Photoelectrochemical characterization of dye sensitized solar cells by tio2 based nanomaterials

Photoelectrochemical Characterization of Dye-sensitized Solar Cells by The-Vinh Nguyen Division of Chemical Engineering and Technology School of Graduate Studies Chonbuk National Univer

Thesis for Doctoral Degree

Photoelectrochemical Characterization of

Dye-sensitized Solar Cells by

February 22, 2005

Division of Chemical Engineering and Technology School of Graduate Studies of Chonbuk National University

The-Vinh Nguyen

타이타니아계 나노입자를 이용한 염료 감응형

태양전지의 광전기 화학적 특성화

Photoelectrochemical Characterization of Dye-sensitized

Solar Cells by TiO2-based Nanomaterials

2005 年 2 月 22 日 全北大學校大學院 化學工學科 구웬더빈

February 22, 2005 Division of Chemical Engineering and Technology

School of Graduate Studies of Chonbuk National University

The-Vinh Nguyen

타이타니아계 나노입자를 이용한 염료 감응형

태양전지의 광전기 화학적 특성화

Photoelectrochemical Characterization of Dye-sensitized

Solar Cells by TiO2-based Nanomaterials

이 論文을 工學博士 學位 論文으로 提出함

2004 年 10 月 12 日 全北大學校大學院 化學工學科 구웬더빈

October 12, 2004 Division of Chemical Engineering and Technology

School of Graduate Studies of Chonbuk National University

The-Vinh Nguyen

Table of Contents

List of Tables ……… vi

List of Figures ……… vii

Abstract ……… xiii

1 General Introduction ……… 1

2 Literature Survey ……… ……8

2.1 Types of renewable energy ……….…….9

2.2 Types of solar energy ……….10

2.3 Solar radiation and air mass ……… 13

2.4 Thermodynamic efficiency limitations in photochemical conversion…15 2.5 Solid state solar cells ……… 24

2.6 Dye-sensitized solar cells (DSCs) ……… 34

2.6.1 Brief history ……… 34

2.6.2 Major components and principle of DSC ……….35

2.6.2.1 Nanoporous titanium dioxide ……… 35

2.6.2.2 Dye sensitizers: metal complex versus organic dyes …… 38

2.6.2.3 Electrolytes ……… 43

2.6.2.4 Operation principle of DSC ……….……45

2.6.2.5 Comparison to solid state solar cell ……….48

2.6.2.6 Solid-state DSC ……… 48

2.7 Photoelectrochemical definitions………50

2.7.1 Current and Potential……….50

2.7.2 Ohm’s law ……….50

2.7.3 DC capacitance ……….51

2.7.4 AC impedance ……… 51

2.7.5 Short circuit current, ISC ………52

2.7.6 Open circuit potential, VOC ……… 52

2.7.7 Fill factor ……… 52

2.7.8 Overall conversion efficiency ……… 54

2.7.9 Incident monochromatic photon-to-current conversion efficiency, IPCE ………54

2.8 Photoelectrochemical theory……… 55

2.8.1 Space charge region ……… 55

2.8.2 Semiconductor photoelectrochemistry ……….56

2.8.3 Elementary analysis of impedance spectra ……… 58

3 Experimental Methods ……… ……… 64

3.1 Preparation of TiO2-based substrates ……….65

3.1.1 Materials ……….……… 65

3.1.2 Substrate preparation ………65

3.2 Fabrication of DSCs ……… ………65

3.2.1 Materials ……….……… 65

3.2.2 Cell fabrication ……….66

3.3 Physicochemical and morphological characterization ……… 68

3.4 Photoelectrochemical characterization ……… 68

3.4.1 Three electrode single compartment ……….68

3.4.2 Two electrode configuration ……….71

3.4.3 I-V curve characterization of DSCs ……… 71

4 Cathodic Electrodeposition of TiO 2 and TiO 2 /SiO 2 Nanocomposite Films for DSCs ………74

4.1 Introduction ……….…… 75

4.2 Experimental ……… 77

4.2.1 Materials ……… 77

4.2.2 Preparation of TiO2-based films and corresponding DSCs…… 77

4.2.3 Characterization of TiO2-based films and corresponding DSCs…78 4.3 Results and discussion ………79

4.3.1 Characterization of as-prepared TiO2 and TiO2/SiO2 films…… 79

4.3.1.1 Potentiostatic characterization ……….79

4.3.1.2 Photocurrent density characterization ……….84

4.3.1.3 FE-SEM image and EDX analysis ……… 91

4.3.1.4 XRD characterization ……… 92

4.3.1.5 AC impedance characterization ……… 95

4.3.2 Characterization of thermal treated TiO2 and TiO2/SiO2 films…102 4.3.2.1 FE-SEM image and EDX analysis……….102

4.3.2.2 XRD characterization ………102

4.3.2.3 Photocurrent density characterization ……… 102

4.3.2.4 AC impedance characterization ………109

4.3.3 Characterization of DSCs ……… 114

4.4 Conclusions ……… 118

5 Optimization of TiO 2 Substrates for DSCs ……… 120

5.1 Introduction ……….….121

5.2 Experimental ………123

5.2.1 Materials ……….123

5.2.2 Preparation of TiO2 film and corresponding DSCs……….123

5.2.3 Characterization of TiO2 films and DSCs ……… 123

5.3 Results and discussion ……… 124

5.3.1 Characterization of TiO2 nanoparticles and thin film………… 124

5.3.2 Photovoltaic performance of DSC ……….136

5.4 Conclusions ……….….141

6 Charge Storage and Transfer in DSCs: A Study by Electrochemical Impedance Spectroscopy ……… 142

6.1 Introduction ……….….143

6.2 Experimental ………145

6.2.1 Materials ……….145

6.2.2 Preparation of TiO2 films and corresponding DSCs ………… 145

6.2.3 Characterization of TiO2 films and DSCs ……… 145

6.3 Results and discussion ……… 146

6.3.1 The effect of dye sensitizers on the performance of DSCs… 146

6.3.2 Charge storage and transfer in DSCs ……… 148

6.3.2.1 AC impedance characterization of DSCs………148

6.3.2.2 Charge storage in the porous TiO2 films of DSCs: a comparison between bare-TiO2 cells and corresponding DSCs ……… 155

