Dye stability and performances of dye-sensitized solar cells with different nitrogen additives at elevated temperatures-Can sterically hindered pyridines prevent dye degradation?

The homogeneous kinetics of the nucleophilic substitution reactions between the ruthenium dye N719 and eight pyridines and 1-methylbenzimidazole have been investigated in 3-methoxypropionitrile at 100 degrees C. The half lives of N719 with the additives 4-tert-butylpyridine (0.5 M) and 1-methylbenzimidazole (0.5 M) were 57 and 160 h, respectively. Sterically hindered pyridines like 2,6-lutidine did not react with N719. The efficiencies of dye-sensitized solar cells (DSC, area = 8.0 cm(2)) prepared with 1-methylbenzimidazole (MBI), 4-tert-butylpyridine (4-TBP), 2,6-lutidine and without any additive were 7.1%, 6.2%, 6.0% and 4.8%, respectively. The cells were stored in dark at 85 degrees C and their I-V curves and impedance spectra were measured at regular time intervals. The N719 dye degradation in the cells were monitored by a new dye extraction protocol combined with analysis of the dye extract by HPLC coupled to mass spectrometry. After 300 h storage in dark at 85 degrees C 40% of the initial amount of N719 dye was degraded in DSC cells prepared with MBI and the efficiency was decreased to 40% of its initial value. DSC cells prepared with 2,6-lutidine or no additives showed smaller thermal dye and efficiency stability at elevated temperatures than DSC cells prepared with the none sterically hindered additives MBI and 4-TBP. In the cells prepared with 2,6-lutidine or no additive higher contents of the iodo products [RuL2(NCS)(iodide)](+) and [RuL2(3-MPN)(iodide)](+) were found than in cells prepared with 4-TBP and MM. It is suggested that sterically hindered pyridines have smaller complexation constants with l than unsterically hindered additives. This may explain the observed faster nucleophilic substitution rates of uncomplexed I-3(-) with N719 in DSC cells prepared with sterically hindered pyridines. The EIS analysis showed that the lifetime of the injected electrons in the TiO2 tau(eff) is reduced by a thermally induced change in the TiO2 vertical bar dye vertical bar electrolyte interface. (C) 2010 Elsevier B.V. All rights reserved.