol as a solvent were measured. The absorption spectra

analysis was carried out in the wavelength range from 390 to 700 nm as shown in

Fig. 4.1.

400 500 600 700

ab

so

rba

nce

(a

.u.)

wavelenght (nm)

mandarine

Orange

Lemon

Fig. 4.1. UV–Vis absorption spectra of natural dyes extracted from lemon, mandarin,

and orange leaves in ethanol solution.

37

Figure 4.1 demonstrates that the ethanol extracts of lemon, mandarin, and orange,

whose colors are green, reach maximum absorption peaks at 665 nm and 435 nm. The

main component of these three extracts is chlorophyll [32]. From this figure, it is

evident that these natural extracts absorb in the visible region of light spectrum and

hence fulfill the primary criterion for their use as sensitizers in DSSCs.

4.1.2 J-V characterization of DSSCs

The photovoltaic performances of DSSCs using natural dyes as photosensitizer were

determined by recording the current density-voltage (J-V) curves as displayed in Fig.

4.2, Fig. 4.3, Fig. 4.4 and Fig. 4.5.

-1.0

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2)

Lemon

Mandarin

Orange

Fig. 4.2. J–V curves for the DSSCs sensitized by lemon, mandarin and orange leaves

extracts.

38

-0.3

-0.2

-0.1

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

J(m

A/c

m2

)

V(v)

Peach

Loquat

Pomegranate

Fig. 4.3. J–V curves for the DSSCs sensitized by peach, loquat and pomegranate leaves

extracts.

-0.4

-0.3

-0.2

-0.1

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2

)

Almond

Fig

Pomelo

Fig. 4.4. J–V curves for the DSSCs sensitized by almond, fig and pomelo leaves

extracts.

39

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2

)

Avocado

Apple

Olive

Fig. 4.5. J–V curves for the DSSCs sensitized by olive, apple and avocado leaves

extracts.

Fig. 4.6, Fig. 4.7, Fig. 4.8 and Fig. 4.9 show the power versus voltage curves for the

DSSCs sensitized by natural dyes. The highest maximum power was obtained in the

case of lemon leaf extract while the lowest maximum power was obtained in the case

of loquat.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

0.3

0.4

0.5

P(m

W/c

m2

)

V(v)

Lemon

Mandarin

Orange

Fig. 4.6. Power versus voltage curves of the DSSCs using lemon, mandarin and orange

leaves dyes.

40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

P(m

W/c

m2

)

V(v)

Peach

Loquat

Pomerganate

Fig. 4.7. Power versus voltage curves of the DSSCs using peach, loquat and

pomegranate leaves dyes.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

P(m

W/c

m2)

V(v)

Almond

Fig

Pomelo

Fig. 4.8. Power versus voltage curves of the DSSCs using almond, fig and pomelo

leaves dyes.

41

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

P(m

W/c

m2)

V(v)

Olive

Apple

Avocado

Fig. 4.9. Power versus voltage curves of the DSSCs using olive, apple and avocado

leaves dyes.

The values of photovoltaic parameters of the DSSCs sensitized with natural dyes are

calculated using the above figures and listed in Table 4.1.

Table 4.1 Photovoltaic parameters of the DSSCs sensitized by 12 kinds of natural dyes

extracted from plant leaves. The result of the DSSC sensitized by Ru(N719) are also

included in the Table.

Dye Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

Avocado 0.22 0.55 0.12 0.47 0.45 0.056

Pomelo 0.20 0.58 0.13 0.49 0.55 0.063

Apple 0.20 0.56 0.11 0.43 0.42 0.047

Fig 0.23 0.58 0.15 0.44 0.49 0.066

Peach 0.24 0.54 0.13 0.43 0.43 0.055

Orange 0.34 0.55 0.22 0.46 0.54 0.10

Mandarin 0.76 0.57 0.55 0.47 0.59 0.25

42

Lemon 0.95 0.59 0.77 0.46 0.63 0.35

Loquat 0.18 0.50 0.09 0.38 0.38 0.034

Pomegranate 0.17 0.49 0.10 0.39 0.46 0.04

Olive 0.27 0.52 0.15 0.40 0.42 0.06

Almond 0.31 0.58 0.19 0.42 0.44 0.08

Ru (N719) 4.76 0.57 3.53 0.38 0.50 1.35

As displayed in Table 4.1 open circuit voltage (Voc) varies from 0.49 to 0.59 V, and

the short circuit photocurrent densities (JSC) changes from 0.17 to 0.95 mA/cm2. The

highest Voc (0.59 V) and JSC (0.95 mA/cm2) were obtained with the DSSC sensitized

by lemon leaves extract; the efficiency of which reached 0.35%.

