30

CHAPTER – II

2 Introduction to DSSC

Photoelectrochemical Solar Cells (PSCs), consisting of a

photoelectrode, a redox electrolyte, and a counter electrode, have

been studied extensively. Several semiconductor materials,

including single-crystal and polycrystal forms of n- and p-Si,

n- and p-GaAs, n- and p-InP, and n-CdS, have been used as

photoelectrodes. These materials, when used with a suitable redox

electrolyte, can produce solar light-to-current conversion efficiency

of approximately 10%. However, under irradiation, photo corrosion

of the electrode in the electrolyte solution frequently occurs,

resulting in poor stability of the cell, so efforts have been made

worldwide to develop more stable PSCs.

Oxide semiconductor materials have good stability under

irradiation in solution. However, stable oxide semiconductors

cannot absorb visible light because they have relatively wide band

gaps. Sensitization of wide band gap oxide semiconductor

materials, such as TiO2, ZnO, and SnO2, with photosensitizers,

such as organic dyes, that can absorb visible light has been

extensively studied in relation to the development of photography

technology since the late nineteenth century. In the sensitization

process, photosensitizers adsorbed onto the semiconductor surface

absorb visible light and excited electrons are injected into the

conduction band of the semiconductor electrodes. Dye-sensitized

oxide semiconductor photoelectrodes have been used for PSCs.

Gerischer and Tributsch studied a ZnO electrode sensitized by

organic dyes including rose bengal, fluorescein, and Rhodamine B

[1, 2]. In early studies, however, singlecrystal and polycrystal

31

materials, which cannot absorb a large amount of dye, were used

for the photoelectrode, which resulted in low Light-Harvesting

Efficiency (LHE) and, consequently, low photon-to-current

conversion efficiencies. Additionally, the organic dyes that were

used had a narrow absorption range in visible light, which also

contributed to low solar cell performance. Thus, to improve light-

harvesting efficiencies and cell performance, researchers used two

approaches: developing photoelectrode with larger surface areas

that could adsorb large amount of dye and synthesizing dyes with

broader absorption ranges. Significant improvements in the

performance of a Dye-Sensitized Solar Cell (DSSC, or Gratzel cell)

have been mainly due to the development of high-performance

nanoporous TiO2 thin-film electrodes that have a large surface

area capable of adsorbing a large amount of photosensitizer, and

due to the synthesis of new Ru complex photosensitizers capable

of absorbing in the wide visible and near-IR region from 400 to 800

or 900 nm.

Ru bipyridyl complexes are suitable photosensitizers because the

excited states of the complexes have long lifetimes and oxidized

Ru(III) has long-term chemical stability. Therefore, Ru bipyridyl

complexes have been studied intensively as photosensitizers for

homogeneous photocatalytic reactions and dye-sensitization

systems. An Ru bipyridyl complex, N719 having carboxyl groups as

anchors to adsorb onto the semiconductor surface, was

synthesized and single-crystal TiO2 photoelectrodes sensitized by

the Ru complex were studied in 1979 to 1980 [3, 4].

Recent drastic improvements in the performance of DSSC have

been made by Gratzel and co workers at the Swiss Federal

Institute of Technology (EPFL). They achieved solar energy

efficiency (η) of 7 to 10% under AM1.5 irradiation using a DSSC

32

consisting of a nanocrystalline TiO2 thin-film electrode having

a nanoporous structure with large surface area, a novel Ru

bipyridyl complex, and an iodine redox electrolyte [5, 6]. They

also developed a Ru terpyridine complex that absorbs in the near-

IR region up to 900 nm as a photosensitizer for a nanocrystalline

TiO2 photoelectrode: the resulting cell obtained η = 10.4% under

AM1.5 with a short-circuit photocurrent density, , of 20.5

mAcm−2, an open-circuit voltage, , of 0.72 V, and a fill factor

(ff ) of 0.70 [7, 8].

The DSSC is an attractive and promising device for solar cell

applications that have been intensively investigated worldwide, and

its PV mechanism is well understood [9–18]. Recently, commercial

applications of the DSSC have been under intensive investigation.

The cost of commercially fabricating DSSCs is expected to be

relatively low because the cells are made of low-cost materials and

assembly is simple and easy.

2.1 Basic principle of DSSCs

The process describing the conversion of light into electrons in

dye-sensitized solar cells is shown in Figure 2.1. Consisting of an

electron-conducting phase (n-type semiconductor) and a hole

conducting phase considered as “mediator” (redox species or hole

conductors) forming a “bulky” heterojunction, DSSCs are majority

carrier devices where electrons and holes are separated in two

chemical phases. In these cells, light is absorbed by the surface

anchored dye, leading to a photoexcited state. This mode of carrier

generation is also observed in organic bulk junction solar cells

where a light-absorbing organic polymer works as the sensitizer

and a fullerene derivative as the electron acceptor. According to

Gregg classification, these devices belong to “excitonic solar cells”.

