Novel Photovoltaic Devices Derived from Mechanically Robust, Network-forming Multiblock Ionomers

ABSTRACT

Al-MOHSIN, HEBA ABDULLAH. Novel Photovoltaic Devices Derived from Mechanically Robust, Network-forming Multiblock Ionomers (Under the direction of Drs. Richard Spontak and Ahmed El-Shafei)

The conversion of sun light to electrical power using photovoltaic device that are

based on organic materials depends sensitively on the molecules order arrangement

of these materials on the nanoscale. Block copolymers self-assembly are strong

candidate materials to enhance hybrid device performance. The formation of high

internal area and continuously connective material with periodicities on the 10 nm

length scale can be achieved by self-assembly of block copolymer. When some

blocks of conventional block copolymers contain ionic moieties, such as sulfonic acid

groups, they are termed as block ionomers. These ionomers have unique electrical

and morphological properties. The non-polar block is incompatible with the polar

ionic group that causes microphase separation between the different blocks in

backbone of the block copolymer. In this study, sulfonated Block ionomer (SBI), with

ionic block being in the middle block allows to control its swelling and contained the

mechanical strength. First of all, physically cross-linked hydrogels generated from

SBI applied with tow different photosensitive dyes to generate photovoltaic

elastomer gels (PVEGs) that are competitive with other biomimetic or ionic

photovoltaic systems reported recently. The sequentially order of introducing these

dyes to the SBI samples and the degree of sulfonation in the ionomers on the

light-harvesting performance of these photovoltaic elastomer gels are examined. In

addition, SBI films with both ionic block and rigid block show high mechanical

strengths. As well as, Raman spectroscopy and electron microscopy provide insight

into the nano- and atomic-scale mechanism of ion transport. The use of a pure polar

solvent (tetrahydrofuran, THF) resulted in coexisting nonpolar cylinders and lamellae.

In the second part, In order to elucidate the effect of the solvent polarity in the SBI

morphology and photovoltaic performance, films casted from a mixed nonpolar/polar

solvent (toluene/isopropyl alcohol, TIPA). The SBI formed a distinct morphology of

microphase-separated nano-domains with a continuous pathway through which ions

and other polar species could diffuse. As well, the solvent annealed SBI-cast films

were investigated. The later presented a lamellar structure. These transitions in

morphologies are reflected in the PVEGs performance. In addition, besides the

flexibility of the film, this dye-containing system yields solid-like gel electrolytes with

high tensile strength.

Finally, A demand to produce clean low-cost easily fabricated and reliable solar cells

has generated interest in using SBI in dye-sensitized solar cells (DSSCs). We

introduced a new approach by using a SBI examined as a polymer gel host for

iodine/iodide electrolytes for quasi-solid state DSSC. This polymer gel showed a

good penetration into TiO2 and high power conversion efficiency. Furthermore, we

established a relationship between the hydrophobicity of the gel and dye used in the

synthesis of DSSC. These results illuminate how to choose and produce the proper

components for high-efficiency DSSC. Also, it is clear that small portion of

Poly(ethylene glycol) dimethyl ether improved the current; however, it does not

Novel Photovoltaic Devices Derived from Mechanically Robust, Network-forming Multiblock Ionomers

by

Heba Abdullah Al-Mohsin

A discussion submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the Degree of

Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina

2015

APPROVED BY

________________________ ________________________

Richard Spontak Ahmed El-Shafei Committee Co-Chair Committee Co-Chair

________________________ ________________________

DEDICATION

This work is dedicated to my parents for their unconditional love and inspirational

prayers, to my parents-in-law for their encouraging and positive attitudes, to my dear

husband, Wajih, for his patience and support, and to my little ones Zahra and Fatima

for the delightful and cheerful moments they have created in my life.

BIOGRAPHY

I completed my B.S in Physics with honors from King Faisal University in Saudi

Arabia in 2003. I also completed a master’s Degree in Solar Energy at the Physics

Department of King Saud University (KSU) in Saudi Arabia in 2008. The title of my

thesis was "The Effect of Substrate and Buffering Layer on The Optical and

Structural Properties of GaN Thin Films." I worked for one year as a teaching

assistant in the physics laboratory of KSU. During that year, I learned a lot of skills

necessary to teach students and to work in the laboratory. I then received a full

scholarship from the Ministry of Higher Education of Saudi Arabia to pursue my Ph.D. in the U.S. I have attended NCSU since August 2010. First, I completed a

non-thesis master’s degree in Material Science and Engineering in 2012. After that, I

joined the Fiber and Polymer Science Program under the direction of Drs. Richard J.

Spontak and Ahmed El-Shafei to complete my Ph.D. degree. My goal is to use

ACKNOWLEDGEMENTS

First of all, I do indeed thank Allah, the most merciful, for his blessing. He guided

and aided me to bring this dissertation to light.

I would like to express my sincere thanks to my advisor Dr. Richared Spontak. It has

been an honor for me to complete this research under his guidance. Without his

encouragement and support, this work would never be possible. My special thanks

to Dr. Ahmed El-Shafei for giving me a chance to work with him in his lab and for his

advice in my research. My thanks to Dr. Orlin Velev and Dr. Xiangwu Zhang for serving on my committee. I would also like to thank all the members of the Polymer

Morphology Group with special thanks to Kenneth Minerrt and all the members of

Dr. El-Shafei’s group.

I am especially grateful to the Ministry of Higher Education of Saudi Arabia for a

doctoral fellowship and for Dr. Abdulaziz Rabatchi for his advice and support

regarding the fellowship. I owe a special thanks to all my family for their continuous

support, understanding, and encouragement during my studies. Also, my thanks to

all of my friends especially Israh Almeraj, Nawres Ali, and Eman Alhajji. Last and

foremost, I would like to express my deepest thanks to my husband and my

daughters for their sacrifice and love.

TABLE OF CONTENTS

LIST OF TABLES ... vii

LIST OF SCHEMES..………...viii

LIST Of FIGURES ... ix

Chapter 1. Literature review: Block copolymers roll in DSSC and hydrogelphotovoltaics ... 1

1-1 Introduction ... 2

1-2 DSSC mechanism of operation ... 7

1-3 DSSC components ... 8

1-3-1 Photoanode (working electrode) ... 9

1-3-2 Counter electrodes ... 12

1-3-3 Photosensitizers ... 16

1-3-4 Electrolytes ... 20

1-4 Block copolymer in DSSCs ... 26

1-4-1 Electrodes based on block copolymers ... 26

1-4-2 Solid-state DSSC by incorporating block copolymer ... 30

1-4-3 Block copolymer for quasi-solid state electrolytes ... 31

1-5 Water-based DSSC ... 33

1-6 Ionic photovoltaics ... 40

1-7 Evaluation of DSSC performance ... 42

1-8 Sulfonated block ionomer (SBI) ... 46

1-9 Conclusions ... 50

1-10 References ... 52

Chapter 2. Highly Flexible Aqueous Photovoltaic Elastomer Gels Derived from Sulfonated Block Ionomers ... 61

