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

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 N-2 atmosphere, ampules at temperatures between 400 and 600 degrees C, and a SnCl2 treatment at 250-500 degrees C on the properties of SnS films and SnS/CdS solar cells prepared by close-spaced sublimation is reported. The ampule and N-2 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 degrees 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.