Mg(OOCCH3)2 Interface Modification after Sensitization to Improve

Oct 26, 2009 - with Al2O3 after sensitization could improve the conversion efficiency and .... Furthermore, Mg(OOCCH3)2 could also act as a carrier la...
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Mg(OOCCH3)2 Interface Modification after Sensitization to Improve Performance in Quasi-solid Dye-Sensitized Solar Cells Rui Gao, Liduo Wang,* Beibei Ma, Chun Zhan, and Yong Qiu* Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China Received July 22, 2009. Revised Manuscript Received October 9, 2009 In this paper, a simple yet efficient method is proposed to improve the performance of dye-sensitized solar cells (DSCs) by modification after sensitization using Mg(OOCCH3)2. With modification of Mg(OOCCH3)2, a blue shift of the absorption peak and optical band gap were observed in the UV-vis spectrum. As shown in the Fourier transform infrared spectrum, the intermolecular hydrogen bonding of N3 dye, which caused the aggregation of dye molecules, was weakened. As shown in the I-V characteristic, the conversion efficiency of the DSCs was improved by the treatment of Mg(OOCCH3)2. Furthermore, the charge recombination was retarded as evidenced by the decreased dark current and the slowed decay rate of the dye excited state, which were characterized by the I-V curve in dark and transient photovoltage spectra. The mechanism of this modification process was also proposed further. Modification with Mg(OOCCH3)2 facilitated the electron injection from the dye molecule to the conductive band of TiO2 by raising the excited state energy level of the dye molecule. This energy level rising was evidenced by the results of the cyclic voltammetry test and the blue shift of the optical band gap. Furthermore, Mg(OOCCH3)2 worked as an insulating barrier layer at the sensitized TiO2/electrolyte interface, thereby retarding the charge recombination in DSCs.

1. Introduction Dye-sensitized solar cells (DSCs) have attracted considerable attention since 1991.1 Investigations have focused on both the efficiency2-5 and stability6-11 of the DSCs for their future practical use, as it could be a low-cost alternative to inorganic photovoltaic devices. And a conversion efficiency higher than 11% has been achieved.12 Despite the discovered advantages, DSCs still suffer from a range of energy losses. The charge recombination may cause an open-circuit voltage reduction, hence decreasing the conversion efficiency.13,14 The conversion efficiency of DSCs can be heightened if all the parameters are optimized.15 *To whom correspondence should be addressed. E-mail: chldwang@mail. tsinghua.edu.cn, [email protected]. Tel.: (008610) 62788802. Fax: (008610) 62795137. (1) O’ Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (2) Kuang, D. B; Klein, C.; Ito, S.; Moser, J.; Baker, R.; Zakeeruddin, S.; Gr€atzel, M. Adv. Funct. Mater. 2007, 17, 154. (3) Hu, L. H.; Dai, S. Y.; Weng, J.; Xiao, S. F.; Sui, Y. F.; Huang, Y.; Chen, S. H.; Kong, F. T.; Pan, X.; Liang, L. Y.; Wang, K. J. J. Phys. Chem. B 2007, 111, 358. (4) Hara, K.; Sugihara, H.; Tachibana, Y.; Islam, A.; Yanagida, M.; Sayama, K.; Arakawa, H. Langmuir 2001, 17, 5992. (5) Jung, H. S.; Lee, J. K.; Nastasi, M.; Lee, S. W.; Kim, J. Y.; Park, J. S.; Hong, K. S. Langmuir 2005, 21, 10332. (6) Nakade, S.; Kanzaki, T.; Kambe, S.; Wada, Y.; Yanagida, S. Langmuir 2005, 21, 11414. (7) Sommeling, P. M.; Sp€ath, M.; Smit, H. J. P.; Bakker, N. J.; Kroon, J. M. J. Photochem. Photobio. A: Chem. 2004, 164, 137. (8) Gr€atzel, M. C. R. Chim. 2006, 9, 578. (9) Figgemeier, E.; Hagfeldt, A. Int. J. Photoenergy 2004, 6, 127. (10) Meng, Q. B.; Takahashi, K.; Zhang, X. T.; Sutanto, I.; Rao, T. N.; Sato, O.; Fujishima, A. Langmuir 2003, 19, 3572. (11) Sathiya Priya, A. R.; Subramania, A.; Jung, Y. S.; Kim, K. J. Langmuir 2008, 24, 9816. (12) Gr€atzel, M. J. Photochem. Photobiol. A: Chem. 2004, 164, 3. (13) Diamant, Y.; Chen, S. G.; Melamed, O.; Zaban, A. J. Phys. Chem. B 2003, 107, 1977. (14) Bandaranayake, K. M. P.; Senevirathna, M. K. I.; Weligamuwa, P.; Tennakone, K. Coord. Chem. Rev. 2004, 248, 1277. (15) Frank, A. J.; Kopidakis, N.; van de Lagemaat, J. Coord. Chem. Rev. 2004, 248, 1165.

