Increasing the Collection Efficiency in Selenium Thin-Film Solar Cells Using a Closed-Space Annealing Strategy

Elemental selenium is an interesting candidate for the top cell in tandem solar cells due to its wide bandgap of E_G ≈ 1.95 eV as well as its monatomic simplicity. To realize high-efficiency selenium solar cells, it is crucial to optimize the crystallization process of the selenium thin-film photoabsorber. However, the high vapor pressure of selenium restricts the processing conditions to a compromise between the growth of large crystal grains and the formation of pinholes. In this study, we introduce a closed-space annealing (CSA) strategy designed to suppress the sublimation of selenium, enabling thermal annealing processes at higher temperatures and for longer periods of time. As a result, we consistently improve the carrier collection and the overall photovoltaic device performance in our selenium solar cells. By characterizing the carrier dynamics in our devices, we conclude that the observed improvements result from a reduction in the charge-transfer resistance rather than an increase in the carrier diffusion length. The CSA strategy is a promising method for controlling the surface morphology and roughness without reducing crystal grain sizes, which paves the way for further advancements in the efficiency and reproducibility of selenium thin-film solar cells.