pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Synthesis of TiO2 Inverse Opal Electrodes for Dye-Sensitized Solar Cells Ju-Hwan Shin,† Ji-Hwan Kang,† Woo-Min Jin,† Jong Hyeok Park,‡ Young-Sang Cho,§ and Jun Hyuk Moon*,† †
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea, ‡ Department of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea, and § Department of Powder Materials, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea Received July 30, 2010 Engineering of TiO2 electrode layers is critical to guaranteeing the photoconversion efficiency of dye-sensitized solar cells (DSSCs). Recently, a novel approach has been introduced for producing TiO2 electrodes using the inverted structures of colloidal crystals. This paper describes a facile route to producing ordered macroporous electrodes from colloidal crystal templates for DSSCs. Using concentrated colloids dispersed in a volatile medium, the colloidal crystal templates were obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Here, the effects of the structural properties of the inverse opal TiO2 electrodes on the photovoltaic parameters of DSSCs were investigated. The photovoltaic parameters were measured as a function of pore ordering and electrode film thickness. Moreover, DSSC applications that used either liquid or viscous polymer electrolyte solutions were investigated to reveal the effects of pore size on performance of an inverse opal TiO2 electrode.
Introduction Dye-sensitized solar cells (DSSCs) are nanotechnology-based photovoltaic devices that have been extensively studied with a view to increasing their photoelectric conversion efficiency.1-3 A typical DSSC comprises two sheets of a transparent conducting substrate separated by a redox electrolyte solution. One of the substrates is coated with a nanocrystalline TiO2 film sensitized by dye molecules, and the other is coated with a platinum catalyst. The photovoltaic effect in DSSCs is induced by transport of the photogenerated electrons from the excited dye molecules. The dyes are regenerated by the oxidation of the electrolyte, which is regenerated by the Pt-coated electrode. A key factor that contributes to the efficiency of DSSCs is the nanocrystalline structure of the TiO2 layer.4 Incident light is collected by the dye molecules attached to the surface of the TiO2 layer, and the photogenerated electrons are transferred through the TiO2 layer. Typically, a large internal TiO2 surface area favors maximal dye absorption, thereby improving light-harvesting and photocurrent generation in the DSSCs. Porous electrode films composed of TiO2 nanocrystalline particles of size around 30 nm are widely used, but the disordered particle assemblies and tight packing in the film reduce electron transport and ion diffusion from the electrolyte solution. In this regard, much effort has been recently devoted to engineering the morphology of the TiO2 electrodes. For example, the preparation of TiO2 electrodes by the assembly of nanotubes, nanowires, nanorods, *Corresponding author. E-mail:
[email protected].
(1) O’Regan, B. C.; Durrant, J. R. Acc. Chem. Res. 2009, 42, 1799. (2) Gratzel, M. Acc. Chem. Res. 2009, 42, 1788. (3) Peter, L. M. Phys. Chem. Chem. Phys. 2007, 9, 2630. (4) Wang, Z. S.; Kawauchi, H.; Kashima, T.; Arakawa, H. Coord. Chem. Rev. 2004, 248, 1381. (5) Park, J. H.; Lee, T. W.; Kang, M. G. Chem. Commun. 2008, 2867. (6) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (7) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364.