6.3.2.3 The effect of the thickness of TiO2 film on the charge storage and transfer in DSC ……… 158

6.3.3 Characterization of DSCs……… 163

6.4 Conclusions ……… 166

7 Conclusions ………168

References … ……… 171

Acknowledgement ……….182

Table 4.2 Photovoltaic performance data of DSCs fabricated with various

TiO2-based films and Eosin Y dye sensitizer ………117

Table 5.1 Structural properties of TiO2 particles and the photocurrent density

and amount of adsorbed dye of the corresponding TiO2 thin films

………126

Table 5.2 Photovoltaic performances of DSCs fabricated with various TiO2

and Eosin Y or ruthenium 535 bis-TBAa as dye sensitizers….138

Table 6.1 Effect of dye sensitizer on the performance of DSCs fabricated

using HK-450-22 and P25-raw as TiO2 substrates .…………147

Table 6.2 Photovoltaic performance of DSCs fabricated using HK-450-based

films as a function of TiO2 film thickness ……… ………165

List of Figures

Fig 2.1 Schematic diagram of the norbornadiene/quadricyclene energy

cycle Right: energy diagram of reaction, Ef* = formation energy for activated complex = minimum solar energy needed for conversion, ∆E = energy gained by reaction reversal, Eb* = energy barrier……… … 12

Fig 2.2 Spectral distribution of solar radiation Air mass 1.5 global

spectrum [37]………16

Fig 2.3 Spectral irradiance of the sun at mean earth-sun separation

[40]……….20

Fig 2.4 Energy diagram of semiconductors with intrinsic, n-type and

p-type properties EC = energy of conduction band, EV = energy of valence band, EF = energy of Fermi level, ED and EA = energy of donor and acceptor dopant level, respectively………28

Fig 2.5 Light absorption by an intrinsic semiconductor The impinging

photon induces an electron transition from the valence band into the conduction band, if hν ≥ Eg……… 28

Fig 2.6 The charge dipole and space charge region produced at a p-n

junction……… 33

Fig 2.7 The p-n junction under illumination Left: A photon induced

hole-electron pair is separated by the local field of the junction Right: The origin of the photovoltage EP EB = Energy barrier created by the p-n junction, EP = Energy equivalent of the photovoltage… 33

Fig 2.8 FE-SEM micrograph of nano-porous TiO2 layer Side and top view

Layer thickness ≈ 11.5 µm, particle diameter ≈ 25 nm…………37

Fig 2.9 Schematic representation of electron transitions in a DSC k0 = rate

constant for decay of excited state (Dye* → Dye) kinj= rate

constant for electron injection into TiO2 (Dye* → e−TiO2), ker1 = rate constant for electron recombination with oxidized dye (e− + Dye+), ker2 = rate constant for electron recombination with redox mediator (e− + R), R−/R = redox mediator system, commonly

Fig 2.12 Space charge region formation at n-type semiconductor-solution

interface:  = electron;  = hole; - + Immobilized donor state or ions in the electrolyte (a) Flat band situation; (b) accumulation layer; (c) depletion layer; (d) inversion layer ……… 57

Fig 2.13 Schematic diagram of the current-potential curves in the dark and

under illumination for a semiconductor-liquid junction An n-type semiconductor is assumed and I1 and I2 are the incident light intensities……… 59

Fig 2.14 The “crossover” effect illustrating excess current flow under

illumination in the forward bias regime for an n-type semiconductor-liquid junction……….………… 59

Fig 2.15 (a) Equivalent circuit of an electrochemical cell, (b) Subdivision of

Zf into Rs and Cs, or into RCT and Zw……….63

Fig 3.1 The schematic diagram of a typical DSC with (a) surface image

and (b) cross-section image; (c) the real surface image ……… 67

Fig 3.2 An electrochemical experiment in a three electrode single

compartment under potentiostatic control……… 70

Fig 3.3 Two electrode configuration connected to potentiostat and lock-in

amplifier……….72

Fig 3.4 Experimental set-up for I – V curve characterization…….…… 73

Fig 4.1 Potentiostatic curves of bare TiO2 film (a) and TiO2/SiO2 film

(b)……… …80

Fig 4.2 The effect of SiO2 concentration in bath on potentiostatic

curve… 81

Fig 4.3 The effect of bath pH on potentiostatic curve……… ….82

Fig 4.4 The effect of deposition potential on potentiostatic curve……….83

Fig 4.5 The effect of bath pH on the photocurrent density of resulting

TiO2/SiO2 film; TCO glass (a), pH 9 (b), pH 2 (c), pH 7 (d), pH 4 (e), pH 5 (f).……… 85

Fig 4.6 The influence of deposition potential on the photocurrent density

of resulting TiO2/SiO2 film; -6 V (a), -3 V (b), -2 V (c), -5 V (d), -4

V (e)……… 86

Fig 4.7 The influence of SiO2 concentration in bath on the photocurrent

density of resulting TiO2/SiO2 film; TiO2(5)/SiO2(3) (a), TiO2(5)/SiO2(1.5) (b), TiO2(5)/SiO2(1) (c), TiO2(5)/SiO2(2.5) (d),

