Optimization of the Performance of Dye-Sensitized Solar Cells Based on Pt-Like TiC Counter Electrodes

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1 DOI: /ejic Optimization of the Performance of Dye-Sensitized Solar Cells Based on Pt-Like TiC Counter Electrodes Yudi Wang, [a] Mingxing Wu, [a] Xiao Lin, [a] Anders Hagfeldt, [b] and Tingli Ma* [a] Keywords: Electrochemistry / Solar cells / Heterogeneous catalysis / Titanium / Iodine Titanium carbide (TiC) has been used as a counter electrode (CE) catalyst in place of Pt for triiodide reduction in dye-sensitized solar cells (DSCs). Its catalytic activity can be comparable to that of Pt. The effect of the thickness of the TiC layer on the performance of the DSCs was investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and Tafel polarization measurements. We found that as the film thickness increased, the fill factor and short-circuit current density increased, which led to improved power-conversion efficiency. When the TiC film was thicker than 20 μm, however, no significant continuous improvement in the catalytic activity was observed. A high power-conversion efficiency of 6.46% was obtained for the DSC by using a TiC CE at the optimized film thickness, a value similar to that obtained with a Pt CE. Introduction Since the novel dye-sensitized solar cell (DSC) was first reported by Grätzel and co-workers, remarkable advances have been made in this field. [1 4] Although DSCs possess advantages such as good plasticity, environmental friendliness, and simple fabrication, their high cost of production and poor stability have limited their commercial applications to date. Normally, a DSC consists of a dye-sensitized porous thin film of TiO 2 as the photoanode, a counter electrode (CE), and an electrolyte containing the iodide/triiodide redox couple. Counter electrodes, as one of the significant components in DSCs, generally consist of Pt deposited on F- doped tin oxide (FTO) glass, the Pt acting as a catalyst to reduce triiodide to iodide in the electrolyte. In previous research we investigated the effect of the Pt film thickness on the counter electrode performance in DSCs. [5,6] We found that the power-conversion efficiency (PCE) of a DSC did not increase in proportion to the increase in the thickness of the Pt film; it was possible to obtain a high PCE with a very thin Pt film of 2 nm. Such results suggest that the production cost can be somewhat reduced by reducing the amount of platinum as it is a scarce and noble metal. However, a more promising way to reduce the cost of DSCs [a] State Key Laboratory of Chemicals, School of Chemial Engineering, Dalian University of Technology, Dalian, China Fax: tinglima@dlut.edu.cn [b] Department of Physical and Analytical Chemistry, Uppsala University, Box 259, Uppsala, Sweden is to explore other low-cost materials with high catalytic activity to replace Pt. Recently, much research has focused on developing new catalysts as substitutes for expensive Pt; for instance, several varieties of carbonaceous materials, [7 13] such as carbon black, activated carbon, carbon nanotubes, and graphite, have been explored. Murakami et al. found that the PCEs of DSCs are strongly dependent on the thickness of the carbon layer; [14] the fill factor increased as the carbon layer thickness increased up to 10 µm. Furthermore, conductive organic polymers [15 19] and inorganic materials [20 28] have also been employed as catalysts for counter electrodes. For these polymer catalysts, the film thickness was important, because it affected both catalytic activity and resistance. TiC is a versatile material exhibiting good hardness, a high melting point, durability, and conductivity. It has commonly been used as a reinforcing material in metal-based composites and has been widely used in the mechanical, electrical, chemical, environmental, fusion reactor, and national defense industries. In the past, we introduced titanium carbide (TiC) into DSCs as the CE; highly efficient catalytic activity was obtained as a result. [29] However, the effect of the thickness of the TiC layer on the properties of the counter electrode has not been studied systematically. In this work, therefore, a comprehensive investigation of TiC counter electrodes of different thicknesses was carried out, including a study of the electrochemical catalytic activity, charge-transfer resistance of the TiC/electrolyte interface, and the stability in catalyzing the triiodide reduction, with the purpose of clarifying the effects of TiC film thickness on the performance of the DSCs; it was found that a thickness of 20 μm was sufficient to obtain a high-performance TiC CE. Eur. J. Inorg. Chem. 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3557

