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Dye-sensitized solar cell based on nanocrystalline ZnO thinfilm electrodes combined with a novel light absorbing dyeCoomassie Brilliant Blue in acetonitrile solution

Pankaj Srivastava*, Lal Bahadur

Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi e 221 005, India

a r t i c l e i n f o

Article history:

Received 10 October 2011

Received in revised form

3 December 2011

Accepted 10 December 2011

Available online 4 January 2012

Keywords:

Dye sensitized solar cell

Zinc oxide

Nanocrystalline thin film

Coomassie Brilliant Blue

* Corresponding author. Fax: þ91 542 236812E-mail addresses: pankaj@bhu.ac.in, pan

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.064

a b s t r a c t

In this work, characterization of dye-sensitized solar cells (DSSC) using nanocrystalline

ZnO thin film electrodes combined with a novel light absorbing dye Coomassie Brilliant

Blue (CBB), in acetonitrile solution is reported. The absorption spectrum of this dye in

acetonitrile solution indicates appreciable absorption in the range of 500e700 nm with

a sharp peak at 597 nm indicating its possible use as a photosensitizer for ZnO. The cur-

rentevoltage and efficiency characteristics of a DSSC based on this dye and ZnO acceptor

are measured for two methods of depositing the ZnO. Better response is achieved for

nanocrystalline ZnO thin films than for sprayed films in terms of cell output.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction compounds of Ru(II) with different polypyridyl derivatives

The necessity of exploiting solar energy has led to the devel-

opment of various means for its conversion into some

conveniently usable form (electrical or heat energy). The high

cost of very efficient solid-state photovoltaic cells is the main

deterrent to their large-scale application. Dye-sensitized solar

cells (DSSCs) (dye-coated wide band-gap metal oxide semi-

conductor electrode/electrolyte/counter electrode) have

shown a significant promise as a cost-effective, efficient and

an environmentally friendly alternative to solid photovoltaic

devices [1e10]. Since Gratzel and co-workers reported

achieving an unprecedented high light-to-electrical conver-

sion efficiency with a DSSC based on a nanocrystalline TiO2

thin film electrode sensitized by RuII(2,20-bipyridle-4,40-dicor-boxylate)2 (NCS)2 [1], a number of other coordination

7.kaj_bhuin@rediffmail.com2011, Hydrogen Energy P

have been synthesized and used as sensitizers [5,11e15]. In

the last decade metal-centered dyes [16,17] have invariably

been the best performing and most widely researched sensi-

tizers in combination with nanocrystalline TiO2, reaching

efficiencies as high as 11% [18]. Such cells have also shown

remarkable photochemical stability on long-term operation.

The semiconductor electrode is the key component of

DSSC. The effective surface area and porosity of thin films can

be greatly enhanced if they are prepared from nanosized

colloidal particles of the semiconductor. Such films facilitate

greater adsorption of dyemolecule on their surfaces, which in

turn improves absorption of incident light for its conversion

into electrical energy. It was for this reason that inmost of the

work conducted on such cells, nanocrystalline thin films of

semiconductors prepared from their colloids have been used

(P. Srivastava).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 8 6 3e4 8 7 04864

[19e29]. ZnO and TiO2 semiconductor thin film electrodes

have recently been employed extensively in photo-

electrochemical (PEC) cells for potential applications [30e34].

Several factors are important for a dye to act as an efficient

photosensitizer: the ability to adhere strongly on the surface

of a semiconductor, absorption of light in the visible and NIR

region for efficient light harvesting, high extinction coefficient

and an excited state reduction potential negative enough for

efficient electron injection into the conduction band of the

semiconductor being the main requisite. Recently, novel

compounds such as mercurochrome [35], phthalocyanines

[36], hemicyanine [37], metalloporphyrins [38], polyenes [39],

coumarins [40], triphenylamine [41], styryls [42], ferrocene

[43], and indoline [44] based dyes have been studied for their

possible application as photosensitizers and efforts need to be

continued to search for newer one. Coomassie Brilliant Blue

(CBB) (shown below), widely used for staining proteins in

biochemical sciences and having the primary requisite prop-

erties to act as photosensitizer, has yet not received attention

of theworkers in this field to explore its possible application in

dye-sensitized solar cells.

