Enhancement of Photovoltaic Performance of Dye-Sensitized Solar

Feb 13, 2013 - Chemistry, Huaqiao University, Quanzhou 362021, China. ABSTRACT: Highly crystalline SnO2 nanorods with lengths of about 200 nm and ...
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Enhancement of Photovoltaic Performance of Dye-Sensitized Solar Cells by Modifying Tin Oxide Nanorods with Titanium Oxide Layer Guanglu Shang, Jihuai Wu,* Shen Tang, Lu Liu, and Xiaopei Zhang Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China ABSTRACT: Highly crystalline SnO2 nanorods with lengths of about 200 nm and diameters of 40−80 nm are synthesized by a hydrothermal method. Because of an efficient electron transport channel along the one-dimensional structure, a dye-sensitized solar cell (DSSC) based on the SnO2 nanorods shows a fast electron transport and a long electron lifetime, resulting in higher power conversion efficiency than the DSSC SnO2-based nanoparticles. To suppress charge recombination at SnO2 nanorods and electrolyte/ dye interfaces, we modify SnO2 nanorod electrode with TiO2 compact layer and TiCl4 post-treatment, and the DSSC exhibits a power conversion efficiency of 4.67%.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted a great deal of attention as a promising candidate for future green energy due to their facile, low-cost, and environmentally friendly fabrication process.1,2 Since Gratzel reported the efficient TiO2-based DSSCs, many attempts have been made to enhance the photon−electron conversion efficiency of this low-cost solar cell.3−7 Recently, a new record power conversion efficiency of more than 12% by using a porphyrin-sensitized nanocrystalline TiO2 photoanode together with cobalt (II/III)-based redox electrolyte has been reported.3 Nevertheless, the present benchmark power conversion efficiency obtained by researchers is still too low in comparison to the theoretical value (32%) predicted for a single junction cell.8 As the key part of a DSSC, the wide-band gap semiconductor metal oxide plays two important roles as a carrier for dye molecules and a transporter for the injected electrons. The photovoltaic performance of a DSSC was found to strongly depend on the photoanode material since it influences both the photocurrent and photovoltage.9 As a wide band gap metal oxide, TiO2 is found to exhibit the highest power conversion efficiency owing to its specific optoelectronic properties.10−13 Besides TiO2, other semiconductor metal oxides, such as ZnO, Nb2O5, and SnO2, have also been investigated as potential alternatives to TiO2.14−17 Among them, SnO2 is an excellent metal oxide semiconductor with high electron mobility (from ∼100 to 200 cm2·V−1·s−1), indicating a faster transport of photogenerated electrons compared to TiO2 for DSSC applications.18 In addition, when TiO2 is used in DSSC, the dye attached on a TiO2 surface degrades very slowly, owing to the generation of an electron−hole pair by UV light absorption, thus affecting the long-term stability of DSSC.19 However, owing to the wider band gap of SnO2 (3.8 eV) compared to that of TiO2 (3.2 eV),2 SnO2 is less likely to generate holes in the valence band through direct photon absorption, and the © 2013 American Chemical Society

DSSCs based on SnO2 are more robust under UV illumination than those made from TiO2.20 In spite of the above advantages, SnO2-based DSSCs were developed with less success due to at least two weak points: (1) a 300 mV positive shift of the conduction band edge of SnO2 with respect to that of nanocrystalline TiO2, leading to a faster interfacial electron recombination;21 (2) lower isoelectric point of SnO2 compared with anatase TiO2 leads to less adsorption of dye.22 Both of them result in lower photocurrent density and photovoltage, hence lower photovoltaic performance for SnO2 based DSSCs. To solve these problems, several attempts have been tested to enhance the power conversion efficiency of SnO2 based DSSCs.23,24 Among them, coating on a SnO2 photoanode with a very thin insulating oxide layer, such as MgO, Al2O3, or SrTiO3, was found to be an effective method to overcome the high charge recombination in a SnO2 photoanode.25−27 SnO2 nanoparticle film as a photoanode material for DSSC has aroused intensive research because of its larger surface areas to adsorb more dye molecules; however, more defects in the nanoparticles limit its extensive application.28 Recently, many reports have shown that electron transport in the nanoparticle film is via a random route with multiple trapping and detrapping events.29 This hopping mechanism seriously limits the electron diffusion coefficient and the electron collection time. In this regard, one-dimensional (1-D) nanorods with less defects show an advantage in offering direct electron transport pathways, and therefore giving longer electron diffusion time than that in the nanoparticle films.17 In other words, 1-D nanorods possess good electron transport function, which compensates for its insufficiency in smaller specific surface area. Received: September 16, 2012 Revised: February 11, 2013 Published: February 13, 2013 4345