2460 DOI: 10.1021/la902688a

Surface modification is considered an important approach in improving the efficiency and stability of DSCs.16 Many kinds of materials such as Al2O3,17,18 MgO,19 Nb2O5,20 SiO2,21 ZnO,22 or ZrO223 have been used to modify the interface between TiO2 film and dye. Durrant et al. attained a 30% efficiency increase through Al2O3 modification in 2002.24 In the meantime, other insulating materials have also been found to be effective in blocking recombination and increasing the conversion efficiency of DSCs. For instance, CaCO3 was applied to modify the TiO2 electrode in 2006.25 This study found that it could improve both the Isc and Voc, resulting in the conversion efficiency enhancement of DSCs. Many studies have focused on the TiO2/dye interface. However, they have unduly ignored the interface between the dye and electrolyte. In our previous work,26 we found that modification with Al2O3 after sensitization could improve the conversion efficiency and stability of DSCs by prohibiting the aggregation of N3 dyes and spacing the TiO2 and the electrolyte. Al2O3 coated on the sensitized TiO2 was from the hydrolysis of a precursor, (16) Alarcon, H.; Boschloo, G.; Mendoza, P.; Solis, J. L.; Hagfeldt, A. J. Phys. Chem. B 2005, 109, 18483. (17) Liu, Z. Y.; Pan, K.; Liu, M.; Wang, M. J.; Lu, Q.; Li, J. H.; Bai, Y. B.; Li, T. J. Electrochim. Acta 2005, 50, 2583. (18) Zhang, X. T.; Sutanto, I.; Taguchi, T.; Meng, Q. B.; Rao, T. N.; Fujishima, A.; Watanabe, H.; Nakamori, T.; Uragami, M. Sol. Energy Mater. Sol. Cells 2003, 80, 315. (19) Wu, S. J.; Han, H. W.; Tai, Q. D.; Zhang, J.; Xu, S.; Zhou, C. H.; Yang, Y.; Hu, H.; Chen, B. L.; Sebo, B.; Zhao, X. Z. Nanotechnology 2008, 19, 215704. (20) Chen, S. G.; Chappel, S.; Diamant, Y.; Zaban, A. Chem. Mater. 2001, 13, 4629. (21) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. J. Am. Chem. Soc. 2003, 125, 475. (22) Wang, P.; Wang, L. D.; Li, B.; Qiu, Y. Chin. Phys. Lett. 2005, 22, 2708. (23) Menzies, D. B.; Cervini, R.; Cheng, Y. B.; Simon, G. P.; Spiccia, L. J. Sol-Gel Sci. Technol. 2004, 32, 363. (24) Palomares, E.; Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Chem. Commun. 2002, 14, 1464. (25) Wang, Z. S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912. (26) Luo, F.; Wang, L. D.; Ma, B. B.; Qiu, Y. J. Photochem. Photobio. A: Chem. 2008, 197, 375.

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Al(OC(CH3)2)3. However, because the alkalinity of Al(OC(CH3)2)3 was too strong, the modification process was hard to control. To avoid this problem, we focused on a new material, Mg(OOCCH3)2, whose alkalinity was much weaker than that of Al(OC(CH3)2)3, and there was not any hydrolysis reaction. The modification process of Mg(OOCCH3)2 was much more gentle than that with Al2O3, thus we used it in the modification in DSCs. In our study, we discovered that Mg(OOCCH3)2 was a good modification material after sensitization, because it could remarkably increase the conversion efficiency of DSCs. Aside from the effects that were similar to Al2O3, such as separating the TiO2 film and electrolyte, Mg(OOCCH3)2 had some novel features. It could interact with N3 dye through intermolecular force. This interaction changed the electron static of the dye molecule, and then raised the LUMO of N3 to a higher energy level. As a result, it benefited the electron injection from N3 to the conductive band of TiO2, which caused the enhancement of IPCE and conversion efficiency. Furthermore, Mg(OOCCH3)2 could also act as a carrier layer that could be used in multilayer sensitizing with dye/modification material alternating assembly structure.27

