chapter 1: dye-sensitized solar cells: an overview 1.1...

Chapter 1: Dye-sensitized solar cells: An Overview

Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 3

1.1 Introduction

The energy requirements and the concerns for environment have led to an increasing

demand for development of energy sources that are cleaner and also abundant in

nature. In this context, the most important one could be considered as the solar

energy. The vast energy given by the sun could be harnessed in a suitable way so as

to compensate for the world’s energy needs. Therefore, the discovery of dye-

sensitized solar cells could be considered as a crucial step in this direction.

Dye-Sensitized Solar Cells (DSSCs), also known as Gratzel solar cells, were

invented by Brian O’Regan and Michael Gratzel in 1991 (O’Regan and Gratzel

1991). These cells are the only photovoltaic devices that separate two varied

functions, solar light harvesting and charge carrier transport, whereas conventional

cells perform both of these operations simultaneously, consequentially requiring

high purity materials like silicon with higher production costs. Solar cells of

mesoscopic type are based on an interpenetrating network or bulk junctions and

offer the benefits of providing flexible devices, of vital importance for particular

applications. These cells are usually fabricated by low cost standard techniques like

spin coating, dip coating, screen printing which in turn avoids the high-vacuum

energy-intensive steps required for fabrication of other film based solar cells. A

recent life cycle analysis has shown that, in case of dye based cells, the carbon

footprint is lower than that for other PV cells owing to which the energy payback

time for the DSSC will be less than one year. In contrast to silicon cells, the

materials required by DSSC are eco-friendly and abundantly available so that the

technology can be scaled up to terawatt scale without running into feedstock supply

problems. All these factors gives it an advantage over the two other major competing



thin film photovoltaic (PV) cells made out of CdTe and CuIn(As)Se that use toxic

materials of low abundance. However, the major challenge in commercial

application of DSSC is reaching the high efficiency values as those of conventional

cells (Nazeeruddin et al. 2005). The scientific progress in nanocrystalline or

mesoscopic TiO2-based DSSCs has come a long way since the advanced

findings first reported in 1991.

Today, three types of DSSCs are available:

liquid junction cells,

hole-conductor based solid-state cells and

cells containing quasi-solid electrolytes with molten salts, polymers and gels.

It is also possible to make flexible Dye sensitized solar cells that differ in colour and

opacities thereby providing alternatives for replacing glasses in windows and

facades in buildings, other architectural structures and automobiles. The ability to

cheaply manufacture flexible roll-to-roll DSSC will greatly help such applications to

be applied practically. Despite the easy fabrication methods, DSSCs still face a few

engineering challenges, notably sealing methods and the need to prevent the loss of

electrolyte. However, significant progress has occurred in the last three years on

these issues.

1.2 Mechanism of working of DSSCs

The separation of the optical absorption and the charge separation processes is

achieved by DSSCs by intervention of a light-absorbing sensitizer chemisorbed

over a wide band gap semiconductor of mesoporous nanocrystalline morphology

(usually TiO2). In order to ensure high performance, it is necessary that electron



injection from the excited state of the dye into the TiO2 film is faster, owing to

which the following factors are crucial:

(i) rapid dye regeneration,

(ii) slow electron recombination at the TiO2 electrode/electrolyte

interface,

(iii) efficient charge carrier collection by fast electron diffusion in the oxide

layer, and

(iv) electron transport in the electrolyte.

The negative charge carriers are injected by the excited dye into an electron

(n) conductor and the positive charges in a hole (p) conductor or an

electrolyte upon photo-excitation. Thus the negative and positive charges move

in different phases to the front and back contacts of the photocell, their

recombination occurring across the interface separating the nanocrystalline

oxide film from the hole conductor material. The open circuit photovoltage

(Voc) produced by the cell corresponds to the difference of the quasi-Fermi

level. The liquid junction DSSCs have reached >11% certified energy

conversion efficiencies with volatile electrolytes having a boiling point near

100°C, whereas their solid-state equivalents using hole-conductors are now at

5.1% efficiency. The biggest advantage of this cell configuration is that the solar

energy conversion process involves only majority charge carriers. Electron and holes

are generated in different phases and their recombination across the interface is

blocked to a maximum extent by the presence of the sensitizer. However, in order to

harvest solar light efficiently one has to develop sensitizers capable of absorbing



maximum sunlight across the entire visible and near IR region up to 900 nm, and

upon excitation generate charges with a quantum yield near unity.

The overall process comprises of light absorption by the dye (S) produces an excited

state (S*) that injects an electron into the conduction band of a wide band gap

semiconductor, such as TiO2. The electrons diffuse across the oxide to the

transparent current collector made of conducting glass from where they pass

through the external circuit thus doing electrical work. From there they re-enter

the cell through the back contact (cathode) by reducing a redox mediator (ox). The

reduced form of the mediator (red) regenerates the sensitizer closing the cyclic

conversion of light to electricity.

1.3 Limitations of DSSCs and some important technique to overcome them

Multi-junction stacked (tandem) solar cells can increase the overall

photovoltaic conversion efficiency by optimal utilization of the solar spectrum

in individual cells. A photovoltaic tandem cell comprising a DSSC as a top cell for

high energy photons and another thin film bottom cell for lower energy photons

(DSSC or inorganic type) can produce, under AM 1.5, solar to electric

conversion efficiencies greater than 15% (Liska et al. 2006). With all of its

advantages, DSSCs should technologically mature further to be widely used in the

PV market. The following is a list of the most urgent actions required for maturing

DSSC as a major PV area:

a) It is a matter of immediate concern that large area panels have not reached the

same efficiency as smaller cells made in laboratories. Presently large modules show

only 5-7% and small devices show close to 11.5% efficiencies.



b) Another issue is with the electrolyte system being volatile or leaking through

cells. This can be resolved by switching over to solid or gel electrolytes without

compensating for the cell efficiency or performance.

c) The material used in DSSCs like glass costs about a third of the total device cost.

