TiO2-Coated Nanoporous SnO2 Electrodes for Dye-Sensitized Solar

Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel. Received August 16, 2001. In Final Form: October 15, 2001. This paper describe...
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TiO2-Coated Nanoporous SnO2 Electrodes for Dye-Sensitized Solar Cells Shlomit Chappel, Si-Guang Chen, and Arie Zaban* Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel Received August 16, 2001. In Final Form: October 15, 2001 This paper describes the synthesis and characterization of a core-shell nanoporous electrode consisting of an inner SnO2 matrix and a thin shell of TiO2. The coating is characterized as a very thin rutile TiO2 layer whose conduction band level is located between the levels of bare SnO2 and TiO2. The TiO2 shell acts as an energy barrier at the electrode-electrolyte interface, thus slowing the interaction between the electrons in the electrode and the electrolyte ions. When applied in a dye-sensitized solar cell, the coated electrode is significantly superior to a bare SnO2 electrode. The increase of all cell parameters improves the conversion efficiency by a factor of 2.2. The combination of improved electron collection efficiency with respect to bare SnO2 and a more positive conduction band with respect to bare TiO2 should make dyes having a relatively positive excited-state potential usable in dye-sensitized systems.

Introduction Efficient light-to-energy conversion of dye-sensitized solar cells (DSSCs) requires that the sensitized semiconductor electrode will have a high surface area.1,2 The high surface area is necessary because of the low absorbance of dye monolayers and the low efficiency of dye multilayers.3-8 In recent years, various high surface area electrodes of wide band gap semiconductors were tested in DSSCs starting with TiO2, which was presented in the first report of efficient DSSCs2 through SnO2,9,10 ZnO,11,12 Nb2O5,13,14 and SrTiO3.14 Most of these electrodes consist of nanosize semiconductor colloids that are sintered on a transparent conducting substrate, thus having a porous geometry. The nanoporous geometry provides the necessary surface area, but at the same time it introduces special characteristics that enhance one of the loss processes of DSSCs. During the operation of a DSSC, the injected electrons diffuse through the TiO2 film toward the conducting substrate, while the oxidized ions move in the opposite direction to be regenerated at the back electrode. * To whom correspondence should be addressed. E-mail: [email protected]. (1) Gerfin, T.; Gratzel, M.; Walder, L. Molecular and Supermolecular Surface Modification of Nanocrystalline TiO2 Films: Charge Separating and Charge Injecting Devices. In Molecular Level Artificial Photosynthetic Materials; Karlin, K. D., Ed.; John Wiley & Sons: New York, 1997; Vol. 44, pp 345-393. (2) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737-740. (3) Gerischer, H. Photochem. Photobiol. 1972, 16, 243. (4) Memming, R. Photochem. Photobiol. 1972, 16, 325. (5) Parkinson, B. A.; Spitler, M. T. Electrochim. Acta 1992, 37, 943. (6) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Meyer, G. J. Inorg. Chem. 1997, 36, 2-3. (7) Bonhote, P.; Moser, J. E.; Vlachopoulos, N.; Walder, L.; Zakiruddin, S. M.; Humphry-Baker, R.; Pechy, P.; Gratzel, M. Chem. Commun. 1996, 10. (8) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49-68. (9) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900-4908. (10) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490-4493. (11) Hoyer, P.; Weller, H. J. Phys. Chem. 1995, 99, 14096-14100. (12) Rensmo, H.; Keis, K.; Lindstrom, H.; Sodergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S. E. J. Phys. Chem. B 1997, 101, 2598. (13) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825-3832. (14) Lenzmann, F.; Krueger, J.; Burnside, S.; Brooks, K.; Gratzel, M.; Gal, D.; Ruhle, S.; Cahen, D. J. Phys. Chem. B 2001, 105, 6347.