2-1 Introduction ... 62

2-2 Experimental section ... 64

2-3 Photovoltaic elastomer gels fabrication ... 66

2-4 Photovoltaic performances ... 67

2-5 Mechanical properties of sulfonated block ionomer ... 74

2-6 Morphology structure of sulfonated block ionomer ... 76

2-7 Conclusions ... 82

2-7 References ... 83

3-1 Introduction ... 87

3-2 Experimental section ... 91

3-2-1 Materials ... 91

3-2-2 Methods ... 92

3-3 Results and discussion ... 95

3-3-1 Morphological characteristics ... 95

3-3-2 Photovoltaic Performance ... 100

3-3-3 Mechanical Performance ... 112

3-4 Conclusions ... 113

3-5 References ... 115

Chapter 4. A Quasi-Solid-State Dye-Sensitized Solar Cell With Highly Flexible Multiblock Copolymer Gel Electrolyte ... 119

4-1 Introduction ... 120

4-2 Experimental section ... 124

4-2-1 Materials ... 124

4-2-2 Fabrication of Dye-Sensitized Solar Cells ... 124

4-2-3 Instrumentation ... 126

4-3 Results and discussion ... 127

4-3-1 SBI penetrations into TiO2 ... 127

4-3-2 Liquid electrolyte interaction with SBI ... 129

4-3-3 Degree of sulfonation effect on photovoltaic performance ... 130

4-3-4 The effect of dye polarity on DSSC performance ... 133

4-3-5 PEGDME effect ... 138

4-4 Conclusions ... 140

4-5 References ... 141

Chapter 5. Conclusions and future works…….……….………....145

APPENDIX………….. ... 152

I. Comparsion between liquid state and QS-DSSC. ... 153

II. Effect of iodine /iodide liquid electrolyte on mechanical properties of block copolymer. ... 154

III. Stability of QS-DSSC using SBI ... 155

LIST OF TABLES

Table 1-1 Confirmed efficiencies measured under the global AM1.5 spectrum (1000 W/m2) at 25 °C. AIST = Japanese National Institute of Advanced Industrial Science and

Technology………. 4

Table 1-2 Sulfonated pentablock copolymer as a function of ion exchange capacity and degree of sulfonation………. 49

Table 2-1 Specimen details and performance metrics of the PVEGs

investigated here……… 70

Table 3-1 Photovoltaic properties of PVEG prepared by casting solvents

varying in polarity………. 102

Table 3-2 Photovoltaic properties of PVEG prepared from cosolvents

varying in composition……… 105

Table 4-1 Photovoltaic characterizations for different degree of sulfonation of SBI specimens with N719 sensitizer………. 133

LIST OF SCHEMES

Scheme 3-1 Chemical structures of (a) the SBI, (b) 9,10-dimethoxy-2-anthracenesulfonic acid sodium salt [DAS- Na+] and (c) tris(2,2'-bipyridine) dichlororuthenium(II) hexahydate [(Ru(bpy)3)2+ _(Cl-_{)2]……… 90} Scheme 3-2 Illustrations of SBI-2.0-70TIPA depicted in the core-shell

LIST Of FIGURES

Figure 1-1 a) Different nanostructures produced by BCP self-assembly. b) Theoretical phase diagram of diblocks. Where f is the volume fraction of one component. χ is the Flory–Huggins interaction

parameter and N indicates the degree of polymerization. ……... 5

Figure 1-2 Number or research articles published per year in the field of block copolymers in photovoltaics. Inset: data for block copolymer in DSSCs. These data source from web of science on September 1st _{2015………... 6}

Figure 1-3 Scheme of operative sequence of dye-sensitized solar cells………..……….. 8

Figure 1-4 a) A cross-sectional view and a (b) top view of nanotube films prepared by anodizing Ti foil……….. 10

Figure 1-5 Schematic view of a photoanode with compact TiO2 layer……… 10

Figure 1-6 a) Relationship between the pressures applied to the TiO2 film and the plastic-substrate DSSC efficiency. b) Effect of an anti-reflective (AR) film on transmittance of ITO-PEN films…………. 12

Figure 1-7 Effect of carbon layer thickness on parameter performances of DSSC……….. 15

Figure 1-8 Proposed complexation mechanism between PPy and SWCNT……….. 16

Figure 1-9 Molecular structures of commercial Ru based metal complex dyes……… 18

Figure 1-10 Schematical structure of metal- free organic dye……… 19

Figure 1-11 Chemical Structures of chlorin e6.….……… 20

Figure 1-12 Schematic drawing of the Grotthus mechanism. ……… 22

Figure 1-13 The fabrication of DSSC cells containing carbon-TiO2 beads in the photoanode part………. 27

Figure 1-15 Schematic illustration of the formation of Yb3+-TiO2-HNPs……. 29 Figure 1-16 Figure 1-16. Schematic represented of gyroid network

replication from PFS-b-PLA templates and the assembly of hybrid solar cells………... 32

Figure 1-17 Long-term behavior of the solar-to-electricity energy conversion efficiency of the DSSCs employing the liquid electrolyte (LE), PHSMI-g- Jeffamine (G-UV), Jeffamine–PHSMA (M-UV), and PHSMA (H-UV) electrolytes after UV irradiation of 360

J/cm2………... 33

Figure 1-18 Electric circuit system of water-based dye-sensitized solar

cell……….. 35

Figure 1-19 Power conversion efficiency and short-circuit photocurrent density (Jsc) of the DSSC with and without 0.02 M Triton X-100 and 2.2 M water.……….. 36

Figure 1-20 One sun I–V curves vs water content for water-based DSSC…. 36 Figure 1-21 Schematic depicting the local-concentration control of the

I3−/I− redox couple by the hydrogen-bonding interaction between the surfactant and the carboxyl group of the dye……… 38

Figure 1-22 a) Molecular structures of hydrophobic dye (D35) and hydrophilic dye (V35). b) The estimation of the contact angles (θc), reveal the hydrophobicity of D35 and hydrophilicity of

V35……….. 40

Figure 1-23 Schematic of the hydrogel photovoltaic cells (HGPVs)………….. 41 Figure 1-24 a) Images illustrating the progressive infusion of dye and

electrolytes through the gel-vascular network at 5 and 15 min after injection of the solution and the injection rate is 10 ml/min. b) I–V curves of the µ-FGPVs………

42

Figure 1-25 Fill Factor From the I-V Sweep……….. 45 Figure 1-26 Structure of the sulfonated pentablock copolymer……….. 47 Figure 1-27 Ions Sorption in sulfonated pentablock………. 49 Figure 1-28 The mechanical properties of the sulfonated pentablock