Fig 4.8 Photocurrent densities of bare TiO2 and TiO2/SiO2 films………88

Fig 4.9 FE-SEM cross-section (a) and surface images at high (b) and low

(c) magnification of TiO2(5)/SiO2(2) film……… 93

Fig 4.10 XRD patterns of bare TiO2 and TiO2/SiO2 films at – 4V deposition

potential (a) and TiO2(5)/SiO2(2) films prepared under different deposition potentials (b)……… 94

Fig 4.11 Nyquist plots of bare TiO2 and TiO2/SiO2 films as prepared

Numbers on the lines are frequencies in kHz……….………….101

Fig 4.12 XRD patterns of bare TiO2 (a) and TiO2/SiO2 (b) films as a

function of calcination temperature Numbers on the peaks are the intensities (cps) of anatase phase in the films………103

Fig 4.13 Photocurrent densities of bare TiO2 (1) and TiO2/SiO2 films (2) as

a function of calcination temperature; 75oC (a), 150oC (b), 250oC (c), 350oC (d), 450oC (e), 550oC (f)……….104

Fig 4.14 Photocurrent densities at zero DC bias of TiO2 and TiO2/SiO2 films

as a function of calcination temperature……… 107

Fig 4.15 Nyquist plots of TiO2/SiO2 film calcined at different temperatures

at high (a) and low (b) frequencies Numbers on the lines are frequencies in kHz……… 110

Fig 4.16 Nyquist plots of bare TiO2 and TiO2/SiO2 films prepared by

d o c t o r - b l a d e m e t h o d I n s e t s h o w s t h e p l o t s a t h i g h frequencies……….112

Fig 4.17 Photocurrent densities of TiO2/SiO2 films prepared by doctor-blade

method with different contents of SiO2; TiO2(5)/SiO2(0.75) (a), TiO2(5)/SiO2(0.5) (b), TiO2(5)/SiO2(0.25) (c), TiO2(5)/SiO2(0) (d)……… 113

Fig 4.18 Typical I-V curves of DSCs fabricated by using electrodeposited

TiO2/SiO2 films as prepared and calcined at 450oC………116

Fig 5.1 X-ray diffraction patterns of HKs and P25 as a function of

pre-thermal treatment temperature……… 127

Fig 5.2 FE-SEM images of various TiO2 thin films prepared by

corresponding TiO2 particles All of the films were prepared with the same TiO2 concentration in the TiO2 paste by the same procedures Surface images of P25-raw (a), HK-raw (c), HK-450 (e) and HK-900 (g) Cross-section images of P25-raw (b), HK-raw (d), HK-450 (f) and HK-900 (h) The insets show the surface images at 100 K magnification………128

Fig 5.3 Photocurrent densities of HK films as a function of pre-thermal

treatment temperature……… 129

Fig 5.4 UV-Vis diffuse reflectance spectra of P25-raw and HK films as a

function of pre-thermal treatment temperature………131

Fig 5.5 FT-IR spectra of P25-raw (a), P25-900 (b), HK-raw (c), HK-450

(d), HK-900 (e) and TiO2-SiO2 mixed oxide (f)……….132

Fig 5.6 Nyquist plots of HK films prepared by TiO2 pre-treated at

corresponding temperatures; (■) raw, (□) 250oC, (▼) 450oC, (○)

700oC and (●) 900oC Numbers on the lines are frequencies in kHz The insets show an equivalent simple circuit model of the experimental cell (a) and Nyquist plots at high frequencies ( b ) … … … 1 3 5

Fig 5.7 Typical graph of photocurrent-voltage for DSC fabricated with

HK-450 and Eosin Y dye sensitizer The cell performance has been measured under 100 mW/cm2 with 0.25 cm2 active area The short-circuit current ISC, open-circuit voltage VOC, fill factor FF, and overall conversion efficiency η are 3.03 mA/cm2, 0.62 V, 0.57, and 1.061 %, respectively……… 137

Fig 6.1 Typical AC impedance spectrum of DSC under visible

illumination (a) RS , (b) RS + RTiO2/TCO + RCE , (c) RS + RTiO2/TCO +

RCE + RCT , (d) RS + RTiO2/TCO + RCE + RCT + RE , (e) - Zim = 1/ωC

∝ 1/CH + 1/CSC≈ 1/CSC (where, RS: the resistance of electrolyte, the sheet resistance of TCO and Pt coated TCO glass; RTiO2/TCO: the resistance of the flow of charge across the TiO2/TCO interface;

RCE: the resistance of Pt/electrolyte interface; RCT: the resistance of charge transfer in TiO2 layer and between TiO2 and electrolyte; RE: charge transfer resistance in electrolyte; CH, CSC: the capacitance of the Hel mh o ltz lay er; apparent space char ge lay er at TiO2/electrolyte; C: capacitance; ω: angular frequency)… ….149

Fig 6.2 AC impedance spectra of HK-450-22 and P25-raw-based DSCs in

dark.……….150

Fig 6.3 AC impedance spectra of HK-450-22 and P25-raw-based DSCs

under visible illumination Numbers on the arrows are frequencies

DC bias of ± 10 mV over the open-circuit potential………… 152

Fig 6.4 AC impedance spectra of HK-450-22-based DSC (a) and

P25-raw-based DSC (b) at different DC bias……… 154

Fig 6.5 Photocurrent densities of bare-TiO2 cells of HK-450-22 and

P25-raw (a) and corresponding DSCs (b) ……….156

Fig 6.6 AC impedance spectra of bare-TiO2 cells of HK-450-22 and

P25-raw……… …157

Fig 6.7 FE-SEM images of cross-section (a) and surface (b) of HK-450

films as a function of thickness ……….159

Fig 6.8 The effect of TiO2 film thickness on the AC impedance spectra of

DSCs under visible illumination DC bias of ± 10 mV over the open-circuit potential ……… 160