The porous geometry that permits electrolyte presence through the entire electrode provides a high surface area for the recombination of the photoinjected electrons and the holes in the dye layer, or in the electrolyte (ions).15-18 Furthermore, the small size of the individual colloidal particles in the nanoporous electrode cannot support a high space charge.15,19-21 Thus, in the absence of band bending at the semiconductor surface, there is no energy barrier that can slow the recombination process, as usually occurs in bulk electrodes. Therefore, slowing the recombination rate has become a major task in attempts to increase the efficiency of DSSCs. The effort to improve the efficiency of DSSCs by suppression of the recombination process is evident from the literature.22-24 Most of these efforts involve two basic approaches: the first approach physically blocks of the electrode area that is not covered with dye. The second approach involves the formation of an energy gradient that directs the electrons toward the substrate. The physical blocking involves adsorption of insulating molecules or polymerization of an insulating layer on the semiconductor surface after the dye adsorption.8,25 This approach faces the complexity of mutual effects between the insulating layer and the dye. The energy gradient approach involves composite material nanoporous electrodes in which the two materials differ by their conduction band potential.22-24 Arranging these materials in the correct geometry is expected to drive the electrons to the (15) Hagfeldt, A.; Lindquist, S. E.; Gratzel, M. Sol. Energy Mater. Sol. Cells 1994, 32, 245. (16) Tachibana, Y.; Moser, J. E.; Gratzel, M.; Klug, D.; Durrant, J. R. J. Phys. Chem. 1996, 100, 20056. (17) Oskam, G.; Bergeron, B. V.; Meyer, G. J.; Searson, P. C. J. Phys. Chem. B 2001, 105, 6867. (18) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. Rev. B 2001, 6320, 5321. (19) Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J. J. Am. Chem. Soc. 1996, 118, 6716. (20) Bisquert, J.; Garcia-Belmonte, G.; Fabregat-Santiago, F. J. Solid State Electrochem. 1999, 3, 337. (21) Cahen, D.; Hodes, G.; Gratzel, M.; Guillemoles, J. F.; Riess, I. J. Phys. Chem. B 2000, 104, 2053. (22) Bedja, I.; Kamat, P. V. J. Phys. Chem. 1995, 99, 9182. (23) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S. an efficient dye-sensitized phtoelectrochemical solar cell made from oxides of tin and zinc. Chem. Commun. 1999, 15-16. (24) Tada, H.; Hattori, A. J. Phys. Chem. B 2000, 104, 4585. (25) Gregg, B. A.; Pichot, F.; Ferrere, S.; Fields, C. L. J. Phys. Chem. B 2001, 105, 1422.

10.1021/la015536s CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

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approximately 200 mV. This barrier increases the cell performance to more than two times the performance of a pure SnO2 electrode. However, despite this improvement, the new electrode is inferior to a pure TiO2 electrode in cases when dyes capable of injecting into TiO2 are used. Experimental Section

Figure 1. Different geometries of composite material nanoporous electrodes showing that only in the coated matrix (c) the photoinjected electrons do not face energy barriers (doted arrows) during the diffusion to the current collector.

Electrode Preparation. The SnO2 nanoporous matrix was prepared from colloids of 18 nm diameter. Normally the matrix thickness was 2.5 µm with high dark resistance. The colloids and the matrix preparation are reported elsewhere.27 The TiO2 coating was prepared by dipping the sintered SnO2 matrix in a 1 M tetrabutyl orthotitanate in dry butanol solution for 30 s, followed by washing with dry butanol and sintering at 500 °C in air for 30 min. The dipping was performed under a dry nitrogen atmosphere. The thickness of the electrode, measured with a Mitutoyo, Surftest SV 500 profilometer, did not change upon coating. Alumina coating was performed by spin coating using an aluminum oxide colloidal suspension (20% in H2O, Alfa/Aesar). DSSC. The dye (cis-di(isothiocyanato)-N-bis(4,4′-dicarboxy2,2′-bipyridine)ruthenium(2)) (Solaronix SA) was adsorbed by immersing the electrodes overnight in a 0.5 mM dry ethanol solution. The oxidation potential of the dye (ca. 1.09 V vs NHE in acetonitrile) is sufficient to allow injection into both SnO2 and TiO2.13 The relative amount of dye adsorbed on the electrodes was calculated from their visible absorption spectra. The spectra were measured by a Hewlett-Packard 8453 spectrophotometer using an undyed, nanocrystalline SnO2 electrode as a reference. The performance of the electrodes in the DSSCS was measured using the standard sandwich-type cell reported elsewhere13 using 0.5 M LiI/0.05 M I2 in 1:1 acetonitrile-NMO (3-methyl-2-oxazolidinone) as electrolyte. Surface Analysis. Scanning electron microscopy (SEM) of bare SnO2 and TiO2-coated SnO2 films were obtained using a JSM-840 SEM (JEOL, Japan). The SEM was equipped with an energy dispersive X-ray analysis (EDAX) which provides the quantitative ratio of the two semiconductors at the outer surface of the electrode. X-ray photoelectron spectroscopy (XPS) was used to determine the percentage of TiO2 throughout the film by the successive removal of electrode material. Limited etching was performed by sputtering. For the XPS examination of electrode areas that are close to the substrate, the electrode material was peeled using adhesive tape. Optical and Spectroelectrochemical Measurements. The transmission of the two types of electrodes was measured by a Cary 500 spectrophotometer. Fluorescence was measured by a SLM Aminco series 2 spectrofluorometer. Spectroelectrochemical experiments were performed using a Teflon cell equipped with two windows, one of which was the tested electrode. The electrochemical cell contained three electrodes; the nanoporous electrode served as the working electrode, a platinum wire served as counter electrode, and a Ag/AgCl/ saturated in KCl(aq) served as a reference electrode. Nitrogen bubbling of the aqueous electrolyte (0.2 M LiClO4) prior to experiments was performed to eliminate dissolved oxygen. The pH of the electrolyte was adjusted to 1.7 by the addition of concentrated HClO4. The cell was incorporated in a HewlettPackard 8453 spectrophotometer to measure the absorbance during the potential scan. applied potential was controlled by an Eco Chemie potentiostat.