2. Experiment Method 2.1. Preparation of the Photoanode. The TiO2 colloid was prepared with a hydrothermal method, which has been well documented in the previous report.28 To prepare nanoporous TiO2 film, transparent conductive FTO glass (12 Ω square-1) was completely cleaned and then a thin compact TiO2 film (about 8 nm in thickness) was deposited on the FTO by dip coating in order to improve ohmic contact and adhesion between the following porous TiO2 layer and the conductive FTO glass. The doctor blade technique was then adopted to prepare the porous TiO2 layer with the thickness of the porous layer being controlled by an adhesive tape. Afterward, the film was thermotreated at 450 °C for 30 min. When cooled to 110 °C, the TiO2 electrode was sensitized by immersion in 0.3 mmol L-1 N3 absolute ethanol solution for 12 h and cleaning with absolute ethanol. Modification was performed as follows: the sensitized nanoporous TiO2 electrode was dipped into a 15 mmol L-1 ethanol solution of Mg(OOCCH3)2 for 30 s, and then kept in air for 30 min. 2.2. Preparation of the Electrolyte. The preparation procedure for the polymer gel electrolytes includes two steps. First, liquid electrolyte was prepared. Second, poly(ethylene oxide) (PEO) was slowly added into the liquid electrolyte slowly and heated under strong stirring until the polymer gel electrolyte became homogeneous. The composition of the liquid electrolyte is as follows: 0.1 mol L-1 LiI, 0.1 mol L-1 I2, 0.6 mol L-1 1,2-dimethyl-3-propyl imidazolium iodide (DMPII), and 0.45 mol L-1 N-methyl-benzimidazole (NMBI). The solvent was 3-methoxypropionitrile (MePN);29 the weight ratios (versus liquid electrolyte) for the PEO in the electrolyte was 10.0%. 2.3. Fabrication of the DSCs. A chemically platinized conductive glass was used as the counter-electrode. When assembling the DSCs, the polymer gel electrolyte was sandwiched by a sensitized TiO2 electrode and a counter-electrode with two clips; the space between the two electrodes was controlled by an adhesive tape with a thickness of 30 μm; and the DSCs were not sealed. Finally, the DSCs were baked at 80 °C to ensure the polymer could penetrate into the nanoporous electrode. (27) Ma, B. B.; Gao, R.; Wang, L. D.; Luo, F.; Zhan, C.; Li, J. L.; Qiu, Y. J. Photochem. Photobio. A: Chem. 2009, 202, 33. (28) Burnside, S. D.; Shklover, V.; Barbe, C.; Comte, P.; Arendse, F.; Brooks, K.; Gr€atzel, M. Chem. Mater. 1998, 10, 2419. (29) Huo, Z. P.; Dai, S. Y.; Wang, K. J.; Kong, F. T.; Zhang, C. N.; Pan, X.; Fang, X. Q. Sol. Energy Mater. Sol. Cells 2007, 91, 1959.

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2.4. Characterization. The UV-vis reflectance absorption spectra was measured with a Hitachi U-3010 spectroscope. The Fourier transform infrared (FTIR) spectrum was measured with a Perkin-Elmer Spectrum GX FTIR spectrometer. Photocurrentvoltage (I-V) and dark current measurements were performed using a Keithley model 4200-SCS semiconductor characterization system with real-time plotting and analysis with an active area of 0.25 cm2. The transient photovoltages of the DSCs were studied by probing the cells with a weak laser pulse at 532 nm, which was generated by a frequency-doubled Nd:YAG laser. The transient photovoltage signal was tested under the open-circuit condition and recorded utilizing a TDS220 oscilloscope (Tektronix). The electrochemical data were obtained by employing a Princeton Applied Research potentiostat/galvanostat model 283 with a scanning rate of 150 mV s-1. The cyclic voltammograms test was carried out in a conventional photoelectrochemical cell equipped with a 2 mm platinum electrode as the working electrode, a platinum disk electrode as the counter-electrode and an Ag/AgCl reference electrode in pure acetonitrile solvent. The solution concentration was 2  10-3 M, with 0.1 M TBATFB electrolyte. The AC impedance was tested by CHI 660 electrochemical workstation, USA. Under illumination, the amplitude of the alternative signal was 100 mV and open circuit voltage (Voc) was applied as the bias voltage so that there was no electric current flowing. In dark condition, the bias voltage for ac impedance measurement was -0.8 V, and the frequency ranged from 0.1 to 105 HZ.

