Synthesis and structural optimization of functional photosensitizers for dye sensitized solar cells

ABSTRACT Since Gratzel and O'Regan reported high solar-cell performances for sensitized solar cells DSSCs based on polypyridyl ruthenium II complex dyes adsorbed on a nanocrystalline n-t

M.S Thesis

Synthesis and Structural Optimization of

Functional Photosensitizers for Dye-sensitized Solar Cells Graduate School of Yeungnam University

Department of Chemical Engineering Major in Chemical Engineering

LE THI THUY

Advisor: Professor Jae Hong Kim

February 2018

M.S Thesis

Synthesis and Structural Optimization of

Functional Photosensitizers for Dye-sensitized Solar Cells Advisor: Professor Jae Hong Kim

Committee member Kim, Jae Hong signature

Committee member Han, Yoon Soo signature

Graduate School of Yeungnam University

February 2018

Le Thi Thuy’s M.S Thesis is approved

ACKNOWLEDGEMENT

Firstly, I would like to express my deepest gratitude to my advisor, Professor Jae Hong Kim for his invaluable advice, motivation and never-ending encouragement throughout my studies His guidance always challenged me knowledgeably and provided a perfect atmosphere that I needed to grow as a researcher I strongly believe that without the help from Prof Kim I could not have enough will to live in Korea, not even mentioning about completing my thesis today

Besides my supervisor, I would like to appreciate other members of M.S thesis defense committee, Prof Kwang-Soon Ahn and Prof Yoon Soo Han for their contribution of time and understanding comments to improve the quality of this M.S thesis

Next, I am extremely thankful to Dr Suresh for his backing and support me in all time of research And I also would like to say thank you to members of LOFM for their assistance, support and friendship during the time I stayed in Korea Without them, I would face a lot of troubles in my experiments

Last but not least, I would like to thank my family for their unconditional love, reassurance and encouragement through all my life especially during the time I live overseas

February 2018

Le Thi Thuy

ABSTRACT

Since Gratzel and O'Regan reported high solar-cell performances for sensitized solar cells (DSSCs) based on polypyridyl ruthenium (II) complex dyes adsorbed on a nanocrystalline n-type semiconductor TiO2 electrode in 1991, DSSCs have expected extensive consideration as a new generation of maintainable photovoltaic devices because of their high incident-solar-light-to-electricity conversion efficiency, colorful and attractive natures, and low cost of production A characteristic DSSCs is created with a dye-absorbed wide band gap oxide semiconductor electrode, such as TiO2, ZnO, or NiO; a liquid electrolyte containing

dye-I-/I3- redox couples; and a platinum-coated counter electrode In the working electrode of the device, the dye sensitizer is very important factor; its function is light harvesting and electronic transition Up to now, the sensitizer can be divided into two general classes: the metal complex sensitizers and the metal-free organic sensitizers The DSSCs using the metal-complex sensitizer such as N3, N179 or black dye have been achieved high efficiency over 13% However, the metal-complex sensitizers have some difficulties such as limited resource, low molar extinction coefficient (ε) and high cost, which will limit their applications in DSSCs of large-scale To get rid

of these problems, the focus has been shifted to metal-free organic sensitizers owing

to compared with metal complexes, metal-free organic dyes have also attracted significant attention caused by the benefits of easier preparation and purification, higher structural flexibility, environmental kindliness and prevention of noble metals

This thesis began with the series of organic sensitizers triphenylamine-based metal-free organic dyes (D1-D3) with different electron acceptors, such as 2-cyanoacetic acid, rhodanine-3-acetic acid or 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid, connected through anthracene and thiophene π-spacers were synthesized and applied for DSSCs The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in DSSCs was measured Electrochemical studies showed that the LUMO energy levels can be tuned by changing the anchoring groups with different electron withdrawing ability The power conversion efficiencies of the DSSCs based on D1-D3 decrease as the electron withdrawing ability of their anchoring groups increase in the order of D1

< D2 < D3 and D1-based device showed the higher power conversion efficiency of 1.27%

In the study on the phenothiazine as an electron donor, three novel dyes with single donor-acceptor (T1), double donor-acceptor (T2) and multiple anchoring group (T3) organic dyes have been synthesized to investigate the influence of the hexyloxy benzene unit between the two chromophores and the different number of anchoring groups on the performance of DSSCs The T3 with double branches phenothiazine bases device shows a broader and higher IPCE as well as photo-current density (Jsc) with an improved photovoltage (Voc) In contrast, T1 with single branch presents a reasonably low IPCE within the whole spectral region, along with

Jsc and Voc of 10.51mA/cm2 and 0.67 V, respectively The higher Jsc and Voc gained

with the device based on T2 dyes and show a highest conversion efficiency of 5.02%