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2.5. Measurements and Characterization. XRD measurements were carried out using powder X-ray diffraction (XRD PANALYTICAL, X’Pert PRO, Cu Kα, 40 mA, λ = 0.15406 nm). The surface morphology of samples was characterized by scanning electron microscopy (SEM, Hitachi S-4800). Dye uptake was estimated by treating the dyeadsorbed electrodes in 0.05 M NaOH solution and measuring the absorption spectrum using a UV−vis spectrophotometer (Shimadzu, UV2450). The specific surface area was calculated using a Hiden IGA100B surface area analyzer. The roughness factors were calculated as a ratio of the total inner pore surface area to active film area.30 The current density−voltage (J−V) measurements and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI660D, Shanghai Chenhua Device Company, China) under irradiation with a simulated solar light with intensity of 100 mW·cm−2 from a xenon arc lamp (XQ-500W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere.

Herein, we report the synthesis of SnO2 nanorods by a simple hydrothermal method and their use in DSSC. We compared the performance of the DSSCs based on SnO2 nanorods and nanoparticles, and observed higher power conversion efficiency was correlated with electronic properties of the nanorod structure. To enhance the performance of the DSSCs, we also modified SnO2 nanorod electrode with TiO2 compact layer and TiCl4 post-treatment.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Tin oxide, sodium stannate trihydrate, sodium hydroxide, absolute ethanol, titanium tetrachloride, hexadecyl trimethyl ammonium bromide sodium iodide, tetrabutyl ammonium iodide, iodine, ethyl cellulose, terpineol, and acetonitrile were all purchased from Sinopharm Chemical Reagent Co., Ltd., China. Sensitized dye N719 (RuL2(NCS)2, L = 4, 4′-dicarboxylate-2, 2′-bipyridine) was purchased from Solaronix SA (Switzerland). All reagents were used without further treatment. Conducting glass plates (FTO glass, fluorine-doped tin oxide overlayer, sheet resistance 8 Ω·cm−2, purchased from Hartford Glass Co., USA) were used as substrates for precipitating SnO2 films. 2.2. Synthesis of SnO2 Nanorods. In a typical procedure, 20 mL of absolute ethanol was slowly added into 20 mL of 0.188 M sodium stannate trihydrate/0.35 M sodium hydroxide aqueous solution at ambient room temperature under stirring conditions. To ensure complete reaction, the mixed solution was stirred for 30 min to obtain a white suspension. The suspension was transferred into an autoclave and reacted hydrothermally at 200 °C for 48 h. After the autoclave was naturally cooled to room temperature, the products were centrifuged and rinsed thoroughly with distilled water several times, dried in air at 60 °C for 24 h, and calcined at 500 °C for 2 h. Finally, SnO2 nanorods were obtained. 2.3. Preparation of SnO2 Paste. To prepare SnO2 nanorod paste, 0.2 g of ethylene cellulose was added to 5 g of turpentine oil. Then SnO2 nanorods were added to 10 mL of absolute ethanol. Finally, the above two solutions were mixed by stirring for 2 days to yield the slurry. For a comparison, the preparation technique for the SnO2 nanoparticle paste was similar to the SnO2 nanorod paste except the paste was made using commercially available SnO2 nanoparticles with about 50 nm in size. 2.4. Fabrication of DSSCs. A TiO2 compact layer was prepared by immersing an FTO glass into a 0.2 M TiCl4 aqueous solution at room temperature overnight, followed by sintering at 500 °C for 30 min. After cooling to room temperature, SnO2 paste was spread uniformly onto FTO and the TiCl4 pretreated FTO substrates, respectively. TiCl4 posttreatment was performed by dipping the SnO2 nanorod electrodes into 0.2 M TiCl4 aqueous solution for 40 min at 80 °C until the TiCl4 was hydrolyzed, then washing the electrodes with distilled water to remove residual TiCl4, and finally sintering at 500 °C in air for 30 min. The active area of the photoelectrodes was about 0.16 cm−2 and the thickness of the films was about 10 μm for all DSSCs. Afterward, the asprepared photoelectrodes were immerged in 2.5 × 10−4 M dye N719 absolute ethanol solution for 24 h to adsorb the dye adequately. And then the photoelectrodes were sandwiched together with platinized FTO counter electrodes. The redox electrolyte was injected into the aperture between the dyesensitized SnO2 film electrode and counter electrode.23 Thus, a SnO2-based DSSC was assembled.

