Electron transport and back reaction in dye sensitised nanocrystalline photovoltaic cells

The transport and back reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, D_n, as a function of illumination intensity. It was found that D_n increased with intensity (D_n∝I^−0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τ_n, which is determined by the rate of back reaction with I₃⁻ ions in the electrolyte. It was found that τ_n decreased with light intensity (τ_n∝I^−0.5). The electron diffusion length, L_n=(D_nτ_n)^1/2, is therefore only weakly dependent on light intensity. The values of L_n were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in D_n and τ_n are discussed and related to the fundamental processes taking place in the DSNC.