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Effect on Cell Efficiency following Thermal Degradation of Dye-Sensitized Mesoporous Electrodes Using N719 and D5 Sensitizers Kristofer Fredin,*,† Kenrick F. Anderson,‡ Noel W. Duffy,§ Gregory J. Wilson,‡ Christopher J. Fell,‡ Daniel P. Hagberg,| Licheng Sun,| Udo Bach,† and Sten-Eric Lindquist‡ Materials Engineering, Monash UniVersity, Clayton Campus, VIC 3800, Australia, CSIRO Energy Technology, Mayfield West, NSW 2304, Australia, CSIRO Energy Technology, Clayton Laboratories, VIC 3169, Australia, and Royal Institute of Technology (KTH), Teknikringen 30, 114 27 Stockholm, Sweden ReceiVed: April 14, 2009; ReVised Manuscript ReceiVed: August 25, 2009
This work examines the comparative durability of two common dyes at temperatures that may be experienced during fabrication of dye-sensitized solar cells (DSCs) such as through the application of thermoplastics for encapsulation or the use of a molten solid-state hole conductor. Dye-sensitized electrodes were heated in an atmosphere of air or nitrogen and thereafter used as working electrodes in DSCs. Electrodes sensitized with N719 appeared more sensitive to thermal degradation than electrodes sensitized with D5, although absorbance measurements suggest similar first-order degradation rates for the two dyes. Intensity modulated photovoltage spectroscopy and intensity modulated photocurrent spectroscopy were used to measure the effect of heating on electron lifetime and transport. It was found that the electron diffusion length may be as low as 10% for heated samples, compared to that of the unheated counterpart, and therefore, we assess recombination as an additional efficiency limiting process in our experiments. Introduction Dye-sensitized solar cells (DSCs) have attained widespread attention for more than a decade because of their potential as a low cost alternative to conventional silicon solar cells.1-5 The photoactive electrode of a DSC consists of a mesoporous semiconductor film on a transparent, conducting substrate. The electrode forms part of a photoelectrochemical cell, for which the electrolyte fills the pores within the film. Absorption of sunlight is achieved by coating the internal surface of the pores with a sensitizer, typically a dye. Although many dyes have been investigated for this design of solar cell, inorganic ruthenium-based dyes, for example N3 and N719,2,6 still remain among the most effective when I-/I3- is used as the redox couple. One concern for practical DSC applications is the standard use of organic solvent electrolytes because of longterm durability problems such as leakage. Promising alternative electrolytes include room temperature ionic liquids,7-9 polymers,10 and solid-state hole conductors,11-14 however, devices incorporating these materials have not yet matched the best power conversion efficiencies reported for solvent-based DSCs. One issue for consideration by DSC developers is the potential for loss of performance due to thermal degradation of the sensitizing dye that may occur when the device is heated after the dye is applied. Examples of such heat treatments include sealant application and, recently, the use of a solid-state hole conducting material via a melt process.14 Herein, we have investigated the influence of heat treatment of dye-sensitized electrodes with respect to the resultant photovoltaic efficiency when used as working electrodes in DSCs. For the purpose of comparison, two different dyes were examined: the widely used * To whom correspondence should be addressed. E-mail: kristofer.
[email protected]. Pone: +61 3 9905 4924. Fax: +61 3 9902 0325. † Monash University. ‡ CSIRO Energy Technology, Mayfield West. § CSIRO Energy Technology, Clayton Laboratories. | Royal Institute of Technology (KTH).
Figure 1. Molecular structures of the N719 and D5 sensitizers.
inorganic ruthenium complex cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719) and the more recently developed organic molecular 3-(5-(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D5)15 (Figure 1). In the work presented here, it is expected that different cell performance parameters will be observed due to the different structural and chemical natures of the sensitizers, i.e., N719 is a metal complex, while D5 is an organic chromophore. Experimental Methods Glass substrates with a fluorine-doped tin oxide (FTO) coating were cleaned and coated with a dense TiO2 blocking layer using a spray pyrolysis method.16 Areas being used for contacts were masked during the procedure. Mesoporous films 4 mm × 4 mm were deposited by screen printing TiO2 paste onto the blocking layer. The thickness of the mesoporous films after sintering was 6 µm. Separate samples were prepared for absorbance measurements; these had no blocking layer, were 7 mm × 7 mm, and were approximately 1.5 µm thick. All films were subsequently sintered at 500 °C for 30 min. Electrodes being used in cells were then post-treated with TiCl417,18 and immersed while still warm overnight in a dye bath containing 0.5 mM N719 in a 1:1 mixture of acetonitrile and tert-butyl alcohol. A second set
10.1021/jp9055208 CCC: $40.75 2009 American Chemical Society Published on Web 10/05/2009
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Figure 2. Cell performance as a function of heat treatment temperature, T, as shown by the selected properties: (a) maximum solar power conversion efficiency, η; (b) short-circuit current, jsc; (c) open-circuit voltage, Voc; and (d) fill factor, ff. Filled boxes (9) represent N719 treated under ambient air. Filled circles (b) represent N719 treated under nitrogen. Open boxes (0) represent D5 treated under ambient air. Open circles (O) represent D5 treated under nitrogen. Straight and dotted lines represent N719 and D5 references at ambient air conditions.
