(http://cdn.intechopen.com/pdfs-wm/23331.pdf). 24. ... M.R. Nishantha, Y.P.Y.P Yapa1 and V.P.S. Perera, Proceedings of the Technical Sessions,. Vol. 28, p.
To Study the Electrical and Optical Properties of the Components and I-V characteristics of the Dye Sensitized Solar Cells Fabricated by using Natural Dyes A Dissertation
Submitted to the Department of Physics GoldenGate Int. College (T.U.), Kathmandu, Nepal In the partial fulfillment for the requirement of Master’s Degree of Science in Physics
By
Rajesh Pathak T.U. Registration no. 5-1-33-141-2004 Roll no.13934 April, 2015 1
RECOMMENDATION
This is to certify that Mr. Rajesh Pathak has carried out the dissertation work entitled “To Study The Electrical And Optical Properties of The Components and I-V Characteristics of The Dye Sensitized Solar Cells Fabricated By Using Natural Dyes” under my supervision and guidance. I recommend this dissertation to be approved for the partial fulfillment of Master‟s Degree of Science in Physics.
.……………………………… Prof. Dr. Shankar Prasad Shrestha (Supervisor) Department of Physics Patan Multiple Campus Lalitpur, Nepal
2
ACKNOWLEDGEMENT
Behind the work complied in these pages lie the steady encouragement, fruitful guidance constant enthusiasm of my esteemed Prof. Dr. Shankar Prasad Shrestha. I owe the greatest debt of gratitude to him for suggesting the research problem followed by meticulous suggestions ever-willing help and his concern extended to me during the period of my research work, which obviously make the completion of work with lots of innovative ideas. Without his supervision and constant help this dissertation would not have been possible.
I am very much thankful to the Department of Physics, Patan Multiple Campus, Lalitpur who allow me to work in the Thin Film Research Laboratory running under an authority of my thesis supervisor.
I would also like to thank GoldenGate Int. College (GGIC) for their financial support and providing the necessary chemicals and materials required along with their persistent encouragement. I would like to give special thanks to the members of the Physics Department, GGIC and they are Prof. Dr. Bhadra Pokharel, Assoc. Prof. Mr. Homnath Poudel, Assoc. Prof Leela Pradhan , Lecturer Mr. Min Dhakal for their guidance and insightful comments.
Also, I would like to say thank you to all my colleagues for all their help regarding the all areas of knowledge that the work involved, for their every single advices and co-operation. I would like take the name of Mr. Madan Saud, Mr. Bishnu Kharel, Mr. Tikaram Neupane, Mr. Bheshraj Chand, Mr. Biplav Dahal, Mrs. Alina Basnet and Yubak raj Poudel.
Finally, not the least, I express my deep gratitude to my parents for their moral support, continuous encouragement and their best wishes throughout my life. Rajesh Pathak 3
EVALUATION
We certify that we have evaluated the dissertation entitled “To Study The Electrical and Optical Properties of The Components and I-V Characteristics of The Dye Sensitized Solar Cells Fabricated by Using Natural Dyes” submitted by Mr. Rajesh Pathak. In our opinion, this thesis meets all the pre-requisites in scope and quality as dissertation in partial fulfillment for the requirement of Master's Degree of Science in Physics, and has been approved by the undersigned members of the Evaluation Committee.
Evaluation Committee
………………………
…………………...........
Prof. Dr. Shankar Prasad Shrestha
Asst. Prof. Mr. Homnath Poudel
(Supervisor)
(M.Sc. Physics Program Director)
………………………
.......................................
Internal Examiner
External Examiner
Date: ……………………….
4
Abstract
Natural dye-sensitized solar cells (DSSC) are one of the most promising devices for the solar energy conversion due to their low production cost and low environmental impact. The synthesis and performance study of Zinc oxide (ZnO) nanostructure based DSSCs is reported in the present paper. ZnO seed layer on the FTO coated (by spray pyrolysis method) glass substrate were prepared by spin coating method and then the ZnO nanostructure were fabricated using hydrothermal process. Nearly the same sheet resistance of each of the ZnO seed layer and each of the FTO glass substrate confirmed that we can reproduce the same kinds of materials in the laboratory, however, the absorbance spectrum before and after dye loading are not much unifor m confirming difficulties to reproduce them. We fabricated the dye-sensitized solar cells (DSSCs) based on natural dyes extracted from the different parts of the avocado fruit (nut of the avocado fruit, inner skin of the avocado fruit and outer skin of the avocado fruit) and from the leaves of guava and Lawsonia Inermis (mehendi) and investigated the performance based on I-V characteristics. The electrical and optical measurements of the different components of the DSSCs were taken using four probe method and UV-VIS spectrophotometer (Ocean optics, USB 2000) respectively. The solar cells were assembled using a hydrothermally grown ZnO nanostructure and platinum coated counter electrode on FTO coated glass substrate. The I-V characteristics of the DSSCs were studied under an incident irradiation of 100 and 1000 W/m2 in the Halogen lamp and the available different power intensity in the sunlight. The best performance was for the DSSC sensitized with nut of the avocado fruit, which to our knowledge have been used for the first time , with short circuit current (Isc) of 123 µ𝐴 , open circuit voltage (Voc) of 315 mV and an efficie nc y of 0.01147% under an incident irradiation of 1000 W/m2 in sunlight, and Isc of 16 µ𝐴 and 5.4 µ𝐴 with Voc of 148 mV and 210mV under an incident irradiation of 1000 W/m2 and 100 W/m2 in the Halogen lamp respectively. Key Words: Dye-Sensitized Solar Cells (DSSCs), Natural dyes, Spectrophotometer, Vander Pauw Four point Probe method.
5
List of Figures
Fig. 2.1 The regenerative working principle of the dye-sensitized nanostructured
11
Solar cell Fig. 2.2 Typical configuration of DSSC
13
Fig. 2.3 Parameters of sheet resistance
23
Fig. 2.4 The Current –Voltage Characteristics of Solar Cell
25
Fig. 3.1 Experimental set up (home made) for spray Pyrolysis unit
30
Fig. 3.2 Experimental set up (home made) for spin coating
31
Fig. 3.3 Experimental set up (home made) for the hydrothermal growth
32
Fig. 3.4 UV-visible spectrophotometer (USB2000, Photonics)
34
Fig. 3.5 Experimental set up (home made) for the measurements of
35
sheet resistance Fig. 3.6 Experimental set up for the I-V characterization of DSSC
36
Fig. 4.1 Absorbance Spectra of all the ZnO Seed Layer (in combined form)
41
used to make WE Fig. 4.2 (a) Absorbance Spectra of ZnO nanostructure (BA,BC,BE,BG,BI,BK) at
44
different positions before loading different dye Fig. 4.2 (b) Absorbance Spectra of ZnO nanostructure (BB,BD,BF,BH,BJ,BL) at
47
different positions after loading different dye Fig. 4.3 (a) Combined absorption spectra of different natural dyes extracted
49
in distilled water Fig. 4.3 (b) Combined absorption spectra of different natural dyes extracted 6
49
in Ethanol Fig. 4.4 Absorption spectra of the ZnO seed layer coated on FTO thinfilm,
51
ZnO nanostructure before and after loading (a)at 700 C (a) and (b) at room temperature with avocado fruit dye in the Sample 3z23 and that of dye extracted in DW Fig. 4.5 Absorption spectra of the ZnO seed layer coated on FTO thinfilm,
52
ZnO nanostructure before and after loading (a) at 700 C and (b) at room temperature with inner covering of avocado fruit dye in the Sample 3z16 and that of dye extracted in DW Fig. 4.6 Absorption spectra of the ZnO seed layer coated on FTO thinfilm,
53
ZnO nanostructure before and after loading (a) at 700 C and (b) at room temperature with leaves of guava dye in the Sample 3z18 and that of dye extracted in DW Fig. 4.7 Absorption spectra of the ZnO seed layer coated on FTO thinfilm,
54
ZnO nanostructure before and after loading (a) at 700 C and (b) at room temperature with Lawsonia Inermis (mehendi) dye in the Sample 3z13 and that of dye extracted in DW Fig. 4.8 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after at (a) 700 C and (b) at room temperature with avocado fruit dye loading in the Sample 3z17 and that of dye extracted in DW
7
55
Fig. 4.9 (a) I-V characteristics of Sample 3z23 (power density 100W/m2 in HL)
60
Fig. 4.9 (b) I-V characteristics of Sample 3z23 (power density 1000W/m2 in HL)
60
Fig. 4.9 (c) I-V characteristics of Sample 3z23 (power density 1000W/m2 in Sunlight)
60
2
Fig. 4.10 (a) I-V characteristics of Sample 3z16 (power density 100W/m in HL)
61
Fig. 4.10 (b) I-V characteristics of Sample 3z16 (power density 1000W/m2 in HL)
61
Fig. 4.10 (c) I-V characteristics of Sample 3z16 (power density 600W/m2 in Sunlight)
61
Fig. 4.11 I-V characteristics of Sample 3z15
(power density 1050 W/m2 in Sunlight)
62
Fig. 4.12 I-V characteristics of Sample 3z18 (power density 1000W/m2 in Sunlight)
62
Fig. 4.13 I-V characteristics of Sample 3z20 (power density 1050W/m2 in Sunlight)
63
Fig. 4.14 I-V characteristics of Sample 3z21 (power density 240 W/m2 in Sunlight)
63
8
List of Tables Table 4.1: The table showing the sheet resistances of the ZnO seed layer
38-39
coated on FTO thin film corresponding to the types of natural dyes used. Table 4.2 The table showing the sheet resistance of the FTO glass
39-40
substrate that is used for making counter electrode (CE). Table 4.3 The table showing the Short circuit current Is c and Open circuit voltage voc for different natural dye sensitized solar cell in light source conditions.
9
different
65
List of Acronyms CdTe
Cadmium Telluride
CIGS
Copper Indium Gallinum Selenide
DSSC
Dye-Sensitized Solar Cell
ITO
Indium Tin Oxide
UV-VIS
Ultra Viloet-Visible
FTO
Flourine doped Tin Oxide
XRD
X-ray Diffraction
SEM
Scanning Electron Microscope
TCO
Transparent Conducting Oxide
EIS
Electrochemical Impedance Spectroscopy
CV
Cyclic Voltammetry
J-V
Current Density-Voltage
ZnO
Zinc Oxide
AM
Air Mass
WE
Working Electrode
CE
Counter Electrode
IR
Infrared Radiation 10
HTM
Hole Transport Material
SPT
Spray Pyrolysis Technique
HMTA
Hexamethylenetetramine
DEA
Di-Ethanolamie
KI
Potassium Iodide
HOMO
Highest Occupied Molecular Orbital
LUMO
Lowest Unoccupied Molecular Orbital
DW
Distilled Water
HL
Halogen Lamp
FF
Fill Factor
Voc
Open Circuit Voltage
Isc
Short Circuit Current
Jsc
Short Circuit Current Density
11
TABLE OF CONTENTS
Recommendation
i
Acknowledgement
ii
Evaluation
iii
Abstract
iv
List of figures
v
List of tables
vi
Acronyms
vii
Table of contents
viii
12
CHAPTER ONE
1-8
INTRODUCTION 1.1 Background Information And Motivation
1
1.2 Literature Review
3
1.3 The Aim with Summary
7
1.4 Thesis Outline
7
CHAPTER TWO
9-27
THEORETICAL BACKGROUND 2.1 Dye-Sensitized Solar Cells (DSSCs)
9
2.1.1 Working Principles
9
2.1.2 Advantages And Drawbacks
12
2.2 Components of Dye Sensitized Solar Cell
12
2.2.1 Substrate
13
2.2.2 Transparent Conducting Oxide
13
2.2.3 Oxide Semiconductor
14
2.2.4 Sensitizer
15
2.2.5 Electrolyte
15
2.2.6 Sealing
17
2.2.7 Counter Electrode
17
2.3 Film Deposition, Nanostructure Growth And Natural Dye Extraction Techniques 13
18
2.3.1 Spray Pyrolysis
18
2.3.2 Spin Coating Process
19
2.3.3 Nanostructure Growth Techniques
20
2.3.4 Natural Dye Extraction Techniques
21
2.4 Characterization Techniques
21
2.4.1 UV-VIS Spectrophotometer And Optical Absorption
21
2.4.2 Sheet Resistance And Resistivity
23
2.4.2.1 Four Point Probe Method
24
2.4.3 Parameters of Solar Cell And I-V Characterization
CHAPTER THREE
24
28-37
EXPERIMENTAL METHODS AND CHARACTERIZATIONS 3.1 Introduction
28
3.2 Glass Substrate Cleaning Method
28
3.3 Preparation of Solutions
28
3.3.1 FTO Solution
And Film Deposition
28
3.3.2 ZnO Solution
And Seed Layer Deposition
30
3.3.3 Solution for Hydrothermal
31
3.3.4 Dye Solution Extraction And Dye loading
32
3.3.5 Electrolyte Solution
33
3.3.6 Platinum Chloride Solution for Counter Electrode
33
3.4 Assembling of Solar Cell
33 14
3.5 Characterization
34
3.5.1 Optical Measurement
34
3.5.2 Electrical Measurement
35
3.5.3 I-V Characterization
37
CHAPTER FOUR
38-64
RESULT AND DISCUSSION 4.1 Reproducibility of Samples
37
4.1.1 Sheet Resistance of The ZnO Seed Layer
37
4.1.2 Sheet Resistance of The FTO Glass Substrates Used for Making CE
39
4.1.3 UV-VIS Absorption Spectroscopy Analysis
40
4.1.3.1 Absorbance Spectra of all ZnO Seed Layer (combined form)
40
Used for Making WE 4.1.3.2 Absorbance Spectra of Hydrothermally Grown ZnO Nanostructure
41
Before And After Dye Loading 4.2 Absorbance of Natural Dyes
48
4.3 Absorption Effect on The Working Electrode
50
(ZnO Semiconductor Photoanode) 4.4 I-V Characteristics of Dye-Sensitized Solar Cells
15
58
CHAPTER FIVE
65-66
CONCLUSION 5.1 Conclusion
65
5.2 Future Work
65
References
67-70
16
TABLE OF CONTENTS
Recommendation
i
Acknowledgement
ii
Evaluation
iii
Abstract
iv
List of figures
v
List of tables
vi
Acronyms
vii
Table of contents
viii
CHAPTER-ONE
INTRODUCTION
1-9
1.1 Background Information And Motivation
1
1.2 Literature Review
3
1.3 The Aim with Summary
7
1.4 Thesis Outline
8
CHAPTER-TWO
MATERIALS, DEVICES, FABRICATION CHARACTERIZATION TECHNIQUES
2.1 Dye-Sensitized Solar Cells (DSSCs)
9-30
9
2.1.1 Working Principles
9
2.1.2 Advantages And Drawbacks
12 17
2.2 Components of Dye Sensitized Solar Cell
13
2.2.1 Substrate
14
2.2.2 Transparent Conducting Oxide
14
2.2.3 Oxide Semiconductor
15
2.2.4 Sensitizer
15
2.2.5 Electrolyte
16
2.2.6 Sealing
18
2.2.7 Counter Electrode
18
2.3 Film Deposition, Nanostructure Growth And Natural
19
Dye Extraction Techniques 2.3.1 Spray Pyrolysis
19
2.3.2 Spin Coating Process
20
2.3.3 Nanostructure Growth Techniques
22
2.3.4 Natural Dye Extraction Techniques
22
2.4 Characterization Techniques
23
2.4.1 UV-VIS Spectrophotometer And Optical Absorption
23
2.4.2 Sheet Resistance And Resistivity
24
2.4.2.1 Four Point Probe Method
25
2.5 Parameters of Solar Cell And I-V Characterization
26
CHAPTER-THREE EXPERIMENTALS AND THE TECHNUQUES
30-40
3.1 Introduction
30
3.2 Glass Substrate Cleaning Method
30
18
3.3 Preparation of Solutions
31
3.3.1 FTO Solution
31
3.3.2 ZnO Solution
32
3.3.3 Solution for Hydrothermal
33
3.3.4 Dye Solution Extraction And Dye loading
34
3.3.5 Electrolyte Solution
35
3.3.6 Platinum Chloride Solution for Counter Electrode
35
3.4 Assembling of Solar Cell
35
3.5 Characterization
36
3.5.1 UV-VIS Spectrophotometer
36
3.5.2 Dual Display Multimeter
37
3.5.3 Amprobe, Dual Display Multimeter And Resistance Box
38
CHAPTER-FOUR
RESULT AND DISCUSSION
4.1 Introduction
40
4.2 Electrical Characterization
40
4.2.1 Sheet Resistance of The ZnO Seed Layer
40
4.2.2 Sheet Resistance of The FTO Glass Substrates Used for Making Counter Electrode
42
4.3 UV-VIS Absorption Spectroscopy Analysis
43
4.3.1 Transmittance Spectrum of FTO Glass Substrate Used for Making Counter Electrode
43
4.3.2 Absorbance of Natural Dyes
46
4.3.3 Absorbance of Hydrothermally Grown ZnO Nanostructure Before And After Dye Loading
48
19
4.3.4 Absorption Effect on The Working Electrode (ZnO Semiconductor Photoanode)
52
4.4 I-V Characteristics of Dye-Sensitized Solar Cells
64
CHAPTER-FIVE
CONCLUSION
5.1 Conclusion
71
5.2 Future Work
72
References
73
20
CHAPTER ONE INTRODUCTION 1.1 Background Information and Motivation Energy is essential for the sustainability of the human civilization. We are entirely dependent on the supply of energy for living, working and developing the infrastructures. Presently, the global energy supplies depend predominantly on fossil fuels (e.g. oil, natural gases and coal) and nuclear technologies. The global concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities primarily due to fossil fuel use and agriculture respectively.
