Semitransparent Cu₂O Films Based on CuO Back Layer for Photoelectrochemical Water Splitting and Photovoltaic Applications

Cuprous oxide (Cu₂O) as an intrinsic p-type semiconductor is promising for solar energy conversion. The major challenge in fabricating Cu₂O lies in achieving both high transparency and high performance in a tandem device. The Cu₂O photocathodes often employ gold as the back contact layer. However, it is not an optimal choice in tandem device due to its poor transmission, scarcity, and electron-hole recombination at the interface of Au and Cu₂O. Here, we presented a facile method that utilizes the earth-abundant material copper oxide (CuO) to fabricate highly transparent Cu₂O devices. The maximum transmittance of the Cu₂O film on CuO (FTO/CuO/Cu₂O) increased from 42 % to 58 % compared with Cu₂O film on Au (FTO/Au (3 nm)/Cu₂O) in 550-800 nm. After coating atomic layer deposition (ALD) layers and hydrogen evolution reaction (HER) catalyst, the photocurrent density at 0 V (versus RHE) of the semitransparent Cu₂O photocathode with CuO as the back layer for photoelectrochemical (PEC) water splitting reached -4.9 mA cm^-2, which showed a 24.5 % improvement compared with FTO/Au/Cu₂O photocathode. Moreover, expanding the CuO layer strategy to the field of solar cells enables Cu₂O solar cells to achieve a PCE of 2.37 %.