Polaron mobility modulation by bandgap engineering in black phase α-FAPbI3

Abstract

Lead halide hybrid perovskites (LHP) have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability. The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation. However, the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation. Here, the bandgap modulated thin films of α-FAPbI3 with different element substitution is investigated by the time resolved Terahertz spectroscopy. We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap, even though the formation of polaron will not be affected apparently. Intuitively, the large polaron mobility in (FAPbI3)0.95(MAPbI3)0.05 thin film is ∼30% larger than that in (FAPbI3)0.85(MAPbBr3)0.15. The larger mobility in (FAPbI3)0.95(MAPbI3)0.05 could be assigned to the slowing down of the carrier scattering time. Therefore, the physical origin of the higher carrier mobility in the (FAPbI3)0.95(MAPbI3)0.05 should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution. In addition, (FAPbI3)0.95(MAPbI3)0.05 will lose fewer active carriers during the polaron cooling process than that in (FAPbI3)0.85(MAPbBr3)0.15, indicating lower thermal dissipation in (FAPbI3)0.95(MAPbI3)0.05. Our results suggest that besides the smaller bandgap, the higher polaron mobility improved by the substitution engineering in α-FAPbI3 can also be an important factor for the high PCE of the black phase α-FAPbI3 based solar cell devices.