Diffusion Length in Nanoporous Photoelectrodes of Dye-Sensitized Solar Cells under Operating Conditions Measured by Photocurrent Microscopy.

We determined the carrier diffusion lengths in nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy. The diffusion lengths were found to be 60-100 µm for the conventional cells. In addition, we found a correlation between the carrier diffusion lengths and the cell efficiency, which proved that improvement in the diffusion length is one of the crucial factors for optimizing device performance. The diffusion length was measured for various operating conditions by varying parameters such as solar light intensity and applied electrical voltage. In particular, we observed electric-field-driven, carrier transport phenomena (i.e., drift current) in modified cells. Fitting with the drift-diffusion model enabled us to extract the electric field strengths present in the TiO2 nanoporous layer.