TABLE OF CONTENTS

Chapter 1: INTRODUCTION 1

1.1 Background 1

1.2 Introduction of dye-sensitized solar cells (DSSCs) 4

1.2.1 Operating principle of dye-sensitized solar cells 5

1.2.2 Fabrication of DSSCs 6

1.3 Scope of work and research objective 8

Chapter 2: CHARACTERIZATION OF DYE-SENSITIZED SOLAR CELLS 9 2.1 Key component of dye-sensitized solar cells 9

2.1.1 Transparent conducting glass 10

2.1.2 TiO 2 as the photoelectrode 10

2.1.3 Dye Sensitizer 11

2.1.4 Electrolyte 13

2.1.5 Counter electrode 15

2.2 Key efficiency parameters of dye-sensitized solar cells 16

2.2.1 Incident photon to current conversion efficiency (IPCE) 16

2.2.2 Current-voltage characteristics (J/V curves) 17

2.2.3 Electrochemical impedance spectroscopy (EIS) of DSSCs 19

Chapter 3: EFFECT OF ANCHORING GROUP IN ANTHRACENE/THIOPHENE-BRIDGED TRIPHENYLAMINE BASED ORGANIC DYES FOR DYE-SENSITIZED 22

3.1 Introduction 22

3.2 Experiment details 24

3.2.1 Materials and instruments 24

3.2.2 Synthesis 25

3.2.3 Assembly and Characterization of the DSSCs 29

3.3 Results and discussion 30

3.3.1 Design and synthesis 30

3.3.2 Optical properties 31

3.3.3 Electrochemcal properties 34

3.3.4 Photovoltaic properties 35

3.4 Conclusion 39

Chapter 4: SYNTHESIS AND PHOTOVOLTAIC PERFORMANCE OF NOVEL PHENOTHIAZINE SENSITIZERS CONTAINING HEXYLOXY BENZENE UNIT AND MUTI -ACCEPTOR FOR DYE-SENSITIZED SOLAR CELLS 41

4.1 Introduction 41

4.2 Experimental 44

4.2.1 Synthesis 44

4.2.2 Instrumental Analysis 50

4.2.3 Assembly and Characterization of the DSSCs 50

4.3 Results and discussion 52

4.3.1 Absorption properties in solution and on TiO 2 films 52

4.3.2 Electrochemcal properties 54

4.3.3 Photovoltaic performances of the DSSCs 55

4.4 Conclusions 63

Chapter 5: CONCLUSION 65

REFERENTS 67

LIST OF FIGURES

Figure 1.1 Best research-cell efficiencies 3

Figure 1.2 Schematic representation of operational principles of DSSCs 5

Figure 1.3 Fabrication process of DSSCs 7

Figure 2.1 Typical configuration of a DSSCs 9

Figure 2.2 Molecular design of a D–π–A organic dye sensitizer for DSSCs 11

Figure 2.3 A typical ICPE spectrum of a DSSCs 16

Figure 2.4 A typical J/V curve of a DSSCs 17

Figure 2 5 Electrochemical impedance spectroscopy of DSSCs 20

Figure 3.1 Chemical structures of dye sensitizers D1-D3 24

Figure 3.2 Scheme of preparation route for dye sensitizers D1-D3 25

Figure 3.3 UV-vis absorption spectra of dye sensitizers D1-D3 in solution 32

Figure 3.4 UV-vis absorption spectra of dye sensitizers D1-D3 on TiO2 film 33

Figure 3.5 Cyclic voltammogram of dye sensitizers D1-D3 35

Figure 3.6 IPCE spectra for the DSSCs based on dye sensitizers D1-D3 36

Figure 3.7 J-V curves for the DSSCs based on dye sensitizers D1-D3 37

Figure 3.8 EIS spectra of the dye sensitizers D1-D3 39

Figure 4.1 Chemical structures of dye sensitizers T1-T3 43

Figure 4.2 Scheme of preparation route for dye sensitizers T1, T2, and T3 44

Figure 4.3 UV-vis absorption spectra of dye sensitizers T1-T3 in solution 52

Figure 4.4 UV-vis absorption spectra of dye sensitizers T1-T3 on TiO 2 film 54

Figure 4 5 Cyclic voltammogram of dye sensitizers T1-T3 55 Figure 4.6 J-V curves for the DSSCs based on dye sensitizers T1-T3 56 Figure 4.7 IPCE spectra for the DSSCs based on dye sensitizers T1-T3 57 Figure 4.8 Electrochemical impedance spectra measured under illuminated condition 59 Figure 4.9 Electrochemical impedance spectra measured in the dark for DSSCs sensitized by T1-T3 60 Figure 4.10 Voc decay curves of DSSCs with T1- T3 based organic photosensitizers 62 Figure 4.11 The electron life-time derived as a function of Voc 63