Fig 6.9 The effect of TiO2 film thickness on the AC impedance spectra of

DSCs under visible illumination; 12 µm (■), 17µm (●) and 22 µm (▲) DC bias of ± 10 mV over the short-circuit potential (a) and –0.3 V (b)……… 161

Fig 6.10 Photocurrent density of DSC as a function TiO2 film thickness; 12

µm (a), 17µm (b) and 22 µm (c)……….164

Photoelectrochemical Characterization of

Dye-sensitized Solar Cells by

The-Vinh Nguyen Division of Chemical Engineering and Technology School of Graduate Studies

Chonbuk National University

Abstract

The synthesis and characterization of TiO2 nanoparticles and TiO2/SiO2nanocomposites for the dye-sensitized solar cell (DSC) have been studied in this thesis TiO2 nanoparticles and thin films were modified in terms of specific surface area, crystallinity, thickness and its nanocomposite with SiO2nanoparticles The characteristics of nanostructured TiO2-based substrates were further investigated by using nitrogen adsorption for BET specific surface area, X-ray diffraction, UV-Vis spectroscopy, UV-Vis diffuse reflectance spectroscopy, field emission scanning electron microscopy equipped energy-dispersive X-ray element analysis system and FT-IR spectroscopy The photoelectrochemical properties of DSC electrode materials were examined with dark current, photocurrent density and AC impedance spectroscopy for understanding the charge storage and transfer in the resulting DSCs

In cathodic electrodeposition of TiO2 and TiO2/SiO2 thin films for DSC, the highest photocurrent density was produced on TiO2-based films prepared by electrodeposition under -4 V of deposition potential, bath pH of 5, and ca

28.57 wt % of SiO2 in the bath The presence of SiO2 in an optimal bath condition resulted in the high amount of electrodeposited TiO2 in TiO2/SiO2film relative to bare TiO2 counterpart The thermal treatment (in the range of

150oC – 550oC) of as-prepared TiO2-based films was found to decrease the charge transfer resistance at the TiO2-based film/electrolyte interface Meanwhile, such thermal treatment could significantly increase the charge storage in the TiO2/SiO2 film This was attributed to the increasing of the bond strength between TiO2 and SiO2 as well as TiO2 nanoparticles where SiO2might act as an effective energy barrier between TiO2 network and redox electrolyte The role of SiO2 as an energy barrier consequently brings about the improvement of photocurrent density of the derived TiO2/SiO2 film and the performance of corresponding DSC

In order to optimize the characteristics of TiO2 substrate, the effect of thermal treatment on pure anatase TiO2 with high specific surface area (ca 334

pre-m2/g, HK) on the photovoltaic performance of DSC was investigated Before coating the raw HK TiO2 (HK-raw) nanoparticles on transparent conducting oxide (TCO) coated glass for DSC fabrication, pre-thermal treatment of HK-raw by calcining at 450oC (HK-450) was an essential step to achieve the optimum properties in terms of morphological feature, crystallinity, specific surface area and photocurrent density HK-450 film showed the high adsorption of dye, high photocurrent density and low interface resistance between TiO2 and TCO glass, RTiO2/TCO and TiO2 and redox electrolyte, RCT, resulting in the superior photovoltaic performance of the DSC fabricated with HK-450 Accordingly, the optimization between the morphological feature, specific surface area and photocurrent density of TiO2 substrate is promising to accomplish the improved overall conversion efficiency of DSC

Finally, it was aimed to elucidate the charge storage and transfer in DSCs by

AC impedance spectroscopy, which was found as an efficient tool to elucidate the electronic behavior of DSC Under visible illumination and open circuit potential, the imaginary part of AC impedance spectroscopy of DSC

characterized the charge storage of apparent space charge region of the TiO2film in DSC rather than the charge storage of the whole dye adsorbed TiO2network P25-raw-based DSC was found to show the prompt charge transfer processes between the dye adsorbed TiO2 film/electrolyte and the dye adsorbed TiO2 film/TCO glass interface Meanwhile, high charge storage in the dye adsorbed porous TiO2 film was observed on HK-450-22-based DSC For a given TiO2 material, only if the thickness of TiO2 film was less than the optimum value was the charge storage found to reinforce the charge transfer in the resulting DSC Otherwise, the thicker the thickness of TiO2 film, the higher the charge storage and the slower the charge transfer of the resulting DSC Accordingly, the existence of an optimized thickness of TiO2 substrate was inherent for the highest overall conversion efficiency of a DSC fabricated with

a given TiO2 material The combination of the charge storage or the concentration of photo-excited electrons in the porous TiO2 film and the prompt charge transfer between it and the TiO2 film /TCO glass interface was worthwhile to note for improvement on the overall conversion efficiency of DSC

타이타니아계 나노입자를 이용한 염료

감응형 태양전지의 광전기 화학적 특성화

전북대학교대학원 화학공학과

구웬더빈

Abstract

본 논문은 염료감응형 태양전지 (dye-sensitized solar cell, DSC)

변형되었다 제조된 나노 물질과 박막은 BET 비표면적 측정, XRD, UV-Vis, UV-Vis DRS, FE-SEM, EDX와 FT-IR에 의해 물리화학적 특성을 조사 하였다 DSC 전극 물질의 광전기 화학적 특성인 전하 축적과 이동은 광전류의 밀도 그리고 AC 임피던스

