Counter electrodes for DSC Application of functional materials as catalysts

Review

Counter electrodes for DSC:Application of functional materials

as catalysts

Takurou N.Murakami a ,Michael Gra ¨tzel

b,*

a Faculty of Biomedical Engineering,Toin University of Yokohama,1614Kurogane-cho,Aoba-ku,Yokohama city 225-8502,Japan

b

Laboratory of Photonics and Interfaces,Swiss Federal Institute of Technology in Lausanne (EPFL),CH-1015Lausanne,Switzerland

Received 23May 2007;received in revised form 14September 2007;accepted 16September 2007

Available online 22September 2007

Dedicated to Michael Gra ¨tzel.

Abstract

Counter electrodes (CEs)of dye-sensitized solar cells (DSCs)can be prepared with di?erent materials and methods.This review covers

recent research on CEs using platinum,graphite,activated carbon,carbon black,single-wall carbon nanotubes,poly(3,4-ethylenedioxy-thiophene)(PEDOT),polypyrrole,and polyaniline as catalysts for reduction of triiodide.Moreover,for the ultimate in low-cost counter electrodes,we have prepared a carbon-black-loaded stainless steel electrode for use as a novel CE.This counter electrode exhibits good photovoltaic performance;the e?ciency reaches 9.15%(16.3mA cm à2J sc ,785mV V oc ,and 71.4%?ll factor)with SUS-316stainless steel,equivalent to the performance with an FTO-glass substrate.ó2007Published by Elsevier B.V.

Keywords:Dye-sensitized solar cells;Counter electrodes;Carbon;Stainless steel

Contents 1.Introduction ...............................................................................5732.Platinum counter electrodes ....................................................................5733.Metal substrates for counter electrodes ............................................................5744.Carbon material ............................................................................5745.Highly efficient carbon black counter electrodes......................................................5746.Carbon catalyst on metal substrates for counter electrodes ..............................................5777.Polymer materials ...........................................................................5788.Polymer–carbon composites ....................................................................5789.Polymer and Pt composite .....................................................................57810.

Conclusion ................................................................................579Acknowledgement ...........................................................................579References.................................................................................

579

0020-1693/$-see front matter ó2007Published by Elsevier B.V.doi:10.1016/j.ica.2007.09.025

*

Corresponding author.Tel.:+41(0)216933112;fax:+41(0)216936100.E-mail address:michael.graetzel@ep?.ch (M.Gra ¨tzel).

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Available online at https://www.360docs.net/doc/9513274604.html,

Inorganica Chimica Acta 361(2008)

572–580

1.Introduction

The counter electrode is one of the most important com-ponents in the dye-sensitized solar cell (DSC).The task of the counter electrode (CE)is the reduction of the redox species used as a mediator in regenerating the sensitizer after electron injection,or collection of the holes from the hole conducting material in a solid-state DSC.At pres-ent,several di?erent kinds of CEs have been introduced,for example:platinized transparent CEs,carbon CEs,and conductive polymer CEs.The counter electrode is cho-sen according to the particular application of the DSC.For power-producing windows or metal-foil-supported DSCs,one must employ a transparent counter electrode,e.g.a small amount of platinum deposited on F -doped tin oxide coated glass (FTO-glass)or plastic.On the other hand,to make an inexpensive cell one should consider a low cost material such as a carbon catalyst for the CE.Or,for ‘‘champion cells’’,one must choose the material for the CE with the lowest possible sheet resistance and a high rate of reduction of the redox electrolyte.In this paper,we review several di?erent kinds of CEs with respect to their application in DSCs.2.Platinum counter electrodes