LIST OF TABLES

Table 3.1 Photophysical and electrochemical data of dye sensitizers D1-D3 33

Table 3.2 Photovoltaic data of the DSSCs based on dye sensitizers D1-D3 38

Table 4.1 Photophysical and electrochemical data of dye sensitizers T1-T3 53

Table 4.2 Photovoltaic data of the DSSCs based on dye sensitizers T1-T3 57

Table 4.3 EIS analysis of the DSSCs under illumination condition 60

Table 4.4 EIS analysis of the DSSCs under dark condition 61

Chapter 1: INTRODUCTION

1.1 Background

As the world is converting more progressive in economy and technology, more energy is being consumed to keep up with the growth and demand on energy boomed over past decades Currently, the energy demands are still highly dependent

on fossil fuels, natural gases and coal with percentages of 36.4%, 23.5% and 27.8%, respectively [1] Nevertheless, the world will shortly come to an end of fossil fuels due to its non-renewable Meanwhile, the extravagant use of fossil fuels actually causes irreparable environmental destruction, geopolitical pressures, and disastrously weather changes [2] The Sun is a winner among all energy sources, and the Earth obtains 174 petawatts (PW) of incoming solar radiation at the upper atmosphere in a year The total solar energy absorbed by the Earth’s surface is approximately 3850 zettajoules (ZJ) per year, which is more energy in one hour than what the world used

in one year The amount of solar energy reaching the surface of the planet is so enormous that in one year it is about twice as much as what will ever be obtained from all of the Earth’s non-renewable resources of coal, oil, natural gas, and mined uranium combined A solar cell, or photovoltaic cell (PV), is a device that converts sunlight directly into electricity by taking advantage of the photoelectric effect Among all the renewable energy technologies, photovoltaic technology is considered

as the most promising one Solar PV has been turned into a multi-billion, growing industry, and the most potential of any renewable technologies The

fast-abundant, clean, safe, and affordable photovoltaic technology has been considered to

be the most promising one among all the novel energy technologies [3]

Solar cell technologies are traditionally divided into three generations First generation solar cells are mainly based on silicon wafers and typically demonstrate a performance about 15-20 % These types of solar cells dominate the market and are mainly those seen on rooftops The benefits of this solar cell technology lie in their good performance, as well as their high stability However, they are rigid and require

a lot of energy in production The second generation solar cells are based on amorphous silicon, CIGS and CdTe, where the typical performance is 10-15% Since the second generation solar cells avoid use of silicon wafers and have a lower material consumption it has been possible to reduce production costs of these types

of solar cells compared to the first generation The second generation solar cells can also be produced so they are flexible to some degree However, as the production of second generation solar cells still include vacuum processes and high temperature treatments, there is still a large energy consumption associated with the production of these solar cells Further, the second generation solar cells are based on scarce elements and this is a limiting factor in the price Third generation solar cells use organic materials such as small molecules or polymers Thus, polymer solar cells are

a sub category of organic solar cells The third generation also covers expensive high performance experimental multi-junction solar cells which hold the world record in solar cell performance This type has only to some extent a commercial application because of the very high production price A new class of thin film solar cells

currently under investigation is perovskite solar cells and show huge potential with record efficiencies beyond 20% on very small area Polymer solar cells or plastic solar cells, on the other hand, offer several advantages such as a simple, quick and inexpensive large-scale production and use of materials that are readily available and potentially inexpensive Polymer solar cells can be fabricated with well-known industrial roll-to-roll technologies that can be compared to the printing of newspapers Although the performance and stability of third generation solar cells is still limited compared to first and second generation solar cells, they have great potential and are already commercialized Research interest in polymer solar cells has increased significantly in recent years and it is now possible to produce them at a price that enables projects such as the free OPV initiative [4]

Dye-sensitized solar cells (DSSCs) is a major of third generation solar cells, it

is a photovoltaic devices in which a dye is used as the light absorber and a

Figure 1.1 Best research-cell efficiencies

semiconductor electrode (essentially titanium) allows for the charge separation and the transport of the electron to the external circuit This technology offers the following advantages: low production and investment cost, flexible design opportunities and feedstock availability to large scale application

The history of the dye-sensitized concept started in the late 1960s by sensitization of an organic dye, perylene, with an n-type zinc oxide semiconductor, which showed very poor power con-version Since then much effort has been put in

to improve the power conversion efficiency Major revolution came in 1991 when Gratzel and O’Regan reported a DSSC device with an efficiency of 7.1% [5] Thenceforward, DSSCs are being considered to be a prospective alternative to expensive conventional inorganic solar cells Unlike silicon solar cells, electrons and holes in a DSSC are transported in two different phases, TiO2 and electrolyte respectively, and because of which the chances of recombination in the cell become low Besides, DSSCs do not require ultrahigh pure materials unlike inorganic solar cells

