Incompatible Length Scales in Nanostructured Cu₂O Solar Cells

Electrodeposited Cu 2 O‐ZnO heterojunctions are promising low‐cost solar cells. While nanostructured architectures improve charge collection in these devices, low open‐circuit voltages result. Bilayer and nanowire Cu 2 O‐ZnO heterojunction architectures are systematically studied as a function of the Cu 2 O layer thickness, ZnO nanowire length, and nanowire seed layer. It is shown that a thick depletion layer exists in the Cu 2 O layer of bilayer devices, owing to the low carrier density of electrodeposited Cu 2 O, such that the predominant charge transport mechanisms in the Cu 2 O and ZnO are drift and diffusion, respectively. This suggests that the low open‐circuit voltage of the nanowire cells is due to an incompatibility between the nanostructure spacing required for good charge collection (<1 μm) and the heterojunction thickness necessary to form the full built‐in potential that inhibits recombination (>2 μm). The work shows the way to improve low‐cost Cu 2 O cells: increasing the carrier concentration or mobility in Cu 2 O synthesized at low temperatures.