The lowest efficiency was obtained with the DSSC sensitized by Loquat leaves extract.

The efficiencies of DSSCs sensitized with lemon, mandarin and orange dyes are

significantly higher than those of the DSSCs sensitized by other natural dyes in this

work. Chlorophyll plays an important role in plant photosynthesis. The DSSCs

fabricated using chlorophyll derivatives as sensitizers showed a relatively high

conversion efficiency. However, the DSSCs sensitized by the leaves of loquat, olive,

almond, pomegranate, peach, fig, apple and avocado did not exhibit high conversion

efficiencies. These results can be explained as follows:

Obviously, the efficiency of the cell sensitized by the Ru-complex dye

is significantly higher than that sensitized by the natural extracts. This

is due to a higher intensity and broader range of the light absorption of

the Ru on TiO2.

In general, the results obtained in this study using chlorophyll pigment gives

less efficiency comparing with other natural dyes such as anthocyanins and

betalains [33]. This is because the week bonds between the dye and TiO2

molecules through which electrons can transport from the excited dye

molecules to the TiO2 film. The functional group necessary to interact with the

TiO2 surface is a carboxylic or other peripheral acidic anchoring group. Several

possible chemical functional groups are able to bind the dye to the TiO2. The

43

best anchoring groups for metal oxides are phosphonic acids followed by

carboxylic acids and their derivatives, such as acid chlorides, amides, esters or

carboxylate salts. The carboxylic group is the most frequently used anchoring

group and its present in the anthocyanin pigments. The presence of carboxylic

groups presents mainly two advantages: a better anchoring between the dye

and the TiO2 surface and an efficient dye regeneration by iodine/iodide redox

couple. Thus, the absence of the effective anchoring group in Chlorophyll

structure and depending on its ester group in chelation which is far

effective to the carboxyl group, leads to a lower performance. This result

indicates that the interaction between the sensitizer and the TiO2 film is

significant in enhancing the energy conversion efficiency of DSSCs. The

chemical structure of chlorophyll is shown in Fig. 4.10.

As can be seen from the above results, the photoelectric conversion efficiency

of lemon leaf extract is higher than the photoelectric conversion efficiency of

other extract dyes. This is because, after the lemon leaf extract is adsorbed on

the surface of TiO2 nanoparticles, the absorption intensity is higher. In addition,

there is a higher interaction between TiO2 nanoparticles and the chlorophyll in

lemon leaf extract, giving the produced DSSCs better charge-transfer

performance, clearly improved efficiency. The reason is that the electrons can

be transported from excited lemon leaf extract dyes molecule to TiO2 thin film

is much higher than other dyes. Also, the difference in chemical structure of

these pigments would also directly reflect the efficiency results [34].

44

Fig. 4.10. The basic molecular structure of Chlorophyll. [35]

4.2 Optimizing of the extracting temperature and time of lemon leaves

dye.

The performance of the dye-sensitized solar cell mainly depends on the dye used as a

sensitizer. The dye performance is affected by several factors such as pH, storage

temperature, extracting temperature and extraction time. The extracting temperature

and time play a critical role for DSSC performance. Hence, it is important to evaluate

the optimum conditions regarding the extracting temperature and time of natural dyes.

In this study, different extracting temperatures were tested for lemon leaves pigments.

Moreover, different times of extraction were examined.

