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 been attracted much attention owing to their outstanding optoelectronic and eco-friendly properties. These materials face severe issues like poor environmental stability, low formation energy, and faster oxidation of tin from Sn2+ to Sn4+ state leading to poor film quality and self-doping. In this work, we have fabricated FA0.75MA0.25SnI3 perovskite thin films via solution processing method and studied 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. Micro-strain of PCDTBT doped FA0.75MA0.25SnI3 perovskite reduced without any change in the crystal structure. Reduction in electron trap density have been observed owing to improved film quality and enlarged perovskite grains. We have observed that Sn4+ content in 0.050wt% PCDTBT doped FA0.75MA0.25SnI3 perovskite film get reduced as shown in X-ray Photoelectron Spectroscopy (XPS) results. The reduction in Sn4+ (cause of self-doping) content shows that PCDTBT doping, maintains the stability of Sn2+ in FA0.75MA0.25SnI3 perovskite thin film. A decrement in hole density from 3.2x1018 cm-3 for pristine to 1.3x1017 cm-3 for 0.050wt% PCDTBT doped FA0.75MA0.25SnI3 perovskite has been observed from C-V measurement which is consistent with XPS results. Thus, PCDTBT doping in perovskite films can effectively tackle the severe issues of tin oxidation and defects in lead-free tin halide perovskite photoactive layer for solar cell application.