The main focus in the recent times has been onincreasing efficiency of solar cells with low cost fabri�cation techniques. One important new class of solarcells that can significantly lower energy requirementsin comparison to other conventional solar cells andwhich may greatly reduces cost, is the dye�sensitizedsolar cell. DSSC, originally developed in the early 90sin Switzerland, showed that when a thin TiO2 layer isdeposited on conducting glass substrate with optimumamount of dye, absorbed in the film and the applica�tion of an electrolyte between two electrodes, it waspossible to convert solar energy in the form of photonsto electricity at about 11% efficiency [1].
The cost of traditional solar cells based on siliconp–n junction devices inhibits a larger use of this tech�nology. In this sense the dye sensitized solar cells,DSSC, have emerged as one of the most promisingdevices due to their reduced cost, low environmentalimpact and reasonable efficiency for conversion ofsolar energy into electricity [2–4].
A suitable dye, where the difference betweenHOMO and LUMO is small enough for low energyphotons (photons with longer wavelength) to exciteelectrons will help the TiO2 to absorb light in the visi�ble spectrum. The wide band gap of TiO2 is not onlyassociated with in sensitivity to the visible spectrumbut also with stability. Metal oxides are due to the sta�bility they exhibit advantageous to work with but forthe sake of efficiency, they need be combined with adye, absorbing visible light.
1 The article is published in the original.
The use of natural products such as organic dyes insolar cells offers promising prospects for the advanceof this technology, since photoexcitable dyes are sub�stances that cede electrons easily while the use of syn�thetic dyes involves several problems, such as theirsynthesis, purification and use, as well as the fact thatthey require rare metals [5–7].
The performances of natural extracts are usually bet�ter than those obtained by using commercial and puri�fied analogous compounds. The improved natural dyeperformance is probably due to a mixture of dyespresent in such extracts [8]. Different dyes promotelight harvesting in distinct wavelengths and have partic�ular electron injection quantum yields leading to theenhancement of photocurrent and photo voltage [9].
The main disadvantage of application of TiO2 asphotoanode is related to poor absorption of sun lightdue to its wide energy gap (ca. 3 eV). Several tech�niques have been led to higher sunlight conversionefficiency. The most promising among them seems tobe the TiO2 doping by foreign metal oxides [10].
The DSSC consists of sensitizing dye, nanoporousmetal oxide film, electrolyte and opposite electrode.The metal oxide films plays a key role in the enhance�ment of photoelectric conversion efficiency of DSSCand many studies focus on the relation between filmstructure and photo catalytic activity as well as thepower conversion efficiency of DSSC [11–13].
In the present paper we applied WO3 coated TiO2
nanoparticle to natural dye sensitized solar cell as aphotoelectrode to reduce recombination rate by pro�viding energy barrier. The cell performance of the WO3
coated TiO2 dye sensitized solar cell is then investi�gated.
Novel Dye Based Photoelectrode for Improvement of Solar Cell Conversion Efficiency1
Mridula Tripathia, Ruby Upadhyaya, b, and Ashutosh Pandeyb, c
a, bC.M.P. Degree College University of Allahabadb, cMotilal Nehru National Institute of Technology, India
Received April 26, 2012
Abstract—We have explored the application of natural dyes extracted from beetroot in Dye sensitized solarcell (DSSC). The main pigment is betacyanin which was obtained by separation and purification from theextract. The photo electrochemical performance of the DSSC based on these dyes showed that the photovoltage and photocurrent 435 mV, 9.86 mA, respectively. The overall conversion efficiency of nano WO3coated TiO2 dye�sensitized solar cells exhibits a higher conversion efficiency of 2.2%. The photo electro�chemical performance of beetroot extract demonstrate that betacyanin dye was the most effectual compo�nent of the sensitizer for DSSC because of the simple preparation technique, widely available and lowcheap cost.
DOI: 10.3103/S0003701X13010131
RENEWABLE ENERGY SOURCES
APPLIED SOLAR ENERGY Vol. 49 No. 1 2013
NOVEL DYE BASED PHOTOELECTRODE FOR IMPROVEMENT 55
2. EXPERIMENTAL
2.1. Extraction and Purification of Dye
The betacyanin dye (Fig. 1) extracted with ethanolwas obtained by the following steps: fresh beetrootwere washed with water and vacuum dried at 60°C.After crushing, these materials were immersed inabsolute ethanol at room temperature in the dark forone week. Then the solids were filtrated out, and thefiltrates were concentrated in rotavopor at 40°C andthese ethanolic extracts were refined by chromato�gram method. After that the natural betacyanin dyesensitizer alcohol solution was prepared.
