International Journal of PhotoenergyVolume 2011 (2011), Article ID 821045, 6 pagesdoi:10.1155/2011/821045

Research Article

Dye-Sensitized Solar Cells Based on

Zeng Chen, Shengjun Li, and Weifeng Zhang

Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics & Electronics, Henan University, Kaifeng 475001, China

Received 10 March 2011; Revised 2 June 2011; Accepted 7 June 2011

Academic Editor: Masanori Tachiya

Copyright © 2011 Zeng Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Bismuth titanate (Bi₄Ti₃O₁₂) particles were synthesized by hydrothermal treatment and nanoporous thin films were prepared on conducting glass substrates. The structures and morphologies of the samples were examined with X-ray diffraction and scanning electron microscope (SEM). Significant absorbance spectra emerged in visible region which indicated the efficient sensitization of Bi₄Ti₃O₁₂ with N3 dye. Surface photovoltaic properties of the samples were investigated by surface photovoltage. The results further indicate that N3 can extend the photovoltaic response range of Bi₄Ti₃O₁₂ nanoparticles to the visible region, which shows potential application in dye-sensitized solar cell. As a working electrode in dye-sensitized solar cells (DSSCs), the overall efficiency reached 0.48% after TiO₂ modification.

1. Introduction

Dye-sensitized solar cells (DSSCs) have attracted the attention of scientists all over the world because of their relatively high efficiency and low-cost production [1, 2]. In dye-sensitized solar cells, the most important part is the nanoporous electrode made of wideband gap semiconductor for supporting dye molecules and transporting photo-injected electrons. Previous research has been limited to simple binary oxides, including TiO₂, ZnO [3], SnO₂ [4], Nb₂O₅ [5], WO₃ [6], and In₂O₃ [7], and so forth. Little attention has been carried out on the ternary oxides, except for SrTiO₃ [8] and Zn₂SnO₄ [9]. However, the multication oxides show more advantages for the tuning of materials’ chemical and physical properties by altering the compositions [10, 11]. It is interesting to investigate the potential applications of new ternary oxides in DSSCs for higher photoelectric conversion efficiency.

Bismuth titanate, Bi₄Ti₃O₁₂, behaves as an n-type semiconductor with an indirect band gap of 3.2 eV [12, 13]. It is an important ceramic material widely studied on its nonvolatile memory, optical memory, piezoelectric, and photocatalytic properties [14–17]. Its photovoltaic properties, however, have scarcely been reported. In the previous work, we have investigated the surface photovoltage response of thin compact Bi₄Ti₃O₁₂ film on fluorine-doped tin oxide conductive glass substrate [13]. In this work, nanoporous Bi₄Ti₃O₁₂ electrode was prepared and sensitized with N3 dye and the photoelectric properties was investigated.

2. Experimental

2.1. Preparation and Characterization of Nanocrystal Bi₄Ti₃O₁₂ Electrode

Bi(NO₃)₃·5H₂O and Ti(OC₄H₉)₄ were used as the bismuth and titanium sources, respectively, for the hydrothermal synthesis of Bi₄Ti₃O₁₂. The two sources were dissolved in glacial acetic acid to form two transparent solutions. A little acetylacetone were added as stabilizer. Then, bismuth nitrate solution was slowly added to the tetrabutyl titanate solution. Eight percent excess Bi(NO₃)₃·5H₂O was added to compensate for the Bi loss during the thermal annealing. After magnetic stirring for 24 hours, sodium carbonate (Na₂CO₃) aqueous solution was added dropwise to the mixture under vigorous stirring to form a yellow colloid. The mixture was autoclaved at 190°C for 12 h. The resulting precipitates were ultrasonic dispersed and washed thoroughly with deionized water and ethanol successively, then dried at 60°C in an oven. The sample was sintered at 690°C for two hours to form Bi₄Ti₃O₁₂.