Figure 2-1 Schematic illustration of the photovoltaic cell employed in this

study………... 67

Figure 2-2 In (a, c, d), current density presented as a function of voltage for three different case studies: (a) fabrication procedure (labeled), (c) block ionomer used (see Table 1) and the cycle number for SBI2.0 (labeled), and (d) dye concentrations (labeled as [Ru(bpy)3]/[DAS] and expressed in mM) used in conjunction with SBI2.0. In all cases, the lines serve to connect the data. The inset displays the dependence of the short-circuit current density (Jsc) on open-circuit voltage (Voc) extracted from the data in (a), and the line serves as a guide for the eye. Both dye concentrations in SBI2.0 are 5 mM in (a), and the sequential dye protocol is used to generate the specimens in (c) and (d). In (b), UV-Vis absorbance spectra acquired from the SBI2.0 material upon addition of each dye and combinations thereof (labeled). All the lines connect the data, and the dotted line corresponds to an aqueous solution of Ru(bpy)_{3 without} polymer. All data have been collected at a power density of 1 kW/m2 under AM1.5G standard light spectrum conditions……… ₇₇ Figure 2-3 Mechanical properties measured from the hydrated SBI2.0

system alone, as well as with either Ru(bpy)_{3 or a mixture of} both dyes (labeled). In (a), quasi-static uniaxial tensile tests provide the nominal stress as a function of strain. In (b), dynamic rheological analyses yield the frequency spectra of the storage and loss moduli (G' and G", labeled and identified by line type). Corresponding values of tan # deduced from the data in (b) are displayed in (c), and all the lines in (a) – (c) serve to connect the data. The inset in (c) shows the water uptake of SBI2.0 after addition of either Ru(bpy)3 _{at different} concentrations (?) or an equimolar (5 mM) mixture of both dyes (dashed line), and the solid line serves as a guide for the eye… 79

in diameter and appearing as small, dark specks. A schematic illustration (d) depicts the variety of complexes that can form between the dye molecules and charged units fixed on the hydrated midblock of the ionomer (identified in the figure

legend)……… 80

Figure 3-1 TEM micrographs of SBI-2.0 cast from (a) 85TIPA and (b) THF where dark regions are Pb-stained S/sS domains and light regions are unstained T and EP domains……… 96

Figure 3-2 Different slices for TIPA casted film at different depth (a) 10, (b) 20, and (c) 30 nm……… 97 Figure 3-3 2D TEM image for SBI immersed in DI water at room

temperature………... 98

Figure 3-4 1-D SAXS scattering profile for THF-and 70 TIPA- SBI-2.0

film……….. 100

Figure 3-5 The photocurrent density- voltage (J_V) curves of hydrogel elastomer photovoltaic (HGEPV) cell prepared by using 70 TIPA under 1 kW/m2(AM 1.5 G) irradiation at tow different dyes

concentration……… 102

Figure 3-6 a) Shows the photocurrent density –voltage characteristics, (b) 1-D SAXS scattering profile for SBI-2.0 films casted from different toluene concentration………... 104

Figure 3-7 a) The photocurrent density- voltage (J-V) curves of hydrogel elastomer photovoltaic (HGEPV) cell prepared by using 70 TIP SBI-2.0-TIPA Solvent annealing films under 1 kW/m2(AM 1.5 G) irradiation at tow different dyes concentration. b) TEM micrograph of an SBI-2.0 film cast from THF and subsequently vapor annealed using THF as the annealing solvent. Dark regions correspond to Pb-stained S/sS domains whereas light regions indicated unstained T and EP domains. (c) 1-D SAXS scattering profile (right) for THF vapor annealed SBI-2.0 films initially cast from 85TIPA and THF……… 107

Figure 3-9 Shows thickness effect on photovoltaic performance. The inset shows 5 mM Das in water solution at different thickness, and the dolid line serves as a guide for the eye……….. 111

Figure 3-10 Stress-strain curves of 70TIPA films with wet state, and

Ru(bpy)3………. 113

Figure 4-1 Schematic illustration of the QS-DSSC using Sulfonated block ionomer used in this study……….. 126

Figure 4-2 a) Cross-section SEM image of QS-DSSC with SBI-THF-HI 30 iodolyte. b) Shows relatively constant elemental concentrations within each respective domain……….. 129

Figure 4-3 1-D SAXS scattering profile for SBI 2.0- TIPA film and SBI 2.0 TIPA with ionic liquid (HI-30)……….. 130

Figure 4-4 a) The J_V curve of in situ gelation of SBI-THF solution and iodolyte (HI-30) with different degree of sulfonation using N 719 sensitizer under AM 11.5 illumination. b) IPCE Spectra of devices assembled with different degree of sulfonation…………. 132

Figure 4-5 Schematic represent the morphology of SBI with different degree of sulfonation and the liquid electrolyte in the ionic

domains………. 132

Figure 4-6 The chemical structures of the three different dyes that used in

this study……… 134

Figure 4-7 J-V curve of the QS-DSSC with SBI 2.0- THF and different

dyes……… 136

Figure 4-8 J-V curve of the QS-DSSC with SBI 2.0- TIPA and different

dyes……… 136

Figure 4-9 IPCE spectra of the QS-DSSC sensitized with N 719 and SBI cast from solvents differing in polarity……….. 137 Figure 4-10 IPCE spectra of the QS-DSSC sensitized with HD-15 and SBI

cast from solvents differing in polarity……….. 137 Figure 4-11 Characteristic of QS-DSSC based on SBI2.0-THF and N719

sensitizer With different PEGDME concentration under AM 1.5 illumination; a- short-circuit photocurrent density and solar energy conversion efficiency. b- fill factor and open-circuit

Chapter 1

1-1 Introduction

The global effort to identify alternative and sustainable energy resources to

overcome climate change caused by petroleum-based pollution has inspired great

scientific interest. Of those considered, solar energy is one of the most promising

candidates for producing renewable and clean energy. On average _∼120,000 TW

per year of the Sun's energy reaches Earth’s surface, which is about four orders of

magnitude larger than the current rate of the total power consumption of the world.1 Solar cells, also called photovoltaic cells, transform light into electricity through the

use of different materials. There are many avenues to produce solar energy, such as

amorphous or crystalline silicon cells, thin film technology, organic photovoltaics

(OPV), and hybrid cells including dye-sensitized solar cells (DSSCs). Tremendous

research efforts have been devoted to constructing lightweight, inexpensive,

environmentally friendly, and flexible photovoltaics.

Crystalline silicon (c-Si) dominates the market today, with thin-film technologies

based on cadmium-telluride (CdTe), copper–indium–gallium–selenide (CIGS) and

silicon possessing only 10% of the market.2 However, c-Si, which possesses a high PV efficiency, costs significantly more than amorphous-Si due to expensive

manufacturing considerations. On the other hand, thin-film silicon devices based on

either amorphous silicon (a-Si:H) or nanocrystalline silicon are less expensive but

have more defects than c-Si and suffers from performance degradation upon light

soaking which lowers the efficiency compare to c-Si.3

The lack of viable alternatives to silicon opens the door for organic materials to be

been devoted to organic photovoltaic (OPV) cells since the late 1970s. Continued

interest in using organic rather than inorganic materials reflects several key

advantages: they are lightweight, inexpensive, mechanically robust, flexible, and

environmentally friendly.4 A major difference between organic and inorganic solar cells is in their operational mechanisms.5 In an inorganic solar cell, the energy of an absorbed photon from incident light is used to excite an electron from the valence

band to the conduction band. This will generate an electron–hole pair. The key to

generating electricity is to then separate this pair and transport the electron to an

electrode. Conversely, in an organic solar cell, the electron–hole pair is bound

together and its constituents are not free to move separately. Dissociation of these

pairs can be achieved upon application of high electric fields or at the interface

between two materials with different energy levels.

Another technology for utilizing sunlight is bioelectrochemical solar energy

conversion in which artificial photosynthesis is used to produce energy.