2 Results and Discussion Characterization of the TiC Film with Various Thicknesses by SEM Four kinds (A D) of TiC films of different thicknesses coated on FTO glasses were prepared for use as counter electrodes; the amounts of TiC deposited on the FTO glass are listed in Table 1. Figure 1 shows the top view of the TiC film, which comprises small particles with a size of around 30 nm in diameter. The cross-section images of the TiC films, as shown in Figure 2, show that the TiC film thicknesses of A, B, C, and D are 6, 12, 20, and 42 μm, respectively. Table 1. Photovoltaic parameters of the DSCs using TiC CEs with different film thicknesses. CE Amount of TiC Thickness V oc J sc FF η [mg cm 2 ] [μm] [V] [ma cm 2 ] [%] A B C D Pt Y. Wang, M. Wu, X. Lin, A. Hagfeldt, T. Ma Cyclic Voltammetry Analysis of TiC Electrodes with Different Thicknesses in the Triiodide/Iodide Electrolyte Cyclic voltammetry (CV) was conducted to assess the electrochemical catalytic activities of the four TiC electrodes for triiodide reduction; Figure 3 a shows the cyclic voltammograms. Two pairs of reversible redox peaks were observed for all of these electrodes. The relatively negative pair of redox peaks can be assigned to the redox reaction in Equation (1), whereas the relatively positive pair can be assigned to the redox reaction in Equation (2). As the thickness of the TiC film increased to 20 μm, the peak-current density increased, and the potential separation between the oxidation and reduction peaks decreased, which indicates good reversibility and a high electron-transfer velocity. This outcome subsequently results in an enhancement of the electrochemical catalytic activity. [30,31] For the C and D electrodes, the peak current density and the potential separation between the redox peaks are similar to each other; this indicates that, beyond 20 μm, further increasing the thickness does not result in any obvious improvement in catalytic activity. I 3 +2e h 3I (1) 3I 2 +2e h 2I 3 (2) Figure 1. SEM image of TiC. Figure 3. (a) Cyclic voltammograms of TiC electrodes with four different film thicknesses of the triiodide/iodide redox couple (scan rate: 10 mvs 1 ). (b) Cyclic voltammograms of the C electrode at various scan rates. Relationship between the peak-current densities of the anode (squares) and cathode (triangles) and the square root of the scan rate (c) for the redox reaction in Equation (1) and (d) for the redox reaction in Equation (2). Figure 2. SEM images of the four TiC CEs in cross-section. Figure 3b shows the cyclic voltammograms of the C electrode with a 20 μm TiC film at various scan rates. The peakcurrent density and the potential separation between the redox peaks both increased with increasing scan rate. A higher scan rate caused the diffusion layer on the surface of the catalyst to become thinner and thus increased the diffusion rate, which led to higher peak-current density and larger electrochemical polarization, resulting in high over Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012,

3 Dye-Sensitized Solar Cells Based on TiC Counter Electrodes potential and poor reversibility. Figure 3c and d show the linear relationship between the peak-current density and the square root of the scan rate, which is in good agreement with Equation (3) in which i p is the peak-current density, n is the number of electrons involved in the reaction at the electrode, A is the area of the electrode, D is the diffusion coefficient, V is the scan rate, and C is the concentration of the species involved in the reaction. This phenomenon has been observed in earlier research. [27] This result suggests that the CV experiments should be performed at low scan rate to show the real electrochemical reaction mechanism. i p = ( )n 3 /2AD 1 /2V 1 /2C (3) Effect of TiC Thickness on the Performance