Hence, the title investigation was undertaken in which the

sensitization of photocurrent by CBB at nanocrystalline ZnO

thin film electrodes has been studied in acetonitrile solution

and a comparison has been made with the performance

observed with sprayed ZnO thin film electrodes. Acetonitrile

has been used as the medium of the electrolyte because

semiconductor thin film electrodes have been found to be

fairly stable in this medium under the operating conditions of

the cell.

2. Experimental details

2.1. Materials

Acetonitrile (Merck, HPLC grade) used as the medium of

electrolyte solution, was purified as described in our earlier

report [7]. Ethanol (Merck, India) was dried before using it for

preparing ZnO-sol. Zn(NO3)2.6H2O (Merck), Zn(CH3COO)2.2H2O

(Merck), LiOH.H2O (Alfa product) and Coomassie Brilliant

Blue (Loba Chemie) were used as received. Anhydrous

NaClO4 (Fluka) and hydroquinone (Merck) used as supporting

electrolyte and redox reagent respectively, in photo-

electrochemical experiments were added without any further

purification.

2.2. Methods

Thin films of ZnO have been prepared by spraying an

aqueous solution of 10�2 M Zn(NO3)2.6H2O on ultrasonically

cleaned non-conducting glass substrate (Blue star, India) at

400 � 20 �C, using a thermostatically controlled vertical

furnace and subsequently annealing in hydrogen atmo-

sphere for 1 h, to make it conducting. The nanocrystalline

ZnO thin films were prepared on conducting glass substrate

(F: SnO2, surface resistivity 15e20 U/,, Pilkington Group Ltd)

using the organometallic precursor consisting of dense ZnO

sol which was prepared following the procedure of Spanhel

and Anderson [45]. The plate (conducting glass substrate)

was heated up to 80 �C before dipping it in the sol. The

substrate was dipped vertically and kept immersed in the sol

for 10 min. This process was repeated 5e6 times and finally

the films were annealed in air at 400 �C for 1 h. In this way

thin films of w2 mm thickness were obtained. To make the

ohmic contact, a thin copper wire was attached on the

surface of the film with the help of silver paste (Eltech

Corporation, India). The occluded solvent in the silver paste

was evaporated by drying in air and the bare part of the

copper wire and the silver coated area of the film were

covered with the Araldite and air dried before use. The

semiconductor thin film electrodes were kept immersed in

dye solution (1 mM, prepared in acetonitrile) for about 12 h to

fix the dye on their surface.

2.3. Apparatus and instruments

Unless stated otherwise, all voltammetric experiments were

carried out in a three-electrode, single compartment cell. For

photoelctrochemical characterization an optically flat quartz

window (Oriel Corporation, USA) for the illumination of the

working semiconductor electrode was used. A spiral platinum

wire was used as counter electrode. The sodium chloride-

saturated calomel electrode [SSCE (aq.), E0 ¼ þ0.236 V vs.

NHE], served as reference electrode. For determining the

power output of the test dye-sensitized ZnO electrode based

PEC cell, a carbon rod, obtained from a Novino dry cell, was

used as counter electrode. Prior to each experiment, the

cell solution was degassed by bubbling purified Nitrogen

through it.

A bi-potentiostat (Model No. AFRDE 4E, Pine Instrument

Company, USA), along with XeY1-Y2 recorder (Houston

Instruments model 2000), was used for all current-potential

measurements. Semiconductor electrode was illuminated

with condensed (with silica lenses obtained from Oriel

Corporation, USA) light beam of a 150 W Xenon arc lamp

(Oriel Corporation, USA). IR & UV filters in the form of 6

inches long water column & long-pass filter (Model No.

51280, Oriel Corporation, USA) respectively, were used in

front of the sample and the corresponding lights being

referred to as ’white light’ and ‘visible light’ (l > 420 nm).

