Highly Efficient Dye-Sensitized Solar Cells by Using a Mesostructured

Sep 3, 2010 - Dye-sensitized solar cells (DSSCs) have attracted considerable attention since they were first reported by Grätzel in 1991 owing to the...
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Ind. Eng. Chem. Res. 2010, 49, 9111–9116

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Highly Efficient Dye-Sensitized Solar Cells by Using a Mesostructured Anatase TiO2 Electrode with High Dye Loading Capacity Wei Shao,† Feng Gu,† Chunzhong Li,*,† and Mengkai Lu‡ Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China, and State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China

The growth and assembly of TiO2 nanostructures with enhanced charge transfer and light harvesting have attracted much attention for fabricating highly efficient dye-sensitized solar cells. In this study, we report an environmentally friendly and easy synthetic route for fabrication of mesostructured anatase TiO2 by controlling the hydrolyzing of n-butyl titanate in boric acid solution. The well-defined mesoporous TiO2 aggregates are obtained by the tensorial reaction-limited aggregation probability. The photovoltaic measurements indicate that the mesoporous TiO2 layer enhances the dye loading capacity, the electron transfer efficiency, and the photocurrent of the cell, contributing to the significant improvement of the energy conversion efficiency of the dye-sensitized solar cells. Introduction Dye-sensitized solar cells (DSSCs) have attracted considerable attention since they were first reported by Gra¨tzel in 1991 owing to the advantages of low cost, high efficiency, and simplicity of fabrication.1 For DSSCs, both optical absorption and charge separation processes can be realized by the association of a sensitizer as light-absorbing material with a wide band gap semiconductor (i.e., TiO2, ZnO, SnO2) of nanocrystalline morphology.2-7 One of the key factors for DSSC is the TiO2 electrode with a high surface area essential to load a large amount of adsorbed dye and achieve a high photocurrent.1,8,9 Nowadays, the growth and assembly of TiO2 nanostructures with enhanced light harvesting effect and electron transport property are emerging to be one of the most effective approaches for fabricating highly efficient DSSCs. Among numerous TiO2 nanostructures (e.g., rods, tubes, wires, and tetrapads) for DSSCs, mesoporous TiO2 can enhance the light harvesting within the electrodes without sacrificing the accessible surface for dye loading, therefore attracting a great deal of attraction.10 Zukalova et al. prepared ordered mesoporous TiO2 nanocrystalline films via layer-by-layer deposition with Pluronic P123 as a template and found the mesoporous TiO2 films showed enhanced solar conversion efficiency resulting from a remarkable enhancement of the short circuit photocurrent, due to the huge surface area accessible to both the dye and the electrolyte.11 Kim et al. introduced a long-range ordered mesoporous TiO2 film between the TiO2 nanoparticle layer and F-doped SnO2 conducting glass substrate during DSSC fabrication and found short-circuit photocurrent density (Jsc) was increased from 12.3 to 14.5 mA/cm.2,12 These reports mostly presented interesting structural, morphological, and photovoltaic properties of TiO2 photoanode films derived from alkylphosphate surfactants, octadecylamine, and triblock copolymer as templates within the matrix, but few studies on the photovoltaic properties depending on dye loading capacity and charge transfer of such a mesostructure are available in the literature. Herein, we demonstrate a facile method, free of organic structure-directing agents or pretreated substrates, for the * To whom correspondence should be addressed. E-mail address: [email protected]. † East China University of Science and Technology. ‡ Shandong University.

