Electron Transfer Dynamics in Dye Sensitized Nanocrystalline Solar Cells Using a Polymer Electrolyte

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Abstract

Transient absorption spectroscopy was employed to study electron-transfer dynamics in dye sensitized nanocrystalline solar cells incorporating a polymer electrolyte, poly(epichlorohydrin-co-ethylene oxide) containing NaI and I2. Solar cells employing this solid-state electrolyte have yielded solar to electrical energy conversion efficiencies of up to 2.6%. Electron-transfer 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 TiO2 electrode was found to be insensitive to electrolyte composition or cell operating conditions. Regeneration of the dye ground state by electron transfer from I- ions in the polymer electrolyte exhibited half times of 4−200 μs, depending upon the concentration of NaI in the polymer electrolyte. A long-lived product of the regeneration reaction was observed and assigned to the I2- radical. At low NaI concentrations, kinetic competition was observed between this regeneration reaction and charge recombination of the oxidized dye with electrons injected into the semiconductor. The decay kinetics of the dye cation, and the yield of I2-, were found to be unchanged by illumination of the cell under either short circuit or open circuit (Voc = 0.75 V) conditions. From these observations, we conclude that the charge recombination dynamics in this cell are not strongly dependent upon the TiO2 Fermi level over this voltage range. Analogy with studies of recombination dynamics in three electrode photoelectrochemical cells employing a redox inactive liquid electrolyte suggest this observation may be related to the Lewis base nature of the polymer employed.

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