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A Kay-core-shell-chen mater-2002

A Kay-core-shell-chen mater-2002
A Kay-core-shell-chen mater-2002

Dye-Sensitized Core-Shell Nanocrystals:Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an Insulating Oxide

Andreas Kay and Michael Gra¨tzel*

Institut de Chimie Physique,Ecole Polytechnique Fe′de′rale de Lausanne,

CH-1015Lausanne,Switzerland

Received November6,2001.Revised Manuscript Received March25,2002

Coating nanocrystalline SnO2electrodes for dye-sensitized solar cell applications with a thin layer of ZnO,TiO2,ZrO2,MgO,Al2O3,Y2O3,or other insulating oxides was found to improve dye adsorption and increase the sensitized photocurrent.The surface of the oxide coating is more basic than SnO2,which renders dye attachment by its carboxyl groups more favorable.At the same time,the photovoltage and fill factor are strongly enhanced,resulting in much better energy conversion efficiencies.This change is ascribed to inhibition of electron back transfer from SnO2to the redox electrolyte(I3-)by the insulating oxide.The optimum coating thickness is only a few angstroms,suggesting that electron transfer from the excited dye attached to the oxide surface to the underlying SnO2occurs by tunneling through the insulator layer.Different methods for coating SnO2nanocrystals with a thin layer of an insulating oxide were compared.The best results were obtained by adding a soluble metal salt to the SnO2colloid,which is converted to the oxide on firing of the electrode.Specific adsorption of the metal ions on the SnO2surface guarantees homogeneous coating at near-monolayer coverage.Although the energy conversion efficiency of such modified SnO2 electrodes is still inferior to that of the usually employed TiO2electrodes,the larger band gap of SnO2offers the advantage of better long-term stability of the solar cell in the presence of UV light.Surface modification of nanocrystalline TiO2electrodes with very thin insulating oxide layers also improves the photovoltage but diminishes the current.Because of a blue shift in the photocurrent onset of insulator-coated TiO2,the solar cell stability in UV light is again enhanced.

Introduction

In the dye sensitization of large-band-gap semicon-ductors for photovoltaic solar cell applications,the highest energy conversion efficiencies have been achieved with titanium dioxide.1Other semiconductors,such as tin oxide,2zinc oxide,3and niobium oxide,4have been employed with less success.Therefore,it was surprising when Tennakone et al.5reported recently that electrodes prepared from a mixture of tin dioxide colloid and zinc oxide powder yield efficiencies superior to those of either of the oxides alone and comparable to those of titanium dioxide electrodes.The authors initially attributed the efficiency increase to ballistic electron motion through SnO2nanoparticles,which were thought to surround the larger ZnO grains.We achieved a similar enhancement by adding a small amount of zinc acetate to the tin dioxide colloid.Because the original recipe5employed

(1)(a)Gra¨tzel,M.Prog.Photovoltaics Res.Appl.2000,8,171.(b) Nazeeruddin,M.K.;Pe′chy,P.;Renouard,T.;Zakeeruddin,S.M.; Humphry-Baker,R.;Comte,P.;Liska,P.;Cevey,L.;Costa, E.; Shklover,V.;Spiccia,L.;Deacon,G.B.;Bignozzi,C.A.;Gra¨tzel,M.J. Am.Chem.Soc.2001,123,1613.(c)O’Regan,B.;Gra¨tzel,M.Nature 1991,335,737.(d)Nazeeruddin,M.K.;Kay,A.;Rodicio,I.;Humphry-Baker,R.;Mu¨ller,E.;Liska,P.;Vlachopoulos,N.;Gra¨tzel,M.J.Am. Chem.Soc.1993,115,6382.(e)Asbury,J.B.;Hao,E.;Wang,Y.;Ghosh, H.N.;Lian,T.J.Phys.Chem.B2001,105,4545.(f)Hinsch,A.;Kroon, J.M.;Spa¨th,M.;Roosmalen,J.A.M.;Bakker,N.J.;Sommeling,P.; van der Burg,N.;Kinderman,P.;Kern,R.;Ferber,J.;Schill,C.; Schubert,M.;Meyer, A.;Meyer,T.;Uhlendorf,I.;Holzbock,J.; Niepmann,R.In Proceedings of the Sixteenth European Photovoltaic Solar Energy Conference;James&James(Science Publishers)Ltd: London,2000;p32.

(2)(a)Bedja,I.;Hotchandani,S.;Kamat,P.V.J.Phys Chem.1994, 98,4133.(b)Fungo,F.;Otero,L.;Durantini,E.N.;Silber,J.J.;Sereno, L.E.J.Phys.Chem.B2000,104,7644.(c)Ferrere,S.;Zaban,A.; Gregg,B.A.J.Phys.Chem.B1997,101,4490.

