2, University of Oxford, Oxford, , United Kingdom
Lead halide perovskites exhibit surprisingly long charge carrier lifetimes for a direct bandgap semiconductor. Recently, we found evidence for a slightly indirect bandgap in MAPbI3 by using pressure to change photoelectric properties. This indirect bandgap is induced by Rashba splitting, and could be one reason for the long lifetimes.
The Rashba splitting requires an electric field across the lead atom, which could originate from the organic cation methylammonium. We recently investigated MAPbI3, which has a methylammonium (MA) cation with a dipole moment of 2.3 Debye inside the lead iodide cage. We use FAPbI3 (Dipole moment of formamidinium (FA) = 0.2 Debye), CsPbI3 (Cesium(Cs) has no dipole moment) to compare materials with the same cage structure but different cation dipole moments. We study photoluminescence and time resolved photoluminescence under pressure to track the changes in optoelectronic behavior of different lead iodine perovskite derivatives.
We find that the cation has a large influence on the magnitude of the Rashba splitting. MAPbI3 with large dipole moment has a clear side peak in the PL spectrum. FAPbI3 with its smaller dipole moment has an asymmetric PL shape, but no distinct side peak and CsPbI3 with no cation dipole moment has a perfectly symmetric PL peak. We quantify the asymmetry of the photoluminescence spectrum, and the lifetime of each material to understand the influence of the cation on lead halide based semiconductor perovskites. Our results lead to a structural understanding of the Rashba-split bandgap of metal halide perovskites, which is crucial for designing novel perovskite materials.