Stabilizing Cu₂S for photovoltaics one atomic layer at a time

Stabilizing Cu₂S in its ideal stoichiometric form, chalcocite, is a long-standing challenge that must be met prior to its practical use in thin-film photovoltaic (PV) devices. Significant copper deficiency, which results in degenerate p-type doping, might be avoided by limiting Cu diffusion into a readily formed surface oxide and other adjacent layers. Here, we examine the extent to which PV-relevant metal-oxide over- and underlayers may stabilize Cu₂S thin films with desirable semiconducting properties. After only 15 nm of TiO₂ coating, Hall measurements and UV-vis-NIR spectroscopy reveal a significant suppression of free charge-carrier addition that depends strongly on the choice of deposition chemistry. Remarkably, the insertion of a single atomic layer of Al₂O₃ between Cu₂S and TiO₂ further stabilizes the active layer for at least 2 weeks, even under ambient conditions. The mechanism of this remarkable enhancement is explored by in situ microbalance and conductivity measurements. Finally, photoluminescence quenching measurements point to the potential utility of these nanolaminate stacks in solar-energy harvesting applications.