1.2 Introduction of dye-sensitized solar cells (DSSCs)

A DSSCs consists of a photoanode, which is made up of a wide band gap semiconductor (TiO2, SnO2, ZnO, etc.) with a monolayer of dye molecule adsorbed

on it, an electrolyte (tri-iodide and iodide redox couple) and a conductive substrate coated with a catalyst (Pt, carbon, etc.) as cathode The wide band gap semiconductor, which absorbs ultra-violet light, is sensitized with the dye molecule that absorbs maximum in the visible range of the solar spectrum and hence, makes efficient use of

the sunlight The nanoparticles of the semiconductor provide a large surface area for adsorption of the dye on it, leading to absorption of sufficient amount of light by the photoanode However, these nanoparticles of the semiconductor have to be sintered together in order to have electronic contact between the particles and allow electronic conduction through the layer The porous semiconductor (TiO2) layer is made on a conducting glass substrate (F:SnO2/FTO coated glass), which is externally connected

to the cathode The cathode is again a conductive glass substrate with a catalyst such

as Pt deposited on it The dye molecules commonly used in DSSCs are polypyridyl complexes of ruthenium and osmium Iodide and tri-iodide redox couple in acetonitrile is a popular liquid electrolyte used in DSSCs

1.2.1 Operating principle of dye-sensitized solar cells

Figure 1.2 Schematic representation of operational principles of DSSCs

The generation of photocurrents in the DSSC occurs through the processes shown in the Figure 1.2 Initially the dye sensitizer absorbs a photon (sunlight) to generate the photoexcited state of the dye (1), then the photoexcited dye injects an

electron into the conduction band [-0.5 V vs a normal hydrogen electrode (NHE)] of TiO2 [(2); the dye sensitizer absorbs sunlight (hυ), by which an electron is usually excited from the HOMO to the LUMO of the dye and then the photogenerated electron is injected from the LUMO of the dye to the conduction band of TiO2] The resulting oxidized dye is subsequently reduced back to its original neutral state through electron donation from the I- ions in the redox mediator; this process is usually called dye regeneration or re-reduction (3) The injected electrons move through the network of interconnected TiO2 nanoparticles to arrive at the fluorine-doped tin oxide (FTO) and then through the external circuit to the counter electrode (Pt-coated glass) The I- ion is regenerated by the reduction of I3- at the counter electrode through the donation of electrons from the external circuit (4) and then the circuit is completed

During this electron flow cycle, however, there are undesirable side processes: the electrons injected into the conduction band of the TiO2 electrode may recombine either with oxidized dye [(5); recombination] or with I3- at the TiO2 surface [(6); dark current], and radiation less relaxation of the photoexcited dye [(7); decay], resulting

in lowering of the photovoltaic performances of DSSCs [6]

1.2.2 Fabrication of DSSCs

The fluorine-doped tin oxide (FTO) transparent conducting glasses (Pilkington, 15 U/cm2) were cleaned with methanol, D.I water and acetone The cleaned FTO glasses (Pilkington, 15 U/cm2) were coated with transparent TiO2

pastes (20-30 nm in diameter, Dyesol Ltd.) using the doctor blade technique,

followed by sintering at 450 0C for 30 min The TiO2 particle scattering layer (200

nm in diameter, Dyesol Ltd.) was deposited on the transparent nanoporous TiO2

films, followed by sintering at 450 0C for 30 min Two layers of TiO2 films were treated with a 40 mM of a TiCl4 aqueous solution at 70 0C for 30 min and then sintered at 450 0C for 30 min After cooling to 100 0C, the TiO2 films were immersed

in dye solutions at 25 0C for 24 h in the dark and the residual dye was rinsed off with acetonitrile to provide the working electrode The platinum paste was deposited on the FTO glasses using the doctor blade technique, followed by sintering at 450 0C for

30 min to give the counter electrodes The working electrodes and Pt counter electrodes were assembled into a sealed sandwich cell with a 60 mm thick Surlyn film (Dupont), which was then filled with an electrolyte solution through pre-drilled holes on the Pt counter electrode The electrolyte was used for the devices depend on the conditions of the fabrications

Figure 1.3 Fabrication process of DSSCs

1.3 Scope of work and research objective

The photovoltaic performances of DSSCs have been progressed by applying new metal free organic dyes, optimizing the device components and carrying out some fundamental studies The most important advantages of using organic dyes as sensitizers in DSSCs include their easily tunable physicochemical properties, through suitable molecular design and well established synthetic procedures, along with their ease of purification and high molar absorption coefficients The objective of the present work is basically to improve the power conversion efficiency of the DSSCs device by discovering novel organic dyes In addition, this research assists to achieve more understanding into structure property relationship of organic dyes, and the photovoltaic performance of DSSCs devices based on these organic sensitizers The ultimate target is to reach high conversion efficiency in DSSCs based on organic dyes, while retaining their stability under standard reporting conditions