Photosynthesis works by producing an electron-hole pair upon absorption of sunlight

to generate fuels. The electron is transferred to photosystem I, while the hole is used

to oxidize water and release hydrogen as a source of fuel.6 This can be achieved by different technologies,7 _{though artificial photosynthetic systems in general require} dramatic improvements in efficiency and durability before they can be considered for

practical applications, which is a great challenge.8-9

A high-potential technology that has been intensively studied in the past two

decades is dye-sensitized solar cells (DSSCs). The ability of DSSCs to mimic

scientists. Unlike traditional solar cells, DSSCs are able to work successfully in

low-light conditions and are less susceptible to thermal energy loss.10 Grätzel and co-workers,11 _{reported the first successful cell in 1991. This cell was developed through} the introduction of mesoporous titanium dioxide (TiO2). In general, DSSC

components include an electrode, a photosensitizer and an electrolyte. Each

component plays a role in the performance of the DSSC.12-13 A comparison between different types of photovoltaics in terms of efficiency is summarized in Table 1-1.14

Table 1-1. Confirmed efficiencies measured under the global AM1.5 (1000 W/m2) spectrum at 25°C. AIST= Japanese national institute of advanced industrial science and technology.

Classification Efficiency %

Area (cm2)

Voc

(V)

Jsc

(mA/cm2) Fill factor %

Test center Si (crystalline) 25.6±0.5 143.7 0.74 41.8 82.7 AIST

Si (AMORPHOUS) 10.2±0.3 1.001 0.89 16.36 69.8 AIST

Organic thin film 11.0±0.3 .993 0.79 19.40 71.4 AIST

DSSC 11.9±0.4 1.005 0.74 22.47 71.2 AIST

Candidate materials for photovoltaic devices that are non-toxic, low cost and

environmentally friendly are block copolymers (BCP). Block copolymers consist of

blocks of different homopolymers. A diblock copolymer is a block copolymer made of

two types of homopolymers, A and B. The existence of a covalent bond between A

nanodomains can be categorized into body-centered-cubic spheres, hexagonally

packed cylinders, bicontinuous gyroids, and lamellae as shown in Figure (1-1,a).16 These morphologies are strongly dependent on the Flory–Huggins interaction

parameter (χ), the volume fraction of the components (f), and the degree of

polymerization (N) and can be depicted on Phase diagram as the one shown in

Figure (1-1,b).

Figure 1-1. a) Different nanostructures produced by BCP self-assembly. b) Theoretical phase diagram of diblocks. Where f is the volume fraction of one component. χ is the Flory–Huggins interaction parameter and N indicates the degree of polymerization. Reproduced from reference [16].

These types of material, BCPs, have been used in organic photovoltaics (OPV)

since 1925. However, a large increase in the number of publications on this topic

has emerged in the last decade. Academic and industrial interests have heightened

the importance of BCP in photovoltaics. Between 2011-2014, the number of

Web of Science database using the keywords “block copolymer photovoltaic or solar

cell’’. BCPs have the potential to be used as donor–acceptor active layers known as

bulk heterojunctions (BHJ) in OPV,17 _{or as structure directors in hybrid solar cells,} including dye-sensitized cells, and a combination of these two approaches by mixing

conjugated polymers and inorganic semiconductors.18-19 By narrowing the search using a key word “ block copolymer and DSSC”, we found this research can be

consider as a new emerging topic, see Figure 1-2. Using BCP just start in the last decade and showed a staidly increasing in last few years.

Figure 1-2. Number of research articles published per year in the field of block copolymers in photovoltaics. (Web of Science, search performed September 1st 2015).

In this paper, we shall begin with an overview of the mechanisms of operation and

device components. Then, reviewing the role of block copolymers in these

components, and different electrolytes that have been used. Additionally, a brief

recent success of water-based DSSCs and ionic photovoltaics will be mentioned.

Next, introducing the main photovoltaic characterization measurements. Finally, we

will highlight the sulfonated block ionomer (SBI) properties and how this material can

be applied into photovoltaics.

1-2 DSSC mechanism of operation

The conversion of the incident solar irradiation to electrical current occurs through a

four-step process:20

• Step one: As a solar photon is absorbed, an electron is excited from the

ground state of the sensitizer to its excited state.

• Step two: The excited electron is injected into the conduction band of TiO2.

• Step three: Through the electric field, the electron will be transported to the

conducting anode and re-introduced into the cell on a counter electrode.

• Last step: The electrolyte will donate electrons to the oxidized dye.

Subsequently, the oxidized species of the electrolyte is reduced and able to

Figure 1-3. Schematic of the operative sequence of dye-sensitized solar cells. Reproduced from [20].

1-3 DSSC components

DSSCs consist of a photoanode, a counter electrode, an electrolyte, and a

sensitizer. Each of these components plays a key role in the performance of the cell.

1-3-1 Photoanode (working electrode)

DSSCs have been used, 22 such as ZnO, SnO2, Nb2O5, and Zn2SnO4, but TiO2 still gives the highest efficiency.23 A targeted review comparing the most common metal oxide in DSSCs is available.24 _{This section will not focus on reviewing the different} electrodes but rather examine the properties of TiO2 that are necessary for DSSCs.

TiO2 has advantages such as good chemical stability, commercial availability with

effectively low cost, no toxicity and good biocompatibility. TiO2 has a high refractive

index (n=2.4-2.5)23 and is used as a dye in white paint. This high refractive index provides efficient scattering of light that enhances the light absorption. In addition, in

nature, TiO2 has several crystal forms such as rutile, anatase and brookite. The

preferred structure in DSSCs is anatase (pyramid-like crystals) due to the large band

gap (3.2 compare to 3.0 for rutile). Furthermore, TiO2 has a high dielectric constant

(є = 80 for anatase), which provides good electrostatic shielding of the injected

electron from the oxidized dye molecule attached to the TiO2. This prevents electron

recombination before reduction of the dye by the redox electrolyte.4

Film morphology is known to critically affect DSSC performance due to its impact on

the electron recombination rate. Therefore, numerous efforts have been made to

optimize the morphology of the nanostructured electrode. Nanoparticles,

one-dimensional nanostructures such as nanotubes12 _{(Figure 1-4) and nanorods have} also been used in DSSCs because electrons can easily migrate through these

structures, greatly reducing electron travel time. Moreover, these nanotubes and

nanorods have special geometries with high surface area and aspect ratios that

The introduction of a compact TiO2 under layer (~50 nm thick),25 as shown in Figure 1-5, was critical for enhancing cell performance as it reduced back recombination from the fluorine doped tin oxide (FTO) electron collector and prevented contact

between the redox couple in the electrolyte and FTO. The main layer is a ∼10 µm

thick film of mesoporous TiO2 that provides a large surface area for dye adsorption

and good electron transport to the substrate. Additionally, an ultrathin (~1 nm) and

ultrapure coating of a mesoporous layer deposited using TiCl4 treatment further

increases dye adsorption.26

Figure 1-4. (a) A cross-sectional view and a (b) top view of nanotube films prepared by anodizing Ti foil. Reproduced from [12].

More kinetic data, such as electron diffusion coefficients, electron diffusion lengths

and ionic diffusion coefficients, are needed to determine and correlate

nanostructures to optimize the photoelectrode and efficiency. Increasing both the

amount of adsorbed dye molecules and the electron transport to the FTO from the

TiO2 still remains a challenge. A review of the current state of photoelectrode

structures and DSSC performance has been published recently.27

To create a flexible photovoltaic, a plastic substrate may replace the transparent

conductive oxide glass substrate in the working electrode.13 _{Yamaguchi et al.}28 _used TiO2 pasted on ITO-coated polyethylene naphthalate (ITO-PEN) as a photoanode.