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RESULTS AND DISCUSSION 3.1. Morphology and Phase. Figure 1a shows the typical SEM image of SnO2 nanorods. It is observed that the samples

Figure 1. SEM images of SnO2 nanorods at different magnifications (a, b), SnO2 nanoparticles (c), top view SEM images of the SnO2 nanorod photoanode without (d) and with (e) TiCl4 treatment, and crosssectional SEM image of the SnO2 nanorod photoanode with TiCl4 treatment (f).

contain numerous well-defined nanorods. The high-magnification image in Figure 1b shows that the diameters of SnO2 nanorods are around 40−80 nm with lengths of about 200 nm. Figure 1c shows that the diameters of SnO2 nanoparticles are about 50 nm. Figure 1d shows the top view SEM image of the SnO2 nanorod photoanode; it can be found that the morphology of SnO2 nanorods does not obviously change after preparation of the slurry, coating on the FTO glass, and sintering of the paste. Coated with TiO2, the SnO2 nanorod film still maintains the nanorod structure, but has a much rougher surface due to surface coating of a layer of TiO2, as shown by an SEM image in Figure 1e. Figure 1f shows the cross-section SEM image of the SnO2 nanorod photoanode with a thickness of about 10 μm. Figure 2 shows the powder XRD patterns from the asprepared SnO2 nanorods and commercially available SnO2 nanoparticles annealed in air at 500 °C. The diffraction peaks 4346

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Table 1. Characteristics of the Films Based on SnO2 Nanorods and Nanoparticles photoanode films SnO2 nanoparticles SnO2 nanorods

BET specific surface area (m2·g−1)

roughness factor

amount of dye loading ( ×10−7 mol·cm−2)

15.1

412

3.23

12.2

365

3.04

observed higher JSC for photoanode fabricated with SnO2 nanorods could be justified. The other difference between SnO2 nanorods and nanoparticles is the increase in FF when SnO2 nanorods were used as an electron transport medium in the photoanode. The EIS spectra of the DSSCs using SnO2 nanorods and nanoparticles were measured and shown in Figure 4. In Figure 4a, two semicircles can be recognized and fitted according to an equivalent circuit model (the inset in Figure 4a); the fitted data are put together in Table 2. The first semicircle (R1) in the high-frequency region (>1000 Hz) represents the charge transfer at the counter electrode−electrolyte interface, while the second semicircle (R2) in the midfrequency (10−100 Hz) region reflects the charge transfer at the photoelectrode− electrolyte interface.31 It can be seen that the R1 values are all similar, about 0.8 Ω for both SnO2 nanorods and SnO2 nanoparticles. As given in Table 2, the R2 value of the SnO2 nanorod film is a bit smaller than that of the SnO2 nanoparticle film. Here, R2 is commonly thought to be mainly determined by the charge recombination resistance, with partial contribution from transport resistance. The observed lower R2 of SnO2 nanorods can be attributed to the 1-D nature of the SnO2 nanorod, as it provides a direct conduction path for electrons injected from the sensitizing dye. Hence, it is expected to reduce the electron diffusion time from the point of injection to the back contact in SnO2 nanorods. However, electron transport in SnO2 nanoparticles proceeds in a random pathway by a trap-limited diffusion process. Because of the trap limited diffusion process in SnO2 nanoparticles, the electrons must pass through a series of interparticle hopping step to reach the collection electrode. The electrons lifetime (Γeff) could be calculated from the midfrequency ( f max) as 1 Γeff = 2Πfmax (3)

Figure 2. XRD patterns of SnO2 nanorods and nanoparticles annealed at 500 °C for 2 h.

of both SnO2 nanorods and nanoparticles are quite similar to those of bulk SnO2, which can be indexed as the tetragonal rutile phase of SnO2 (JCPDS card no. 41-1445). No impurity peaks were observed in both SnO2 nanorods and nanoparticles, indicating the high purity of the final products. 3.2. Comparison between SnO2 Nanorods and Nanoparticles. Tto compare the performance of DSSCs fabricated with SnO2 nanorods and nanoparticles, respectively, the J−V behavior was measured under a simulated solar light irradiation with an intensity of 100 mW·cm−2. As shown in Figure 3,

Figure 3. J−V curves of the DSSCs based on SnO2 nanorods and nanoparticles.