desired direction by energy considerations; i.e., the electrons will favor the material having the more positive conduction band. This approach requires a very specific electrode design which ensures that the electrons will not encounter energy barriers (the more negative material) while diffusing to the current collector. As indicated in Figure 1, only the coated matrix design (Figure 1c, denoted below as core-shell) ensures free electron diffusion to the current collector. Recently we reported on a core-shell nanoporous electrode in which we applied the energy gradient approach.26 The electrode was made by coating a nanoporous TiO2 matrix with a Nb2O5 layer. The conduction band potential of the Nb2O5 is approximately 100 mV negative of the potential of the TiO2.13 Consequently, the electrons injected into the electrodes are driven away from the electrode surface into the TiO2 core. This activity slows the recombination rate and increases the cell’s efficiency (Figure 1c). In other words, the coating forms an energy barrier at the electrode-electrolyte interface. In this work, we report on the synthesis, characterization, and application of a nanoporous TiO2-coated SnO2 electrode. The fabrication of this core-shell electrode is motivated by the variety of dyes that have two advantages: they are stable and have a high absorption coefficient; however, these dyes are unable to inject into TiO2 due to a relatively positive excited-state potential. The SnO2-TiO2 core-shell electrode, which has a conduction band more positive than those of a bare or Nb2O5coated TiO2 electrode,13 enables efficient use of these dyes compared with the single material alternatives. The energy barrier formed by the TiO2 was found to be

Results and Discussion In this work, we compare two nanoporous electrodes: a standard SnO2 electrode (denoted “bare”) and the TiO2coated SnO2 electrode (denoted “coated”). The comparison relates to the electrodes’ characteristics and to their performances in dye-sensitized solar cells (DSSCs). The DSSC is a very complicated system that requires electrodes to be similar in order to achieve a meaningful comparison. Thus, the bare electrode and the matrix of the coated

(26) Zaban, A.; Chen, S. G.; Chappel, S.; Gregg, B. A. Chem. Commun. 2000, 2231.

(27) Chappel, S.; Zaban, A. Sol. Energy Mater. Sol. Cells 2002, 71, 141.

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Figure 3. The photocurrent and power as a function of the voltage for two typical DSSCs containing (a) the bare SnO2 electrode and (b) the coated electrode.