They used the press method to control the TiO2 thickness to around 6-7 µm by

applying pressure in the range from 0-190 MPa. A significant effect was observed in

photocurrent density as a result of better adhesion strength between the TiO2

particles and the ITO-PEN substrate (Figure 1-6,a). In addition, they used UV-O3

treatment to increase the hydrophilicity of the ITO-PEN surface and adhesion of TiO2

paste with substrate. Also, they placed an anti-reflection film on the outer surface of

DSSC to increase the light transmitted by the substrate, which directly improved the

cell performance. The effect of the anti-reflection film on the light transmittance is

shown in Figure (1-6,b). With these fabrication conditions, the efficiency reached

Figure 1-6. a) Relationship between the pressures applied to the TiO2 film and the plastic-substrate DSSC efficiency. b) Effect of an anti-reflective (AR) film on transmittance of ITO-PEN films. Reproduced from [28].

1-3-2 Counter electrodes

The counter electrode plays a vital role in the DSSC performance and is used to

complete the electrical circuit of the solar cell and to transfer electrons arriving from

the external circuit back to the redox electrolyte.29 The counter electrode should have a high conductivity and a high exchange current density for the reduction of the

oxidized form of the redox couple. Presently, platinum (Pt) is the optimal material for

counter electrodes since it is an excellent catalyst for triiodide (I3−) reduction, has a

low charge transfer resistance and good chemical stability against electrolytes.30 Pt can be deposited using different techniques such as sputtering, dip coating,

electrodeposition, vapor deposition and spray pyrolysis. The common method used

for lab scale DSSC fabrication is sputtering. However, dip coating is the simplest

method for Pt nanoparticle deposition, which produces platinum electrodes with

good conductivity.31

An alternative counter electrode to Pt includes carbon and carbon allotropes such as

carbon black, carbon nanotubes and graphene. A high power conversion efficiency

of 9.1% was achieved under 100 mW cm-2 _{light intensity by using carbon black on} FTO as the counter electrode.32 A clear improvement in fill factor has been observed by increasing the carbon film thickness, as shown in Figure 1-7. By increasing the thickness of the carbon film up to 14.47 µm, the surface area and porosity (rough

morphology) are both increased, consequently creating large numbers of reduction

sites. Thus, charge transfer resistance between the electrolyte and counter electrode

(Rct) was reduced. This decrease in Rct was critical for the improvement in fill factor,

and resulted in the improved efficiency. A recent, noteworthy review discussing the

different carbonaceous materials and their uses as counter electrodes in DSSC has

been published by Poudel et al.33 Carbon and its allotropes have poor electrocatalytic activity compared to Pt, yet remain an attractive material due to their

chemical stability and low cost.

Other candidate materials for counter electrodes with low cost, high-conductivity,

good stability, and good catalytic activity for triodide reduction are conjugated

polymer composites such as, polypyrrole, polyaniline (PANI), and

poly(3,4-ethylenedioxythiophene) (PEDOT). Furthermore, conjugated polymer composites

are potentially able to eliminate the need for Pt and transparent conductive oxide

(TCO) since they are capable of functioning as both the redox catalyst and the

electrical conductor. For example, PEDOT, which has advantages such as optical

replace both the Pt and the TCO. Lee et al.34 reported a power conversion efficiency of 5.08% for PEDOT film deposited on a glass substrate by in situ polymerization.

This efficiency is similar to the 5.88% observed using a Pt/TCO electrode under the

same test conditions. Another example, Poly

[3,4-ethylenedioxythiophene:para-toluenesulfonate] (PEDOT:PTS),35 a highly conducting conjugated polymer composite, was prepared by vapor phase polymerization on plain glass. It was used

as the counter electrode in a DSSC by itself, without conductive fluorine doped tin

oxide (FTO) or tin doped indium oxide (ITO) coatings.36 _{A power conversion} efficiency of 5.25% was reported under 100 mW cm-2.

Combining a conducting polymer with a carbonaceous material will produce an

electrolyte comparable to Pt with an electro-catalytic ability for triodide reduction

and high charge mobility. A flexible composite electrode has been fabricated by in

situ chemical polymerization of polyaniline (PANI) and flexible graphite (FG)37 _to substitute for the expensive Pt counter electrode (CE) used in dye-sensitized solar

cells (DSSCs). The polymerization conditions, such as reaction time and initial

monomer concentration, have been examined to control the thickness of the PANI

film. An overall conversion efficiency of 7.36% was found due to low sheet

resistance and easy charge transfer. Also, this efficiency was comparable to 7.45%

of that with Pt electrode under the same test condition. Another group studied the

PANI nanoparticles deposited on a conducting FTO glass as a counter electrode for

DSSC.38 They found PANI nanoparticles having diameters of approximately 100 nm exhibited an overall energy conversion efficiency of 7.15%, which is higher than the

conditions. The conducting researchers discovered that increasing the surface area

of PANI electrodes while also maintaining uniform and tight attachments to FTOs

improve catalytic activity and trap liquid electrolytes in DSSCs. Consequently, the

outstanding photoelectric properties, simple preparation process and inexpensive

cost allow PANI electrodes to be impressive DSSC counter electrodes.

Figure 1-7. Effect of carbon layer thickness on parameter performances of DSSCs. Reproduced from [32].

Another successful CE has been fabricated from polypyrrole (PPy) single wall

carbon nanotube (SWCNT) complexes.39 SWCNTs are good electron acceptors, and similarly pyrroles are fairly good electron donors, therefore, at elevated

temperatures around 131 ºC, electron transport between PPy and SWCNT is easily

which can be covalently shared with a carbon atom in the conjugation structure of

SWCNT (-C=), as shown in Figure 1-8. The group used Fourier transform infrared spectrometry (FTIR) to find a band shift that verified the occurrence of covalent

interactions between PPy and SWCNT. Using PPy-2 wt%-SWCNT complex as a

counter electrode for DSSCs provides a remarkable power conversion efficiency of

8.30% in comparison with that of 6.31% from PPy-only CEs.

Figure 1-8. Proposed complexation mechanism between PPy and SWCNT. Reproduced from [39].

1-3-3 Photosensitizers

The performance of DSSCs depends mainly on the sensitizer present. Dye, when

used as a sensitizer, will absorb sunlight and transform solar energy into electrical

energy, therefore the photosensitizer possess some primary characteristics such

as:23,40

• Intense absorption in the visible and near-infrared (near-IR) regions of solar

spectra. Strong adsorption onto the oxide semiconductor surface through the

phosphates, which enhance the interaction between the dye and oxide

semiconductor;

• Highly efficient electron transfer resulting from the lowest unoccupied, molecular

orbital (LUMO) of the dye being higher than the conduction band of the oxide

semiconductor;

• Efficient regeneration and the ability to avoid electron recombination, resulting

because the dye’s oxidized state has a more positive potential than that of the

redox electrolyte;

• A resistance to aggregates formation on the oxide semiconductor surface.

Also, dyes need to be photostable and have good chemical and thermal stability.

Ruthenium (Ru) based metal complex dyes are the most prevalent sensitizers, and

the molecular structures of N3, N719, and black dye are shown in Figure 1-9. Power

conversion efficiencies of more than 10% are achieved using Ru based sensitizers

due to their broad absorption spectra, suitable excited and ground state energy

levels, and good chemical stability.