Thus it is absolutely necessary to move from heavier glass substrates to lighter

plastic materials. This will give us significant advantages in terms of lower cost,

flexibility, the possibility of roll-to-roll manufacturing, light weight and hence

lower transportation and packaging costs, wider application scope including in

the areas of sustainable architecture of buildings and utilities, portable devices,

transportation, health care in remote and rural locations, and vaccine storage

(refrigeration) in mobile rural health centres.

d) The cells should be designed in such a way that it absorbs a substantial portion of

red and almost the entire IR, unlike Si and other inorganic PV devices. DSSCs

should break the barrier of 12% single cell conversion efficiency by taking

advantage of the presently under-utilized and untapped red and near-IR

spectral regions of sunlight. This will make DSSC performance on par with

Si, CdTe and CIGS solar cells.

e) Flexible cells require thinner semiconductor oxide layers to adapt to roll-

to-roll product lines, as well as to reduce material costs. Thinner layers will also

have better adherence to pliable substrates.

f) This thinner mesoporous oxide layer will then require dyes with higher

molar absorptivity in order to harvest all the available light within a shorter path

length. Only organic dyes and Q-dots can meet this requirement.



g) Commercial TCO deposited conducting polymer films are generally ITO coated

polymer films, which often show modification upon heating, which in turn results in

increased haziness to light transmission. Furthermore, ITO is much more

expensive than FTO (fluoride-doped tin oxide). Therefore, it is necessary to

develop cheaper FTO coated highly transparent and photochemically stable

polymeric substrates for successfully maturing sensitised flexible solar cells to

compete in the market.

1.4 Criteria for designing sensitizers for dye-sensitized solar cells

There is an ideal set of conditions for a dye to be satisfied so as to suit application in

DSSC.

1. The dye should have anchoring groups (for example, -COOH, -H2PO3, -

H2SO3, etc) to strongly bind the dye onto the semiconductor surface.

Suitable anchoring groups such as carboxylate or phosphonate can firmly

graft to the semiconductor oxide surface. The lowest unoccupied molecular

orbital (LUMO) of the dye should be localized near the anchoring group.

2. The light absorption of the dye should cover the whole visible region and

even a part of the near-infrared (NIR), so that a large part of the solar energy

is absorbed. It should also have high values of molar extinction co-efficient.

3. The HOMO level of the dye should be below the energy level of the redox

system to allow efficient regeneration of the oxidized dye.

4. The LUMO level should be sufficiently above the conduction band edge of

the semi-conductor electrode (typically TiO2) to ensure efficient electron

injection.



5. Unwanted dye aggregation on the semiconductor surface should be avoided

through optimization of the molecular structure of the dye or by addition of

co-adsorbents.

6. Good thermal, photochemical and electrochemical stability of the dye

molecule.

Based on these requirements, several different kinds of dyes including transition

metal complexes, porphyrins, phthalocyanines and metal free organic dyes have

been designed and applied successfully in DSSCs over the last two decades. Lately,

in addition to Ru, polypyridine dyes, pure organic dyes are increasingly studied as

attractive sensitizers. Due to their larger structural variety, organic dyes can

be designed to adjustable absorption spectral responses, higher molar

extinction coefficients, and with environmental compatibility. They are also

cheaper to manufacture. The successful organic sensitizers have three major

parts; donor, linker and acceptor components. The HOMO energy level has

delocalised π-characteristics over the entire molecule. The LUMO energy level also

has π-character, where the electron density is shifted towards the acceptor group.

Thus, when exposed to sunlight, an intramolecular charge separation occurs

between the donor and the acceptor components of the chromophore in a

manner better described as a push-pull effect supported by the HOMO and LUMO.

The general character of the orbitals is independent of the linker length. The

absorption spectrum of the dye can be extended towards the near IR region

by decreasing the gap between HOMO and LUMO energy levels, which is

possible by increasing the conjugation in the dye molecule.



The conjugation could be increased either by a π-conjugated carbon-carbon

double bond (C=C) or by a suitable linker, like thiophene. However, in the

case of a carbon-carbon double bond (C=C), the stability of the dye may be

affected due to photoinduced trans to cis isomerisation. Hence, it is preferable to

increase the conjugation by means of a linker that consists of different π-conjugated

ring moieties like thiophene, benzene or pyrrole. Such linkers extend the conjugation

without affecting the stability of the dye.

1.5 Sensitizers used for dye-sensitized solar cells

Dye-sensitized solar cells have become one of the most promising low-cost

alternatives for the photovoltaic conversion of solar energy compared with the

conventional solid p-n junction photovoltaic devices. This cell was invented by

Michael Gratzel et. al. in 1991 and is also known as Gratzel cell.1 Although

commercially available solar cells are mainly based on silicon based inorganic cells,

however dye based solar cells are highly promising and cost-effective alternative for

the photo-voltaic energy sector. Currently, these flexible and tunable solar cells

based on ruthenium(II)–polypyridyl complexes as key sensitizers have reached an

overall power conversion efficiency (η) greater than 11% under standard (Global Air

Mass 1.5) illumination (Chiba et al. 2006; Nazeeruddin et al. 2001; Gao et al. 2008;

Nazeeruddin et al. 2005; Nazeeruddin et al. 2007).

The main functioning of DSSC lies in the mesoporous oxide layer involving

nanometre-sized particles sintered together for better electronic conduction. The

metal oxide usually chosen is titanium dioxide (anatase) but many reports of other

wide-gap oxides such as ZnO, SnO2 and Nb2O5 have also been done (Rensmo et al.