(3)(a)O’Regan,B.;Schwartz,D.T.;Zakeeruddin,S.M.;Gra¨tzel, M.Adv.Mater.2000,12,1263.(b)Bauer,C.;Boschloo,G.;Mukhtar, E.;Hagfeldt, A.J.Phys.Chem.B2001,105,5585.(c)Keis,K.; Lindgren,J.;Lindquist,S.E.;Hagfeldt,https://www.doczj.com/doc/d54560152.html,ngmuir2000,16,4688.

(4)(a)Lenzmann,F.;Krueger,J.;Burnside,S.;Brooks,K.;Gra¨tzel, M.;Gal,D.;Ru¨hle,S.;Cahen,D.J.Phys.Chem.B2001,105,6347.

(b)Eguchi,K.;Koga,H.;Sekizawa,K.;Sasaki,K.J.Ceram.Soc.Jpn. 2000,108,1067.(c)Barros,D.D.;Abreu,P.P.;Alves,O.L.;Franco,

D.W.J.Sol-Gel Sci.Technol.2000,18,259.

(5)(a)Tennakone,K.;Kumara,G.R.R.A.;Kottegoda,I.R.M.; Perera,https://www.doczj.com/doc/d54560152.html,mun.1999,15.(b)Bandara,J.;Tennakone, K.J.Colloid Interface Sci.2001,236,375.(c)Kumara,G.R.A.;Konno, A.;Tennakone,K.Chem.Lett.2001,180.(d)Kumara,G.R.R.A.; Tennakone,K.;Perera,V.P.S.;Konno,A.;Kaneko,S.;Okuya,M.J. Phys.D:Appl.Phys.2001,34,868.(e)Tennakone,K.;Kottegoda,I. R.M.;De Silva,L.A.A.;Perera,V.P.S.Semicond.Sci.Technol.1999, 14,975.(f)One of the reviewers drew our attention to the more recent work of Tennakone et al.on MgO-coated nanocrystalline SnO2,which was published during the final stage of our manuscript preparation (Tennakone,K.;Bandara,J.;Bandaranayake,P.K.M.;Kumara,G. R.A.;Konno,A.Jpn.J.Appl.Phys.2001,40,L732and presentations given at the International Workshop on Nanostructures in Photovol-taics,Dresden,Germany,Jul28-Aug4,2001,as well as the200th Meeting of the Electrochemical Society,San Francisco,CA,Sep2-7, 2001).In that work,the authors concluded that MgO is first dissolved by acetic acid and then forms an insulating shell covering the SnO2 particles that allows electron injection by tunneling but suppresses recombination.

2930Chem.Mater.2002,14,2930-2935

10.1021/cm0115968CCC:$22.00?2002American Chemical Society

Published on Web06/26/2002

acetic acid,which dissolves the zinc oxide to some extent,it appeared that the efficiency improvement was due,instead,to the formation of a thin zinc oxide layer covering the tin dioxide particles.5f Because ZnO has a higher conduction band edge than SnO2,the open-circuit photovoltage of the cell,given by the difference between the conduction band edge and the redox potential of the electrolyte,will thus be enhanced.Moreover,the col-oration of the zinc-modified tin oxide electrode by the dye carrying carboxyl attachment groups is much stronger than that of the unmodified powder.This is expected from the higher isolectric point(iep)of ZnO (iep at pH9)compared to SnO2(iep at pH4-5),7 resulting in better light absorption and,thus,a higher photocurrent.

Intrigued by these observations we investigated the surface modification of SnO2by other insulating oxides that might form a tunneling barrier between the dye sensitizer and the SnO2conduction band.It is well-established in solid-state metal-insulator-semiconduc-tor(MIS)photovoltaics that such an insulating layer can enhance solar cell efficiency.In the case of dye-

sensitized photoelectrochemical cells,the tunneling layer would suppress the dark current arising from the reduction of the redox electrolyte at the semiconductor surface,hence improving the photovoltage.In addition, the photocurrent can be enhanced,as already men-tioned,by stronger dye adsorption on the modified semiconductor surface.

Experimental Section

SnO2Colloid Preparation.The results reported below were obtained with electrodes prepared from commercially available colloidal SnO2(Alfa Aesar,15wt%in H2O,stabilized with NH4OH at pH9.5,BET surface area)56m2/g).To prevent cracking and peeling of thicker films(>1μm)during sintering and also to enhance light absorption by scattering, we added SnO2powder(Ventron99.9%,BET surface area) 6.3m2/g),so that the nanoporous electrodes consisted of SnO2 nanoparticles(diameter≈15nm)and larger particles(~140 nm).For example,0.3g of SnO2powder was ground in a small porcelain mortar with0.1mL of SnO2colloid and stepwise diluted under grinding with0.9mL of SnO2colloid(giving a total of0.17g of colloidal SnO2).A drop of Triton X-100was added to help spreading on the substrate.Electrodes prepared form other SnO2colloids or without the addition of SnO2 powder gave similar results.