Bi₄Ti₃O₁₂ particles were suspended, mixing 0.6 g with 0.1 g of ethylcellulose in 5 mL of 1-butanol. Thin porous Bi₄Ti₃O₁₂ films were prepared by spreading the suspension onto conducting glass substrates (30 Ω/□), followed by heating at 450°C for 30 min in air to eliminate the ethylcellulose. TiO₂ coated Bi₄Ti₃O₁₂ was also prepared by adding tetrabutyl titanate to the colloid of Bi₄Ti₃O₁₂ in 1-butanol before the doctor blading process.

The crystalline phases of the samples were characterized by DX-2500 X-ray diffractometer (XRD) with a monochromatized Cu Kα irradiation ( nm). The morphologies of the samples were determined using a HitachiS-4300F field emission scanning electron microscope (SEM). The absorption spectrum was measured by a UV-vis-NIR photospectrometer (Varian Cary 5000). To investigate the charge separation behavior at the surface of the samples, the surface photovoltage spectroscopy (SPS) was carried out with a home-built apparatus including a 500 W xenon lamp (CHF XQ500 W), a double-grating monochromator (Zolix SP500), a lock-in amplifier (SR830-DSP), a light chopper (SR540), and so on [18].

2.2. Fabrication and Characterization of Dye-Sensitized Solar Cells

Nanoporous Bi₄Ti₃O₁₂ films were dipped into the 5 × 10⁴ mol ethanol solution of cis-bis(thiocyanato)-,-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) (N3 dye) for more than 48 h to absorb enough sensitizer. The counter electrode was Pt foil. The mixture of 0.6 M dimethylpropylimidazolium iodide, 0.1 M iodine, 0.5 M 4-tertbutylpyridine, and 0.1 M lithium iodide in methoxy acetonitrile was selected to be the electrolyte of DSSCs. The counter electrode and the dye-sensitized Bi₄Ti₃O₁₂ electrode were clamped firmly together. The redox electrolyte solution was introduced into the porous nanocrystalline Bi₄Ti₃O₁₂ film by capillary action.

The photoelectrochemical characteristics of DSSCs were measured by the photocurrent-voltage curve (I-V curves) measurement with an Electrochemical Workstation (Shanghai Chen-hua Instrument Co, China) under the simulated solar light. A 500 W xenon lamp was used as the light source. The incident light intensity was 100 mW·cm⁻² measured by a Radiation Meter (FZ-A, Beijing Normal University, China) and the active cell area was 0.25 cm². The flat band potential was measured by the Mott-Schottky analysis method with the potential range from 0 to 1.0 V [19, 20].

3. Results and Discussion

3.1. Structures and Morphologies

The XRD pattern of the as-prepared Bi₄Ti₃O₁₂ and TiO₂ coated Bi₄Ti₃O₁₂ nanoparticles is shown in Figure 1. The samples exhibit well-crystallized perovskite phase of Bi₄Ti₃O₁₂ according to the JCPDS database card number 47-0398. The average particle size calculated is around 56 nm by Scherrer formula. There was no diffraction peak for TiO₂ after the coating process.

Figure 1: X-ray diffraction pattern of Bi₄Ti₃O₁₂- and TiO₂-coated Bi₄Ti₃O₁₂ nanoparticles annealed at 690°C for 2 h.

Figure 2 shows the surface morphology image of the Bi₄Ti₃O₁₂ film (a) and TiO₂ coated Bi₄Ti₃O₁₂ (b) in top views. It can be seen that the Bi₄Ti₃O₁₂ film grown on FTO substrates possesses a high degree of porosity which was the key factor to obtain efficient dye-sensitized solar cells. The size of the nanoparticles is in the range of 100–200 nm which is much larger than the XRD result. The thickness of the Bi₄Ti₃O₁₂ film is about 15 μm. After the modification, most of the Bi₄Ti₃O₁₂ nanoparticles were coated with TiO₂.

(a)

(b)

Figure 2: SEM micrographs the thin film. (a) Bi₄Ti₃O₁₂ top view, (b) TiO₂ coated Bi₄Ti₃O₁₂.

