Current status of n-type SnS: Paving the way for SnS homojunction solar cells

Abstract

Orthorhombic SnS is a promising thin-film solar-cell material composed of safe and abundant elements with suitable optical properties for photovoltaic application. For approximately two decades, SnS solar cells have employed heterojunction structures with p-type SnS and other n-type semiconductors because undoped SnS typically exhibits p-type electrical conduction. However, their conversion efficiency has remained stagnant at 4%-5% for a long time. A breakthrough is required to significantly improve their conversion efficiencies before SnS solar cells can be put into practical use. Therefore, this comprehensive review article establishes the current state of the art in SnS solar cells, with an aim to accelerate both fundamental research and practical applications in this field. We discuss issues specific to SnS heterojunction solar cells, the advantages of the homojunction structure, and summarize recent advances in the n-type conversion of SnS by impurity doping, which is required to form a homojunction. The latter half of this article describes the latest research on the fabrication of n-type single crystals and films of halogen-doped n-type SnS, which is prepared via a doping system suitable for practical use. We conclude the article by summarizing the current status and future work on SnS homojunction devices, including the development of high-efficiency multi-junction SnS solar cells by band gap engineering.