끝으로 본 연구에서는 AC임피던스 분광법에 의해 DSC의 전하의 축적과 이동 및 DSC의 전기적인 행동을 명료하게 밝힐 수 있었다 가시광선 하에서 open-circuit potential 과 AC 임피던스 분광법의

Chapter 1

General Introduction

Historically, the world energy supply was based on renewables Wood was used for cooking, water and space heating Water powered mills were found throughout Europe, especially in Southern Germany and Switzerland, while wind mills belonged to the scenery of the Netherlands and Northern Europe The first renewable energy technologies were primarily simple mechanical applications and did not reach high energetic efficiencies

Industrialization in the 18th century changed the primary energy use from renewable resources to sources with a much higher energetic value such as coal and oil In the early days, pollution by the burning of fossil fuels was not a critical issue Due to their superiority by providing cheap power at any location independent from the availability of wind or water sources, they quickly became the most used energy sources The emerging industry did not pursue the technical improvement of technologies such as small-scale hydro and wind power any further The promise of unlimited fossil fuels was much more attractive and rapid technical progress made the industrial use of oil and coal economical Renewable technologies like water and wind power probably would not have provided the same fast increase in industrial productivity as fossil fuels did

Nevertheless, there is a common apprehension that the world’s fossil fuels are declining and soon they will disappear With the oil crisis in 1973’s when almost overnight the price for crude oil tripled, it was the driving force behind renewed efforts and searches of alternative energy sources Nuclear fission was one of the first energy carriers identified and strongly sponsored However, today we know the problems associated with this kind of energy and its large scale and long-term dangers which are inherent to its technology Meanwhile, there are other non-fossil and renewable sources of energy, like solar, wind, tide, ocean thermal exchange, ocean current, geothermal, biomass etc., which have great potential but so far have been barely harnessed

It was known that the amount of solar energy intercepted by the Earth and hence the amount of energy flowing in the “solar energy cycle” is about

5.4×1024J per year [1] The total worldwide demand for end-use energy has been estimated to be 8×109tce per year (tce = tons of coal equivalent) Since one tce = 2.931×1010J [2], this equals 1.09×1020J and allows us to reach the following conclusion:

“The energy of less than 10 minutes of sunshine on the Earth is equal to the total yearly human energy consumption”

The above statement should provide sufficient justification and motivation for anybody to undertake research in the area of solar energy

Among various types of solar energy conversion methods that will be presented in the Chapter 2 photovolatics technology has already demonstrated its effectiveness and holds great promise in electrical generation for the world The electricity obtained is direct current and can be used directly to operate direct current devices, converted to an alternating current or stored for later use Because sunlight is universally available, photovoltaic devices provide many benefits that make them usable and acceptable to all inhabitants of our planet Photovoltaic systems are modular, and so their electrical power output can be

engineered for virtually any application, from low-powered consumer uses (e.g

wristwatches or calculators) to high capacity solar power plant stations Moreover, incremental power additions are easily accommodated in photovoltaic systems, unlike more conventional approaches based on use of fossil or nuclear fuel, which require multimegawatt plants to be economically feasible In order to illustrate the potential of solar cells, it has been estimated [3] that a photovoltaic power plant of an area of 140km2 located in an average part of the US could generate all the electricity needed by the country, assuming realizable conditions and a system efficiency of 10%

In spite of the great potential with over 100 million watts of conventional solar cell are currently produced each year worldwide, no solar cell technology has produced an efficient, reliable, and cost effective solar module that can be widely used to replace fossil fuel energy resources

A research group in Lausanne, Switzerland has used our knowledge of photosynthesis and photography to produce a new kind of solar cell that may meet the challenge [4-8] The nanocrystalline dye sensitized solar cell is a photoelectrochemical cell that resembles natural photosynthesis in two respects: (1) it uses an organic dye like chlorophyll to absorb light and produce a flow of electrons, and (2) it uses multiple layers to enhance both the light absorption and electron collection efficiency Like photosynthesis, it is a molecular machine that is one of the first devices to go beyond microelectronics technology into the realm of what is known as nanotechnology To create the nanocrystalline solar cell, a solution of nanometer size particles of titanium dioxide, TiO2, is distributed uniformly on a glass plate which has previously been coated with a thin conductive and transparent layer of tin dioxide (SnO2) These are not exotic materials Large amounts of TiO2 powder are used in the manufacture of white paint, and large areas of tin dioxide-coated glass are used

in buildings as heat reflective (and energy saving) windows The titanium dioxide film is dried and then heated to form a porous, high surface area TiO2structure that, when magnified, looks like a thin sponge or membrane This TiO2 film on the glass plate is dipped into a solution of a dye such as a red ruthenium containing organic dye or green chlorophyll derivative Many dyes can be utilized, but they must both possess a chemical group which can attach and adsorb to the titanium dioxide surface, and they must have energy levels at the proper positions necessary for electron injection and sensitization A single layer of dye molecules coats and attaches to each particle of the TiO2 and acts

as the absorber of sunlight To complete the device, a drop of liquid electrolyte containing iodide is placed on the film to percolate into the pores of the membrane A counter electrode of conductive glass, which has been coated with a thin catalytic layer of platinum or carbon, is placed on top, and the sandwich is illuminated through the TiO2 side

Currently, dye-sensitized solar cells (DSC) are under intense investigation for their respectable photo-conversion efficiency, low cost materials and

production, and life cycle assessment in mass production Long-term stability

of more than two years under outdoor conditions and 10,000 hours under simulated sunlight (400W/m2) has been confirmed [9] The project Joule 3 in the European Community [10] obtained the following results for DSCs based

on pure liquid electrolytes: (1) A minor decrease in performance of initially 5% solar efficient cells has been found after 2,000 hours at 60oC storage in the dark; (2) After 3,400 hours under combined thermal stress and continuous 1-sun equivalent light soaking at 40oC, good stability with 15% decrease in maximum power could be demonstrated

