An investigation of the photosubstitution reaction between N719-dyed nanocrystalline TiO2 particles and 4-tert-butylpyridine

The effect of high concentrations of the solar cell additive 4-tert-butylpyridine (4-TBP) on the stability and lifetime of the sensitizer [Ru(Hdcbpy)2(NCS)2]2−, 2(n-C4H9)4N+, (H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid), known as N719, has been evaluated based on kinetic data obtained from simple model experiments. In these experiments, colloidal solutions of N719-dyed nanocrystalline TiO2 particles in acetonitrile were irradiated with 532-nm laser light in the presence of 0–1 mol/l of 4-TBP. Five degradation products were identified using LC–ESI-MS: the 4-tert-butylpyridine substitution product [Ru(H2dcbpy)(Hdcbpy)(NCS)(4-TBP)] (SP) and the products [Ru(H2dcbpy)2(NCS)(CN)], [Ru(H2dcbpy)(Hdcbpy)(NCS)(CH3CN)], [Ru(H2dcbpy)(Hdcbpy)(NCS)(H2O)] and [Ru(H2dcbpy)2(CN)2] (3–6). The sum of the quantum yields of the five products, Φdeg = (1.3 ± 0.2) × 10−4, was found to be independent of 4-TBP concentration. Based on this observation, a degradation mechanism was proposed, in which the reaction proceeds through the rate-determining formation of a common intermediate complex, I = [RuII(H2dcbpy)2(NCS)(NCS·)]+. An average degradation rate of kdeg = 6 × 10−3 s−1 was obtained from the value of Φdeg and the back electron-transfer rate, kback of the reaction TiO2 + e− | N719+ →TiO2 | N719, obtained by means of photo-induced absorption (PIA) measurements. The lifetime of the solar cell sensitizer N719 was estimated to be between 34 years, based on kdeg and an average literature value of the regeneration rate, kreg = 2 × 106 M−1 s−1, of the reaction between TiO2 | N719+ and iodide. We conclude that the addition of 4-TBP to dye-sensitized solar cells (DSSC) does not decrease the lifetime of the N719 dye during normal solar cell operation at room temperature.