synthesis of mesoporous anatase TiO2 through a reaction-limited aggregation in boric acid solution. By adjusting the concentration of boric acid, the hydrolysis rate as well as the motion of the TiO2 nanoparticles could be adjusted. We focus on the changes of physical and chemical properties of the mesoporoous TiO2 by the substitution of Ti4+ through B3+ in the TiO2 lattice. The corresponding performance of the fabricated solar cell is given close attention. The mesoporous TiO2 possesses high surface area and increases the dye loading, accordingly, increasing the photocurrent and cell efficiency. Experimental Section Formation of Mesoporous TiO2. In the typical synthesis, 3.4 mL of tetra-n-butyl titanate (TBT) was introduced dropwise into 100 mL of boric acid aqueous solution (0-1.70 M) at room temperature and aged for 72 h to obtain a white precipitate. The precipitate was filtered and washed with distilled water and absolute ethanol by centrifugation several times. After being dried in an oven at 60 °C overnight, the sample was calcined at 450 °C (1 °C/min heating rate) and kept for 3 h. To investigate the effects of the experimental conditions, parallel experiments were carried out by changing the concentration of boric acid solution. We named a series of mesoporous TiO2 as sample A to sample D by adjusting the concentration of boric acid solution of 0, 0.43, 0.85, and 1.70 M. Preparation of TiO2 Films. In the fabrication of DSSCs, the P25 TiO2 paste was first prepared. An amount of 2.4 g of P25 TiO2 nanopowders was dispersed in acetylacetone ethanol aqueous solution (10 vol %) and grinded for 0.5 h. Amounts of 3.2 mL of distilled water and 0.04 mL of Triton X-100 were introduced into the above mixture. After 0.5 h grinding, the result sol was printed onto F-doped SnO2 conducting glass (Nippon Sheet Glass, SnO2:F, 15 ohm/sq) with an active area of 0.25 cm2 using the screen printing technique, which was then heated at 450 °C for 30 min. For the bilayer structure, the mesoporous TiO2 particle layer was deposited by the spincoating method on annealed P25 TiO2 films and heated over the same heating profile as before. The resulting TiO2 films were immersed in anhydrous ethanol containing 5 × 10-4 M purified N719 and kept for 72 h. Pt counter electrodes were prepared on the FTO glasses using 0.7 mM H2PtCl6 solution, followed

10.1021/ie901692z  2010 American Chemical Society Published on Web 09/03/2010

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Figure 2. (a) TEM image and (b) HRTEM image of the mesoporous TiO2. The inset of (a) shows the ED pattern.

Figure 1. XRD patterns of the prepared TiO2 products. The inset shows the small-angle diffraction patterns.

by heating at 400 °C for 20 min in air. The redox electrolyte used was 0.1 M LiI, 0.05 M I2, 0.6 M 1,2-dimethyl-3propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in dried acetonitrile. The two electrodes were sealed together with a hot-melt polymer film (60 µm thick, Surlyn, DuPont). Characterization. A transmission electron microscope (JEM2100) and high-resolution transmission electron microscope (JEM-2010) were used to characterize the samples. The X-ray diffraction (XRD) patterns of the samples were measured by using a Japan Rigaku D/max 2550 VB/PC diffractometer with Cu KR radiation (λ ) 0.15418 nm). Infrared spectra of the samples were recorded with a Nicolet Avatar 360 ATR-FTIR instrument (Nicolet Instrument, USA). X-ray photoelectron spectroscopy (XPS) spectra were recorded by a PHI 5000C ESCA spectrometer using Mg Ka radiation (hν ) 1253.6 eV). The shift of the binding energy due to relative surface charging was corrected using the C 1s level at 284.6 eV as an internal standard. The bandgap energy of TiO2 powders and absorbance spectra with dye-covered TiO2 films were obtained using a Scan UV-vis-NIR spectrophotometer (Varian, Cary 500) equipped with an integrating sphere assembly. Nitrogen adsorptiondesorption isotherms were measured on an AUTOSORB-1 analyzer (Quantachrome Instruments). A Keithley 2400 source meter and a 500 W xenon lamp (Shanghai Danguang) were used to measure the current-voltage characteristics. The incident monochromatic photon to current conversion efficiency (IPCE) spectra were measured using an Oriel 300 W xenon arc lamp and a lock-in amplifier M 70104 (Oriel) under monochromator illumination, which was calibrated with a monocrystalline silicon diode. The impedance spectra were studied by applying electrochemical impedance spectroscopy (EIS, AUTOLAB PGSTAT302N) under 100 mW · cm-2 illumination at applied bias of Voc, respectively. Results and Discussion X-ray diffraction patterns of the TiO2 samples obtained with and without using boric acid are shown in Figure 1. The diffraction peaks, corresponding to the reflections from the 101, 004, 200, 105, 211, 204, 116, 220, and 215 crystal planes, respectively, identify the sample as anatase TiO2 (JCPDS 21-1272). The average crystallite size of the TiO2 sample without using boric acid estimated based on the Scherrer formula13 is 11.1 nm, while the size of the sample with boric acid is 8.6 nm. B3+ ions were incorporated into the TiO2 lattice during the reaction process, and the possible

