Alkali Metals Doping for High‐Performance Planar Heterojunction Sb₂S₃ Solar Cells

Sb 2 S 3 as a stable light harvesting material has received increasing attention for solar cell applications. To improve the device efficiency, much effort has been put into the materials synthesis, interfacial engineering, and device structure design. Here, it is demonstrated that doping of alkali metal (Li, Na, K, Rb, Cs) ions essentially affects the morphological, crystal, optical and electrical properties of Sb 2 S 3 films. The structural and compositional analysis shows that the doping is in form of alkali metal‐sulfur chemical bond at both surface and grain boundaries of Sb 2 S 3 films. Compared with the pristine Sb 2 S 3 , we observe that Li and Na doping are not able to improve the device efficiency, while K, Rb, and Cs notably increase energy conversion efficiency. The most effective enhancement is found in Cs‐doped Sb 2 S 3 , where the carrier concentration, crystallinity, and film formability show remarkable improvement. The device based on the Cs‐Sb 2 S 3 film delivers a power conversion efficiency of 6.56%, which is the highest efficiency in planar heterojunction Sb 2 S 3 solar cells, regardless of whether the films are fabricated by a solution process or thermal deposition. This research identifies that doping of heavy alkali metals is an effective approach to improve the performance of Sb 2 S 3 solar cells.