KCN Chemical Etching of van der Waals Sb 2 Se 3 Thin Films Synthesized at Low Temperature Leads to Inverted Surface Polarity and Improved Solar Cell Efficiency

Recent developments in Sb Se van der Waals material as an absorber candidate for thin film photovoltaic applications have demonstrated the importance of surface management for improving the conversion efficiency of this technology. Sb Se thin films' versatility in delivering good efficiencies in both superstrate and substrate configurations, coupled with a compatibility with various low-temperature deposition techniques (below 500 °C and often below 350 °C), makes them highly attractive for advanced photovoltaic applications. This study presents a comparative analysis of the most effective chemical etchings developed for related thin film chalcogenide technologies to identify and understand the most appropriate surface chemical treatments for Sb Se in substrate configuration, synthesized using a sequential process at very low temperatures (320 °C). Eight different chemical etchings were tested and investigated, and the results show that only KCN-based solutions lead to an improvement in the solar cell's performance, primarily due to an increase in the fill factor. Surface analysis of the samples shows that KCN etching produces very Sb-rich surfaces that do not affect the properties of the bulk. It is proposed that this Sb-rich interface inverts the surface polarity, creating a "buried junction" with CdS, thereby explaining the improvement of the fill factor of the devices, as confirmed by device modeling. The results of this study underscore the importance of surface management in low-temperature synthesized Sb Se absorbers, where Sb-rich interfaces are crucial for achieving high-efficiency devices. This research contributes to ongoing efforts to improve the performance of Sb Se thin film photovoltaic technology and could pave the way for the development of more efficient solar cells with optimized interfaces.