1997; Hoyer and Weller 1995; Green et al. 2005). The key component comprises of

dye that is responsible for sensitization of DSSC. The monolayer of the dye is

attached to the nanocrystalline film of TiO2. The solar cell works on the principle of

continuous cycle of oxidation and reduction of dye assisted by the redox electrolytic

system. The cycle is initiated by the photo-excitation of dye followed by injection of

a dye electron into the conduction band of the oxide film. The original state of the

dye is subsequently restored by electron donation from the electrolyte via

regeneration reaction. The redox electrolyte usually comprises an iodide/triiodide

redox system that is dissolved in an organic solvent. Several other alternatives for

this organic solvent has been thought upon with more emphasis on lesser volatility

of the solvent. In this context, attention is increasingly being focused on ionic

liquids, gelled electrolytes, polymer electrolytes and deep eutectic solvents

(Stathatos et al. 2003; Wang et al. 2003b; Nogueira et al. 2004; Durr et al. 2005).

The regeneration of the oxidized dye to normal state is obtained by gain of electron

by iodide ion of redox system. The iodide ion is in turn regenerated by the

reduction of triiodide at the counter electrode, with the electrical circuit being

completed via electron migration through the external load.

The dye component being the key material has been explored extensively via change

in structural features, chromophores, conjugation bridges, ligands, metal ion, and

nature of complexes (Spiccia et al. 2004; Argazzi et al. 2004; Polo et al. 2004). This

systematic study has resulted in the development of mononuclear and polynuclear

(Bignozzi et al. 2000) dyes based on metals such as RuII (Nazeeruddin et al.

1993; Islam et al. 2001; Nazeeruddin et al. 1997), OsII (Kuciauskas et al. 2001;

Argazzi et al. 2004), PtII (Islam et al. 2001; Geary et al. 2005), ReI (Hasselmann et



al. 1999), CuI (Alonso-Vante et al. 1994) and FeII (Jayaweera et al. 2001). Besides

transition-metal complexes, a variety of organic molecules have been explored,

including coumarin (Hara et al. 2005) squaraine (Alex et al 2005), indoline

(Horiuchi et al. 2004), hemicyanine (Chen et al. 2005) and other conjugated

donor–acceptor organic dyes (Hara et al. 2005, Thomas et al. 2005, Kitamura et al.

2004, Hara et al. 2003) and the best efficiency reported was 8% (Horiuchi et al.

2004). In addition, porphyrin dyes (Campbell et al. 2004, Wang et al. 2005) and

phthalocyanine dyes (Komori et al. 2003) have also been explored.

1.5.1 Overview of organic sensitizers employed for DSSCs

Although the most efficient sensitizers to date are ruthenium complexes, metal free

organic dyes have been attracting attention because of their ease of synthesis,

high molar extinction coefficient, tunable absorption spectral response from the

visible to the near infrared (NIR) region, environmental friendliness and

inexpensive production techniques. The organic sensitizers can be divided in three

major parts; donor, linker and acceptor components.

The HOMO energy level has π-characteristics being delocalized over the entire

molecule and the LUMO energy level also has π-character where the electron

density is shifted towards the acceptor group of the chromophores. Thus, when

subjected to sunlight, an intramolecular charge separation occurs between the donor

and acceptor components of the chromophores also known as pronounced push-pull

effect supported by the HOMO-LUMO concept. The general characters of the

orbitals are independent of the linker length.



Development in the use of organic sensitizers began when Hara et al. (Hara et al.

2003a; Hara et al. 2005a) started using oligoenes containing dialkylaminophenyl

groups as the donors and cyanoacrylic acid as the acceptor [1]. The use of these dyes

as sensitizers in liquid electrolyte systems yielded overall power conversion

efficiencies of up to 6.8%.

[1]

Among the metal-free organic dyes studied in DSSCs, coumarin-based dyes are

promising sensitisers owing to their good photo-response in the visible region, good

long-term stability under one sun soaking (Wang et al. 2007) and appropriate

lowest unoccupied molecular orbital (LUMO) levels matching the

conduction band of TiO2. Some coumarin sensitizers (Hara et al. 2001; Hara et al.

2003b; Hara et al. 2003c; Hara et al. 2003d; Hara et al. 2005b; Hara et al. 2005c) as

donor groups has reached efficiencies of upto 8.2 % values comparable to the

standard N719 sensitizer. The importance of coumarin is understood from the fact

that on coumarin moiety [2] was replaced N,N-dimethylaminophenyl (DMA) donor

group [3] (Kitamura et al. 2004), a significant hypsochromic shift of the maximum

absorption peak is observed indicating that the coumarin is a stronger donor than the

DMA group.



COOH

CNN

CH3

CH3

N

CH3 CH3

CH3

CH3

O O

COOH

CN

[2] [3]

[4]

A coumarin dye reported as NKX 2510 [4] was reported to have an efficiency of

4.7%. However, it was noted that in comparison with the reported dye NKX-2311

[2] with an efficiency of 5.2%, NKX 2510 [4] is blue-shifted. It was assumed that

the ring structure of coumarin in dye NKX-2311 [2] might be responsible for such a

behavior. In order to further obtain a red shift, it would be desirable to increase π-

conjugation by extending the methine unit (–CH=CH–) of [2]. However, such an

extension by more double bonds would increase the instability of the dye molecule,

owing to the possibility of isomer formation. Also, report by K. Hara (Hara et al.

2001; Hara et al. 2003b) indicated that among a series of coumarin dyes with

extended methine units [5], the highest values of Jsc and efficiency is obtained with

two methine units as in dye NKX-2311 [2]. This is mainly due to the H-aggregation

of the dye due to strong interactions between dye molecules on the TiO2 surface that

results in reduced values of Jsc and Voc.

NC2H5

C2H5

O O

COOH

CN



[5]

It has been observed that introduction of thiophene moieties improves the solar cell

performance mainly by broadening the absorption spectra thereby resulting in a

large photocurrent and also relatively lowers the positions of the LUMO levels of

the dyes. Also when the number of thiophene units are increased (as in [6]), the

LUMO are much lowered due to increased π-π stacking leading to insufficient

driving force for electron injection (Hara et al. 2003c; Hara et al. 2005c).

