Electron transfer dynamics in dye sensitized nanocrystalline solar cells using a polymer electrolyte

Transient absorption spectroscopy was employed to study electron-transfer d ynamics in dye sensitized nanocrystalline solar cells incorporating a polym er electrolyte, poly (epichlorohydrin-co-ethylene oxide) containing NaI and I-2, Solar cells employing this solid-state electrolyte have yielded solar to electrical energy conversion efficiencies of up to 2.6%. Electron-trans fer kinetics were collected as a function of electrolyte composition, white light illumination, and device voltage and correlated with current/voltage characterization of the cell. The yield of electron injection from the dye excited state into the Tio, electrode was found to be insensitive to elect rolyte composition or cell operating conditions. Regeneration of the dye gr ound state by electron transfer from I- ions in the polymer electrolyte exh ibited half times of 4-200 Ais, depending upon the concentration of NaI in the polymer electrolyte. A long-lived product of the regeneration reaction was observed and assigned to the I-2(-) radical. At low NaI concentrations, kinetic competition was observed between this regeneration reaction and ch arge recombination of the oxidized dye with electrons injected into the sem iconductor. The decay kinetics of the dye cation, and the yield Of I-2(-), were found to be unchanged by illumination of the cell under either short c ircuit or open circuit (V-oc = 0.75 V) conditions, From these observations, we conclude that the charge recombination dynamics in this cell are not st rongly dependent upon the TiO2 Fermi level over this voltage range. Analogy with studies of recombination dynamics in three electrode photoelectrochem ical cells employing a redox inactive liquid electrolyte suggest this obser vation may be related to the Lewis base nature of the polymer employed.