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Ordered Crystalline TiO2 Nanotube Arrays on Transparent FTO Glass for Efficient Dye-Sensitized Solar Cells Bing-Xin Lei, Jin-Yun Liao, Ran Zhang, Jing Wang, Cheng-Yong Su, and Dai-Bin Kuang* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, People’s Republic of China ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: August 2, 2010
The fabrication of highly ordered one-dimensional TiO2 nanotube arrays on transparent conductive glass is of key interest for constructing dye-sensitized solar cells (DSSCs) from the front-side illumination mode. We report the formation of large-scale free-standing TiO2 nanotube arrays via sonication of a TiO2 nanotube on Ti foil prepared by an anodization process, which can be further transferred to the fluorine-doped tin oxide conductive glass via a drop of TiO2 sol containing Ti(OBu)4 and polyethylene glycol. The photovoltaic performance of DSSCs based on 20.8 µm length TiO2 nanotube arrays on FTO glass reached 8.07%, which is higher than that of a TiO2 nanoparticle electrode (7.58%) because of the reduced electron combination and efficient light-harvesting efficiency for the former. It is also observed that the power conversion efficiency of DSSCs measured from the front-side illumination mode (8.07%) is higher than that of the back-side illumination mode (7.29%) owing to the light absorption by the iodine electrolyte and light reflection by the Pt counter electrode for the latter. Introduction Titanium dioxide (TiO2) has been currently attracting widespread academic and industrial attention owing to its excellent properties and important applications in photocatalysis,1 sensors,2 water splitting,3 and dye-sensitized solar cells (DSSCs).4 A DSSC with a high energy conversion efficiency and low production cost has been considered as a promising competitor to the well-developed, but relatively expensive, silicon solar cell.4,5 A typical DSSC is assembled with a nanocrystalline TiO2 film covered by a monolayer of dye molecules (N719) on FTO glass, redox electrolyte (I-/I3-), and counter electrode (Pt/FTO glass).4 A more than 11% photovoltaic performance was achieved for DSSCs based on random three-dimensional networks of TiO2 nanoparticles with a size of 10-30 nm.5 The randomly interconnected pore structure of the TiO2 nanoparticle film elicits a short electron diffusion length (10-30 µm), measured by intensity-modulated photocurrent and photovoltage spectroscopies (IMPS and IMVS, respectively), which increases the recombination probability of the electrons with redox species and an oxidized dye.6 This result limits the effort of increasing the absorption of low-energy photons (red and near-infrared wavelengths) through thickening of the TiO2 film, which does not improve the overall energy conversion efficiency.7,8 Hence, photoanode materials with hierarchical or/and ordered architectures exhibiting efficient light scattering, longer electron diffusion lengths, and shorter electron-transport time constants will be of significant importance for the improvement of power conversion efficiency of DSSCs.7,9-14 Very recently, hierarchical ZnO hollow spheres or SnO2 octahedra consisting of nanoparticles have been first prepared via a facile sonochemical method and exhibited highly efficient photovoltaic performance in DSSCs.15,16 * To whom correspondence should be addressed. Phone: +86 20 84113015. Fax: +86-20-84113015. E-mail address: kuangdb@ mail.sysu.edu.cn.
Compared with nanoparticles, highly ordered one-dimensional (1D) nanoarrays (nanowires,12,17 nanorods,11,18,19 and nanotubes14,20-24) were found to be superior in chemical and photoelectrochemical performance due to their one-dimensional channel for carrier transportation, in which the efficiency of charge collection is improved owing to a more rapid electron transport and slower charge recombination.25 Considerable efforts have been devoted to the development of TiO2 nanotube (TNT)-based DSSCs. The ionic liquid electrolyte based flexible DSSC with a power conversion efficiency of ∼3.6% has been first reported by using TiO2 nanotubes/Ti photoanodes.20 Recently, the photovoltaic performance of DSSCs based on TNT/Ti photoelectrodes and a volatile liquid electrolyte has been achieved, ∼7%. However, such TNT/Ti-based DSSCs require back-side illumination, which limits the enhancement of the photovoltaic performance because the counter electrode partially reflects light and the electrolyte absorbs photos in the near UV region.21,26 Hence, the fabrication of TNTs on a transparent conductive substrate for front-side illuminated DSSCs is desired and will be expected to improve photovoltaic performance. Several methods have been reported for the fabrication of TNTs on the transparent conductive FTO glass and their applications in DSSCs. Transparent TNT/FTO glass was prepared via an anodic oxidation of titanium thin film that was sputtered onto FTO conductive glass,7,27 which involves the sputtering of Ti-films onto FTO glass, thus leading to a high fabrication cost. Park and co-workers21 transferred and adhered TNT arrays onto the FTO glass using Ti isopropoxide solution as a paste; however, the TNT/FTO has a very small area (0.03-0.15 cm2) owing to the structural destruction during onestep annealing. Nanoparticle paste was also used to adhere the free-standing TNT membrane onto FTO glass; however, the micrometer-thick nanoparticle layer at the TNT-FTO interface limits the role of the TNT in the solar cell performance.7,14,28 Hence, a simple process of fabricating large-scale TNT arrays on FTO glass is very desirable.