Depletion of fossil fuel reserves in the near future along with dangerous
environmental pollution and a very tight safety regulation in modern nuclear energy necessitates imperatively the use of alternative renewable energy sources. Therefore, renewable energy resources and their technology can provide direct and rapid benefit at local and national level contributing to a more sustainable global energy balance. Renewable energy sources such as the solar radiation, wind, hydro-mechanical and geotherma l energy are clean, cheap, available in plentiful supply and they do not contaminate the environme nt directly. Although there are numbers of renewable energy sources we are not in a position to make proper use of all these sources. Some renewable energy sources have relatively high cost and some have boundaries to select them because of the lack of modern technology or their availability in the difficult geographical locations. We are concerning and searching for the appropriate source in terms of cost, easy to make use of that, environmentally friendly and easily accessible. In countries like Nepal, due to geographical structure and where large part of the population, still live in remote areas without access to electrical grid, transportation and living the life under extreme poverty, solar energy technology can provide substantial socio-economic benefit. Also solar energy is available at any location on the earth’s surface and considered to be one of the most promising renewable energy sources for our present and future energy needs. Solar cells (photovoltaic cells) are the smallest basic unit of solar electric devices that convert solar energy directly into electrical energy. The first generation of solar cells, from industrializa tio n point of view, are based on crystalline silicon solar cells and are the most efficient and widely used 21
solar cells to date. The theoretical efficiency limit for this type of device is about 31% [1]. The manufacture of silicon-based solar cells involves high purity silicon, the manufacturing processes of which are extremely expensive. The high cost of the first generation solar cells severely restricts their widespread application in the future. To meet the demand of reducing material and purification costs second generation solar cells (thin film solar cells) have been developed. Thin film solar cells are based on thin layers of various semiconductor materials such as amorphous silicon, cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS). Even though the thin film solar cell requires less material, the complex production processes of the different combinations of rare materials are expensive. So we may not be able to produce them in large-scale in the upcoming days. Moreover, the use of toxic materials as, e.g., cadmium and rare elements as telluride and indium is another problem in this technology.
An alternative solar cell technology is the dye-sensitized solar cell (DSSC) also known as Grätzel cell [2], which has attracted much attention because they are inexpensive, light weight, portable, flexible and transparent relative to classical solid state heterojunction solar cells [3,4]. Although the efficiencies of DSSCs at the present stage are lower than those conventional solar cells, the high ratio of the performance and price still identifies DSSCs as an attractive potential solar cell technology to be commercialized. The record efficiency of a DSSC is 12.3% based on dye-sensitized mesoporous TiO 2 electrodes in contact with an electrolyte containing a cobalt complex redox electrolyte for dye regeneration [5]. These cells were first reported in 1991 and since then there have been significant improvements to the efficiency of the cell as well as gaining a much better understanding of the processes involved in a DSSC and their effect on the working of the cell DSSC are based on nanocrystalline metal oxide electrodes. The fundamental difference between DSSCs and p-n junction solar cells is the functional components. In the latter, the semiconductor assumes both the task of light absorption and charge carrier transport, while these two functions are separated in the DSSC. Accordingly, alternative, readily available and efficient photosensitizers at low cost is a scientific challenge for semiconductor based photovoltaic devices. Recently, research has focused on the easily available dyes extracted from natural sources and as a photosensitizer because of its large 22
absorption coefficients,
high light-harvesting efficiency,
low cost, easy preparation and
environment friendliness. The use of natural dyes in solar cells offers promising prospects for the advancement of this technology, because fabrication of cost effective solar cells is a scientific challenge. The use of natural pigments cut down the cost of chemical synthesis and high cost of rare metals need for metal organic dye sensitizers. Therefore lot of interest has been drawn on natural dyes which extracted from plant materials. Several natural pigments have been utilized as sensitizers in photovoltaic cells due to their capability of injecting electron from excited pigments to the conduction band of the semiconductor material. Natural dyes are exclusively used for educational, research and commercial purposes representing a low-cost, easily accessible and environment friendly alternative to conventional Ru-complexes. In this thesis work, I have fabricated a dye sensitized solar cell sensitized with a new natural dyes extracted from the nut of avocado fruit, skin of the nut of avocado fruit, outer skin of avocado fruit, leaves of guava and Lawsonia Inermis (mehendi) in distilled water and ethanol solvent. The first four natural dyes have been used for the first time to study the I-V characteristics and efficie nc y of the solar cell. ZnO possesses higher electron mobility and electron diffusion coefficient, and ZnO nanostructure can be gown, which is suitable for the collection of photoinduced electrons. So, ZnO semiconductor was used as the working electrode. 1.2 Literature Review Before starting our experiment on the natural dye sensitized solar cell, we had reviewed the papers, articles, journals and thesis or dissertation by many other professors, research group and scholars. O’regan and Gratzel in 1991, described a photovoltaic cell, created from low-to medium-pur ity materials through low-cost processes, which exhibits a commercially realistic energy-convers io n efficiency [6]. The device was based on a 10-µm-thick, optically transparent film of titanium dioxide particles a few nanometres in size, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. Because of the high surface area of the semiconductor film and the ideal spectral characteristics of the dye, the device harvests a high proportion of the incident solar energy flux (46%) and showed exceptionally high efficiencies for the conversion of incident photons to electrical current (more than 80%). The overall light-to-electric energy conversion yield was 7.1-7.9% in simulated solar light and 12% in diffuse daylight. They showed 23
that the large current densities (greater than 12 mA cm-2 ) and exceptional stability, as well as the low cost, make practical applications feasible. Rothenberger and his co-workers in 1999, presented a model that estimates the enhancement of optical absorption that can be obtained from light scattering in the porous nanocrystalline films used in the cells and from reflection at the back electrode [7]. The model was applied to the optical characterization of two films, a transparent, and a strongly scattering porous titania sample. Wongcharee and his co-workers in 2006, fabricated Dye-sensitized solar cells using natural dyes extracted from rosella, blue pea and a mixture of the extracts [8]. They found that the light absorption spectrum of the mixed extract adsorbed on TiO 2 does not show synergistic light absorption and photosensitization compared to the individual extracts. Instead, the cell sensitized by the rosella extract alone showed the best sensitization, which was in agreement with the broadest spectrum of the extract adsorbed on TiO 2 film. In case that the dyes were extracted at 1000 C, using water as extracting solvent, the energy conversion efficiency of the cells consisting of rosella extract alone, blue pea extract alone and mixed extract was 0.37%, 0.05% and 0.15%, respectively. They discussed the sensitization performance related to interaction between the dye and TiO 2 surface. The explanations were supported by the light absorption of the extract solution compared to extracts adsorbed on TiO 2 and also dye structures. They also reported the effects of changing extracting temperature, extracting solvent and pH of the extract solution. However, the efficiency of a DSSC using ethanol as extracting solvent was found to be diminished after being exposed to the simulated sunlight for a short period, they saw that the efficiency of rosella extract sensitized DSSC was improved from 0.37% to 0.70% when the aqueous dye was extracted at 50̊C instead of 100 ̊C and pH of the dye was adjusted from 3.2 to 1.0. Researchers from Taiwan, Chang and his co-worker in 2010, studied spinach extract, ipomoea leaf extract and their mixed extracts as the natural dyes for a dye-sensitized solar cell (DSSC) [9]. Initially, Spinach and Ipomoea leaves were placed separately in ethanol and the chlorophyll of these two kinds of plants was extracted to serve as the natural dyes for using in DSSCs. In addition, the self-developed nanofluid synthesis system prepared a TiO 2 nanofluid with an average particle size of 50nm. Electrophoresis deposition was performed to let the TiO 2 deposit nanoparticles on the indium tin oxide (ITO) conductive glass, forming a TiO 2 thin film with the thickness of 11.61µm. This TiO 2 thin film underwent sintering at 450◦ C to enhance the compactness of thin 24
film. Finally, the sintered TiO 2 thin film was immersed in the natural dye solutions extracted from spinach and ipomoea leaves, completing the production of the anode of DSSC. This study then further inspected the fill factor, photoelectric conversion efficiency and incident photon current efficiency of the encapsulated DSSC. According to the experimental results of current–volta ge curve, the photoelectric conversion efficiency of the DSSCs prepared by natural dyes from ipomoea leaf extract is 0.318% under extraction temperature of 50 ◦ C and pH value of extraction fluid at 1.0. They also investigated the influence of the temperature in the extraction process of this kind of natural dye and the influence of pH value of the dye solution on the UV–VIS patterns absorption spectra of the prepared natural dye solutions, and the influence of these two factors on the photoelectric conversion efficiency of DSSC. JingbinHan and his co-workers in 2010, fabricated high-density vertically aligned ZnO nanotube arrays on FTO substrates by a simple and facile chemical etching process from electrodeposited ZnO nanorods [10]. They also showed that the morphology of the nanotubes can be readily controlled by electrodeposition parameters for the nanorod precursor. By employing the 5.1 µmlength nanotubes as the photoanode for a dye-sensitized solar cell (DSSC), a full-sun conversion efficiency of 1.18% was achieved. Furthermore, they showed that the DSSC unit can serve as a robust power source to drive a humidity sensor, with a potential for self-powered devices. Researcher group of Nwanya, Ugwuoke and their co-workers, in 2012, assembled the DSSC by using natural dyes extracted from jathropha curcas and citrus aurantium leaves as sensitizers and compared with that of ruthenium dye [11]. TiO 2 films were prepared on FTO glass using the solgel process, the chemical bath deposition method and the slot coating of P-25 Degussa TiO 2 powder. They investigated the chemical, structural, morphology and optical properties of theTiO 2 using Energy Dispersive Spectrum (EDS), X-ray diffraction (XRD), UV-Visible-Infrared spectroscopy and Scanning Electron Microscopy (SEM). They found that the slot coated TiO 2 with jathropha leaf dye extract showed the highest overall efficiency of 1.26% with open circuit voltage of 350mV, short circuit current of 65.5 µA and a fill factor of 0.55. Researcher Arun Kumar and Francis Xavier in 2012, coated TiO 2 thin films by spin coating technique using Ti(OH)2 as a precursor, prepared by sol-gel synthesis [12]. They used Lawsone dye extracted from Lawsonia Inermis.The as-synthesized TiO 2 thin films were subjected to Fourier Transform Infrared Spectroscopy (FT-IR), UV-Vis spectrophotometer, Field dependent dark and 25
photo conductivity and temperature dependent conductivity studies. They found that the field dependent conductivity showed an insignificant rise in photocurrent for TiO 2 which was in conformity with its wide band gap nature and the Lawsone dye-sensitized TiO 2 thin film showed a significant rise of photocurrent over the dark current. Kim and his co-workers in 2013, extracted the natural dye, anthocyanin, as a sensitizer in DSSC, from the flowers of Rhododendron species with three different colors, pink, red and violet, using a simple extraction technique [13]. They treated dyes with nitric and acetic acids to examine their effects on the power conversion efficiency. According to the experimental results, the performance was better with the acetic acid-treated anthocyanin. The Jsc value for the acetic acidtreated pink, red and violet dyes increased to 0.887, 0.932 and 0.737 mA/cm2 compared to nitric acid treated and bare dye. The conversion efficiency of the acetic acid-treated pink, red and violet was 0.35%, 0.36%, 0.28%, respectively, which was higher than the nitric acid-treated and bare dye. Li and his co-workers in 2013, studied the photoelectrochemical optimal conditions for red cabbage extract as natural dye to develop a dye-sensitized solar cells (DSSC) [14]. Red cabbage extract were characterized by various methods, such as electrochemical impedance spectroscopy (EIS), UV-Vis and cyclic voltammetry (CVs). They fabricated the DSSC from a combination of relatively popular materials containing TiO 2 photoelectrode, natural dye, electrolyte containing I−/I3 − redox mediator, and counter electrode. They found that the use of suitable pH for red cabbage improves the DSSC performance. Moreover, they showed that the increase of efficiency due to the purified and immersed time of natural dye content greatly increased the specific activity and total load volume. Gharbi and his co-workers in 2015, employed several natural dyes for application in dye sensitized solar cells (DSSC) [15]. They checked the opportunity to realize good DSSC with dyes available henna and Mallow (Mloukhya). The optical absorption of the extracted dyes diluted in ethanol or distilled water were measured by using UV-Vis spectrophotometer. The absorption in beet and red cabbage was more significant compared to the other dyes. Mallow and henna dyes present a noticeable band in the region 660 nm. The DSSC were assembled using two supporting electrode and counter electrode which are coated with transparent conducting oxide (TCO). The counter electrode was coated by a catalyst Pt (platinum) to speed up the redox reaction with the electrolyte solution. The typical J-V curves of the cells under AM1.5 using a density of power 100W/m2 were 26
measured and it was found that the cells using henna and mallow as dyes present less degradation with time in the photoelectric characteristics. Moreover, the mallow cell shows a good fill factor of 55% and a noticeable photoelectric conversion efficiency of 0.215%. 1.3 The Aim With Summary This thesis work has been devoted to natural dye sensitized solar cell. Our main aim is (i)
to study the electrical and optical properties of the FTO coated glass substrate and Zinc Oxide seed layer for understanding the reproducibility of the samples in the laboratory.
