Facile Synthesis of Mesoporous Tin Oxide Spheres and Their

Aug 27, 2012 - Chemistry, Huaqiao University, Quanzhou 362021, China. ABSTRACT: Mesoporous tin oxide spheres (MS-SnO2) with a main size ranging ...
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Facile Synthesis of Mesoporous Tin Oxide Spheres and Their Applications in Dye-Sensitized Solar Cells Guanglu Shang, Jihuai Wu,* Miaoliang Huang, Jianming Lin, Zhang Lan, Yunfang Huang, and Leqing Fan Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China ABSTRACT: Mesoporous tin oxide spheres (MS-SnO2) with a main size ranging from 100 to 800 nm are synthesized via a simple solution route. These spheres consist of packed nanocrystals and possess a specific surface area of 29.4 m2·g−1 and a main pore size of about 4 nm. Due to large particle size, high specific surface area, and good intercrystalline connections, a dye-sensitized solar cell (DSSC) based on the MS-SnO2 film photoanode shows an energy conversion efficiency of 4.97%, which is higher than that of a DSSC based on current SnO2 nanoparticle film photoanodes. On the other hand, the photovoltaic performance of the DSSC can be improved by modifying the photoanode with a TiO2 blocking layer and TiCl4 post-treatment.

1. INTRODUCTION Since the first report on the dye-sensitized solar cell (DSSC) in 1991 by O’Regan and Gratzel,1 DSSCs have attracted significant attention because of their low-cost and facile fabrication process.2 So far, a light-to-electric conversion efficiency of 12.3% has been obtained.3 In the process of light-to-electric conversion for the DSSC, photons are absorbed by a dye adsorbed on a metal oxide semiconductor with a wide band gap, and excited electrons are generated. These electrons are quickly injected into the metal oxide and are collected by a conducting substrate. The oxidized dye is reduced by iodide ions in the electrolyte, and the resultant triiodide ions accept electrons from the platinized counter electrode to complete a current cycle in the DSSC.4,5 As a wide band gap metal oxide, TiO2 has been widely used in DSSCs.2,4 Besides TiO2, other semiconductor metal oxides, such as ZnO,6,7 Nb2O5,8 and SnO29,10 have also been investigated as potential alternatives to TiO2. Among them, SnO2 has two advantages compared to TiO2: First, SnO2 (3.8 eV) shows a larger band gap than that of TiO2 (3.2 eV) and creates fewer oxidative holes in the valence band under UV illumination, thereby minimizing the dye degradation rate and improving the long-term stability of DSSCs.11 Second, the mobility of charge carriers in SnO2 is faster than that in TiO2.12 However, the DSSCs based on SnO2 show lower energy conversion efficiency than those based on TiO2 so far.13 The inferior photovoltaic properties of SnO2-based DSSCs are © 2012 American Chemical Society

attributed to the faster charge carrier recombination resulting from a 300 mV positive shift in the conduction band edge and poor dye uptake associated with the low isoelectric point.14 Additionally, in the case of SnO2 photoanodes, low open-circuit voltage (VOC) values are frequently observed due to the intrinsically lower value of the conduction band energy level compared to TiO2.2 These issues have been partly overcome by engineering the SnO2 surface with conformal barrier layers using materials such as ZnO, TiO2, ZrO2, MgO, and Al2O3.9,15−18 It is well-known that an efficient photoanode for DSSC should not only have high surface area for dye loading but should also possess a densely packed microstructure for fast electron diffusion and light scattering.19 As for the photoanode consisting of nanoparticles, the poor interconnectivity between particles results in high charge carrier recombination. Recently, one-dimensional (1D) metal oxide architecture has attracted great attention, because the ordered nanomaterials contribute to faster charge carrier transport and slower recombination.20,21 However, the limit of specific surface area of the 1D nanostructures hinders significant improvement of the energy conversion efficiency. One way to overcome the above drawbacks is to introduce hierarchically structured oxide Received: May 1, 2012 Revised: June 23, 2012 Published: August 27, 2012 20140