impurities such as B2O3 were not detected. In the measurement range 0.4-2°, as shown in the inset of Figure 1, only a half peak is observable around a two-theta value of 0.6°, suggesting that the sample is mesoporous but lacked longrange ordered arrangements. The TEM image shown in Figure 2a reveals an irregular aggregation morphology of the mesostructured TiO2. The diameter of the aggregates is about 200-400 nm. The inset of Figure 2a shows the ED pattern, with five diffusing diffraction rings indexed to (101), (004), (200), (211), and (204) planes, respectively, of the tetragonal structure of anatase TiO2. The wormlike nanopores with pore size of ca. 3-4 nm can be seen in the HRTEM image in Figure 2b, forming the special closely interconnected nanostructures. During the reaction process, boric acid solution was used to influence the hydrolysis rate as well as the motion of the formed TiO2 nanoparticles. By adjusting the concentration of boric acid used, the pH value of the solution varied. TBT hydrolyzed into an amorphous TiO2 sol at the beginning and ripened into the well-defined TiO2 nanoparticles with aging. In the case of the boric acid environment, the hydrolysis of TBT was a slow process. After ripening into the TiO2 nanoparticles, the aggregation took place. From the figures, the parallel lattice fringes among almost all the primary building blocks and the grain boundaries can support the substantial growth by oriented aggregation. As diffusionlimited aggregation mostly leads to disordered particles with no structural porosity, the present reaction is believed to proceed via reaction-limited aggregation.14,15 Zhang utilized hard interfaces between metal foils and aqueous solutions to grow oriented nanostructured films of metal chalcogenides.16 The well-defined structural porosity of the mesoporous TiO2 aggregate in this work would then result from the tensorial reaction-limited aggregation probability, branching the rates and the defects involved.14 Figure 3(a) shows the UV-vis absorption spectra. The absorption onset of the mesoporous TiO2 (Sample C) prepared with boric acid is blue-shifted. The bandgap energies of the two samples are estimated to be 3.28 and 3.33 eV (inset of Figure 3(a)), similar to the value of bulk anatase TiO2. This tendency is similar to that of borated titania reported by Fei et al.17 For element-doped TiO2, two types of theories of “bandgap narrowing” and “introduction of donor or acceptor levels” are generally accepted, such as a mixture of N 2p or C 2p with O 2p states.18,19 However, it is also accepted that the UV absorption threshold or optical band gap of TiO2 is a strong function of particle size for diameters less than 10 nm.20 Here, the bandgap of the sample with boron addition is broadened slightly in the case of smaller crystallite size. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements are further carried out to study the presence of boron. From the FTIR spectra shown in Figure 3(b), the absorption at 1378 cm-1 ascribed to tricoordinated boron can be detected in the samples before and after calcinations.21 The

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Figure 3. (a) Absorption spectra of the mesoporous TiO2 (inset shows the corresponding plots of (hν)2 vs photon energy). (b) FT-IR spectra of the mesoporous TiO2 before and after calcination. (c) XPS spectra of the mesoporous TiO2. Table 1. Summary of Experimental Conditions and Results of the Mesoporous TiO2 Samples total boric BET pore average acid surface volume pore roughness sample (M) area (m2/g) (cm3/g) size (nm) porosity factor A B C D

0 0.43 0.85 1.70

96.33 134.00 182.10 177.10

0.16 0.22 0.27 0.28

6.7 6.6 6.0 6.2

0.38 0.46 0.51 0.52

229.3 277.9 342.6 326.4

absorption peaks of pure B2O3 (1202 cm-1) and incorporated BO4 (1096 cm-1) disappear after the calcinations. Figure 3(c) shows the XPS spectra of the mesoporous TiO2 by measuring B binding energy. The peak at 192.1 eV is observed for the presence of boron. The binding energy for B1s is 194.1 eV in B2O3 (B-O bonds) and 188.2 eV in TiB2 (Ti-B bonds).22,23 The results indicate that the boron atom is really incorporated into TiO2 as a mixed state Ti-O-B. Table 1 summarizes the BET surface area, total pore volume, and average pore size results of the mesoporous TiO2 samples. The specific surface area and total pore volume increase drastically when boric acid concentration varies in the range of 0-0.85 M. The highest BET value of the mesoporous TiO2 reaches 182.10 m2/g, while that of P25 is about 50.0 m2/g. Here, we tentatively investigated the porosity (P) and surface roughness factor (R) based on the BET measurements of these samples. The porosities can be calculated by using the following equation:24,25 P ) Vp/(q-1 + Vp), where Vp is the specific cumulative pore volume (cm3 · g-1) and q is the density (g · cm-3) of anatase TiO2. The porosities are approximately 38%, 46%, 51%, and 52%, respectively. The porosity in nanocrystalline TiO2 films employed in DSSCs is typically in the range of 50-65%.24 An estimation of the roughness factor (R) could be induced by the following equation:26 R ) q(1 - P)S, where P is the porosity (%) and S is the specific surface area (m2/g). The calculated roughness factors are approximately 229.3, 277.9, 342.6, and 326.4, showing that the mesoporous TiO2 samples have much higher dye adsorption ability. To investigate the cell performance, two electrodes made from mesoporous TiO2 and commercially available TiO2 (Degussa P25) were prepared using a similar procedure. Figure 4 shows the current-voltage (I-V) curves of these two cells, and the results are summarized in Table 2. The cell based on the mesostructured TiO2 electrode shows a higher short-circuit photocurrent density (Jsc) and lower open-circuit voltage (Voc) and fill factor (FF), compared with that based on the P25 electrode. The higher Jsc mainly originated from the excellent dye loading capacity of the mesoporous TiO2 samples. The dye loading capacity corresponding to the surface area of the electrode film was also studied by UV-vis absorption spectroscopy. To measure the

