Postdeposition Processing of SnS Thin Films and Solar Cells: Prospective Strategy To Obtain Large, Sintered, and Doped SnS Grains by Recrystallization in the Presence of a Metal Halide Flux

Abstract

Postdeposition treatments (PDTs) are common technological approaches to achieve high-efficiency chalcogenide solar cells. For SnS, a promising solar cell material, most PDT strategies to control the SnS properties are overwhelmingly based on an annealing in sulfur-containing ambient atmosphere that is described by condensed-state reactions and vapor-phase transport. In this work, a systematic study of the impact of PDTs in a N₂ atmosphere, ampules at temperatures between 400 and 600 °C, and a SnCl₂ treatment at 250-500 °C on the properties of SnS films and SnS/CdS solar cells prepared by close-spaced sublimation is reported. The ampule and N₂ annealing conditions do not affect the grain size of the SnS layers but significantly impact the concentration of intrinsic point defects, carrier density, and mobility. Annealing at 500-600 °C strongly enhances the hole concentration and decreases the carrier mobility, having detrimental impacts on the device performance. SnCl₂ treatment promotes grain growth, sintering, and doping by mass transport through the melted phase; it adjusts the hole density and improves the carrier mobility in the SnS layers. SnS/CdS solar cells with an efficiency of 2.8% are achieved in the SnCl₂ treatment step, opening new possibilities to further improve the performance of SnS-based devices.