10.1021/jp105780v 2010 American Chemical Society Published on Web 08/18/2010
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SCHEME 1: Schematic Illustration of the Procedure for Fabricating Crystallized TNT/FTO Glass Films
In this article, we report the fabrication of large-scale noncurling crystallized TNT/FTO films and their application in DSSCs. The preparation of TNT/FTO glass is composed of four steps, shown in Scheme 1: (i) fabrication of TNT arrays on Ti foil by anodization, (ii) detachment of the amorphous free-standing TNT array membrane from Ti foil via ultrasonic treatment, (iii) transfer and adhesion of the free-standing TNT array membrane onto FTO glass, and (iv) formation of crystallized TNT/FTO glass via a two-step annealing treatment of the amorphous TNT/film. The prepared TNT/FTO glass was used as a photoanode in DSSCs, and a highly efficient power conversion efficiency (η) of 8.07% was obtained by using a 20.8 µm length TiO2 nanotube, with an open-circuit voltage (Voc) of 0.814 V, a short-circuit current (Jsc) of 15.46 mA cm-2, and a fill factor (FF) of 64.1%. Experimental Section Highly ordered TiO2 nanotube arrays were prepared by anodization of Ti foils (99.7%, 0.25 µm, Aldrich) in a twoelectrode electrochemical cell with a platinum foil as the counter electrode using a high-voltage potentiostat at room temperature. The Ti foils were first physically polished, then sonicated separately in acetone, isopropanol, and ethanol. Finally, they were rinsed with deionized (DI) water and dried in air at 60 °C. The nanotubular layer was obtained by Ti metal anodization in the mixture solution of ethylene glycol and polyethylene glycol with a ratio of 4:1 (v/v) containing 0.25 wt % NH4F and 0.75 wt % DI water. The Ti foil was anodized at 50 V for 1.5-9 h to prepare TiO2 nanotube arrays with various lengths. The anodized samples were rinsed with DI water. Ultrasonic treatment of the anodized samples in DI water was carried out until the TNT membranes were detached from the Ti substrate. The TNT membranes were adhered onto FTO glass with a drop of TiO2 sol containing Ti(OBu)4 and polyethylene glycol. To crystallize amorphous TiO2 into the anatase phase, the TNT/ FTO films were subjected to annealing in a two-step process: annealed at 200 °C for 1 h using a heating rate of 1 °C s-1 to remove organic solvents and further heated to 500 °C and kept for another 3 h to crystallize the amorphous TiO2 nanotube to the anatase phase. The TNT/FTO films were soaked in 0.04 M TiCl4 aqueous solution for 30 min at 70 °C, which improves the photocurrent and photovoltaic performances. The TiCl4-treated TNT/FTO films were rinsed with water and ethanol and then sintered at 500 °C for 30 min. After cooling to ∼80 °C, the TNT/FTO films were sensitized by immersing them into a 5 × 10-4 M solution of the N719 dye (Solaronix SA, Switzerland) in acetonitrile/tert-butanol (volume ratio ) 1:1) for 16 h. After-
Figure 1. Digital images of TiO2 nanotube arrays at different fabricating processes: (a) TNT membrane grown on a Ti substrate after anodization at 50 V for 4 h, (b) TNT membrane peeled from the Ti substrate via sonication treatment, (c) TNT membrane transferred to the FTO glass substrate using a drop of TiO2 sol, (d) crystallized TNT/ FTO film after a two-step annealing treatment, (e) sensitized TNT/ FTO film with N719 dye solution, and (f) a full solar cell based on the TNT/FTO glass electrode.
