Extended Light Harvesting with Dual Cu 2 O‐Based Photocathodes for High Efficiency Water Splitting

Cu 2 O is one of the most promising light absorbing materials for solar energy conversion. Previous studies with Cu 2 O for water splitting usually deliver high photocurrent or high photovoltage, but not both. Here, a Cu 2 O/Ga 2 O 3 /TiO 2 /RuO x photocathode that benefits from a high quality thermally oxidized Cu 2 O layer and good band alignment of the Ga 2 O 3 buffer layer is reported, yielding a photocurrent of 6 mA cm −2 at 0 V versus reversible hydrogen electrode (RHE), an onset potential of 0.9 V versus RHE, and 3.5 mA cm −2 at 0.5 V versus RHE. The quantum efficiency spectrum (incident photon to current efficiency, IPCE) reveals a dramatically improved green/red response and a decreased blue response compared with electrodeposited Cu 2 O films. Light intensity dependence and photocurrent transient studies enable the identification of the limitations in the performance. Due to the complementary IPCE curves of thermally oxidized and electrodeposited Cu 2 O photocathodes, a dual photocathode is fabricated to maximize the absorption over the entire range of above band gap radiation. Photocurrents of 7 mA cm −2 at 0 V versus RHE are obtained in the dual photocathodes, with an onset potential of 0.9 V versus RHE and a thermodynamically based energy conversion efficiency of 1.9%.