33

The term refers to the creation (by light absorption) of a molecular

excited-state as the first step in the series of events that lead to

charge separation and collection in the devices [4-5]. In the bulk

heterojunction cell, the exciton must first diffuse several nm to the

polymer/fullerene interface for example, where it can dissociate to

form an electron-hole pair, with the hole in the polymer phase and

the electron in the fullerene network. In the DSSC by contrast, the

exciton is created by excitation of the dye at the interface so that it

can dissociate readily to create an electron-hole pair, with the

electron injected in the conduction band of the semiconductor and

the hole located, initially at least, on the dye molecule. The dye is

then regenerated by the mediator, which ensures the transfer of

the hole to the counter electrode either in a diffusion or hopping

mechanism depending on the mediator nature.

Figure 2.1 Working Principles in Illuminated Dye-Sensitized

Solar Cells under Open Circuit Conditions.

34

Charge separation in a conventional photovoltaic built on p-n

junction is fundamentally different from the one in the DSSCs.

Figure 2.2a shows energy band diagram of a conventional p-n

junction after thermal equilibration of positive and negative charge

carriers. Due to the concentration difference between the p and the

n type semiconductor, holes move to the n region and electrons to

p region. The uncompensated charges induced by this diffusion

generate a built-in electric field at the junction and impair further

percolation of charge carriers since the orientation of the electric

field is contrary to the direction of the carrier diffusion. At

equilibrium, no net charge diffusion occurs and a depletion region

is formed, which is also referred to as a space charge layer. Upon

illumination as shown in Figure 2.2b, absorption of photons with

energy higher than a threshold, the band gap, results in generation

of excitons which interact via columbic forces.

Figure 2.2 a) Energy Band Diagram of Conventional p-n Junction

under Short Circuit Conditions, b) Charge Separation under

Illumination

35

Considering that excitons will recombine after a certain time with

emission of photons or phonons (heat), therefore only those created

in or close to the space charge layer can be separated by the built-

in electric field and contribute to the photocurrent. Since both

electrons and holes coexist in the same chemical phase, these cells

are called minority carriers devices and their efficiencies are highly

dependent on the ability of photogenerated minority carriers (for

example, electrons in a p-type material) to be collected out of

devices before recombining with the majority carriers (holes, in this

case)[6].

Following the above description, the difference between the

conventional solar cells and dye sensitized solar cells can be

summarized as follows:

Upon illumination, light absorption and charge carrier

transport are separated in dye sensitized solar cell, whereas

both processes are established in the semiconductor in the

conventional solar cell.

In the DSSCs, the nanoparticles of oxide semiconductor are

simply too small to sustain a build-in electric field and

thereby the charge transport occurs via diffusion. In a

conventional p-n junction the presence of an electric field is

necessary for an efficient charge separation.

DSSCs are majority charges carrier devices in which the

electron transport occurs in the TiO2 and the hole by the

mediator. Thereby, the recombination processes can be only

confined at the interface. Inside a p-n junction minority and

majority charge carriers coexist in the same bulk volume.

Hence these cells are very sensitive to the presence of the

trace impurities or defects, which can act as recombination

centers.

36

2.2 Electron Transfer Dynamics

DSSCs are photoelectrochemical devices where several electron

transfer processes are in parallel and in competition. The presence

of a local electrostatic field is not required to achieve good

collection efficiencies as it is the case for conventional p-n junction

cells. Figure 2.3 summarizes the electron transfer processes

occurring at the dye-sensitized heterojunction.

Figure 2.3 Schematic Diagram of Electron-Transfer Processes at

the Dye-Sensitized Solar Cell

Recent studies of the electron injection dynamics from the

electronically excited state of [perylene dye] and [Ru(II)polypyridyl

complexes] into the conduction band of the TiO2 demonstrated that

the electron injection rate constant ( ) are relatively similar

was reported to be 5×1013 s-1 for perylene derivatives and >

37

4×1014 s-1 for Ru-complexes [7, 8]. Assuming, that the injection

kinetics do not significantly change upon replacement of the

electrolyte mediator by the solid-state charge transport material,

the injection of electrons by the sensitizer in both the electrolyte

cell and solid-state solar cell is expected to show similar

characteristics [9].