856 DOI: 10.1021/la104512c
or electrospun fibers has been attempted in an effort to improve the efficiency.5-13 Recently, a novel approach was introduced for engineering TiO2 pore structures by employing the inverted structures of particle assemblies.14-16 Specifically, colloidal crystals of monodisperse particles were applied as sacrificial templates for porous TiO2 electrodes. Although the introduction of macropores decreased the surface area and efficiency of DSSCs, several potential advantages of this type of structured electrode may offset this effect: ordered TiO2 networks can provide fast electron transport paths, and ordered macropores can enhance light scattering, facilitate infiltration of the electrolyte solution, and promote mass transport. In particular, this type of macroporous electrode may be important for the development of DSSCs with polymer or gel electrolytes, which possess a high viscosity and contain relatively large polymer molecules.17,18 The polymer electrolytes has been studied to accelerate practical applications of DSSCs because polymers can prevent the leakage and volatilization of liquid electrolytes. However, the mesoscale pores in conventional TiO2 electrodes often limited the use of polymeric electrolytes.18 (8) Feng, X. J.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781. (9) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nature Mater. 2005, 4, 455. (10) Wang, D. A.; Yu, B.; Wang, C. W.; Zhou, F.; Liu, W. M. Adv. Mater. 2009, 21, 1964. (11) Chuangchote, S.; Sagawa, T.; Yoshikawa, S. Appl. Phys. Lett. 2008, 93. (12) Mukherjee, K.; Teng, T. H.; Jose, R.; Ramakrishna, S. Appl. Phys. Lett. 2009, 95. (13) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114. (14) Kwak, E. S.; Lee, W.; Park, N. G.; Kim, J.; Lee, H. Adv. Funct. Mater. 2009, 19, 1093. (15) Yang, S. C.; Yang, D. J.; Kim, J.; Hong, J. M.; Kim, H. G.; Kim, I. D.; Lee, H. Adv. Mater. 2008, 20, 1059. (16) Kuo, C.-Y.; Lu, S.-Y. Nanotechnology 2008, 19, 095705. (17) Somani, P. R.; Dionigi, C.; Murgia, M.; Palles, D.; Nozar, P.; Ruani, G. Sol. Energy Mater. Sol. Cells 2005, 87, 513. (18) Kang, M.-S.; Kim, J. H.; Kim, Y. J.; Won, J.; Park, N. G.; Kang, Y. S. Chem. Commun. 2005, 889.
Published on Web 12/14/2010
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In this paper, we first report a facile route to the fabrication of TiO2 inverse opal (io-TiO2) electrodes for DSSCs. We employed a concentrated colloidal dispersion in an aqueous ethanol solution, which accelerated crystallization within a few minutes, much faster than has been previously achieved. Previous techniques used the dip-coating method or crystallization in a thin gap for depositing the colloidal crystals.14,16 These methods, however, are time-consuming due to the use of a low concentration and waterbased colloidal dispersion, typically requiring between several hours and several days to complete crystallization over an area of several square centimeters.19 Second, we extended the investigation of io-TiO2 electrodes to evaluate the structural effects on the photovoltaic parameters of DSSCs. In particular, we considered the effects of macropore ordering and thickness in io-TiO2 electrodes on the DSSC efficiency. The ordering of pores in the io-TiO2 layer was directly reflected in the ordering of the colloidal crystal template, and the ordering of the colloidal crystals was directly affected by the processing time allowed for self-assembly.20,21 In general, shorter crystallization times resulted in a higher defect density and less long-range order in the colloidal assemblies. Therefore, characterizing the effects of colloidal crystalline ordering on DSSC efficiency can be used to optimize the processing time for assembly of the colloidal particles. Briefly, we found that the ordering of pores in the inverse opal electrode did not affect the photovoltaic parameters or the efficiency. The thickness of the electrode was carefully controlled because as the thickness increased, two mutually compensating effects emerged: the number of photogenerated electrons increased due to the additional adsorbed dyes and the number of electrons decreased due to recombination with the electrolytes in solution.4,22,23 In this study, the io-TiO2 electrode could be prepared at an optimal thickness for maximum efficiency. However, compared to the conventional TiO2 electrodes, the electrode efficiency saturated early with respect to the effective thickness of TiO2 due to the high scattering characteristics of the io-TiO2 electrode structure. Finally, we compared applications of both liquid and viscous polymeric electrolytes in io-TiO2 electrodes and compared the efficiency in both cases to that of conventional nanocrystalline TiO2 electrodes. Briefly, our results showed a smaller drop in efficiency for the io-TiO2 than for the nanocrystalline TiO2 electrodes upon replacement of the liquid electrolyte with a viscous polymer electrolyte solution. This observation indicated facile infiltration of the polymeric electrolyte solution into 3D ordered macropores in io-TiO2 electrodes.