The monochromatic light was obtained with the use of

a monochromator (Oriel model 77250 equipped with model

7798 grating), and the corresponding photocurrent was

measured with the help of a digital multimeter (Philips

model no. 2525) in combination with the potentiostat. The

intensities of light were varied with neutral density filters

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 8 6 3e4 8 7 0 4865

(50490-50570, Oriel Corporation, USA) and measured with

a digital photometer (Tektronix model J16 with J 6502

sensor). The lamp intensity was found to fluctuate by �5%

setting a limit for the relative error in the IPCE measure-

ments. The absorption spectra were recorded on Cary 2390

spectrophotometer (Varian). Impedance measurements were

made using EG & G, PARC 378/3 system. The surface

morphology of the semiconductor thin film was examined

through SEM (JEOL Model No. JSM 840A) and Atomic Force

Microscope (Burleigh Metric 2000).

3. Results and discussion

3.1. Surface characterizations of ZnO thin films

The surface morphology of ZnO thin film was probed by SEM

and AFM and the photomicrographs are shown in Fig. 1 (aed).

It is clear from these photomicrographs that the nano-

crystalline film is composed of ultra-small particles, which are

in close contact with each other and such a three-dimensional

network of nanocrystallites is expected to make the thin ZnO

film highly porous (a). However there is no uniformity of the

film surface in the sprayed films (b). Further, sprayedmaterial

Fig. 1 e SEM and AFM micrograpgs of ZnO thin films prepar

is deposited without any continuous link. AFM images indi-

cate the depth profile (or roughness) of the film’s surface.

Nanocrystalline ZnO thin films possessed large surface area (c)

as compared to that of sprayed films (d), which are essentially

required for greater adsorption of dye molecules on its

surface.

3.2. Optical property of Coomassie Brilliant Blue

The optical property is the most crucial one for deciding the

ability of the compound to act as photosensitizer. The absorp-

tion spectrum of CBB taken in acetonitrile solution (Fig. 2)

indicates appreciable absorption with a sharp peak at 597 nm.

The onset of absorption, which occurs at around 700 nm is

extended up tow500 nm, covering a large fraction of the visible

region of the solar radiation. Hence it fulfils the primary

requirement for its possible use as sensitizer for extending the

spectral response of ZnO to visible range of solar radiation.

3.3. Electrochemical redox behaviour of the dye inacetonitrile medium

The redox behaviour of dye was studied at platinum electrode

in acetonitrile medium using 0.1 M NaClO4 supporting

ed by sol-gel (a & c) and spray pyrolysis method (b & d).

Fig. 2 e Absorption spectrum of 0.002 mM solution of

Coomassie Brilliant Blue in acetonitrile solution.

Fig. 3 e Cyclic voltammogram of CBB (0.001 mM) at

platinum electrode in acetonitrile solution containing 0.1 M

NaClO4 supporting electrolyte, figure on the curves being

the scan rates (mVsL1).

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electrolyte and the cyclic voltammogrms obtained at different

scan rates (60e300 mV�1) are shown in Fig. 3. From these

voltammograms it is evident that CBB can undergo reduction

at �0.700 � 0.025 V vs. SSCE. Therefore, CBB can act as an

acceptor-type dye and the reductive redox potential provides

electron accepting energy level of the dye ðE0D=D� Þ in the ground

state.

3.4. Determination of flat band potential (Vfb) of ZnOElectrode

Fig. 4 represents the MotteSchottky plots determined at

three frequencies (2.5, 6.3 and 10 KHz) for sprayed (a) and

nanocrystalline (b) ZnO thin film electrodes in acetonitrile

solution containing 0.1 M NaClO4, 0.01 M H2Q and 0.01 mM

Coomasie Brilliant Blue. From this figure, it is evident that

frequency dispersion effect was observed for both types of

ZnO electrodes showing the presence of surface states. The

MotteSchottky plots for nanocrystalline ZnO thin film

electrodes were found to be non-linear while such plots for

sprayed thin film electrodes were linear. The non-linearity

in the case of nanocrystalline electrodes makes the extrap-

olation at the potential axis rather uncertain. Because of this

reason we could not say anything regarding the conduction

band differences between the two ZnO preparations.

Nevertheless, in spite of frequency dispersion effect all the

curves in Fig. 3a (for sprayed film) converge at the same

point at the potential axis and this provides the flat band

potential Vfb ¼ �0.55 V vs. SSCE for the ZnO electrode. This

gives the position of the Fermi level ðEFÞ on electrochemical

potential scale with respect to the reference electrode

(SSCE).