[6]

Amongst aromatic amines, triphenyl amine has been explored by many researchers.

A simple triphenyl amine based dye [7] without any substitution gave an efficiency

of 2.5% (Xu et al. 2008). However, when bridges like thiophene [8] and

ethylenedioxythiophene (EDOT) [9] is incorporated between triphenyl amine and

cyanocarboxylic acid units, a red-shift is observed in the absorption of the dyes

thereby improveing the efficiencies to 5.2% and 7.3%, respectively (Liu et al. 2008).



[7] [8]

[9] [10]

Further improvement can be seen by simple structural modification done by Hwang

et al. by including a benzene bridge in conjugation with double bond unit [10] that

yielded excellent efficiency of 9.1% (Hwang et al. 2007). It is further observed that

replacing the benzene by thiophene (Hagberg et al. 2007; Hagberg et al. 2006;

Hagberg et al. 2008) or a bi-thiophene unit (Thomas et al. 2008) lowers the

efficiency to 5.9 %, 6.9% and 6.15 % respectively. The increase in efficiency of

over 30 by 1% exhibits the significance of methoxy donor groups in improving cell

response. Li et al have also reported triphenyl amine derivatives with methyl

substitutions on two phenyl rings and inserting bithiophene [11] or a

thienothiophene unit [12] that gave 6.6 to 7% (Li et al. 2008). A benzothiadiazole

unit in triphenyl amine sensitizer and shifted the absorption to the near-infrared

region (Velusamy et al. 2005). These dyes showed absorption gave efficiencies of

3.8 and 1.15%, respectively. An interesting fact to be observed is that replacement



of benzene ring by a thiophene ring gave a sharp drop in efficiency which is less

than half of the former. This trend could be attributed to dramatic decrease in the

short-circuit current.

[11] [12]

Triphenyl moiety was also investigated in branched sensitizers and where higher

efficiency of [14] (7.2%) than[13] (5.4%) shows the effect of methoxy groups on

the photocurrent (Hagberg et al. 2008). The lower value of photocurrent could be

caused due to narrower IPCE spectrum. It was further shown by Thomas and co-

workers that on incorporation of branched oligothiophene backbone into

triphenylamino moiety [15] prevents the back electron transfer process and enhances

the efficiency of charge transfer in the electronically excited state of the dye giving

increased efficiency of 6% (Thomas et al. 2008)



[13] [14]

[15]

1.5.2 Near-infrared absorbing dyes

One of the factors limiting the efficiency of DSSCs can be non-optimized

photocurrent arising from the poor optical response of the dyes in the NIR region.

Hence, it is crucial to increase the light harvesting capability of the dyes towards the

near-IR region. This is mainly because of the widened scope of improving efficiency

of dye-sensitized solar cells. The conventional DSSCs are very effective in the

region between 400–800 nm after which though they reduce to almost zero value of

IPCE. This means that they usually do not absorb all the photons in the visible and

near-infrared region so that a big fraction of photons extending till wavelength of



1200 nm are not used. Since traditional silicon devices makes use of these photons,

it is crucial to increase the absorption range to obtain efficiency values closer to

these semiconductor cells.

Apart from the usual type of cells, there have been reports of a tandem cell

incorporating a standard ‘dye-sensitised’ solar cell (DSSC) connected in series with

a second cell sensitised by an organic dye, capable of absorbing photons in the near

IR region without loss in Voc.

The application of near-IR dyes also gives rise to the possibility of using them in

transparent solar cells (Burke et al. 2007)

1.5.2.1 Metal free non-ionic dyes:

The interest in application of infrared radiation - absorbing dyes in dye-sensitized

solar cells has greatly enhanced in the recent years. The absorption spectrum of the

dye can be extended towards the near IR region by decreasing the gap between

HOMO and LUMO energy levels which is possible by increasing the conjugation in

the dye molecule. The conjugation could be increased either by a π-conjugated

carbon-carbon double bond (C=C) or by a suitable linker like thiophene. However,

in case of a carbon-carbon double bond (C=C), the stability of the dye is affected

due to photoinduced trans to cis isomerisation.

Hence, it is preferable to increase the conjugation by means of a linker that consist

of different π-conjugated ring moieties like thiophene, benzene or pyrrole. Such

linkers extend the conjugation without affecting the stability of the dye thereby

improving the solar cell performance. Lin and co-workers introduced a

benzothiadiazole unit in the diphenylamine moiety [16] (Velusamy et al. 2005)



where they were successful in extending the absorption towards the near-infrared

region with absorption maxima for the dye being 541 nm and overall efficiency of

about 1.15%.

[16]

Novel phenoxazine metal-free dye [17] was designed and synthesized by Sun et al

for panchromatic dye-sensitized solar cells. It shows a broader IPCE spectrum from

300 nm to 920 nm with a maximum value 67% at 580 nm and overall solar energy-

to-electricity conversion efficiency (η) of 3.0% (Tian et al. 2009).

[17]

Recently, a new method of molecular design has been developed by Sun et.al. where

the anchoring group in the tetrahydroquinoline moiety is separated from the acceptor

5

O

N

CH3

S

N

S

O

HOOC

S

N

O

S CH3

N SS

NC COOH

N

SN



units that permit tuning HOMO-LUMO level by modification in the acceptor groups

(Hao et al. 2009). One of the dyes synthesized by them showed value of IPCE as

high as 86% at 660 nm and overall solar energy to electricity conversion efficiency

of around 3.7%. The absorption spectrum of this dye extends into the near-IR region

which could be due to the change in the acceptor group from cyanoacetic acid to

malononitrile. The fact that the HOMO levels of the dye are more positive than the

iodine/triiodide redox potential value also offers a good scope for optimizing

absorption in the near-IR region.