Figure 4. J-V characteristics of the fabricated DSSCs. The mesoporous TiO2 was derived when 0.8 M boric acid solution was used. Table 2. Summarized Cell Performance Results of the Fabricated DSSCs

DSSC 1 2 3

photoelectrode

absorbed Voc Jsc FF Dye (×10-7 (V) (mA/cm2) (%) η (%) mol · cm-2,a)

0.67 mesoporous TiO2b layer P25 layer 0.78 mesoporous 0.77 TiO2/P25 bilayer

16.00

0.49

5.20

2.95

11.23 15.68

0.61 0.57

5.27 6.94

0.93 1.84

a Dye-adsorbed films with a dimension of 0.25 cm2 were used for estimating the adsorbed dye concentration. b The mesoporous TiO2 sample chosen was Sample C.

absorbed dye amount, each film was desorbed by an equal volume of 0.05 M aqueous NaOH solution, and the absorbance of the solution was quantified with a UV-vis spectrophotometer. From Figure 5a, we find that higher surface area of the electrode film may result in the improved dyeloading amounts, and the detailed results are summarized in Table 2. Figure 5b shows the UV-vis reflectance spectra of the electrode film derived from mesoporous TiO2 and P25 TiO2. After dye loading, the reflectance of the TiO2 film decreases drastically in the range of 400-600 nm, which is mainly due to light absorption by the dye molecules. The reflectance of the film derived from mesoporous TiO2 is higher than that from P25 TiO2, indicating a higher lightscattering ability. As Voc is correlated with the Fermi level of TiO2, we can speculate that the utilization of boron acid during the experiment may affect the energy bandgap structure of the resultant TiO2 samples. After boron doping, the negative charge of the B′Ti ion has to be compensated by creation of oxygen vacancies in the host, which would affect

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Figure 5. (a) Absorbance spectra of the dyed and undyed films. (b) Diffused reflection of the films.

conducting substrate resulted in an unavoidable light loss.28 Figure 7 shows the incident-photon-to-current efficiency (IPCE) spectra of the fabricated solar cells, which display a significant enhancement in the IPCE of the cell based on the mesoporous TiO2 electrode compared to that based on the P25 electrode. The improvement can be attributed to the enhanced electron injection and charge-transfer efficiency as well as to the higher amount of dye absorption.

Figure 6. Electrochemical impedance spectra of the cells.

Figure 7. IPCE spectra of the cells based on the TiO2 electrodes.

the position of the conduction band minimum, and the difference between the flat band energy of TiO2 and redox potential of the dye will be decreased. Therefore, a smaller Voc was obtained. For FF, it is known that a low total series resistance of the solar cell contributes to the good FF value. Herein, the internal resistances of the DSSCs are studied via electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 Hz-100K Hz. In Figure 6, two semicircles are observed and attributed to charge transportation at the Pt counter electrode and the electron transfer at the TiO2/dye/electrolyte interface. The total resistance of the cell derived from mesoporous TiO2 is higher than that derived from P25 TiO2, which can attribute to the poor contact between the TiO2 layer and the FTO glass due to the irregular shapes of the mesoporous TiO2.27 The strong backscattering of light due to the large irregular aggregates near the