of DSCs Figure 4 shows the photocurrent density voltage (I V) curves of the DSCs with TiC layers of different thicknesses as CEs. The photovoltaic parameters are collated in Table 1. The DSC with C as the CE yielded a high PCE of 6.46 %, which matches the photovoltaic performance of the DSC with a Pt CE (7.23%). This result demonstrates that TiC possesses high catalytic activity towards triiodide reduction. With an increase in the thickness of the TiC film, the opencircuit voltage (V oc ) remained unaffected, whereas the PCE of the DSCs increased from 4.05 to 6.46 %, which was attributed to the increase in the short-circuit current density (J sc ) and the fill factor (FF). When the TiC layer was thicker than 20 μm, the PCE ceased to increase due to a slight decrease in FF. This can be explained as follows: If the TiC film is too thick, the film may crack slightly due to TiC film body stress. This phenomenon has also been observed in a previous study. [32] impedance spectroscopy (EIS) was performed on the symmetrical dummy cells. Figure 5a shows the Nyquist plots of the dummy cells. The high-frequency intercept on the x-axis corresponds to the series resistance (R s ). The left semicircle in the high-frequency region arises from charge transfer (R ct ) and the corresponding capacitance (C μ ) at the electrode/electrolyte interface, whereas the other semicircle in the low-frequency region arises from the Nernst diffusion impedance (Z N ) of the triiodide/iodide in the electrolyte. The EIS parameters shown in Table 2 were calculated by fitting the impedance plots using the Zview software based on an equivalent circuit diagram, as shown in Figure 5b. The four electrodes display similar R s and Z N with variations observed in the values of R ct and C μ. For the thinnest TiC electrode, a large R ct of Ω was observed. However, it decreased drastically to 5.35 Ω on increasing the film thickness, which implies an acceleration of the triiodide reduction rate. The C μ value reveals opposite trends; it increases from 9.03 to μf with increasing film thickness, which indicates an enlargement of the specific surface area of the catalyst. The lower R ct and higher C μ values may result in higher J sc and FF values, leading to better photovoltaic performance. When the TiC film was thicker than 20 μm, no marked difference in the EIS parameters was visible, which confirms that the C and D electrodes have almost the same catalytic activity. Figure 5. (a) Nyquist plots of the dummy cells based on TiC electrodes of different film thicknesses. (b) Equivalent circuit diagram. Figure 4. I V curves of the DSCs using TiC CEs with different film thicknesses. Investigation of the Catalytic Activities of the TiC Electrode with Different Film Thicknesses in an Iodide/ Triiodide Electrolyte by Electrochemical Impedance Spectroscopy and the Tafel-Polarization Test To detect the electrochemical process at the four kinds of TiC electrodes of different thicknesses, electrochemical Table 2. EIS parameters of the dummy cells using TiC electrodes with different film thicknesses. CEs R s [Ω] R ct [Ω] C μ [μf] Z N [Ω] A B C D Tafel polarization curves, a powerful electrochemical characterization measurement, were also derived with the dummy cells to verify the electrochemical catalytic activities Eur. J. Inorg. Chem. 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 of the four CEs. Theoretically, as shown in Figure 6, the Tafel curve can be divided into three zones. The curve at very high potential arises from the limiting diffusion zone, which depends on the transport of triiodide and iodide in the electrolyte. The curve at relatively low potential but higher than 120 mv corresponds to the Tafel zone, in which the voltage (E) is a linear function of the logarithmic current density (logi) [Equation (4), in which R is the gas constant, T is the temperature, α is the distribution coefficient, F is Faraday s constant, and n is the number of electrons involved in the reaction at the electrode]. The curve at very low potential is attributed to the electrochemical reaction, called the polarization zone, in which the voltage (E) is a linear function of current density (I). The slope of this region represents the charge-transfer resistance (R ct ), which is inversely proportional to the exchange-current density (I 0 ), an important indicator for assessing the electrochemical reaction activity [Equation (5)]. With regard to Equation (4), we can acquire information on I 0 in the Tafel zone by extending the line of the voltage to zero, and the current density (I) is then derived from there. Thus, the steep slope of the Tafel zone has a large I 0 and a small R ct value. 