3.5. Energy level diagram

The thermodynamic feasibility of electron injection from

photo-excited dye molecule (D*) into the conduction band of

ZnO electrode [D*/ Dþ þ e� (ZnO)] and subsequent regener-

ation of dyemolecule to its original form can be accessed from

the energy level diagramdepicting the relative positions of the

electron exchange energy levels of all the cell components.

Using the value of Vfb of ZnO electrode, reductive redox

potential of the dye determined from the cyclic voltammetry,

photoexecitation energy of the dye corresponding to its lmax

(597 nm) obtained from the absorption spectrum, and the

redox potential of the supersensitizer (H2Q) in acetonitrile [46],

the energy level diagram was constructed and the same is

shown in Fig. 5. Based on the relative positions of various

electron exchange energy terms it can be inferred that the dye

molecules being in their ground state, represented byðE0D=D� Þ,

can be photo-excited to occupy the energy level E0D�=D� on

absorption of light of energy corresponding to lmax ¼ 597 nm.

Further, the excited dye molecules (D*) can be reduced (D�) onreacting with reduced species (H2Q) of the redox reagent.

These reduced dye molecules having the extra exchange

energy level corresponding to E0D=D� can inject electrons into

the conduction band of the semiconductor (ZnO) electrode

and thereafter get converted back to its initial form (D� / D).

In this way the dye molecules can mediate the photo-induced

charge transfer process occurring at the semiconductor elec-

trode and provide the means to convert light energy into

electrical energy. However, the overall efficiency of the system

cannot be predicted only on the basis of these thermodynamic

parameters alone, the kinetic aspects of various processes

involved also need to be explored.

Fig. 4 e MotteSchottky plots for (a) sprayed and (b) Nanocrystalline thin film electrodes in acetonitrile solution containing

0.1 M NaClO4and 0.01 M hydroquinone at three different measuring frequencies.

Fig. 5 e Schematic energy level diagram showing electron

injection from photo-excited dye molecules to the

conduction band of ZnO.

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4. Photoelectrochemical studies

4.1. Current epotential (ieV) curves

Fig. 6 shows the dynamic current-potential curves for bare

(curves aec) and dye-sensitized (curves a0ec0) nanocrystallineZnO electrodes in dark (curves a and a0), under illumination

with visible light (curves b and b0) and white light (curves c and

c0). For comparison, the similar curves obtained with sprayed

ZnO electrode in absence (curve d) and presence (d0) of dye

under visible light illumination is also included in the figure.

These current-potential curves indicate that in dark, the

current remains at a very low level in absence (a) as well as in

the presence of the dye (a0) in the potential range used.

Whereas under illuminated condition, there is a significant

enhancement in photocurrent when ZnO electrode is sensi-

tized by dye (curves b0 and c0) as compared to that observed

with bare electrode (curves b and c). With sprayed ZnO elec-

trode, the enhancement in photocurrent on sensitization by

dye (compare curve d0 with curve d) was found to be less than

that observed with nanocrystalline ZnO electrode (compare

curve b0 with b) under similar illumination condition. Further,

the shape of curve b0 obtained with nanocrystalline ZnO

electrode was almost of ideal nature while the curve

d0 obtained with sprayed ZnO electrode under identical

condition is quite distorted. This shows the improved

performance of the test dye on nanocrystalline ZnO electrode

than the sprayed ones. With the use of this dye on nano-

crystalline ZnO, a photovoltage of 360 mV (onset of potential,

exclusively induced by dye on visible light illumination),

photocurrent of 160 mA cm�2 and fill factor of 0.4 could be

achieved. With similar particulate ZnO films sensitized by

Rhodamine B, Mercurochrome, and Squarine dye earlier,

comparable open circuit photovoltages of 260, 520 & 610 mV

respectively could be achieved [4,47,48]. Still lower photo-

voltage & photocurrent values for sprayed films/CBB are again

evidencing better response of the nanocrystalline thin films

than for sprayed films.