Figure 2. SEM surface pictures of the (a) bare SnO2 electrode and (b) coated electrode.

electrode were fabricated and sintered at the same batch. After the TiO2 coating, both the bare and the coated electrodes were sintered a second time together. The thickness of both electrodes was measured by a profilometer showing a similar value of 2.5 µm. Figure 2 presents SEM pictures of the outer side of the two electrodes. These pictures do not show any significant difference in the morphology of the two electrodes. Both the bare and the coated electrodes were transparent to visible light. Finally, the absorption spectra of the electrodes showed that the same amount of dye adsorbed to the surfaces. Figure 3 presents the photocurrent and power as a function of the voltage for two typical DSSCs containing the bare and the coated electrodes under one sun conditions. The coated electrode performed better than the bare electrode with respect to all cell parameters. The photocurrent increased from 5.4 to 6.7 mA cm-2, the photovoltage from 320 to 480 mV, and the fill factor from 28.4% to 33.6%. As a result, the cell’s conversion efficiency increased by a factor of 2.2 from 0.51% to 1.125%. The performance enhancement of the DSSC achieved by the TiO2 coating may be attributed to the energy barrier formed at the SnO2 matrix surface. As described above, this barrier is expected to slow the recombination process of the photoinjected electrons back to the oxidized dye or ions. However, because the conduction band potential of the TiO2 coating is ca. 0.5 V more negative than the potential of the SnO2,13,28 one expects a greater performance improvement. In particular, the photovoltage that is highly affected by the ability to accumulate electrons in the core SnO2 increased only by 160 mV. There are three possible reasons for this partial improvement, all related to the coating characteristics: (1) The TiO2 does (28) Bedja, I.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1994, 98, 4133.

not fully cover the matrix, thus leaving bare SnO2 that suppresses the coating effect. (2) The TiO2 layer is too thin to either attain the physical properties of bulk material or diminish the influence of the SnO2 matrix on its properties. (3) The TiO2 layer consists of a rutile rather than an anatase structure. In this case, the difference in conduction band potential between the SnO2 and the TiO2 is only ca. 0.3 V.29 In the remainder of this paper, we describe the characterization of the SnO2-TiO2 core-shell electrode performed by spectroelectrochemistry, dark current analysis, fluorescence measurements of dyed electrodes, misfit calculations, and chemical analysis techniques. The results discussed below suggest the following: a very thin rutilestructured TiO2 layer whose conduction band is more positive than bare anatase TiO2 covers the entire SnO2 matrix. Therefore, we cannot clearly determine whether the rutile structure, thickness of the coating, or both (options 2 and 3 above) control the core-shell electrode properties. However, the results clearly show that the coating decreases the reaction between electrons in the SnO2 matrix and the electrolyte, i.e., the recombination rate. Coverage Fraction of TiO2. Spectroelectrochemical measurements of the bare and coated electrodes were used to determine the coverage fraction of the TiO2 layer. These experiments test the change in the UV-vis spectrum of the electrode as a function of applied bias. In the context of nanoporous semiconductor electrodes, spectroelectrochemistry is commonly used to determine the conduction band potential.30 Scanning the bias toward the conduction band increases the electron concentration in the semiconductor. Consequently, the electrode absorption increases at long wavelengths,31 while bleach appears at energies above the band gap (short wavelengths).32 The (29) Burnside, S.; Moser, J. E.; Brooks, K.; Gratzel, M.; Cahen, D. J. Phys. Chem. B 1999, 103, 9328. (30) Fitzmaurice, D. Sol. Energy Mater. Sol. Cells 1994, 32, 289305. (31) Pankove, J. Optical Processes in Semiconductors; Dover Publications: New York, 1971. (32) Burstein, E. Phys. Rev. 1952, 93, 632.

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Figure 5. Cyclic voltammograms of the (a) bare and (b) coated SnO2 electrodes recorded during the SEC measurement of Figure 3.

Figure 4. Difference absorption spectra of (a) bare SnO2 electrode and (b) TiO2-coated SnO2 electrode, at 0.1, -0.1, -0.3, -0.5, and -0.7 V (the arrow points in the negative bias direction), measured at pH 1.75, V vs Ag/AgCl, in 0. 2 M LiClO4.

