Highly Stable n–i–p Structured Formamidinium Tin Triiodide Solar Cells through the Stabilization of Surface Sn2+ Cations

Abstract Improving the performance, reproducibility, and stability of Sn‐based perovskite solar cells (PSCs) with n–i–p structures is an important challenge. Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N ‐di‐ p ‐methoxyphenyl‐amine)9,9′‐spirobifluorene], a hole transporting material (HTM) with n–i–p structure, requires the oxygen exposure after addition of Li‐TFSI [Lithium bis(trifluoromethanesulfonyl)imide] as a dopant to increase the hole concentration. In Sn‐based PSC, Sn 2+ is easily oxidized to Sn 4+ under such a condition, resulting in a sharp decrease in efficiency. Herein, a formamidinium tin triiodide (FASnI 3 )‐based PSCs fabricated using DPI‐TPFB [4‐Isopropyl‐4′‐methyldiphenyliodonium tetrakis(pentafluorophenyl)borate] instead of Li‐TFSI are reported as a dopant in Spiro‐OMeTAD. The DPI‐TPFB enables the fabrication of PSCs with an efficiency of up to 10.9%, the highest among FASnI 3 ‐based PSCs with n–i–p structures. Moreover, ≈80% of the initial efficiency is maintained even after 1,597 h under maximum power point tracking conditions. In particular, the encapsulated device does not show any decrease in efficiency even after holding for 50 h in the 85 °C/85% RH condition. The high efficiency and excellent stability of PSCs prepared by doping with DPI‐TPFB are attributed to not only increasing electrical conductivity by acting as a Lewis acid, but also stabilizing Sn 2+ through coordination with Sn 2+ on the surface of FASnI 3 .