DSSC fabricated with SnO2 nanoparticles shows an opencircuit voltage (VOC) of 0.525 V, a short-circuit current density (JSC) of 4.02 mA·cm−2, and a photovoltaic performance fill factor (FF) of 46.6%, resulting in a light-to-electric energy conversion efficiency (η) of 0.986%. While the DSSC based on SnO2 nanorods has VOC, JSC, FF, and η of 0.530 V, 4.58 mA·cm−2, 49.8%, and 1.21%, respectively, the increase in JSC and FF of the DSSC based on SnO2 nanorods compared with the performance of the DSSC based on SnO2 nanoparticles is clearly noticeable. To investigate the reason for observed higher JSC for SnO2 nanorods, we compared the roughness factor and dye adsorption amount of the two films, and found that SnO2 nanorods show lower internal surface area and inherent surface roughness factor, leading to inferior dye loading (Table 1). If JSC is correlated to the adsorbed dye amount, higher JSC is expected for the DSSC fabricated with SnO2 nanoparticles, but the JSC results of SnO2 nanoparticles and nanorods indicate different results. It can be assumed that the 1-D nature of the SnO2 nanorods facilitates the charge transport and hence

where f max is the peak frequency in the midfrequency range.26 As shown in Figure 4, the f max value for the SnO2 nanorods is 12.1 Hz, much smaller than the values for SnO2 nanoparticles (62.3 Hz), indicating a longer electron lifetime in SnO2 nanorod film. An increase in the Γeff value reveals the effective retardation of the charge recombination reaction between photoinjected electrons with I3− during the electron transport through the film of SnO2 nanorods, as compared to SnO2 nanoparticle film. The results suggest that SnO2 nanorods favor the electron transport through a longer distance with less diffusive hindrance compared to the other film. 3.3. Modification of Photoanode. Figure 5 shows the J− V characteristics of the DSSCs based on four kinds of films (DSSC-1, bare SnO2 nanorods; DSSC-2, TiO2 compact layer + SnO2 nanorods; DSSC-3, SnO2 nanorods + TiCl4 posttreatment; DSSC-4, TiO2 compact layer + SnO2 nanorods + TiCl4 post-treatment), and the detailed photovoltaic parame4347

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Figure 4. Fitted results of Nyquist plots from EIS of the DSSCs based on SnO2 nanorods and nanoparticles (a) and Bode-phase plots (b).

tigated. After the TiCl4 post-treatment of the SnO2 photoelectrodes, the η of DSSC-3 has improved 231% compared to that of DSSC-1, which is due to the increased JSC, VOC, and FF. It is observed that the JSC, VOC, FF, and η of DSSC-3 (VOC = 0.689 V, JSC = 10.12 mA·cm−2, FF = 57.3%, and η = 4.00%) are considerably higher than those for DSSC-1. It is expected that the conduction band edge of SnO2 is 300 mV more positive than that of TiO2, and this leads to a greatly decreased opencircuit photovoltage of SnO2 photoelectrode. In addition, the higher JSC and FF of the DSSC-3 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. After the introduction of both TiO2 compact layer and TiCl4 post-treatment, the DSSC-4 shows the highest efficiency of 4.67%. Figure 5 also shows the dark current of the DSSCs as a function of the applied potential. It can be observed that the dark current is in the order DSSC-1 > DSSC-2 > DSSC-3 > DSSC-4. The dark current comes from the recombination between photogenerated electrons in anode and I3− ions in electrolyte. Owing to lower conduction band energy level for SnO2 than TiO2,2 the SnO2 establishes a bridge between TiO2 and FTO glass to transfer the photogenerated electrons, and reduces the recombination between photogenerated electrons in anode and I3− ions in electrolyte, so the dark current for DSSC-2 is smaller than that for DSSC-1. It is believed that the TiO2 compact layer improves adherence between the SnO2 and FTO surface and provides more electron pathways from SnO2 film to FTO, which is helpful for electron transfer and subsequent suppression of dark current, so the dark current for DSSC-3 is smaller than that for DSSC-2. Furthermore, an energy barrier at the SnO2−electrolyte interface is established by the TiCl4 post-treatment, the back electron transport is retarded, and the interfacial recombination can be reduced, which can also suppress dark current, so the DSSC-4 shows the lowest dark urrent among four cells. The larger dark current means a smaller dark-voltage start point, although the darkvoltage start point is not the same as the light-voltage drop point; similar phenomena also were reported by Yang et al. and Kang et al.32,33 Recently, Yang et al.34 reported a DSSC based on a threedimensional highly doped fluorinated SnO2 (FTO) nanoparticulate film serving as conductive core for low resistance and a thin, low-doped conformal TiO2 shell for a large resistance to recombination. EIS reveals that the electron transit time is reduced by orders of magnitude, whereas the