Fig. 7 e The action spectra (IPCE-l) of dye-modified (curve a)

and bare (curve b) nanocrystalline ZnO thin film electrodes

in acetonitrile solution containing 0.1 M NaClO4, 10L2 M

hydroquinone, and 0.002 mM dye (only in the case of curve

a). Curve (c) represents the absorption spectrum of

0.002 mM dye solution in acetonitrile.

Fig. 6 e Currentepotential curves for bare and dye-

sensitized nanocrystalline ZnO electrodes respectively in

dark - curves a & a’, under visible light (422 mWcmL2)

illumination - curves b & b’, and under white light

(556 mWcmL2) illumination - curves c & c’. Curves d and

d0 represent ieV curves for sprayed ZnO slectrode in

absence and presence of dye respectively under visible

light illumination. Solution composition: 0.1 M NaClO4,

0.01 mM CBB and 10 mM hydroquinone for curves a0, b0, c0

and d0 while for curves a, b, c, and d the dye was not used.

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4.2. Spectral dependence of photocurrent

In order to confirm conclusively the sensitization of photo-

current by the test dye, the spectral dependence of the

photocurrent (iphoto vs. l) was determined. For this purpose

the photocurrent was measured at each wavelength (l) of the

incident monochromatic light and the incident photon-to-

current conversion efficiency (IPCE) was calculated using the

following relation,

%IPCE ¼ 1240� iscðAcm�2Þ�2 � 100 (1)

Fig. 8 e Absorption spectra of dyed nanocrystalline ZnO

electrode coated with 0.002 mM CBB (curve a). Curve b is

the (APCE-l) plot for nanocrystalline ZnO sensitized by CBB.

lðnmÞ � IincðWcm ÞIinc being the intensity of the incident light and isc is the cor-

responding short-circuit photocurrent of the PEC cell

(ZnO film/dye -containing electrolyte/Pt). In Fig. 7, curve

a shows the (IPCE vs. l) plot for dye-sensitized nanocrystalline

ZnO electrode in acetonitrile solution and curve b is same for

the bare electrode (without dye). The nature of the action

spectrum (curve a) resembles fairly the absorption spectrum

of the dye in solution (curve c) indicating the sensitization of

photocurrent by dye. However, the photocurrent response is

narrowed down to a very small range of visible spectrum

(curve a). It is also to be noted that there is small red shift

(w8 nm) in the photocurrent peak as compared to the

absorption peak of dye in solution. This is probably due to

interaction of the dyemolecules with ZnO surface resulting in

slight decrease in the excitation energy of the adsorbed dye

molecules.

In calculating the IPCE values the intensity of incident

(not the absorbed) light is used and hence it does not take into

account the actual light harvesting efficiency of the cell. So,

IPCE values were converted to absorbed photon-to-current

conversion efficiency (APCE) using the following relation:

APCE ¼ IPCEð1� 10�AbsÞ (2)

Abs is the absorbance of the dyed electrode. For this purpose

the absorption spectrum of dyed electrode was recorded and

the same is shown in Fig. 8 (curve a). The values of Abs ob-

tained from this curve were used to calculate the APCE values.

The (APCE vs. l) plot is returned in Fig. 8 (curve b). The

maximum APCE value was found to be 4.5%. Nevertheless,

with similar particulate ZnO thin film photoelectrodes sensi-

tized by Rhodamine B, Rao et al. have found 8% [4] while

Gratzel et al. could get 6e7% with monolayer of Ru based

complex [50].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 8 6 3e4 8 7 0 4869

However, the absorption spectrum of dye-coated ZnO

electrode is almost flat (Fig. 8, curve a) indicating aggregation

of dye molecules leading to multi-layer formation. Although

dye agglomerates contained in over layers lead to enhanced

light absorption resulting in a broad absorption spectrum,

they do not participate in electron injection [49]. Dye mole-

cules contained in the first layer only, which are in direct

contact with ZnO film, efficiently inject electrons into the

semiconductor from their photo-excited state. Not only that,

the over layers may obstruct the dye molecules in the first

layer from absorbing light which ultimately reduces the effi-

ciency. These might be the reasons for narrow action spec-

trum and lower IPCE/APCE values.