(ii)
to study optical properties of hydrothermally growned ZnO nanostructure, differe nt natural dye extract and after loading different dyes on the ZnO nanostructure
(iii)
to fabricate and study the I-V characteristics of the natural dye sensitized solar cell.
The ambition of this thesis work is to develop and compare the very new natural dye as an alternative dye sensitizer. 1.4 Thesis Outline Chapter 1: In this chapter introduction, need for renewable energy sources and why solar cell is more important topic to be discussed, history reviews of dye-sensitized solar cells and the aim of the thesis with summary is considered, presented and discussed briefly. Chapter 2: The working mechanisms of DSSC, components of DSSC, and the mechanisms of major fabrication process used during the research, such as spray pyrolysis, spin coating, hydrother ma l growth for nanostructures, etc. and the electrical and optical characterization will be discussed in this section.
Chapter 3: A detailed description on the experimental procedures carried out in this research will be provided, including the equipment models and different techniques involved during the film deposition and the condition settings. This section also include the electrical and optical properties of the components, and I-V characterization of the dye sensitized solar cell using natural dyes. 27
Chapter 4: This chapter focuses on the results and observations made during the research. The obstacles of the fabrication process and methods of overcoming these problems have also been given in this chapter. Finally, a short description on the research application and possible future works are added with reserved conclusion. A list of references is given at the end of this work for further readings.
CHAPTER TWO THEORETICAL BACKGROUND 2.1 Dye-Sensitized Solar Cells (DSSCs) 2.1.1 Working Principles The DSSC is composed of two electrodes, the working electrode (WE) and a counter electrode (CE) and also an electrolyte containing a reduction-oxidation couple such as I-/I3 -.The working electrode consists of a dye-sensitized to the mesoporousfilm or nanostructure of semiconductor 28
that is attached to a conducting substrate. The conducting substrate is usually a glass plate coated with a thin layer of a transparent conducting oxide or TCO-layer, such as fluorine doped tin oxide (F:SnO 2 ) or indium tin oxide(ITO). The counter electrode is also composed of a conducting substrate, but with an additional catalyst such as platinum (Pt), carbon, etc. that helps to decrease the charge transfer resistance. Dye-sensitized solar cells operate with some remarkable analogies to the natural process of photosynthesis. Like the chlorophyll in plants, a monolayer of dye molecules (sensitizers) absorbs the incident light, giving rise to the generation of positive and negative charge carriers. When the sunlight is incident on DSSCs, the electrons in the dye jump from their ground state i.e. highest occupied molecular orbital (HOMO) to their excited state i.e. lowest unoccupied molecular orbital (LUMO), likely to the process of photosynthesis, where sunlight is absorbed and stored chemical energy is formed. The electrons are then injected into the conduction band of the semiconducting material and travel through the semiconducting material to the electron collecting glass substrate. Electrons then travel from the anode through the circuit and performing generating current around to the counter electrode. The dye is in oxidized state but is regenerated by the reducing agent of the electrolyte. In turn, the oxidized species of the electrolyte is reduced to its original state once the electron has passed through the load and is collected at the counter electrode. At this point, the circuit is complete, and the cyclic flow of electrons continues to pass from the anode to the cathode [16]. In other words, the absorption of light in the DSSC occurs by dye molecules and the charge separation by electron injection from the dye to the TiO 2 or Zno nanowire at the semiconductor electrolyte interface [16]. When the light is absorbed by the dye (S), then dye is promoted into an electronically excited state (S ∗) and it injects an electron into the conduction band of a large bandgap semiconductor film or nanostructure for example that of TiO 2 or ZnO within a very short time that we cannot observe [2]. The electrons are transported through the semiconductor by diffusion before reaching the anode of the cell, generally SnO 2 :F coated glasssubstrate, and then to the external circuit [17]. A single layer of dye molecules however, can absorb only less than one percent of the incoming light [2]. Piling the dye molecules on top of each other, we obtain a thick dye layer that increases the optical thickness of the layer. But only the dye molecules in direct contact to the semiconductor electrode surface can separate charges and contribute to the current generation. To overcome this difficulty the Grätzel group developed a porous nanocrystalline TiO 2 electrode structure in order to increase the internal surface area of the electrode to allow large 29
enough amount of dye to be contacted at the same time by the TiO 2 electrode and the electrolyte. To increase the surface area for more adsorption of the dye and as a result of the absorption of the sunlight, nanorods, nanowires, nanoflowers have been grown on the thinfilm of the semiconductor [18]. The incoming photon is absorbed by the dye molecule adsorbed on the surface on the semiconductor nanostructure or film and an electron from a molecular ground state S is excited to a higher lying excited state S*. The excited electron is injected to the conduction band of the semiconductor material because of the lower energy level, leaving the dye molecule to an oxidized state S+ (lack of electron). The injected electron penetrate gradually through the porous nanocrystalline structure of semiconductor to the transparent conducting oxide layer of the glass substrate (negative electrode, anode) and finally through an external load to the counter-electrode (positive electrode, cathode). At the counter-electrode the electron is transferred to triiodide in the electrolyte to yield iodine, and then the iodine in the electrolyte reduced the oxidized dye. The operating cycle can be summarized in chemical reaction terminology as [19]:
Anode:
S + hν → S*
Absorption
S* → S++ e-
(Semiconductor) Electron injection
2S+ + 3I-→ 2S + I3 -
Regeneration
Cathode:
I3 - + 2e-(Pt) → 3I-
Cell:
e-(Pt)+hν→e-(Tio2 )
30
Fig. 2.1 The regenerative working principle of the dye-sensitized nanostructured solar cell [20].
2.1.2 Advantages and Drawbacks As DSSC has been considered as the most efficient third-generation solar cell because of its important advantages over other p-n junction solar cells Moreover, taking into account the production cost and assuming the cell life time, the cost of electricity produced by a DSSCs would be approximately 10% of the cost of electricity produced by a single crystal silicon solar cell [21]. That means the cost to efficiency during the long life time of DSSC is less as compared to the previous generation solar cells. The materials used in the DSSCs are relatively environmenta lly 31
friendly and the neophyte architecture such as flexible sheets and flexible photovoltaics fiber can be constructed. Also, we can even use materials of lower purity in DSSC and assembling or fabrication methods are generally simple and inexpensive as well. However, the conversion efficiencies of the dye sensitized solar cell are relatively lower (slightly above 11%) compared to that of the conventional solar cell (silicon based solar cells have recorded of about 25% efficiency) [1,5]. At low temperature the electrolyte can freeze it, ending the power production and at high temperature the electrolyte can expand causing the problems in sealing the panel. Also the volatile organic solvents in electrolytic solution are volatile so we unable to use them in large proportion in the outdoor and their integration into flexible structures is another problem [22]. Thus we need to replace the liquid electrolyte with a solid that has been a major ongoing field of research nowadays. 2.2 Components of Dye Sensitized Solar Cell The current DSSC design involves a set of different layers of components stacked in serial, including glass substrate, transparent conducting layer, TiO 2 nanoparticles or Zno nanowire, dyes, electrolyte, and counter electrode covered with sealing gasket. The typical configuration is shown in Fig. 2.2.
Fig. 2.2 Typical configuration of DSSC [23] 32
2.2.1 Substrate Substrates play a key role as the foundation for optoelectronic devices. The mechanical strength, optical transparency, and maximum processing temperature are among the critical properties of these substrates that determine its eligibility for various applications. Solar cell substrates require high optical transparency but also prefer high optical haze to increase the light scattering and consequently the absorption in the active materials.
The optoelectronic device industry
predominantly utilizes glass substrates and plastic substrates for flexible electronics, however , other types of substrate like poly(ethylene terephthalate) (PET), renewable cellulose nanofibers , textiles, metal or polymer foil, wood fiber, etc. can also be used. Mainly, in the context of Nepal, we use glass substrate for the dye-sensitized solar cell because we are familiar with it as it is easily available in the market, environmental friendly, can withstand temperatures of up to ~450 to ~5000 C that are necessary when annealing the other component layers and coatings. 2.2.2 Transparent Conducting Oxide In the front of the DSSC there is a layer of glass substrate which is coated with a thin layer of transparent conducting layer. This allows sunlight penetrating into the cell while conducting electron carriers to outer circuit. ITO have greater than 85% transmittance performs best among all TCO substrates [23]. However, the window material of the thin film or dye-sensitized solar cell allows the visible region of solar spectrum to pass through but reflect the infrared (IR) radiation. Such windows material can be obtained by the preparation of transparent and conducting oxide (TCO) coatings such as indium oxide (In2 O3 ), cadmium stannate (Cd2 SnO 4 ), tin oxide (SnO 2 ), cadmium oxide (CdO), cadmium indate (CdIn2 O4 ), etc [23]. Transparent Conductive Oxide (TCO) substrates are adopted, including F-doped or In-doped tin oxide (FTO or ITO) and Aluminumdoped zinc oxide (AZO), studies and application on these highly conducting semiconductors have attracted the interest of many researchers because of their wide applications in both commercia l and research field. These films are very efficient in reflecting broadband infrared heat radiation in a manner similar to highly conducting metal-like materials and in transmitting the light in the visible region as if they are insulators. AZO thin films are also widely studied because the materials are cheap, nontoxic and easy to obtain [24]. Among the different transparent conductive oxides, SnO 2 films doped with fluorine or antimony seem to be the most appropriate for use in solar cells, owing to its low electrical resistivity and high optical transmittance. SnO 2 is chemically inert, 33
mechanically hard, and can resist high temperatures [25]. Also ITO contains rare, toxic and expensive metal materials, some research groups replace ITO with FTO [25]. 2.2.3 Oxide Semiconductor The semiconductor layers are the most important parts of a DSSC. they form the heart of the solar cell. There are a number of different semiconductor materials that are suitable for the conversion of energy of photons into electrical energy, each having advantages and drawbacks. A close match energy levels is needed for the efficient injection of photo-excited electrons from the dye into the electrode. Sensitized wide band gap semiconductors such as nanoporous or nanostructured TiO 2 , ZnO are used that resulted in high chemical stability of the cell due to their resistance to photocorrosion [26]. A dense network of nanostructures such as, nanowires, nanorods, nanotubes, nanoplates, etc. of semiconductor oxide should be favorable for electron collection because the nanowire morphology provides more direct conduction paths for electrons to transport from the point of injection to the collection electrode as a result the efficiency of theDSSC is improved [18]. 2.2.4 Sensitizer One of the key components of the DSSC is the sensitizing dye that harvest the incident light for the photon-to-electron conversion [27]. The desirable sensitizer should harvest all photons below a threshold wavelength of about 920 nm i.e. whole visible region and even the part of the nearinfrared (NIR). Secondly, to strongly bind the dye onto the semiconductor surface, the photosensitizer should have anchoring groups (-COOH, -H2 PO 3 ,-SO3H, etc.). In addition to this, the excited state level of the photosensitizer should be higher in energy than the conduction band edge of n-type semiconductor (n-type DSSCs), so that an efficient electron transfer process between the excited dye and conduction band (CB) of the semiconductor can take place. Moreover, the photosensitizer should be photostable and thermal stable [28]. Transition metal coordinated compounds (e.g., ruthenium polypyridyl complexes) were used as sensitizers in the earlier DSSC because of their strong visible absorption, long excitation lifetime and efficient metal-to-ligand charge transfer [2,3]. Although highly effective, with current maximum efficiency of 11% [3], the costly synthesis (Ru dye costs >$1,000/g) and undesired 34
environmental impact of those prototypes, the cheaper, simpler, and safer dyes as alternatives is the matter of fact to be studied. Some other sensitizer are Porphyrins, Phthalocyanines and some other organic dyes. The desirable sensitizer should harvest all photons below a threshold wavelength of about 920 nm. Natural dyes are exclusively used for educational purposes representing a low-cost and environmentally friendly alternative to conventional Ru-complexes. The best results so far are the anthocyanins extracted from Jaboticaba and Calafate yielding I SC = 9 mAcm-2 , VOC=0.59 V and 6 mAcm-2 , 0.47 V respectively [29]. 2.2.5 Electrolyte The properties of the electrolyte, another key components in DSSC, have great influence on the efficiency and stability of the devices. The function of the redox shuttle is to transfer electrons from the counter electrode to the oxidized dye formed when the photo-excited electron is injected into the photo-anode, completing the electrochemical circuit. The redox electrolytes used in a DSSC can be liquid electrolyte, quasi-solid state electrolyte or solid-state hole conductors. The electrolyte, containing the components such as the redox couple, solvent, additives, cations, etc in DSSC should have following characteristics [30,31]. a. The electrolyte must have long-term chemical stability, thermal stability, optical stability, electrochemical stability, and interfacial stability, which prevents the desorption and degradation of the dye from the oxide surface. b. The electrolyte should not exhibit a significant absorption in the range of visible light. For the electrolyte containing I– /I3 – redox couple, since I3 – shows color and reduces the visible light absorption by the dye, and I3 – ions can react with the injected electrons and increase the dark current. Thus, the concentration of I– /I3 – must be optimized. c. After the dye injects electrons into the conduction band of the oxide semiconductor, the oxidized dye must be reduced to its ground state quickly. Thus, while selecting the electrolyte, we should take into account the electrolyte redox potential and regeneration of itself. d. The electrolyte must be able to permit the fast diffusion of charge carriers so that conductivity increases. 35
e. They should be non-corrosive with DSSC components. Several redox couples, such as Br-/Br2 , SCN-/SCN 2 , Co(II)/Co(III), I-/I3 -, etc.have been used but I-/I3 - redox couple has been demonstrated up to now as the most efficient, however, it can seriously corrode the glass and/or TiO 2 resulting in poor long term stability [32]. In fact, cations, particular ly small cations such as protons (Li+, etc.) play an important role in the photoelectric performance of DSSC. For example, the addition of LiI into liquid electrolyte can enhance the Jsc of DSSCs. The reason is that the small-radius Li+ can deeply penetrate into the mesoporous dye-coated nanocrystalline TiO 2 film and form an ambipolar Li+- e– with the electrons in the conduction band of TiO 2 , which increases the transport speed of electrons in nanocrystalline TiO 2 network and enhances the Jsc of DSSCs [33]. The influence of nitrogen-containing heterocyclic electric additives such as 4-tert-butylpyridine, pyridine could enhance Voc but decrease Jsc. There are two kinds of solid-state DSSCs, one uses hole transport materials (HTMs) as medium, the other uses a solid-state electrolyte containing iodide/triiodide redox couple as medium. Higher photovoltaic performance for DSSCs using iodide/triiodide redox couple than that using HTMs is due to the fact that the iodide/triiodide can efficiently recreate oxidized dyes, and the dark reactions in these solid-state electrolytes are lower than that in HTMs. Another reason is that the interfac ia l contact properties of these solid-state electrolytes are better than that of HTMs.This kind of solid state electrolyte has a good prospect in practical DSSC. 2.2.6 Sealing Sealing the DSSCs has long been a difficult question because of the corrosive and volatile liquid iodide electrolyte used in the cells. Being directly related to the long term stability of the cells it seems to be one of the main technological challenges of the DSSC technology [29]. A suitable sealing material should at least [34]. 1. be leak-proof to the electrolyte components and impermeable to both ambient oxygen and water vapor, 2. be chemically inert towards the electrolyte and other cell components, and 3. adhere well to the glass substrate and TCO coating.
