Evaluation of the charge-collection efficiency of dye-sensitized nanocrystalline TiO2 solar cells

Intensity modulated photovoltage spectroscopy (IMVS) and intensity modulate d photocurrent spectroscopy (IMPS) are used to evaluate the charge-collecti on efficiency of dye-sensitized nanocrystalline TiO2 solar cells. The charg e-collection efficiency of the photoinjected electrons from dye sensitizati on is estimated from the respective time constants for charge recombination at open circuit tau(oc) and the combined processes of charge collection an d charge recombination at short circuit tau(sc) obtained by IMVS and IMPS m easurements. Three models are developed for relating the charge-collection efficiency to tau(oc)/tau(sc). The first model determines the charge-collec tion efficiency from tau(oc)/tau(sc) without considering the underlying phy sical processes measured by IMVS and IMPS. The second model obtains tau(oc) /tau(sc) by simulating the frequency response of IMVS and IMPS from the tim e-dependent continuity equation for simplified conditions. The third model determines the time constants for IMVS and IMPS from electron-concentration profiles calculated for constant light intensity and more realistic condit ions. To obtain a realistic steady-state electron concentration profile, a nonlinear dependence of the rate of recombination on the electron concentra tion in the TiO2 film is considered. Furthermore, the continuity equation i s modified to account for charge trapping and detrapping. For the first tim e, expressions are derived for calculating the time constants from the stea dy-state electron concentration profile. The validity of this method is dem onstrated for the second model from which the exact IMPS and IMVS responses are calculated. The three models are compared with each other. A simple ex pression is derived for calculating the charge-collection efficiency from t he measured values of tau(oc)/tau(sc) and the light intensity dependence of tau(oc).