spectroelectrochemical measurements of the nanoporous SnO2 electrodes performed in this work demonstrate this behavior, except that the spectra at short wavelengths are altered by an additional process that occurs during the potential scan. This process will be used to determine the coverage fraction. Figure 4 presents the absorbance change of bare and TiO2-coated SnO2 electrodes at various potentials. The experiments were performed at pH 1.7 in a potential range of +0.4 to -0.75 V vs Ag/AgCl. As the potential is scanned negatively, the absorption at long wavelengths increases in both electrodes. However, regarding the bleach below 350 nm, there is a clear difference between the bare and coated electrode. In the case of the coated electrode, the bleaching increases during the scan negatively, while at similar conditions for the bare electrode we observe the development of a peak having a maximum at ca. 330 nm. This peak overlaps the expected bleach. We considered two processes that may generate the 330 nm absorbance in the SnO2 electrode: SnO2 reduction to SnO and Li+ intercalation. The intercalation process was eliminated since similar spectra were recorded when the spectroelectrochemical measurement was performed in a Li+ or H+ free electrolyte (dry acetonitrile with tetrabutylammonium perchlorate). Furthermore, Li+ intercalation should not occur in the potential range used in this experiment.33 On the other hand, SnO2 reduction can occur in the potential range of the spectroelectrochemical measurement. The literature value for SnO2 reduction to SnO is between -0.5 and -0.6 V.28,34 Figure 5 presents cyclic voltammograms of the bare and coated SnO2 electrodes recorded during the spectroelectrochemistry (SEC) experiment. A clear difference between the two voltammograms may be characterized by the reduction-oxidation peaks centered around -0.55 V, which appear only in the curve of the bare SnO2. Thus, the bare (33) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (34) Laitinen, H. A.; Vincent, C. A.; Bednarski, T. M. J. Electrochem. Soc. 1968, 115, 1024-1028.

Figure 6. Dark current measurement of the (a) bare SnO2 electrode and (b) the coated electrode in a standard DSSC.

SnO2 electrode undergoes a reduction during the SEC measurement. Also only the bare SnO2 voltammogram shows a small pick around -0.45 V, which is usually attributed to surfacestate charging. This may indicate a change in the surface state distribution. Both processes seem to be affected by the internal resistance and nonuniform potential distribution that are associated with this type of electrode.20,35 Figures 4 and 5 show that under similar conditions the SnO2 electrode undergoes reduction, while the coated electrode is passivated by the TiO2. We assume that the passivation results from the physical separation between the SnO2 and the solution. However, regardless of the passivation nature, the results displayed in Figures 4 and 5 show that full coverage is achieved because the coated electrode shows no sign of reduction. Surface Energy Barrier. To examine the coating effect on the recombination process, we measured the dark current of DSSCs containing the bare and coated electrodes. The dark current is not a direct measurement of the recombination rate; however, it is meaningful when two similar cells are compared. Figure 6 presents the dark currents of the cells presented in Figure 3 showing that at each given potential the dark current of the coated electrode is lower than that of the bare electrode. This difference indicates a decrease of the electron recombination rate, presumably due to the energy barrier formed between the SnO2 matrix and the solution. We wanted to eliminate the possibility that the lower dark current measured at the coated electrode is due to a coverage of the exposed conducting substrate by the TiO2. Therefore, we performed a similar comparison for electrodes in which these pinholes were electrochemically isolated. The isolation was performed by the electro(35) Zaban, A.; Meier, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 7985.

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Chappel et al. Table 1. Fluorescence Emission at 642 nm of Electrodes Excited at 493 nm normalized fluorescence (au) bare SnO2 coated

Figure 7. A schematic view of the electrode used to study the possible bilayer effect. A layer of nanoporous Al2O3 sintered on top of the SnO2 matrix before the TiO2 coating process.