3.2. UV-vis Absorption Spectra

The UV-vis absorption spectra of N3 dye in ethanol solution (curve a), Bi₄Ti₃O₁₂ nanoparticles (curve b), and N3-sensitized Bi₄Ti₃O₁₂ nanoparticles (curve c) are illustrated in Figure 3. The spectrum (curve b) of Bi₄Ti₃O₁₂ nanoparticles exhibits a typical optical absorption behavior of a wide-band gap semiconducting oxide, having an intense absorption band with a steep edge. When N3 dye molecules are adsorbed onto the surface of the Bi₄Ti₃O₁₂ nanoparticles, a new absorption band around 524 nm emerges in the visible region (curve c) compared with that of bare Bi₄Ti₃O₁₂ nanoparticles (curve b) which evidently originates from the absorption of dye molecules. However, the maximum absorption at 500 nm of pure N3 dye is shifted to 524 nm. The red shift might be attributed to the interaction between the dye molecules and semiconductor surface which shows the potential application in dye-sensitized solar cells [21–23].

Figure 3: UV-vis absorption spectra of N3 (curve a), Bi₄Ti₃O₁₂ nanoparticles (curve b), and N3 sensitized Bi₄Ti₃O₁₂ nanoparticles (curve c).

3.3. Surface Photovoltage Spectroscopy Analysis

In our previous work, it has been found that the Bi₄Ti₃O₁₂ thin compact film exhibited obvious surface photovoltaic response, which is attributed to the electronic transition from the O 2p valence band to the Ti 3d conduction band. Figure 4 shows the SPS spectra of bare and N3 sensitized Bi₄Ti₃O₁₂ nanoparticles. The surface photovoltaic response of the bare Bi₄Ti₃O₁₂ nanoparticles appeared in the wavelength range of 310–420 nm with a maximum at 361 nm. After the absorption of N3, a new SPS response band emerges in the visible region of 420–600 nm, which extends the onset of the surface photovoltaic response of Bi₄Ti₃O₁₂ from 420 to 600 nm. The clear SPV peak indicates that photo-induced electron-hole pairs are easily and effectively separated on the basis of the SPS principle in the visible light.

Figure 4: SPS spectra of Bi₄Ti₃O₁₂ nanoparticles (curve a) and N3-sensitized Bi₄Ti₃O₁₂ nanoparticles (curve b).

3.4. Performance of DSSCs

Based on the previous study, application of Bi₄Ti₃O₁₂ film in dye-sensitized solar cells seems to be possible. However, the adsorption of dye on Bi₄Ti₃O₁₂ film is slow. In this study, all the photoelectrode was soaked in the dye ethanol solution at least for 48 h to achieve better dye adsorption. Figure 5 shows the J-V characteristics of the DSC based on pure Bi₄Ti₃O₁₂, pure TiO₂, and TiO₂-modified Bi₄Ti₃O₁₂ nanoporous electrodes. It can be seen that the photoelectrical performance of Bi₄Ti₃O₁₂ is similar with the simple binary oxides, such as TiO₂ and ZnO. But the overall light to electricity conversion efficiency is as low as 0.02%. The factors which contribute to the conversion efficiency are short-circuit current (), open-circuit voltage (), and the fill factor (FF). The three main parameters are 0.13 mA cm⁻², 0.38 V, and 0.40, respectively (shown in Table 1.).

Table 1: Photoelectrochemical parameters of DSSCs using bare and 5 at % TiO₂-modified Bi₄Ti₃O₁₂ nanoporous electrodes.

Sample
/mA cm⁻²
/mV
ff
η/%

Bare Bi₄Ti₃O₁₂/N3
0.13
0.38
0.40
0.02

TiO₂/N3
1.64
0.65
0.50
0.53

Bi₄Ti₃O₁₂/TiO₂/N3
0.97
0.72
0.69
0.48

Figure 5: characteristics of bare (curve a) and 5 at % TiO₂-modified Bi₄Ti₃O₁₂ (curve b) thin films.