Chapter 2: CHARACTERIZATION OF DYE-SENSITIZED

SOLAR CELLS

2.1 Key component of dye-sensitized solar cells

The DSSCs is composed of a photoanode and a photoinert counter electrode (cathode) sandwiching a redox mediator It consists of five materials: FTO glass substrate, semiconducting electrode, a dye sensitizer, an electrolyte and a counter electrode covered with sealing Surlyn A schematic representation of the dye-sensitized solar cell is showed in Figure 2.1

Figure 2.1 Typical configuration of a DSSCs

2.1.1 Transparent conducting glass

In the front of the DSSCs there is a layer of glass substrate, on top of which covers a thin layer of transparent conducting layer This layer is crucial since it allows sunlight penetrating into the cell while conducting electron carriers to outer circuit Transparent Conductive Oxide (TCO) substrates are adopted, including F-doped or In-doped tin oxide (FTO or ITO) and Aluminum-doped zinc oxide (AZO), which satisfy both requirements ITO performs best among all TCO substrates However, because ITO contains rare, toxic and expensive metal materials, some research groups replace ITO with FTO AZO thin films are also widely studied because the materials are cheap, nontoxic and easy to obtain [7]

2.1.2 TiO 2 as the photoelectrode

Many wide bandgap oxide semiconductors (TiO2, ZnO, SnO2 …) have been examined as potential electron acceptors for DSSCs TiO2 turned out to the most versatile, delivering the highest solar-conversion efficiency TiO2 is chemically stable, non-toxic and readily available in vast quantities It is the basic component of

white paints [8]

TiO2 photoelectrode properties favors natural pigments as sensitized of DSSCs because the conduction band of TiO2 photoelectrode coincides well with the excited level (LUMO) of natural pigments (specially with anthocyanins) The match

in conduction band and LUMO energy levels is an important condition for efficient electrons injection from the dye to the semiconductor photoelectrode to occur [9]

In the standard version of DSSCs, typically film thickness is 2-15μm and the films are deposited using nanosized particles of 10-30 nm The highest solar conversion efficiency is obtained in double layer structures, where an under layer of thickness 2-4 μm is first deposited using larger (200-300 nm) size particles

The most common techniques for the preparation of TiO2 electrodes are doctor blade technique, screen printing, electrophoretic deposition and tape casting method [8]

2.1.3 Dye Sensitizer

In the DSSCs component, the dye sensitizer is very important factor; its function is light harvesting and electronic transition Nowadays, the dye sensitizer can be divided in to two general classes: the metal complex sensitizer and the metal-free sensitizer The metal complex sensitizers (N3, N719 or black dye…) have been achieved high efficiency, over 13%

Figure 2.2 Molecular design of a D–π–A organic dye sensitizer for DSSCs

However, the metal-complex sensitizers have some problem, such as limited resource, low molar extension coefficient (ε) and expensive, which will limit their application in DSSCs of large scale To get rid of these problems, the metal organic sensitizers have been developed for DSSCs Compared with rare and expensive metal complex, organic dye has the advantages of being eco-friendly, having flexible and diverse form of molecular structures, easier preparation and fabrication The organic dye using for DSSCs should be have D-π-A form, the structure show in the Figure 2.2

For the designing of a new sensitizer, it should fulfil the following essential requirements:

 The sensitizer should be strong with broad absorption, it cover the whole visible region and even the part of the near IR region

 The molar extinction coefficient (ɛ) of the sensitizer should be high to avoid multi-layer adsorption of the sensitizer on the semiconductor surface

 The sensitizer should have anchoring groups such as -COOH, -SO3H etc to strongly bind the dye onto the semiconductor surface

 The excited state life-time of the sensitizer should be a sufficiently long span (typically in the ns domain), and excited electrons of the sensitizer should be efficiently injected into the conduction band of the semiconductor to avoid the decay of the excited state dye to the ground state

 The excited state level sensitizer should be more positive energy than the conduction band edge of the semiconductor (n-type DSSCs) so that an

efficient electron transfer process between the excited dye and conduction band of the semiconductor takes place

 For dye regeneration, the oxidation state level of the sensitizer must be more positive (by ca 200–300 mV) than the redox potential of the electrolyte

 Unfavorable dye aggregation on the semiconductor surface should be avoided through optimization of the molecular structure of the dye or by addition of co-absorbers that prevent aggregation

 The sensitizer should be stable electrochemically, photochemically and thermally for longer periods

 The sensitizer should have good solubility in a variety of solvents, be less hazardous, and be low cost and abundantly available [10]

2.1.4 Electrolyte

The electrolyte plays a very important role in the DSSCs by facilitating the transport of charger between the working electrode and counter electrode In general, following are the criteria for materials to serve as electrolytes in DSSCs:

 The redox potential of electrolyte should be negative as compared to the oxidation potential or HOMO of the dye

 The electrolyte should efficiently regenerate the dye after the process of dye excitation and electron injection to conduction band of oxide semiconductor

 It should have high conductivity (~10–3 S.cm–1)

