Dependence of the photocurrent conversion efficiency of dye-sensitized solar cells on the incident light intensity

The incident photon to current conversion efficiency (IPCE) of dye-sensitized solar cells is measured as a function of the incident photon current density. A characteristic dependence of the IPCE on the incident light intensity with a maximum value for absorbed photon current densities in the range of j(gamma ,abs) = 10(17) cm(-2) s(-1) is observed for all cells investigated in this study. The IPCE decreases significantly for large absorbed photon current densities (j(gamma ,abs) > 10(18) cm(-2) s(-1)) and for small absorbed photon current densities (j(gamma ,abs) < 10(16) cm(-2) s(-1)). The decrease of the IPCE at large incident light intensities is due to the diffusion limitation of the electrolyte within the nanocrystalline TiO2 electrode. The decrease of the IPCE at low incident light intensities can be explained by the loss of photogenerated electrons via surface-state-mediated electron transfer into the electrolyte. The strong dependence of the IPCE on the incident light intensity has consequences for the efficiency of a dye-sensitized solar cell at low illumination levels. It also suggests that spectral measurements of the IPCE of dye-sensitized solar cells and of nanocrystalline electrodes cannot be used to predict the solar or white light IPCE unless they were taken with bias light or with very high intensities of monochromatic light, especially at weakly absorbed wavelengths.