36
Parafilm, Hotmelt resin and UV-curing resin can be applied as the sealent of the cells [35]. 2.2.7 Counter Electrode On the back of the DSSC there presents another glass substrate (generally FTO coated) covered with a thin layer catalyst to regenerate I- and is termed as the cathode material. Pt is the best material to make efficient devices technically. The reaction on the surface of counter electrode is the reduction of triiodide such as I3 −+2e→3I−, while the oxidation of iodide occurs on the dye molecule. The efficiency and properties of DSSC are deeply affected by the reduction rate on the counter electrode and the properties of counter electrode materials. The materials used for the good counter electrode should have a good electrocatalytic effect on the reduction of electrolyte (triiodide), high surface area, and high electric conductivity. At present, the platinum coated counter electrode is used due to the high efficiency obtained [36]. However, the platinum coated electrode is highly expensive, so the several authors have developed new counter electrode alternatives, such as carbon [37], CoS [38] and other conductive polymer. 2.3 Film Deposition, Nanostructure Growth And Natural Dye Extraction Techniques Some of the important low cost film deposition techniques are described in the next page. 2.3.1 Spray Pyrolysis The chemical spray pyrolysis technique (SPT) has been, during last three decades, one of the major techniques to deposit a wide variety of materials in thin film form. The prime requisite for obtaining good quality thin film is the optimization of preparative conditions viz. substrate temperature, spray rate, concentration of solution etc. However, in recent years an emphasis has been given to a variety of atomization techniques such as ultrasonic nebulisation, improved spray hydrolysis, corona spray pyrolysis, electrostatic spray pyrolysis and microprocessor based spray pyrolysis [39]. This is the most critical parameter as it enables control over the size of the droplet s and their distribution over the preheated substrates [39]. The enhancement in deposition efficie nc y and improvement in quality of the thin film can be achieved with these atomization techniques. It is observed that the properties of thin films depend very much on the preparative conditions. The properties of the thin film can be easily tailored by adjusting or optimizing these conditions, which in turn are suitable for a particular application. 37
Spray pyrolysis is a thermally stimulated reaction of differe nt compounds. In this technique, thin film is deposited on a hot substrate by spraying an aqueous acidic solution of desired compound (salt). This technique is most widely useful for the preparation of zinc oxide films and has been used for the long time for the production good transparent electrical conductor of SnO 2 on glass. By using simple instrument, a range area of thin film can be prepared in short time. Spray pyrolysis technique is one of the much attractive techniques because a large area of thin film can be prepared in short time by using simple instrument. Also by this method one can obtain the required properties of deposited film by controlling the deposition parameter. All different properties such as adherence, electrical, structural, optical and surface morphology etc and can be controlled by proper selection of substrate, starting material and by optimizing the condition of spray (i.e. flow rate of the solution, carrier gas, solution concentration, substrate temperature, spray time etc.) and annealing. First of all the spray droplets reside on the heated surface, then the solvent evaporation take place leaving behind the solid. This is further reacted in dry state forming required compound on the substrate. Before the droplet reaches the surface of the substrate, the solvent evaporation take place and the dry solid impinges on the heated surface where decomposition reaction occurs at the surface leading to the required product on the substrate. When the droplet reaches to the substrate, then the solvent vaporizes. The vapor thus produce diffused in to the substrate and undergo a heterogeneous reaction and lead to chemical vapor deposition. When aerosol droplets arrive close to the hot substrate, then the reaction occurs. In this technique, the significant deposition parameters are substrate temperature, solution flow rate and carrier gas, solution concentration, nozzle to substrate distance and size of droplets. The function of the solvent liquid is to carry the reactants and to distribute them uniformly over the heated substrate. When the droplets sprayed over the nozzle come in contact with the hot substrate surface, it undergoes pyrolytic decomposition and form crystallites of the desired compound. The volatile product and the excess solvent escape in the vapor phase. On further sintering, the deposited film grain growth takes place which results in a coherent film of the desired substance. The pyrolytic reactions are temperature dependant because temperature plays a crucial role in the formatio n of the film. 2.3.2 Spin Coating Process
38
Spin coating was first studied for coating of paint and pitch. Spin coating has been used for several decades for the application of thin film. A typical process involves depositing a small puddle of a fluid resin onto the center of a substrate and then spinning the substrate at high speed (typically around 3000 rpm) [40]. Centripetal acceleration will cause the resin to spread toward the edge of the substrate leaving a thin film of resin on the surface. Final film thickness and other properties will depend on the nature of the resin (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process. Factors such as final rotational speed, acceleration, dispense volume, final film thickness, solution viscosity, solution concentration (c), spin time, and fume exhaust contribute to how the properties of coated films are defined. One of the most important factors in spin coating is repeatability. A typical spin process consists of a dispense step in which the resin fluid is deposited onto the substrate surface, a high speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. Two common methods of dispense are static dispense, and dynamic dispense. Static dispense is simply depositing a small puddle of fluid on or near the center of the substrate. Higher viscosity and or larger substrates typically require a larger puddle to ensure full coverage of the substrate during the high speed spin step. Dynamic dispense is the process of dispensing while the substrate is turning at low speed. A speed of about 500 rpm is commonly used during this step of the process. This serves to spread the fluid over the substrate and can result in less waste of resin material since it is usually not necessary to deposit as much to wet the entire surface of the substrate. This is a particularly advantageous method when the fluid or substrate itself has poor wetting abilities and can eliminate voids that may otherwise form. After the dispense step it is common to accelerate to a relatively high speed to thin the fluid to near its final desired thickness. Typical spin speeds for this step range from1500-6000 rpm, again depending on the properties of the fluid as well as the substrate. This step can take from 10 seconds to several minutes. The combination of spin speed and time selected for this step will generally define the final film thickness. In general, higher spin speeds and longer spin times create thinner films. A separate drying step is sometimes added after the high speed spin step to further dry the film without substantially thinning it. This can be advantageous for thick films since long drying times may be necessary to increase the physical stability of the film before handling. 2.3.3 Nanostructure Growth Techniques 39
In order to obtain high quality well-aligned nanostructures, the substrate surfaces must be completely free of contaminants. Any contaminated area may cause disrupted structure or hinde r nanowire formation. Therefore, all type of substrates including silicon (Si) wafer, soda-lime silica glass and indium tin oxide (ITO) coated glass are cleaned with acetone), isopropanol and DI water in an ultrasonic bath for about 10 minutes. Then, the substrates are dried under nitrogen flow. The ZnO seeded substrates are immersed into an equimolar aqueous solution of zinc acetate dihydrate (Zn(O 2 CCH3 )2 (H2 O)2 ) or zinc nitrate hexahydrate (Zn(NO 3 )2 .6H2 O) alone or Zinc nitrate hexahydrate in presence of cetyltrimethylammonium bromide(CTAB) as the promoter or Zinc nitrate hydrate along with aluminum nitrate hydrate and hexamethylenetetramine (HMTA) in DI water, depending on the types of nanostructures, for example, nanorod, nanowire, nanotube, nanoflower,etc [41,42]. Sometimes, Sodium hydroxide (NaoH) or ammonia hydroxide is added to the Zinc acetate dihydrate dissolved in doubly distilled water [43]. This solution is kept in a closed bottle to prevent the water evaporation during growth. Growth time and concentration of the solution are used to determine the length and size of the nanostructures. The bottle is dipped into an oil bath or water bath kept at around 90°C for several hours. This process leads to hydroxide ion formation as a result of the decomposition of HMTA, which reacts with the Zn2+ ions to form ZnO crystals. Firstly, the reaction of HMTA and water occurs and gives ammonia into the solution. Then, ammonia reacts with water and this reaction results in the formation of ammonium and hydroxide ions. Finally, these ions react with Zn ion to form ZnO. 2.3.4 Natural Dye Extraction Techniques Natural dyes for the sensitizer in DSSC are extracted from the different parts of plants, herbs, fruits, flowers, etc. The fresh dye extraction parts (roots, barks of stem, petals of flowers, fruits, leafs) of plants are taken from the respective parts and washed with distilled water to remove impurities, dust and outer moisture [44]. Some dyes are directly squeezed and dissolved into the solvent like ethanol and distilled water directly and left undisturbed for few hours or about 24 hours or may be heated, depending upon the type of dyes, in the temperature range of about 60700 C for few hours. The color come out from the dye material and then the colorful solution is extracted out by the filtration process. While some dyes are dried in the sunlight or micro-oven for few days and then crushed into fine powders. After drying and crushing into fine powder using a mortar, then immersed in absolute ethanol or distilled water at room temperature for 24 hr in dark 40
environment or is heated in a water bath maintained at the temperature of around 60 to 70 degree centigrade and then filtered out [45,46,44].
2.4 Characterization Technique 2.4.1 UV-VIS Spectrophotometer and optical absorption Spectrophotometry is one of the branches of spectroscopy where we measure the absorption of light by molecules that are in a gas or vapor state or dissolved molecules/ions. Spectrophotometry investigates the absorption of the different substances between the wavelength limits 190 nm and 780 nm (visible spectroscopy is restricted to the wavelength range of electromagnetic radiation detectable by the human eye, that is above ~360 nm; ultraviolet spectroscopy is used for shorter wavelengths). In this wavelength range the absorption of the electromagnetic radiation is caused by the excitation (i.e. transition to a higher energy level) of the bonding and non-bonding electrons of the ions or molecules. A graph of absorbance against wavelength gives the sample’s absorption spectrum. Spectrophotometry is used for both qualitative and quantitative investigations of samples. The wavelength at the maximum of the absorption band will give information about the structure of the molecule or ion and the extent of the absorption is proportional with the amount of the species absorbing the light. UV-Vis spectroscopy is useful to characterize the absorption, transmission, reflectivity of different materials and records the require portion within UV-Vis spectrum for optical analysis such as band gap, transmittance and absorbance [47]. When the light is shinning on a semiconductor sample, if the energy which depends on wavelength of the individual photons is equal or greater than the semiconductor band gap, then the photons can be absorbed, transferring their energy to an electron. This process elevates the electron from the valence band into the conduction band. The absorption process creates an electron-hole pair (EHP), because it results in an electron in the conduction band, and a hole in the valence band. The electron may be an ion-core electron or a free electron in the solid. If the energy of the incoming photon does not match the required excitation energy, no excitation occurs and the material is transparent to such radiation. For thermal excitation it requires specific minimum amount of energy by absorbing either phonon (heat) or photon (light). The requirement of energy for transition of electrons from valence band to conduction band differs for different materials like insulators, semiconductors and metals.