ward, the TNT/FTO films were rinsed with acetonitrile in order to remove physisorbed dye. To evaluate their photovoltaic performance, the dye-sensitized TNT/FTO films were sandwiched together with a Pt-coated fluorine-doped glass counter electrode. Platinized counter electrodes were fabricated by thermally depositing H2PtCl6 onto the FTO glass. The electrolyte, 0.03 M I2, 0.6 M 1-methyl-3-propylimidazolium iodide (PMII), 0.10 M guanidinium thiocyanate, and 0.5 M tertbutylpyridine in acetonitrile and valeronitrile (85:15), was introduced into the space between the sandwiched full cells. The morphology and structure of the TNT/FTO films were characterized using field emission scanning electron microscopy (FE-SEM, JSM-6330F) and power X-ray diffractometry (XRD, Bruker D8 Advance), respectively. The current-voltage characteristics were measured using a Keithley 2400 source meter under simulated AM 1.5G illumination (100 mW cm-2) provided by a solar simulator (69920, 1 kW Xe lamp with an optical filter, Oriel) to determine the open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and conversion efficiency (η). The incident light intensity (100 mW cm-2) of the light source was calibrated with a NREL-calibrated Si solar cell. The amounts of dyes adsorbed on the TiO2 films were obtained by measuring the UV/vis absorption spectra of solutions containing dyes dissolved from the TiO2 film with 0.1 M NaOH (3 mL). Results and Discussion Figure 1 is the photograph of TiO2 nanotube arrays at the different preparation process, which was anodized at 50 V for
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Figure 2. XRD patterns of as-prepared TNT/Ti arrays (a) and annealed TNT/FTO arrays (b).
4 h. After sonication treatment (30 s), the TiO2 nanotube membrane can be detached (Figure 1b) from the as-prepared Ti foil (Figure 1a). The partially crystalline free-standing TiO2 nanotube membrane can then be adhered onto the FTO glass via a drop of TiO2 sol containing Ti(OBu)4 and polyethylene glycol (Figure 1c) and then transferred to the semitransparent anatase TiO2 nanotube/FTO glass via a two-step annealing (Figure 1d), including the removal of organic solvent at low temperatures and crystallization at higher temperatures (500 °C). Finally, the noncurling large-scale (>1 × 2.5 cm2) anatase TNT/ FTO (Figure 1d) have been successfully obtained, which is much better than the previous report,21 where the TNT/FTO has a very small area prepared via a single-step annealing. The TNT/FTO glass can be sensitized by N719 dye solution (Figure 1e) and further assembled to a full solar cell (Figure 1f) for the photovoltaic application. As-prepared TiO2 nanotube arrays are commonly amorphous by Ti anodization. The phase structure of the as-anodized and annealed TNT/FTO samples was characterized by XRD measurements and are shown in Figure 2. It shows that the as-anodized sample prepared from the mixture solution of ethylene glycol and polyethylene glycol with a ratio of 4:1 (v/ v) containing 0.25 wt % NH4F and 0.75 wt % DI water at 50 V for 4 h at room temperature is partially crystallized because there is an anatase (101) peak (Figure 2, curve a). After calcination, the XRD patterns (Figure 2, curve b) show that all peaks in the pattern can be indexed to the TiO2 anatase phase (JCPDS file no. 21-1272), which illuminates that the partially crystallized TNT/FTO changes into the anatase TiO2 phase via the two-step annealing treatment. Figure 3 shows the FE-SEM images of a sample prepared after 4 h of anodization at 50 V. From the cross-sectional and top-view images of as-anodized TNT/Ti foil (Figure 3a,b), it clearly shows that the TiO2 nanotube with a length of 20.8 µm, a tube diameter of 99 nm, and a tube wall of 27 nm was obtained. After the transfer to the FTO glass and annealing, the TNT/FTO arrays remain compact and no destructive changes are observed though the partially crystalline structure changed to anatase. Moreover, the length, tube diameter, and tube wall remain unchanged via the calcination (Figure 3c,d). Further, the effects of the anodization duration on the physical features of TNT arrays are investigated. The length of TNT arrays can be increased from 11.9 to 37.5 µm by simply increasing the anodization duration from 1.5 to 9 h (data not shown). Photo images of the TNT/FTO films prepared via anodization for 1.5, 4, and 9 h and a two-step heat treatment are given in Figure 4a. It is worth noting that the flat TNT/FTO films have no collapse and curl via the present two-step annealing treatment. The semitransparent TNT/FTO films for the 11.9 and 20.8 µm length
Figure 3. FE-SEM images of as-prepared TNT/Ti (a, b) and annealed TNT/FTO (c, d) arrays: (a, c) top views and (b, d) cross-sectional views.