Table 2. Properties Determined by EIS Measurements DSSCs

R1 (Ω)

R2 (Ω)

f max (Hz)

SnO2 nanorods SnO2 nanoparticles

0.840 0.822

7.38 8.25

12.1 62.3

Figure 5. J−V curves of the DSSCs and dark J−V curves of the DSSCs.

ters are collected in Table 3. It is observed that the JSC, VOC, FF, and η of DSSC-2 (VOC = 0.575 V, JSC = 8.06 mA·cm−2, FF = Table 3. Photovoltaic Parameters of the DSSCs Based on SnO2 Nanorods DSSCs

VOC (V)

JSC (mA·cm−2)

FF (%)

η (%)

DSSC-1 DSSC-2 DSSC-3 DSSC-4

0.530 0.575 0.689 0.757

4.58 8.06 10.12 10.51

49.7 52.2 57.3 58.8

1.21 2.42 4.00 4.67

52.2%, and η = 2.42%) are considerably higher than DSSC-1 (VOC = 0.530 V, JSC = 4.58 mA·cm−2, FF = 49.7%, and η = 1.21%). The introduction of TiO2 compact layer was believed to effectively reduce the charge recombination at the FTO− electrolyte, hence to improve the photocurrent density, photovoltage, fill factor, and power conversion efficiency. In addition, the compact TiO2 compact layer improves adherence between the SnO2 nanorods and FTO surface and provides more electron pathways from SnO2 film to FTO glass, which is helpful for electron transfer and subsequently enhances the electron-transfer efficiency. The influence of TiCl4 posttreatment on the photovoltaic parameters is further inves4348