Platinum-loaded conducting glass has already been widely used as the standard for DSC counter electrodes.The reactions at the CE depend on the type of redox shuttle used to transfer charge between the photoelectrode and the CE.In many cases the iodide–triiodide couple has been employed as the redox mediator.The kinetics of reduction of triiodide at the CEs have been studied with a symmetric two-electrode system and the mass transfer limitation on the photocurrent has been examined by Papageorgiou et al.[1]The catalytic activity was expressed in terms of the exchange current density (J 0),which is calculated from the charge-transfer resistance (R ct )using the equation R ct =RT /nFJ 0,in which R ,T ,n ,and F are the gas con-stant,temperature,number of electrons transferred in the elementary electrode reaction (n =2)and Faraday con-stant,respectively.Electrochemical reduction and thermal deposition were compared as deposition methods for plat-inum on the transparent conductive glass (TCO)substrate.The thermally deposited Pt was more stable and showed a higher exchange current for the I 3à=I àcouple.Hauch and Georg used impedance spectra to investigate R ct on plati-nized TCO CEs prepared by either the electron beam evap-oration method,the sputtering method,or the thermal method [2].The R ct was measured with a symmetric two-electrode system with di?erent solvents for the electrolyte such as acetonitrile,propylene carbonate,3-methoxypropi-onitrile,polyethylene glycol,4-tert -butylpyridine,and mix-tures thereof.The 450nm thick platinum electrode prepared by the sputtering method gave the lowest R ct of 0.05ohm cm 2with acetonitrile as the electrolyte solvent to improve the di?usion coe?cient of I 3à.The Pt layer thickness correlated with the capacitance on the electrodes.The thicker layer showed a higher capacitance,a porous structure being observed by SEM.The thermal deposition of a Pt ?lm (<10nm thick),using H 2PtCl 6as a precursor on a TCO-substrate,produced a low R ct of 1.3ohm cm 2comparable to the R ct of the 40nm thick sputtered Pt,con-?rming the superiority of the thermal deposition method.DSCs with conversion e?ciencies greater than 11%also employ Pt as a catalyst for the CE [3–5]

.

Takurou N.Murakami is an associate professor at

the faculty of biomedical engineering at Toin Uni-versity of Yokohama,Japan.He obtained his phD degree in material science and engineering from Toin in 2005.From 2005to 2007,he was a post-doctoral researcher at Laboratory of Photonic and Interfcaces,Prof.Michael Gra ¨tzel’s group,in Swiss Federal Institute of Technology (EPFL).He was also a research fellow of the Japan Society for the Promotion of Science from 2004to 2006.Murakami joined Toin University in 2007.His

research interests are photoelectrochemical devices,chemical sensor,surface modi?cation of polymer,and materials for medical application.He received Governor Award from Kanagawa prefecture in Japan for Young Scienti?c Research Competition in 1993,Technology Prize from Material Technology of Japan Research Institute in 2001and in 2002,Scienti?c American 50Award in 2005,2nd Honda-Fujishima Prize in 2006,and EPFL Award for Excellence in Research in 2006.

He can be reached at Faculty of Biomedical Engineering,Toin Univ.of Yokohama,1614Kurogane-cho,Aoba-ku,Yokohama city 225-8502,Japan,Tel.:+81459745135;email:

murakami@edu.toin.ac.jp.

Michael Gra

¨tzel Professor at the Ecole Poly-technique de Lausanne,Michael Graetzel directs there the Laboratory of Photonics and Interfaces.He pioneered research on energy and electron transfer reactions in mesoscopic-materials and their optoelectronic applications.He discovered a new type of solar cell based on dye-sensitized mesoscopic oxide particles and pioneered the use of nanomaterials in lithium ion batteries.Author of over 500publications,two books and inventor or co-inventor of over 40patents he was an

invited professor at the University of California at Berkeley,the Ecole

Nationale Supe

′rieure de Cachan (Paris)and is presently part-time dis-tinguished visiting professor at the Delft University of Technology.He was a frequent guest scientist at the National Renewable Energy Laboratory (NREL)in Golden Colorado,was a fellow of the Japanese Society for the Promotion of Science.He has received numerous awards including the Millenium 2000European innovation prize,the 2001Faraday Medal of the British Royal Society,the 2001Dutch Havinga Award,the 2004Italgas Prize,two McKinsey Venture awards in 1998and 2002and the 2005Gerischer Prize.He is holding a doctors degree from the TU Berlin and honorary doctors degrees from the Universities of Uppsala and Turin.