3. Results and Discussion 3.1. DSC Structure with Mg(OOCCH3)2 Modification. In this study, the modification process was conducted by dip coating using an ethanol solution of Mg(OOCCH3)2, which was simple and peaceful. The structure of the DSC-modified with Mg(OOCCH3)2 after sensitization was shown as (TiO2/N3/ Mg(OOCCH3)2/electrolyte/Pt). The effects of the modification on DSCs will be discussed in the following sections. 3.2. UV-Visible Spectrum. The UV-vis absorption spectra of the sensitized TiO2 films with and without Mg(OOCCH3)2 coating are shown in Figure 1a. It was noted that the maximum absorbance did not change too much, which indicated that there was almost no dye desorption. There is a blue shift in the UV-vis spectrum with alkaline material modification in the TiO2/dye interface.30 The same observation was also noted during the modification of the dye/ electrolyte interface. As shown in Figure 1a, there was a 20 nm blue shift of the maximum absorption peak with modification after sensitization. The modification after sensitization with Mg(OOCCH3)2 also resulted in the change of the energy level of N3 dye. It is known that the optical band gap (Eg) for direct interband transitions and the absorption coefficient (A) near the absorption edge have a relationship that complied with the following formula31 ðAhνÞ2 ¼ cðhν -Eg Þ where the optical band gap for the absorption edge can be obtained by extrapolating the linear portion of the plot (Ahv)2 hv to A = 0.32 As shown in Figure 1b, the optical band gap of N3 dye was blue-shifted with the modification of Mg(OOCCH3)2. As a result, the modification with Mg(OOCCH3)2 changed the (30) Wu, X. M.; Wang, L. D.; Luo, F.; Ma, B. B.; Zhan, C.; Qiu, Y. J. Phys. Chem. C 2007, 111, 8075. (31) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318. (32) Zeng, J.; Xin, M. D; Li, K. W.; Wang, H.; Yan, H.; Zhang, W. J. J. Phys. Chem. C 2008, 112, 4159.

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Figure 2. IR spectra for photoanode of TiO2/N3 and TiO2/N3/ Mg(OOCCH3)2.

optical band gap of N3 dye, which had important influence on the performance of DSCs. The shift was due to the intermolecular force between Mg(OOCCH3)2 and dye molecules. 3.3. FTIR. The oversaturation adsorption of N3-dye suffers from the problem of dye aggregation. Wenger et al. reported that the slow component of the electron injection in DSCs arises from the aggregated state of the dye, which may lead to the reductive quenching of N3 derivatives.33 The result of an X-ray analysis of N3 single crystal showed that the interaction between the dye molecules was hydrogen bonding.34 The structural models also demonstrated that there were two or three free carboxylic groups, which did not come in contact with the surface when the N3 dye molecules was absorbed onto TiO2 films.34 These free carboxylic groups may cause the hydrogen bonding aggregation, hence lowering the performance of the DSCs. Therefore, to retard the aggregations, it is important to weaken the hydrogen bond between the N3 dye molecules. In our previous study,26 with Al2O3 modification after sensitization, the alkalinity of Al2O3 could change -COOH to -COO-, thus weakening the aggregation of the dye molecules. This change, which was the deprotonation process of N3 on the dye-sensitized TiO2 film, was observed in the FTIR

spectrum. However, being a less alkaline material, the effects of Mg(OOCCH3)2 are different from that of Al2O3. As shown in Figure 2, the 2114 cm-1 peak assigned to the -SCN group was observed in all films without change, as the -SCN group could only come from the dye molecule, and the group number was consistent with the molecule number. The 3400 cm-1 peak was attributed to the stretching vibrations of the -O-H group. The 1720 cm-1 peak assigned to the -CdO group in COOH and the 1600 and 1380 cm-1 peaks could be attributed to the asymmetric and symmetric stretching vibrations of -COO- group.35 With Mg(OOCCH3)2 coating, the decrease of the 1720 cm-1 peak intensity can be neglected, which was obvious in Al2O3 modification. At the same time, the intensity of 1600 and 1380 cm-1 peaks increased due to the presence of the -COO- in Mg(OOCCH3)2 molecule. That is, the deprotonation of N3 dye was not the main effect brought about by Mg(OOCCH3)2 on the dye molecules. Because of the existence of intermolecular hydrogen bonding, the peak assigned to the stretching vibrations of -O-H group would be shifted to a smaller wavenumber in the FTIR spectra.36 At the same time, the peak would become wider, and the intensity would be weaker.37 As shown in Figure 2, with Mg(OOCCH3)2 coating, the 3400 cm-1 peak shifted to about 3430 cm-1, and became sharper. At the same time, the intensity was stronger. This suggests that the intermolecular hydrogen bonding between free carboxylic groups of N3 dye molecules which cause the aggregation of N3 dye was weakened with Mg(OOCCH3)2 coating. Thus it means that there is a decrease in the aggregation of the dye molecules. The results of the FTIR suggest that the intermolecular force between Mg(OOCCH3)2 and dye molecules reduced the aggregation of the N3 dye by weakening the intermolecular hydrogen bonding of the dye molecules. 3.4. Photovoltaic Performance of DSCs. The currentvoltage characteristics of DSCs with and without Mg(OOCCH3)2 modification under different intensities of simulated sunlight were compared. As shown in Table 1 and Figure 3, under 30 mW/cm2 irradiation, the efficiency of the conventional cell was 8.17%, which increased to 9.93% with Mg(OOCCH3)2 coating. Under 100 mW/cm2 irradiation, the efficiency increased from 4.69% to 5.37%.

(33) Wenger, B.; Gr€atzel, M.; Moser, J. E. J. Am. Chem. Soc. 2005, 127, 12150. (34) Shklover, V.; Ovchinnikov, Yu. E.; Braginsky, L. S.; Zakeeruddin, S. M.; Gr€atzel, M. Chem. Mater. 1998, 10, 2533.

(35) Kilsa, K.; Mayo, E. I.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S.; Winkler, J. R. J. Phys. Chem. B 2004, 108, 15640. (36) Rundle, R. E.; Parasol, M. J. Chem. Phys. 1952, 20, 1487. (37) Gaffney, K. J.; Piletic, I. R.; Fayer, M. D. J. Phys. Chem. A 2002, 106, 9428.

Figure 1. (a) UV-vis spectrum of sensitized TiO2 film with and without Mg(OOCCH3)2 coating; (b) (Ahv)2 vs hv curves of the N3 dye with and without modification of Mg(OOCCH3)2.

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Table 1. Performance of DSCs Based on Photoanodes with Different Materials Modification Voc (V)

Jsc (mA/cm2)

FF%

η%

30 mW/cm TiO2/N3 TiO2/N3/Mg(OOCCH3)2

0.635 0.650

5.41 6.64

71.3 69.0

8.17 9.93

100 mW/cm2 TiO2/N3 TiO2/N3/Mg(OOCCH3)2

0.675 0.675

11.00 13.17

63.1 60.4

4.69 5.37

2

Figure 4. IPCE spectra of cells without and with Mg(OOCCH3)2

Figure 3. I-V characteristics of the cells based on different photoanodes under 30 mW/cm2 (a) and 100 mW/cm2 (b).

Based on the results above, the photovoltaic performance of DSCs modified with Mg(OOCCH3)2 was better compared with that of the conventional cell. An IPCE spectrum of cells with and without Mg(OOCCH3)2 coating was tested. As shown in Figure 4, the IPCE peak of the cell modified with Mg(OOCCH3)2 was higher than that of the unmodified cell. IPCE can be expressed by the following formula:38 IPCEðλÞ ¼ LHEðλÞφinj ηc where LHE(λ) is the light-harvesting efficiency for photons of wavelength; φinj is the quantum yield for electron injected from the excited sensitizer into the conduction band of TiO2; and ηc is the electron collection efficiency. As there was no absorption increase (38) Gr€atzel, M. Inorg. Chem. 2005, 44, 6841.