One of the disadvantages of Ru based metal complex dyes is the limited absorption

in the near-infrared region of the solar spectrum. Other metal ion complexes, like

rhenium, Pt, and Cu complexes have been developed and used as sensitizers in

DSSCs. However, metal complex dyes are expensive due to the rarity of noble

metals and also their complicated synthesis. Therefore, there is a demand to

develop metal-free organic dyes. Organic dyes such as coumarin, indoline,

tetrahydroquinoline, etc. are easier to synthesize, cheaper, and are more

environmentally friendly than metal-complex dyes but characterized by narrow

absorption spectra.41 Organic dyes have the general donor-π bridge-acceptor42 configuration, schematically presented in Figure 1-10, where the π conjugated bridge is mainly based on symmetrical chromophoric units. Recently, the highest

power conversion efficiency based on pure organic dye is 10.2% by using a

dissymmetric π bridge.43 This modified organic dye structure increases the dye stabilization and decreases the recombination rate. As a result, this high efficiency is

coupled with outstanding long-term stability, as demonstrated by devices based on

an ionic liquid electrolyte.

One attempt to increase the spectral response of DSSCs is by using multiple dyes

with different absorption spectra. Co-sensitizing DSSCs with a mixture of Ru

complex (the black dye) and an organic sensitizer (D131) has achieved an efficiency

of 11%.44 A candidate sensitizer for photovoltaic applications is a porphyin-based dye, which has a significant spectral response in the near-IR and good thermal and

chemical stability. Until now, the highest recorded efficiency achieved is 12.3% by

using a cobalt (II/III) based redox couple with porphyrin based dye co-sensitized with

organic dye.45 _{This was a result of suppressing the electron back transfer from the} oxide semiconductor to the redox couple. The benefit of the multiple-dye system is

that an electron will transfer from each dye to the electrode either independently or

will work synergistically to produce current.

Furthermore, natural dyes are another interesting DSSC sensitizer worth

exploring.20,46 _{These dyes can be extracted from biomaterials such as flowers,} leaves, fruits, and vegetables. They are alternatives to expensive organic based

DSSCs and prepared easily when compared to ruthenium complex based dyes.

However, these natural dyes often work poorly in DSSCs due to their weak

interactions with metal oxide electrodes and low charge transfer absorption over the

entire visible range.46 _{Dye-sensitized solar cells with a metal-free derivative of} chlorophyll,47 chlorin e6 is shown in Figure 1-11, has the highest performance with 4.3% efficiency so far reported with raw natural dyes. These solar cells revealed

wide absorption spectra in the visible light region by using co-adsorbing surfactant

agents. Three different types of cholic acids (CAs) were used as a co-adsorbing

high energy conversion efficiency of 4.3% was achieved by optimizing the TiO2

amount loaded, the electrolyte amount, and the adsorbing conditions. The

enhancement of performance is due to the suppression of the intermolecular

aggregation of the dye.

Figure 1-11. Chemical structures of chlorin e6. Reproduced from [47].

1-3-4 Electrolytes

The functions of the electrolytes in DSSCs are to regenerate the dye and to provide

an electrically conducting environment. Generally, there are certain criteria that an

electrolyte must exhibit in order to serve as a functional electrolyte for DSSCs.7,9 They include:

• The redox potential of an electrolyte should be more negative than the

oxidation potential or HOMO of the dye.

• An electrolyte should efficiently regenerate the dye after the process of dye

semiconductor. Electrolytes should not cause desorption of dye from the

photoanode.

• To avoid losses by evaporation or leakage, electrolytes should not react with

the sealant, resulting in degradation.

• The electrolyte should have minimum absorption of light compared to the dye

molecules’ absorption in the visible range.

• Finally, the electrolyte should have high electrical conductivity (~10-3 S-cm–1).

Electrolytes can be classified into three groups based on their physical state: liquid,

quasi-solid, and solid electrolyte. They contain redox ions that transfer electrons

between the photoelectrode and the counter electrode. Additionally, electrolytes

contain organic solvents that provide a medium for redox diffusion, and an additive

that modifies both the potential of the redox couple and the band shift of the

photoelectrode. Different redox ions have been tested in DSSCs, such as I−_/I3−_,

Br−/Br2, SCN-/SCN2, and SeCN-/SeCN248. The most prevalent redox ion used is I−/I3− due to its high performance, including reduction kinetics that slows the

recombination of I3− with conduction band electrons in the oxide semiconductor.

However, the major shortcoming of I−/I3− is the potential loss due to its high dye

reduction driving force (~0.75 V). The performance of DSSCs depends on some

physical properties of electrolytes like viscosity, dielectric constant, donor number

concentration, and redox potential.

vapor pressure, and non-flammability. When they are used in DSSCs, they serve

simultaneously as iodide sources and solvents. Their high viscosity indicates low

ionic diffusion. However, electron transfer occurs by hopping, known as the Grotthus

mechanism.49 Redox couples form well-arranged chains that assist charge transfer through the Grotthus mechanism, as shown in Figure 1-12. This type of transfer facilitates charge transport and explains the enhanced solar efficiency. Grätzel and

coworkers first utilized methyl-­‐‑hexyl-­‐‑imidazolium iodide (MHImI), currently the most

commonly used ionic liquid in electrochemical applications, and reported

improvement in both performance and stability of the solar cell.

Figure 1-12. Schematic drawing of the Grotthus mechanism. Reproduced from [49].

Major drawbacks for liquid electrolytes include leakage and volatility of the solvent,

ultimately affecting the stability of the DSSC. These disadvantages led to an

investigation using solid electrolytes (solid hole conductors). The basic requirements

for solid electrolyte materials consist of being p-type with valance bands that are

compatible with the HOMO level of the dye for regeneration, high hole mobility, good

conductors employed in solid-state DSSCs can be classified as either inorganic or

organic electrolytes.50

Inorganic solid electrolytes must be able to transfer holes from the sensitizer into the

TiO2, and can be deposited within the porous TiO2 layer without dissolving or

degrading the dye. Additionally, inorganic solid electrolytes need to be transparent in

the visible spectrum, or have efficient visible light absorbance. Inorganic p-type

semiconductors based on copper compounds, such as CuI, CuBr, or CuSCN are

reported to meet all of these requirements.51 _{Organic hole conductors have also} been used in fabricating solid-state dye sensitized solar cells. These can be

classified as conductive polymers or molecular hole conductors. The first solid state

DSSC incorporating polymers was in developed in 199652 and used oligoethylene glycol methacrylate (MEO) with a power conversion efficiency of 0.49%. Hence,

many researchers work with materials that can be utilized as hole transport materials

(HTMs) to formulate approaches to improve the performance of this type of

electrolyte. Several organic and inorganic materials were designed and applied

effectively as HTMs. For example, p-type direct band gap semiconductor CsSnI2.95

F.05 doped with nonporous TiO2 and using the dye N719 showed conversion

efficiencies of up to 10.2% because of enhanced visible light absorption on the red

side of the spectrum.53 Another class of promising HTMs is organic conjugated polymers, such as poly (3, 4-ethylenedioxythiophene) (PEDOT) and poly

(3-hexylthiophene) (P3HT), which typically possess plane and continuous pathways

inside the mesopores of the TiO2 layer. Owning these structure facilities the hole

Consequently, the light-harvesting efficiency of the cell improved. The most widely

used molecular hole conductors in solid-state DSSCs are triarylamine-based

compounds, such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′

-spirobifluorene (OMeTAD).