5. Conclusions

In this work we have reported the photosensitization of

nanocrystalline thin film ZnO electrode by extending its

spectral sensitivity to the visible region with a novel quite low

cost light absorbing dye Coomassie Brilliant Blue.With the use

of this dye on nanocrystalline ZnO, a photovoltage of 360 mV

(exclusively induced by dye on visible light illumination) and

photocurrent of 160 mA cm�2 could be achieved. Through this

investigation it has clearly been demonstrated that the same

dye can perform better on nanocrystalline than sprayed thin

film electrodes of the same semiconducting material. Further

improvements can possibly be accomplished by optimizing

system with respect to different forms of nanostructures (viz.

nanorods, nanotubes, flower-like etc.), particle size, film

thickness, redox couple’s concentration, dye loading, etc., and

efforts in these directions are underway.

r e f e r e n c e s

[1] O’Regan B, Gratzel M. A low-cost, high-efficiency solar cellbased on dye-sensitized colloidal TiO2 films. Nature 1991;353:737.

[2] Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R,Muller E, Liska P, et al. Conversion of light to electricity bycis-X2bis(2,20-bipyridyl-4,40-dicarboxylate)ruthenium(II)charge-transfer sensitizers (X ¼ Cl�, Br�, I�, CN�, and SCN�)on nanocrystalline titanium dioxide electrodes. J Am ChemSoc 1993;115:6382.

[3] Kamat PV, Bedja I, Hotchandani S, Patterson LK.Photosensitization of nanocrystalline semiconductor films.modulation of electron transfer between excited rutheniumcomplex and SnO2 nanocrystallites with an externallyapplied bias. J Phys Chem 1996;100:4900.

[4] Rao TN, Bahadur L. Photoelectrochemical studies on dye-sensitized particulate ZnO thin-film photoelectrodes innonaqueous media. J Electrochem Soc 1997;144:179.

[5] Kalyanasundaram K, Gratzel M. Applications offunctionalized transition metal complexes in photonic andoptoelectronic devices. Coord Chem Rev 1998;177:347.

[6] Ferrere S, Gregg BA. Photosensitization of TiO2 by[FeII(2,20-bipyridine-4,40-dicarboxylic acid)2(CN)2]: bandselective electron injection from ultra-short-lived excitedstates. J Am Chem Soc 1998;120:843.

[7] Bahadur L, Srivastava P. Efficient photon-to-electronconversion with rhodamine 6G-sensitized nanocrystalline

n-Zno thin film electrodes in acetonitrile solution. SolEnergy Mater Sol Cells 2003;79:235.

[8] G- Meyer J. Molecular approaches to solar energy conversionwith coordination compounds anchored to semiconductorsurfaces. Inorg Chem 2005;44:6852.

[9] Wang Z-S, Cui Y, Dan-oh Y, Kasada C, Shinpo A, Hara K.Thiophene-functionalized coumarin dye for efficient dye-sensitized solar cells: electron lifetime improved bycoadsorption of deoxycholic acid. J PhysChemC2007;111:7224.

[10] Kushwaha S, Bahadur L. Characterization of somemetal-freeorganic dyes as photosensitizer for nanocrystalline ZnO-based dye sensitized solar cells. Int J Hydrogen Energy 2011;36:11620.

[11] Hagfeldt A, Gratzel M. Molecular photovoltaics. Acc ChemRes 2000;33:269.

[12] Nazeeruddin MK, Pechy P, Renouard T, Zakeeruddin SM,Humphry-Baker R, Comte P, et al. Engineering of efficientpanchromatic sensitizers for nanocrystalline TiO2-basedsolar cells. J Am Chem Soc 2001;123:1613.

[13] Watson DF, Meyer GJ. Electron injection at dye-sensitizedsemiconductor electrodes. Annu Rev Phys Chem 2005;56:119.

[14] Klein C, Nazeeruddin MK, Liska P, Di Censo Davide, Hirata N,Palomares E, et al. Engineering of a novel ruthenium sensitizerand its application in dye-sensitized solar cells for conversionof sunlight into electricity. Inorg Chem 2005;44:178.