Figure 4. (a) Digital images of TNT/FTO films of different thicknesses prepared via anodization for 1.5, 4, and 9 h and a two-step heat treatment (L, nanotube length). (b) Diffused reflectance spectra of TNT/ FTO films of different thicknesses and FTO glass.
TNT arrays are observed; however, it is the nontransparent TNT/ FTO glass for the 37.5 µm length TNT arrays that is prepared after 9 h of anodization. Figure 4b presents the UV-vis diffuse reflectance spectra of TNT/FTO films of different thicknesses and FTO glass. The reflectance of TNT/FTO films increases with the increase of the TNT length, which renders thicker TNT/ FTO films appropriate for fabricating high-efficiency DSSCs. Figure 5 shows the photocurrent-photovoltage (J-V) properties of the TNT/FTO array-based DSSCs as a function of TNT length measured under AM 1.5G illumination (100 mW cm-2). Detailed photovoltaic performance parameters are presented in Table 1. As shown in Figure 5 and Table 1, the TNT length increases from 11.9 to 37.5 µm with the prolonged anodization time from 1.5 to 9 h. The Jsc initially increases from 13.13 to 15.46 mA cm-2 with the increasing length from 11.9 to 20.8 µm, which can be possibly attributed to the increase in the number of adsorbed dye molecules (from 11.4 × 10-8 to 15.8 × 10-8 mol cm-2) from the increased surface area of the TNT film and, therefore, a higher number of photogenerated electrons.
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Figure 5. Photocurrent-photovoltage characteristics of TNT/FTO array-based DSSCs as a function of tube length under AM 1.5G illumination (100 mW cm-2). The nanotube films were prepared at 50 V for 1.5, 2, 4, 6, and 9 h.
Figure 6. Comparison of photovoltaic performance using the same thickness (20.8 µm) of TNT/FTO film and P25/FTO film under AM 1.5G illumination (100 mW cm-2).
TABLE 1: Photovoltaic Performance of TNT/FTO-Based DSSCs as a Function of Tube Length (L) under AM 1.5G Illumination (100 mW cm-2) L/µm
anodization time/h
adsorbed dye/× 10-8 mol cm-2
Jsc/mA cm-2
Voc/V
η/%
FF/%
11.9 16.4 20.8 28.5 37.5
1.5 2 4 6 9
11.4 12.8 15.8 17.2 23.0
13.13 14.76 15.46 13.23 12.36
0.825 0.815 0.814 0.794 0.784
7.140 7.813 8.070 6.988 6.335
65.6 64.9 64.1 66.5 65.4
Moreover, the long TNT would also improve the light-harvesting efficiency in TiO2 nanotube-based DSSCs, which could also be a result of stronger light-scattering effects.25 However, when the TNT length is further increased, for example, from 20.8 to 37.5 µm, the Jsc reduces from 15.46 to 13.23 mA cm-2, which may be due to the diffusion length of the electron being shorter than the length of the TNT at values above 20.8 µm. The Voc of the DSSCs decreases with increasing length of the TNT due to the augmentation of the surface area providing additional charge-recombination sites and enhancing the dark current.8 Moreover, for long nanotubes, the outer TiO2 do not contribute significantly to the photogeneration of conduction band electrons due to the filtering of light by the dyed TiO2 nanotubes located close to the FTO glass. The sharing of photoinjected conduction band electrons by these nanotubes lowers their quasi-Fermi level and hence the Voc.8 A similar result was also observed in previous TiO2 nanoparticle-based DSSCs.8 The fill factor does not show any obvious changes for the different length TNT/ FTO DSSCs. Hence, the photovoltaic performance shows an initial increase and following decrease with the increasing TiO2 nanotube length. Finally, a highest overall photoconversion efficiency of 8.07% is achieved for a 