SnF₂-Doped Cs₂SnI₆ Ordered Vacancy Double Perovskite for Photovoltaic Applications

Cs₂SnI₆, an emerging inorganic perovskite variant is considered an environmentally benign lead-free option for photovoltaic application. Cs₂SnI₆ offers favorable optoelectronic properties as well as high stability due to the presence of the tetravalent oxidation state. Owing to its p-type semiconducting nature, Cs₂SnI₆ has previously been reported as an efficient hole transport material. However, its candidacy as a potential light absorber is still being explored in different strategical investigations. In this study, for the first time, Cs₂SnI₆ has been used in a superstrate <i>n-i-p</i> planar device structure enabled by a spin-coated absorber thickness of ~2 mm on a chemical bath deposited Zn(O,S) electron transport layer. The crystalline Cs₂SnI₆ phase is obtained with CsI, SnI₂, and SnF₂ salts in a solvent mixture of Gamma-Butyrolactone (GBL) and N, N-Dimethylformamide (DMF) and a bandgap of 1.6 eV has been demonstrated. The best device power conversion efficiency reported here is 5.18% with V_OC of 0.81 V, J_SC of 9.28 mA/cm² and a fill-factor of about 68%. The devices exhibit a high V_OC deficit, and higher device efficiency can be expected by improvement of material quality, charge transport and device engineering.