Due to the efficiency drop at higher temperatures (over 60oC) for the based DSCs, attempts have been made to replace the liquid electrolyte with room temperature molten salts [11, 12], inorganic p-type semiconductors [13-16], ionic conducting polymers [17-23], organic hole transport materials [24-26] and novel polymer gel [27, 28] The above systems present high technological interest but their practical use encounters serious problems such as low conversion efficiencies or/and poor electric contact between the photoelectrode and the electrolyte

liquid-Whatever the DSCs are based on liquid, quasi-solid, or solid electrolytes, the use of a large surface area semiconductor is indispensable to provide sufficient light absorption with only one adsorbed mono-layer of dye Therefore, the morphology of TiO2 substrates, such as specific surface area, pore size, pore volume, greatly affects the performance of resulting DSCs Barbé et al reported in detail the correlation between porous structure of the film and preparation conditions such as hydrolysis condition of titanium alkoxide, hydrothermal treatment temperature and sintering temperature [29] For synthesis of nanoporous TiO2, usually titanium alkyloxides are hydrolyzed under acidic conditions, deposited as a thin film on conducting glass and sintered at temperature of 450oC for around 30 min According to this procedure, the derived TiO2 exhibits the specific surface area of around 100

m2/g [29] that is higher than commercial TiO2 (P25, ca 58 m2/g) It has been

inherently accepted that the higher the surface area of TiO2, the higher the current that is generated by the DSC [29, 30] In our recent studies [31, 32], to increase the specific surface area up to 670 m2/g, TiO2 was atomically mixed with SiO2 by sol-gel processes Nevertheless, the crystallinity of the resulting TiO2-SiO2 mixed oxide was found to substantially decrease The irregularity due to the low crystallinity of TiO2-SiO2 compared to bare TiO2 in turn results

in the significant increase in the charge recombination and therefore, decreases the photocurrent density of TiO2-SiO2 In other words, the increase in the surface area of TiO2 substrate of DSC without sacrificing of its photocurrent density seems to be inevitable To some extent, the specific surface area and the crystallinity of TiO2 substrate are closely related to the charge storage and transfer in DSCs, respectively Optimization of the formers could lead to that of the latters

Intrigued by these findings along with the lack of research in detail on the relation between the morphological feature and photoelectrochemical properties of TiO2 substrate and the performance of resulting DSCs, the main contents of this thesis will be focused on the modification of TiO2 substrate in terms of specific surface area, crystallinity and its nanocomposite with SiO2nanoparticles, which are simultaneously characterized by photoelectrochemical methods for elaborating the charge storage and transfer in the resulting DSCs Accordingly, the following is a brief research outline of the content of the thesis:

(i) Modification of TiO2 substrates by deliberate introduction of

amorphous SiO2 nanoparticles in order to increase the specific surface area of the substrates and suppress the charge recombination processes in DSCs Instead of doctor-blade method electrodeposition will be employed to deposit TiO2 and TiO2/SiO2

on the transparent conducting oxide coated glass The effect of calcination temperature on the photoelectrocemical properties of

TiO2-based substrates and the performance of corresponding DSCs will be also elucidated

(ii) The optimization between the specific surface area and the

crystallinity of TiO2 substrate for the improved photovoltaic performance of DSCs

(iii) The charge storage and transfer in DSC correlated with its

photovoltaic performance are investigated by using AC impedance measurement in combination with DC bias under visible illumination

Chapter 2

Literature Survey

2.1 Types of renewable energy

Renewable energy has become a fashionable term and is unalterably attached

to the phrase solar energy A satisfactory definition of the phrase "Renewable Energy" could be adopted from the book of Sørensen [1] as follow:

The term "Renewable Energy Resource" is used for energy flows which are replenished at rates comparable to which they are used

Accordingly, fossil energy (petrol and the like) cannot be a renewable energy resource The crude oil found on our planet was formed over periods of several tens of thousands of years However, we have used almost all of it within only

a few decades and it becomes clear that this does not meet the requirements of our definition of a renewable energy source Nuclear energy obtained from nuclear fission is also clearly not a renewable energy because its main

resources, e.g uranium, were formed during the birth of our planet and are

therefore limited

Similarly, nuclear fusion is not a renewable energy resource, since it depends

on the presence of deuterium However, these fusion reactions are similar to those that occur in the sun and other stars, and provide a much greater energy potential than nuclear fission Overall, in a fusion process deuterium, a hydrogen isotope, reacts to give helium and hydrogen and simultaneously releases heat Although the quantity of starting material is finite, it is so abundant in seawater, that the potential nuclear energy that could be released

by nuclear fusion of deuterium contained in one cubic meter of seawater is

1013Jm-3 [1] Thus, man's energy expenditure might be sustained for a long time Nevertheless, radioactive substances are formed as side products and the technology will not be available in time for this generation to make fusion an important energy resource

The quantity of solar energy absorbed by the Earth is about equal to the amount of heat reradiated back into space Utilization of solar energy by man

usually means temporary storage in a usable form (e.g battery or another

natural process like photosynthesis) However, solar energy is eventually reconverted into heat upon utilization, so that the renewable cycle is completed The same renewable cycle is true for wind, tidal and hydrodynamic energy resources Wood as an energy resource is more complicated to categorize and demonstrates that the definition of renewable energy cannot be sufficiently rigid to allow the adoption of a universal black and white classification scheme Timber needs on average 60−70 years for regrowth, although some species grow to full size within 10−20 years In the wood energy cycle, a tree is felled and burnt for energy conversion within a maximum of a few months, and this cycle occurs on a timescale which is significantly shorter than its "energy flow rate" However, if proper forestry management techniques are employed so that sufficient forest regrowth is ensured, wood can be, and is at the moment considered to be, a renewable energy resource