Upon illumination, the sensitizer is photoexcited in a few femto

seconds (eq. 2.1) and electron injection is ultrafast from excited dye

S* to TiO2 CB (eq. 2.2) on the subpicosecond time scale

(intramolecular relaxation of dye excited states might complicate

the injection process and change the timescale), where they are

rapidly (less than 10 fs) thermalized by lattice collisions and

phonon emissions. The relaxation of the excited dye S* occurring in

the range of nanosecond (eq. 2.3) is rather slow compared to

injection, ensuring the injection efficiency to be unity. The ground

state of the sensitizer is then regenerated by I2 the microsecond

domain (eq. 2.4), effectively annihilating S+ and intercepting the

recombination of electrons in TiO2 with S+ (eq. 2.5) that happens in

the millisecond time range. This is followed by the two most

important processes – electron percolation across the

nanocrystalline film and the redox capture of the electron by the

oxidized relay (back reaction, eq. 2.6), within milliseconds or

even seconds.

Photoexcitation:

*

2 2S TiO h S TiO (2.1)

Charge injection:

*

2 2 2( )S TiO S TiO e CB TiO (2.2)

38

Relaxation:

* '

2 2S TiO S TiO h (2.3)

Regeneration:

2 2 22S TiO I S TiO I (2.4)

Regeneration (in solid-sate DSSC)

2 2S TiO MeOTAD S TiO MeOTAD

Recombination:

2 2 2( )S TiO e TiO S TiO (2.5)

Back reaction:

2 2 2( )e TiO I I (2.6)

2.3 Components of the DSSC

The DSSC is having the following components:

Photoelectrode

Dye material

Electrolyte

Counter Electrode

Transparent Conductive Oxide

Indium/Fluorine Doped Tin Oxide

Aluminium Doped Zinc Oxide

2.3.1 Photoelectrode

The photoelectrode consists of a wide band gap, porous

semiconductor of high surface area that is sensitized for the visible

spectrum by a dye adsorbed on its surface. Titanium dioxide (TiO2)

offers some unique properties making it the preferred

semiconductor for dye sensitized solar cells. Its conduction band

39

edge lies slightly below the excited state energy level of many dyes,

which is one condition for efficient electron injection. The high

dielectric constant of TiO2 (ε = 80 for anatase) provides good

electrostatic shielding of the injected electrons from the oxidized

dye molecule attached to the TiO2 surface, thus preventing their

recombination before reduction of the dye by the redox electrolyte.

Due to the presence of band gap, semiconductors only absorbs

light below a threshold wavelength , the fundamental absorption

edge, which is related to the band gap energy, , by eq. 2.7

( ) 1240 ( )g gnm E eV (2.7)

TiO2 occurs in three crystal modifications, namely rutile, anatase

and brookite. While rutile is the thermodynamically stable phase,

anatase is preferred for dye sensitized solar cells, due to its larger

band gap ( = 3.2 eV for anatase compared to = 3.0 eV for

rutile, corresponding to an absorption edge of ~390 nm and

~410 nm, respectively). The capability of anatase phase absorb

only ultraviolet light, leaving the rest of the visible until the near

infrared of the solar spectrum to the surface anchored dyes,

depending on the property of the sensitizers.

(a) Physical Properties of TiO2

Titanium is the world‟s fourth most abundant metal and ninth

most abundant element. It was discovered in 1791 in England by

Reverend William Gregor, who recognised the presence of a new

element in ilmenite. It was then rediscovered in rutile ore several

years later by a German chemist, Heinrich Klaporth who named it

after Titans, mythological first sons of the goddess Ge (earth in

Greek mythology).

40

Titanium is not found in its elemental state, it occurs mainly in

minerals like rutile, ilmenite, leucoxene, anatase, brookite,

perovskite and spene. It is also found in titanates and many iron

ores. The metal has been detected in meteorites and stars. In fact,

samples brought back from the moon by Apollo 17 contained

12.1% TiO2. The primary source and the most stable form of

titanium dioxide is rutile ore. It was discovered in Spain by Werner

in 1803. Its name is derived from the Latin rutilus, red because of

the deep colour observed in some specimens when the transmitted

light is viewed [10]. Rutile is one of three main polymorphs of

titanium dioxide (TiO2), the other polymorphs being; anatase and

brookite [11-12]. Brookite was discovered in 1825 by A. Levy and

was named after an English mineralogist, H. J. Brooke. In 1801

anatase was named by R. J. Hauy from the Greek word „anatasis‟

meaning extension, due to its longer vertical axis compared to that

of rutile. In all three forms, titanium ( ) atoms are co-ordinated to

six oxygen ( ) atoms; forming TiO2 octahedral [13]. All three forms

differ only in the arrangement of these octahedral. The anatase

structure is made up of corner (vertice) sharing octahedral (Figure

2.4b) resulting in a tetragonal structure [14]. In rutile the

octahedral share edges to give a tetragonal structure (Figure 2.4a)

[15] and in brookite both edges and corners are shared to give an

orthorhombic structure (Figure 2.4) [16].