4.2.1 J-V characterization of DSSCs at different extracting temperature

Figures 4.11 and 4.12 show the J-V characteristic curves and the power of DSSCs

fabricated with lemon leaves at different extracting temperatures. Figure 4.13 shows

the DSSC efficiency as a function of the extracting temperature. As observed from the

figure, the efficiency is enhanced with increasing the extracting temperature from

30°C to 50°C and then it declines towards lower values with further increasing of the

45

temperature. Here, the optimum extracting temperature was found 50º C. This is due

to the greater absorbance of light than the dye extracted at other temperatures. The

reason is that the dye extracted at 50 °C resulted in deep colored solution. In addition,

the extraction solution has the best stability and the degradation rate of pigment is the

slowest under the extraction temperature at 50º C [36]. Solar cell sensitized using the

dye extracted at 50 °C showed a power conversion efficiency of 0.5%, with Voc of

0.55V, Jsc of 1.52 mA/cm2 and FF of 0.60. Solar cell sensitized using the dye extracted

at higher temperatures (60 °C and 75 °C) showed less efficiency. This is due to the

less stability of chlorophyll at higher temperatures [37]. Solar cell sensitized using the

dyes extracted at lower temperatures (30 °C) also gave lower efficiency than that

sensitized using the dyes extracted at 50 °C. This is due to the lighter color of the

extract, which is due to restriction of chlorophyll solubility.

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2)

T=30 C

T=40C

T=50C

T=60C

T=75C

Fig. 4.11. Current density (J) versus voltage (V) characteristics curves for the DSSCs

sensitized at different extracting temperatures.

46

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

0.3

0.4

0.5

0.6

V(v)

P(m

W/c

m2)

T= 30C

T= 40C

T= 50C

T= 60C

T= 75C

Fig. 4.12. Power (P) versus voltage (V) characteristics curves of the DSSCs sensitized

at different extracting temperatures.

30 40 50 60 70 800.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

(%

)

Temprature (oC)

Fig. 4.13. DSSC efficiency (η) versus the extracting temperature (T) of lemon leaves

extract.

Table 4.2 shows the electrochemical parametrs of the DSSCs sensitized with lemon

leaves dye extracted at different temperatures.

47

Table 4.2. Solar cell parameters of the DSSCs sensitized with lemon leaves dye

extracted at different temperatures.

Extracting

Temperature

(ºC)

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

30 0.77 0.57 0.54 0.46 0.56 0.24

40 1.35 0.56 0.98 0.46 0.59 0.45

50 1.52 0.55 1.17 0.43 0.60 0.50

60 1.09 0.56 0.78 0.42 0.53 0.32

75 0.94 0.58 0.66 0.47 0.56 0.31

Therefore, in order to acquire the best photoelectric conversion efficiency, the

extraction temperature of natural dye has also must to be controlled.

4.2.2 J-V characterization of DSSCs at different extracting times

To find out the proper extracting time, we studied the extraction of lemon leaves at 1,

2, 3, 5, 6, 7 and 8 hours at 60 ºC. Figures 4.14 and 4.15 show the J-V characteristic

curves and the power of DSSCs fabricated with lemon leaves at different extracting

time. Figure 4.16 shows the DSSC efficiency as a function of the extracting time. The

extraction process of dye solution usually takes 24 hours in dye-sensitized solar cell

fabrication. Here, it was shown that by changing of extracting temperature, extracting

time can be reduced from 24 hours to 5 hours. This method is ideal for rapid DSSC

fabrication. While immersing photoelectrodes for 24 hours in dye solution gives cells

with an efficiency of 0.35. Changing of extracting temperature to 60 ºC for 5 hours

gave cells with an efficiency of 0.37.

48

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2)

1 h

2 h

3 h

5 h

6 h

7 h

8 h

Fig. 4.14. Current density (J) versus voltage (V) characteristic curves for the DSSCs

sensitized at different extracting times at 60 ºC.

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

P(m

W/c

m2)

V(v)

1 h

2 h

3 h

5 h

6 h

7 h

8 h

Fig. 4.15. Power (P) versus voltage (V) characteristics curves of the DSSCs sensitized

at different extracting times.

The effect of dye extracting time on DSSC efficiency at 60 ºC is presented in Table

4.3.

49

Table 4.3. Photoelectrochemical parameters of the DSSCs sensitized with lemon

leaves dye extracted at various times at 60º C.