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films.22,23 The oxide films are typically constructed with submicrometer-sized spheres consisting of nanoparticles; therefore, they cannot only play a role of light scatterers but can also serve as a supporting matrix for dye adsorption. Here, we report the synthesis of mesoporous SnO2 spheres (MS-SnO2) via the self-assembly method. The MS-SnO2 consisting of nanoparticles is used for assembling DSSCs, showing a higher light-to-electric conversion efficiency compared to DSSCs based on current SnO2 nanoparticles (NP-SnO2) due to a stronger light scattering effect, faster electron transport, and higher surface area for MS-SnO2. It is expected that the photovoltaic performance of the DSSC can be further improved by modifying the MS-SnO2 photoanode with a TiO2 compact layer and TiCl4 post-treatment.

the dye-sensitized SnO2 film electrode and counter electrode.24 Thus, a DSSC with SnO2 photoanode was assembled. 2.5. Measurements and Characterization. Dye-adsorption spectra were measured with a UV−visible spectrophotometer (UV2450, Shimadzu, Japan), and the adsorbed dye was washed from the dye-coated SnO2 films with 0.05 M NaOH ethanol solution. The specific surface area and average pore size were calculated using a surface area analyzer (Hiden IGA100B). MS-SnO2 and NP-SnO2 were evaluated by X-ray diffraction (XRD PANALYTICAL, X’Pert PRO, Cu Kα, 40 mA, λ = 0.15406 nm). Their morphologies and sizes were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The diffuse reflectance spectra of SnO2 film was measured with a UV−vis spectrophotometer (UV2550, Shimadzu, Japan). Electrochemical impedance spectroscopy (EIS) was measured in a symmetrical dummy cell fabricated with two identical photocnodes and using the same electrolyte for the DSSCs.25 The data were recorded on an electrochemical workstation (CHI660C, Shanghai Chenhua Device Company, China). An equivalent circuit model was used to fit the data with Zview software. EIS measurements were conducted at the open circuit voltage of the DSSCs, with an AC amplitude of 10 mV under one-sun illumination (100 mW·cm−2 AM 1.5G). The current density−voltage (J−V) measurements were performed on an electrochemical workstation (CHI660C, Shanghai Chenhua Device Company, China) under a simulated solar light irradiation with an intensity of 100 mW·cm−2 from a xenon arc lamp (XQ-500W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere. The photovoltaic performance fill factor (FF) and light-to-electric energy conversion efficiency (η) of the DSSC were calculated according to the following equations.4

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Sensitized dye N719 (RuL2(NCS)2, L = 4,4′-dicarboxylate-2,2′-bipyridine) was purchased from Solaronix SA (Switzerland). Tin oxide, stannous sulfate, tetrabutyl ammonium iodide, alcohol, iodine, ethyl cellulose, terpineol, and acetonitrile were all analytically pure grade and purchased from Sinopharm Chemical Reagent Co., Ltd., China. All reagents were used without further treatment. Fluorine-doped SnO2 conductive glass (FTO, with sheet resistance 8 Ω·cm−2, purchased from Hartford Glass Co., Fort Wayne, CT, USA) was used as substrate for precipitating SnO2 films. 2.2. Synthesis of MS-SnO2. In a typical procedure, 0.4 g of stannous sulfate was dissolved in 60 mL of solution containing water and ethanol. The solution was vigorously stirred for 60 min to give a white suspension. The product was collected by centrifugation after being washed with distilled water several times. Then the product was dried at 50 °C and calcined at 500 °C for 2.5 h in the air. Finally, MS-SnO2 was obtained. 2.3. Preparation of SnO2 Paste. To prepare the MS-SnO2 paste, 0.35 g of ethylene cellulose was added to 5 g of turpentine oil. Then the obtained MS-SnO2 was added to 10 mL of absolute ethanol. Finally, the above two solutions were mixed by stirring for 48 h to yield the paste. For a comparison, a NP-SnO2 paste was prepared by a technique similar to that used for the MS-SnO2 paste except that commercially available NP-SnO2 was used. 2.4. Fabrication of DSSCs. The TiO2 compact layer was prepared by immersing the FTO into 0.2 M TiCl4 aqueous solution at room temperature for overnight, followed by sintering at 500 °C for 30 min. After being cooled to room temperature, the MS-SnO2 paste was spread uniformly onto FTO (labeled as bare-SnO2) and the TiCl4-pretreated FTO (labeled as TiO2−SnO2) substrates, respectively. TiCl4 posttreatment was performed by dipping the bare-SnO2 and TiO2− SnO2 films into 0.2 M TiCl4 aqueous solution for 40 min at 80 °C until the TiCl4 was hydrolyzed, washing the electrodes with distilled water to remove residual TiCl4, and finally sintering at 500 °C in air for 30 min. The TiCl4 post-treated bare-SnO2 and TiO2−SnO2 films were then labeled as SnO2−TiO2 and TiO2− SnO2−TiO2, respectively. The active area of the photoelectrodes was about 0.16 cm2, and the thickness of the photoanodes was about 10 μm for all DSSCs reported in this paper. Afterward, the as-prepared photoanodes were immersed in 2.5 × 10−4 M dye N719 absolute ethanol solution for 24 h to adsorb the dye adequately. Then the photoanodes were sandwiched together with platinized FTO counter electrodes. The redox electrolyte was injected into the aperture between