[15] Ardo S, Meyer G-J. Photodriven heterogeneous chargetransfer with transition-metal compounds anchored to TiO2

semiconductor surfaces. Chem Soc Rev 2009;38:115.[16] Islam A, Sugihara H, Hara K, Singh LP, Katoh R, Yanagida M,

et al. Dye sensitization of nanocrystalline titanium dioxidewith square planar Platinum(II) diimine dithiolatecomplexes. Inorg Chem 2001;40:5371.

[17] Mayo E, Kilsa K, Tirrell T, Djurovich PI, Tamayo A,Thompson ME, et al. Cyclometalated iridium(III)-sensitizedtitanium dioxide solar cells. Photochem Photobiol Sci 2006;5:871.

[18] Chen C, Wang M, Li J, Pootrakulchote N, Alibabaei L, Ngoc-le C, et al. Highly efficient light-harvesting rutheniumsensitizer for thin-film dye-sensitized solar cells. ACS Nano2009;3:3103.

[19] Kamat PV, Dimitrijevic NM. Colloidal semiconductors asphotocatalysts for solar energy conversion. Solar Energy1990;44:83.

[20] Sodergren S, Hagfeldt A, Olsson J, Lindquist SE. Theoreticalmodels for the action spectrum and the current-voltagecharacteristics of microporous semiconductor films inphotoelectrochemical cells. J Phys Chem 1994;98:5552.

[21] Hagfeldt A, Gratzel M. Light-induced redox reactions innanocrystalline systems. Chem Rev 1995;95:49.

[22] Hoyer P, Weller H. Potential-dependent electron injection innanoporous colloidal ZnO films. J Phys Chem 1995;99:14096.

[23] Solbrand A, Lindstrom H, Rensmo H, Hagfeldt A,Lindquist SE. Electron transport in the nanostructuredTiO2�electrolyte system studied with time-resolvedphotocurrents. J Phys Chem B 1997;101:2514.

[24] Keis K, Lindgren J, Lindquist SE, Hagfeldt A. Studies of theadsorption process of Ru complexes in nanoporous ZnOelectrodes. Langmuir 2000;16:4688.

[25] Kuciauskas D, Freund MS, Grey HB, Winkler JR, Lewis NS.Electron transfer dynamics in nanocrystalline titaniumdioxide solar cells sensitized with ruthenium or osmiumpolypyridyl complexes. J Phys Chem B 2001;105:392.

[26] Schwarzburg K, Willig F. Diffusion impedance and spacecharge capacitance in the nanoporous dye-sensitizedelectrochemical solar cell. J Phys Chem B 2003;107:3552.

[27] Bahadur L, Srivastava P. Improved performance of 2-imidazolin-5-one as sensitizer at nanocrystalline ZnO thinfilm electrode in acetonitrile solution. Semicond Sci Technol2004;19:531.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 4 8 6 3e4 8 7 04870

[28] Hillhouse HW, Beard MC. Solar cells from colloidalnanocrystals: fundamentals, materials, devices, andeconomics. Curr Opin Colloid Interface Sci 2009;14:245.

[29] Damonte LC, Donderis V, Ferrari S, Meyer M, Orozco J,Fenollosa MAH. ZnO-based nanocrystalline powders withapplications in hybrid photovoltaic cells. Int J HydrogenEnergy 2010;35:5834.

[30] Kim DW, Lee S, Jung HS, Kim JY, Shin H, Hong KS. Effects ofheterojunction on photoelectrocatalytic properties ofZnOeTiO2 films. Int J Hydrogen Energy 2007;32:3137.

[31] Amao Y, Takeuchi Y. Visible light-operated saccharide-O2

biofuel cell based on the photosensitization of chlorophyllderivative on TiO2 film. Int J Hydrogen Energy 2008;33:2845.

[32] S‚ ennik E, Colak Z, Kılınc N, Ozturk ZZ. Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor. Int JHydrogen Energy 2010;35:4420.

[33] Hsu C-H, Chen D-H. Photoresponse and stabilityimprovement of ZnO nanorod array thin film as a singlelayer of photoelectrode for photoelectrochemical watersplitting. Int J Hydrogen Energy 2011;36:15538.

[34] Chuang H-Y, Chen D-H. Fabrication andphotoelectrochemical study of Ag@TiO2 nanoparticle thinfilm electrode. Int J Hydrogen Energy 2011;36:9487.