41
Semiconductors are normally transparent in the near infrared and absorbing in the visible spectrum. Thus, the fundamental absorption edge of semiconductors lies approximately, between 0.5 eV (λ ∼ 2500 nm) and 2.5 eV (λ ∼ 500 nm). Within a small energy range around the fundamental absorption edge, semiconductors go, ideally, from high transparency to complete opacity. But the presence of impurities, free conduction electrons or holes, or other defect states may affect the transparency of semiconductor at photon energies smaller than the band-gap. The absorption coefficient is strong function of the wavelength or photon energy in semiconductors. After absorption of photon, transition of electrons can take place according to different process, i) Direct absorption process ii) Indirect absorption process. The absorption coefficient is given by [48]: 𝛾
𝛼 ∝ (ℎ𝜈 − 𝐸𝑔 )
(2.1)
Where; 𝛾 is a constant and its value depends on the allowed transition that is its value is ½ for direct band gap and is 2 for indirect band gap transition. 2.4.2 Sheet resistance and resistivity Materials are generally classified by their electrical resistivity. The range of resistivity of semiconductors from 10-2 to 109 ohm-cm at room temperature, and strongly depend on temperature. At absolute zero, a perfect crystal of most semiconductors will be an insulator having resistivity above 1014 ohm-cm. The properties of as deposited thin films semiconductors can be explained on the bases of electrical resistivity. The sheet resistance for any rectangular shaped thin films, is given by
R
l
(2.2)
db
Where; ρ, l, b, and d are resistivity, length, breadth and thickness of the rectangular shaped thin films which is shown in Fig. 2.3. The sheet resistance is very useful quantity, which is widely used to compare films, particularly those of the same material deposited under similar conditions. For square base, l = b the sheet resistance is given by
42
R RS d
(2.3)
b d
l Fig. 2.3 Parameters of sheet resistance This shows that the resistance RS of one square of film is independent of size of the square and depends only on the resistivity and thickness of the film. The quantity RS is called the ‘sheet resistance’ of the film and is expressed in ohms per square. If the thickness is known then the resistivity can be readily obtained by using the relation
dRS
(2.4)
2.4.2.1 Four Point Probe Method Although two probe method can also be used to measure sheet resistance, four probe method is commonly used because it is more convincing and more accurate method. In the four probe method two probes carry currents and other two probes carry voltages of the spread resistance of the semiconductor. This method gives more reliable resistance than that of Two Probe Method because it avoids contact resistance. For high resolution a square array such as shown in Figure 2.3 is used rather than linear one, known as Square Four Probe Method. To use such a probe, current I is given to adjacent probes and the voltage V generated across the other two is measured. Then the sheet resistance is computed [49] as, 𝑅𝑠 =
𝑉 2𝜋 𝐼 𝑙𝑛 2
𝑉
= 9.06 𝐼
(2.5)
43
For improvement of sheet resistance, the placement of voltage and current may change through number 1, 2, 3and 4 respectively. Finally, the average of these measurements gives more reliable rather than one reading. 2.4.3 Parameters of Solar Cell And I-V Characterization The current-voltage (I-V) characteristics of a solar cell under illumination are used to determine the power conversion efficiency (η). The important parameters of the DSSC are short circuit current (Isc), open circuit voltage (Voc), optimum voltage (Vm), optimum current (Im), fill factor (FF) and efficiency (), that can be carried out from I-V curve of DSSCs under illumination as shown in Fig. 2.4
Fig. 2.4 The Current-Voltage Characteristics of Dye Sensitized Solar Cell [50]. Short Circuit Current Isc: If the output voltage is zero, the cell is said to be short circuited. The short circuit current is equal to the absolute number of photons converted to hole-electron pairs. The Isc depends on the 44
thickness of the electrode, the adsorbed dye molecule, diffusion electrolytes, dipping time, temperature of the cell, and dye loading.
Open Circuit voltage Voc : If the output current is zero, the cell is open circuited and the voltage of the cell is called the open circuit voltage, is depends on wide band gap of semiconductors, redox potential, and the ground state of the dye molecule.
Optimum voltage Vm : Vm is the voltage at the optimum operating point at which the DSSC output power is maximum, is depends on bonds between the dye molecule and oxide semiconductor, and dye loading temperature and time. Optimum Current Im : Im is the current at the optimum operating point which the DSSC output power is maximum, is depends on the intensity of incident light, and connection between material interfaces. Fill Factor (FF) : The fill factor is an important part of the efficiency of the cell. High Voc and Isc are essential in achieving high efficiencies, but paired with a low fill factor, the overall efficiency of the cell will remain low. The ratio of peak output power Vm.Im to Voc .Isc is called the fill factor (FF) of a solar cell. FF =
𝐼m × Vm Isc ×Voc
×100%
Pm
= Isc ×Voc ×100%
(2.6)
45
Where Pm is maximum power. The meaning of fill factor can be understood from its graphical representation. It indicates how much area underneath the I−V characteristic curve is filled by the rectangle described by VmIm in relation to the rectangle VocIsc. The theoretically maximum obtainable FF is a function of the open circuit potential. Efficiency: The energy conversion of a solar cell is defined as the ratio of the output power of the cell and incident irradiance. Maximum efficiency is reached when power delivered to the load is maximum. The power conversion efficiency is given by =
output power
=
FF×JSC×VOC
input power
Pinc
×100%
×100%
(2.7)
Where Pinc is the power density of the light shining on the solar cell and is obtained when the light intensities of the whole spectral range are integrated. I sc is directly proportional to the incident optical power P(light) while Voc increases logarithmically with the incident power. So the overall efficiency of solar cell is expected to increase logarithmically with incident power. The efficie nc y of the solar cell depends on the temperature of the cell, and which is even more important, on the quality of the illumination, i.e., the total light intensity and the spectral distribution of the intens ity. For this reason, a standard measurement condition has been developed to facilitate comparable testing of the solar cells between different laboratories. Specific solar radiation conditions are defined by the Air Mass (AM) value. The Standard Test Condition (STC) for solar cells is the Air Mass 1.5 spectrum, an incident power density of 1000 W/m-2 , and temperature of the cell is 25°C. Because most solar simulators do not provide an ideal AM 1.5 G spectrum, a careful correction needs to be made to account for the spectral mismatch, as is described in detail elsewhere [51]. Because dye-sensitized solar cells have a relatively slow electrical response due to their high interfacial capacity, the voltage scan should be sufficiently slow to avoid errors in the current measurement due to capacitive charging or discharging. Alternatively, the currents from a rapid forward and reverse voltage scan can be averaged. From the I-V curve, the short-circuit current,
46
Isc (or short circuit current density, Jsc), is determined at the V=0 intercept, while the open-circuit potential, Voc, is found at the I =0 intercept. The maximum output power of the solar cell is found where the product |I×V| reaches a maximum (the maximum power point).
47
CHAPTER THREE EXPERIMENTAL METHODS AND CHARACTERIZATIONS 3.1 Introduction This chapter describes about the materials and methods and the characterization employed while assembling the solar cell. In brief, this chapter describes about what materials is used, how much is used and in what technique is used in the stepwise process. In the process, Firstly, the description of deposition of Fluorine doped tin oxide (FTO) on glass substrate, secondly, the deposition of ZnO thin layer on FTO, third, growth of nanostructure on ZnO seed layer, fourth, method of dye extraction and its loading on ZnO nanostrutures, fifth, preparation of electrolyte and counter electrode, and finally, the assembling of solar cell and the i-v characterization is described. 3.2 Glass Substrate Cleaning Method We need the glass substrate having clean and dry surface. The moisture, dust and impurities particles on the surface of the glass substrate affect the thin film deposition by budding, incomp lete adhesion and formation of many impurities. Therefore, it is important to clean substrate properly before deposition of thin film. For the cleaning procedure, first, the glass substrates were put into the water bath with detergent and ultrasonically treated in warm at about 45 0 C for about 30 minutes to remove lotions, waxes, creams or oils from the substrate. Then the substrates were washed with distilled water and acetone once more times. Finally glass substrate washed with distilled water and dried in hot air oven at 1000 C. 3.3 Preparation of Solutions 3.3.1 FTO Solution And Film Deposition The FTO thin films by spray pyrolysis method was prepared by making the precursor solution of dehydrate stannous chloride (SnCl2 .2H2 O) in ethanol and ammonium fluoride (NH4 F) in distilled water [52]. 5gm dehydrate stannous chloride is dissolved on 5ml conc.HCl (36%) and heated at temperature 900 C until completely dissolved. Then the ethanol is poured up to 30ml over all solution. For 15% fluorine doping, 0.1234gm ammonium fluoride dissolved in 30ml doubly distilled water and these two solutions were mixed and stirred for 2 hours at room temperature. 48
After stirred for 2 hours, the solution was filtered with filter paper and kept for 24 hours (aging) before starting spray. The prepared FTO solution was used for thin film deposition using spray pyrolysis method. The cleaned two optical glasses with 75x25x1 mm3 dimensions were used as substrates which are place on the heated iron plate. The substrates were preheated at 450 0 C, for deposition of precursor solution of FTO and the same temperature was maintained by temperature controller while the spray pyrolysis method was carried out. The EPSON LQ-1050 model P18MA printer for the movement of the nozzle and the application T was used to give command through the computer. The heater was made to move horizontally keeping it over the homemade slider. Filtered FTO solution was sprayed by using compressed air as carrier gas. 7ml solution was kept in kit and when the spraying was stopped, 3ml solution remained in the kit. The total deposition time was maintained at 20 minutes for each film, including waiting times. Before removing the substrate from the heater, 5 minutes waiting time was kept in mind to avoid direct excess cooling of the substrate. Finally, these substrates were cleaned with propanol or acetone. Optical transmitta nce measurements of the FTO films were measured using UV-VIS spectrophotometer (USB2000) in the wavelength ranging from 250nm to 1100nm and electrical properties was measured by four probe method.
49
Fig. 3.1 Experimental set up (home made) for spray Pyrolysis unit 3.3.2 ZnO Solution And Seed Layer Deposition The ZnO seed layers were deposited on the FTO glass substrates using the sol-gel spin coating method. Zinc acetate dehydrate [Zn(CH3OO)2 ·2H2 O], ethanol (analytical reagent grade) and diethanolamine (laboratory reagent grade) were used as starting material, solvent and sol stabilizer respectively.
0.6 M solution
was prepared by dissolving
the zinc acetate dihydrate
[Zn(CH3OO)2 ·2H2 O] in mixture of ethanol and di-ethanolamine (DEA) solution at room temperature. The molar ratio of zinc acetate to DEA was 1:1 for all experiments carried out in this study. The resultant solutions were stirred at room temperature for 2 h to yield a clear and homogeneous solution before being aged at room temperature for 24 h [53]. The prepared Zinc acetate precursor solution was dripped on to the FTO coated substrate and rotated at 3,000 rpm for 20 s in order to spread the solution on to the FTO substrate with unifor m thin layer. After deposition by spin-coating, the ZnO seed layers were preheated at 350°C for 10 min to evaporate the solvent and to remove the residual organic materials. The apparatus and 50
experimental set up is shown in the figure below. After this pre-heating, the ZnO seed layers were cooled at a rate of 5 °C/min to prevent from cracking due to change in temperature. The coating and pre-heating procedures were repeated three times and the ZnO seed layers were then postheated at 450°C about 1 hour which is called sintering or annealing.
Fig. 3.2 Experimental set up (home made) for spin coating 3.3.3 Solution For Hydrothermal Zinc nitrate hexahydrate [Zn(NO 3 )2 ].6H2 O, hexamethylene-tetramine (C 6 H12 N4 ) (A.R. grade) and Distilled water were used as starting materials and solvent respectively and molar ratio between zinc nitrate hexahydrate and hexamethylene-tetramine be 1:1. 0.1M zinc nitrate hexahydrate (1.487gm) dissolved in 50 ml distilled water and magnetic stirrer about 30 minutes and also 0.1M Hexa-methine tera-amine(HMTA) (0.70095 gm) dissolved in 50 ml distilled water and magnetic stirrer about 30 minutes. These two solutions were mixed and also stirred in magnetic stirrer about 30 minutes then filtered out using filter paper [54]. For the growth of ZnO nanostructures, hydrothermal method was carried out. The two ZnO coated substrates were kept inside the air tight glass bottle in the slanted position and then the filtered solution was poured gently. The slides were kept in such a way that the ZnO seed layer lies lower part. This pot was kept in the hot water at temperature 75 0 C for three and half hour. After three and half hours, the slides were kept out from solution and rinsed with distilled water. Then slides were dried at room temperature and finally annealed at temperature 350 0 C for 30 minutes. The experimental set up is given in the Fig. 3.3. 51
Fig. 3.3 Experimental set up (home made) for the hydrothermal growth From the SEM image of the previous work carried out in the same process as above, in the Laboratory of Patan Multiple Campus and Amrit Science Campus, we assume that the nanostructure is formed. In the work carried out in each laboratory, the nanosheet was formed [55] 3.3.4 Dye Solution Extraction And Dye Loading The natural dyes, nut of the avocado fruit (avocado fruit), skin of nut of avocado fruit (inner covering of avocado fruit), outer skin of the nut of the avocado fruit (outer covering of avocado fruit), Lawsonia Inermis (mehendi) and red leaves of guava (leaves of guava were taken and washed with distilled water several times. They were then kept in micro-oven for a week. In average 3 hours per day, the temperature of the oven was maintained at 600 C. The dried dyes were crushed into power by the pickle grinding machine. The powder dyes were then dissolved in the distilled water (1gm per 25 ml) and left undisturbed for about 24 hours and then heated at 70 0 C for about 15minutes. After cooling down, the solutions were filtered. The same above mentioned dyes were extracted in ethanol kept at room temperature for 24 hours. After cooling down, the solutio ns were filtered. The hydrothermally grown nanostructured ZnO samples were kept inside the air tight glass bottle containing filtered dye solution and kept over water bath maintained at 70 0 C for 8 hours and then left undisturbed until it reaches 24 hour from the initial time at room temperature. Some other 52
slides of ZnO nanostructure grown samples were loaded with dye solution at room temperature for 24 hours. Then the samples were removed and washed with distilled water gently and kept for drying in the room temperature. 3.3.5 Electrolyte Solution The liquid electrolyte solution which composed of 0.5M potassium iodide (KI) and 0.05M Iodine (I2 ) in acetonitrile solvent was prepared [56]. The potassium iodide and iodine was dissolved in acetonitrile and stirred on magnetic stirrer for about 15 minutes and filtered out. The electrolyte undertakes the responsibility of dye regeneration and charge transport between the working electrodes and counter electrodes. 3.3.6 Platinum Chloride Solution for Counter Electrode The solution for the counter electrode was prepared by taking chloroplatinic acid hexahydrate (H2 PtCl6 . 6H2 O) with 2-propanol as solvent. 5mM solution was prepared (0.008648 gm H2 PtCl6 . 6H2 O in 25ml 2-propanol), stirred on the magnetic stirrer for about 10 minutes and then filtered. The counter electrode was prepared by spreading a drop of 5mM chloroplatinic acid hexahydrate (H2 PtCl6 . 6H2 O) with isopropyl alcohol on separate FTO glass substrate and heated it at about 450°C for 15 min [57]. The main function of counter electrode is to reduce the oxidized form of the redox couple consisting of iodide & triiodide in the electrolyte. 3.4 Assembling of Solar Cell The prepared electrolytic solution was sandwiched between photoanode (working electrode) and counter electrode through capillary action, pressing firmly. A thin layer of parafilm was used as a spacer to avoid short-circuiting between two electrodes. Finally the superglue was used to make sealing air tight and more convenient. A binder clip was fixed externally to maintain the mechanical grip of the cell without any further sealing, which finalized the assembly of the DSSC. The assembled solar cell is shown in the Fig. 3.4.