Titanium dioxide is an n-type semiconductor that has a band gap

of 3.2 eV for anatase, 3.0 eV for rutile, and ~3.2 eV for brookite

[17-24]. Titanium dioxide (TiO2) is the most widely investigated

photocatalyst due to its strong oxidative properties, low cost, non

toxicity, chemical and thermal stability [25-27]. Anatase and rutile

are the most researched polymorphs. Their properties are

summarised in Table 2.1.

41

Figure 2.4 Crystalline Structures of Titanium Dioxide (a)-Rutile,

(B)-Anatase, (C)-Brookite

Table 2.1 Physical and Structural Properties of Anatase and

Rutile

Property Anatase Rutile

Molecular Weight

(g/mol)

79.88 79.88

Melting Point ºC 1825 1825

Boiling Point ºC 2500 ~ 3000 2500 ~ 3000

Specific gravity 3.9 4.0

Light absorption (nm) < 390 <415

Mohr‟s Hardness 5.5 6.5-7.0

Refractive index 2.55 2.75

Dielectric constant 31 114

Crystal structure Tetragonal Tetragonal

Lattice constant (Ǻ) a=3.78 a=4.59

c=9.52 c=2.96

Density (g/cm3) 3.79 4.13

Ti-O bond length (Ǻ) 1.94(4) 1.95(4)

1.97(2) 1.98(2)

In the past few decades there have been several exciting

breakthroughs with respect to titanium dioxide. The first major

breakthrough was in 1972 when Fujishima and Honda reported the

photoelectrochemical splitting of water (2H2O → 2H2 + O2) using a

TiO2 anode and a Pt counter electrode [28]. Titanium dioxide first

showed promise for the remediation of environmental pollutants in

1977 when Frank and Bard investigated the reduction of CN- in

42

water [29-30]. This led into an increasingly well researched area of

TiO2 because of the potential implications for environmental water

and air purification utilising solar energy [31-33]. In 1997 Wang et al

reported TiO2 surfaces with excellent anti-fogging and self-cleaning

abilities which were attributed to the super hydrophilic attributes of

the TiO2 surfaces [34]. Nano sized titanium dioxide was employed to

excellent use in an efficient solar cell, the dye sensitized solar cell

(DSSC) as reported by Gratzel and O‟Regan in 1991 [35] .

(b) Modifications of TiO2 Nanomaterials

The performance of titania nanomaterials in the above mentioned

applications strongly relies on their physicochemical

characteristics such as crystallinity, crystallite size, crystal

structure, specific surface area, thermal stability and quantum

efficiency [36, 37]. For example, in solar applications narrower

band gap energy is favorable to obtain higher photon capture

efficiency. Undoped TiO2 offers a wide band gap, which allows

utilizing only a small fraction of the available solar energy (<5%).

Therefore, it is highly desirable to improve the TiO2 nanomaterials

in order to increase their optical activity by shifting the onset of the

response from the UV to the visible region. Indeed, a great deal of

research is focusing on modifying the properties of TiO2 in order to

achieve these desirable properties. Generally, the modification of

TiO2 nanomaterials can be divided into two main groups, (i) bulk

modification and (ii) surface modification.

(i) Bulk modification: Foreign-element-doping is one of the well-

known methods to enhance the performance of titania

nanomaterials [38]. Usually, two different approaches: 1) Zr, Al, or

Si are added to increase the thermal stability, and surface area [39-

41]. 2) Fe, Cr, V, Mn, is added to shift the absorption edge over a

43

broader range [42-45]. In some cases, simultaneous cation and

anion doping of TiO2 also helps in improving the desirable bulk

properties of TiO2. Wang et al investigated the role of a potential

promoter, ZrO2, in enhancing the activity of TiO2-xNx for the

oxidation of gaseous organic compounds [46]. The nitrogen-doped

photocatalysts were synthesized by reacting amorphous metal

oxide xerogels via a sol-gel process with an ammonia solution,

followed by calcining the products. They reported that ZrO2 helped

to preserve the surface area and prevent grain growth resulting in

higher activity.