FF =

J × Vmax Pmax = max JSC × VOC JSC × VOC

η(%) =

(1)

J × VOC × FF Pmax × 100% = SC × 100% Pin Pin

(2)

−2

where JSC is the short-circuit current density (mA·cm ), VOC is the open-circuit voltage (V), Pin is the incident light power, Pmax is the maximum power output, and Jmax (mA·cm−2) and Vmax (V) are the current density and voltage at the point of maximum power output in the J−V curves, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphology and Phase. Figure 1a shows the FESEM image of the as-synthesized MS-SnO2. The MS-SnO2 shows a spherical shape with a main diameter from 100 to 800 nm. Figure 1b indicates that the surface of MS-SnO2 is rough and that the spheres are packed with nanocrystals. The typical TEM image in Figure 1c shows that the shape of MS-SnO2 is similar to those of the FESEM observations. Noted that SnO2 spheres are in close contact with the neighboring spheres, which may facilitate electron transfer when they are fabricated as films for DSSCs. Shown in Figure 1d is commercially available NP-SnO2 with a mean size of about 50 nm, which is much smaller than that of MS-SnO2. The typical XRD patterns of MS-SnO2 and NP-SnO2 are shown in Figure 2. The diffraction pattern of MS-SnO2 is well matched with NP-SnO2, and all the diffraction peaks are indexed to the rutile structure (JCPDS card no. 41-1445). The 20141

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3.2. Surface Area and Pore Size Distribution. Nitrogen sorptometer measurements were used to characterize the pore structure of MS-SnO2. The nitrogen adsorption−desorption isotherm (Figure 3a) of MS-SnO2 displays the typical IV-type curve, which is usually ascribed to the predominance of mesopores. The Barrett−Joyner−Halenda (BJH) pore size distribution (Figure 3b) indicates a main pore size of 4 nm for MS-SnO2. The BET (Brunauer−Emmett−Teller) specific surface area of MS-SnO2 is calculated as 29 m2·g−1, which is higher than that of NP-SnO2 (15 m2·g−1). 3.3. UV−vis Spectra. By using MS-SnO2 instead of NPSnO2 for photoanodes, the obtained SnO2 film turns from semitransparent to opaque, suggesting the lower transmittance of the MS-SnO2 film in the visible range. The light scattering effect of MS-SnO2 and NP-SnO2 films were quantified by measuring the diffuse reflectance spectra as shown in Figure 4. Figure 1. FESEM images of MS-SnO2 at different magnifications (a, b), TEM image of MS-SnO2 (c), and FESEM image of NP-SnO2 (d).

Figure 4. UV−vis diffused reflectance spectra of MS-SnO2 and NPSnO2 films with the same thickness.