53
Fig. 3.4 Sample of assembled natural dye sensitized solar cell
3.5 Characterization The FTO thin film, ZnO seed layer , hydrothermally grown ZnO nanostructure, natural dye and dye adsorbed photoanode were characterized by optical (absorbance) and electrical (sheet resistance) study. Finally, the I-V characteristics of the fabricated solar cell using natural dyes were observed. The experimental measurement in the present study are characterized as follows. I.
Optical measurement
II.
Electrical measurement
III.
I-V characterization
3.5.1 Optical Measurement The absorbance of the deposited FTO thin films, ZnO seed layer, hydrothermally grown ZnO nanostructure and absorbance of natural dye and natural dye adsorbed electrode was determined by the UV visible spectrophotometer (USB 2000, Photonics), the spectrophotometer used in the present study as shown in Fig. 3.5. A spectrophotometer is employed to measure the amount of light that a sample absorbs. The instrument operates by passing a beam of light through a sample and measuring the intensity of light reaching a detector. To determine the optical transmittance or mainly the absorbance, the integration time (msec), average, boxcar, strobe frequency (msec) and visible intensity of the UV-VIS spectrophotometer was set to 4, 3, 3, 100, and 0 units respectively. The absorbance is measured in the wavelength of ranges of 250-1100nm.
54
Fig. 3.5 UV-visible spectrophotometer (USB2000, Photonics) 3.5.2 Electrical Measurement The electrical measurements of the deposited FTO thin film and ZnO seed layer were carried out by measuring the sheet resistance. For this we used four probe techniques. The experimental setup for the measurements of sheet resistance is shown in the Fig. 3.6. To measure sheet resistance, the four flexible brass sticks cut into equal lengths were mounted on a small square wooden board with their golden plated (to avoid contact defects and barriers) pointed tips as placed in the four corners of a square. The four sticks were made adjustable so as to hold any size of square samples. A simple power-supply of 24V was constructed in laboratory and voltage and current were measured with a digital voltmeter (FLUKE28II) and a general ammeter respectively.
55
Ammeter (200mA) Circuit Board
digital Voltmeter (FLUKE28II) (manual range 600mV)
Thin film coated sample
Fig. 3.6 Experimental set up (home made) for the measurements of sheet resistance 3.5.3 I-V Characterization For the I-V characterization of the assembled dye-sensitized solar cell, the current and voltage were measured by gradually increasing the resistance from zero (at which short circuit current I sc is noted down) to maximum (at which the open circuit voltage Voc is noted down) using the general ammeter and a digital voltmeter (FLUKE28II). The resistance box having five resistor each of 100kΩ was used as a variable resistance. The amprobe (100-Sunlight) was used to measure the power density of the halogen lamp or sunlight. The current and the voltage of the natural dye sensitized
solar cell was noted varying the resistance. The experimental setup for the
measurements of sheet resistance is shown in the Fig. 3.7.
56
Ammeter (200µA) Resistance Box Amprobe (100-Sunlight)
digital Voltmeter (FLUKE28II) (manual range 600mV)
Fig. 3.7 Experimental set up for the I-V characterization of DSSC
57
CHAPTER FOUR RESULT AND DISCUSSION
In this chapter, the results of our present thesis work, structural properties of ZnO nanostructure (hydrothermal method), electrical and optical properties of the FTO (Spray Pyrolysis Method), ZnO thin film or seed layer (Spin Coating Method), optical properties of ZnO nanostructure (Hydrothermal Method) before dye loading, optical properties of the natural dye and then the effect of the different natural dye on the optical properties of the ZnO nanostructure grown hydrothermally after loading the dye on them and finally, the I-V characteristics of the natural dye-sensitized solar cells have been discussed briefly. We have extracted different natural dye from the nut of avocado fruit (avocado fruit, skin of the nut of avocado fruit (inner covering of avocado fruit), outer skin of the nut of avocado fruit (outer covering of avocado fruit), Lawsonia Inermis(mehendi) and red leaves of guava(leaves of guava) in distilled water and ethanol. We have studied their absorption spectra and the effects on the absorption spectra after loading them individually on the assumed nanostructured grown ZnO. 4.1 Reproducibility of Samples In order to test the reproducibility of the samples prepared, we have summarized the electrical properties (sheet resistance) and optical properties (absorbance spectra) of the differe nt components. 4.1.1 Sheet Resistance of The ZnO Seed Layer The electrical properties of the 3-coat ZnO seed layer coated on FTO coated glass substrate, and FTO on glass substrate that were used for making counter electrode is carried out by measuring the sheet resistance. The sheet resistance of the ZnO seed layer was calculated by using the four point probe method using the equation (2.5).
58
The sheet resistance of the each of the 3-coat ZnO seed layer coated sample is summarized in the table 4.1. Table 4.1: The measurements of the sheet resistances of the 3-coat ZnO seed layer coated on FTO thin film and corresponding types of natural dyes loaded
Serial Sample Name
Sheet
Sheet
Resistance
Resistance
No.
1
Average
used (R1+R2)/2
3z12
(R1 ) Ω/□
(R2 ) Ω/□
26.37
38.60
Name of the dye Dye-loading on it after
hydrothermal
Ω/□
method
32.48
Avocado fruit
Temperature
Room temperature (RT)
2
3z13
49.26
51.42
50.34
Lawsonia
Inermis Room
(mehendi)
temperature (RT)
3
3z15
32.03
31.53
31.78
Lawsonia
Inermis 700 C
(mehendi)
4
3z16
42.74
34.68
38.71
Inner covering of 700 C avocado fruit
5
3z17
36.78
47.26
42.02
Outer covering of RT avocado fruit
6
3z18
34.91
37.44
36.18
59
Leaves of guava
RT
7
3z19
37.10
46.40
41.75
Inner covering of RT avocado fruit
8
3z20
41.68
39.40
40.54
Outer covering of 700 C avocado fruit
9
3z21
35.36
31.77
33.56
Leaves of guava
700 C
10
3z23
86.48
79.56
83.02
Avocado fruit
700 C
Here, among the eight different sheet resistance of same sample in four probe point method, we first calculated sheet resistances from the four current and voltage probe point and then again from the four other current voltage probe point and finally, the average was taken. From the table, we see that except sample 3z13 (sheet resistance 51.42) and 3z23 (sheet resistance 83.02), the sheet resistance of all samples are nearly same with slight fluctuations. Since the sheet resistance are highly sensitive to even small amount of impurity and defects, and the thickness of the seed layer, the observed sheet resistance show that except these two samples all other samples can be considered to be reproducibility to some extent. Generally, the low sheet resistance ZnO seed layer is suitable for the conduction of electrons but many other parameters involved made us difficult to correlate the sheet resistance with other parameters involved in the fabrication of the solar cell. 4.1.2 Sheet Resistance of the FTO Glass Substrates Used for Making CE The sheet resistance of each of the FTO glass substrate used for making a counter electrode is listed in the Table 4.2. Table 4.2 The measurements of the sheet resistances of the FTO glass substrate that is used for making counter electrode (CE)
60
Serial
Sample
Sheet
Sheet
Average
No.
Name
Resistance
Resistance
(R1+R2)/2
Photoanode (Sample
name)
used
(R1) Ω/□
(R2 ) Ω/□
Ω/□
while assembling DSSC
1
F7
27.31
27.43
27.37
3z20
2
F8
24.67
24.73
24.7
3z18
3
F24
16.95
16.17
16.56
3z15
4
F27
24.45
24.41
24.43
3z16
5
F36
11.68
11.66
11.67
3z23
6
F43
9.06
9.02
9.04
3z21
An ideal CE should have low electrical resistance and high electrocatalytic activity towards the iodide/triiodide (I-/I3 - ) redox reaction and stable. Since the sheet resistance of each of the samples used is low, these FTO coated glass substrates are suitable to use as counter electrode.
61
4.1.3 UV-VIS Absorption Spectroscopy Analysis The UV-VIS spectral studies of the 3-coat ZnO seed layer coated on FTO glass substrate and ZnO Nanostructure before and after dye loading were carried out by using UV-VIS spectrophotometer in the wavelength range of nearly 350- nearly 1030 nm. To determine the optical absorbance, the integration time (msec), average, boxcar, strobe frequency (msec) and visible intensity of the UV visible spectrometer was set to 4, 3, 3, 100, and 0 units respectively. 4.1.3.1 Band Gap of the Seed Layer To confirm the seed layer to be ZnO material, the band gap of the seed layer on FTO was determined from the plot given in Fig. 4.1. From the figure we see that the straight line portion of the curve intersected x-axis at 3.16 eV which is close to the band gap of ZnO film.
Fig. 4.1 Graph of (αhv)2 versus hv of 3z23 sample with seed layer growth on FTO glass substrate
62
4.1.3.2 Absorbance Spectra of all ZnO Seed Layer (combined form) Used for Making Working Electrode The absorbance spectra of the 3-coated ZnO seed layer coated on FTO glass substrate is shown in the Fig. 4.2.
Fig. 4.2 Absorbance Spectra of all the ZnO Seed Layer (in combined form) used to make Working Electrode Here, C 3z12 to L 3z23 are the sample names of all diffferent ZnO seed layer samples. From the above absorbance spectra, we see that all the 3-coated ZnO seed layer have nearly the same absorbance spectrum. That’s why these samples are suitable for the further processing to fabricate the solar cell. 4.1.3.3 Absorbance Spectra of Hydrothermally Grown ZnO nanostructure Before and After Dye Loading The absorbance spectrum of the hydrothermally grown ZnO nanostructure before and after dye-loading taken at different positions of the same sample are shown in Fig. 4.3 to Fig. 4.8. 63
(A)
(B)
Fig. 4.3 Absorbance spectra of 3z15 ZnO nanostructure (Sample name 3z15) at differe nt positions before dye loading (A) and after mehendi dye loading (B) at 70 0 C
(A)
(B)
Fig. 4.4 Absorbance spectra of ZnO nanostructure (Sample name 3z16) at differe nt positions before dye loading (A) and after inner covering of avocado dye loading (B) at 700 C
64
(A)
(B)
Fig. 4.5 Absorbance spectra of ZnO nanostructure (Sample name 3z18) at differe nt positions before dye loading (A) and after guava dye loading (B) at RT
(A) Fig. 4.6 Absorbance spectra of ZnO
(B) nanostructure (Sample name 3z20) at differe nt
positions before dye loading (A) and after outer covering of avocado dye loading (B) at 700 C
65
(A) Fig. 4.7 Absorbance spectra of ZnO
(B) nanostructure (Sample name 3z21) at differe nt
positions before dye loading (A) and after leaves of guava dye loading (B) at 70 0 C
(A) Fig. 4.8 Absorbance spectra of ZnO
(B) nanostructure (Sample name 3z23) at differe nt
positions before dye loading (A) and after avocado fruit dye loading (B) at 70 0 C In order to test the uniformness of hydrothermally grown ZnO nanostructurewe have studied the absorbance spectra of the same sample at different positions(C,D,E,F,G, H positions) within the same sample. Almost all the figure depicts that even the aborbance spectra of the same sample 66
either before loading the dye or after loading the natural dye are not uniform. This implies that the nanostructure are not uniform. Either there is problem to grow uniform nanostructure in our laboratory or we are not able to produce exact uniform sample. For the comparative study of (the effects of the natural dye loading) of the DSSCs, we have taken the average of the absorbance for each individual sample which is discussed in the next section. 4.2 Absorbance of Natural Dyes The dye used as a photo-sensitizer plays an important role in the operation of DSSCs. The efficiency of the cell is critically dependent on the absorption spectrum of the dye and the anchorage of the dye to the surface of the semiconductor. We have extracted different natural dyes from the nut of the avocado fruit (avocado fruit), skin of nut of avocado fruit(inner covering of avocado fruit), outer skin of the nut of the avocado fruit(outer covering of avocado fruit), Lawsonia Inermis (mehendi) and red leaves of guava in distilled water (DW). Fig 4.7 depicts the absorption spectra of these natural dyes extracted from different plants and parts of the avocado fruit. It can be seen that, the absorption of the dye solutions obtained from the leaves of guava increases sharply at near 675nm towards the lower wavelength range and the maximum absorption is observed near 450 nm.. For the Mehendi dye solution, the narrow absorption peak have been obtained near 675 nm and at around 650 nm the absorption intensity goes increasing sharply towards lower wavelength region with maximum intensity at near 450 nm. The absorbance peak for leaves of guava and mehendi dye extract solution is in 450nm to 675 nm wavelength range. Although the maximum intensity of the absorption spectrum (near 440 nm) is lower than those of that of the guava leaves and outer skin of nut of avocado fruit, the absorption spectra of the nut of the avocado fruit shows higher absorption intensity range which increases less sharply then others from around 525 nm. Similarly, for the outer skin of nut of the avocado fruit and skin of the nut of the avocado fruit, the absorption intensity goes on increasing at around 650 nm towards the lower wavelength range and becomes intense at 412 nm and 405 nm respectively. The strange narrow peak is observed around 675nm for the Lawsoina Inermis (mehendi dye).