2.3RTlogI logi0 E = αnf R ct = RT nfi 0 (5) (4) Y. Wang, M. Wu, X. Lin, A. Hagfeldt, T. Ma scanning was performed with the C electrode. Figure 7 shows the cyclic voltammogram for 10 cycles. The profiles and the peak positions always coincide, which indicates that TiC is an effective and stable catalyst that can catalyze the triiodide reduction. [33] Figure 7. CV curves for the multi-circulative scanning of iodide species using the C electrode. Conclusions TiC exhibits promising properties as a counter-electrode catalyst for DSCs with the advantages of low cost and absence of corrosion. We investigated the effect of the thickness of the TiC CE on the performance of the DSC and found that the DSC fabricated with the TiC CE with an optimal film thickness of 20 μm achieved a high PCE of 6.46%, which matches the performance of a DSC prepared with a Pt CE. Moreover, this research has revealed TiC to be a highly efficient catalyst that can be expected to perform well in the fields of catalytic chemistry, hydrogen generation, fuel cells, and organic matter decomposition. Experimental Section Figure 6. Tafel curves of the dummy cells fabricated with TiC electrodes of different film thicknesses. As shown in Figure 6, in the Tafel zone, large slopes were obtained for the C and D electrodes, which indicates the occurrence of large I 0 and small R ct values for triiodide reduction on the two electrode surfaces, whereas the curves of the A and B electrodes have gentle slopes, which indicates that they cannot behave as well as the other two electrodes in catalyzing the reduction of triiodide. The Tafel results are in good agreement with the EIS experiments and the photovoltaic performance. Stability Test of the TiC Electrode in Iodide/Triiodide Electrolyte Moreover, to study the stability of the TiC electrode in catalyzing the triiodide reduction, CV multi-circulative Preparation of the Photoanode: A semiconducting TiO 2 (Degussa, Germany) film coated on an ultrasonically cleaned FTO glass (Asahi Glass, type-u, 25 Ω/, Japan) was fabricated by the screenprinting technique. The TiO 2 film with a thickness of 12 μm was sintered at 500 C for 30 min. Then the TiO 2 film was cooled to 80 C and subsequently immersed in a m solution of N719 dye (Solaronix SA, Switzerland) in acetonitrile/tert-butyl alcohol (1:1, v/v) for 22 h to obtain the photoanode. Preparation of the Counter Electrode: To obtain a TiC slurry, TiC (500 mg) was dispersed in 2-propanol (10 ml) in an agate pot and then milled with a star ball mill (QM-QX04, Nanjing NanDa Instrument Plant) for 4 h. Next, the prepared TiC slurry was sprayed onto the FTO glass with an airbrush connected to a minicompressor and sintered in a tube furnace under N 2 at 400 C for 30 min to form the TiC CE. The thickness of the TiC film was controlled by varying the quantity of the spray paste. In addition, the Pt CE was prepared according to our previous research. [34 36] Characterization: The thickness of the TiC film coated on the FTO glass was characterized by SEM using the FEI QUANTA450 microscope. The DSCs with an active area of 0.16 cm 2 used for the photovoltaic performance test were fabricated with a photoanode, a CE, and an electrolyte containing 0.06 m LiI, 0.6 m 1-butyl Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012,

