Defect Engineering and Growth of n-Type Tin(II) Sulfide Single Crystals Enabled by Tin(II) Iodide

SnS is a highly promising, environmentally compatible material for next-generation solar cells. However, its reliable application is limited by its p-type conduction due to the stability of hole-generating defects, such as Sn vacancies, across varying stoichiometry. Although n-type SnS has been achieved via halogen doping, controlling the carrier concentration remains challenging due to difficulties in managing dopant concentrations in single crystals. In this study, we aimed to achieve low carrier-concentration n-type SnS by growing single crystals in the presence of SnI2. This approach yielded a carrier concentration of 1013 cm–3 and a mobility of 116 cm2 V–1 s–1 in single crystals with minimal variations in Sn/S ratio. Next, SnI2 was comixed with SnCl2 to achieve precise Cl-doping control, resulting in low-Cl-concentration n-type SnS, with Cl concentrations an order of magnitude lower than those in conventional Cl-doped SnS. The carrier concentration and mobility were 2.5 × 1016 cm–3 and 49.3 cm2 V–1 s–1, respectively. The realization of this ultralow-carrier-concentration n-type SnS using SnI2 is attributed to the SnI2-mediated suppression of hole-producing defect generation during SnS single-crystal growth. Overall, these results demonstrate the feasibility of deploying SnI2 as an effective defect-engineering agent for precise carrier-concentration control toward high-efficiency SnS-homojunction solar cells.