67
Fig. 4.9 (a) Combined absorption spectra of different natural dyes extracted in distilled water
Fig. 4.9 (b) Combined absorption spectra of different natural dyes extracted in Ethanol 68
The different absorbance spectrum or the difference in the intensity or the expansion over the wavelength is due to the different pigments like anthocyanin, merocyanin, carotenes, coumarin, bixin, betalains, betanin, etc.contained in the natural dyes [58]. In the present situation, we are unable to specify the exact molecule responsible for the absorption of light. From the Fig. 4.9 (a) we can see that the natural dyes extracted from red leaves of guava, skin of the nut of avocado fruit and nut of the avocado fruit have broad spectrum over the wavelength and have high intensity of the absorbance. Initially, we worked in the natural dyes extracted in DW and then extract the same dyes in the ethanol. Although the absorbance spectrum for the leaves of guava and lawsonia inermis extracted in the ethanol shows high absorbance and very good broadening and many peaks over the wavelength, overload in work and the time duration compelled us to keep it as the future work. We can see that the absorbance spectrum of the extracted dye solution extracted from the same material shows different absorbance peak in distilled water and ethanol. This may be due to the solubility of the pigments of the dye material corresponding to the solvent used. Those material dissolves well in the solvent have good absorption spectrum. 4.3 Absorption effect on The Working Electrode (ZnO Semiconductor Photoanode) The absorbance gives an idea of the amount of dye adsorbed in the film or semiconductor photoanode of the DSSCs. The absorbance of the ZnO seed layer, hydrothermally grown ZnO nanostructures and then dye loaded to the hydrothermally grown ZnO nanostructure was taken to find out the change in the absorbance spectrum and its effect on the I-V characteristics or performance of the DSSCs. Fig. 4.10 (a,b) to 4.14 (a,b) shows the absorbance of the ZnO seed layer, hydrothermally grown ZnO nanostructure and then after loading dye on it along with the absorbance of the dye extract solution. Each of the figure below shows the absorbance at differe nt wavelength for four different cases. Here, these different curves for different cases (C,D,E,F) are shown to make comparative study to see how the absorbance changes with hydrothermal growth and dye loading with respect to each four cases. The dye was loaded on the ZnO nanostructure at about 700 C and at room temperature separately on separate sample. We see that, somewhere, there is increase in the absorption spectra in intensity or broadening over the wavelength after loading the natural dye by giving the loading temperature of about 70 0 C in the sample but somewhere the 69
room temperature is seen suitable for loading the dye. This may be because of the peak of absorption spectra are in tunes with the HOMO-LUMO energy difference of the different dyes or anchoring property of the dye on the ZnO nanostructure.
(a) 700 C
(b) Room Temperature
Fig. 4.10 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after loading (a)at 70 0 C (a) and (b) at room temperature with avocado fruit dye in the Sample 3z23 and that of dye extracted in DW
70
(a) 700 C
(b) Room Temperature
Fig. 4.11 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after loading (a) at 70 0 C and (b) at room temperature with inner covering of avocado fruit dye in the Sample 3z16 and that of dye extracted in DW 71
(a) 700 C
(b) Room Temperature
Fig. 4.12 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after loading (a) at 70 0 C and (b) at room temperature with leaves of guava dye in the Sample 3z18 and that of dye extracted in DW 72
(a) 700 C
(b) Room Temperature
Fig. 4.13 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after loading (a) at 70 0 C and (b) at room temperature with Lawsonia Inermis (mehendi) dye in the Sample 3z13 and that of dye extracted in DW 73
(a) 700 C
(b) Room Temperature
Fig. 4.14 Absorption spectra of the ZnO seed layer coated on FTO thinfilm, ZnO nanostructure before and after at (a) 70 0 C and (b) at room temperature with avocado fruit dye loading in the Sample 3z17 and that of dye extracted in DW 74
The Fig. 4.10 (a) shows the maximum absorption peak before dye loading (ZnO nanostructure) at 376 nm with absorbance 0.51 unit and after loading the dye of avocado fruit at temperature of about 700 C, we see that the absorbance peak is at 394 nm with absorbance 0.71unit. Similarly, the Fig. 4.12 (b) shows the maximum absorption peak before dye loading (ZnO nanostructure) at 399 nm with absorbance 1.47 unit and after loading the dye of leaves of guava at room temperature, we see that the absorbance peak is at 443 nm with absorbance 2.33unit. In both the figure, the absorbance spectrum of the dye loaded hydrothermally grown ZnO nanostructure shifts relative ly higher in comparison to the dye unloaded ZnO nanostructure. The absorbance spectrum is observed to be broadened after dye loading, with respect to the dye extract solution and hydrothermally grown ZnO nanostructure. The Fig. 4.10 (b) shows the maximum absorption peak before dye loading (ZnO nanostructure) at 406 nm with absorbance 1.71unit and after loading the dye of avocado fruit at room temperature, we see that the absorbance peak is at 409 nm with absorbance 1.77 unit. Similarly, the Fig. 4.14 (a) shows the maximum absorption peak before dye loading at 399 with absorbance unit 1.04 and after loading with the dye of outer covering of avocado fruit at 70 0 C, we see that the absorbance peak is at 394 nm with absorbance 0.83 unit. In both the figure, there is neither clear shifting nor broadening of the absorbance spectrum with respect to the absorbance before dye loading. There is no obvious broadening in comparison to the dye extract as well, in both the cases. In the Fig. 4.11 (b), Fig. 4.12 (a), 4.13 (a) and (b), and 4.14 (b), the maximum absorbance peak after dye loading is decreased and there is no broadening of the absorbance spectrum after loading the dye. In the Fig. 4.11 (b) and Fig. 4.14 (b), the absorbance spectrum shifts higher from about 475 nm and 525 respectively, towards the higher wavelength in comparison to the respective dye extract solution. The Fig. 4.11 (a) shows the maximum absorption peak before dye loading (ZnO nanostructure) at 411 nm with absorbance 2.5unit and after loading the dye of inner covering of avocado fruit at temperature of about 700 C, we see that the absorbance peak is at 411 nm with absorbance 11.44 unit. The absorbance spectrum shifts lower after loading with the dye. We see that there is increase in intensity and expansion or broadening in the absorbance over the wavelength range in comparison to the absorbance of dye extract solution. 75
Almost all the figure above shows that there is increase in the intensity of absorbance with sharp slopes when hydrothermal process is carried out to grow nanostructure on the ZnO seed layer. This is because the surface area (surface to volume ratio) of the naostructure grown is more than the seed layer and absorbs more light than the ZnO seed layer. Similar result were observed by Jiang [59]. Also the figure depicts that the adsorbed dyes on the surface of the hydrothermally grown ZnO nanostructure absorb light on the visible light region in the range of 400nm to around 700nm wavelength. The absorption spectrum decreases sharply with increase in wavelength from around 400 to 700 nm and after that there is slow decrease with increasing wavelength. Like as, there is little or no absorption within the region of infrared. The similar result was shown by Okoli, Laeticia Udodiri [60]. In the Fig. 4.10 (a), Fig. 4.11 (b), Fig. 4.12 (b) and 4.14 (b), we see that the spectrum of all the dye-adsorbed ZnO nanostructure are broadened and less intense compared to the corresponding extracted dye solutions. This broadening is due to the adsorption or binding of the corresponding dye molecule to the semiconductor ZnO nanostructure surface. The lowering in the intensity may be due to the removal of adsorbed ZnO or other impurity during 24 hour soaking of the hydrothermally grown ZnO film on dye solution or due to the only monolayer adsorption of the extracted dye solution. Similar results were obtained by Kim [13]. Although there is broadening of the absorbance, there is increase in the absorbance compared to the dye extract in the Fig. 4.10 (a) that may be due to already high absorbance of the ZnO nanostructure compared to the absorbance of dye extract. The Fig. 4.10 (a) and Fig. 4.12 (b) shows that the intensity of the light absorption increases and absorbance broadens with the adsorption of dye extract from the avocado fruit dye (nut of avocado fruit) and leaves of guava dye on ZnO nanostructure respectively similar to the work of Senthil [61]. It is not necessary that the dye extract having good absorption over the wavelength or having high intensity is suitable for the better performance of the dye sensitized solar cell because the efficie nc y or the performance will be high if the dye is well anchoraged or adsorbed by the photoanode semiconductor materials. The natural dye which may be suitable for the ZnO semiconductor may
76
not be suitable for the some other semiconductor because of the mismatch of the band gap energy of the dye and the semiconductor. The natural dye mainly has -OH and -O ligands and lacks -COOH ligands, whereas the -COOH ligands of the Ru dye will combine with the hydroxyl group of the smiconductor particles to produce an ester and allow easy electron transfer to the conduction band of semiconductor to acquire a rapid electron-transport rate. The hindrance by the dye molecules in the bond formatio n on the oxide semiconductor may also be the reason that causes in the less transportation of the electron from the dye to the conduction band of the semiconductor [13]. 4.4 I-V Characteristics of Dye-Sensitized Solar Cells The I-V curves are used to calculate that short circuit photocurrent density (Jsc), the open circuit voltage (Voc), the fill factor (ff), and the conversion efficiency (η) of DSSC's. The experimenta l set up for measuring the performance of DSSCs using natural dyes was built using a halogen lamp and sunlight as light source. The distance and direction (angle) from the halogen lamp (HL) source and the DSSC sample was maintained in such a way that the light intensity falling on the solar cell sample was 100W/m2 and 1000W/m2 . It was really difficult to get constant power density or light intensity from the sunlight each time or each day, so the I-V measurement under the sunlight was done with the different power density. To measure the power input intensity the Amprobe 100Sunlight was used. The area of the solar cell is calculated by considering 1 cm breadth and 1.5 cm length of the cell approximately. Fig. 4.15 (a,b,c) depict the I-V characteristics for DSSC prepared using the avocado fruit at two different intensity of Halogen lamp, 100W/m2 and1000W/m2 and 1000W/m2 in the Sunlight. In this Figure we see that with increase in power density of Halogen lamp I sc increases but Voc decreases. This is possibly due to heating effect on DSSC due to high intensity of halogen lamp. We have noticed that for high intensity light when we move DSSC close towards the light source, DSSC was heated up resulting in the evaporation of hole transport liquid electrolyte used in the DSSC. Similar is the case for Fig. 4.16 (b) and (c), where, Isc increases with the increase in the power density of halogen lamp but the open circuit voltage is decreased. The long duration for noting the current and voltage and the sealing problem may be the other cause.
77
Comparing the observed Isc value and Voc value of DSSCs exposed at 1000W/m2 in the sunlight, we see from the Fig. 4.15 (c) that the maximum Isc 123µA and maximum Voc 315mV was observed. The natural dye used in this DSSC was extracted from the avocado fruit (nut of the avocado fruit). Fig. 4.16 to Fig. 4.20 depicts the I-V characteristics of DSSC based on different natural dyes under the different power intensity illuminated in halogen lamp and sunlight. The observed values of I sc and Voc for all curves is tabulated in table 4.3. The DSSC prepared with the skin of the nut of avocado fruit (inner covering of fruit) also seems to be better because even at the illumination power density of 600W/m2 in sunlight, Isc and Voc value of 79 µA and 290 mV respectively were observed, which is considerably high (see column with sample name 3z16 in the table 4.3) Also, comparing the Isc and Voc value of DSSCs at power density of 100W/m2 in halogen lamp, we see that the DSSC with dye from nut of the avocado fruit and DSSC with dye from skin of the nut of avocado fruit shows Isc of 5.4 µA and 4.9µA and voc of 210mV and 160mV respectively, which are also considerably better compared to the other DSSC with other dyes. The different I-V curves of the solar cell sensitized with different natural dyes at different light intensity and light source conditions are shown in the Fig. shown in the next page.
78
Fig. 4.15 (a) I-V characteristics of Sample 3z23 (power density 100W/m2 in HL)
Fig. 4.15 (b) I-V characteristics of Sample 3z23 (power density 1000W/m2 in HL)
Fig. 4.15 (c) I-V characteristics of Sample 3z23 (power density 1000W/m2 in Sunlight) 79
Fig. 4.16 (a) I-V characteristics of Sample 3z16 (power density100W/m2 in HL)
Fig. 4.16 (b) I-V characteristics of Sample 3z16 (power density 1000W/m2 in HL)
Fig. 4.16 (c) I-V characteristics of Sample 3z16 (power density 600W/m2 in Sunlight)
80
Fig. 4.17 I-V characteristics of Sample 3z15 (power density 1050 W/m2 in Sunlight)
Fig. 4.18 I-V characteristics of Sample 3z18 (power density 1000W/m2 in Sunlight)
81
Fig. 4.19 I-V characteristics of Sample 3z20 (power density 1050W/m2 in Sunlight)
Fig. 4.20 I-V characteristics of Sample 3z21 (power density 240 W/m2 in Sunlight) 82
Zhou and his co-workers in 2011 using Twenty natural dyes, extracted from natural materials such as flowers, leaves, fruits, traditional Chinese medicines, and beverages as sensitizer for the DSSCs showed that the open circuit voltages varied from 0.337 to 0.689 V, and the short circuit current photocurrent densities (Jsc) ranged from 140 to 269 µA/cm2 [62] . Researcher group of Nwanya, Ugwuoke and their co-workers in 2012, assembled the DSSC by using natural dyes extracted from jathropha curcas and citrus aurantium leaves as sensitizers showed the overall efficiency of 1.26% with open circuit voltage of 350mv, short circuit current of 65.5 µA [11]. El-Agez in collaboration with other researchers fabricated dye sensitized solar cell using natural dye Walnuts, Rhubarbs and pomegranate and found short circuit I sc of 73.3, 82.65, 63.2 in microampere (µA) and open circuit voltage Voc of 0.304, 0.238 and 0.159 volt (V) [26]. Gharbi and his co-workers in 2015, using beet, red cabbage, Spinach, Strawberry Mallow, and Henna as the sensitizer for DSSC showed the open circuit voltage of 370, 461, 460, 465, 600.5 and 470 in mV and photocurrent densities of 0.76, 0.97, 0.55, 1.33, 0.69 in mA/cm2 and 0.66 respectively [15]. We have calculated the efficiency by using the formula given in equation (2.7). Since the power input was in W/m2 , the Jsc was calculated by taking the area of the cell 1cm in breadth and 1.5 cm in length approximately. The efficiency of the fabricated dye sensitized solar cell is low compared to the solar cell fabricated by many other researchers. For example Nwanya, Ugwuoke and their co-workers [11].