Typically, a 20 vol % solution containing highly monodisperse particles in ethanol was applied to fabrication of colloidal crystal templates. The PS colloids were dropped onto the TiO2-coated FTO substrate (1.7 cm 1.5 cm) to cover the substrate and dried in a convection oven at 70 °C for 5-10 min. The amount of deposited PS colloidal solution was varied to control the thickness of the colloidal crystals and, therefore, the io-TiO2 electrodes. The volume of deposited colloidal solution was varied from 10 to 80 μL, which produced io-TiO2 electrodes of thickness 4.5-22 μm. Finally, the nanoparticles of TiO2 (average size 15 nm, dispersed in water, NanoAmor) filled the cavities of the PS colloidal crystals. The average diameter of TiO2 nanoparticles was around 15 nm, and they formed a stable dispersion in water. Because the size of the cavities formed by the closed-packed colloidal crystals was around 200 nm, which was larger than the size of TiO2 particles, TiO2 particles easily infiltrated into the colloidal crystals. After several drops of TiO2 colloids, we spun the substrate at 2000 rpm for several minutes to remove the excess TiO2 colloids from the surface and facilitate the evaporation of solvent. The samples were then calcined in air at 450 °C for 1 h to burn away the PS, leaving behind an io-TiO2. Post-treatment of the TiO2 inverse opal electrodes with TiCl4 aqueous solution was applied for 1 h. Subsequently, the samples were calcined in air at 450 °C for 1 h. Assembly of DSSC. The TiO2 layer was immersed in a dye solution containing 0.5 mM N719 dye (Dyesol) in anhydrous ethyl alcohol (99.9%, Aldrich) for 40 h. The counter electrode was prepared by coating a 0.7 mM H2PtCl6 solution in anhydrous ethanol onto the FTO substrate. The TiO2 electrode with active area of 13-15 mm2 was assembled with the counter electrode, and the gap size between electrodes was controlled using a 60 μm thick polymeric film. The electrolyte solution was injected into the gap. The redox couple/electrolyte solution contained 0.7 M 1-butyl-3methylimidazolium iodide, 0.03 M iodine, 0.1 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine in a solution of acetonitrile and valeronitrile (85:15 v/v). In the case of a polymeric electrolyte solution, a mixture of 0.5 M LiI, 0.05 M I2, and 1.0 g of poly(ethylene oxide) (PEO, Mw 1 000 000) in acetonitrile was prepared. This solution was highly viscous and did not flow over the course of several hours after the bottle was inverted. The polymer electrolyte solution infiltrated into the inverse opal or nanocrystalline TiO2 electrolytes and covered the counter electrode. Characterization. The photocurrent and voltage of the DSSCs were measured using a Source Meter (Keithley Instruments) under simulated solar light, produced by a 150 W Xe lamp (Oriel) and AM 1.5G filters. The intensity of the lamp was adjusted using a Si reference cell (BS-520, Bunko-Keiki) to a power density of 100 mW/cm2. The impedance was measured using an impedance analyzer (Versastat, AMETEK). The transmission spectra of the TiO2 electrodes were measured by UV-vis spectrophotometry (JASCO V550).