(ii) Surface modification: Sensitizing TiO2 with coloured inorganic or

organic compounds can improve its optical absorption in the visible

light region [47, 48]. In addition, modification of the TiO2

nanomaterials surface with other semiconductors can alter the

charge-transfer properties between TiO2 and the surrounding

environment [49].

(c) Electronic Properties and Optical Response of TiO2

The optical response of any material is largely dependent on its

underlying electronic structure. The electronic structure of a

nanomaterial is closely related to its chemical composition,

arrangement, and physical dimensions. The electronic states of

TiO2 are generally considered to consist of three parts: (1) valence

band, (2) lower conduction band, and (3) upper conduction band

(Figure 2.5).

(i) Valence band: The VB consists of three parts: σ bonding of the O

pσ and states in the lower energy region; the π bonding of the

O pπ and states in the middle energy region; and the O pπ

states in the higher energy region.

44

(ii) Lower conduction band: The bottom of the lower conduction

band (CB) consists of the Ti orbital, which contributes to the

metal metal interactions due to the σ bonding of the Ti -Ti

states. At the top of the lower CB, the remaining Ti2g states are

antibonding with the O pπ states.

(iii) Upper conduction band: The upper CB consists of the σ

antibonding orbitals between the O p σ and states. The

chemical composition of TiO2 applicable for solar applications can

be altered by incorporating a dopant. Specifically, the metal (Ti) or

the nonmetal (O) component can be replaced by the dopant

material in order to alter the optical properties.

The following section shows the effects of metal and non-metal

doping on both the electronic properties and the optical response

of TiO2 nanomaterials.

Figure 2.5 Bonding Diagram of a Perfect TiO2 Crystal

45

(d) Metal-Doped TiO2 Nanomaterials

When a doping agent such as V, Cr, Mn, Fe, or Co is introduced

into TiO2, an electron occupied level forms and the electrons are

localized around each dopant. With a dopant with a higher atomic

number, the localized level shifts to a lower energy. The energy of

the localized level due to Co doping is low enough that it lies at the

top of the valence band, while the other metals produce midgap

states [50]. The states due to the 3d dopants shift to a lower energy

level as the atomic number of the dopant increases. The electron

densities around the dopant are large in the VB and small in the

CB compared to the case of pure TiO2. The metal-O interaction

strengthens, and the metal - metal interactions become weak, as a

result of the 3d metal doping. The optical response or the

absorption spectra are also shifted to a lower energy region due to

narrowing of the band gap. This “red shift” is attributed to the

charge-transfer transition between the d electrons of the dopant

and of the TiO2. However, this shift strongly depends on the

preparation method, type and amount of dopant [51].

2.3.2 Dye material

Small band gap energy materials are also used as dye for DSSC.

This is because the photons‟ energy cannot be absorbed if it is

smaller than the material‟s band gap energy. Moreover, the

sensitizer needs to be stable for over 108 times redox reactions for

long lifetime cell. The most common used inorganic materials

utilized in laboratory are three Ru based sensitizers: N3, N719 and

black dye, as shown in Figure 2.6.

Ru based sensitizer has a faster electron injection speed than the

electron recaptured by the oxidised sensitizer. N3 dye cis-

46

RuL2(SCN)2 (L=2,2-bipyridyl-4,4‟dicarboxylate), has an optical

cross section at 530 nm, giving the material the best Light

Harvesting Efficiency (LHE) compared with other sensitizers at

around 99.8%.

LHE is calculated using by the absorption length (α), of the

sensitizer and the nanocrystalline film thickness, d, as shown in

eq. 2.8

( ) 1 10 dLHE (2.8)

Figure 2.6 Chemical Structures of Dye Materials: N3, N719 and

Black Dye [52]

The absorption length, α, can be obtained using the sensitizer‟s

concentration in the nanocrystalline film at full monolayer

coverage, σΩ, and the optical absorption cross-section of the

sensitizer, C.

C (2.9)

47

However, N3 sensitizer could not absorb energies from near red

spectrum. Therefore, new sensitizers such as K19 and K77 have

been developed in the 4 years [53]. N719 exhibits similar

characteristics to N3, but provides higher voltage for DSSC. The

absorption of incident light in the DSSCs is realized by specifically

engineered dye molecules placed on the semiconductor electrode

surface. To achieve a high light-to-energy conversion efficiency in

the DSSC, the properties of the dye molecule as attached to the

semiconductor particle surface are essential. Such desirable

properties can be summarized as:

(i) Absorption: The dye should absorb light at wavelengths up to

about 920 nanometers, i.e. the energy of the exited state of the

molecule should be about 1.35 eV above the electronic ground

state corresponding to the ideal band gap of a single band gap

solar cell [54].