2.2. Synthesis and Structural Characterization of WO3 Coated TiO2 Nanopowder
The WO3 coated TiO2 nanopowder was prepared inthe form of a sol�gel by the hydrolysis process. For thispreparation, first Ti [OCH(CH3)2]4 was added slowly topropanol drop by drop. Deionized water was slowlyadded under vigorous stirring conditions for the dura�tion of 10 min. Then conc. HNO3 solution was addedunder magnetic stirring. Then Na2WO4 solution wasadded dropwise into above mixture under continuousstirring. The resulting mixture was stirred at room tem�perature for 6 hrs. The resulting solid was centri�fuged,washed, dried and calcined to get WO3–TiO2 oxides.
ITO conductive glass with a sheet resistance of 15–20 Ω/cm2 were first cleaned in a detergent solutionusing an ultrasonic bath for 15 min, rinsed with waterand ethanol, and then dried. TiO2–WO3 nanopowderpastes were deposited on the ITO conductive glass bydoctor�blading technique in order to obtainTiO2⎯WO3 film. The film on the substrate was fired inair (oven) at 150°C for 5–10 min. The nano�crystal�line version of the material generally leads to highquantum efficiency.
The structural characterization of ns�TiO2–WO3
electrode material was carried out through XRD byemploying a Philips PW 1710 Diffractometerequipped with a graphite monochromater. Figure 2shows XRD patterns (CuK
α radiation) of the
ns�TiO2–WO3 materials. Analysis of XRD patternsrevealed that on alloying ns�TiO2 with WO3 did notlead to the formation of any new composite material.However, a slight change in the lattice parameter wasinvariably found. The XRD also confirmed the pres�ence of ns�TiO2–WO3. With the help of scanning elec�tron microscope (JEOL�JXA�8100 EPMA), themicro structural characteristics of the synthesizedmixed oxide ns�TiO2–WO3nanopowder were done(Fig. 3). The observed nanostructured characteristicsshowed a very fine grained structure suggestive of anano�crystalline like matrix. By employing theScherer’s equation for the powder diffraction peaks andSEM, the average grain size of the ns�TiO2–WO3 photoelectrodes were found to be 20–40 nm respectively.
N
HO
HO
COOH
NHOOC COOHH
Fig. 1. Structure of betacyanin dye.
70503010 4020 60
Inte
nsi
ty,
a.u.
(020
)
(110
)(2
20)
(101
)
(111
)
(033
)
(220
)(1
32)
(103
)
2θ, deg
TiO2
WO3
Fig. 2. X�ray diffraction pattern of ns�WO3 coated TiO2.
200 nm
Fig. 3. Scanning electron micrograph of ns�WO3 coatedTiO2.
56
APPLIED SOLAR ENERGY Vol. 49 No. 1 2013
MRIDULA TRIPATHI et al.
In order to explore the spectral response theabsorption spectra of TiO2–WO3 and betacyanin dyeon quartz glass substrate was carried out through dou�ble beam spectrophotometer (Systronics, 2201)(Fig. 4). The absorption range after sensitization withdye increases from 200–540 nm.
2.3. Fabrication and Operation principle of DSSC
The WO3 coated TiO2 photoelectrode wasimmersed in an ethanol solution containing a naturalbetacyanin dye for 10–12 h [14]. Figure 5 shows celldiagram which consist of dye�sensitized electrode anda sputtered –Pt counter electrode were assembled to
form a solar cell by sandwiching a redox (I–/ ) elec�trolyte solution. The electrolyte solution was com�posed of 0.5 M LiI, 0.05 M iodine in acetonitrile [15].This electrolyte solution was poured in the dye coatedTiO2–WO3 photoelectrode which was previously pre�pared using paraffin�film as framework to seal the cellsto prevent evaporation of the liquids. The counterelectrode was pressed against the impregnated anodeand clamped firmly in a sandwich configuration. Noleaks (solvent evaporation) were detected.
A schematic presentation of the operating princi�ple of the DSSC is given in Fig. 5. At the heart of thesystem is a mesoporous oxide layer composed ofTiO2–WO3 nanometer sized particles which have beensintered together to allow for electronic conduction totake place. Attached to the surface of the TiO2 nanoc�rystalline film is a monolayer of the charge transferbetacyanin dye. Photoexcitation of the later result inthe injection of an electron into the conduction bandof the oxide. The original state of the betacyanin dye issubsequently restored by electron donation from theelectrolyte, usually an organic solvent containingredox system, such as the iodide/triiodide couple. Theregeneration of the betacyanin dye by iodide interceptsthe recapture of the conduction band electron by theoxidized dye. The iodide is regenerated in turn by the
I3–
reduction of triiodide at the counter electrode, the cir�cuit being completed via electron migration throughthe external load. The voltage generated under illumi�nation corresponds to the difference between theFermi level of the electron in the solid and the redoxpotential of the electrolyte. Overall the device gener�ates electric power from light without suffering anypermanent chemical transformation.