The problems arising with dye-sensitized solar cells based on solid electrolytes

include poor pore filling of the photoanode, high recombination reactions, and

unsuccessful contact with the oxide semiconductor surface as well as the counter

electrodes. However, because of their stability they continue to be promising

candidates for future research in DSSCs. Further information and a recent review of

solid-state electrolytes exist in the literature.54

Another way to eliminate the problem of volatilization of liquid electrolytes is by using

a quasi-solid electrolyte. Polymeric agents can gelate both organic solvents and

ionic liquids and transform them into quasi-solid electrolytes.55 _{In general, quasi-solid} electrolytes have conversion efficiencies lower than liquid electrolytes. This behavior

is related to limitations in the mobility of redox couple components within the

quasi-solid electrolyte, in which charge transport occurs mainly by diffusion of molecules,

not by the Grotthus mechanism. In 2013, this claim was refuted when researchers

showed that gel electrolytes can further increase the stability of DSSCs and have

achieved photovoltaics with efficiencies higher than that of their liquid counterparts.

This improvement is attributed to the high penetration of the gel electrolytes into

TiO2 films, coupled with high ionic conductivity. Lun Chen et al.56 have reported using poly(acrylonitrile- co -vinyl acetate) (PAN-VA) copolymers as a gelators. Two

as base solutions to prepare gel-state electrolytes. The efficiency of gel electrolytes

with ACN is higher than that of the MPN solvent. The solvent effectively increases

the ionic conductivity of the electrolyte and improves the overall cell conversion

efficiency. Further improvement in the cell conversion efficiency was observed by

adding 10 wt% TiO2 nanoparticles as a filler to the gel electrolyte.57 This addition slightly reduced the recombination of photoelectrons in the electrolyte. As a result,

the current and efficiency were both increased. This step has successfully produced

quasi-solid DSSCs with efficiencies of 9.46%, higher than the liquid version that

exhibited an efficiency of 9.04%. Furthermore, combining this technique with the

use of a material with a high molar extinction coefficient, like that of heteroleptic

ruthenium complex (CYC-B11)58 instead of N719, improves the overall energy conversion efficiency up to 10.58% due to an increase in light harvesting. Finally,

(PAN-VA) was solidified by applying external pressure during the solvent

evaporation process. A solid-state DSSC with TiO2 nanoparticles as a filler and

CYC-B11 as a sensitizer was the finished product. The efficiency was 8.65%, which

is considered to be good performance but is still lower than quasi-solid DSSCs.

Another successful polymer gel electrolyte characterized as a quasi-solid state

DSSC with a conversion efficiency higher than the liquid electrolytes is

elastomeric-type copolymers consisting of poly(oxyethylene) and poly(amide-imide) (POE-PAI).59 The relationship between electrolyte concentration and photovoltaic performance

has been examined for this material. The conclusions showed that 76.8 wt%

concentration of liquid electrolyte adsorbed into the POE-PAI gave the highest

1-4 Block copolymer in DSSCs

Block copolymers possessing the ability to self-assemble are strong candidate

materials for DSSCs due to their enhanced device performance. The formation of a

high internal area with a continuously connective material having a periodicity on the

order of tens of nanometers can be exploited to increase both conductivity and light

harvesting. Therefore, they have been examined for their use in electrodes and in

both solid-state and quasi-state electrolytes.

1-4-1 Electrodes based on block copolymers

The properties of nanoparticles are linked to their dimensions, crystallinity

composition, and architecture. Additionally, the ability to control these properties

allow fine-tuning of the optical, electrical, and, catalytic characteristics of the

nanoparticles. Block copolymers have been successfully combined with TiO2

nanoparticles in DSSC systems.60 Jang et al. used a commercially-available triblock copolymer, Pluronic P123 (poly(ethylene poly(propylene

oxide)-block-poly(ethylene oxide)), as a carbon foundation in which the hydrophilic domains

contain TiO2 coated onto FTO glass,61 shown schematically in Figure 1-13. The researchers found that carbon-integrated DSSCs were characterized with higher

efficiencies than neat TiO2 due to enhancement of charge transfer and collection into

the cell. Furthermore, using nanostructured carbon-TiO2 beads improves the

structural properties of working electrodes and increases their stability. Using large

size nanoparticles (~200 nm) improves light scattering and contributes to better

Figure 1-13. The fabrication of DSSC cells containing carbon-TiO2 beads in the photoanode. Reproduced from [61].

Another group, Zheng et al. used amphiphilic star-like triblock copolymer,

poly(4-vinylpyridine)-b-poly(t-butyl acrylate)-b-polystyrene, as a carbon source.62 This triblock copolymer, having well-controlled individual block lengths, possesses a

robust star-like architecture where the inner and outer blocks, P4VP AND PS, are

hydrophilic and the center blocks, PtBA, are hydrophobic. The researchers used the

resulting architecture to produced PS-capped Au/TiO2 core/shell nanoparticles, as

Figure 1-14. Schematic of the synthesis of PS-Capped Au/TiO2 core/shell nanoparticles. Using star-like P4VP-b-PtBA-b-PS triblock copolymers forms this structure. Reproduced from reference [62].

The device efficiency improved due to the interaction of light with the Au

nanoparticles, which increased light absorption, coupled with the enhanced charge

transport supplied by the carbon present on the core/shell nanoparticle surface. For

comparison, the efficiency of DSSCs with only TiO2 is 5.67%, the Au/TiO2 layer

increase the efficiency up to 6.09%, and the highest value is 6.44%, achieved using

an Au/TiO2_{-c top layer.}

Amphiphilic triblock poly(ethylene glycol)-block-poly(propylene glycol) block

poly(ethylene glycol), P123, used as the structure-directing agent.63 _{P123 plays a} vital role in forming a hollow nanostructure, visualized schematically in Figure 1-15.

The device efficiency is improved by doping with rare-earth metal ions, like ytterbium

Figure 1-15. Schematic illustration of the formation of Yb3+-TiO2 HNPs. Reproduced from [63].

Lan et al. proposed that using the block copolymer, Pluronic, F-127 [poly(ethylene

oxide)106-poly(propylene oxide)70- poly(ethylene oxide)106], produced super-porous

TiO2 film which enhance the polymer gel electrolyte penetration into TiO2. The

energy conversion efficiency of approximately 7.93% is achieved, compare to 6.17%

for the mesoporous microstructure of QS-DSSC.64

As mentioned earlier, carbonaceous materials and conjugated polymers are an

alternative to produce flexible devices. Furthermore, decreasing the cost is a key

factor in commercializing any device. Recently, electropolymerized PEDOT was

used in flexible, metal-free cathodes for DSSCs with power conversion efficiencies

reaching 4.5%.65 BCPs are also candidates for the production of flexible conductive electrodes. Polystyrene-block-poly-(2-vinylpyridine) (PS-b-P2VP) diblock

copolymer-templated NiO films have been reported with the ability to be deposited in up to three

appropriate materials for building dye-sensitized photocathodes with current

densities up to 300 µA/cm2.

