Role of nanochemical environments in porous TiO2 in photocurrent efficiency and degradation in dye sensitized solar cells

Elongated dye sensitized solar cells with a thickness gradient of the nanoporous TiO2 front electrode were used to assess the impact of the layer thickness on photocurrent and degradation. The photocurrent efficiency passes through a maximum (in our case at about 12 mu m). Interestingly, the degradation rate also strongly depends on the layer thickness and is about 3 times faster for a 15-mu m cell (in comparison with a 1-mu m cell). To explain these nonanticipated results, a model to describe the I-3(-/I-) concentration within a typical dye sensitized solar cell under steady-state conditions was derived. It includes the nanoporous TiO2 layer and a bulk solution with their different mobilities for the electrolyte species. Using typical parameters from the literature, it turned out that, despite the fact that the initial I- concentration is about 1 order of magnitude larger and the assumed diffusion coefficient is 1.3 times higher, the depletion of the I- concentration at the TiO2/FTO front contact happens to be in the same range as the depletion of the 13 concentration at the back contact. This stresses the importance of iodide in nanoporous environments for both the maximum attainable photocurrent and its role in the regeneration of the oxidized dye. Enhanced degradation rates might be related to poor iodide supply, since the oxidized state cannot be regenerated efficiently.