20.8 µm length TNT sample, with an open-circuit voltage (Voc) of 0.814 V, shortcircuit current density (Jsc) of 15.46 mA cm-2, and fill factor (FF) of 64.1%. Figure 6 shows the J-V curves of DSSCs based on TNT/ FTO and P25/FTO photoelectrodes with the same thickness (20.8 µm) measured under AM 1.5G illumination (100 mW cm-2). The P25/FTO photoanode was prepared via screenprinting of the commercial P25 paste on the FTO glass substrate and also precoating a drop of TiO2 sol containing Ti(OBu)4 and polyethylene glycol. It clearly shows that the photocurrent density of the TNT/FTO-based DSSC (15.46 mA cm-2) is much larger than that of the P25/FTO-based DSSC (13.94 mA cm-2). The photovoltage does not show obvious changes for both photoelectrodes. The fill factor is a little bit lower for the TNT/ FTO solar cell (64.1%) compared with P25/FTO (67.2%). The lower FF is attributed to a larger series (sheet) resistance at the
Figure 7. Photovoltage-decay measurement of a TNT/FTO arraybased DSSC and a P25/FTO-based DSSC.
TNT/substrate interface. As a result, the power conversion efficiency is enhanced from 7.58% to 8.07% by using the TNT/ FTO film replacing the P25/FTO. The TiO2 nanoparticle/FTObased DSSCs normally have significant disorder associated with the individual particles (e.g., defects, size and shape nonuniformities) and the three-dimensional randomly packed particle network.29 Disorder can significantly retard the transport dynamics and lead to the longer transport pathway. However, a TNT has a one-dimensional channel for carrier transportation that promotes electron transport. An important implication of the longer transport pathway is that electrons undergo more trapping and detrapping events and, therefore, spend a longer time in a film before being collected at the electrical contact substrate.30 Hence, a TNT has a faster charge-collection efficiency than a TiO2 nanoparticle. Although the amount of dye (18.9 × 10-8 mol cm-2) adsorbed on P25 is higher than that of the TNT (15.8 × 10-8 mol cm-2) for a similar thickness (20.8 µm), the higher photocurrent density and photovoltaic performance of the TNT/ FTO-based DSSC can be ascribed to higher charge-collection and light-harvesting efficiencies, which is confirmed by the following Voc decay results and was also observed in the previous publications.25 Zhu and co-workers25 have also found that TNT/Ti-based DSSCs have charge-collection and lightharvesting efficiencies 25 and 20% higher, respectively, than the corresponding TiO2 nanoparticle-based DSSCs. The recombination property is further characterized by using open-circuit voltage decay (OCVD) measurements.31 The electron recombination kinetics is investigated by monitoring the transient Voc as a function of time upon switching off the light. After the light is switched off with a shutter under a steady voltage, the Voc decays sharply due to electron recombination, which relates to the electron lifetime. Figure 7 plots the Voc decay as a function of time measured using a TNT/FTO arraybased DSSC and a P25/FTO-based DSSC. It is evident that the DSSC based on TNT/FTO has a significantly slower Voc decay rate than that based on the P25 nanoparticle, suggesting slow
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Figure 9. Photocurrent-photovoltage characteristics of a DSSC based on only TiO2 sol under AM 1.5G illumination (100 mW cm-2). Figure 8. Comparison of the photovoltaic performance of the TNT/ FTO-based DSSC in the front-side and the back-side illumination modes under AM 1.5G illumination (100 mW cm-2).
