In Gratzel cells, fluorescent dye molecules are excited by photons, and the electrons from the excited state of the dye are brought into the conduction band of a semiconductor. A photocurrent is the result.
I assume, that when a metal is used instead of a semiconductor, there is no bandgap and the excited electron falls back immediately into a low energy state providing no photocurrent.
If an insulator is used instead of a semiconductor, would a photocurrent be possible (for example, if the excited electrons have high energy)?
Is it possible to think of the dye as a strong reducing agent, and the semiconductor acceptor as being oxidized when the electron is transferred to its high energetic conduction band?