Extracting

Time (h)

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

1 0.75 0.56 0.51 0.38 0.46 0.19

2 0.86 0.57 0.50 0.42 0.42 0.21

3 1.09 0.56 0.78 0.42 0.53 0.32

5 1.08 0.55 0.87 0.43 0.62 0.37

6 1.30 0.54 0.83 0.42 0.49 0.34

7 1.26 0.52 0.86 0.39 0.51 0.33

8 1.09 0.56 0.82 0.38 0.51 0.31

0 1 2 3 4 5 6 7 8 9

0.20

0.25

0.30

0.35

Time (h)

Fig. 4.16. DSSC efficiency (η) versus the extracting time (t) of lemon leaves extract

at 60º C.

4.3 Effect of pre-treatment of FTO glass substrates

Acidic pre-treatment are used in the fabrication of high performance photoanodes for

dye-sensitized solar cells (DSSCs). In this study, hydrochloric (HCl), phosphoric

(H3PO4) and nitric (HNO3) acids pretreatment were used with fluorine doped tin oxide

50

(FTO) before fabricating TiO2 films. The effects of previous acids pretreatment on

some important parameters of solar cells, such as short-circuit current (Jsc) and fill

factor, were investigated.

4.3.1 J-V characterization of DSSCs with the pre-treatment of FTO glass

Fig. 4.17, Fig. 4.19 and Fig. 4.21 show the J–V characteristic curves of DSSCs with

the pre-treatment of FTO glass substrate using lemon, mandarin and orange leaves dye

respectively, while Fig. 4.18, Fig. 4.20 and Fig. 4.22 show the power versus voltage

curves obtained for DSSCs sensitized using lemon, mandarin and orange leaves dye

respectively.

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

V(v)

J(m

A/c

m2)

untreated

H3PO

4

HCL

HNO3

Fig. 4.17. Current density (J) versus voltage (V) characteristic curves for the DSSCs

with the pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids

using lemon leaves dye.

51

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

0.3

0.4

0.5

0.6

P(m

W/c

m2)

V(v)

Untreated

HCL

HNO3

H3PO

4

Fig. 4.18 Power (P) versus voltage (V) characteristic curves of the DSSCs with the

pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids using lemon

leaves dye.

-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.000.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2)

Untreated

HNO3

HCL

H3PO

4

Fig. 4.19. Current density (J) versus voltage (V) characteristic curves for the DSSCs

with the pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids

using mandarin leaves dye.

52

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

P(m

W/c

m2)

V(v)

Untreated

HNO3

HCL

H3PO

4

Fig. 4.20. Power (P) versus voltage (V) characteristic curves of the DSSCs with the

pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids using

mandarin leaves dye.

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6

V(v)

J(m

A/c

m2)

Untreated

HNO3

HCL

H3PO

4

Fig. 4.21. Current density (J) versus voltage (V) characteristic curves for the DSSCs

with the pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids

using orange leaves dye.

53

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

P(m

W/c

m2)

V(v)

Untreated

HNO3

HCL

H3PO

4

Fig. 4.22. Power (P) versus voltage (V) characteristics curve of the DSSCs with the

pre-treatment of FTO glass substrates with HCL, H3PO4 and HNO3 acids using orange

leaves dye.

Table 4.4 shows the photovoltaic parameters of the DSSCs sensitized by lemon leaves

dye and pre-treated with hydrochloric (HCl), phosphoric (H3PO4) and nitric (HNO3)

acids, while Tables 4.5 and 4.6 show the photovoltaic parameters of the DSSCs

sensitized by mandarin and orange leaves dye and pre-treated with hydrochloric (HCl),

phosphoric (H3PO4) and nitric (HNO3) acids respectively.

Table 4.4 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using lemon leaves dye.

FTO

Pre-treatment

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

Untreated 0.95 0.59 0.77 0.46 0.63 0.35

HCl

(0.1M, 5min) 1.38 0.58 1.08 0.46 0.62 0.49

H3PO4

(0.1M, 5min) 1.44 0.57 1.06 0.46 0.59 0.48

HNO3

(0.1M, 5min) 1.35 0.58 1.08 0.46 0.63 0.49

54

Table 4.5 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using mandarin leaves dye.

