Elementary photoelectronic processes at a porphyrin dye/single-walled TiO2 nanotube hetero-interface in dye-sensitized solar cells: a first-principles study.

A unique one-dimensional (1D) sandwich single-walled TiO2 nanotube (STNT) is proposed as a photoanode nanomaterial with perfect morphology and large specific surface area. We have thoroughly examined the elementary photoelectronic processes occurring at the porphyrin dye/STNT hetero-interface in dye-sensitized solar cells (DSSCs) by theoretical simulation. It is desirable to investigate the interfacial photoelectronic processes to elucidate the electron transfer and transport mechanism in 1D STNT-based DSSCs. We have found that the photoexcitation and interfacial charge separation mechanism can be described as follows. A ground-state electron of the dye molecule (localized around the electron donor) is first promoted to the excited state (distributed electron donor), and then undergoes ultrafast injection into the conduction band of the STNT, leaving a hole around the oxidized dye. Significantly, the injected electron in the conduction band is transported along the STNT by means of Ti 3d (x2-y2) orbitals, offering a unidirectional electron pathway toward the electrode for massive collection without the observation of trap states. Our study not only provides theoretical guidelines for the modification of TiO2 nanotubes as a photoanode material, but also opens a new perspective for the development of a novel class of TiO2 nanotubes with high power-generation efficiency.