5 Dye-Sensitized Solar Cells Based on TiC Counter Electrodes methylimidazolium iodide, 0.03 m I 2, 0.5 m 4-tert-butylpyridine, and 0.1 m guanidinium thiocyanate in acetonitrile. The current voltage (I V) characteristics of the DSCs were measured under simulated AM 1.5 illumination (100 mw cm 2, PEC-L15, Japan) with a Keithley digital source meter (Keithley 2601, USA). The symmetrical dummy cell was assembled by first stacking two identical electrodes facing each other and then filling with the electrolyte similar to the method described above. The cell was used in electrochemical impedance spectroscopy (EIS) experiments and the Tafel polarization test. EIS measurements were carried out in the dark at room temperature with a computer-controlled potentiostat (Zennium Zahner, Germany) over a frequency range of MHz, setting the AC amplitude at 10 mv and the bias at 0.75 V. The EIS spectra were fitted by Zview software. The Tafel polarization curve was derived from an electrochemical workstation system (LK-9805, Tian Jin Lanli Inc.) at a scan rate of 10 mv s 1. Cyclic voltammetry (CV) was carried out in a three-electrode system in an acetonitrile solution containing 0.1 m LiClO 4,10mm LiI, and 1 mm I 2 at a scan rate of 10 mv s 1 with a counter electrode of Pt and an Ag/Ag + reference electrode. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No ). It was also supported by the National High Technology Research and Development Program for Advanced Materials of China (Grant No. 2009AA03Z220). [1] B. O Regan, M. Grätzel, Nature 1991, 353, [2] M. Grätzel, Nature 2001, 414, [3] M. K. Nazeeruddin, A. Kay, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, J. Am. Chem. Soc. 1993, 115, [4] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, [5] X. Fang, T. Ma, G. Guan, M. Akiyama, T. Kida, E. Abe, J. Electroanal. Chem. 2004, 570, [6] A. Hauch, A. Georg, Electrochim. Acta 2001, 46, [7] A. Kay, M. Grätzel, Sol. Energy Mater. Sol. Cells 1996, 44, [8] K. Suzuki, M. Yamamoto, M. Kumagai, S. Yanagida, Chem. Lett. 2003, 32, [9] H. Lindstrom, A. Holmberg, E. Magnusson, S. E. Lindquist, L. Malmqvist, A. Hagfeld, Nano Lett. 2001, 1, [10] E. Ramasamy, W. J. Lee, D. Y. Lee, J. S. Song, Appl. Phys. Lett. 2007, 90, [11] P. Joshi, Y. Xie, M. Ropp, D. Galipeau, S. Bailey, Q. Q. Qiao, Energy Environ. Sci. 2009, 2, [12] M. X. Wu, X. Lin, T. H. Wang, J. S. Qiu, T. L. Ma, Energy Environ. Sci. 2011, 4, [13] S. K. Koo, S. H. Seo, D. Y. Lee, J. Mater. Chem. 2010, 20, [14] T. N. Murakami, S. Ito, Q. Wang, M. K. Nazeeruddin, T. Bessho, I. Cesar, P. Liska, R. Humphry-Baker, P. Comte, P. Péchy, M. Grätzel, J. Electrochem. Soc. 2006, 153, A2255 A2261. [15] J. H. Wu, Q. H. Li, L. Q. Fan, Z. Lan, P. J. Li, J. M. Lin, S. C. Hao, J. Power Sources 2008, 181, [16] Q. H. Li, J. H. Wu, Q. W. Tang, Z. Lan, Q. J. Li, J. M. Lin, L. Q. Fan, Electrochem. Commun. 2008, 10, [17] K.-M. Lee, C.-Y. Hsu, P.-Y. Chen, M. Ikegami, T. Miyasaka, K.-C. Ho, Phys. Chem. Chem. Phys. 2009, 11, [18] H. Q. Jiang, S. Sakurai, K. Kobayashi, Electrochem. Solid- State Lett. 2009, 12, F13 F16. [19] Q. Tai, B. Chen, F. Guo, S. Xu, H. Hu, B. Sebo, X.-Z. Zhao, ACS Nano 2011, 5, [20] M. K. Wang, A. M. Anghel, B. Marsan, N. C. Ha, N. Pootrakulchote, S. M. Zakeeruddin, M. Grätzel, J. Am. Chem. Soc. 2009, 131, [21] Q. W. Jiang, G. R. Li, X. P. Gao, Chem. Commun. 2009, [22] G. R. Li, F. Wang, Q. W. Jiang, X. P. Gao, P. W. Shen, Angew. Chem. 2010, 122, 3735; Angew. Chem. Int. Ed. 2010, 49, [23] M. X. Wu, X. Lin, A. Hagfeldt, T. L. Ma, Angew. Chem. 2011, 123, 3582; Angew. Chem. Int. Ed. 2011, 50, [24] M. X. Wu, X. Lin, A. Hagfeldt, T. L. Ma, Chem. Commun. 2011, 47, [25] M. X. Wu, Q. Y. Zhang, J. Q. Xiao, C. Y. Ma, X. Lin, C. Y. Miao, Y. J. He, Y. R. Gao, A. Hagfeldt, T. L. Ma, J. Mater. Chem. 2011, 21, [26] Q. W. Jiang, G. R. Li, S. Liu, X. P. Gao, J. Phys. Chem. C 2010, 114, [27] M. X. Wu, Y. D. Wang, X. Lin, N. Yu, L. Wang, L. L. Wang, A. Hagfeldt, T. L. Ma, Phys. Chem. Chem. Phys. 2011, 13, [28] X. Lin, M. X. Wu, Y. D. Wang, A. Hagfeldt, T. L. Ma, Chem. Commun. 2011, 47, [29] Y. D. Wang, M. X. Wu, X. Lin, Z. C. Shi, A. Hagfeldt, T. L. Ma, J. Mater. Chem. 2012, 22, [30] E. Ramasamy, J. Lee, Carbon 2010, 48, [31] R. S. Nicholson, Anal. Chem. 1965, 37, [32] M. X. Wu, X. Lin, T. H. Wang, J. S. Qiu, T. L. Ma, Energy Environ. Sci. 2011, 4, [33] H. C. Sun, D. Qin, S. Q. Huang, X. Z. Cuo, D. M. Li, Y. H. Luo, Q. B. Meng, Energy Environ. Sci. 2011, 4, [34] T. L. Ma, X. M. Fang, M. Akiyama, K. Inoue, H. Noma, E. Abe, J. Electroanal. Chem. 2004, 574, [35] X. M. Fang, T. L. Ma, G. Q. Guan, M. Akiyama, E. Abe, J. Photochem. Photobiol. A: Chem. 2004, 164, [36] X. M. Fang, T. L. Ma, M. Akiyama, G. Guan, S. Tsunematsu, E. Abe, Thin Solid Films 2005, 472, Received: March 31, 2012 Published Online: June 22, 2012 Eur. J. Inorg. Chem. 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim