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, Sn2+ is easily oxidized to Sn4+ 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, approximate to 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 degrees 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 Sn2+ through coordination with Sn2+ on the surface of FASnI(3).