(ii) Energetics: To minimize energy losses and to maximize the

photovoltage, the exited state of the adsorbed dye molecule should

be only slightly above the conduction band edge of the TiO2, but

yet above enough to present an energetic driving force for the

electron injection process. For the same reason, the ground state

of the molecule should be only slightly below the redox potential of

the electrolyte.

(iii) Kinetics: The process of electron injection from the exited state

to the conduction band of the semiconductor should be fast

enough to outrun competing unwanted relaxation and reaction

pathways. The excitation of the molecule should be preferentially

of the MLCT-type.

48

(iv) Stability: The adsorbed dye molecule should be stable enough

in the working environment (at the semiconductor-electrolyte

interface) to sustain about 20 years of operation at exposure to

natural daylight, i.e. at least 108 redox turnovers [55].

(v) Interfacial properties: Good adsorption to the semiconductor

surface.

(vi) Practical properties: For example the high solubility to the

solvent is used in the dye impregnation. These can be considered

as the prerequisites for a proper photovoltaic sensitizer. However,

the factors that actually make the dye-sensitization work efficiently

and yield good photovoltaic performance in the practical cell.

2.3.3 Electrolyte

Electrolyte is the material that filled between the spaces of the

nanoporous electrode. The purpose of electrolyte is to donate

electrons to oxidised sensitizer to prevent the excited electrons

recaptured by the sensitizer. It has to be a transparent material

that allows the light to go through and, at the same time, has good

conductivity and fast redox reaction. Moreover, it needs to have

long term stability in many aspects including chemical, optical and

especially the interfacial stability that relates to desorption and

degradation of dye from oxide film. The most commonly used is

liquid iodide/triiodide redox couple dissolved in organic solvents.

Organic solvent is the major material that gives the iodide/triiodide

ion dissolution and diffusion environment [56]. The ideal

characteristics of the redox couple for the DSSC electrolyte [57]:

49

1. Redox potential thermodynamically (energetically) favorable

with respect to the redox potential of the dye to maximize

cell voltage.

2. High solubility to the solvent to ensure high concentration of

charge carriers in the electrolyte.

3. High diffusion coefficients in the used solvent to enable

efficient mass transport.

4. Absence of significant spectral characteristics in the visible

region to prevent absorption of incident light in the

electrolyte.

5. High stability of both the reduced and oxidized forms of the

couple to enable long operating life.

6. Highly reversible couple to facilitate fast electron transfer

kinetics.

7. Chemically inert toward all other components in the DSSC.

2.3.4 Counter Electrode

Counter electrode in DSSC needs to provide high conductivity as it

needs to provide the liquid electrolyte electrons to complete the

redox reaction in very short time for lifetime stability and

preventing the electron recapture. Currently, the most common

used material is Pt. This is because Pt has high electron mobility

that can regenerate the electrolyte rapidly. Moreover, literatures

show that, for example, using gold as the counter electrode and

found that the electrolyte corrodes gold [58]. Pt, on the other hand,

has high stability against electrolyte‟s corrosives characteristic.

2.3.5 Transparent Conductive Oxide (TCO)

TCO is a wide bandgap n type semiconductor that consists of high

concentration of free electrons. The most common ones are Tin

50

doped Indium Oxide (ITO), Fluorine doped Tin Oxide (FTO) and

Aluminium doped Zinc Oxide (AZO or Al:ZnO) due to the good

electrical conductivity, high transparency and the low material

costs. The research in TCO began popular about one century ago

when K. Badeger published a report in 1907 proposed the method

of cadmium sputtering with thermal oxidation to produce CdO thin

film [59]. Since then, numerous reports on transparent film

deposition emerged. Undoped SnO2, In2O3 and ZnO were materials

that are researched widely at the start. Doping with other materials

showing much better characteristics was discovered later.

Moreover, electronic devices utilizing this technique such as

resistors, light trapping anti-reflection coatings, and thin film solar

cells have been developed. Sixty years later, Holland reviewed the

efforts that have been done in this field in his publication,

describing the methods of fabrication, characteristics and

applications. The research in TCO has not yet reaches its acme as

abundant research projects are set to investigate a better TCO

material with higher electron mobility [60]. Apart from the material

itself, methods of depositing TCO were also examined closely. The

common processes for fabricating TCO are evaporation [61-63],

sputtering [64], reactive ion etching [65], chemical vapor deposition

(CVD) [66, 67], spray pyrolysis [68], solution dipping , and chemical

solution growth.