He was elected honorary member of the Socie

′te ′Vaudoise des Sciences Naturelles.

T.N.Murakami,M.Gra ¨tzel /Inorganica Chimica Acta 361(2008)572–580

573

3.Metal substrates for counter electrodes

Metal substrates such as steel and nickel are di?cult to

employ for liquid-type DSCs because the Ià=I

3àredox

species in the electrolyte are corrosive for these metals. However,if these surfaces are covered completely with anti-corrosion materials such as carbon or?uorine-doped SnO2,it is possible to employ these materials as the sub-strate of the CE.Metals can be bene?cial to obtain a high FF for large-scale DSCs because of their low sheet resis-tance.Ma et https://www.360docs.net/doc/9513274604.html,ed Pt-covered stainless steel(SUS304) and nickel as the CEs and obtained conversion e?ciencies under full sun of 5.24%and 5.13%,respectively[6,7]. They compared di?erent deposition methods for Pt on stainless steel,such as sputtering,electroplating,and ther-mal deposition of platinum nanoparticles.In this case,the sputtering method achieved the best results as mentioned above with12.4mA cmà2J sc,703mV V oc,and a?ll fac-tor(?)of60%.

4.Carbon material

DSCs are well known as potentially low-cost photovol-taic devices[3];from this perspective,the application of low-cost materials should be important.The standard cat-alyst on the CE in most of the publications is platinum because of the high catalytic activity and high corrosion stability against iodine in the electrolyte.As a precious metal Pt is subject to price variations[8].In the future, large solar conversion systems producing electric power on the terawatt scale will prefer materials that are abun-dantly available.Some reports claim the risk of corrosion of Pt in triiodide-containing solutions to generate plati-num iodides such as PtI4[9]although this was not con-?rmed for Pt nanoclusters thermally deposited on FTO glass.It is nevertheless desirable to develop alternative low-cost materials for CEs,which should be corrosion-resistant and exhibit good catalytic activity for the reduc-tion of the triiodide ion.In1996Kay and Gra¨tzel achieved a conversion e?ciency of6.7%using a mono-lithic DSC embodiment based on a mixture of graphite and carbon black as the current collector[10].The graph-ite increases the lateral conductivity of the CE and the carbon black,having a very high surface area,provides

catalytic activity for the reduction of I

3àin place of plat-

inum.The performance imparts con?dence that carbon

can act as a catalyst for the reduction of I

3àin DSCs

without Pt and is an adequate material to make series-connected layered structures.Imoto et https://www.360docs.net/doc/9513274604.html,pared sev-eral types of activated carbon with di?erent surface areas ranging from1000m2gà1to2000m2gà1as the CE cata-lyst,assessing in addition the activity of several di?erent types of activated carbon,glassy carbon,and graphite [11].The surface area of the glassy carbon and graphite was three orders of magnitude lower than those of the activated carbon catalysts.In the preparation of the latter electrodes,a certain amount of carbon black was included.In their results,the electrodes consisting of the lower sheet resistance materials,graphite and glassy car-bon,gave lower J sc values and?ll factors,indicating the importance of the roughness of the carbon materials in achieving a better performance.They demonstrated an improvement in the J sc and FF with increasing thickness (>30l m)of the carbon material.

Carbon nanotubes have a high longitudinal conductiv-ity.Suzuki et https://www.360docs.net/doc/9513274604.html,ed single-wall carbon nanotubes (SWCNTs)for the CE[12],which were deposited on both FTO-glass and a Te?on membrane?lter achieving conver-sion e?ciencies of3.5%and4.5%,respectively.SWCNTs are good triiodide reduction catalysts.In addition,the sheet resistance of the SWCNTs on the Te?on membrane (1.8ohm sqà1)is four times lower than that of FTO-glass (ca.8–15ohm sqà1)rendering them an attractive choice for large-area DSC modules.