chemical polymerization of polyphenoxide using the method reported by Gregg et al.25 Here also, the dark current of the coated electrode was lower than the current of the bare electrode throughout the potential scan, indicating that the different currents do not result from the substrate effects. During the coating process, the SnO2 matrix is dipped into a TiO2 precursor solution. Under certain conditions, the dipping may result in the formation of a colloidal TiO2 layer on top of the matrix. It is therefore possible that a side reaction of the coating provides most of the observed effect; i.e., the measured system resembles a bilayer structure. Thus, the electrons injected into the porous TiO2 layer reach the substrate via the SnO2 layer and the improvement is attributed to the inherent properties of the TiO2 that drive the electrons toward the current collector. The SEM pictures of the coated electrodes (Figure 2) and the thickness measurements did not reveal an outer TiO2 layer. However, to eliminate this possibility, we fabricated a DSSC containing the electrode described in Figure 7. A layer of nanoporous alumina was sintered on top of a standard SnO2 matrix. This bilayer electrode was coated with TiO2 using the same procedure that was performed for coating the simple SnO2 matrixes. Having the alumina on top of the SnO2 disables electron transport from a TiO2 layer that may be formed at the outer part of the electrode to the current collector. Thus, if the performance enhancement that is achieved by the TiO2 coating results from a formation of a TiO2 layer on top of the SnO2, the cell containing the alumina layer should not be effected by the TiO2 coating. Measurements of this solar cell showed an increase of both the photovoltage and maximum power with respect to a noncoated electrode. This finding indicates that the core-shell configuration which forms an energy barrier at the SnO2 surface derives the efficiency increase. Conduction Band Potential. As mentioned above, the performance of the solar cells indicates that the conduction band potential of the coated electrode is located between the potentials of bare SnO2 and bare TiO2. Fluorescence measurements of a dye adsorbed to these nanoporous electrodes were used to study their relative band position. The fluorescence intensity of the adsorbed dye contains information regarding the injection efficiency in the measured system. This is because the injection process is an alternative nonradiative decay path for the excited electrons.9,36 In other words, high injection efficiency decreases the fluorescence intensity. To relate the injection efficiency to the relative band position of the electrodes, one must assume that (1) the dye energetics and (2) the injection kinetics are independent of the type (36) O’Regan, B.; Moser, J.; Anderson, J.; Gratzel, M. J. Phys. Chem. 1990, 94, 8720.

170 240

normalized fluorescence (au) bare TiO2 bare Al2O3

570 691

of semiconductor to which the dye injects to. The first assumption provides a common reference potential for the comparison between the different systems, while the second assumption relates the injection efficiency to the relative position of the excited state of the dye and the conduction band of the semiconductor. Both assumptions are not necessarily true. It was shown that the dye energetics is affected by the surrounding area, which includes the semiconductor electrode.37 However, because the measurement is performed without an electrolyte, one expects that the electrode effect will be minimal.37 The injection kinetics is also affected by the type of acceptor semiconductor.13,38 However, these studies show that the injection into SnO2 is slower than that into TiO2, which means that higher injection efficiency into SnO2, in comparison with TiO2, necessitates a more positive conduction band potential of the SnO2. The perylene-based dye used in the fluorescence measurements has an excited-state band that allows injection into the more positive SnO2, but this dye is practically incapable of injecting into anatase TiO2.10 Table 1 presents the fluorescence intensity of four dyed electrodes: bare SnO2, TiO2, Al2O3, and TiO2-coated SnO2. The intensity was normalized to the amount of dye adsorbed to each film. The alumina, having a very negative conduction band, serves as a reference since it should not allow any injection.39 The other three electrodes show an increasing amount of injection in the following order: bare TiO2 < TiO2-coated SnO2 < bare SnO2. The results thus indicate that the conduction band potential of the coated electrode is located between the potentials of bare TiO2 and bare SnO2, as suggested by the solar cell results. Crystal Structure of the TiO2 Coat. The TiO2 coat likely consists of either the anatase or the rutile structure. The third structure that can be formed at room temperature, the brookite, is not stable at the electrode sintering temperature.40,41 If formed, the brookite should transform to one of the other two structures during the sintering process.40 The crystal structure of the TiO2 coat has a significant effect on the overall properties of the coreshell electrode. The following major parameters are important to DSSC: (1) The conduction band potential at the electrode surface which determines the height of the surface energy barrier and the relative dye-semiconductor energetics. (2) The photochemical activity of the surface which affects the stability of the solar cell. (3) The semiconductor-dye coupling that can affect the injection efficiency (although recent studies of rutile-based DSSCs show that this structure is not necessarily less efficient than anatase).42 (37) Zaban, A.; Ferrere, S.; Gregg, B. A. J. Phys. Chem. B 1998, 102, 452. (38) Asbury, J. B.; Hao, E.; Wang, Y. Q.; Ghosh, H. N.; Lian, T. Q. J. Phys. Chem. B 2001, 105, 4545. (39) Vinodgopal, K.; Hua, X.; Dahlgren, R. L.; Lappin, A. G.; Patterson, L. K.; Kamat, P. V. J. Phys. Chem. 1995, 99, 10883-10889. (40) Zachau-Christiansen, B.; West, K.; Jacobsen, T.; Atlung, S. Solid State Ionics 1988, 28-30, 1176-1182. (41) Schuisky, M.; Harsta, A.; Aidla, A.; Kukli, K.; Kiisler, A. A.; Aarik, J. J. Electrochem. Soc. 2000, 147, 3319-3325. (42) Park, N. G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 8989.