All electrochemical measurement were carried out byusing a Princeton Applied Research (PAR) model 173Potentiostate/Galvanostate PAR 175 universal program�mer coupled with a Housten 2000 X–Y/t recorder. A1000 W Xenon�Mercury lamp (Oriel Corporation, USA)was used as illumination source. The intensity of incidentradiation was adjusted and fixed at 100 mW/cm2 of activearea 0.5 cm2. The fill factor (FF) and conversion effi�ciency (η) of DSSC were determined according to ff =(I × V)max/(Isc × Voc) and η = Isc × Voc × FF/Intensity(mW/cm2) based on I–Vcurve.
3. RESULTS AND DISCUSSION
As is clear from XRD and SEM studies the WO3
does not react with base but forms only islands over thesurface of TiO2 It is also clear from the scanning elec�tron micrograph pictures (Fig. 3) the coated film fromthe suspension of WO3 deposited TiO2 has also a mod�erate porosity and high surface area to which dye mol�ecules could be absorbed. The increased photo�activ�ity is also thought to be due to low concentration ofdefects and impurities.
The absorption spectrum of betacyanin dye coversthe entire solar spectrum. It exhibits an intramolecularcharge transfer (ICT) band at 540 nm. It shows broadabsorption spectra with an extended absorption edge inthe solid state. From the result it is understood beet rootdye absorbed light range from 200–540 nm wavelength.
The beetroot extract contained the betacyanin dyewhich is the core composition for natural dye inDSSC. The carboxylic and hydroxyl groups in betacy�
Fig. 5. Working principle of a dye� sensitized photovoltaicdevice.
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NOVEL DYE BASED PHOTOELECTRODE FOR IMPROVEMENT 57
anin dye can be bound with the surface of TiO2–WO3
porous film and thus result in the high photoelectricconversion effect.
Figure 6 shows the resulting photocurrent, photo�voltage curve for the DSSC fabricated using betacya�nin dye modified film from the suspension of the WO3
the photovoltage and photocurrent 435 mV, 9.86 mArespectively. The conversion efficiency of nano WO3
coated TiO2 dye�sensitized solar cells exhibits a higherconversion efficiency of 2.2%. The improvement inthe photocurrent with WO3 modification is thought tobe due to improved spectral response of the photoan�ode due to the presence of surface of WO3.
By providing inherent energy barrier that led to adecrease in recombination, the value of fill factor (FF)and conversion efficiency (η) were 52% and 2.2%respectively, without any heat treatment or static pressprocessing.
The light absorption of the betacyanin dye onTiO2–WO3 coated DSSC is high because of higherinteraction between TiO2–WO3 and betacyanin dyewhich leads to a better charge transfer and less recom�bination rate. Moreover, betacyanin in the beetrootextract has a shorter distance between the dye skeletonand the point connected to WO3 coated TiO2 surface.This could facilitate an electron transfer from betacy�anin in the beetroot extract to the WO3 coated TiO2
surface and could be accounted for a better perfor�mance of betacyanin dye sensitization.
4. CONCLUSIONS
In conclusion, it can therefore be said that the car�boxylic group present in betacyanin dye permits thenecessary contact and electronic coupling between thesensitizer and WO3 coated TiO2 surface resulting in anultrafast electron transfer from the dye into the semi�
conductor, the sensitizer performs efficient light har�vesting and provides an enhanced spectral response ofthe TiO2 electrodes to visible light.
The sensitizing dye itself does not provide a con�ducting functionality but is distributed at an interfacein the form of immobilized molecular species; it is evi�dent that for charge transfer each molecule must be inintimate contact with both conducting phases.
Photo electrochemical parameters of 0.5 cm2
active photo electrode area devices sensitized by beta�cyanin dye were constant for 12 weeks of continuousevaluation. Moreover the cell remained stable evenafter 24 weeks with a fairly good efficiency. Therefore,beetroot dye sensitized WO3–TiO2 dye sensitized solarcell opens up a perspective of commercial feasibilityfor designable polychrome modules, inexpensive andenvironmentally friendly dye cells.
5. ACKNOWLEDGMENT
The present work is financially supported by theDepartment of Science and Technology, Govt. ofIndia. We are thankful to Dr. O.N. Srivastava, Depart�ment of Physics, BHU, Varanasi for stimulating dis�cussions and provide us instrumental support.
REFERENCES
1. O’Regan, B. and Gratzel, M., Nature, 2001, vol. 353,pp. 737–740.
2. Polo, A.S., Itokazu, M.K., and Murakami Iha, N.Y.,Coord. Chem. Rev., 2004, vol. 248, pp. 1343–1361.
