Electron injection efficiency and diffusion length in dye-sensitized solar cells derived from incident photon conversion efficiency measurements

Injection efficiency, η_inj and diffusion length, L, in dye-sensitized solar cells have been derived from the spectral response (incident photon to current efficiency, IPCE) of the cells under front side or backside illumination. Values of L from IPCE are found to be ~2 times shorter than the values of L derived from the normal small perturbation transient method. IPCE-derived values of L (2 to more than 40 μm) and η_inj (63-90%) are found to correlate with the photocurrent (and indirectly with the photovoltage) of the different cells indicating the extent to which each factor limits the cell efficiency. IPCE spectra varied with light intensity, so that diffusion lengths derived from both methods show similar trends, e.g., L from IPCE is found to increase 3 times when the light intensity is increased 10 times up to approximately 0.1 sun where L tends to plateau or peak. The values for η_inj derived from the spectral response are shown to be in quantitative agreement with those determined from picosecond transient emission spectroscopy. To illustrate the utility of this method, L and η_inj were measured on cells with and without the TiCl ₄ chemical bath treatment. The results show that the increase in photocurrent after the TiCl₄ treatment is due to around a 2-fold increase in L despite a 3-fold reduction in the electron diffusion coefficient. The increased L can be explained by a factor of 10 decrease in electron recombination rate.