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Table 2. Characteristics of Anatase/Rutile TiO2 and Rutile SnO2 SnO2-rutile TiO2-rutile TiO2-anatase crystal structurea lattice constant (Å)a band gap (eV)

tetragonal A ) 4.738 C ) 3.188 3.729

tetragonal A ) 4.5936 C ) 2.9587 328,29

tetragonal A ) 3.784 C ) 9.515 3.228

a Wyckoff, R. W. G. Crystal Structures, 2nd ed.; University of Arizona Press: Tucson, AZ, 1965; Vol. 1.

The simplest structure characterization tool, the X-ray diffraction (XRD), did not show any peaks of the TiO2. The inability to detect the TiO2 by XRD indicates that the coating is either too thin or not fully structured. However, the literature provides many examples for the use of SnO2 as mineralizers for the synthesis of rutile TiO2.43 Moreover, based on the lattice matching rules, TiO2 will likely grow on the SnO2 matrix in the rutile structure. The basic rule of epitaxial growth requires parallelism of two lattice planes which have networks of identical form and of closely similar spacings. The difference between the network spacings is usually expressed in terms of the misfit percentage, which is defined as 100(b - a)/a, a and b being the corresponding network spacings in the substrate and overgrowth, respectively. Epitaxial growth requires that the misfits will not be greater than 15%.44 The SnO2 matrix used in this study has a cassiterite structure which resembles the rutile. Table 2 shows unit cell parameters of this SnO2 which closely resemble those of the rutile TiO2. Table 2 also presents the unit cell parameters of anatase TiO2 which differ significantly from those of the SnO2. The misfit of cassiterite SnO2 and rutile TiO2 is less than 4%, while the anatase mismatches the cassiterite SnO2 by 34%. Thus, it seems that a rutile TiO2 grows with a parallelism of the {101} planes of the two materials. The conduction band potential of rutile TiO2 is ca. 0.2 V positive in comparison to anatase TiO2.29 The misfit calculations showing that the TiO2 coating of the SnO2 has a rutile structure confirms the results presented above regarding the conduction band potential of the surface of the core-shell electrode. In other words, the lower open circuit photovoltage value and the higher injection efficiency, measured in comparison with bare anatase TiO2, should be related to the rutile structure of the TiO2 coating. However, in the absence of a direct measurement of the coating’s crystal structure, we still cannot eliminate the other possibility which attributes these effects to a thin, not fully structured, layer. Chemical Analysis. XPS, EDAX, and UV-vis transmission were utilized for a quantitative analysis of the coated layer. The measured atomic percentage of TiO2 in the film was 4-10% (the remainder being SnO2) depending on the technique used. XPS analysis showed 4.3% atomic concentration of Ti 2p at the electrode outer layer. Etching of the film by sputtering showed a decrease of the Ti concentration to a constant value of 2.27 (Figure 8). A similar value was measured when more material was removed from the electrode by adhesive tape. Figure 9 presents the UV-vis transmission spectra of bare SnO2, bare TiO2, and the coated electrodes. The spectrum of the coated film was fitted to a superposition of the bare TiO2 and SnO2 spectra. Good fitting was obtained for a combination of 90% SnO2 and 10% TiO2. We note, however, (43) Kumar, K. N. P.; Keizer, K.; Burggraaf, A. J.; Okubo, T.; Nagamoto, H. J. Mater. Chem. 1993, 3, 923. (44) Pashely, D. W. Adv. Phys. 1956, 5, 173-240.

Figure 8. Ti atomic concentration obtained from by XPS as a function of the sputtering duration. The remaining percentage is Sn.

Figure 9. Transmission measurement of the (a) bare SnO2 electrode, (b) bare TiO2 electrode, and (c) coated electrode. Fitting of the coated electrode spectra was done by overlapping the bare electrodes spectra (d) in a ratio of 10:90.

that unlike the bare SnO2 reference, the bare TiO2 reference is not necessarily similar to the coated layer. In other words, the spectrum of the TiO2 layer may differ slightly from the spectrum of the TiO2 reference due to physical phenomena such as size, surface, and structure. EDAX measurement of the outer layer of the coated electrode provided a value of 8% for the atomic Ti concentration. This results in an average of ca. 1000 nm penetration. To estimate the coating thickness from the values of TiO2 percentage in the film, one must assume uniform coating, spherical shape of the SnO2 nanoparticles, and ordered particle array. This estimate results in a value of ca. 0.5 nm which corresponds to two unit cells of TiO2. We note however, that none of the characterization methods provided information regarding the uniformity of the coated layer. Furthermore, the SnO2 particle shape is not a perfect sphere and the film is not well ordered. Optimization of the Electrode Performance to DSSC. Several TiO2 coating procedures were tested in this work. The best electrodes in terms of solar cell efficiency were made by three successive dippings of the SnO2 matrix in 1 M tetrabutyl orthotitanate, each dipping followed by 500 °C sintering in air for 30 min. The work reported above relates to this fabrication procedure. To obtain the best coating conditions, we were required to perform an optimization study. In this study the performance of the core-shell electrode was tested by five parameters: dye absorption, photocurrent, photovoltage, fill factor, and conversion efficiency. The preparation conditions were varied by factors such as the type and concentration of the Ti precursor, the number and duration of dippings, and the sintering conditions. As a result, we generated a multidimensional array from which the optimal conditions for the coating was extracted. The text here provides an example of the optimization process regarding the number of successive dippings in the TiO2 precursor. Table 3 shows the performance of

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Table 3. Solar Cell Parameters for Cells Containing Electrodes That Were Coated by Different Dipping Times (zero coating is the reference electrode)a sample

absorbance

Pmax (µW)

Voc (mW)

Jsc (mA/cm2)

FF (%)

0 1 2 3 4

0.438 0.459 0.463 0.481 0.436

497 587 893 1041 974

320 340 413 452 486

5.7 6.2 6.6 6.7 6.4

27 28 33 34 31

a

The electrode thicknesses in these cells were ∼2.5 µm.

DSSCs containing electrodes that underwent several dippings. Each dipping lasted 30 s and was followed by a rinse of excess precursor with the dry solvent and sintering at 500 °C for 30 min. Table 3 shows that increasing the number of dippings up to three times improved all parameters. At the fourth dipping, the cell performance decreased with respect to all parameters apart from Voc. We attribute this behavior primarily to TiO2 partially blocking the matrix. This blockage affects the dye adsorption, the ion migration, and thus the photocurrent and fill factors. We note however, that the increase of Voc at the forth dipping may indicate that an optimal coating should be thicker than the coating achieved by three dippings if the pore size of the matrix was bigger. In other words, Table 3 suggests that the coating properties should be at least in part attributed to the coating thickness. At this stage larger SnO2 colloids that will form larger pores are not available.27

Conclusions The synthesis and characterization of TiO2-coated SnO2 nanoporous electrodes for DSSC applications were reported. These core-shell electrodes consist of an inherent energy barrier at the electrode-electrolyte interface. This barrier decreases the recombination rate and thus increases the conversion efficiency of the cell, in comparison with a bare SnO2 electrode. The TiO2-coated SnO2 electrode is designed for systems that utilize dyes having an excited state that cannot inject into the standard TiO2. The results presented show that a thin layer probably having the rutile structure is formed at the coating. We were not successful in fabricating a thicker layer without partially blocking the porous structure. Therefore it was not possible to determine whether the thickness of the coating, its structure, or both control the coating properties, primarily with respect to the conduction band potential at the electrode’s surface. The fabrication of larger pore size matrix should solve this ambiguity and probably further increase the efficiency of this SnO2TiO2 core-shell electrode. Acknowledgment. This research was supported by The Israel Science Foundation founded by The Israel Academy of Science and Humanities. S.C. was supported by Israel Ministry of Science, The Levi Eshkol schlolarship. We thank E. Levy for help with the XRD measurements. LA015536S