FTO

Pre-treatment

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

Untreated 0.76 0.57 0.55 0.47 0.59 0.25

HCl

(0.1M, 5min) 1.11 0.53 0.77 0.45 0.58 0.34

H3PO4

(0.1M, 5min) 1.12 0.53 0.71 0.44 0.52 0.31

HNO3

(0.1M, 5min) 1.27 0.53 0.85 0.44 0.55 0.37

Table 4.6 Photovoltaic parameters of the DSSCs with the pre-treatment of FTO glass

substrates by HCl, H3PO4, and HNO3 acids using orange leaves dye.

FTO

Pre-treatment

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

Untreated 0.34 0.55 0.22 0.46 0.54 0.10

HCl

(0.1M, 5min) 0.62 0.54 0.46 0.46 0.63 0.21

H3PO4

(0.1M, 5min) 0.53 0.55 0.34 0.45 0.52 0.15

HNO3

(0.1M, 5min) 0.60 0.53 0.42 0.44 0.42 0.18

It can be seen from the J-V curves that the performances of DSSCs with acids

pretreatment are much better than the DSSCs without acids pretreatment. As shown in

Tables 4.4, 4.5, and 4.6, the short-circuit current (Jsc), open-circuit voltage, conversion

efficiency, and filling factor of DSSCs with acids pretreatment are greatly improved.

The pre-treatment of the FTO with HCL and HNO3 showed an improved efficiency of

140% using lemon leaves dye, while the pre-treatment of the FTO with HCL using

55

orange leaves dye showed an improved efficiency of 210%. The main reason for these

results is that the acids pretreatment of FTO enhance the bonding strength between the

FTO substrate and the porous-TiO2 layer. Aslo more dye were adsorbed on TiO2.

Meanwhile the acids pretreatment block charge recombination between electrons in

the FTO with holes in I-/Ι-3 redox couple and a layer of dense TiO2 on FTO is formed

and impede carriers recombination between Ι-3 and FTO effectively, thus, the short-

circuit photocurrent density is improved, so the conversion efficiency is also increased

[38]. The reason behind that may be attributed to decreasing of the FTO sheet

resistance by acids treatment.

4.4 Effect of surface post-treatment of TiO2 layer

The role of photoelectrode is one of the most important factors to get high efficiency

in DSSCs. So, lots of efforts have been reported to enhance the dispersion and adhesion

relating to porosity, dispersion and modification between FTO and TiO2 particles as a

photoelectrode. In this study, we investigated the influence of acidic post-treatment of

TiO2 photoelectrode on DSSCs efficiency. Hydrochloric acid (HCl) and nitric acid

(HNO3) were used to investigate the effect of surface post-treatment of TiO2 film on

the efficiency of DSSCs.

4.4.1 J-V characterization of DSSCs with post-treatment of TiO2 electrode

DSSCs with electrodes that untreated with acids, together with acids post-treated TiO2

electrodes are tested under 100 mW/cm2 intensity. Their characteristic J-V curves are

illustrated in Fig. 4.23. The power versus the voltage for all DSSCs with post-treatment

of TiO2 electrode with the two acids are shown in Fig. 4.24.

56

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

V(v)

J(m

A/c

m2)

Untreated

HNO3

HCL

Fig. 4.23 Current density (J) versus voltage (V) characteristic curves of DSSCs with

post-treatment of TiO2 electrode using HCL and HNO3 acids.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70.0

0.1

0.2

0.3

0.4

0.5

P(m

W/c

m2)

V(v)

Untreated

HNO3

HCL

Fig. 4.24 Power (P) versus voltage (V) characteristic curves of the DSSCs with post-

treatment of TiO2 electrode with HCL and HNO3 acids.

57

Table 4.7 Photovoltaic parameters of the DSSCs with post-treatment of TiO2 electrode

by HCL and HNO3 acids.