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Figure 5. (a) Normalized transient photovoltage spectra for cells without and with Mg(OOCCH3)2 coating; (b) dark currents of cells without and with Mg(OOCCH3)2 coating.

in the UV-vis spectrum, the light harvest efficiency (LHE(λ)) of the cell modified with Mg(OOCCH3)2 changed little compared with the conventional one. Moreover, the modification after sensitization did not react with the TiO2 film, thus the electron collection efficiency (ηc) did not change much. Thus the increase of IPCE after modification was mainly caused by enhancing the DOI: 10.1021/la902688a

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Figure 6. Schematic of possible interaction between Mg(OOCCH3)2 and N3 dye with modification after sensitizing.

injection quantum yield (φinj). It was also consistent with the result that the efficiency enhancement was mainly contributed to the increase of Jsc. The photovoltage transient measurements carried out under the open-circuit condition denoted the decay rate of the dye molecule in its excited state. The slower the decay was, the longer the life of the dye molecule in its excited state was. Figure 5a shows the normalized transient photovoltage curves of the dye-sensitized solar cells with and without Mg(OOCCH3)2 coating. As seen in the figure, the decay was slower in cells with Mg(OOCCH3)2 coating, which indicated that the life of the dye molecule in its excited state was extended. This is considered to be caused by the change of the dye molecular structure brought about by modification with Mg(OOCCH3)2. Figure 5b shows that the dark current decreased after Mg(OOCCH3)2 modification, which indicates the electron recombination in DSC was retarded. This is also consistent with the photovoltage transient measurements. The results shown in Figure 5 suggest that after modification with Mg(OOCCH3)2, the dark reaction at the interfaces in DSCs was reduced. 3.5. Mechanism and Discussion. Based on the above results, with Mg(OOCCH3)2 modification after sensitization, the conversion efficiency of DSCs was enhanced, and the dark reaction was weakened. A possible mechanism is hereby proposed to explain the effects of Mg(OOCCH3)2 modification after TiO2 was sensitized. It was noted that an optical band gap blue shift of N3 dye appeared upon the modification of Mg(OOCCH3)2. This shows that the energy gap of the dye molecule became wider than that before the modification. The energy level of the N3 molecule in its excited state was higher than that of the conductive band of TiO2,39 that was why the electron could be injected from the dye molecule into the TiO2 film. As the energy gap of dye molecule became wider after the modification, it was considered that the electron static of N3 dye was changed by Mg(OOCCH3)2 modification through intermolecular force (Figure 6). The intermolecular force between N3 dye and Mg(OOCCH3)2 raised the excited state of the N3 dye to a higher energy level. It benefited the electron injection from the excited state of the N3 dye molecule to the conductive band of TiO2, which could explain the enhancement of φinj and the obvious improvement of Jsc. To further explore the energy level raising of the excited state of dye molecules, we tested the cyclic voltammograms of the N3 dye and N3/Mg(OOCCH3)2 in pure CH3CN solvent. As shown in Figure 7, their respective oxidation potential was found to be 0.95 and 0.92 V, with almost no change taking place, and the reduction potential was -1.07 V and -1.33 V, respectively, which changed more negative. From the result of the cyclic voltammograms, the energy levels of the TiO2 and the N3 dye without and (39) Hagfeldt, A.; Gr€atzel, M. Acc. Chem. Res. 2000, 33, 269.

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Figure 7. Cyclic voltammograms of N3 and N3/Mg(OOCCH3)2 in CH3CN.

Figure 8. The energy levels of the TiO2 and the N3 dye without and with Mg(OOCCH3)2 modification.

with Mg(OOCCH3)2 modification are shown in Figure 8. It could be observed that the excited state of the dye molecule became higher after Mg(OOCCH3)2 modification, suggesting that the mechanism we proposed was reasonable. DSCs can be considered as a leaking capacitor in dark condition.40 As such, the resistance of the back reaction from TiO2 to the triiodide ions in the electrolyte was analyzed through AC impedance technique under dark condition. The resistance at the interface of the sensitized TiO2/electrolyte was presented by the semicircle in intermediate frequency regime of the Nyquist plots.41 (40) Bisquert, J. J. Phys. Chem. B 2002, 106, 325. (41) Wang, Q.; Moser, J.; Gr€atzel, M. J. Phys. Chem. B 2005, 109, 14945.