To improve the photoelectrical performance of Bi₄Ti₃O₁₂ electrode, the efficient way is coating the photoelectrode with other semiconducting oxide, such as Al₂O₃, ZnO, and so on [4, 24]. Here, we modified the Bi₄Ti₃O₁₂ film with TiO₂ by addition of 5% (a.t.) tetrabutyl titanate to the colloid of Bi₄Ti₃O₁₂ in 1-butanol before the doctor blading process. Remarkable improvements were found in every aspect of short-circuit current (), open-circuit voltage (), and the fill factor (ff) (shown in Table 1.). The was improved by nearly seven times. The and fill factor also showed an improvement of 340 mV and 0.29. The overall efficiency reached 0.48%. The Property of the pure TiO₂ electrode prepared by the doctor blading of the 1-butanol solution of tetrabutyl titanate was also given in Figure 5. It can be seen that the TiO₂ modified Bi₄Ti₃O₁₂ also shows higher open circuit voltage than the pure TiO₂ electrode. To explore the causes for the enhancement of the photoelectrochemical properties of DSCs, the dye absorption, flat band potential, and dark current were investigated.

The restriction of charge recombination at the semiconductor/dye/electrolyte interface is an efficient method for the improvement of and FF of DSSCs [25, 26]. The dark current arises from the reduction of triiodide and oxidized dye molecule by conduction band electrons of semiconductor films [25, 26]. The dark current measurement is an apparent analysis on the interface charge recombination. Figure 6 addresses the dark current for the dye-sensitized solar cells using pure Bi₄Ti₃O₁₂, pure TiO₂, and TiO₂-modified Bi₄Ti₃O₁₂ nanoporous electrodes. The TiO₂-modified Bi₄Ti₃O₁₂ shows lower recombination than pure Bi₄Ti₃O₁₂, and pure TiO₂ which contributes to the elevation of and fill factor.

Figure 6: Dark current characteristics of bare (curve a) and 5 at % TiO₂-modified Bi₄Ti₃O₁₂ (curve b) thin films.

The flat band potential () of pure Bi₄Ti₃O₁₂, pure TiO₂, and TiO₂ coated Bi₄Ti₃O₁₂ electrode were evaluated through the Mott-Schottky analysis method. This analysis method is based on the capacitance versus applied potential measurement. The relationship between the capacitance and the applied potential can be expressed by the following equation: [27]where is the space-charge capacitance, ε is the dielectric constant of the semiconductor, is the permittivity of free space, is the dopant density, is the flat band potential, and V is the applied potential. Figure 7 shows the Mott-Schottky curves of pure Bi₄Ti₃O₁₂, pure TiO₂, and TiO₂ coated Bi₄Ti₃O₁₂ films. The plot of versus polarization potential shows an X-intercept corresponding to . The flat band potential of pure Bi₄Ti₃O₁₂ and pure TiO₂ was determined to be about −0.58 V and −0.47 V (versus Ag/AgCl). But the of TiO₂ coated Bi₄Ti₃O₁₂ shifts significantly toward the cathodic potential which should cause the improvement of open-circuit voltage () [27, 28].

Figure 7: Mott-Schottky plots of bare (curve a) and 5 at % TiO₂-modified Bi₄Ti₃O₁₂ (curve b) thin films, , the flat band potential.

The amount of dye absorption is one of the important factors on the short-circuit current of DSSCs. The dye absorbed on Bi₄Ti₃O₁₂ electrode is about 1.4E-8 mol cm⁻² which contributes to the low photocurrent. However, no significant change was found in our study before and after TiO₂ coating on the Bi₄Ti₃O₁₂ films. So other controlling process should exist, such as the electron transfer process from excited dye molecular to the conduction band of Bi₄Ti₃O₁₂ and the electron transport process in the Bi₄Ti₃O₁₂ film. Though the overall efficiency of Bi₄Ti₃O₁₂ is severely lower than that of TiO₂, there are many ways to ameliorate the performance for this multication oxide, the variation of the relative Bi/Ti ratio, the partial replacement of Bi or Ti by other metal ion. From this work we can conclude that Bi₄Ti₃O₁₂ film is a potential photoanode material for DSSCs.