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(9) Wang, W.; Zhao, Q.; Li, H.; Wu, H. W.; Zou, D. C.; Yu, D. P. Transparent, Double-Sided, ITO-Free, Flexible Dye-Sensitized Solar Cells Based on Metal Wire/ZnO Nanowire Arrays. Adv. Funct. Mater. 2012, 13, 2775−2882. (10) Kim, Y. J.; Lee, M. H.; Kim, H. J.; Lim, G.; Choi, Y. S.; Park, N. G.; Kim, K.; Lee, W. I. Formation of Highly Efficient Dye-Sensitized Solar Cells by Hierarchical Pore Generation with Nanoporous TiO2 Spheres. Adv. Mater. 2009, 21, 3668−3673. (11) Mou, J.; Zhang, W.; Fan, J.; Deng, H.; Chen, W. Facile Synthesis of ZnO Nanobullets/ Nanoflakes and Their Applications to DyeSensitized Solar Cells. J. Alloys Compd. 2011, 509, 961−965. (12) Sauvage, F.; Chen, D.; Comte, P.; Huang, F.; Heiniger, L. P.; Cheng, Y. B.; Caruso, R. A.; Graetzel, M. Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads Achieve Power Conversion Efficiencies Over 10%. ACS Nano 2010, 4, 4420−4425. (13) Li, Y.; Jia, L.; Wu, C.; Han, S.; Gong, Y.; Chi, B.; Pu, J.; Jian, L. Mesoporous (N, S)-Codoped TiO2 Nanoparticles as Effective Photoanode for Dye-Sensitized Solar Cells. J. Alloys Compd. 2012, 512, 23−26. (14) Dong, Z. H.; Lai, X. Y.; Halpert, J. E.; Yang, N. L.; Yi, L. X.; Zhai, J.; Wang, D.; Tang, Z. Y.; Jiang, L. Accurate Control of Multishelled ZnO Hollow Microspheres for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2012, 24, 1046−1049. (15) Kang, J. H.; Myung, Y.; Choi, J. W.; Jang, D. M.; Lee, C. W.; Park, J.; Cha, E. H. Nb2O5 Nanowire Photoanode Sensitized by a Composition-Tuned CdSxSe1−x Shell. J. Mater. Chem. 2012, 22, 8413− 8419. (16) Wang, Y.; Li, J.; Hou, Y.; Yu, X.; Su, C.; Kuang, D. Hierarchical Tin Oxide Octahedra for Highly Efficient Dye-Sensitized Solar Cells. Chem.Eur. J. 2010, 16, 8620−8625. (17) Gubbala, S.; Russell, H.; Shah, H.; Deb, B.; Jasinski, J.; Rypkema, H.; Sunkara, M. Surface Properties of SnO2 Nanowires for Enhanced Performance with Dye-Sensitized Solar Cells. Energy Environ. Sci. 2009, 2, 1302−1309. (18) Caruso, F. Nanoengineering of Particle Surfaces. Adv. Mater. 2001, 13, 11−22. (19) Agrell, H. G.; Lindgren, J.; Hagfeldt, A. Degradation Mechanisms in a Dye-Sensitized Solar Cell Studied by UV−VIS and IR Spectroscopy. Solar Energy 2003, 75, 169−180. (20) Park, N. G.; Kang, M. G.; Ryu, K. S.; Kim, K. M.; Chang, S. H. Photovoltaic Characteristics of Dye-Sensitized Surface-Modified Nanocrystalline SnO2 Solar Cells. J. Photochem. Photobiol. A 2004, 161, 105−110. (21) Green, A. N. M.; Palomares, E.; Haque, S. A.; Kroon, J. M.; Durrant, J. R. Charge Transport versus Recombination in DyeSensitized Solar Cells Employing Nanocrystalline TiO2 and SnO2 Films. J. Phys. Chem. B 2005, 109, 12525−12533. (22) Kay, A.; Grätzel, M. Dye-Sensitized Core-Shell Nanocrystals: Improved Efficiency of Mesoporous Tin Oxide Electrodes Coated with a Thin Layer of an In. Chem. Mater. 2002, 14, 2930−2935. (23) Ramasamy, E.; Lee, J. Ordered Mesoporous SnO2-Based Photoanodes for High-Performance Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 22032−22037. (24) Chen, J.; Li, C.; Xu, F.; Zhou, Y.; Lei, W.; Sun, L.; Zhang, Y. Hollow SnO2 Microspheres for High-Efficiency Bilayered Dye Sensitized Solar Cell. RSC Adv. 2012, 2, 7384−7387. (25) Choi, S. Y.; Kim, M. H.; Kwon, Y. U. Evolution of Photoluminescence across Dimensionality in Lanthanide Silicates. J. Phys. Chem. C. 2012, 14, 3576−3582. (26) Park, N.; Kang, M.; Kim, K.; Ryu, K.; Chang, S. Morphological and Photoelectro chemical Characterization of Core-Shell Nanoparticle Films for Dye-Sensitized Solar Cells: Zn-O Type Shell on SnO2 and TiO2 Cores. Langmuir 2004, 20, 4246−4253. (27) Ko, Y.; Kim, M.; Kwon, Y. Method to Increase the Surface Area of Titania Films and Its Effects on the Performance of Dye-Sensitized Solar Cells. Bull. Korean Chem. Soc. 2008, 29, 463−466. (28) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. BandEdge Engineered Hybrid Structures for Dye-Sensitized Solar Cells Based on SnO2 Nanowires. Adv. Funct. Mater. 2008, 18, 2411−2418.

recombination resistance remains in the range of traditional nanoparticle TiO2 photoelectrodes. The preparation procedure for 3D FTO(core)−TiO2(shell) is more complex than the SnO2 nanorod (core)−TiO2 (shell) by us, and the efficiencies for the both are almost the same. Their results further demonstrate the feasibility of DSSC based on SnO2 nanorod (core)−TiO2 (shell) structure photoanode in this study by us.

4. CONCLUSION In summary, the SnO2 nanorods with fast electron transport properties and slow charge recombination rates can be used as anode materials in DSSC. The overall power conversion efficiency of the DSSC based on SnO2 nanorods reaches 1.21%; the improvement performance could be attributed to the 1-D structure of the SnO2 nanorods that enhances electron transport and electron lifetime. Furthermore, by modifying the SnO2 electrode with a TiO2 compact layer and TiCl4 posttreatment, we fabricated a TiO2−SnO2−TiO2 structured DSSC; the DSSC exhibits a power conversion efficiency of 4.67%. The results given herein show that the combination of the advantageous features of fast electron transport and slow interfacial electron recombination indicate a promising strategy to obtain high-efficiency DSSC.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 595 22693899. Fax: +86 595 22692229. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (No. 2009AA03Z217) and the National Natural Science Foundation of China (Nos. U1205112, 20123501110001).

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REFERENCES

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The Journal of Physical Chemistry C

Article

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dx.doi.org/10.1021/jp309193n | J. Phys. Chem. C 2013, 117, 4345−4350