The above discussion does not take into account the environmental impact of the energy cycle, such as the greenhouse effect and the release of toxic fumes, undesirable chemicals or dangerous radiation into the atmosphere Ideally, the environment must include the biosphere of the Earth as well as all other resources directly required for our existence In order to survive we have to make sure not to adversely alter it Consequently, a positive attitude toward renewable energy resources is essential and ultimately only the use of this kind

of energy will be acceptable

2.2 Types of solar energy

(a) Thermal Solar Energy Conversion

The most commonly encountered domestic type of solar energy usage involves the use of solar radiation to directly heat water This method is observed as black patches on roofs of houses and is only used to heat water for domestic purposes, for which otherwise electricity or natural gas would be used

(b) Thermoelectric Solar Energy Conversion

Solar energy is converted into heat which in turn is converted into electricity

In this method, conventional fuels, such as coal or natural gas, commonly used

to heat water and to generate the steam required to rotate electricity producing turbines in power plants, are replaced by solar energy This technology requires large sun collection systems and has been applied to construct small to medium sized power plants

(c) Photoelectric Solar Energy Conversion

The direct conversion of solar energy into electricity is the best known form

of solar energy conversion This method requires devices such as solar cells

(d) Chemical Solar Energy Conversion

This path represents the conversion of solar energy into chemical energy and

is important because of its potential to overcome problems with long term storage and transport of energy Use of endergonic reactions (with a positive change in Gibb's free energy) are especially suited for this purpose Two examples of photochemical reactions of this kind are the concerted Paterno-Büchi rearrangement reaction (see below) and photocatalytic splitting of water into hydrogen and oxygen

A classical example for the former type of reaction is the norbornadiene cycle [33, 34] shown in Figure 2.1 Norbornadiene is photoisomerized via an endergonic 2π+2π Paterno-Büchi cycloaddition to quadricyclene The energy

of one photon is thereby stored in one molecule of quadricyclene, which is an energetically higher state than the starting material Thus, by choosing an appropriate catalyst [35] the reaction barrier (Eb*, see Figure 2.1 right) can be lowered to allow the exothermic back reaction to take place Via use of this mechanism, the system can store energy at a high volumetric energy density of approximately 1000Jcm−3

hν = Ef*

activated complex

Eb*product(quadricyclene)

LIGHTstorage

Fig 2.1 Schematic diagram of the norbornadiene/quadricyclene energy cycle Right: energy diagram of reaction, Ef* =

formation energy for activated complex = minimum solar energy needed for conversion, ∆E = energy gained by reaction reversal, Eb* = energy barrier

This very elegant reaction combines organic photochemistry, catalysis and solar technology The simplicity of the scheme is striking and it is unfortunate that large scale applications have never been realized

2.3 Solar radiation and air mass

The energy of the sun is created by the nuclear fusion reaction of hydrogen and helium which occurs inside the sun at several million degrees The mass difference that occurs in this process is converted into energy The hot sun is in radiation equilibrium with the cold universe, and this gives rise to its surface temperature of 5800K Because all elements are ionized to some degree at this temperature, their spectral lines are strongly broadened so that the gaseous surface of the sun radiates like a black body The solar energy that reaches Earth is determined by the radiation of the sun and the distance between the Earth and the sun The solar radiation power just outside the Earth's atmosphere

is 1.367kWm−2[36] and this value is known as the solar constant

Radiation is partly absorbed and scattered during the course of its journey through the atmosphere Water and carbon dioxide absorb in the infrared radiation and ozone absorbs ultraviolet radiation Scattering of radiation is

caused by Rayleigh scattering by molecules, Mie scattering by aerosols and cirrus scattering by clouds For example, in central Europe (Germany) on a

cloudless summer day with the sun at the zenith, up to 70% of the total solar radiation can be due to diffuse (scattered and reflected) light and only 30% of light actually hits the surface perpendicular to the sun's path Furthermore, due

to the nature of scattering, the diffuse light component contains a higher fraction of the higher energy ultra violet radiation Thus, solar cell devices depending on the light arriving at a certain angle will be significantly limited in efficiency so that cells that can collect solar radiation over a large angle and are tuned for optimal performance under diffuse light conditions will be preferred

Specific solar radiation conditions are defined by the Air Mass (AM) value

The spectral distribution and total flux of radiation just outside the Earth's atmosphere, similar to the radiation of a black body of 5800K, has been defined

as AM-0 In passing through the atmosphere the radiation becomes attenuated

by complex and varying extinction processes mentioned above At the equator

at sea level at noon when the incidence of sunlight is vertical (α=90°, sun in zenith) and the light travels the shortest distance through the atmosphere and air ("air-mass") to the surface, the spectral solar radiance and flux (1.07kWm-2)

is defined as AM-1 However, if the angle of light incidence is smaller than 90°, the light has to travel through more air-mass than under AM-1 conditions The relative pathlength through the atmosphere by the shortest geometrical path is given by:

