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 SnI 2 . This approach yielded a carrier concentration of 10 13 cm –3 and a mobility of 116 cm 2 V –1 s –1 in single crystals with minimal variations in Sn/S ratio. Next, SnI 2 was comixed with SnCl 2 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 × 10 16 cm –3 and 49.3 cm 2 V –1 s –1, respectively. The realization of this ultralow-carrier-concentration n-type SnS using SnI 2 is attributed to the SnI 2 -mediated suppression of hole-producing defect generation during SnS single-crystal growth. Overall, these results demonstrate the feasibility of deploying SnI 2 as an effective defect-engineering agent for precise carrier-concentration control toward high-efficiency SnS-homojunction solar cells.