 It should infiltrate the pores of the photoanode and establish contact with both the electrodes

 It should not cause desorption of the dye from the photoanode

 It should not react with the sealant and degrade it leading to poor stability of the cell

 The absorption of light by the electrolyte in the visible range, in which dye molecules absorb, should be minimum

 It should not undergo any chemical change leading to loss of its functionality

 It should be stable up to ~80 0C [11]

An organic electrolyte consists of a redox couple, a solvent and additives Among them, the redox couple is the most important component since it is directly linked to the open circuit voltage (Voc) of DSSCs There are many redox couples such I-/I3-, Br-/Br3-, SCN-/(SCN)2, SeCN-/(SeCN)-

3 etc However, the most popular redox couple is I3-/I-, because it has better solubility, fast dye regeneration process, low light absorption in the visible region, appropriate redox potential, and a very slow recombination rate between the TiO2 injected electrons and I3- [9]

The solvent is responsible for the diffusion and dissolution of the I-/I3- ions A number of solvents had been studied in DSSCs, such as acrylonitrile (AcN), ethylenecarbonate (EC), propylene carbonate (PC), 3-methoxypropionitrile (MePN) and N-methylpyrrolidone (NMP) Each particular solvent has its own donor number, and contributes to the Voc and short-circuit photocurrent (Jsc) A solvent with a high donor number can increase the Voc and decrease the Jsc by lowering concentration of

I3- The lower I3- concentration helps to slow the recombination rate and as a result, it increases the Voc [9]

2.1.5 Counter electrode

The counter electrode is where the redox mediator reduction occurs The oxidized ions in electrolyte diffuse toward the counter electrode and accept electrons from the external circuit Requirements of a material to be used as a counter electrode in DSSCs is to have a low charge transfer resistance, optimum thickness, high surface area, porous nature good adhesiveness to the transparent conducting oxide, high reflectance of transmitted light, good electrochemical stability in the electrolyte and high exchange current density At the same time, it aids to carry the photocurrent over the whole width of the DSSCs Therefore, the counter electrode must be a good conductor and it needs to have a low overvoltage for the reduction of the redox mediator

Platinum (Pt), thus far, is the preferred material for the counter electrode since

it is an excellent catalyst for I3 –

reduction The Pt counter electrode is ~200 nm in thickness, and it can be fabricated by sputtering, screen printing or pyrolysis of

H2PtCl6 solution onto the FTO substrate The platinized TCO substrate exhibits electrocatalytic activity, which improves the reduction of I3- by facilitating electron exchange, and it has a high light reflection due to the mirror-like effect of Pt However, Pt is a rare metal, hence not cost effective for large scale production Besides the high cost Pt corrodes with the redox mediator I3- which leads to the generation of platinum iodides like PtI4 which is undesirable This means the Pt counter electrode has a durability issue Therefore, other materials such as carbon

nanotube,graphite, conductive polymer etc., are being investigated as an alternative

to Pt [9]

2.2 Key efficiency parameters of dye-sensitized solar cells

2.2.1 Incident photon to current conversion efficiency (IPCE)

The incident photon to current conversion efficiency (IPCE) is a measure of the efficiency of the solar cell to convert the incoming photons to photocurrent at different wavelengths This is done by measuring the resulting photocurrent of the solar cell when illuminated by monochromatic light The IPCE is a measure of the product of different efficiencies such as light harvesting efficiency (LHE), the quantum yield of electron injection from the excited dye into the TiO2 conduction band ɸinj, the efficiency of regeneration ηreg, and the collection efficiency of the photo-generated charge carrier ηcoll

IPCE = LHE × ɸinj × ηreg × ηcoll (2.1)

Figure 2.3 A typical ICPE spectrum of a DSSCs

For calculating the IPCE experimentally one use the following equation:

𝐼𝑃𝐶𝐸(%) = 1240×𝐽𝑠𝑐 (𝑚𝐴𝑐𝑚−2)

𝜆(𝑛𝑚)×𝑃𝑖𝑛(𝑚𝑊𝑐𝑚 −2 ) (2.2) The IPCE spectrum is very useful for the evaluation of a new dye sensitizer for DSSCs A typical IPCE spectrum is shown in Figure 2.3 [6]

2.2.2 Current-voltage characteristics (J/V curves)

Measurement of the J/V curves under standard 1.5 AM simulated sunlight (100 mWcm-2) is an easy and useful method for the evaluation of the photovoltaic performance of a DSSC A typical J/V curve is depicted in Figure 2.4 The performances of DSSCs are universally represented by the following four key factors: Voc, Jsc, FF, and ɳ

Figure 2.4 A typical J/V curve of a DSSCs

1 Open-Circuit Photovoltage (V oc )

The Voc value is the difference in electrical potential between two terminals

of a cell under illumination when the circuit is open The Voc value can be expressed