Experimental Section Fabrication of the io-TiO2 Electrode. The monodisperse PS particles were synthesized by dispersion copolymerization of styrene. The styrene (1 M) and thermal initiator, 2,20 -azobis(2-methylbutyronitrile) (1 wt % of monomer), were dissolved in ethanol. The size of particles, controlled by a stabilizer, poly(N-vinylpyrrolidone) (Mw = 49 000, Junsei Chemicals Co.), was 900 nm as a result of adding 1.0 g (per 100 mL of ethanol) of the stabilizer. The PS particles were dispersed in a water-ethanol. The concentration of particles was increased, and the amount of ethanol in the colloidal solution was controlled to decrease the rate of crystallization without losing much of crystalline order. (19) Moon, J. H.; Yang, S. Chem. Rev. 2010, 110, 547. (20) Chung, Y. W.; Leu, I. C.; Lee, J. H.; Hon, M. H. Langmuir 2006, 22, 6454. (21) Teh, L. K.; Tan, N. K.; Wong, C. C.; Li, S. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 1399. (22) Hsiao, P. T.; Tung, Y. L.; Teng, H. S. J. Phys. Chem. C 2010, 114, 6762. (23) Jarernboon, W.; Pimanpang, S.; Maensiri, S.; Swatsitang, E.; Amornkitbamrung, V. Thin Solid Films 2009, 517, 4663.
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Results and Discussion In order to fabricate TiO2 inverse opal electrodes for DSSCs, the colloidal particles were assembled into face-centered cubic (FCC) arrays on a blocking layer-coated FTO substrate. Specifically, we employed colloidal crystals of about 900 nm diameter particles. The previous result on the optimization of macropore size shows that larger pores about 1 μm shows higher efficiency due to enhanced adhesion between their inverted structure and the substrate.14 Colloidal particles dispersed in a water-ethanol mixture were employed to fabricate a colloidal crystal template. The amount of ethanol in the colloidal solution was controlled to decrease the rate of crystallization without losing much of crystalline order. Typically, a 20 vol % solution containing highly monodisperse particles in ethanol (∼20 vol %) yielded colloidal crystals with large crystalline domains on the order of 30 μm30 μm, as shown DOI: 10.1021/la104512c
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Figure 1. SEM images of (a) PS colloidal crystals and (b) the surface of io-TiO2 (scale bar: 5 μm). Inset picture of io-TiO2 on a substrate (4 cm 5 cm).
in Figure 1a. TiO2 nanoparticles with an anatase crystalline phase and an average diameter of 15 nm were spin-coated onto the colloidal crystals. Subsequent removal of the colloidal crystals left behind the io-TiO2. The TiO2 layer was post-treated for 1 h to enhance the interfacial contact between the io-TiO2 structure and the underlying substrate. Figure 1b shows typical SEM images of io-TiO2. The io-TiO2 contained air cavities 730 nm in diameter, which was connected with smaller pores 130 nm in diameter. The wall thickness of TiO2 is 200 nm. The inset image shows large-area and uniform io-TiO2 film (1.7 cm 1.5 cm). First, we studied the influence of degree of disorder within the macropore arrays of the io-TiO2 electrodes on the photovoltaic parameters. Since the disorder, such as point and line defects, drying cracks, or stacking faults in colloidal crystals, is known to be unavoidable, their inverted structures also contain this type of defects in pore arrays. In colloidal crystallization, the defects are more severe as the processing time decreased because the fast evaporation of solvent is not allowed enough time for self-organization. Here, binary colloidal particles are used to induce disorder into the colloidal crystals in a controlled manner. Previously, it was reported that the polydispersity of particles in a colloidal assembly was critical for producing ordered structures.24 With about 10% polydispersity in particle diameter, it can severely deteriorate the crystalline structure.25 Figures 2a, 2b, and 2c show SEM images of colloidal crystals composed of 900 nm diameter monodisperse colloidal particles in which were dispersed respectively 0%, 10%, and 20% particles 1.2 μm in diameter. Also shown are their io-TiO2 structures. As the amount of larger colloidal particles increased, the size of crystalline domains clearly decreased with the formation of numerous point and line defects. Accordingly, the io-TiO2 displayed less organized macropores and large cracks between the crystalline domains, as are visible in Figures 2a-c. The disorder in the colloidal crystals was quantified by calculating the dimensionless pair correlation functions using (24) Allard, M.; Sargent, E. H. Appl. Phys. Lett. 2004, 85, 5887. (25) Rengarajan, R.; Mittleman, D.; Rich, C.; Colvin, V. Phys. Rev. E 2005, 71.