Surface Modification.For the surface modification of SnO2,different procedures were compared(see Results and Discussion).The best efficiencies were obtained by dissolving magnesium,aluminum,or ytterbium nitrate;ZrOCl2;TiCl4; Zn(CH3-COO)2;or Sc(CF3-SO3)3in acetic acid and then adding the SnO2colloid/powder dispersion.Acidification of the basic SnO2colloid resulted in immediate gellation,but upon agitation and sonification,the colloid became more or less liquid again.

For example,10μL of2.6M Y(NO3)3and10μL of acetic acid were mixed in a1.5-mL Eppendorf cup with0.2mL of SnO2colloid/powder dispersion and sonicated for1min in an ultrasonic bath.In cases of Mg,Al,and Y,the pH was adjusted to5by the addition of ammonium hydroxide(25%NH3in water)to enhance cation adsorption(see Results and Discus-sion).

Photoelectrode Preparation and Characterization. Transparent conducting glass(SnO2:F from Nippon Sheet Glass,sheet resistance)10?/square)was cut into3×2.5cm2pieces,cleaned by ultrasonification in1%Triton X-100, and rinsed with distilled water.The longer edges were covered with scotch tape(40μm thick),and70μL of the colloid was spread over the surface with a glass rod sliding on the scotch tape spacer.After drying at room temperature,the electrode was heated for10min on a hot plate at500°C.The film thickness determined with an Alpha-Step200apparatus (Tencor Instruments)was around8μm.

The electrode was cut into pieces of dimensions1.25×1 cm2,which were again heated for10min at500°C and transferred while still hot(~80°C)into0.5mL each of0.25 mM N719dye[bis(tetrabutylammonium)-cis-bis(thiocyanato)-bis(2,2′-bipyridine-4,4′-dicarboxylato)ruthenium(II)]1d in50% acetonitrile/50%tert-butyl alcohol.After soaking for at least 1day at room temperature,the colored electrode was dried in a stream of dry air and clamped against a counter electrode consisting of a thermally platinized880-nm-thick sputtered platinum layer on ITO glass.Electrolyte(5μL of0.6M dimethylpropylimidazolium iodide,0.1M iodine,0.5M4-tert-butylpyridine,and0.1M lithium iodide in methoxy acetoni-trile)was admitted by capillary action,and the electrode was illuminated perpendicularly with a parallel light beam directed through a mask of0.44cm2aperture.

Photocurrent/voltage characteristics and photoaction spectra were measured as described before.1b For long-term stability tests,9the solar cells were filled with0.6M dimethylpropyl-imidazolium iodide,0.1M iodine,and0.5M4-tert-butylpyri-dine inγ-butyrolactone as the electrolyte,hermetically sealed, and irradiated at open circuit under a Suntest CPS plus lamp (ATLAS GmbH,1000W/m2,AM1.5,T)55°C).

Results and Discussion

Unmodified Dye-Sensitized Nanocrystalline SnO2 Electrodes.Figure1a shows the photocurrent/voltage characteristics of a dye-sensitized nanocrystalline SnO2 electrode.Glass fibers of10μm diameter were inserted as a spacer between the photo-and counter electrodes in this case,because direct contact reduces the open-circuit photovoltage to less than300mV(depending on the contact pressure).

The coloration of the SnO2film was much weaker than that of a TiO2electrode of comparable active surface area.This is explained by the more acidic

(6)(a)Scaife,D.E.

Solar Energy1980,25,41.(b)Harris,L.A.;

Wilson,R.H.Annu.Rev.Mater.Sci.1978,99.

(7)Parks,G.A.Chem.Rev.1965,65,177.

(8)Papageorgiou,N.;Maier,W.F.;Gra¨tzel,M.J.Electrochem.Soc. 1997,144,876.

(9)Solar cell stability was tested by Dr.P.Liska in our laboratory. Figure 1.Photocurrent-voltage characteristics of dye-sensitized nanocrystalline photoelectrochemical cells with modified SnO2photoelectrodes at an irradiance of1000W/m2, AM1.5(lower curves in darkness):(a)unmodified SnO2,(b) SnO2coated with40μmol of ZnO/m2or0.59nm,(c)SnO2 mixed with ZnO powder.

Dye-Sensitized Core-Shell Nanocrystals Chem.Mater.,Vol.14,No.7,20022931

character of the SnO2surface,whose isoelectric point

is at pH4-5as compared to pH6.2for TiO2(anatase),7

decreasing dye adsorption.Consequently,the short-

circuit photocurrent is more than two times lower than

the current with TiO2(I sc)6.4mA/cm2at100mW/

cm2).