1.5.2.2 Ionic sensitizers:

Ionic dyes could be regarded as potential sensitizers for DSSCs owing to their

tunable absorption in the red to near-infrared region and also high absorptivities (ca.

105 Lmol_1cm_1). The ionic dyes tend to form strong self-associated aggregates

(Mishra et al. 2000) in solution or at the solid–liquid interface in three different

patterns which are:

red-shifted J-aggregates,

blue-shifted H-aggregates and

both red- and blue-shifted herringbone aggregates.

This aggregation phenomenon has been shown to be used to the advantage of

improving efficiency of the dyes by broadening of IPCE curves (Sayama et al. 2002;

Ehret et al. 2001; Guo et al. 2005; Mann et al. 2008). However the disadvantage lies

in the aggregation after adsorption on titanium dioxide that results in lesser

interaction of dye LUMO with TiO2 conduction band. These colorants also shows

cis-trans isomerisation that gives a route for decay thereby affecting injection



efficiency. Amongst ionic dyes, much research has been done in development of

dyes from class of cyanine dyes (Sayama et al. 2001; Ehret et al. 2001) merocyanine

dyes (Khazraji et al. 1997; Sayama et al. 2000; Sayama et al. 2003) and hemicyanine

dyes (Stathatos et al. 2001; Wang et al. 2000) have been investigated in such

applications due to their high absorptivity mainly in the red to the near-IR region.

The class of hemicyanine dyes belong to a group of interesting molecules, with

electron donor and electron acceptor linked by a p-conjugation bridge (D–p–A). The

electron injection can occur with a high quantum yield on formation of a charge

separated excited state that is possible because of the structural features. This case is

similar to that of the ruthenium metal complexes where electrons for injection are

provided by metal to ligand charge transfer (Nazeeruddin et al.1993). Among

several hemicyanine dyes reported in literature, several structural modifications have

been tried like inclusion of hydroxy group as in [19] that can improve adsorption on

TiO2 via hydrogen bonding. The efficieny improves by 2.3 % after this modification

(Yao et al. 2003).



[18] [19]

Sheng et al synthesized a series of new benzothiazolium hemicyanine dyes and

incorporated different functional groups like carboxyl, hydroxyl, or sulfonate

anchoring groups on the dye skeleton (Sheng et al. 2005). They showed that the

efficiency was maximum in compounds with both carboxyl and hydroxy group

rather than with carboxyl group alone. In addition, the combination of sulphonate

and hydroxyl group did not serve to be a good anchor as carboxyl that resulted in

lower efficiency.

[20] [21]



[22] [23]

Tian and co-workers found that D–p–A-based hemicyanine dye generated a high

JSC value of 13.8 mAcm-2, while the cell efficiency was only 2.1% because of its

relatively low VOC value of 0.36 V and low fill factor of 0.41 (Meng et al. 2003).

Arakawa and co-workers studied a series of benzothiazole merocyanines with

differing chain lengths of alkyl and found that the conversion efficiency and the

IPCE value increased with an increasing length of the alkyl side chain attached to

the benzothiazole ring and with a decreasing number of methylene units between the

carboxylic acid group and the dye chromophores (Sayama et al. 2002; Sayama et al.

2000). The dye with the longest alkyl side chain was the best sensitizer, generating

an efficiency of 4.5% . In addition, dyes prepared with different numbers of C-C

double revealed that rate of electron transfer decreased with increase in conjugation

(Sayama et al. 2003).

Squaraine molecule is quite prospective due to its intense absorption towards the

near-infrared region, high extinction coefficient and inherent stability towards cis–

trans photoisomerization owing to the presence of the rigid squaric acid ring in the

methine chain. Gratzel et al reported for the first time an organic dye from the

squarine class (Burke et al. 2007) [24] with extended absorption in the near IR

region showing an intense peak at 679 nm. This dye had a symmetrical squaraine



moiety with two carboxylic acid groups which was successfully applied in both

liquid and solid-state solar cells with solar energy to electricity conversion

efficiencies of 3.7 and 1.5% respectively.

[24]

An unsymmetrical squaraine sensitizer [25] reported by Gratzel and co-workers

showed good absorption at about 640 nm with an IPCE of 85% and overall

conversion efficiency of 4.5% (Yum et al. 2007).

[25]

N+

CH3CH3

N

CH3

CH3

O

O-

O-

O

O-O

HN+Et3

HN+Et3

N+

CH3CH3

N

CH3

CH3

O

O-CH3

H 17 C 8

COOH



Das et al have compared the efficiencies of novel symmetrical and unsymmetrical

squaraine dyes synthesized by them and concluded that unsymmetrical squaraines

exhibit higher efficiencies than symmetrical squaraines (Saji et al. 2005)

1.5.3 Phthalocyanines

Phthalocyanines are planar macrocycles that constitute four isoindole units with an

18 atom, 18 π-electron aromatic cloud. Phthalocyanines possessing highly

delocalized cyclic p-electron systems show intense absorptions in the red region (Q

band) and these absorptions could be shifted to near-IR region by making slight

modifications in the molecule (Claessens et al. 2001). Phthalocyanines have proven

to be useful sensitizers for dye-sensitized solar cells owing to their good stability,

broad absorption in the visible spectral region and photoelectrochemical properties

(Kaliya et al. 1999). One of the major reasons for the growing interest in this class

can be attributed mainly to the strong absorption of visible light in the far-red/near-

IR region (Aranyos et al. 2001).

Due to these properties, phthalocyanines are potentially attractive for the conversion

of solar energy to electricity as conventionally used organic dyes do not absorb

efficiently in near-IR region (Hagfeldt et al. 1995; Hagfeldt et al. 2000; Nazeeruddin

et al. 1993). Apart from this, phthalocyanines also exhibit good photostability and

lesser possibility of desorption from the titanium dioxide surface after adsorption.