of electrodes were treated in a similar manner in a dye bath containing 0.25 mM D5 in acetonitrile. Dye soaking for the samples for absorbance measurements was similar, except that the D5 samples had a reduced soaking time of 1 min. Electrodes subject to heat treatment (henceforth referred to as treated electrode and subsequently treated cell) were placed on a hot plate (IKA C-MAG HS7) that had stabilized at the treatment temperature and left for 5 min. Two sets of samples were treated, one set under ambient air conditions and the other set under a nitrogen atmosphere. The electrodes were protected from exposure to light during heating and cooling. A number of control electrodes were also prepared without heat treatment, for the purpose of comparison. Cell assembly involved the application of a 25 µm thick thermoplastic gasket (Dupont SURLYN) to the electrodes and pressing on the normal DSC counter electrode at 120 °C. This step was conducted as quickly as possible (approx 30 s) to minimize the impact of this heating step on the experimental results. The electrolyte, comprising 0.03 M I2, 0.5 M 4-tertbutylpyridine, 0.6 M 1-butyl-3-methylimidazolium iodide, and 0.1 M guanadinium thiocyanate in an 85:15 volume mixture of acetonitrile and valeronitrile,19 was introduced via a predrilled hole in the counter electrode using a backfilling procedure, after which the hole was sealed. Current-voltage (I-V) characteristics for the cells were measured at an irradiance of 1 sun using a Newport Class A
solar simulator and calibrated silicon reference cell. The reference cell is fitted with a color filter to approximate the spectral response of the devices, and hence, the effect of spectral mismatch is expected to be small, for the purpose of this experiment. In an earthed Faraday dark-box, the cells were illuminated with a small sinusoidal intensity modulation, superimposed (∼1%) on a much larger constant illumination level, using a light-emitting diode (λ ) 470 nm). The characteristic intensity modulated photovoltage spectroscopy (IMVS) time constant, τIMVS,20,21 was obtained from the photovoltage response as a function of modulation frequency. In a similar way, the characteristic intensity modulated photocurrent spectroscopy (IMPS) time constant, τIMPS,22,23 was obtained from the photocurrent response. Measurements of τIMVS and τIMPS were taken at a number of different photovoltage and photocurrent levels, which were set by varying the irradiance, using neutral density filters and a calibrated silicon photodiode as reference. Measurements of dye absorption were performed using a Varian 5000 UV-vis-NIR spectrophotometer. Absorbance was calculated from transmittance measurements, with a pristine (nondyed) mesoporous TiO2 electrode on FTO glass used as the baseline reference. An integrating sphere was used to ensure complete capture of the transmitted light.
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Results and Discussion The solar power conversion efficiency, η, was calculated from the I-V characteristics for each test cell using the standard method. The efficiencies for test cells treated at various temperatures as well as their corresponding short-circuit currents, jsc, open-circuit voltages, Voc, and fill factors, ff, are shown in panels a-d of Figures 2, where the values for the control samples are presented by horizontal markers. The efficiencies measured for the N719 and D5 control samples were 5.3% and 4.5%, respectively, as displayed in Figure 2a. While cells from the N719 and D5 test series exhibit progressively poorer performance when treated to higher temperatures, the N719 devices appear to be affected significantly more than the D5 devices. For example, after heating at 150 °C, N719 sample efficiency has reduced approximately 50%, while D5 sample efficiency has reduced by less than 5%. The presented results also show that that the reduction in cell efficiency is less pronounced for cells treated under low partial pressure of oxygen and moisture in comparison to an ambient air atmosphere, when heated to 200 °C and above. However, the exact surface chemistry of the TiO2-dye interface remains to be investigated for dye-sensitized electrodes heated under various conditions. The reduced efficiency for heat-treated electrodes occurs through reductions in jsc and Voc, whereas ff remains practically unchanged (Figures 2b-d). Of value in understanding the comparative degradation mechanisms for the two dyes is the issue of whether the reduction in electrode efficiency is driven by reduced dye absorption or dominated by some other mechanism such as increased recombination or reduced charge injection efficiency. We have used absorbance spectroscopy to examine electrodes heat treated to 120 and 200 °C. Because of the strong absorbance of the D5 molecule, the dye soaking time for the D5 samples is reduced to 1 min to allow for a meaningful interpretation of the absorbance data. We represent the effect