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the two-dimensional radial distributions mapped by the SEM images.25 The number of oscillations in the pair correlation function clearly decreased as the percentage of added particles increased, indicating that the in-plane order deteriorated, as shown in Figures 2d-f. Table 1 shows the photovoltaic parameters, including the short-circuit photocurrent density (Jsc), the open-circuit voltage (Voc), and the fill factor (FF) of DSSCs containing io-TiO2 electrodes inverted from the samples. The overall conversion efficiency (η) was calculated using the equation η=Jsc Voc FF/P, where P is the power density of the incident light (100 mW/cm2). Similar quantities of dye adsorption in all samples suggested that the samples’ effective TiO2 surface areas did not vary. As shown in Table 1, all samples showed similar efficiencies and photovoltaic parameters. Therefore, the defects and poor connectivity of pores in the TiO2 structure did not critically affect the photovoltaic performance of the DSSCs. In other words, even with defects, the TiO2 network was sufficiently connected that electron transport through the TiO2 substrate was not hindered. Additionally, the size and fraction in macropores were sufficiently large that transport of ions from the electrolyte solution was also not hindered. This result implies that colloidal crystallization, which is usually time-consuming and must be carefully controlled, is not critical, and therefore, the processing time can be minimized to produce colloidal crystal templates. Next, the effect of io-TiO2 electrode thickness on the efficiency of DSSCs was investigated. Previous investigations of the use of nanocrystalline TiO2 electrodes showed that as the thickness increased, Jsc increased due to the accumulation of dye molecules. However, electron recombination with the oxidized redox species of the electrolyte solution concurrently increased, and the average path to the FTO substrate lengthened, thereby lowering Jsc.26,27 The Voc and FF were also affected by the recombination rate, mass transport limits, and series resistances and, therefore, decreased with increasing film thickness.27 As a result, for the case of nanocrystalline TiO2 electrodes, the optimum thickness was previously reported to be 15-20 μm.4,26,27 Figure 2 shows the photovoltaic parameters of DSSCs for io-TiO2 films of various thicknesses. The dependence of efficiency on thickness was mainly attributed to the Jsc. Jsc was found to increase from 3.5 to 7.6 mA/cm2 as the film thickness increased from 4.5 to 14.5 μm, but further increases in thickness up to 22 μm slightly decreased the Jsc. As the thickness increased, the increase in Jsc was obvious due to the buildup of photosensitized dye molecules, which was proportional to the thickness. The decrease in Jsc was attributed to the increased rate of electron recombination. It has been reported that the recombination rate scales linearly with the internal surface area of TiO2 electrodes and, therefore, the thickness of the electrode.4,20 These competing effects result in first an increase and then a slight decrease in Jsc as the thickness of the inverse TiO2 opal electrodes is increased. The relative rates of charge carrier generation and recombination can be characterized by electrochemical impedance spectroscopy (EIS).28 The impedance spectrum of a DSSC containing liquid electrolytes yields an estimate for the relative resistances associated with charge transport at the Pt counter electrode, the TiO2/dye/electrolyte interface, and Nernstian diffusion in the electrolyte. It is known that the frequency at which the imaginary part (fmax) is maximum within the frequency range 1-100 Hz is (26) Wei, M. D.; Konishi, Y.; Zhou, H. S.; Yanagida, M.; Sugihara, H.; Arakawa, H. J. Mater. Chem. 2006, 16, 1287. (27) Hsiao, P. T.; Tung, Y. L.; Teng, H. S. J. Phys. Chem. C 2010, 114, 6762. (28) Kern, R.; Sastrawan, R.; Ferber, J.; Stangl, R.; Luther, J. Electrochim. Acta 2002, 47, 4213.
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Figure 2. SEM images of binary colloidal crystals and their inverted TiO2 structures prepared with varying volume concentrations of larger particles: (a) 0%, (b) 10%, or (c) 20%. The pair correlation functions (d), (e), and (f) of the corresponding SEM images of (a), (b), and (c), respectively, plotted against the normalized radial distance. Table 1. Photovoltaic Characteristics of Dye-Sensitized Solar Cells Containing io-TiO2 Electrodes in Which the Ordering of the Pores Was Controlled samples (vol % of larger particles)
adsorbed dye [μmol/cm2]
Jsc [mA/cm2]
Voc [V]
fill factor
η [%]
0 10 20
0.025 0.024 0.026
4.4 4.0 4.1
0.77 0.80 0.79
0.62 0.63 0.62
2.1 2.0 2.0
inversely proportional to the electron lifetime.29,30 Here, the EIS spectrum was measured for films with various thicknesses under open-circuit conditions and under the illumination. Figure 4 shows the imaginary part of the impedance analysis over the frequency range, and fmax was maximized at a thickness of 14.5 μm. In other words, the electron lifetime was maximal at that thickness, in accordance with the trend in the Jsc. The Voc gradually decreased as the electrode thickness increased, as shown in Figure 3. This trend resulted from increased charge recombination, which scales with the thickness of the TiO2 electrode. In the case of FF (not shown), it was also decreased with the thickness, but the dependence was relatively low. The combined variations in Jsc, Voc, and FF yielded an efficiency that increased to a maximum of 3.0% at a thickness of 14.5 μm and decreased thereafter. We compared our efficiency to that of previous result containing conventional nanocrystalline TiO2 electrodes (P-25) with the similar thickness.26 The result showed that Jsc of 14 mA/cm2 and the efficiency of 7.2% for these DSSCs. This (29) van de Lagemaat, J.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (30) Qian, J. F.; Liu, P.; Xiao, Y.; Jiang, Y.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Adv. Mater. 2009, 21, 3663.
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Figure 3. Dependence of the photovoltaic parameters;short-circuit currents (Jsc), twice the efficiency (2η), and the open-circuit voltages (Voc);on the io-TiO2 electrode thickness in DSSCs.
efficiency was 2-fold higher than the efficiency of io-TiO2. The low efficiency was mainly attributed to a lower Jsc by 50%, and this low Jsc was due to the lower specific area of io-TiO2 resulting from the introduction of macropores. Meanwhile, considering structural conversion of io-TiO2 structure to nonmacroporous film, the io-TiO2 electrode showed a maximum efficiency at an effective thickness of 3.6 μm (15 μm 0.24, where 0.24 indicates the filling fraction of TiO2 in the inverse opal structure), which was 4-5 times lower than the optimum conventional nanocrystalline TiO2 electrode thickness of 15-20 μm. Early saturation of the efficiency may have been due to the scattering io-TiO2 electrode structure. The high light absorbance (about 5% average transmittance) of the io-TiO2 electrode was DOI: 10.1021/la104512c
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Figure 4. EIS responses of DSSCs with the designated thickness of io-TiO2 electrodes under open-circuit conditions.
replacement of the liquid electrolyte with a polymer gel electrolyte solution decreased the efficiencies of nc-TiO2- and inverse opal TiO2-based DSSCs by 72% and 27%, respectively. Although the decrease in efficiency was attributed to the decrease in ionic conductivity and transport by the addition of PEO, the nc-TiO2 electrode showed a larger efficiency drop upon applying the polymer gel electrolyte solution. This can be explained by the fact that the relatively large radius of PEO molecules (∼63.0 nm for Mw 1 000 000 PEO in a good solvent), which was comparable to the size of the mesopores in nc-TiO2 electrodes, limited the infiltration of PEO into the nanocrystalline TiO2 mesopores, resulting in a decrease in efficiency due to less contact between the electrolyte and the dye on TiO2.18 Instead, the larger pores in the io-TiO2 electrodes facilitated infiltration of this polymeric solution, resulting in a smaller decrease in efficiency.