4. Conclusions

In conclusion, Bi₄Ti₃O₁₂ film could be efficiently sensitized with N3 dye. The sensitized Bi₄Ti₃O₁₂ film shows significant absorbance and surface photovoltaic response in visible light which indicates the emergence of chemical interaction between the dye molecules and semiconductor surface. This ternary oxide exhibited similar photoelectrical performance with TiO₂. But poor performance was obtained because of the severe charge recombination and lower conduction band edge. After TiO₂ modification, the three main parameters short-circuit current (), open-circuit voltage (), and the fill factor (FF) are 0.97 mA cm⁻², 0.72 V, and 0.69, respectively. The overall efficiency reached 0.48%. Potential improvement can be anticipated by the decrease of the particle size and optimization of the compositions.

Acknowledgments

This work was supported by the Basic and Frontier Technology Research Programs of the Department of Science & Technology of Henan Province (no. 112300410004), the Key Technologies R & D Program of Henan Province (no. 092102210005), and the Research Fund of Henan University (no. 2010ZRZD07).

References

B. O'Regan and M. Grätzel, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991.

A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, “Dye-sensitized solar cells,” Chemical Reviews, vol. 110, no. 11, pp. 6595–6663, 2010.

S. Rani, P. Suri, P. K. Shishodia, and R. M. Mehra, “Synthesis of nanocrystalline ZnO powder via sol-gel route for dye-sensitized solar cells,” Solar Energy Materials and Solar Cells, vol. 92, no. 12, pp. 1639–1645, 2008.

A. Kay and M. Grätzel, “Dye-sensitized core-shell nanocrystals: improved efficiency of mesoporous tin oxide electrodes coated with a thin layer of an insulating oxide,” Chemistry of Materials, vol. 14, no. 7, pp. 2930–2935, 2002.

K. Sayama, H. Sugihara, and H. Arakawa, “Photoelectrochemical properties of a porous Nb₂O₅ electrode sensitized by a ruthenium dye,” Chemistry of Materials, vol. 10, no. 12, pp. 3825–3832, 1998.

H. Zheng, Y. Tachibana, and K. Kalantar-Zadeh, “Dye-sensitized solar cells based on WO₃,” Langmuir, vol. 26, no. 24, pp. 19148–19152, 2010.

S. Mori and A. Asano, “Light intensity independent electron transport and slow charge recombination in dye-sensitized In₂O₃ solar cells: in contrast to the case of TiO₂,” Journal of Physical Chemistry C, vol. 114, no. 30, pp. 13113–13117, 2010.

S. Burnside, J. E. Moser, K. Brooks, M. Grätzel, and D. Cahen, “Nanocrystalline mesoporous strontium titanate as photoelectrode material for photosensitized solar devices: increasing photovoltage through flatband potential engineering,” Journal of Physical Chemistry B, vol. 103, no. 43, pp. 9328–9332, 1999.

B. Tan, E. Toman, Y. Li, and Y. Wu, “Zinc stannate (Zn₂SnO₄) dye-sensitized solar cells,” Journal of the American Chemical Society, vol. 129, no. 14, pp. 4162–4163, 2007.

M. W. Chu, M. Ganne, M. T. Caldes, and L. Brohan, “X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds: Bi4-xLaxTi₃O₁₂(x = 0, 0.5, 0.75, 1.0, 1.5, and 2.0),” Journal of Applied Physics, vol. 91, no. 5, pp. 3178–3187, 2002.

L. Wang, C. Chen, Z. Tang, C. Lu, and B. Yu, “Dependence of Zr content on electrical properties of Bi3.15Nd0.85Ti3-xZrxO₁₂ thin films synthesized by chemical solution deposition (CSD),” Vacuum, vol. 85, no. 2, pp. 203–206, 2010.

L. Pintilie, I. Pintilie, and M. Alexe, “Photoconductive properties of Bi₄Ti₃O₁₂/Si heterostructures with different thickness of the Bi₄Ti₃O₁₂ film,” Journal of the European Ceramic Society, vol. 19, no. 6-7, pp. 1473–1476, 1999.

W. C. Wang, H. W. Zheng, X. Y. Liu et al., “Surface photovoltage characterization of sol-gel derived Bi₄Ti₃O₁₂ ferroelectric thin film on F-doped SnO₂ conducting glass,” Chemical Physics Letters, vol. 488, no. 1–3, pp. 50–53, 2010.