However, the property of lesser desorption is due to low solubility of

phthalocyanines in organic media (Zipplies et al. 1986) owing to the presence of

functional groups like carboxylate or sulphonate that further makes purification and

characterization processes very tedious in nature. At the same time, metal



phthalocyanines also suffer from disadvantages like aggregation of dye due to strong

intermolecular interaction between the phthalocyanine rings that easily induce face-

to-face stacking to form a thick layer on the surface. Due to this aggregation, the

injection efficiency of into the oxide conduction band decreases. These limitations

urge for the need of improvement so as to explore this class of compound as

efficient sensitizers for DSSC.

There have been many reports of phthalocyanine derivatives used as photo-

sensitizers in dye-sensitized solar cells especially in the near-IR region (Yanagisawa

et al. 2002; He et al. 2002). Gratzel et al have synthesized a series of unsymmetrical

zinc phthalocyanines bearing an anchoring carboxylic functional group linked to the

phthalocyanine ring through different spacers (Gratzel et al. 2009). In this

comparative study, they have shown that the nature of the spacer group plays a

significant role in the electron injection from the photo-excited dye into the

nanocrystalline TiO2 semiconductor, the recombination rates and the efficiency of

the cells. The incident monochromatic IPCE value for phthalocyanines bearing an

insulating spacer is as low as 9%, whereas for dye with a conducting spacer an

outstanding IPCE 80% was obtained.

Rawling and co-workers have reported four monomeric complexes and two dyads

consisting of ruthenium phthalocyanine and bis(bipyridyl)-ruthenium(II)

chromophores which vary in peripheral substitution and axial ligand anchoring

groups (Rawling et al. 2009). The results highlight the importance of molecular size,

and thus the dye coverage of the electrode surface in the design of new sensitizing

dyes.



Komori et al have reported dye-sensitized solar cell (DSSC) with visible and near-

infrared sensitization of nanocrystalline TiO2 films using a series of four aluminum

phthalocyanines. For all the DSSCs using aluminum phthalocyanines, IPCE values

at near-infrared region (700 nm) were larger than those at visible region (500 nm)

(Komori and Amao 2003). It has been shown by Masaru et. al. that bis(4-

carboxypyridine)-phthalocyaninato ruthenium(II) sensitizer has an incident photon-

to-current conversion efficiency (IPCE) of 21% at 640 nm and overall conversion

efficiency (η) of 0.61% that is a good value for solar cells based on a phthalocyanine

dye.

Vivian and co-workers have shown successful sensitization of nanostructured TiO2

electrodes with tetradimethoxyphenyl phthalocyanine and tetra-phenyl

phthalocyanine derivatives that exhibits IPCEmax of 9 and 5% respectively at 650 nm

(Vivian et al. 2001). Shen and co-workers have reported some derivatives containing

functional groups on the periphery of a metallated phthalocyanine exhibiting

maximum IPCE values of 1–4% in the near-IR region (Shen et al. 1995).

However, some have even reported high values of IPCE as 45% by use of co-

adsorbers which prevent aggregation of dye molecule (Nazeeruddin et al. 1999).

Masaru et al have derived Zinc phthalocyanine dyes for nano-structured TiO2 solar

cells103 with an IPCE value of 1.6% at 690 nm and Ruthenium phthalocyanine dyes

with axial pyridine ligands having IPCE of 23% at 630 nm (Masaru 2004). The

overall conversion efficiencies were 0.03% and 0.40% for the zinc and ruthenium

complexes respectively. The noticeable difference in efficiencies could be due to the

formation of face-to-face aggregation in the former case whereas this aggregation

could be avoided both in solution and on the surface due to axial pyridine ligands in



the latter case. Torres et al. have demonstrated the use of phthalocyanine derivative

with t-butyl substituents on peripheral positions anchored to TiO2 films for efficient

photocurrent generation in dye sensitized photoelectrochemical solar cells

(Palomares et al. 2004).

1.5.4 Porphyrins

In natural photosynthesis, which is the highest photon-to-electron energy conversion

system in the earth, porphyrin-based chromophores play important roles not only as

strongly light-harvesting pigments but also as sequential intermolecular energy and

electron transfer components. The use of porphyrins as light harvesters on

semiconductor is particularly attractive due to the similarity of the DSC’s and

photosynthetic working principle, namely the excited state electron transfer process.

Due to these reasons, the energy generating process of dsscs is generally referred to

as ‘‘artificial photosynthesis”. Thus, many researchers have been attracted to

porphyrins-based chromophores in the various field of science such as biology,

chemistry and photo physics (Imahori et al. 2003; Choi et al. 2004). The advantages

of porphyrins in dye-sensitized solar cells include:

1. Porphyrins exhibit long-lived (> 1 ns) singlet excited state and only weak

singlet/triplet mixing.

2. They have an appropriate LUMO level that resides above the conduction

band of the TiO2 and HOMO level that lies below the redox couple potential.

This is a very important requirement of colorants in dye-sensitized solar

cells.



3. Presence of very strong absorption in the 400 - 450 nm region (Soret band)

as well as in the 500 - 700 nm region (Q-bands).

However, porphyrins suffer from the drawbacks like poor light-harvesting properties

thus limiting its cell performance in dye sensitized TiO2 cells.

One promising way to surmount this problem relating to the poor light-harvesting

properties of porphyrins is to modulate the electronic structures of porphyrins, so

that one can match the light-harvesting properties with the solar energy distribution

on the earth. Elongation of the π conjugation and loss of symmetry in porphyrins

cause broadening and a red shift of the absorption bands together with an increasing

intensity of the Q bands relative to that of the Soret band. On the basis of this

strategy, the cell performance of porphyrin-sensitized solar cells has been improved

intensively by the enhanced light absorption. Actually, some push-pull-type

porphyrins have disclosed remarkably high power conversion efficiency (6-7%) that

was close to that of the ruthenium complexes.