Both films show a high reflectance in the wavelength range from 400 to 450 nm. However, the reflectance of the MS-SnO2 film is much higher than that of the NP-SnO2 film from 500 to 800 nm, revealing that the former has a higher light-scattering effect than the latter. This is mainly attributed to a larger particle size of MS-SnO2; as we know, the scattering only occurs when the particle size is comparable to the wavelength of incident light. For the NP-SnO2 film, the light scattering

Figure 2. XRD pattern of MS-SnO2 and NP-SnO2 after annealing at 500 °C for 2.5 h.

high intensity of diffraction peaks of MS-SnO2 demonstrates the high crystallinity. No impurity peak is observed, indicating the high purity of the rutile phase.

Figure 3. N2 adsorption−desorption isotherm (a) and BJH pore size distribution curve (b) of MS-SnO2. 20142

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values are 5 Ω and 6 Ω for MS-SnO2 and NP-SnO2, respectively. The RCT values are 7 Ω and 9 Ω for MS-SnO2 and NP-SnO2, respectively. The smaller RS and RCT for MSSnO2 indicates smaller charge carrier transport resistance and faster charge carrier rate for MS-SnO2. This is because MSSnO2 consisting of tightly packed nanocrystals favors the electron fast transport with less diffusive hindrance compared to NP-SnO2. In the bode-phase plots of two films, two characteristic frequency peaks could be found, and the electron lifetime (Γeff) could be calculated from the middle frequency (f max) as: 1 Γeff = 2Πfmax (3)

effect is usually ignored due to the small size of NP-SnO2, which is far away from the wavelength of visible light. The improved light scattering effect can thus increase the light traveling path, resulting in higher light harvesting efficiency and a corresponding higher photocurrent. Figure 5 shows the UV−vis adsorption spectra of dye washed from MS-SnO2 and NP-SnO2 films, respectively, which reflects

where f max is the maximum frequency of the midfrequency peak.27 As shown in Figure 6b, the f max value for MS-SnO2 is 14 Hz, much lower than the value for NP-SnO2 (68 Hz). These data suggest that electrons in MS-SnO2 film have a longer lifetime than that in NP-SnO2 film. This can be ascribed to the reduction of electron loss in the grain boundaries caused by the decrease of the intercrystalline contacts. 3.5. Photovoltaic Performance. Figure 7 shows that the η (1.26%) of the DSSC based on MS-SnO2 is higher than that of Figure 5. Adsorption spectra of the dye desorbed from MS-SnO2 and NP-SnO2 films with the same thickness.

the difference in light harvesting ability of the films. Compared to the NP-SnO2 film, the MS-SnO2 film has higher adsorption ability, indicating that MS-SnO2 could adsorb more N719 dye. According to Beer's law,26 the dye loading of MS-SnO2 and NP-SnO2 films were calculated to be 4.13 × 10−7 and 3.23 × 10−7 mol·cm−2, respectively. The significant enhancement in dye loading of MS-SnO2 film can be attributed to the higher surface area compared to NP-SnO2. 3.4. EIS Spectra. Figure 6 displays the EIS spectra of the DSSCs based on MS-SnO2 and NP-SnO2 films, respectively. In the Nyquist plots of EIS spectra shown in Figure 6a, two welldefined semicircles can be observed from high-frequency (>1000 Hz) to midfrequency (10−100 Hz). The first semicircle (RS) represents to the series resistance, and the second semicircle (RCT) reflects the charge-transfer at the photoanode−electrolyte interface. From the the figure, the RS

Figure 7. J−V curves of DSSCs based on MS-SnO2 and NP-SnO2 films with the same thickness.

Figure 6. Nyquist plots (a) and Bode-phase plots (b) of DSSCs based on MS-SnO2 and NP-SnO2. 20143

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Figure 8. J−V curves of DSSCs (a) and dark J−V curves of DSSCs (b) based on MS-SnO2.