J. Hou, R. V. Kumar, Y. Qu, and D. Krsmanovic, “Controlled synthesis of photoluminescent Bi₄Ti₃O₁₂ nanoparticles from metal-organic polymeric precursor,” Journal of Nanoparticle Research, vol. 12, no. 2, pp. 563–571, 2010.

F. Huang, X. Lu, C. Chen et al., “Room-temperature multiferroic properties of Bi4.15Nd0.85Ti₃FeO₁₅ thin films prepared by the metal-organic decomposition method,” Solid State Communications, vol. 150, no. 35-36, pp. 1646–1649, 2010.

A. Z. Simões, B. D. Stojanovic, M. A. Ramirez, A. A. Cavalheiro, E. Longo, and J. A. Varela, “Lanthanum-doped Bi₄Ti₃O₁₂ prepared by the soft chemical method: rietveld analysis and piezoelectric properties,” Ceramics International, vol. 34, no. 2, pp. 257–261, 2008.

G. B. Kumar and S. Buddhudu, “Optical, thermal and dielectric properties of Bi₄(TiO₄)₃ ceramic powders,” Ceramics International, vol. 36, no. 6, pp. 1857–1861, 2010.

D. Cavalcoli and A. Cavallini, “Surface photovoltage spectroscopy—method and applications,” Physica Status Solidi C, vol. 7, no. 5, pp. 1293–1300, 2010.

G. Redmond, A. O'Keeffe, C. Burgess, C. MacHale, and D. Fitzmaurice, “Spectroscopic determination of the flatband potential of transparent nanocrystalline ZnO films,” Journal of Physical Chemistry, vol. 97, no. 42, pp. 11081–11086, 1993.

M. Wang, Q. L. Zhang, Y. X. Weng, Y. Lin, and X. R. Xiao, “Investigation of mechanisms of enhanced open-circuit photovoltage of dye-sensitized solar cells based the electrolyte containing 1-hexyl-3- methylimidazolium iodide,” Chinese Physics Letters, vol. 23, no. 3, pp. 724–727, 2006.

P. V. Kamat, “Photochemistry on nonreactive and reactive (semiconductor) surfaces,” Chemical Reviews, vol. 93, no. 1, pp. 267–300, 1993.

C. Bauer, G. Boschloo, E. Mukhtar, and A. Hagfeldt, “Interfacial electron-transfer dynamics in Ru(tcterpy)(NCS)₃-sensitized TiO₂ nanocrystalline solar cells,” Journal of Physical Chemistry B, vol. 106, no. 49, pp. 12693–12704, 2002.

Y. Zhang, H. Zhang, Y. Wang, and W. F. Zhang, “Efficient visible spectrum sensitization of BaSnO₃ nanoparticles with N719,” Journal of Physical Chemistry C, vol. 112, no. 23, pp. 8553–8557, 2008.

L. Li, R. Chen, G. Jing, G. Zhang, F. Wu, and S. Chen, “Improved performance of TiO₂ electrodes coated with NiO by magnetron sputtering for dye-sensitized solar cells,” Applied Surface Science, vol. 256, no. 14, pp. 4533–4537, 2010.

E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, and J. R. Durrant, “Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers,” Journal of the American Chemical Society, vol. 125, no. 2, pp. 475–482, 2003.

J. Van De Lagemaat, N. G. Park, and A. J. Frank, “Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO₂ solar cells: a study by electrical impedance and optical modulation techniques,” Journal of Physical Chemistry B, vol. 104, no. 9, pp. 2044–2052, 2000.

X. Yin, H. Zhao, L. Chen et al., “The effects of pyridine derivative additives on interface processes at nanocrystalline TiO₂ thin film in dye-sensitized solar cells,” Surface and Interface Analysis, vol. 39, no. 10, pp. 809–816, 2007.

S. J. Roh, R. S. Mane, S. K. Min, W. J. Lee, C. D. Lokhande, and S. H. Han, “Achievement of 4.51% conversion efficiency using ZnO recombination barrier layer in TiO₂ based dye-sensitized solar cells,” Applied Physics Letters, vol. 89, no. 25, Article ID 253512, 2006.