However, the incident photon-to-current efficiencies (IPCEs) of the substituted

porphyrins are found to be only 5–7%. In contrast, the IPCEs of high- efficiency

DSSCs normally reached 70–80% which is the desired result. Low IPCEs mean that

the photons absorbed by the porphyrin dye are not converted to photocurrent

efficiently.

This is attributing to two effects associated with the porphyrin dyes:

(1) Aggregation of the porphyrin molecules

(2) Geometrical structure of the anchoring group.



Organic dyes have been shown to aggregate strongly on the TiO2 surface. Emission

quenching measurements revealed that the aggregation reduces the injection yield of

the photocarriers from the photoexcited dye into the conduction band of TiO2 (Kroon

et al. 2007; Wang et al. 2007). In addition, the aggregation effect becomes stronger as

the molecule size increases. This would explain the results that the substituted

porphyrins, which are larger molecules, have lower IPCEs than the unsubstituted

porphyrin.

The second reason for low IPCEs is due to the geometrical structure of the anchoring

group. Electron injection efficiency depends on the substituent and the position at

which the substituent is anchored to the porphyrin molecule. In order to produce

large conversion efficiencies, the porphyrin macrocycle should have strong

electronic coupling to TiO2. If an anchoring group is linked directly to the

macrocycle, the distance reduces and the coupling becomes strong thereby

improving efficiency. Such a case has been reported by Wang et al in which the

porphyrin DSSCs exhibited efficiency of 5% where the anchoring COOH group was

directly attached to the β position of the macrocycle (Wang et al. 2005). The

orientation of the anchoring group with respect to the macrocycle also affects

electronic coupling. The preferred orientation is to have the anchoring group parallel

to the macrocycle. Hence, although the absorption bands of the substituted porphyrin

have been successfully red-shifted to the infrared region but to improve the

conversion efficiency, it is crucial to reduce aggregation of the porphyrin molecules

and design anchoring groups with better structure.

Although, incident-photon-to-current conversion efficiency of 85% has been

reported with many dyes but its main absorption region is concentrated in the short-



wavelength region of the visible spectrum, i.e., 400–700 nm. However, the solar

spectrum covers ultraviolet, visible and near-infrared (NIR). Indeed, over 60% of the

total solar photon flux is at wavelengths λ > 600 nm with approximately 50% in the

red and NIR spectrum at 600 < λ < 1000 nm. This means that the most of the dye

utilizes only one half of the solar photon flux. Hence, much effort has been devoted

to develop new dyes with broader absorption ranges, especially in the red and NIR

regions.

Various porphyrins have been used for the photosensitization of wide-band-gap

semiconductors (SCs) like NiO, ZnO and TiO2, the most common being the free-

base and zinc derivatives of the meso-benzoic acid substituted porphyrin. In

contrast, other workers have tested pyrrole-β substituted porphyrins as

photosensitizers for nanocrystalline TiO2 semiconductor. It is observed that the zinc

porphyrin with extended π-conjugated acrylic acid derivative has shown the

efficiency as high as of 5.2% (Campbell et al. 2004; Wang et al. 2005). Giribabu et

al. showed that a photosensitizer having two rhodanine acetic acid groups at meso-

positions of a zinc porphyrin [meso-Rhod-Zn-Rhod] has been characterized with the

maximum of 0-76% in AH3 redox electrolyte (Giribabu et al. 2008). Cherian et al

reported the energy conversion efficiency of Tetra (4-carboxyphenyl) porphyrin

(TCPP) adsorbed strongly onto nanoparticulate TiO2 to be about 3% (Cherian et al.

2000).

Tamaki et al have synthesized novel directly-linked heterometallic porphyrin dimers

and used as sensitizers for dssc that showed high photon- to-electron conversion

efficiency of about 2.81%. Takahashi et al have reported a dimer used as

photosensitizing dye in an Al/dye/Au sandwich-type cell that showed better energy



conversion yield of 4.8 %. The dye dimer is the porphyrin heterodimer(HD)

consisting of H2pyp3p(Cl) and Zntpp(OMe)2 or the mixed dimer consisting of

5,10,15,20-tetraphenyl- porphyrinatozinc(Zntpp) and 3-ethyl-5-[(3-ethyl-2(3H)-

benzothiazolylidine) ethylidene]- 2-thioxo-4-thiazolidinone(MC) (Takahashi et al.

1997). Hideyuki et al. attempted to synthesise porphyrins covalently linked to

viologen by methylene chain. This type of molecule showed increase of electron

transfer yield from the photoexcited singlet state of porphyrin to viologen by

changing the distance between the porphyrin and the bonded viologen (Hideyuki et

al. 1997)

Lee et al reported porphyrin substituted with triarylamine in which a comparative

study was done by taking (Zinc (II) 5, 15-bis (4-carboxylphenyl) porphyrin as a

reference and two other porphyrin derivatives with one and two triphenylamine

groups respectively. The absorption spectra reveal that the substitutions resulted in

large redshifts in both the Soret band (60 nm) and the Q bands (125 nm), as well as

enhancement of optical absorption. The incident photon-to-current conversion

efficiency is 24% for the reference porphyrin and 5–7% for other porphyrins at the

Soret peak (Lee et al. 2009).



1.6 Q-dots as sensitizers

The efficiency of conversion in dye sensitised solar cells is mainly restricted by the

non equilibrium conditions due to power extraction, the entropy of light and

the free energy wastage under polychromatic radiation. This factor is by far the

most significant source of loss as all incident radiation lower in energy than the band

gap is not absorbed and all energy greater than the band gap is lost as heat. The

demonstration of efficient Impact Ionization in semiconductor quantum dots (QD)

by Klimov and others has created significant enthusiasm as to the possibility of

multiple electron generation (Klimov 2006).

