Efficient electron injection in non-toxic silver sulfide (Ag₂S) sensitized solar cells

α-Ag₂S, with a direct forbidden bandgap of about 1.0 eV, is a non-toxic low bandgap semiconductor which can readily be deposited in the form of a thin film by chemical bath deposition. In a solar cell configuration, it can potentially provide a high short-circuit current due to the infrared absorption, and is compatible with the polysulfide electrolyte. Its practical use in a solar cell depends, however, critically on band alignment between the Ag₂S, the oxide anode and the electrolyte redox potential. Here we examine the conduction band (CB) offsets in the nanostructured α-Ag ₂S sensitized TiO₂ and SnO₂ electrodes by X-ray Photoelectron Spectroscopy, and show that they can significantly differ from the extrapolated bulk values. The much higher CB offset for SnO ₂/Ag₂S interface (∼0.6 eV) compared with that of ∼0.2 eV for TiO₂/Ag₂S, supplied a sufficient injection driving force and was favorable for the electron separation at the heterojunction. When fabricated into solar cells, a dramatically higher current density under AM 1.5 illumination for the SnO₂/Ag₂S heterojunction was obtained, which was contributed by the efficient electron injection.