For highly efficient DSSCs, a bilayer-structure electrode was developed with an underlayer composed of P25 TiO2 and an overlayer of mesoporous TiO2. The energy-conversion efficiency of the cell was improved greatly, about 31.7%, compared to the cell derived from P25 TiO2. Such a bilayer structure can guarantee the dye loading capacity, and a higher Jsc will be obtained (2.95 × 10-7 mol · cm-2, 16.0 mA · cm-2 for mesoporous TiO2 electrode, 0.93 × 10-7 mol · cm-2, 11.23 mA · cm-2 for P25 TiO2 electrode). The presence of the P25 TiO2 underlayer can result in the better contact between the layer and the FTO glass, and a higher FF can be obtained (0.67 V for mesoporous TiO2 electrode and 0.77 V for bilayer TiO2 electrode). Under light irradiation, photoexcited electrons in the dye are first injected into the conduction band of mesoporous TiO2 and then into the conduction band of P25 TiO2. The presence of the P25 TiO2 underlayer may optimize the overall energy level of the electrode film, and the Voc can be increased to 0.77 V, similar to the value of the cell derived from P25 TiO2. Table 2 listed the results. Additionally, morphology and surface orientation of nanocrystalline TiO2 play important roles for DSSC performances.29 For DSSCs, charge recombination occurs with dye cations and I3- in the electrolyte when electrons meet them at the surface of particles. Electron recombination lifetimes could be influenced by the TiO2 film surface states, and the conditions of surface and boundaries between particles can affect diffusion coefficients of electrons.30 The cell performance of the mesoporous TiO2 samples (Samples A-D) is illustrated in Figure 8. It can be noticed that the rising tendency for η is lower than that of the surface area. Nakade et al. demonstrated that the diffusion coefficient decrease was related to the increase of film surface area and condition of grain boundaries.31 Peter et al. revealed that charge injection efficiency was reduced with decreasing particle size.32 In this work, the increased surface area caused more dye to be adsorbed, leading to the increase of Jsc. However, the TiO2 particle size decreased from 11.1 to 8.6 nm with increasing concentration of boric acid from 0 to 0.85 M, which would lead to more grain boundaries and the trap site distribution

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Figure 8. J-V characteristics of the fabricated DSSCs with a mesoporous TiO2/P25 TiO2 bilayer.

and density. The synergetic effect of these two points made the rising tendency for η slow down. Conclusions In this study, we have demonstrated successful fabrication of mesoporous anatase TiO2 without any organic structuredirecting agents or pretreated substrates with the assistance of boric acid solution. The effects of boric acid on the formation process, the structure characteristic, and the optical properties of the mesoporous anatase TiO2 have been investigated in detail. The experimental results revealed that the formation of mesoporous anatase TiO2 was subjected to reaction-limited growth. By adjusting the boric acid concentration, the mesostructure and the surface area would change. The dye loading capacity and photoinduced electron transfer can be enhanced drastically. Accordingly, improved photocurrent and cell efficiency were realized. Acknowledgment This work was supported by the National Natural Science Foundation of China (20706015, 50703009, 20925621, 20906027), the Shanghai Rising-Star Program (09QH1400700), the Special Projects for Key Laboratories in Shanghai (09DZ2202000, 10DZ2211100), the Major Basic Research Program of Shanghai (10JC1403300, 10JC1403600), and the Special Projects for Nanotechnology of Shanghai (0952nm02100). Literature Cited (1) Oregan, B.; Gra¨tzel, M. A Low-cost, High-efficiency Solar-cell Based on Dye-sensitized Colloidal TiO2 Films. Nature 1991, 353, 737. (2) Gra¨tzel, M. Dye-sensitized Solar Cells. J. Photochem. Photobiol., C 2003, 4, 145. (3) Gra¨tzel, M. Perspectives for Dye-sensitized Nanocrystalline Solar Cells. Prog. PhotoVoltaics 2000, 8, 171. (4) Chen, Y. J.; Stathatos, E.; Dionysiou, D. D. Sol-gel Modified TiO2 Powder Films for High Performance Dye-sensitized Solar Cells. J. Photochem. Photobiol., A 2009, 203, 192. (5) Hasin, P.; Alpuche-Aviles, M. A.; Li, Y. G.; Wu, Y. Y. Mesoporous Nb-Doped TiO2 as Pt Support for Counter Electrode in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2009, 113, 7456. (6) Hamann, T. W.; Jensen, R. A.; Martinson, A. B. F.; Van Ryswyk, H.; Hupp, J. T. Advancing Beyond Current Generation Dye-sensitized Solar Cells. Energy EnViron. Sci. 2008, 1, 66. (7) Saito, M.; Fujihara, S. Large Photocurrent Generation in Dyesensitized ZnO Solar Cells. Energy EnViron. Sci. 2008, 1, 280.

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ReceiVed for reView October 28, 2009 ReVised manuscript receiVed July 9, 2010 Accepted August 16, 2010 IE901692Z