TiO2

post-treatment

Jsc

(mA/cm2)

Voc

(V)

Jm

(mA/cm2)

Vm

(V)

FF

η

%

Untreated 1.08 0.61 0.70 0.46 0.48 0.32

HCl

(0.1M, 5min) 1.36 0.58 0.95 0.42 0.50 0.39

HNO3

(0.1M, 5min) 1.25 0.58 0.91 0.46 0.57 0.41

Acid post-treated cells showed enhanced current density in comparison with normal

cell. HCl and HNO3 treated cells showed higher current densities of 1.36 mA/cm2 and

1.25 mA/cm2, and reduced open circuit voltage 0.58V respectively. HNO3 treated cell

showed over 128% improvement of conversion efficiency (0.41) with 1.25 mA/cm2

short circuit current, 0.58V open circuit voltage and 0.57 fill factor. These observations

could be conferring that the acid contributed regular arrangement of the photoelectrode

by the dispersion of TiO2 nanoparticles. This dispersion is one of the factors that makes

much chemisorption sites for the dyes. Also, the reason behind that behavior may be

attributed to the improvement of TiO2 film electrical conductivity by enhancing the

neck points between the nanoparticles, increasing dye loading and minimizing the

recombination rate between the TiO2 film and the mediator. The positively charged

TiO2 surface formed by nitric acid treatment induces high electrostatic attraction

between the reaction sites and anionic dyes, resulting in a much faster adsorption

reaction [39]. The HNO3 treatment enhanced the dispersion of TiO2 particles,

increased the surface area and porosity of the sintered TiO2 films, increased the amount

of adsorbed dye molecules on the TiO2 electrode, and reduced the charge-transfer

resistance [40]. The Isc improvement upon HNO3 treatment of TiO2 has also been

related to the enhanced charge collection efficiency by surface protonation of TiO2 and

the retarded backward electron transfer by anion (NO3−) adsorption on the TiO2

surface.

58

4.5 Electrochemical impedance spectroscopy analysis

In order to study the internal resistances and electron transfer kinetics of the cells

sensitized by lemon leaves dye, EIS was employed using AUT 85276 Potentiostat-

Galvanostat with FRA 32 Module. DSSCs sensitized by lemon leaves dye without

treatment and with FTO treated with nitric acid were studied.

4.5.1 EIS of the DSSC sensitized by lemon leaves without any treatment

The EIS measurements were carried out in the dark and under an illumination of

100 mW/cm2 for the DSSC sensitized by lemon leaves without any treatment at -0.6 V,

applied voltages.

As observed from Fig. 4.25 and Table 4.8, there is a decrease in the charge-transfer

resistance (RCT) upon exposure to an illumination of 100 mW/cm2 for the DSSC

sensitized by lemon leaves without any treatment at the chosen applied voltages. This

can be ascribed to a difference in the local I3− concentration. Under illumination, I3

− is

formed "in situ" by dye regeneration at the mesoporous TiO2/electrolyte interface,

whereas in the dark, I3− is generated at counter electrode and penetrates the

mesoporous TiO2 films by diffusion. Electrochemical circle fit was obtained for the

DSSC sensitized by lemon leaves without any treatment in the dark and under an

illumination at chosen applied voltage as shown in Fig 4.25. Hus and Mansfeld

proposed equation 4.1 that can be used to calculate the estimated interfacial

capacitance from a depressed semicircle model (a Constant Phase Element in parallel

with a resistor).

𝐶 = 𝑄0(𝜔𝑚𝑎𝑥)(𝑛−1) (4.1)

where C is estimated interfacial capacitance, Qo is the CPE coefficient, ωmax is the

characteristic frequency at which the imaginary part of the impedance reaches its

maximum magnitude, and n is the exponent. A summary of the equivalent circuit

component values obtained from modeling the DSSCs sensitized by lemon leaves

without any treatment in the dark and under an illumination at chosen applied voltage

can be found in Table 4.8.

59

0 1 2 3 4 50.0

0.5

1.0

1.5

2.0

2.5 -0.6v dark

-0.6v illumination

-Z"(

K

)

Z'(K)

Fig. 4.25. Nyquist plots of the DSSC sensitized by lemon leaves without any treatment

at -0.6 V in the dark and under an illumination of 100 mW/cm2.

Fig. 4.26. The equivalent circuit for the DSSCs sensitized by lemon leaves without

any treatment.

where RS is the series resistance, CPE is the constant phase element, and RCT is the

charge-transfer resistance.

Table 4.8 EIS results from data-fitting of Nyquist plots to the equivalent circuit model

in Fig. 4.26 without treatment.