recombination kinetics and a long electron lifetime in the TNT/ FTO array-based DSSC compared with the P25 nanoparticle anode. The longer electron lifetime indicates that more electrons surviving from the back-reaction contribute to the improvement in photocurrent.32 Figure 8 is the current-voltage curves of DSSCs based on TNT/FTO glass measured from back-side and front-side illuminations mode under AM 1.5G illumination (100 mW cm-2). It clearly shows that the Jsc of the back illumination mode (13.18 mA cm-2) is significantly smaller than that of the front illumination mode (15.46 mA cm-2) because the counter electrode (Pt) and the electrolyte (I-/I3-) attenuate the incident light intensity for the back-side illumination mode.26 The FF of back-side illumination mode (67.5%) is higher than that of the front illumination mode (64.1%) because the resistance of the electrolyte for the back-side illumination mode is smaller than that for the front-side illumination mode.33 These results are in agreement with previous nanoparticle-based DSSCs.33 The opencircuit voltage of both illumination modes shows a similar value of 817 ( 3 mV. Finally, the photovoltaic performance of the TNT/FTO glass-based DSSC measured from the front-side illumination (8.07%) is higher than that of the back-side illumination mode (7.29%) due to the increasing Jsc for the former. Growth of TNTs on a transparent conductive substrate is desirable for the application in DSSCs. Recently, the fabrication of TNTs on FTO glass, such as the anodization of sputtering Ti metal on FTO glass7,22,24,27 or the anodization of Ti metal and then transfer to FTO glass via TiO2 sol,14,21,28 has made some progress. However, the anodization of sputtered Ti metal on FTO glass has a high cost and it is hard to get a long nanotube, which limits the widespread applications.24,27 Recently, titania nanoparticle paste was used to adhere a freestanding TNT membrane to FTO glass;14,21,28 however, the micrometer-thick nanoparticle layer (∼3 µm, η ) 1.7-2.0%) contributes significantly to the power conversion efficiency and limits the role of the nanotubes in determining the solar cell performance. Park et al used Ti isoprosoxide solution to adhere amorphous free-standing TNT arrays to FTO glass, which results in very small fragments (0.03-0.15 cm-2) due to the curing and collapsing of the TNT arrays during the fixation and sintering.21 Here, we develop a sonication to detach the TNT from the Ti foil and then use a drop of TiO2 sol containing Ti(OBu)4 and polyethylene glycol to adhere the free-standing TNT to FTO glass; after a two-step and mild annealing treatment, large-scale noncurling crystalline TNT/FTO glass has been successfully obtained. To clarify the effect of the TiO2 sol layer on the photovoltaic performance, a comparison experiment was performed. The DSSC was assembled using
only TiO2 sol as a photoelectrode (without a TiO2 nanotube), and the J-V curve is shown in Figure 9. The Jsc, Voc, FF, and η are 0.18 mA cm-2, 0.739 V, 45.1%, and 0.06%, respectively, which are much lower than those of the TNT/FTO-based DSSC. Hence, the present TiO2 sol does not show an obvious influence on the power conversion efficiency of the TNT/FTO glass-based DSSC. The present high efficiency DSSC is mainly attributed to the ordered TNT/FTO glass photoelectrode for the front-side illumination. Conclusions In summary, we demonstrate the fabrication of large-scale, noncurling anatase crystalline TiO2 nanotube arrays on a transparent conductive substrate (FTO glass) via ultrasonic treatment of anodized TNT/Ti foil, adhesion of free-standing amorphous TiO2 nanoarray membrane to the FTO glass using a drop of a mixture solution of Ti(OBu)4 and polyethylene glycol, and a two-step annealing process to crystallize the asprepared TNT/FTO glass. The influences of anodization time on the TiO2 nanotube length and the photovoltaic performance have been investigated. The highest power conversion efficiency of 8.07% is obtained based on a 20.8 µm length TNT/FTO glass photoelectrode that was prepared via 4 h of anodization at 50 V. For the TNT/FTO glass-based DSSC, the photovoltaic performance of the front-side illumination mode (8.07%) is found to be higher than that of the back-side illumination mode (7.29%) due to the reduction of incident light intensity for the latter. Moreover, the photovoltaic performance of the DSSC based on TNT/FTO glass exhibits an increase from 7.57% to 8.07% compared to TiO2 nanoparticle/FTO glass due to better mass transport as enhanced light harvesting and electron collection efficiencies for the former. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20873183 and U0934003); the Natural Science Foundation of Guangdong Province (8151027501000030); the Open Foundation of the Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education; Shenzhen Key Laboratory of Special Functional Materials, Shenzhen University, Shenzhen; and the Foundation of Sun Yat-Sen University. References and Notes (1) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2811. (2) Gouma, P. I.; Mills, M. J.; Sandhage, K. H. J. Am. Ceram. Soc. 2000, 83, 1007. (3) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (4) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (5) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (6) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949.
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