5.Highly e?cient carbon black counter electrodes

Carbon blacks are used on a large scale,for example,in printing toners,and cheap industrial mass production methods are already in place.They are electrically conduct-ing and show catalytic activity for the reduction of triio-dide.In this section,we would like to review our previous work on the optimization and characterization of carbon black CEs deposited on FTO-glass[13].We pre-pared carbon layers of various thicknesses(Fig.1)and whose e?ect on the cell performance was examined (Fig.2).The?ll factor(FF)increases from46%to72% with an increase in the thickness of the carbon layer up to10l m.For layers thicker than10l m the FF remains constant at around70%.On the other hand,the short cir-cuit photocurrent(J sc)is una?ected by changes in the thick-ness of the carbon layer.The open circuit voltage(V oc) decreases slightly while the conversion e?ciency increases with an increasing thickness of the carbon layer.Clearly the latter e?ect is linked to the improvement in the?ll fac-tor.I–V curves for each thickness of the carbon layer are shown in Fig.3and the performance of these cells is listed in Table1.The highest e?ciency was9.1%for a14.5l

m Fig.1.SEM image of the carbon layer for the counter electrode.

574T.N.Murakami,M.Gra¨tzel/Inorganica Chimica Acta361(2008)572–580

thick layer of carbon.The photocurrent for carbon layers of3.09,2.14,and0.85l m thickness decreases exponen-tially with increasing forward bias voltage near the V oc. This can be attributed to heterogenous electron transfer overvoltage losses due to insu?cient catalytic activity for the reduction of triiodide in the thinner carbon layers.Sim-ilar I–V curves have previously been observed with carbon or conductive polymers as CEs.

Furthermore,we measured the impedance spectra to investigate the relation between the thickness of the carbon layer and the cell performance.At?rst,we attempted to measure the spectra of the DSC with di?erent thicknesses of the carbon layer.However,the measured impedance plots of the cells do not clearly indicate the charge-transfer resistance between electrode and electrolyte(R ct)of each component of the cell because there are in some cases strong overlapping of the R ct on the TiO2surface and that

Table1

Photoelectric performances of the cells in Fig.3

Thickness(l m)J sc(mA cmà2)V oc(mV)FF(%)g(%)

14.4716.879068.59.1

9.7916.877064.68.4

4.7316.976064.18.2

3.0916.576959.37.5

2.1416.277255.2 6.9

0.8516.276946.3 5.8 T.N.Murakami,M.Gra¨tzel/Inorganica Chimica Acta361(2008)572–580575

on the carbon surface.For these reasons,a dummy cell was

employed consisting of platinized FTO-glass and carbon coated FTO-glass electrodes placed face-to-face in a sand-wich con?guration.Fig.4shows the Nyquist plots for these cells that were ?tted to a simulated model of the simple equivalent circuit shown in Fig.5.

The arch in the impedance spectra due to Pt-based CEs generally appears at 1–100kHz while carbon lowers the frequency range of the response to 1–100Hz [2,14].Conse-quently,in the Nyquist plots,starting from the zero Z 0end of the scale the ?rst and second semicircle re?ect the imped-ance of the platinum and carbon electrode,respectively.For the 19.5l m thick carbon layer the CE charge-transfer resistance becomes so small that a third semicircle repre-senting the Warburg impedance for triiodide di?usion emerges,Fig.4c.Impedance values are summarized in Table 2.The charge-transfer resistance of the CE is a mea-sure of its catalytic activity for reducing the triiodide ion.Thus the 350ohm cm 2charge-transfer resistance of the 0.65l m thick carbon layer is orders of magnitude greater than those of the 2.6l m and 19.5l m thick layers.This is in keeping with the high ?ll factor achieved with the thicker ?lms.The value of 2.95ohm cm 2obtained with the 19.5l m carbon layer approaches the value of 1.17ohm cm 2for the platinized FTO.However,the series resistance increases with the ?lm thickness,the value being 21.6ohm for the 19.5l m carbon layer,almost twice as much as the value of 11.5Ohm measured for the Pt electrode.Only at a thickness of 2.6l m or below does the series resistance of the carbon/FTO electrode approach that of Pt/FTO.The observed increase in the surface area and decrease of the a parameter for the CPE with the carbon layer thickness clearly re?ect the porosity of the carbon electrode.These results con?rm the trends observed for the impedance char-acteristics of the CE in real solar cells,supporting the valid-ity of our above interpretation.The contribution of the high surface area in the thick carbon layer considerably improves the cell performance.The BET surface area of this carbon layer is 163.9m 2g à1.The BJH average pore diameter and pore volume are 37.8nm and 0.89cm 3g à1,respectively.The relation between layer thickness and layer weight is constant at ca.65l g cm à2l m à1.This means that a carbon layer of 0.36cm 2geometric area and 1l m thick-ness has 38cm 2of active surface area and a porosity of 57%.With this information,we furthermore attempted to normalize the charge-transfer resistance to the active sur-face area of the carbon layers.The results summarized in Table 2show practically the same value for the 2.6l m (5443X cm 2)and 19.5l m thick carbon ?lms

