Effect of Grain Boundary Cross-Section on the Performance of Electrodeposited Cu₂O Photocathodes

Abstract

Large-grained Cu2O photocathodes in a superstrate configuration on a F-doped SnO2 (FTO) coated glass substrate are synthesized via two-step electrodeposition. Only submicrometer sized grains were obtained during single-step electrodeposition in the potential window (−0.31 to −0.7 V vs Ag/AgCl) of stable Cu2O formation. We observe reductive decomposition of the Cu2O to Cu metal in the potential range of −0.7 to −0.98 V; bulk reduction of Cu2+ in the solution to Cu metal occurs only beyond −0.98 V. In the potential window of stable Cu2O deposition, only the growth of the few nuclei occurs until a certain time. Minimal nucleation on the pristine FTO sites occurs during this period of deposition. The time to secondary nucleation is ∼6 min at −0.31 V and ∼15 s at −0.37 V. Interrupting the deposition at −0.31 V after 6 min and increasing the potential to −0.37 V leads to uniform, large grains (∼3 μm) of Cu2O. Photoinduced conducting atomic force microscopy reveals shunting and the presence of sub-bandgap states at the grain boundaries of Cu2O. Also, the lower carrier concentration (∼1016 cm–3) in the large-grained Cu2O film obtained from Mott–Schottky analysis suggests a lower rate of Auger recombination. Thus, lowering the grain boundary cross-section in the two-step deposited film leads to a 30% increase in photocurrent at 0.0 V vs RHE.