1-4-2 Solid-state DSSC by incorporating block copolymer

With its ability to self-organize into long-range ordered crystalline fibrils upon

spin-drying, poly(3-hexylthiophene (P3HT) copolymerized with

poly(2,5-dihexyloxy-p-phenylene)(ppp) forms an all-conjugated diblock copolymer (PPP-b-P3HT). It has

been applied as a hole transport material for the fabrication of solid-state

dye-sensitized solar cells (SS-DSSCs). The cell performance with the block copolymer

was better than the homopolymer P3HT. This result can be explained by the fact

that PPP block facilitates an intimate contact between the copolymer and the dye

molecules absorbed on the nanoporous TiO2 layer as well as increasing hole

mobility. This all-conjugated block copolymer SS-DSSC exhibits a promising power

conversion efficiency of 4.65%.67 Recent studies by Crossland et al. have demonstrated that the nanostructure formed by microphase-separated diblock

copolymers containing poly(4-fluorostyrene)-b-poly(D,L-lactide) can be used to

template hybrid solar cells with a bicontinuous gyroid morphology, as shown in

Figure 1-16. Their design, however, leads to a metallated system that is inflexible and possesses low conversion efficiency.68,69

Snaith and co-workers have conducted extensive research using block copolymers

to investigate the influence of TiO2 morphologies on solid-state DSSC efficiency.

The researchers used polyisoprene-block-ethyleneoxide (PI-b-PEO) copolymers as

structure directing agents to synthesize mesoporous TiO2. A monolithic mesoporous

films. The thickness is controlled by varying the copolymer molecular weight and the

TiO2-to-block copolymer ratio during fabrication. A mesoscopic length scale (~10

nm) solid-state electrode was achieved. This demonstrated that block copolymers

have an important role not only in the morphological aspects, but also in the

crystallization and electronic characterization, of TiO2.70 Furthermore, a solvent exchange step added to the sample preparation procedure slowed the solvent

evaporation and enhanced the solubility of the polymer and TiO2. As a result, this

allowed the utilization of the amphiphilic block copolymer (PI-b -PEO) self-assembly

to crystalize TiO2 at temperatures up to 700 °C, preventing porosity failures and

improving conductivity. The power conversion efficiency for this system reached

2.48 %.71-72

1-4-3 Block copolymer for quasi-solid state electrolytes

One solution to solvent leakage and liquid electrolyte evaporation is to utilize

quasi-solid state electrolytes in DSSCs. As mentioned earlier, different polymers have

been applied for the development of quasi-solid state DSSC (QS-DSSC)

electrolytes. In this section, examples of block copolymers applied as a gelation

agent are examined. In 2012, Hong et al. used the cross-linking properties of block

copolymers to achieve in situ gelation in a liquid electrolyte by exposing it to

ultraviolet radiation (UV).73 The solar to electric energy conversion for the liquid and gel electrolyte were comparable. However, the gel electrolyte has remarkable

Figure 1-16. Schematic represented of gyroid network replication from PFS-b-PLA templates and the assembly of hybrid solar cells. Reproduced from [69].

Jung assembled iodine-free DSSCs using doped (Cl-) polypyrrole with alkyl

imidazolium iodide as the redox couples, instead of I3-/I- redox couples in the liquid

electrolyte. Subsequently, a block copolymer poly(vinyl alcohol-b-ethylene)

(PVA-EL) was added in order to change the liquid into a gel, the conductivity of which

reached 5.56 S/cm, increasing the QS-DSSC efficiency even higher than the liquid

state without the block copolymer.74

A third example of quasi-solid state electrolytes are those made up of

PSn-b-PEOm-b-PSn block copolymers, which afforded efficiencies up to 6.7%, comparable with

liquid versions (7.3%). Block copolymers with a higher PS:PEO ratios had similar

resistance to liquid electrolytes since different volume ratios in the block copolymers

cause different morphologies. Given this phenomenon, the most favorable condition

for electrolyte diffusion was phase separation, however, this resulted in a disordered

morphology. Even though the presence of the polymer affected the Pt/electrolyte

interface by increasing counter electrode charge transfer resistance (Rct), the

lifetimes of electrons in the photoanode were comparable.75

1-5 Water-based DSSC

The demand to produce an environmentally friendly, low cost, easily fabricated and

reliable solar cell resulted in the emergence of the water-based DSSC. In 2002,

Mikoshiba et al. achieved a high conversion efficiency of up to 4.2% by applying a

buthylpyridine) with up to 10 wt% of water and without other volatile organic

solvents.76 The improved photovoltaic performance was due to the decrease in the viscosity of the electrolyte, which led to an increase in conductivity. One major

concern with aqueous electrolytes was that the high surface tension between water

and the TiO2 surface showed less hydrophilicity compared to other organic

electrolytes. As a result, this lowered the permeation of the aqueous electrolyte in

the mesoporous structure. Murakami et al. attempted to use ozone/UV post

treatment to increase the wettability of the mesoporous layer. Consequently, this

treatment increased the current density by 20% compared to the untreated device.77 Furthermore, adding water to DSSCs promoted the loss of photocurrent due to dye

detachment. The water-based DSSC is a suitable applicant for use in nontoxic and

biologically friendly solar energy devices. In 2003, Kaneko et al. used natural

products such as agarose or k-carrageenan in a water medium with the

dye-sensitized nanoporous TiO2 film in a quasi-solid DSSC to produce artificial

photosynthetic systems with energy conversion efficiency reaching 0.58%.78 In 2007, Lai et al. used different natural dyes with gold nanoparticles (Au NPs) in a

water-based DSSC schematically presented in Figure 1-18. Au NPs can act as a Schottky

barriers, promoting the efficiency of the photoelectric up to 1.49%.79 _{Comparing the} literature published on the various components of DSSCs, water based-DSSCs hold

less interest due to a misunderstanding of the role of water in their photovoltaic

performance and stability. Kim and co-worker added a non-ionic surfactant, Triton

X-100, to the water electrolytic solution in 3-methoxpropionitrile. Triton X-100 seems to

water solubility in the organic electrolytic solution. Based on this experiment, the

TiO2 film sensitized with the N719 dye yielded a higher energy conversion efficiency

of 5.9%, compared to 5.3% without the Triton X-100 coupled with better stability as

can be seen in Figure 1-19.80 This finding opens the door to the optimization of other components in DSSCs.

Figure 1- 18. Electric circuit system of water-based dye-sensitized solar cell ( : dye). Adopted and reprinted from [79].

Law et al. successfully used two new hydrophobic Ru dyes to avoid desorption of

the dye into the water-based electrolyte. The energy conversion efficiency reached

up to 5.7% with 20% water,81 _{which was slightly better than with no water in the} electrolyte as shown in Figure 1-20.

Figure 1-20. One sun I–V curves vs water content for water-based DSSC. Adopted and reprinted from reference [81].

Altering the redox couple is a promising research direction for the development of

water-based DSSCs. In 2012, Tian et al. introduced the water-soluble organic redox

couple 1-ethyl-3-methyl-imidazolium4-methyl-1,2,4-triazole-3-thiolate (TT−_EMI+_)/ 3,3′-dithiobis[4-methyl-(1,2,4)-triazole] (DTT) and used it in a device alongside

hydrophobic organic dyes. An optimal efficiency of 3.5% was obtained. This redox

couple shows higher efficiency and much better stability in water than in