What factors reduce carrier mobility of organic-inorganic hybrid perovskite for (BA)2MAGe2I7 and (BA)2MASn2 I7?

To overcome the poor environmental stability and toxicity caused by lead, germanium/tin-based perovskite becomes a promising material for solar cells and optoelectronics. An important parameter for semiconductors is mobility, which determines the response speed of carriers to electric field. Here we elaborate on those factors that restrict carrier mobility for (BA)(2)MAGe(2)I(7) and (BA)(2)MASn(2)I(7). The mobility is calculated by deformation potential theory (DPT) and polar optical phonon (POP) models. Compared with inorganic materials, they exhibit low elastic modulus, which limits the mobility calculated from longitudinal acoustic phonon (LAP) model. Optical phonons (OP) with low energy play an important role in mobility. POP model reveals that the mobility reduces with the decrease of phonon energy and the increase of dielectric constant difference (Delta epsilon). In these materials, OP are distributed over a wide energy range, some of them are in the low energy part. Atomic structure is easy to produce distortion in electric field, which leads to a large Delta epsilon. Furthermore, the tensile strain inevitably exists in two-dimensional materials, and effective masses for electrons and holes increase sharply with the biaxial tensile strain, which is also an external factor leading to the decrease of mobility.