Modification of the properties of bulk semiconductors is very hard to bring about

given the quasi-continuous spacing of energy levels which makes changes to

the Eg nearly impossible. This is not the case in semiconductor quantum dots

whose discrete energy levels allow for greater versatility. When an electron from

the valence band is excited to the conduction band, it leaves behind a positive empty

space called a hole. This is the electron-hole pair, the exciton, which is responsible

for the electric conduction in these materials.

Contrary to bulk semiconductors where the exciton is free to move, the

exciton finds itself confined in a physically restrained space when in QDs

characterized by a size that is comparable to or smaller than the Bohr exciton

radius. Impact ionisation can also occur in principle under such circumstances.

Impact ionization is an inverse Auger process by which an excited charge carrier

loses energy by the creation of a new charge carrier. When a high-energy electron

relaxes back to the band edge by transferring at least electron in the valence band,



this electron has enough energy to jump the band gap and produce an additional

exciton. This excess energy can thereby be used for multi-exciton generation and

potentially improve photovoltaic conversion efficiencies. In bulk semiconductors

impact ionization (ImI) must compete with energy relaxation by electron-phonon

scattering. The situation is different in QDs where the discrete character of

the spectrum makes electron-photon relaxation a lot less efficient. Quantum dots

are traditionally synthesized in organic media (Geissbuehler et al. 2005). This

synthetic route gives rise to good monodispersities as well as high quantum

yields. However, the long chain TOP/TOPO capping ligands are not conducive to

electron injection essential in solar cell sensitization. Ligand exchange can be

performed on the synthesized QDs but this gives rise to dramatic decreases of

photoluminescence. Precipitation of the dots is also often observed. Aqueous

synthesis is therefore preferred even though the quantum yields are generally lower

and the size distribution larger. Another important issue is the interaction of the

QDs with TiO2 or other mesoporous oxide layers used as the wide bangap

semiconductor in the sensitised solar cells.

Efficient sensitization will depend on how well the QDs will also adsorb to

the oxide surface.

The following types of Q-dot sensitized solar cells can be studied.

1) Charge transfer based solar cells

2) Energy transfer based solar cells



1.7 Flexible DSSCs

The replacement of rigid and heavy substrates by light-weight flexible materials

can allow the low-cost fabrication of cells through roll-to-roll mass production

methods. The techniques of TiO2 film fabrication are a very important step

in the production of DSSC (Kroon et al. 2007; Ito et al. 2008). The currently most

preferred deposition technique is screen-printing yielding controllable thickness and

uniform morphologies. The screen-printed layer is heated to 450 °C to obtain good

working layers. This limits its use to only conducting glass substrates, excluding the

use of plastic substrates. Methods such as ductor-blade coating, spraying methods

and electrophoretic deposition, have been applied in the past to deposit

mesoporous TiO2 layers on top of ITO-coated PET films. However, the

cell efficiencies reported for such deposition methods are not satisfactory. The

poor efficiencies reported for ITO-PET based DSSC are mainly attributed to the

poor necking of TiO2 particles and inadequate adherence to the flexible substrates.

This is mainly because of the limitation in sintering temperatures employed.

Another reason is the thinner TiO2 layers used in flexible cells. As mentioned

earlier, this problem can be solved by using new sensitizers with higher molar

extinction coefficients. To circumvent the problem of low-temperature sintering,

lift-off methods have been tried as well (Wang et al. 2005; Wang et al. 2006).

However, practical difficulties preclude the use of this technique in mass-scale

manufacturing. The high temperature sintering step could be retained, if the flexible

cells are manufactured using metal foils. In this case the illumination should enter

through a transparent plastic counter electrode (back side illumination). Ti, stainless

steel, W and Zn have been proposed as the most suitable foil substrates for



making such flexible DSSCs An efficiency of 4.2% under 100mW/cm2 light

intensity has been reported for a DSSC fabricated on an

ITO/SiO2/stainless steel substrate.

Processing of oxide layers on plastic materials after deposition by a screen-printing

route or ductor blade method is thus problematic. TiO2 printing inks which can

form well necked (particle-to-particle) layers, offer better adhesion with high

conductivity and no left over carbon residue at the lower temperature on flexible

and plastic foil substrates are essential to make a breakthrough and yield

flexible plastic-based sensitised solar cells reaching double digit energy conversion

efficiencies.



1.8 Conclusions

The importance of the low cost/high efficiency sensitized solar cells suggests

that its need to be developed. Current SSC conversion efficiencies need to catch

up with other more mature technologies (rigid modules) which have solar energy

conversion efficiencies beyond 13-15%. While the flexible SSCs can have

efficiencies in the range 5-10%, definitely comparable with more matured

technologies (flexible) it is very important to note that these cells have an added

advantage over other PV technologies in that they are superior solar energy

harvesting devices under diffuse light conditions. The net effect of this will be

to bring the operational efficiency of SSCs into line with current technologies.

Reduction in the cost of PV to as low as €0.5/Wp with a concomitant commercially

viable efficiency will dramatically broaden the scope for applications involving

power generation via solar energy harvesting. In particular, lightweight, flexible

SSCs with conformal shape adaptability will see service not only in urban

domestic and commercial settings, but also in isolated, rural locations and under

field conditions (e.g. refrigeration of medical care). SSCs will also have immediate

indoor as well as outdoor applications and will supplement the current widespread

use of personal electronic devices such as battery chargers. Materials can be

developed that can be useful in rigid cell structures which currently have a defined

market of their own e.g. building integrated PV. These new materials will then be

able to take the rigid DSSC efficiencies closer or beyond the levels of greater rigid

PV cells.

chapter 1: dye-sensitized solar cells: an overview 1.1...

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