2 Short-Circuit Photocurrent Density (J sc )

The Jsc value is the photocurrent per unit area (mAcm-2) when a DSSCs under irradiation is short-circuited The Voc value corresponds to the difference between Ef

of the electron in TiO2 and the redox potential of the electrolyte (I3-/I-), whereas Jsc is related to the interaction between TiO2 and the dye sensitizer, as well as the absorption coefficient of the dye sensitizer The Jsc value strongly depends on the photophysical and electrochemical properties and molecular structures of the dye sensitizers The Jsc value can be derived by integrating the IPCE spectra to give Equation (2.4):

𝐹𝐹 =𝐽𝑚𝑝 𝑉𝑚𝑝

𝐽𝑠𝑐𝑉𝑜𝑐 (2.5) The FF is determined from the J/V curve and is an indication of how much of the area of the rectangle for JscVoc is filled by that described by JmpVmp (Figure 2.4) Thus, the maximum FF value is unity However, the FF value is attenuated by the series resistance of the cell, which includes the sheet resistances of the substrate and counter electrode, electron transport resistance through the photoanode, ion transport resistance, and the charge transfer resistance at the counter electrode Therefore, careful fabrication of the cell is important for attaining high photovoltaic performance [6]

4 Solar Energy-to-Electricity Conversion Yield (ɳ)

The ɳ value of a DSSC is defined as the ratio of the maximum output electrical power of the DSSC to the energy of incident sunlight (I0) [Equation (4)] and is therefore determined by Voc, Jsc, FF, and I0 (generally AM 1.5, 100 mW cm-2)

ɳ(%) =𝐽𝑠𝑐 (𝑚𝐴𝑐𝑚 −2 𝑉𝑜𝑐(𝑉)𝐹𝐹

𝐼0(𝑚𝑊𝑐𝑚 −2 ) (2.6)

2.2.3 Electrochemical impedance spectroscopy (EIS) of DSSCs

Electrochemical impedance spectroscopy (EIS) is one of the most important tools to explain the charge transfer and transport processes in various electrochemical systems including DSSCs Impedance spectroscopy is a powerful method for characterizing the electrical properties of materials and their interfaces Analysis of EIS spectrum of a DSSC provides information about several important charge transports, transfer, and accumulation processes in the cell These are (i) charge transport due to electron diffusion through TiO2 and ionic diffusion in the electrolyte

solution; (ii) charge transfer due to electron back reaction at the FTO/electrolyte interface and recombination at the TiO2/electrolyte interface and the regeneration of the redox species at counter electrode/electrolyte interfaces; and (iii) charging of the capacitive elements in the cells including the interfaces, the conduction band, and surface states of the porous network of TiO2 [12]

Figure 2 5 Electrochemical impedance spectroscopy of DSSCs

The most widely used representation of the results of EIS measurements is a Nyquist plot Figure 2.5 represents a characteristics impedance spectrum (Nyquist plot) along with the electrical transmission line model of equivalent circuit for DSSCs The transmission line model is purely a combination of resistant (R) and capacitance (C).The ohmic serial resistance (Rs) corresponds to the electrolyte and FTO resistance The high frequency region represents the series resistance (Rce), corresponding to the diameter of the first semicircle; the larger semicircle in the mid-

frequency region reflects the charge transfer/recombination resistance (RTiO2) at the TiO2/dye/electrolyte interface [13]

Chapter 3: EFFECT OF ANCHORING GROUP IN

sensitizer is still one of the most crucial assignments to endorse the development of DSSCs [33]

It is well known that organic dye-sensitizers comprises of donor, bridge/spacer, and acceptor/anchoring units (D-π-A) and this structure is related with the potent intramolecular charge transfer (ICT), thus resulting in the facile charge transfer from excited dye sensitizer (via anchoring unit) into the semiconductor surface [34] For an excellent dye-sensitizer, selecting a proper anchoring unit is exceptionally crucial, which governs the binding strength of the dye-sensitize over the semiconductor surface and charge transfer rate [35-36] In most of the organic dye-sensitizers, 2-cyanoacetic acid (CA) is commonly employed into the D-π-A structure as an anchoring unit Also, there are few reports on the effect of different anchoring units on the D-π-A structure

π-In this work, we have synthesized three organic dyes which have triphenylamine as donor group with different acceptors in their charge-transfer chromophoric system for DSSCs application Anthracene and thiophene conjugate groups were inserted between the donor and acceptor units as the π-spacers, this extended π-conjugation can definitely enhance the light-harvesting effect The anchoring groups employed were 2-cyanoacetic acid (CA), rhodanine-3-acetic acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (OPCA) and Figure 3.1 shows corresponding dye molecular structures The UV-vis, electrochemical, incident photon-to-current conversion efficiency (IPCE), and currentdensity-photovoltage (J-V) curves were studied