(6072X cm 2

)while the value for the 0.65l m thick carbon ?lm (25162X cm 2)is four times higher than the

others.

Fig.4.Nyquist plots of electrochemical cells consisting of a carbon coated FTO-glass of various layer thicknesses as a working electrode and a platinized FTO-glass as a counter electrode.Electrolyte composition and coated area of carbon layer are the same as that in Fig.2.The platinum deposited area was larger than that of the carbon.The cells were measured without applied voltage (0V).b and c expand the range of the ordinate and abscissa from a and b.

576T.N.Murakami,M.Gra ¨tzel /Inorganica Chimica Acta 361(2008)572–580

This is attributed to the incomplete coverage of the FTO glass by the carbon ?lm and insu?cient exposure of buried carbon to the electrolyte.We con?rmed the relation between the applied area of the carbon on the CE and the cell performance (Fig.6).In this case,the size of the square photo electrode is 4mm by 4mm equal to 0.16cm 2.If we make an electrode 1mm larger on each side,5mm by 5mm,the area is 0.25cm 2.In these experi-ments,the cell performances saturated at 0.25cm 2render-ing it bene?cial to use an additional 1mm on each side of the CEs.

6.Carbon catalyst on metal substrates for counter electrodes FTO-glass is one of the major contributors to the mate-rials and energy cost of DSCs.Thus we attempted to use a highly e?cient carbon black catalyst applied on a stainless steel substrate as the CE.This concept,a combination between low-cost catalyst and low-cost substrate,ulti-mately a?ords a low-cost CE.We used SUS-304and SUS-316for the substrates because of the higher corrosion stability compared to other kinds of stainless steel.The preparation method for the carbon layer was the same as

Table 2

Impedance parameters of the platinum and the carbon counter electrodes of various thicknesses estimated from impedance spectra in Fig.4and the equivalent circuit in Fig.5Catalyst on FTO-glass

R ct (CB)(geometric)a (ohm ?cm 2)CPE (CB):A (S s a )CPE (CB):a

R s

(ohm)R ct (CB)

(active surface)a (ohm cm 2)Heat deposited platinum 1.52 3.97·10à50.9411.519.5l m thickness of carbon 2.96 5.89·10à30.8621.660722.6l m thickness of carbon 19.99.05·10à40.9220.054430.65l m thickness of carbon

368

1.11·10à4

0.94

16.3

25162

a

R ct (geometric),charge transfer resistance normalized with respect to geometric area (0.36cm 2)of the carbon layer;R ct (active surface),charge transfer resistance normalized active surface area (=thickness ·38cm 2)of the carbon layer.

T.N.Murakami,M.Gra ¨tzel /Inorganica Chimica Acta 361(2008)572–580577

for an FTO-glass substrate.Similarly,the photoelectrode,dye,and electrolyte were maintained the same as before.The I-V performances of these cells are shown in Fig.7.With the SUS-304steel substrate we obtained,J sc =15.4mA cm à2,V oc =780mV,and FF =73.7%,corre-sponding to 8.86%conversion e?ciency,while the SUS-316support gave J sc ,=16.3mA cm à2,V oc =785mV,and FF =71.4%Fill yielding 9.15%e?ciency,This perfor-mance is very similar to that observed with a FTO-glass substrate.

7.Polymer materials

Conducting polymers are also suitable materials for CEs.Thus Saito et https://www.360docs.net/doc/9513274604.html,ed poly(3,4-ethylenedioxythioph-ene)(PEDOT)doped with p -toluenesulfonate (PEDOT-TsO)or polystyrenesulfonate (PEDOT-PSS)as catalysts for CEs of DSCs [15,16].The preparation methods of these CEs are attractive:PEDOT-TsO was produced by oxida-tion of the monomer 3,4-ethylenedioxythiophene by Fe(III)tris-p -toluenesulfonate,the two components being spin coated together and then heated at 110°C for 5min for polymerization;PEDOT-PSS was deposited by the spin coating of a dispersion of the polymer.It is not necessary to apply a temperature >150°C using this method render-ing it suitable for plastic substrates.PEDOT-TsO showed better performance than PEDOT-PSS.Cyclic voltammetry experiments con?rmed that PEDOT-TsO shows a similar electro-catalytic e?ect for I 3àreduction to that of Pt depos-its.The R ct of the electrode with a 1.5l m thick layer of PEDOT-PSS is 0.8–0.9Ohm cm 2in an ionic liquid electro-lyte (0.5M I 2in 1-methyl-3-hexylimidazolium iodide)and 0.4–0.5Ohm cm 2in a volatile electrolyte (3-methoxyaceto-nitrile used as solvent).These values are comparable to the R ct of the I à=I 3àredox system on platinized TCO-glass prepared by the thermal method (1.3Ohm cm 2by Hauch et al.and 0.07Ohm cm 2by Papageorgiou et al.[2,1]).The V oc and FF increase with the thickness of the PEDOT-TsO.An e?ciency of 4.6%was reached with this

material,the PV performance parameters being J sc =

11.2mA cm à2,V oc =670mV and FF =61%.These poly-meric electro-catalysts will also be applicable for ?exible DSCs.

8.Polymer–carbon composites

Hole-conducting materials such as polypyrrole and polyaniline can be used along with carbon black as a com-posite catalyst for the CE.Kitamura et https://www.360docs.net/doc/9513274604.html,pared plat-inum-sputtered FTO-glass and carbon black deposited FTO-glass as CEs for solid-state DSCs,which use a poly-pyrrole as the hole-transport material [17].A carbon CE can improve the ?ll factor and the e?ciency will then increase.A modest 0.62%e?ciency was reported for 10mW cm à2irradiance (J sc =0.104mA cm à2,V oc =716mV,and FF =78%)The porous structure of the car-bon increases the interface between the polypyrrole and CE to improve the charge exchange.On the other hand,Ikeda et https://www.360docs.net/doc/9513274604.html,ed a polyaniline-loaded carbon black (PACB)as a CE [18].A viscous mixture of PACB with the ionic liquid 1,3-diethyleneoxide-derivatized imidazoli-um iodide (EOI)was formed and this paste was applied between the photoelectrode and bare FTO-glass as the CE.The best concentration of the PACB is 10%in the PACB-EOI mixture,yielding 3.48%conversion e?ciency (J sc =12.8mA cm à2,V oc =580mV,FF =47%).The sig-ni?cant di?erence between Kitamura’s method and Ikeda’s methods is the dispersion of the carbon black in the poly-mer material,which is related to an increase in the interface between these materials,and the addition of the EOI,which can penetrate into the pores of TiO 2for transferring the charge.