2.3.6 Indium doped Tin Oxide (ITO)

ITO is one of the most used TCO materials in industries and

laboratories for the past decades due to its high transmittance

(around 80% to 90%) and high conductivity [69]. However, when

the material is placed at temperature over 300 °C, its conductivity

drops dramatically. This is due to the decrease in oxygen vacancies

in high temperature, resulting in the decrease of electric carriers.

51

Moreover, the scarcity of the expensive Indium material resulting in

high material costs [70]. In addition to that, the toxicity of the

material and the ease of reacting with hydrogen plasma, cause the

researchers to look for a better substitution.

2.3.7 Fluorine doped Tin Oxide (FTO)

FTO is another type of TCO that have been widely used, especially

in solar cells. This is due to its good stability at high temperature

and its competitive cost in comparison with ITO. SnO2 itself is a

semiconductor with very low conductivity and wide band gap

(around 4 eV). An extrinsic dopant, such as Sb or F, is added into

the material. Fluorine doped SnO2 is more commonly used than the

material doped with Sb. This is due to the variation in resistivity

with the amount of Sb doped. Another advantage of FTO is that it

has high transmittance (> 80 % or 85 % depending on the

thickness), especially in visible wave region [71]. Its resistivity can

be as low as 2 × 10-4 cm, depending on the thickness of the

film [72].

2.3.8 Aluminium doped Zinc Oxide (AZO or Al:ZnO)

An alternative material that has been widely discussed for ITO

substitution is Al doped ZnO. ZnO is a wide band gap n type

semiconductor with wurtzite structure, categorized in II-VI group. It

has high exciton binding energy, approximately 60 meV, which is

higher than the 25 meV of GaN. Therefore, ZnO has been a

promising material in optoelectronic fields. However, undoped ZnO

suffers from low conductivity (around 1-100 cm), not compatible

for use as electrodes. Therefore, like FTO, an extrinsic doping is

normally added into the material to increase the carrier

concentration, resulting in conductivity increase, at the same time,

52

maintain the transmittance [73]. Both Zinc and Aluminium are

inexpensive and abundant material. This gives AZO an advantage

when comparing with ITO as it is relatively cheap [74].

Transparent conducting oxide (TCO)-coated glass was used as the

substrate for the TiO2 photoelectrode. For high solar cell

performance, the substrate must have low sheet resistance and

high transparency. In addition, sheet resistance should be nearly

independent of the temperature up to 500 ºC because sintering of

the TiO2 electrode is carried out at 450 to 500 ºC. Indium–tin oxide

(ITO) is one of the most famous TCO materials. In spite of having

low resistance at room temperature, ITO resistance increases

significantly at high temperature in air. Usually, fluorine-doped

SnO2 is used as the TCO substrate for DSSCs.

2.4 Preparation of nc-DSSC

Apart from the materials used in preparing cells and the methods

of processing them onto the substrates affects the solar cell overall

performance, the technique of how to assemble the TiO2 deposited

substrate and the Pt coated substrate together is another essential

aspect in DSSC fabrication. A space or gap need to be present

between the TiO2 electrode and the counter electrode for electrolyte

injection. The size of the gap determines the amount of liquid

electrode being injected. This space is provided by the spacers

placed around the active working area. Therefore, the material has

to be thicker than dye absorbed TiO2 electrode thickness. Moreover,

as it was mentioned in the section Counter Electrode that the

electrolyte has corrosive characteristics, the stability of the spacer

material from the corrosion is essential. In addition to that, the

method of injecting the electrolyte into the space also determines

the cell‟s performance since improper injection may cause air

53

bubbles stuck inside the cell. Therefore, two narrow channels from

the edge of the substrate to the two opposite sides of the active

working area are normally used in research for easy electrolyte

injection and air bubble removal [75].

2.5 Processing of TiO2 films

The temperature profile used in the sintering process has a great

impact on the quality of the film and the typical TiO2 sintering

profile was reported by L. N. Lewis et al [76]. The maximum

temperature should not exceed 550ºC, because the phase

transition from anatase to rutile starts in this temperature region,

along with grain coarsening resulting in loss of nanostructure. The

heating rate should be very slow. In the temperature interval 200ºC

< T < 350ºC, organic materials such as dispersants and organic

solvents used during the TiO2 film deposition stage decompose. The

decomposition process induces mechanical stress into the TiO2

layer. If the heating is done too fast, the adhesion to the FTO

substrate is not firm. Consequently, cracks form within the layer

and the film becomes brittle. The cooling rate of the sintered TiO2

electrode also needs to be slow in order to minimize the stress

within the TiO2 layer [77]. Following sintering the film is subjected

to a post-treatment using TiCl4 solution. Typically, this treatment

entails the immersion of the sintered film into 0.05 M TiCl4

aqueous solution at 60ºC for 30 min. in a closed vessel. After that,

the water is evaporated and the electrode is sintered again at 450ºC

for 30 min. This post-treatment result was enhanced the cell

efficiency due to development of improved inter-particle bridges in

the TiO2 film [78, 79].

