Tuning Spectral and Electrochemical Properties of Porphyrin-Sensitized Solar Cells

Phenylethynyl-substituted porphyrin (PE1) sensitizers bearing a nitro, cyano, methoxy, or dimethylamino phenylethynyl substituent were prepared to examine the electron-donating or -withdrawing effects of dyes on the photovoltaic performance of the corresponding dye-sensitized solar cells. The overall efficiencies of power conversion of the devices show a systematic trend Me2N-PE1 > MeO-PE1 > CN-PE1 > NO2-PE1, for which Me2N-PE1 has a device performance about 90% of that of a N719-sensitized solar cell under the same experimental conditions. The superior performance of Me2N-PE1 is attributed to the effective electron-donating property of the dye that exhibits broadened and red-shifted spectral features. According to frontier orbitals based on DFT calculations, the electrons are effectively injected from the dye to TiO2 for Me2N-PE1 and MeO-PE1 upon excitation, but that driving force reverses for NO2-PE1. Electrochemical tests indicate that both LUMO and HOMO levels show a systematic trend Me2N-PE1 > MeO-PE1 > CN-PE1 > NO2-PE1, consistent with the trend of variation of the short-circuit currents in this series of sensitizers.