of heat treatment on absorbance by subtracting the data for heat-treated electrodes from the data for the corresponding untreated electrode. The absorbance data for the unheated dye-sensitized electrodes are shown in Figure 3a, and the resulting differential absorbance data are shown in panels b and c of Figure 3 for N719 electrodes and D5 electrodes, respectively. The data show that peak absorbance decreases with treatment temperature for both dyes, and that the effect is of a similar magnitude for each. For both dyes, peak absorbance of the unheated electrode is around 0.35, and in both cases, the peak value decreases by approximately ∆A ) 0.05 for the 120 °C treatment and by approximately ∆A ) 0.15 for the 200 °C treatment. Provided that neither dye produces decomposition products absorbing close to the peak and assuming first-order kinetics, this suggests that the rate constants for degradation are similar for N719 and D5. Considering the different molecular structures for these dyes, this is a notable observation. Although it is clear that a reduced light-harvesting efficiency has a negative impact on the short-circuit current and opencircuit voltage, eventually leading to reduced efficiency, a quantitative estimation relating to the effect is however difficult considering the shorter soaking time for the samples sensitized with D5 during the absorbance measurements, and the fact that the irradiation spectrum of the solar simulator does not allow for a simple interpretation of the absorbance data. However, we can calculate that a 1.5 µm thick film sensitized with N719 absorbs 55% of the incident light at the absorption peak, which implies that a 6 µm thick film absorbs 96%, using the
Figure 3. (a) Absorbance spectrum of the N719 (straight line) and D5 (dotted line) unheated dye sensitized electrodes. (b) Difference in absorbance for films sensitized with N719 and heat treated at 120 °C (solid) and 200 °C (dashed) compared to the control film. (c) Difference in absorbance for films sensitized with D5 and treated at 120 °C (solid) and 200 °C (dashed) compared to the control film.
Lambert-Beer relationship. In a similar way, a 6 µm thick film heated at 150 °C absorbs 94%, and heat treated at 200 °C, the sample would still absorb 84% of the incident light at the absorption peak. Considering that jsc is reduced from 10.0 to 5.9 mA cm-2 and 2.4 mA cm-2 for samples heat treated at 150 and 200 °C, respectively, we find it plausible that there are additional effects on the efficiency limiting parameters, eventually leading to a more strongly reduced jsc, for both sensitizers. Further, stronger overall absorbance in the D5 electrodes may have meant that utilization of the incident light was still effective after some of the dye has degraded, leading to an appearance that the D5 dye is less affected by heating than the N719.
Degradation of N719 and D5 Sensitizers
Figure 4. IMVS time constant dependence on Voc for cells sensitized with N719 (a) and D5 (b) and heat treated to 150 °C (b), 200 °C (2), and 250 °C (1). Data for the reference cell is represented by 9.
The influence of heating on the electron recombination rate was studied using the IMVS technique. The measured values of the characteristic time constant, τIMVS, with respect to Voc, are presented in panels a and b of Figures 4 for the N719 and the D5 cells, respectively. As is shown for both dyes, the trendlines for the treated cells are progressively shifted toward lower values of τIMVS, spanning several orders of magnitude, demonstrating that heat treatment results in a shorter electron lifetime in the device. The influence of heating on the electron transport rate was studied using the IMPS technique. The measured values of the characteristic time constant, τIMPS, with respect to jsc are presented in panels a and b of Figure 5 for the N719 and D5 cells, respectively. As shown in Figure 5a, the trendlines are shifted toward lower values of τIMPS, implying a faster electron transport and hence an increased electron diffusion coefficient in heated cells sensitized with N719. In Figure 5b, we see no such discernible difference, and therefore, the effect of heating on the electron diffusion coefficient in cells sensitized with D5 is viewed as small or negligible. A reduced electron lifetime may have an effect on the electron diffusion length,24-27 provided that a shorter lifetime is not compensated for by an increased electron diffusion coefficient. For cells sensitized with N719, the electron lifetime decreases by up to 4 orders of magnitude for heated cells. Despite that the electron diffusion coefficient increases with up to 2 orders of magnitude, the product still decreases with an order of 2, in
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Figure 5. IMPS time constant dependence on jsc for cells sensitized with N719 (a) and D5 (b) and heat treated to 150 °C (b), 200 °C (2), and 250 °C (1). Data for the reference cell is represented by 9.