measured over the visible range, even for electrodes 4.5 μm thick. The high absorbance even for thin film electrodes indicated strong scattering characteristics of the io-TiO2 opal film. Therefore, we expect that the depth to which light cannot penetrate may increase in proportion to the thickness.31,32 The increased fraction of unexposed electrode surface may yield additional charge recombination centers that resulted in loss of photocurrent density and concurrently decrease of efficiency. Finally, we applied a PEO-based polymeric electrolyte solution in place of a liquid electrolyte. Poly(ethylene oxide) has been widely studied as a polymer electrolytes because of good chemical stability and solubility of salts. In particular, PEO has been used as a polymer solvent for solid-state electrolytes or applied to gelate liquid electrolyte for polymer gel electrolyte.33 Here, we compared our results to the properties of conventional nanocrystalline TiO2 electrodes. Table 2 shows photovoltaic parameters for DSSCs containing conventional nanocrystalline TiO2 (Dyesol, 5 μm thickness) and io-TiO2 electrodes (10 μm thickness) with a liquid or polymer electrolyte solution. The results showed that
Conclusions We have provided a facile route to the fabrication of colloidal crystal templates and their inverted TiO2 electrodes. Monodisperse colloids were dispersed in a volatile medium and deposited onto a substrate. A colloidal crystal template was obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Using this facile fabrication method, we investigated the effects of pore ordering and thickness of io-TiO2 electrodes on the photovoltaic parameters and efficiency. The ordering as well as the connectivity of the macropores, which are key issues for the self-assembly processes, were not critical to the photovoltaic performance. The thickness of the electrodes yielded a maximum efficiency of 3.0% at 14.5 μm due to the compensating effects of increased dye loading and simultaneous increase in charge recombination. With respect to the thickness of the TiO2 layer, the efficiency saturated early, at low thicknesses, compared to the saturation in conventional TiO2 electrodes due to their highly scattering characteristics. Moreover, we investigated DSCC applications that used a polymeric electrolyte solution at the io-TiO2 electrodes. Replacing the liquid electrolyte solution with the polymeric electrolyte solution decreased the efficiency in io-TiO2 DSCCs to a lesser degree than in conventional mesoporous electrodes. Despite the facile fabrication of TiO2 electrodes and the potential advantages of io-TiO2, the efficiency io-TiO2 remained low compared to the efficiency of conventional nanocrystalline TiO2 electrodes. Thus, further investigations, for example posttreatment or introduction of a blocking layer beneath the io-TiO2 layer, must be conducted to improve the efficiency.
(31) Ni, M.; Leung, M. K. H.; Leung, D. Y. C. Can. J. Chem. Eng. 2008, 86, 35. (32) Ito, S.; Zakeeruddin, S. M.; Comte, P.; Liska, P.; Kuang, D. B.; Gratzel, M. Nature Photon. 2008, 2, 693. (33) Shi, Y.; Zhan, C.; Wang, L.; Ma, B.; Gao, R.; Zhu, Y.; Qui, Y. Phys. Chem. Chem. Phys. 2009, 11, 4230.
Acknowledgment. This work was supported by the National Research Foundation of Korea (KRF-2008-313-D00295, NRF2010-0011024). The Korea Basic Science Institute is also acknowledged for the measurement of scanning electron microscope.
Table 2. Photovoltaic Properties of DSSCs Containing Conventional Nanocrystalline TiO2 or io-TiO2 Electrodes with Liquid and Polymeric Electrolytes electrode
electrolyte Voc [V] Jsc [mA/cm2]
nanocrystalline TiO2 liquid polymeric liquid io-TiO2 polymeric
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0.73 0.74 0.75 0.74
9.1 2.6 6.0 4.5
FF
η [%]
0.64 0.62 0.62 0.62
4.3 1.2 2.8 2.1
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