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

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-FAPbI3with 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(FAPb I3)0.95(MAPbI3)0.05thin film is ～30% larger than that in(FAPb I3)0.85(MAPbBr3)0.15.The larger mobility in(FAPb I3)0.95(MAPb I3)0.05could be assigned to the slowing down of the carrier scattering time.Therefore, the physical origin of the higher carrier mobility in the(FAPb I3)0.95(MAPbI3)0.05should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution.In addition,(FAPbI3)0.95(MAPbI3)0.05will lose fewer active carriers during the polaron cooling process than that in(FAPb I3)0.85(MAPbBr3),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-FAPbI3can also be an important factor for the high PCE of the black phase α-FAPbI3based solar cell devices.