TiO2- and ZnO-based solar cells using a chlorophyll a derivative sensitizer for light-harvesting and energy conversion

TiO2- and ZnO-based solar cells sensitized by a chlorophyll a derivative (methyl trans-32-carboxy-pyropheophorbide a) were fabricated and compared. The TiO2-based solar cell produces higher values for the short-circuit photocurrent (Jsc), open-circuit photovoltage (Voc), and energy-to-electricity conversion efficiency (η) than the ZnO-based solar cell. The observed ATR-FTIR data on the dye-sensitized semiconductor electrodes and the spectra estimated from the density functional theory (DFT) suggest that the dye sensitizer is bound to TiO2 with both the bidentate chelating and monodentate modes but is bound to ZnO with the monodentate mode exclusively. The frontier orbitals of the dye molecule bound to semiconductors suggest that the HOMO-2 and LUMO + 2 orbitals of the dye sensitizer do not participate in electron transfer processes for the dye–ZnO system, resulting in a lower Jsc value and a relatively narrow response for the incident photon-to-current conversion efficiency in the solar cell. Transition component analysis based upon the time-dependent DFT results explains well the experimental UV–vis spectra and difference in the η values between the dye-sensitized solar cells based upon TiO2 and ZnO nanocrystalline electrode materials.