Direct Estimation of the Electron Diffusion Length in Dye-Sensitized Solar Cells

The diffusion length is a key parameter that controls the electron collection efficiency in dye-sensitized solar cells (DSCs). In this work, we carry out a direct estimation of this parameter by means of the laser beam-induced current (LBIC) technique. The DSC devices are prepared on transparent conducting glass substrates, which were divided in two electrically isolated parts by means of a groove. The LBIC measurement is conducted by moving a highly focused laser spot over the DSC across the groove and monitoring the open-circuit voltage yielded by the solar cell. The resulting voltage profile can be fitted to a simple diffusion-recombination model such that the electron diffusion length can be extracted. Measurements carried out on DSC with various oxides (TiO2/ZnO) and electrolytes (organic, ionic-liquid) yield diffusion lengths in the 10-35 mu m range, with longer values found for higher illumination and for cells of better efficiency.