αsin

AM p

p AM

o

)(

The so-called AM-1.5 conditions are achieved when the sun is at an angle of 41.8° above the horizon and results in the spectral distribution shown in Figure 2.2 and a solar flux of 963Wm-2 This angle of incidence is commonly encountered in western countries and hence AM-1.5 is taken as a standard condition for solar cell testing and referencing In the last few years, the AM-1.5 spectrum has been standardized by both the International Organization of Standardization (ISO 9845-1:1992) and by the American Society for Testing and Materials (ASTM E892-87:1992), although the latter standard is more commonly referred to in respect of solar cell testing For convenience, the flux

of the standardized AM-1.5 spectrum has been corrected to 1000Wm−2 However, despite availability of standards, great care has to be taken when results reported in the literature are compared For example, often AM-1.5 conditions are reported, but this may only mean that a radiance of 1000Wm−2has been used, with the proper spectral distribution being neglected

2.4 Thermodynamic efficiency limitations in photochemical conversion

The primary concern of photochemists and chemists when they run a reaction

is the mass yield of product In chemical reactions whose aim is to convert chemicals into fuels, in electrochemical reactions which convert electricity to chemical potential, or vice versa, and in photochemical reactions which convert light into chemical potential or work, the free energy yield is of equal importance The laws of thermodynamics impose limitations on the efficiency

of the conversion of light energy into chemical potential Free energy losses in the sequence of steps during a photochemical process have several origins that will be considered in the following order [38]: (1) non-equilibrium conditions

at maximum power, (2) entropy of the radiation source, (3) entropy increase on scattering or absorption of the original radiation, (4) inefficiency of polychromatic radiation Further limitations associated to the storage of the chemical potential will not be discussed here

2.4.1 Maximum power extraction

General to all reactions, whether photochemical or not, is the loss of free energy caused by non-equilibrium conditions due to finite power extraction Consider a chemical reaction in which a reactant A at chemical potential µA is

converted into a product B at chemical potential µB

500 1000 1500 2000 2500 3000 3500 4000 0.0

The rate of storage of chemical potential in the product B per unit of volume

is J⋅ µ B , where the flux J = – d[A]/dt = d[B]/dt If A and B are in equilibrium,

the rates of the forward and back reactions are equal and the net flux to the

product J = j f – j b = 0 Under non-equilibrium conditions where the forward reaction takes place with J > 0, there is a net overall entropy increase and µ B <

µA If K is the equilibrium constant for reaction A ' B in ideal conditions, the

change in chemical potential is given by the van’t Hoff isotherm:

∆µ = µB – µA = – RT ln K + RT ln (

] [

] [

A

B

By putting K = k f / k b , where k f and k b are the rate constants of reactions

A→B and B→Α , respectively, and by substituting the fluxes defined by jf = k f

[A] and j b = k b [B], one obtains the expression of the free energy loss in a

spontaneous reaction:

where ϕ = J / jf The conversion power P of the reaction is given by the rate of

production of chemical potential in the form of the product B at the potential

µB : P = J⋅ µ B = J⋅ (µA + ∆µ) At maximum power, the reaction flux J is given by:

A

µ ϕ ϕ

The amount of chemical potential converted in a photochemical reaction is typically of the order of 1–2 eV If µA = 1 eV, one calculates from the latter

equations ϕ = 0.972, ∆µ = – 0.093 eV, and ηp = 0.91

If the product B is involved in a leakage reaction to yield an undesired

product C with a rate constant k l, Equation (2.6) can be re-written :

A

µ ϕ ϕ

κ ϕ

with κ = kl / (k b + k l),, and the free energy conversion efficiency at maximum

power :

A A b

l p

k

k

µ

µ µ ϕ

Assuming a leakage reaction with k l = k b, and µA = 1 eV, one obtains κ = 0.5,

ϕ = 0.986, ∆µ = – 0.110 eV, and ηp = 0.89 If k l is increased by a factor of ten,

the efficiency decreases slightly to ηp = 0.85

2.4.2 Limitations due to the entropy of light

The fact that radiation possess entropy imposes additional constraints on the possible changes in a material system interacting with light These constraints determine, in particular, the efficiency of processes involving the utilization of radiant energy Let consider a photochemical reaction without leakage, where the only fates of the product A* are reaction to give the final product (with flux

i) and reverse reaction (with flux j b) The potential of the reactants is composed

of the chemical potential µA of A and the potential µR of the radiation or, by analogy with chemical potentials, the partial molar free energy of the absorbed light quanta

The change in potential during the light absorption process is given by:

∆µ = µA* - µA - µR = RT ln (1 – ϕ) (2.11)

If the radiation is monochromatic at wavelength λ, its total energy is QR

[J⋅ Einstein–1] = NA⋅ hc / λ , where NA is Avogadro’s number, and the entropy associated with it ∆SR = ∆Q R / T R The effective temperature T R of the monochromatic radiation of wavelength λ and of a given spectral irradiance

Iλ is expressed by the formula:

2 1 ln(

1

5 2

hc T

B

where Ω is the solid angle subtended by the source at the receiver (including

any optical concentrator) The spectral irradiance Iλ is the energy of the radiation incident on a unit area per unit time and unit wavelength interval at a given wavelength λ Thus, we may write the dimension of Iλ as, for example,

[Iλ] = W m–2 nm–1 Expression (2.12) for T R is the same as the Planck formula for a black-body giving the same spectral irradiance Iλ at the same wavelength

λ for unit wavelength interval and unit solid angle Thus rays of light propagating in a specified direction and delivering at the receiver a spectral

irradiance Iλ possess a temperature equal to that of a black-body emitting radiation and giving rise to the same irradiance

The entropy of the radiation ∆SR is lost when the light disappears in the

absorption process An equivalent amount of entropy must then be created in

the absorber at ambient temperature T A Therefore, the maximum energy

available to do work at temperature T A is given by:

R

A R R R

T

T T

R r

T

T T Q