similarity to cells sensitized with D5. Therefore, the electron diffusion length may be as low 10% for heated samples, compared to their unheated counterpart. With regard to this significant reduction, we assess a reduced electron diffusion length, equivalent to an increased recombination current, as an additional efficiency limiting factor in our experiments. Conclusions Mesoporous electrodes sensitized with N719 and D5 were subjected to different elevated temperatures in the range 120-250 °C for 5 min under air and nitrogen atmospheres and then tested as working electrodes in dye-sensitized solar cells. Heated electrodes sensitized with N719 displayed reduced efficiency, and a clear relationship with the heating temperature was observed. For electrodes sensitized with D5, mild heat treatment had only a minor influence on the efficiency, but above 200 °C the effect was more severe. The effect of heating on cell efficiency was found to be less pronounced for electrodes treated under low partial pressure of oxygen and moisture in comparison to an ambient air atmosphere. Absorbance measurements showed that a reduction in shortcircuit current should be expected for heated samples due to a reduced light harvesting efficiency of the sensitizers. Further, the data also suggests that the degradation rate is similar for N719 and D5, which is a notable observation considering their different molecular structures.
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Intensity modulated photovoltage spectroscopy and intensity modulated photocurrent spectroscopy were used to measure the effect of heating on electron lifetime and transport. It was found that the electron lifetime is shorter in the heat-treated samples for both sensitizers, and that the electron transport is faster in heat-treated samples sensitized with N719, whereas no discernible difference is measured for the cells sensitized with D5. It follows that the electron diffusion length may be as low as 10% for samples heated to 250 °C, compared to that of the unheated counterpart, and therefore, we assess recombination as an additional efficiency limiting process in our experiments. Acknowledgment. This work was made possible in part through funding from the following organizations: Swedish Research Council, Australian Research Council, Australia’s International Consortium for Organic Solar Cells (ICOS), Victorian Consortium for Organic Solar Cells (VICOSC), and Commonwealth Scientific and Industrial Research Organisation (CSIRO). We also thank JGC Catalysts and Chemicals, Ltd., Kitakyushu-Shi (Japan), for providing samples of the TiO2 screen-printing paste. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (3) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (4) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, 638. (5) Kroon, J. M.; Bakker, N. J.; Smit, H. J. P.; Liska, P.; Thampi, K. R.; Gra¨tzel, M.; Hinsch, A.; Hore, S.; Durrant, J. R.; Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. New Concepts and Materials for World-Class Dye Sensitized Solar Cells, 19th European Photovoltaic Solar Energy Conference, Paris, June 7-11, 2004. (6) Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Gra¨tzel, M. J. Phys. Chem. B 2003, 107, 8981.
Fredin et al. (7) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Gra¨tzel, M. J. Electrochem. Soc. 1996, 143, 3099. (8) Wang, P.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. Chem. Mater. 2004, 16, 2694. (9) Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Chem. Commun. 2005, 740. (10) Zhang, F.; Jespersen, K.; Bjo¨rstro¨m, C.; Svensson, M.; Andersson, M.; Sundstro¨m, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Ingana¨s, O. AdV. Funct. Mater. 2006, 16, 667. (11) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (12) Schmidt-Mende, L.; Zakeeruddin, S.; Gra¨tzel, M. App. Phys. Lett. 2005, 86, 013504. (13) Schmidt-Mende, L.; Gra¨tzel, M. Thin Solid Films 2006, 500, 296. (14) Fredin, K.; Johansson, E.; Blom, T.; Hedlund, M.; Leifer, K.; Rensmo, H. Synt. Met. 2009, 159, 166. (15) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Comm. 2006, 2245. (16) Kavan, L.; Gra¨tzel, M. Electrochim. Acta 1995, 40, 643. (17) Sommeling, P. M.; O’Regan, B. C.; Haswell, R. R.; Smit, H. J. P.; Bakker, N. J.; Smits, J. J. T.; Kroon, J. M.; van Roosmalen, J. A. M. J. Phys. Chem. B 2006, 110, 19191. (18) O’Regan, B. C.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J. J. Phys. Chem. C 2007, 111, 14001. (19) Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M.; Liska, P.; Pechy, P.; Gra¨tzel, M. Prog. PhotoVolt. 2007, 15, 603. (20) Schlichtho¨rl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8139. (21) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 8916. (22) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281. (23) Fisher, A. C.; Peter, L. M.; Ponomarev, E. A.; Walker, A. B.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 949. (24) Nissfolk, J.; Fredin, K.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. B 2006, 110, 17715. (25) Snaith, H. J.; Moule, A. J.; Klein, C.; Meerholz, K.; Friend, R. H.; Gra¨tzel, M. Nano Lett. 2007, 7, 3372. (26) Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O’Regan, B. C. J. Phys. Chem. C 2009, 113, 1126. (27) Peter, L. M. J. Phys. Chem. C 2007, 111, 6601.
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