The open-circuit photovoltage(V oc)475mV)is also

much lower than that obtained with TiO2(typically V oc ≈700mV).This is expected because the conduction band edge of SnO2is more positive than that of TiO2.6

Finally,the fill factor of the I-V curve is very poor(ff

)0.4,whereas TiO

2

typically gives ff≈0.7),resulting in an energy conversion efficiency of onlyη)1.2%in full sun.

ZnO-Coated SnO2Electrodes.Zinc acetate was added to the SnO2colloid to deposit a thin layer of ZnO on the SnO2surface during firing(final relative com-position)0.22g of ZnO+1.0g of SnO2colloid+1.7g of SnO2powder,corresponding to40μmol of ZnO per square meter of SnO2),yielding a much more strongly colored electrode.The better dye adsorption on the ZnO-modified SnO2surface is easily explained by the basic

character of ZnO(iep at pH9),7which favors attachment of the dyes carboxyl groups.Accordingly,the photocur-rent(Figure1b)is much higher with ZnO(I sc)11.2 mA/cm2)than without ZnO.

The open-circuit voltage(U oc)670mV)is also greatly improved,indicating that electrons are injected from the dye into the more negative conduction band of ZnO.6 The increased voltage and fill factor(ff)0.69)show that back electron transfer from SnO2is efficiently suppressed by the ZnO layer,resulting in an energy conversion efficiency ofη)5.1%in full sun.Also,a spacer is no longer necessary to prevent a shunt between the photo-and counter electrodes.

A mixed SnO2/ZnO electrode(relative composition)

1.15g of ZnO powder+1.0g of SnO2colloid,thickness )15μm)prepared according to the method of Tenna-kone et al.5shows very similar I-V characteristics (Figure1c).Because the preparation involves acetic acid,which partially dissolves the ZnO powder,we assume that the improved efficiency is,in fact,due to the formation of a ZnO layer around the SnO2particles, as in the case of direct addition of zinc acetate.The optimum ZnO content is much higher in this case(53 wt%)5e because only the most reactive part of the ZnO powder is dissolved by the weak acetic acid.In contrast, direct addition of zinc acetate yields equivalent cell performance only for7.5wt%ZnO,showing that a large amount of large ZnO particles is not required for efficient charge separation as claimed.5e

Assuming a uniform coating5f due to the specific adsorption of Zn2+on SnO2,11the thickness of the ZnO layer in Figure1b estimated from the amount of zinc acetate added(giving0.22g of ZnO)and the surface area of the SnO2colloid(67m2)corresponds to3.3mg of ZnO/m2or0.59nm(taking the bulk density of ZnO as F) 5.6g/cm3),which equals to only about two monolayers of ZnO on the L15-nm SnO2particles. The impregnation of an already sintered colloidal SnO2electrode with zinc acetate solution followed by firing to deposit a ZnO coating around the SnO2 particles also improved the I-V characteristics.How-ever,the photovoltage and fill factor were always inferior to those obtained by the addition of zinc acetate to the SnO2colloid before sintering.

TiO2-and ZrO2-Modified SnO2.Coating the SnO2 particles with TiO2by adding TiCl4to the colloid (resulting in0.1g of TiO2+1.0g of SnO2colloid+1.7 g of SnO2powder or18μmol of TiO2/m2of SnO2)does not improve the dye adsorption as much as ZnO does, in accordance with the more acidic surface of TiO2(iep at pH4.7for rutile,pH6.2for anatase).7Still,the I-V curve(Figure2b)is strongly improved by the TiO2 coating,which corresponds to1.4mg of TiO2/m2or a thickness of0.35nm.

Interestingly,coating the SnO2with insulating ZrO2 (band gap energy E g)5.2eV)6b by adding ZrOCl2to the colloid(giving0.19g of ZrO2+1g of SnO2colloid +1.7g of SnO

2

powder or25μmol of ZrO2/m2of SnO2 or0.5nm)also improves the efficiency(Figure2c). Because the conduction band level of ZrO2(E cb)-1.4 V NHE at pH7)6b lies above the excited-state level of the dye(E S//S+)-1.3V NHE),1d electron injection into the conduction band of ZrO2is not possible.Even though the band structure of a0.5-nm-thick layer of ZrO2on SnO2might be different from that of a bulk ZrO2crystal, the experiment shows that the insulating ZrO2layer is thin enough to allow electron tunneling from the excited dye to the conduction band of the underlying SnO2.The increased photovoltage and fill factor are again ex-plained by inhibition of the back electron transfer from SnO2to I3-through the ZrO2coating.

Suppression of the back current by a thin insulating tunneling layer is widely applied in solid-state photo-voltaic cells of MIS(metal-insulator-semiconductor) structure and has also been observed in photoelectro-chemical cells.10To further exploit this approach with the dye-sensitized solar cell,we investigated insulating oxides other than ZrO2as the tunneling layer.

SnO2Electrodes with Insulating Oxide Coat-ings.Modifying the surface of the SnO2colloid with SiO2by adding Si(O-CH2-CH3)4along with HCl as a

(10)(a)Howe,A.T.;Fleisch,T.

H.J.Electrochem.Soc.1987,134,

72.(b)Howe,A.T.J.Chem.Soc.,https://www.doczj.com/doc/d54560152.html,mun.1983,1407. Figure 2.Photocurrent-voltage characteristics of dye-sensitized nanocrystalline photoelectrochemical cells with modified SnO2photoelectrodes at an irradiance of1000W/m2, AM1.5(lower curves in darkness):(a)unmodified SnO2,(b) SnO2coated with18μmol of TiO2/m2or0.35nm,(c)SnO2 coated with25mol of ZrO2/m2or0.50nm.

2932Chem.Mater.,Vol.14,No.7,2002Kay and Gra¨tzel

catalyst resulted in very poor dye adsorption,because of the very acid character of the SiO 2surface (iep at pH 2.0).7Much better coloration and photocurrent/voltage characteristics (Figure 3)were obtained with the basic oxides MgO (iep at pH 12),Al 2O 3(iep at pH 9),and Y 2O 3(iep at pH 9).7

In fact,yttrium oxide,which is used as an insulating layer in semiconductor devices because of its high dielectric constant,17gave the highest energy conversion efficiencies,and we therefore report the influence of different coating thicknesses of this oxide (Figure 4).At 3μmol of Y 2O 3/m 2or 0.13nm (Figure 4b),the short-circuit photocurrent was already twice that of a pure SnO 2electrode (Figure 4a),because of the much better dye adsorption.However,both the open-circuit photo-voltage and the fill factor were still low,indicating back electron transfer from uncovered areas of SnO 2.At 6μmol of Y 2O 3/m 2or 0.27nm (Figure 4c),the short-circuit photocurrent remained about the same,but the photo-voltage and fill factor were much improved.Further doubling of the coating thickness to 12μmol of Y 2O 3/m 2or 0.54nm (Figure 4d)led to an additional gain in voltage,but the photocurrent was slightly suppressed by the tunneling barrier.

The best results for different oxides are summarized in Table 1.As described in the Experimental Section,the oxide coatings were prepared by dissolving the magnesium,aluminum,or ytterbium nitrate;ZrOCl 2;TiCl 4;Zn(CH 3-COO)2;or Sc(CF 3-SO 3)3in acetic acid and then adding the SnO 2colloid/powder dispersion.The SnO 2colloid of Alfa Aesar is electrostatically stabilized by NH 4OH (pH 9.5),which imparts a negative charge to the SnO 2particles (iep at pH 4-5).7Mixing with acetic acid decreases the pH to about 3and results in gelling of the SnO 2colloid.However,in the presence of multivalent cations,such as Mg 2+,Al 3+,and Y 3+,the gel becomes liquid again upon agitation.This is a result of the specific adsorption of these cations,11which impart a positive charge to the SnO 2particles and peptize the gel.

Subsequent increases of the pH to 5and even 9by the addition of NH 4OH to enhance cation adsorption or precipitate the hydroxides on the SnO 2surface did not further improve the electrode efficiency.The addition of Y(NO 3)3alone (without acetic acid)to the basic SnO 2colloid led to irreversible gelling.The presence of other acids,such as propionic,lactic,and nitric acids,and also of glycerol gave electrode efficiencies similar to those obtained in the presence of acetic https://www.doczj.com/doc/d54560152.html,parable results were also obtained with a HNO 3-stabilized (pH 1.5)SnO 2colloid 12upon addition of Al(NO 3)3.

We conclude that acidic conditions or cation chelating molecules such as glycerol are required initially to ensure homogeneous cation adsorption on the SnO 2surface to form a continuous oxide layer on firing.As already mentioned for the ZnO-modified elec-trodes,the (repeated)impregnation of an already sin-tered colloidal SnO 2electrode with a solution of mag-nesium,aluminum,or ytterbium nitrate or TiCl 4,etc.,followed by drying and firing to form an oxide coating on the SnO 2particles also improved the I -V charac-teristics.However,the photovoltage and fill factor were always inferior to those obtained by cation adsorption on colloidal SnO 2.The same is true for cathodic deposi-

(11)(a)James,R.O.;Healy,T.W.

J.Colloid Interface Sci.1972,40,65.(b)Wiese,G.R.;Healy,T.W.J.Colloid Interface Sci.1975,51,435.(c)Fuerstenau, D.W.;Manmohan, D.;Raghavan,S.In Adsorption from Aqueous Solution ;Tewari,P.H.,Ed.;Plenum Press:New York 1981;p 93.(d)Jang,H.M.;Fuerstenau,D.W.Colloids Surf.1986,21,235.(e)Hunter,R.J.Foundations of Colloid Science ;Clarendon Press:Oxford,U.K.,1989;Vol.II,Chapter 12,p 709.(12)The HNO 3-stabilized SnO 2colloid was prepared by F.Arendse in our laboratory by hydrolysis of SnCl 4,dialysis,peptization with HNO 3,and autoclavation at 230°C.

(13)Lee,J.;Tak,Y.J.Ind.Eng.Chem.1999,5,139.

(14)Barbe,C.J.;Arendse,F.;Comte,P.;Jirousek,M.;Lenzmann,F.;Shklover,V.;Gra ¨tzel,M.J.Am.Ceram.Soc.1997,80,3157.

(15)Jarzebski,Z.M.;Marton,J.P.J.Electrochem.Soc.1976,123,333C.

(16)Hawn,D.D.;Armstrong,N.R.J.Phys.Chem.1978,82,1288.(17)(a)Sharma,R.N.;Rastogi,A.C.J.Appl.Phys.1993,74,6691.(b)Zhang,S.Q.;Xiao,R.F.J.Appl.Phys.1998,83,3842.(c)Araiza,J.J.;Cardenas,M.;Falcony,C.;Mendez-Garcia,V.H.;Lopez,M.;Contreras-Puente,G.J.Vac.Sci.Technol.A 1998,16,3305.(d)Hunter,M.E.;Reed,M.J.;El-Masry,N.A.;Roberts,J.C.;Bedair,S.M.Appl.Phys.Lett .2000,76,1935.

Figure 3.Photocurrent -voltage characteristics of dye-sensitized nanocrystalline photoelectrochemical cells with modified SnO 2photoelectrodes at an irradiance of 1000W/m 2,AM 1.5(lower curves in darkness):(a)unmodified SnO 2,(b)SnO 2coated with 12μmol of MgO/m 2or 0.13nm,(c)SnO 2coated with 6μmol of Al 2O 3/m 2or 0.16nm,(d)SnO 2coated with 6μmol of Y 2O 3/m 2or 0.27nm.

Figure 4.Photocurrent -voltage characteristics of dye-sensitized nanocrystalline photoelectrochemical cells with SnO 2photoelectrodes coated with Y 2O 3layers of different thicknesses at an irradiance of 1000W/m 2,AM 1.5(lower curves in darkness):(a)unmodified SnO 2,(b)SnO 2coated with 3μmol of Y 2O 3/m 2or 0.13nm,(c)SnO 2coated with 6μmol of Y 2O 3/m 2or 0.27nm,(d)SnO 2coated with 12μmol of Y 2O 3/m 2or 0.54nm.

Dye-Sensitized Core -Shell Nanocrystals Chem.Mater.,Vol.14,No.7,20022933

tion of Y(OH)3on a colloidal SnO 2electrode from Y(NO 3)3solution.13This indicates the presence of un-covered SnO 2areas resulting from a less homogeneous oxide coating.

Insulating Coatings on TiO 2Electrodes.Colloidal TiO 2(anatase)prepared by hydrolysis of titanium isopropoxide,peptization with HNO 3,and autoclavation at 230°C (surface area )100m 2/g,pH 1.0)14was surface-modified by the addition of magnesium,alumi-num,or ytterbium nitrate.On addition of NH 4OH to pH 5,no gelling occurred,as would have been the case in the absence of these cations,indicating their specific adsorption on TiO 2.

The adsorption of multivalent cations on TiO 2has been extensively studied.11TiO 2is a special case because of its very high dielectric constant ( )80-180),which favors the specific adsorption of multivalent cations by electrostatic shielding.According to the cited studies,the formation of a homogeneous layer of adsorbed cations is expected under our experimental conditions,which is then converted into a continuous insulating oxide layer by firing.

The I -V characteristics of transparent thin layer electrodes prepared from such modified TiO 2colloids are shown in Figure 5.An increase in the open-circuit voltage with respect to that of the unmodified TiO 2electrode is observed,despite the very low insulator coverage,but the photocurrent diminishes,resulting in an overall loss of energy conversion efficiency (Table 1).The decrease in the photocurrent probably results from poor electrical contact between the particles caused by their insulating shell reducing the charge collection efficiency.Coating the titiania nanoparticles after they have been sintered to give the mesoporous film may overcome this problem.

UV Action Spectra and Cell Stability.None of the modified SnO 2or TiO 2electrodes is as efficient in solar energy conversion as the pure TiO 2electrode (Table 1).However,TiO 2(anatase),with a band gap of E g )3.2eV,absorbs UV light below λg )400nm,which leads to the formation of strongly oxidizing valence band holes

(E vb )3V NHE at pH 7)that can attack the electrolyte solvent of the solar cell.

Most of the valence band holes will oxidize I -to I 3-in the same way as the oxidized dye,while the corre-sponding conduction band electron created by UV excitation will contribute to the photocurrent and reduce I 3-back to I -at the counter electrode.On the other hand,any holes that oxidize the solvent irreversibly rather than regenerating I 3-will result in an unrecov-erable loss of I 3-,which is required as an electron acceptor at the counter electrode.

Stable solar cell performance under several months of UV illumination has been obtained with very inert solvents such as propio-or valeronitrile.1f Yet,cells with more reactive solvents such as γ-butyrolactone,which is preferred to acetonitrile because of its higher boiling point,can degrade within a few days of illumination through the loss of I 3-.From this point of view,SnO 2is an interesting alternative to TiO 2,because its larger

Table 1.Photoelectric Parameters of Dye-Sensitized Nanocrystalline Solar Cells with SnO 2or TiO 2Photoelectrodes

Coated by Different Oxides at Indicated Coverage a

colloid modified with resulting oxide coating oxide coverage (μmol/m 2)equivalent coating thickness (nm)short-circuit photocurrent I sc (mA/cm 2)

open-circuit photovoltage V oc (mV)

fill factor ff (%)

energy conversion efficiency η(%)

SnO 2--00 6.448040 1.2SnO 2ZnO powder ZnO ??11.269067 5.2SnO 2Zn(CH 3-COO)2ZnO 400.5911.267069 5.1SnO 2TiCl4TiO 2180.3510.966054 3.9SnO 2ZrOCl 2ZrO 2250.5012.955047 3.4SnO 2Mg(NO 3)2MgO 120.1312.660051 3.8SnO 2Al(NO 3)3Al 2O 360.169.761061 3.6SnO 2Y(NO 3)3Y 2O 330.1313.653049 3.7SnO 2Y(NO 3)3Y 2O 360.2713.861059 4.9SnO 2Y(NO 3)3

Y 2O 3120.5410.768060 4.4SnO 2Sc(CF 3-SO 3)3Sc 2O 3100.3511.061061 4.2SnO 2La(NO 3)3La 2O 380.4014.051046 3.3SnO 2Sm(NO 3)3Sm 2O 380.4014.053049 3.6SnO 2Nd(NO 3)3Nd 2O 380.4013.152048 3.2SnO 2Ga(NO 3)3Ga 2O 390.2811.358049 3.3SnO 2In(NO 3)3In 2O 350.208.2500 1.8TiO 2--0012.970066 5.9TiO 2Mg(NO 3)2MgO 4.60.057.080072 4.0TiO 2Al(NO 3)3Al 2O 3 2.30.0610.074070 5.2TiO 2

Y(NO 3)3

Y 2O 3

2.3.

0.10

6.8

770

75

3.9

a

Irradiance )1000W/m 2,AM 1.5.

Figure 5.Photocurrent -voltage characteristics of dye-sensitized nanocrystalline photoelectrochemical cells with modified TiO 2photoelectrodes at an irradiance of 1000W/m 2,AM 1.5(lower curves in darkness):(a)unmodified TiO 2,(b)TiO 2coated with 4.6μmol of MgO/m 2or 0.05nm,(c)TiO 2coated with 2.3μmol of Al 2O 3/m 2or 0.06nm,(d)TiO 2coated with 2.3μmol of Y 2O 3/m 2or 0.10nm.

2934Chem.Mater.,Vol.14,No.7,2002Kay and Gra ¨

tzel

band gap (E g )3.5eV)15corresponds to an absorption edge of λg )360nm,so that fewer valence band holes will be created under illumination.

A comparison of the photocurrent action spectra of TiO 2and SnO 2/Al 2O 3electrodes without dye sensitizer (Figure 6)indeed shows a blue-shifted absorption edge for SnO 2/Al 2O 3.Unfortunately,the SnO 2/Al 2O 3electrode exhibits a weak tail extending up to 480nm,which might be due to the indirect absorption edge at 2.5eV.15Still,solar cells with dye-sensitized SnO 2/Al 2O 3elec-trodes and γ-butyrolactone as the electrolyte solvent were stable for 10weeks under continuous illumination with simulated sunlight (AM 1.5)including ultraviolet photons,9whereas an unmodified TiO 2electrode showed I 3-consumption within a few days.Cells with MgO-and Y 2O 3-coated SnO 2electrodes were less stable,despite the fact that their UV action spectra were similar to those of Al 2O 3-coated SnO 2electrodes.Presumably,MgO and Y 2O 3,which,in contrast to Al 2O 3,are both slightly water soluble,are slowly desorbed by the electrolyte from the SnO 2surface.

Interestingly,coating a TiO 2electrode with a thin layer of Al 2O 3(cf.Figure 5c),or with a thin layer of Y 2O 3or MgO,also results in a blue shift of the photoaction spectrum (Figure 6c).Again,solar cells with dye-sen-sitized TiO 2/Al 2O 3photoelectrodes and γ-butyrolactone as the electrolyte solvent were much more stable than cells with unmodified TiO 2or TiO 2/Y 2O 3photoelectrodes.It is well-known in pigment applications of TiO 2that coating the paint with an insulator such as Al 2O 3or SiO 2prevents chalking,which occurs as a result of photode-gredation of the organic binder at the TiO 2surface.In the case of dye-sensitized photoelectrochemical cells,the insulator coating has to be very thin so that electron injection from the excited dye into the conduction band of TiO 2is still possible.Further studies will be necessary to optimize the efficiency and long-term stability of this new type of solar cell.

Conclusion

Surface modification of nanocrystalline SnO 2elec-trodes with a thin layer of ZnO,TiO 2,ZrO 2,MgO,Al 2O 3,Y 2O 3,or other insulating oxides was found to improve

dye adsorption and thus increase the sensitized photo-current.This difference is ascribed to the more basic surface of the oxide coating as compared to SnO 2,favoring dye attachment through its carboxylic acid groups.At the same time,the photovoltage and fill factor of the cell are strongly enhanced,resulting in much better energy conversion efficiencies.This is explained by the suppression of electron back transfer from SnO 2to the redox electrolyte (I 3-)in the presence of the insulating oxide.

The optimum coating thickness is only a few ang-stroms,suggesting that electron transfer from the excited dye attached to the oxide surface to the underly-ing SnO 2occurs by tunneling through the insulator layer.Thicker layers yield a higher open-circuit photo-voltage but result in a decreased photocurrent.

The best results were obtained by adding a soluble metal salt to the SnO 2colloid,which was converted to the oxide upon firing of the electrode.Posttreatment of nanocrystalline SnO 2electrodes with a metal salt solu-tion always gave poor fill factors,indicating incomplete coverage of the SnO 2surface.The more homogeneous oxide coating in the case of metal salt addition to the colloid is explained by specific adsorption of the metal ions on the SnO 2surface.Although this should also result in a thin insulating oxide layer in the contact area between SnO 2particles,the electron transfer at these grain boundaries seems not to be impaired if the coating is sufficiently thin.Probably,sintering at 500°C allows sufficient interdiffusion of SnO 2to yield good electrical contact.

The energy conversion efficiency of such modified SnO 2electrodes is still inferior to that of pure TiO 2electrodes.However,the larger band gap of SnO 2makes the solar cell less sensitive to UV degradation when solvents are employed that react with the valence band holes generated by band gap excitation.

Surface modification of nanocrystalline TiO 2elec-trodes with very thin layers of MgO,Al 2O 3,or Y 2O 3also improves the photovoltage but diminishes the current.Interestingly,the insulating oxide coating results in a blue shift in the photocurrent onset of TiO 2.The suppression of the UV photocurrent through band gap excitation of TiO 2can be explained either by inhibition of hole transfer to the electrolyte by the insulator or by trapping of charge carriers in surface states followed by electron/hole recombination.This observation might be useful for improving the long-term stability under UV irradiation of dye-sensitized solar cells.

As an alternative to the oxide coatings on SnO 2investigated here,surface modification with organome-tallic compounds such as functionalized silanes (e.g.,γ-aminopropyltrimethoxysilane)16can be envisaged to improve dye attachment and suppress back electron transfer.However,the long-term stability of such coat-ings might be inferior to that of simple oxide coatings.Acknowledgment.Thanks are due to Paul Liska for testing the long-term stability of solar cells,Francine Arendse for preparation of the HNO 3-stabilized SnO 2colloid,Pascal Comte for supplying the TiO 2colloid,and Robin Humphry-Baker for helping with the data acqui-sition.Support of this work by the Swiss National Energy Office is gratefully acknowledged.

CM0115968

Figure 6.UV action spectra of unsensitized nanocrystalline photoelectrochemical cells.The incident photon-to-current conversion efficiency (IPCE)is plotted as a function of excita-tion wavelength:(a)unmodified TiO 2,(b)SnO 2coated with 6μmol of Al 2O 3/m 2or 0.16nm,(c)TiO 2coated with 2.3μmol of Al 2O 3/m 2or 0.06nm.

Dye-Sensitized Core -Shell Nanocrystals Chem.Mater.,Vol.14,No.7,20022935

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