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

Cuprous oxide (Cu 2 O) as an intrinsic p‐type semiconductor is promising for solar energy conversion. The major challenge in fabricating Cu 2 O lies in achieving both high transparency and high performance in a tandem device. The Cu 2 O 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 Cu 2 O. Here, we presented a facile method that utilizes the earth‐abundant material copper oxide (CuO) to fabricate highly transparent Cu 2 O devices. The maximum transmittance of the Cu 2 O film on CuO (FTO/CuO/Cu 2 O) increased from 42 % to 58 % compared with Cu 2 O film on Au (FTO/Au (3 nm)/Cu 2 O) 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 Cu 2 O 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/Cu 2 O photocathode. Moreover, expanding the CuO layer strategy to the field of solar cells enables Cu 2 O solar cells to achieve a PCE of 2.37 %.