9.Polymer and Pt composite

DSCs based on plastic substrates have also received attention in recent years because of both the availability of the roll-to-roll manufacturing method and the light module weight.For plastic DSCs,the CEs must be made from plastic substrates.Ma et al.deposited Pt on indium tin oxide coated poly(ethylene naphthalate)(ITO-PEN)by sputtering at room temperature to achieve a plastic CE [6].The performance of the cell with this plastic CE is 12.9mA cm à2J sc ,693mV V oc ,60%?ll factor,and 5.39%e?ciency.These values are quite comparable to the cell performance with platinized FTO-glass as the CE (13.1mA cm à2J sc ,695mV V oc ,60%?ll factor,and 5.39%e?ciency)in their study.Ito et al.also used ITO-PEN as the CE for ?exible DSCs [19].They deposited Pt by the electrochemical deposition method with an electro-lyte of 5mM H 2PtCl 6aqueous solution and achieved 13.6mA cm à2J sc ,780mV V oc ,68%?ll factor,and 7.2%e?ciency for the cell irradiated through the CE.Wei et al.made polyvinylpyrrolidone-capped platinum (PVP-capped Pt).ITO-glass was dipped in a suspension of this material to prepare PVP-capped Pt coated ITO-glass

for

Fig.7.Typical photocurrent–voltage curves of cells using a carbon layer on a stainless steel substrate as counter electrodes.The area of electrodes,sensitizing dye and electrolyte composition are the same as those in Fig.2.

578T.N.Murakami,M.Gra ¨tzel /Inorganica Chimica Acta 361(2008)572–580

the CE.They prepared this CE at temperatures of less than 60°C during the process and achieved2.84%conversion e?ciency and5.66ohm cm2R ct in the DSC.This method is also applicable to conductive plastic substrates because of the low-temperature process.

10.Conclusion

There are many di?erent kinds of CEs.The cell perfor-mances with individual CEs are shown in Table3.They can be divided into various groups:CEs prepared with a heating process,without a heating process,and with Pt and without Pt.The CEs prepared with a heating process show better results than without heating,however,these processes are impossible to apply to plastic substrates and non-heating processes are suitable for this application. Platinum is an e?cient catalyst but it can be replaced by other materials to obtain similar performances.Transpar-ent CEs can be realized with a platinum catalyst on con-ducting glass.A combination of a low sheet resistance material such as SWCNT and a highly catalytically active material such as carbon black is promising for large DSC modules.

Acknowledgement

We wish to thank Dr.Nick Evans(EPFL,Switzerland) for valuable discussions in the preparation of this manuscript.References

[1]N.Papageorgiou,W.F.Maier,M.Gra¨tzel,J.Electrochem.Soc.144

(1997)876.

[2]A.Hauch,A.Georg,Electrochim.Acta46(2001)3457.

[3]M.Gra¨tzel,J.Photochem.Photobiol.A.Chem.164(2004)3.

[4]Md.K.Nazeeruddin, F.D.Angelis,S.Fantacci, A.Selloni,G.

Viscardi,P.Liska,S.Ito,B.Takeru,M.Graetzel,J.Am.Chem.Soc.

127(2005)16837.

[5]Y.Chiba,A.Islam,Y.Watanabe,R.Komiya,N.Koide,L.Han,

Jpn.J.Appl.Phys.45(2006)L638.

[6]T.Ma,X.Fang,M.Akiyama,K.Inoue,H.Noma,E.Abe,J.

Electroanal.Chem.574(2004)77.

[7]X.Fanga,T.Ma,M.Akiyama,G.Guan,S.Tsunematsua,E.Abe,

Thin Solid Film472(2005)242.

[8]D.R.Wilburn,D.I.Bleiwas,Platinum-Group Metals—World Supply

and Demand,US Geological Survey Open-File Report,2004,p.1224.

[9]E.Olsen,G.Hagen,S.E.Lindquist,Sol.Energy Mater.Sol.Cell63

(2000)267.

[10]A.Kay,M.Gra¨tzel,Sol.Energy Mater.Sol.Cell44(1996)99.

[11]K.Imoto,K.Takatashi,T.Yamaguchi,T.Komura,J.Nakamura,K.

Murata,Sol.Energy Mater.Sol.Cell79(2003)459.

[12]K.Suzuki,M.Yamamoto,M.Kumagai,S.Yanagida,Chem.Lett.32

(2003)28.

[13]T.N.Murakami,S.Ito,Q.Wang,Md.K.Nazeeruddin,T.Bessho,I.

Cesar,P.Liska,R.Humphry-Baker,https://www.360docs.net/doc/9513274604.html,te,P.Pe′chy,M.

