Recently, the research team led by Professor Jingshan Luo of the College of Electronic Information and Optical Engineering at Nankai University published the latest research results in the field of the photoelectrocatalytic water splitting for hydrogen production in the internationally renowned journal Nature Communications. The research team introduced dual buffer layers using atomic layer deposition (ALD) to significantly improve the photovoltage of the Cu₂O photocathode. 

Professor Jingshan Luo has been focusing on improving the photocatalytic activity of Cu₂O for many years. With the team, they achieved a saturation photocurrent density of the Cu₂O photocathode of 10 mA cm^-2 using nanowire structure for the first time internationally (Nano Letters, 16, 1848-1857, 2016). Subsequently, using a Ga₂O₃ buffer layer, they successfully increased the onset potential of the Cu₂O photocathode to the recorded value of 1 V (vs. RHE) (Nature Catalyst, 1, 412-420, 2018). Improving the photovoltage of the Cu₂O photocathode helps to improve the solar-to-hydrogen (STH) conversion efficiency of un-biased tandem devices. The current onset potential of the Cu₂O photocathode can reach 1 V (vs. RHE), which is still lower than the theoretical value of 1.6 V. Thus, there is significant room for improvement.

Previous studies mostly focused on optimizing the band alignment between Cu₂O and the n-type buffer layer to improve the photovoltage of Cu₂O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. According to literature analysis and electrochemical impedance spectroscopy (EIS) measurements results, doctoral students Jinshui Cheng and Linxiao Wu, guided by Professor Jingshan Luo, found that a potential barrier existed at the interface of the n-type buffer layer (Ga₂O₃) and the protective layer (TiO₂), which decreased the photovoltage and fill factor of the Cu₂O photocathode. Next, they inserted a second buffer layer (ZnGeO_x) using atomic layer deposition (ALD) between Ga₂O₃ and TiO₂ to form an energy-level gradient, which eliminated the potential barrier and improved the band alignment. As a result, the photovoltage of the Cu₂O photocathode was successfully increased to 1.07 V (vs. RHE).

The authors revealed the transport behavior of photo-generated charge carriers at different interfaces by comparing the resistance changes of photocathodes with a single buffer layer (Ga₂O₃) and dual buffer layers (Ga₂O₃/ZnGeO_x) under different bias. The operando EIS measurements showed that, when compared to the single buffer layer device, the dual buffer layers device could let the Cu₂O photocathode eliminate the unfavorable potential barrier and charge transfer resistance at the Ga₂O₃/TiO₂ interface. In addition, the author revealed that an energy-level gradient was formed between Ga₂O₃ and TiO₂ after the introduction of the ZnGeO_x buffer layer, thereby optimizing the band alignment between the buffer layer and the protective layer, reducing the energy loss, and ultimately improving the photovoltage of the Cu₂O photocathode.

Figure a, Schematic diagram of the energy-level structure and charge transfer behaviour of the Cu₂O photocathode with dual buffer layers. b, Current density-voltage curves of different Cu₂O photocathodes under illumination (AM 1.5G, 100 mW cm^-2). c, Statistical onset potential for different Cu₂O photocathodes. d, Resistance values and their change trends of the Cu₂O photocathode with dual buffer layers under different bias voltages

In summary, this work emphasizes the importance of the band alignment between the n-type buffer layer and the protective layer and proposes dual buffer layers strategy to improve the photovoltage of the Cu₂O photocathode, providing a new idea for improving the performance of other heterojunction devices.

Nankai University is the sole completion unit and communication unit for this work. Professor Jingshan Luo from the College of Electronic Information and Optical Engineering at Nankai University is the corresponding author of this paper, and Jinshui Cheng, a 2020 doctoral student from the College of Electronic Information and Optical Engineering at Nankai University, is the first author.

Article Link: https://www.nature.com/articles/s41467-023-42799-x