[35] Hara K, Horiguchi T, Kinoshita T, Sayama K, Sugihara H,Arakawa H. Highly efficient photon-to-electron conversionwith mercurochrome-sensitized nanoporous oxidesemiconductor solar cells. Sol Energy Mater Sol Cells 2000;64:115.

[36] He J, Benko G, Korodi F, Polyvka T, Lomoth R, Akermark B,et al. Modified phthalocyanines for efficient Near-IRsensitization of nanostructured TiO2 electrode. J Am ChemSoc 2002;124:4922.

[37] Yao QH, Meng FS, Li FY, Tian H, Huang CH. Photoelectricconversion properties of four novel carboxylatedhemicyanine dyes on TiO2 electrode. J Mater Chem 2003;13:1048.

[38] Nazeeruddin MK, Humphrey-Baker R, Officer DL,Campbell WM, Burrell AK, Gratzel M. Application ofmetalloporphyrins in nanocrystalline dye-sensitized solarcells for conversion of sunlight into electricity. Langmuir2004;20:6514.

[39] Kitamura T, Ikeda M, Shigaki K, Inoue T, Anderson NA, Ai X.Phenyl-conjugated oligoene sensitizers for TiO2 solar cells.Chem Mater 2004;16:1806.

[40] Wang ZS, Hara K, Y Dan-oh, Kasada C, Shinpo A, Suga S.Photophysical and (photo) electrochemical properties ofa coumarin dye. J Phys Chem B 2005;109:3907.

[41] Koumura N, Wang ZS, S Mori, Miyashita M, Suzuki E, Hara K.Alkyl-functionalized organic dyes for efficient molecularphotovoltaics. J Am Chem Soc 2006;128:14256.

[42] Takuya D, Nagasaka K, Funabiki K, Jin JY, Tsukasa Y,Minoura H. Flexible zinc oxide solar cells sensitized by styryldyes. Dyes Pigm 2008;77:59.

[43] Kumar A, Chauhan R, Molloy KC, Kociok-Kohn G, Bahadur L,Singh N. Synthesis, structure and light-harvesting propertiesof some new transition-metal dithiocarbamates involvingferrocene. Chem A Eur J 2010;16:4307.

[44] Ambade SB, Mane RS, Han S-H, Lee S-H, Sun M-Mo, Joo O-S.Indoline-dye immobilized ZnO nanoparticles for whopping5.44% light conversion efficiency. J Photochem Photobiol AChem 2011;222:366.

[45] Spanhel L, Anderson MA. Semiconductor clusters in the sol-gel process: quantized aggregation, gelation, and crystalgrowth in concentrated zinc oxide colloids. J Am Chem Soc1991;113:2826.

[46] Eggins BR, Chambers JQ. Proton effects in theElectrochemistry of the quinone hydroquinone system inaprotic solvents. J Electrochem Soc 1970;117:186.

[47] Hara K, Horiguchi T, Kinoshita T, Sayama K, Sugihara H,Arakawa H. Highly efficient photon-to-electron conversionof mercurochrome-sensitized nanoporous ZnO solar cells.Chem Lett 2000;29:316.

[48] Otsuka A, Funabiki K, Sugiyama N, Yoshida T, Minoura H,Matsui M. Dye sensitization of ZnO by unsymmetricalsquaraine dyes suppressing aggregation. Chem Lett 2006;35:666.

[49] Dabestani R, Bard AJ, Campion A, Fox MA, Mallouk TE,Webber SE, et al. Sensitization of titanium dioxide andstrontium titanate electrodes by ruthenium(II) tris(2,20-bipyridine-4,40-dicarboxylic acid) and zinc tetrakis(4-carboxyphenyl)porphyrin: an evaluation of sensitizationefficiency for component photoelectrodes in a multipaneldevice. J Phys Chem 1988;92:1872.

[50] Redmond G, Fitzmaurice, Gratzel M. Visible lightsensitization by cis-Bis(thiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato)ruthenium(II) of a transparentnanocrystalline zno film prepared by sol-gel techniques.Chem Mater 1994;6:686.