Figure 3.1 Chemical structures of dye sensitizers D1-D3

3.2 Experiment details

3.2.1 Materials and instruments

All reactions were carried out under nitrogen atmosphere Solvents were distilled from appropriate reagents All starting materials and reagents were purchased from Sigma-Aldrich, TCI, and ACROS Co 1H spectra were recorded on Varian Mercury NMR 300 MHz spectrometer Chemical shifts were reported in parts per million down fields from tetramethylsilane (TMS) as an internal standard in appropriate deuterated solvents The optical spectra of dyes in solution were recorded with Agilent 8453 UV-vis spectrophotometer Electrochemical data were recorded

using CV-BAS-Epsilon The cyclic voltammogram curves were obtained from a three electrode cell in 0.1 M Bu4NPF6 chloroform solution at a scan rate of 100 mV

s-1, Pt wire as a counter electrode and an Ag/AgCl reference electrode

three times with chloroform The combined organic fractions were washed with brine and dried over anhydrous MgSO4 The solvent was removed under reduced pressure and the residue was recrystallized by using ethanol to give a white powder (3.020 g, 76%) 1H NMR (300 MHz, CDCl3): 7.62-7.21 (m, 7H), 7.19-7.00 (m, 5H), 6.99-6.93 (m, 2H)

2 N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (2)

Under an inert atmosphere, a degassed solution of 1 (3.000 g, 9.250 mmol), bis(pinacolato)diboron (3.054 g, 12.255 mmol), KOAc (3.018 g, 32.225 mmol), and Pd(dppf)2Cl2 (0.338 g, 0.05 mmol) in dry dimethoxyethane (30 mL) was heated at reflux for 15 h After this period, the mixture was cooled to room temperature, filtered, and diluted with CH2Cl2 (50 mL) The organic solution was washed with

H2O (2x30 mL) and brine, then dried (anhydrous MgSO4) and evaporated The residue was separated by column chromatography using hexane/CH2Cl2 (9/1 v/v) to give the compound 2 as white solid product (2.170 g, 63%) 1H NMR (300 MHz, CDCl3): 7.74-7.59 (d, 2H), 7.19-7.08 (d, 4H), 7.07-6.90 (t, 4H), 1.48-1.16 (t, 12H)

3 5-(10-bromoanthracen-9-yl)thiophene-2-carbaldehyde (3)

A 50 mL of three neck round bottom flask was charged with bromoanthracen-9-boronic acid (0.770 g, 2.000 mmol), Pd(PPh3)4 (0.232 g, 10 mol%), THF (20 mL) and 2 M aqueous K2CO3 (2 mL), then the flask was purged with nitrogen gas with 5 evacuate/refill cycles Then 5-bromo-2-thiophenecarboxaldehyde (0.460 g, 2.400 mmol) was added under inert atmosphere The tube was sealed and heated at 70 0C with vigorous stirring for 15 h Upon

10-cooling to ambient temperature, the organics were extracted three times with CH2Cl2 The combined organic fractions were washed with brine and dried over MgSO4 The solvent was removed under reduced pressure and the residue was purified by silicagel column chromatography using hexane/ CH2Cl2 (8/2, v/v) as eluent to give as

a yellow powder (0.420 g, 57%) 1H NMR (300 MHz, acetone-d6): 10.14 (s, 1H), 8.7-8.62 (d, 2H),8.25 (d, 2H), 7.88-7.77 (m, 4H), 7.68-7.57 (t, 2H), 7.53-7.46 (d, 1H)

4 5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophene-2-carbaldehyde (4)

To a mixture of compound 3 (0.918 g, 2.500 mmol), compound 2 (0.900 g, 2.500 mmol) and K2CO3 (1.029 g, 8.75 mmol) in toluene/ethanol (15/5 mL) was added Pd(PPh3)4 (0.347 mg, 0.3 mmol) under inert atmosphere After stirring for 24

h at 110 0C, water (10 mL) and dichloromethane (30 mL) were added The organic layer was separated, and the aqueous layer was extracted with dichloromethane (2×10 mL) The organic layer and the dichloromethane extracts were combined and dried (anhydrous MgSO4), and then filtered The organic solvent was completely removed by rotary evaporation The solid residue was purified by column chromatography using hexane/ CH2Cl2 (8/2, v/v) as eluent to give a pale yellow solid (0.916 g, 70%) 1H NMR (300 MHz, CDCl3): 10.08 (s, 1H), 8.21-7.97 (d, 1H), 7.90-7.75 (m, 5H), 7.50-7.40 (t, 5H), 7.39-7.20 (m, 10H), 7.15-7.02 (t, 3H)

5.(Z)-2-cyano-3-(5-(10-(4-(diphenylamino)phenyl)anthracen-9-yl)thiophen

2yl)acrylic acid (D1)

Compound 4, carbaldehyde (0.100 g, 0.188 mmol), dissolved in acetic acid (15 mL) was condensed