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not too long, for example, 10 and 30 s. However, it increased significantly with longer modification time. As modification time rose to 2 min, the diameter expanded to more than double its original size compared with that of the unmodified cell. As shown in Figure 6, Mg(OOCCH3)2 interacted with the dye molecule through the intermolecular force between the carboxyls from Mg(OOCCH3)2 and the dye molecule. The dye molecules accepted electron from electrolyte through the -NCS group, which did not interact with Mg(OOCCH3)2. As a result, the electron transfer would not be retarded in the illumination condition when the Mg(OOCCH3)2 is not too thick. Thus, when the modification time was not very long, a thin Mg(OOCCH3)2 layer would have little impact on electron transfer at the sensitized TiO2/electrolyte interface. When the modification time was too long, there would be much more Mg(OOCCH3)2 forming a much thicker layer, which could resist the electron transfer at the sensitized TiO2/electrolyte interface. In our study, it was noted that as a novel modification material after sensitization, Mg(OOCCH3)2 had proper alkalinity, and it could raise the excited state energy level of N3 dye through intermolecular force. As a result, the Mg(OOCCH3)2 modification benefited the electron injection and improved the photovoltaic performance of DSCs.

4. Conclusion

Figure 9. Nyquist plots of DSCs with different modification time of Mg(OOCCH3)2 in dark (a) and under illumination of 30mW/ cm2 (b).

When the diameter of the middle frequency semicircle was bigger, the electron recombination at the sensitized TiO2/electrolyte interface was slighter.29 Figure 9a shows the Nyquist plots of DSCs based on photoanodes with different coating times of Mg(OOCCH3)2, which were measured at -0.8 V bias voltage in dark condition. Compared with the DSCs based on photoanode without Mg(OOCCH3)2, the diameter of the middle frequency semicircle of the cells that underwent Mg(OOCCH3)2 modification became bigger. The longer the modification lasted, the bigger the radius became. The diameter increase indicated a decrease of the electron recombination occurring at the sensitized TiO2/ electrolyte interface. This was probably because Mg(OOCCH3)2 worked as an insulating barrier layer to separate the sensitized TiO2 film from the electrolyte and then retarded the recombination. The longer the modification time was, the thicker the barrier layer was. As a result, the resistance at the sensitized TiO2/electrolyte increased more when the dipping time became longer. Thus the longer the dipping time, the slighter the recombination at the interface became. Under an illumination condition, the DSCs could be taken as diodes.42 The resistance at the TiO2/dye/electrolyte interface was also presented by the middle frequency semicircle in the Nyquist plots. As shown in Figure 9b, the diameter of the middle frequency semicircle changed slightly when the modification duration was (42) Koide, N.; Islam, A.; Chiba, Y.; Han, L. Y. J. Photochem. Photobio. A: Chem. 2006, 182, 296.

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This paper applied a novel modification material after sensitization, Mg(OOCCH3)2, to improve the performance of DSCs. With the modification of Mg(OOCCH3)2, a blue shift of absorption peak and optical band gap were observed in the UVvis spectrum. As shown in the FTIR spectrum, the intermolecular hydrogen bonding of N3 dye, which caused the aggregation of dye molecules, was weakened. The photovoltaic performance of DSCs modified with Mg(OOCCH3)2 was improved. In fact, with the modification of Mg(OOCCH3)2, the conversion efficiency increased from 8.17% to 9.93% under an illumination of 30 mW/cm2, and from 4.69% to 5.37% under 100 mW/cm2. In addition, the charge recombination was retarded, which was shown in the I-V curve in dark and transient photovoltage spectra. A possible mechanism of the effects of Mg(OOCCH3)2 modification was proposed. The excited state energy level of N3 dye became higher, which was caused by the intermolecular force between Mg(OOCCH3)2 and the dye molecules. This raising of energy level facilitated electron injection from the dye molecules to the conduction band of TiO2. And this was proved by the results of the UV-vis spectrum and cyclic voltammetry test. Furthermore, Mg(OOCCH3)2 worked as an insulating barrier layer between the sensitized TiO2 and the electrolyte, thus retarding the charge recombination. In summary, this study provides a simple and efficient way to improve the performance of DSCs by facilitating charge injection and prohibiting the charge recombination at the same time. Mg(OOCCH3)2 can also be used in DSCs based on ZnO photoanode to improve their performance. In addition, it can work as an addictive in electrolyte. Furthermore, Mg(OOCCH3)2 can also act as a modification material that is used in multilayer sensitizing with dye/modification material alternating assembly structure. Further study on the multilayer sensitizing is ongoing. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant No. 50873055 and the National Key Basic Research and Development Program of China under Grant No. 2006CB806203 and No. 2009CB930602. DOI: 10.1021/la902688a

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