83
Table 4.3 The measurements of Short circuit current Is c , Open circuit voltage voc and efficie nc y η for different natural dye sensitized solar cell in different light source conditions.
Sample
3z15
3z20
3z18
3z23
3z16
3z21
3z23
3z16
3z23
3z16
1050
1050
1000
1000
600
240
1000
1000
100
100
Light
Sunlig
Sunli
Sunli
Sunli
Sunli
Sunli
Halog
Halog
Halog
Sunli
Source
ht
ght
ght
ght
ght
ght
en
en
en
ght
lamp
lamp
lamp
name
Powerde nsity (W/m2 )
Isc (µA)
80
5.5
40
123
79
3.5
16
12.2
5.4
4.9
voc (mV)
245
198
288
315
290
163
148
133
210
160
Efficienc
0.0049
0.000
0.002
0.011
0.008
0.000
0.000
0.000
0.001
0.001
y (η) ٪
7
19
24
47
92
38
65
04
84
27
Natural
Lawso
Outer
Leav
Avoc
Inner
Leav
Avoc
Inner
Avoc
Inner
dye used
nia
cover
es of ado
cover
es of ado
cover
ado
cover
inermi
ing of guava
s
avoca
(mehe
do
fruit
ing of guava
ing of fruit
ing of
avoca
avoca
avoca
do
do
do
ndi)
84
fruit
CHAPTER FIVE CONCLUSIONS 5.1 Conclusion The electrical and optical properties of the components of the fabricated natural Dye-Sensitized solar cells have been studied for the reproducibility of the components. Nearly same sheet resistance of each of the ZnO seed layer and each of the FTO glass substrate confirmed that we can reproduce same kinds of materials in the laboratory for assembling solar cell. The absorbance spectrum before and after dye loading are not much uniform confirming difficulties to reproduce them.The band gap of the seed layer confirms that the photoanode semiconductor was ZnO. Natural dyes extracted from the nut of avocado fruit, skin of the nut of avocado fruit, outer skin of the nut of avocado fruit, Lawsonia Inermis (mehendi) and leaves of guava have been used as sensitizers in dye sensitized solar cells. We have extracted dyes from locally available avocado fruit and its inner and outer covering and the leaves of guava, which to our knowledge have been used for the first time. The I-V measurement made on our DSSCs shows that the solar cells using avocado fruit and inner covering of avocado fruit have interesting values of short circuit current of 123 µA and 79 µA and open circuit voltage of 315 mV and 290 mV respectively. Looking at the I-V curves of the solar cell sensitized with the different parts of avocado fruit, mehendi and leaves of guava, we conclude that the high efficiency of solar cell can be achieved from the avocado fruit dye as compared to other dye sensitized solar. 5.2 Future Work
Since the main target is to fabricate a solar cell of low cost and of very new material we can use Choerospondias axillaris (Lapsi), only found in Nepal, to prepare activated carbon for making counter electrode instead of using expensive chloroplatinic acid hexahydrate, which of course will be a very new material used for the counter electrode.
85
We can study the absorbance and I-V characteristics of the assembled solar cell from same above mentioned natural dyes extracted in the ethanol or many other natural dyes extracted in distilled water or in ethanol. We can also calculate the efficiency of the solar cell by changing the different parameters and materials. For example, the concentration of solutions used, the number of coatings for making seed layer, the time variation and the chemicals used for the hydrothermal growth, the time variation and the temperature variation while loading the dye, use of solid state electrolyte would be more beneficial, etc. can be used to study performance of DSSC.
86
References 1.
W. Shockley, H. J. Queisser, J. Appl. Phys., Vol.32, p.510 ( 1961).
2.
B. O'Regan, M. Gratzel, Nature, Vol.353, p. 737 (1991).
3.
M. Gratzel, Inorg. Chem., Vol. 44, p. 6841 (2005)
4.
M. Gratzel, Nature, Vol. 414, p. 338 (2001).
5.
A. Yella, H.W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C.Y.Yeh, S. M. Zakeeruddin, M. Gratzel, Science, Vol. 334, p. 1203 (2011).
6.
B. O’regan and M. Gratzel, Nature, Vol. 353, p. 737-740 (1991)
7.
G. Rothenberger, P. Comte, M. Gratzel, Solar Energy Materials & Solar Cells, Vol. 58, p. 321-336 (1999)
8.
K. Wongcharee, V. Meeyoo, S. Chavadej, Solar Energy materials and Solar Cells, Vol. 91, p. 566-571 (2007)
9.
H. Changa, H.M. Wub, T.L. Chenc, K.D. Huangd, C.S. Jwoe, Y.J. Loa, Journal of Alloys and Compounds, Vol. 495, p. 606–610 (2010).
10.
J. Han, F. Fan, C. Xu, S. Lin, M. Wei, X. Duan and Z. L. Wang, Nanotechnology, Vol. 21, p.7 (2010)
11.
A.C. Nwanya, P.E. Ugwuoke, P. M. Ejikeme, O.U Oparaku and F.I. Ezema, Int. J. Electrochem. Sci., Vol.7, p. 11219-11235 (2012)
12.
D. A. Kumar, F. P. Xavier and J. M. Shyla, Scholars Research Library Archives of Applied Science Research, Vol. 4 (5), p. 2122-2132 (2012)
13.
H. J Kim, Y. T. Bin, S.N. Karthick, K.V. Hemalatha, C. J. Raj, S.Venkatesan, S. Park, G. Vijayakumar, Int. J. Electrochem. Sci, Vol. 8, p. 6734-6743 (2013)
14.
Y. Li, S. H. Ku1, S. M. Chen, M. A. Ali, F. M. A. AlHemaid, Int. J. Electrochem. Sci., Vol. 8, p. 1237-1245 (2013)
15.
A. Torchani, S. Saadaoui, R. gharbi , M. Fathallah, Current Applied Physics, Vol. 15, p. 307-312 (2015)
16.
J. Halme, Master’s thesis, Helsinki University of Technology, (2002)
17.
S. A. Haque, E. Palomares, B. M. Cho, Journal of the American Chemical Society, Vol.127, p. 3456-3462 (2005).
87
18.
N. Memarian, I. Concina, A. Braga , S.M.Rozati , A. Vomiero , Sberveglieri. Angew Chem Int Ed, Vol. 50, p. 1-5. (2011)
19.
D. Matthews, , Solar Energy Materials & Solar Cells, Vol. 44, p. 119-155 (1996).
20.
M. Grätzel, Journal of Photochemistry and Photobiology A: Chemistry, Vol.164, 3-14, (2004)
21.
G. Smestad, C. Bignozzi, and Argazzi, Sol Enerer Mater Sol. Cells, Vol. 32, p. 259 (1994).
22.
Ecole Polytechnique Federale de Lausanne, Science Daily, Vol. 3 (2008).
23.
Y. Jiao, F. Zhang and S. Meng, Dye Sensitized Solar Cells Principles and New Design (http://cdn.intechopen.com/pdfs-wm/23331.pdf)
24.
A. L. Dawar, J. C. Joshi, J. Mater. Sci., Vol.9, p. 1, (1984)
25.
B. Thangaraju, Thin Solid Films Vol. 402, p. 71 (2002).
26.
T. M. El-Agez1, A. A. El Tayyan, A. Al-Kahlout, S. A. Taya1, M. S. Abdel-Latif, DyeSensitized Solar Cells Based on ZnO Films and Natural Dyes, International Journal of Materials and Chemistry, Vol. 2(3), p. 105-110 (2012)
27.
M.K. Nazeeruddin, P.pechy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A. B.
28.
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev. Vol. 110, p. 6595-6663, (2010)
29.
N. Yamanaka, R. Kawano, W. Kubo, N. Masaki, T. Kitamura, Y. Wada, M. Watanabeand S. Yanagida, Chem. Commun, 740 (2005)
30.
A. F. Nogueira, C. Longo, M. A. De Paoli. Coord. Chem. Rev. Vol. 248, p. 1455 (2004).
31.
B. Li, L. D. Wang, B. Kang, P. Wang, Y. Qiu. Sol. Energy Mater. Sol. Cells, Vol. 90, p. 549 (2006).
32.
F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc., Vol. 130, p. 10720, (2008)
33.
C. L. Olson. J. Phys. Chem. B Vol. 110, p. 9619 (2006).
34.
H. Pettersson., T. Gruszecki, Solar Energy Materials & Solar Cells, Vol. 70, p.203-212 (2001)
88
35.
H. Pettersson., T. Gruszecki, , Solar Energy Materials & Solar Cells, Vol. 70, p.203-212 (2001)
36.
T. N. Murakami, M. Grätzel, Inorg. Chim. Acta, Vol. 361, p. 572 (2008)
37.
A. Kay, M. Grätzel, Sol. Energy Mater. Sol. Cells, Vol. 44, p. 99 (1996)
38.
M. Wang, A. M. Anghel, B. Marsan, N.-L. Cevey Ha, N. Pootrakulchote, S. M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc., Vol. 131, p. 15976 (2009)
39.
S. Aydin, G. Turgut, M. Yilmaz, M. Ertugrul, Journal of Science and Technology, Vol. 1, p. 1-3, (2011)
40.
S. Ahmadi, N. Asim, M.A.Alghoul, F.Y.Hammadi, K. Saeedfar, N.A.Ludin, S. H.Zaidi, and K.Sopian, International Journal of Photoenergy, Vol. 2014, p. 19 (2014)
41.
D. Chu,w Y. Masuda, T. Ohji, and K. Kato, J. Am. Ceram. Soc., Vol. 93, p. 887-893 (2010)
42.
P. Rai, J. N. Jo, I-H Lee, Y. -tae Yu, Materials Chemistry and Physics, Vol. 124, p. 406412 (2010)
43.
M. Movahedi and A. R. Mahjoub, Journal of Applied Chemical Researches Winter, Vol. 3, p. 12 (2010)
44.
A.Pamain, T. Pogrebnaya, C. K. King'ondu, Research Journal in Engineering and Applied Sciences, Vol. 3(5), p. 332-336 (2014)
45.
M.R. Nishantha, Y.P.Y.P Yapa1 and V.P.S. Perera, Proceedings of the Technical Sessions, Vol. 28, p. 54-58 (2012)
46.
S. A. Taya, T. M. El-Agez, H.S. El-Ghamri, M. S. Abdel-Latif, International Journal of Materials Science and Applications, Vol.2, p. 37-42 (2013)
47.
Y. Zhang, M. K. Ram, E. K. Stefanakos, D. Y. Goswami , Synthesis, Characterization, and Applications of ZnO Nanowires, Journal of Nanomaterials, 1-20 (2012)
48.
C. Kittel, Introduction to Solid State Physics, Willey-India Edition, Seventh Edition, (2009)
49.
S. Hasegawa, I. Shiraki, F. Tanabe, R. Hobara, T. Kanagawa, T. Tanikawa and I. Matsuda, Surface Review and Letters, Vol. 10, p. 963-980 (2003)
50.
J. Cubas, S. Pindado and C. de Manuel, Energies, Vol. 7, p. 4098-4115, (2014)
51
F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. H. Baker, P. Wang, S. M. Zakeeruddin, and M. Gratzel, J. A. Chem. Soc., Vol.130, p. 32 (2008) 89
52.
D.Tatar, G. Turgut and B. DÜZGÜN, Rom. Journ. Phys., Vol. 58, p. 143-158, (2013)
53.
S. Ilican, Y. Caglar, M. Caglar, Journal of optoelectronics and advanced materials, Vol. 10, p. 2578-2583 (2008)
54.
T. H. Meen, W. Water, Y. S. Chen, W. R. Chen, L. W. Ji, C. J. Huang, IEEE, Vol. 1, p. 617-620, (2007).
55.
G. N. Gyawali, Master’s thesis, GoldenGate Int. College (T.U.) (2014)
56.
B. Yuliarto,W. Septina, K. Fuadi, F. Fanani, L. Muliani,and Nugraha, Hindawi Publishing Corporation, Advances in Materials Science and Engineering, Vol. 2010, p.6 (2010)
57.
S. S. Kanmani, K. Ramachandran, and S. Umapathy, Hindawi Publishing Corporation International Journal of Photoenergy, Vol. 2012, p. 8 (2012)
58.
H. Hug, M. Bader, P. Mair, T. Glatzel, Applied Energy, Vol.115, 216-225 (2014)
59.
C.Y. Jiang, X.W. Sun, G.Q. Lo, D.L. Kwong and J. W. Wang, Appl. Phys. Lett., Vol. 90, (2007)
60.
Okoli, Laeticia Udodiri; Fabrication and characterization of dye sensitized solar cell using natural dye from rosella plant, (2010).
61.
T. S. Senthil, N. Muthukumarasamy, and M. Kang, ZnO Nanorods Based Dye Sensitized Solar Cells Sensitized using Natural Dyes Extracted from Beetroot, Rose and Strawberry, Bull. Korean Chem. Soc., Vol. 35, p. 4 (2014)
62.
H. Zhou, L. Wu, Y. Gao, T. Ma, Dye-sensitized solar cells using 20 natural dyes as sensitizers, Journal of Photochemistry and Photobiology A: Chemistry, Vol. 219, p. 188194 (2011)
90