54

2.6 DSSCs Photovoltaic Cell Performance

A photovoltaic cell is a device, which converts incident light to

electrical energy. Generation of electrical power under illumination

is achieved by the capability of the photovoltaic device to produce

voltage over an external load and current through the load at the

same time. This is characterized by the current-voltage (I-V) curve

of the cell at certain illumination and temperature (Figure 2.7).

When the cell is short circuited under illumination, the maximum

current, the short circuit current ( ), is generated, while under

open circuit conditions no current can flow and the voltage is at its

maximum, called the open circuit voltage ( ). The point in the

IV-curve yielding maximum product of current and voltage, i.e.

power, is called the Maximum Power Point (MPP). Another

important characteristic of the solar cell performance is the fill

factor (ff).

(a) Open Circuit Photovoltage (Voc)

The is the difference in potential between the two terminals in

the cell under light illumination when the circuit is open. It is

dependent on both the Fermi level of the semiconductor and the

level of dark current. The theoretical maximum of the cell is

determined by the difference between the Fermi level of the

semiconductor and the redox potential of the hole-conductor. It is

measured when the current through the cell is equal to zero (open

circuit).

55

Figure 2.7 Illustration of Current-Voltage Characteristics of a

Solar Cell.

(b) Short Circuit Photocurrent (Jsc)

Jsc is the photocurrent per unit area (mA/cm2) when an

illuminated cell is short circuited. It is dependent on several

factors such as the light intensity, light absorption, injection

efficiency and regeneration of the oxidized dye.

It is strongly related to the IPCE and theoretical values on the

can be calculated from the IPCE spectrum. Figure 2.7 shows an

illustration of current-voltage.

(c) Fill Factor (FF)

The fill factor measures the ideality of the device and is defined as

the ratio of the maximum power output per unit area to the

product of and Several factors can influence the ff, such as a

56

high inner resistance (e.g. a bad counter electrode), which will give

a low fill factor and a decreased overall efficiency.

max max

oc sc

V JFF

V J

(2.10)

(d) Solar Energy to Electricity Conversion Efficiency (η)

The overall solar energy to electricity conversion efficiency of a

solar cell is defined as the ratio of the maximum output of the cell

divided by the power of the incident light. It can be determined by

the photocurrent density measured at short circuit ( ), the open

circuit photovoltage ( ), the fill factor of the cell (ff), and the

intensity of the incident light ( ) as shown in eq. 2.11.

100%oc sc

in

V J FF

P

(2.11)

Since it is dependent on all the three first factors under standard

conditions it is of great importance to optimize each one of them for

high overall efficiency.

2.7 Characteristics

The photovoltaic (PV) mechanism of DSSC is different from that of

conventional p-n-type solar cells. The DSSC has other unique

characteristics such as the following:

(i) High energy conversion efficiency: A DSSC efficiency equal to that

of the amorphous-Si solar cell has been obtained during laboratory

development and efficiencies greater than 10% may be possible.

57

(ii) Low-cost fabrication: The DSSC is very simple to construct and

is made of low-cost materials. Fabrication costs will therefore be

less than that for conventional solar cells. For example,

US$0.60/W, which may be competitive for conventional solar cells,

has been estimated for a DSSC with 10% efficiency [10, 15].

(iii) Abundant supply of component materials: Oxide semiconductors

such as TiO2, dye, and iodine are abundantly available. Although

metal deposits of Ru are limited, the amount of Ru complex used in

the DSSC is only 1 × 10−7 mol cm−2.

(iv) Good potential for colourful, adaptable consumer products:

Colourful and transparent solar cells can be made using various

kinds of dyes, depending on the use of the cell. For example,

transparent solar cells could be used in place of windowpanes.

Additionally, the use of a plastic substrate, rather than glass, is

possible if low temperature processing of the TiO2 film preparation

(<250ºC) is available and would expand the use of DSSC.

(v) Low potential for environmental pollution: The TiO2, dyes, and

iodine used in the DSSC are nontoxic. The only component that

could potentially cause harm is the organic solvents used in the

electrolyte solution. Future research should be directed toward

developing a solid-state electrolyte.

(vi) Good recyclability: The organic dye photosensitizers adsorbed on

the electrode can be removed by washing the electrode with alkali

solutions or combustion, providing recyclability of the DSSC.

58

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