Applied voltage RS(Ω) RCT (kΩ) C(µF)

-0.6v dark 34.55 4.35 1.77

-0.6v illumination 33.19 1.48 1.62

60

4.5.2 EIS of the DSSC sensitized with lemon leaves with pre-treated FTO

Figure 4.27 shows the Nyquist plots for DSSC sensitized by lemon leaves with pre-

treatment of FTO glass substrates with nitric acid in the dark at -0.6 V applied voltages.

Electrochemical circle fit was obtained for the DSSCs sensitized by lemon leaves and

pre-treatment of FTO glass substrates with nitric acid in the dark at different applied

voltages as shown in Fig 4.27. A summary of the equivalent circuit component values

obtained from modeling the DSSCs sensitized by lemon leaves with pre-treatment of

FTO glass substrates with nitric acid in the dark and under an illumination at chosen

applied voltage can be found in Table 4.9.

0.00 0.25 0.50 0.75 1.000.0

0.1

0.2

0.3

0.4

0.5 -0.6v dark

-0.6v illumination

Z'(k)

Fig. 4.27. Nyquist plots of the DSSC sensitized by lemon leaves with nitric acid

pretreatment of FTO glass substrates at -0.6 V

Table 4.9 EIS results from data-fitting of Nyquist plots to the equivalent circuit model

in Fig. 4.27 for the DSSC sensitized by lemon leaves with nitric acid pre-treatment of

FTO glass substrates.

Pre-treatment of FTO

with HNO3 RS(Ω) RCT(kΩ) C(µF)

-0.6v dark 36.4 0.80 1.18

-0.6v illumination 35.6 0.89 1.20

61

CONCLUSION

Twelve natural dyes obtained from leaves of plants were used as sensitizers in DSSCs.

The absorption spectra of these dyes were measured. The J-V characteristics curves of

the fabricated cells were carried out. The performance of the cells were optimized. The

photoelectrochemical performance of the DSSCs based on these dyes showed that the

Voc ranged from 0.49–0.59 V, and Jsc was in the range of 0.17–0.95 mA/cm2. The

DSSC sensitized by lemon leaves extract exhibited the highest conversion efficiency

of 0.35% among the 12 extracts.

It was found that the efficiency of the DSSC can be enhanced by adjusting extracting

temperature of the extracts. The optimum extracting temperature of the lemon leaves

extract were found at 50º C. At the optimum conditions, the lemon leaves sensitized

DSSC showed efficiency as high as 0.50%.

It was also found that the efficiency of the DSSC can be enhanced by the pre-treatment

of the FTO with phosphoric acid (H3PO4), nitric acid (HNO3) and hydrochloric acid

(HCl). The pre-treatment of the FTO with phosphoric acid (H3PO4) improved

efficiencies by 137%, 124% and 150% for the DSSCs sensitized by lemon, mandarin

and orange leaves, respectively. The pre-treatment of the FTO with nitric acid (HNO3)

improved efficiencies by 140%, 148% and 180% for the DSSCs sensitized by lemon,

mandarin and orange leaves respectively, where as the pre-treatment of the FTO with

hydrochloric acid (HCL) improved efficiencies by 140%, 136% and 210% for the

DSSCs sensitized by lemon, mandarin and orange leaves respectively.

The nitric acid (HNO3) and hydrochloric acid (HCL) post-treated electrodes showed

an efficiency of 0.41% and 0.39% while the DSSC fabricated with untreated electrodes

gave an efficiency of η=0.32%. The HNO3 and HCL treated electrodes showed an

efficiency enhancement of about 128% and 121% for the DSSCs sensitized by lemon

leaves dye.

The highest efficiency of 0.5% has been obtained by lemon leaves which contains

chlorophyll pigment extracted at 50º C for 3 hours.

62

It can be concluded that natural dyes as sensitizers for DSSCs are promising

alternatives to organic dyes. They offer environmental friendliness, low-cost

production, simple preparation technique and wide availability. Although the

efficiencies obtained with the natural dyes are below the requirements for large scale

production, the results are hopeful and can boost additional studies oriented to the

search of new natural sensitizers and to the optimization of solar cell components

compatible with such dyes. However, there still remains room for further development

for the commercialization of this technology and there are still many challenges that

need to be faced, and a lot of improvements within the field have to be done, before

dye sensitized solar cells make a large impact on the society.

63

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