Stabilization of Sn²⁺ in FA0.75MA0.25SnI₃ perovskite thin films using electron doner polymer PCDTBT and an improvement in charge transport properties for perovskite solar cells

Lead-free tin halide perovskites for the fabrication of perovskite solar cells have attracted considerable attention due to their outstanding optoelectronic and ecofriendly properties. These materials face severe issues, such as poor environmental stability, low formation energy and faster oxidation of tin from the Sn 2+ to Sn 4+ state, leading to poor film quality and self-doping. In this work, we have fabricated FA 0.75 MA 0.25 SnI 3 perovskite thin films via a solution processing method and studied the conjugated polymer poly [N-9′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadia-zole)] (PCDTBT)-induced effects in perovskite thin films. The micro-strain of PCDTBT-doped FA 0.75 MA 0.25 SnI 3 perovskite reduced without any change in the crystal structure. Reductions in electron trap density have been observed due to improved film quality and enlarged perovskite grains. We have observed that the Sn 4+ content in 0.050 wt% PCDTBT-doped FA 0.75 MA 0.25 SnI 3 perovskite film gets reduced, as shown in the x-ray photoelectron spectroscopy (XPS) results. The reduction in Sn 4+ (cause of self-doping) content shows that PCDTBT doping maintains the stability of Sn 2+ in FA 0.75 MA 0.25 SnI 3 perovskite thin film. A decrement in hole density from 3.2 × 10 18 cm −3 for pristine films to 1.3 × 10 17 cm −3 for 0.050 wt% PCDTBT-doped FA 0.75 MA 0.25 SnI 3 perovskite has been observed from C – V measurement, which is consistent with the XPS results. Thus, PCDTBT doping in perovskite films can effectively tackle the severe issues of tin oxidation and defects in the lead-free tin halide perovskite photoactive layer for solar cell application.