Figure 8b shows the dark J−V characteristics of the DSSCs employing the bare-SnO2, TiO2−SnO2, SnO2−TiO2, and TiO2−SnO2−TiO2 electrodes, respectively. The dark current onset occurs at low forward bias for bare-SnO2, whereas it shifts by a few hundred millivolts for TiO2−SnO2, SnO2−TiO2, and TiO 2−SnO 2−TiO2 . The TiO2 compact layer improves adherence between the SnO2 and FTO surface and provides more electron pathways from SnO2 to FTO, which is helpful for electron transfer and to subsequently suppress dark current. The modification by TiCl4 post-treatment can also suppress dark current, which is due to that the conduction-band edge of SnO2 positive shift 300 mV than that of TiO2. It establishes an energy barrier at the SnO2−electrolyte interface after TiCl4 post-treatment, the back electron transport is retarded and the interfacial recombination is reduced.

the DSSC based on NP-SnO2 (0.99%). This can be attributed to the following factors. First, the specific surface area of MSSnO2 is higher than that of NP-SnO2, which leads to enhanced dye loading and light harvesting. Second, compared to NPSnO2, MS-SnO2 provides a faster electron transfer rate due to a decreased number of contact barriers between primary particles. Third, MS-SnO2 is expected to enhance the diffusibility of electrolyte in the DSSC because of the mesoporous structure. It is well-known that the efficient diffusion of I3−/I− to regenerate the dye is important to the photovoltaic response of the DSSC. Finally, light scattering from MS-SnO2, which extends the distance that light travels within the photoanode film and provides the photons with more opportunities to be absorbed by the dye molecules, results in a significant increase in the light harvesting capability of the photoanode. All these factors contribute to the enhancement of photovoltaic performance of the DSSC based on MS-SnO2. Figure 8a shows the J−V characteristics of the DSSCs based on four type films, and the photovoltaic parameters of these DSSCs are summarized in Table 1. Compared to bare-SnO2,

4. CONCLUSION In conclusion, mesoporous tin oxide spheres (MS-SnO2) consisting of nanocrystals have been successfully synthesized via a simple solution route. Due to the large particle size and large specific surface area, MS-SnO2 can enhance the light harvesting for the photoanodes. Meanwhile, their good crystallinity and interparticle connections caused by partially oriented attachment of primary particles result in the effective charge transport in the MS-SnO2 film. These advantages make MS-SnO2 become an ideal candidate for the photoanode of DSSCs. DSSCs assembled with MS-SnO2 as photoanodes exhibit higher energy conversion efficiency compared to NPSnO2. After being modified with a TiO2 compact layer and TiCl4 post-treatment, the DSSCs exhibit a high power conversion efficiency of 4.97%.

Table 1. Photovoltaic Parameters of the DSSCs Based on MS-SnO2 DSSCs bare-SnO2 TiO2−SnO2 SnO2−TiO2 TiO2− SnO2− TiO2

VOC (V)

JSC (mA·cm−2)

FF (%)

η (%)

adsorbed dye ( × 10−7 mol·cm−2)

0.549 0.683 0.754 0.745

5.13 7.02 10.9 10.6

44.9 55.6 54.0 63.1

1.26 2.67 4.42 4.97

4.13 4.92 5.72 6.38

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AUTHOR INFORMATION

Corresponding Author

the performance of TiO2−SnO2, in terms of η, VOC, and FF, is significantly improved to 2.67%, 0.683 V, and 55.6%, respectively. After TiCl4 post-treatment, the VOC and JSC of TiO2−SnO2 are highly increased to 0.754 V and 10.9 mA·cm−2, respectively. As a result, the η is further improved to be 4.42%, corresponding to a 2.5 times increment compared to that of bare-SnO2. After the introduction of both the TiO2 compact layer and TiCl4 post-treatment, TiO2−SnO2−TiO2 shows the highest efficiency of 4.97%, which is much higher than that of bare-SnO2. The TiO2 compact layer and TiCl4 post-treatment have been applied in TiO2-based DSSCs to enhance the photovoltaic performance of the cells,28,29 however, it is rare to see reports for application of the TiO2 compact layer and TiCl4 post-treatment in SnO2-based DSSCs.

*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. 90922028, 50842027).

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