Gra¨tzelzel,J.Electrochem.Soc.153(2006)A2255.

[14]T.Hoshikawa,M.Yamada,R.Kikuchi,K.Eguchi,J.Electrochem.

Soc.152(2005)E68.

[15]Y.Saito,T.Kitamura,Y.Wada,S.Yanagida,Chem.Lett.31(2002)

1060.

[16]Y.Saito,W.Kubo,T.Kitamura,Y.Wada,S.Yanagida,J.

Photochem.Photobiol.A.Chem.164(2004)153.

[17]T.Kitamura,M.Maitani,M.Matsuda,Y.Wada,S.Yanagida,

Chem.Lett.30(2001)1054.

Table3

Typical performances of DSCs with di?erent types of counter electrodes

E substrate Catalyst on CE Electrolyte solvent Cell

area

(cm2)Light

intensity

(mW cmà2)

J sc

(mA cmà2)

V oc

(mV)

FF

(%)

g

(%)

Authors Year Reference

Graphite and carbon black Acetonitrile0.410011.3482671 6.67Kay and

Gra¨tzel

1996[10]

FTO-glass Carbon black Polypyrrole(solid state)100.104716780.62Kitamura

et al.

2001[17]

ITO-glass PEDOT-TsO3-Methoxyacetonitrile0.3510011.267061 4.60Saito et al.2002[16] Glass Single wall carbon

nanotubes

3-Methoxyacetonitrile0.251009.7a750a62a 4.5Suzuki et al.2003[12]

FTO-glass Activated carbon Acetonitrile0.051007.9380861 3.89Imoto et al.2003[11] Au PEDOT–(solid state)100 2.6680510.93Fukuri et al.2004[21] Stainless steel Pt3-Methoxypropionitrile0.210012.470360 5.24Ma et al.2004[6]

Au/FTO-glass PEDOT Poly(3-thiophenylacetic acid)

(solid state)100 2.5365540.5Senadeera

et al.

2005[22]

FTO-glass Carbon black Acetonitrile+Valeronitrile0.1610016.8790699.1Murakami

et al.

2006[13]

FTO-glass Carbon black and

polyaniline 1,3-diethyleneoxideimidazolium

iodide(semisolid state)

0.2410012.858047 3.48Ikeda et al.2006[18]

ITO-glass Polyvinyl pyrrolidone-

capped platinum

3-Methoxypropionitrile0.2510010.566041 2.84Wei et al.2006[20]

ITO-PEN Pt Acetonitrile+Valeronitrile10013.6780687.2Ito et al.2006[19]

FTO-glass Pt Acetonitrile+Valeronitrile0.1610017.738467511.18Nazeeruddin

et al.

2006[4] Stainless steel Carbon black Acetonitrile+Valeronitrile0.1610016.378571.9.15Murakami

et al.

2007

a The values are read from the?gures in the referenced manuscripts.

T.N.Murakami,M.Gra¨tzel/Inorganica Chimica Acta361(2008)572–580579

[18]N.Ikeda,K.Teshima,T.Miyasaka,https://www.360docs.net/doc/9513274604.html,mun.2006(2006)

1733.

[19]S.Ito,N.L.Cevey Ha,G.Rothenberger,P.Liska,https://www.360docs.net/doc/9513274604.html,te,S.M.

Zakeeruddin,P.Pe′chy,Md.K.Nazeeruddin,M.Gra¨tzel,Chem.

Commun.2006(2006)4004.[20]T.C.Wei,C.C.Wan,Y.Y.Wang,Appl.Phys.Lett.88(2006)103122.

[21]N.Fukuri,Y.Saito,W.Kubo,G.K.R.Senadeera,T.Kitamura,Y.

Wada,S.Yanagida,J.Electrochem.Soc.151(2004)A1745.

[22]R.Senadeera,N.Fukuri,Y.Saito,T.Kitamura,Y.Wada,S.

Yanagida,https://www.360docs.net/doc/9513274604.html,mun.2005(2005)2259.

580T.N.Murakami,M.Gra¨tzel/Inorganica Chimica Acta361(2008)572–580