What factors reduce carrier mobility of organic-inorganic hybrid perovskite for ( BA ) 2 MAGe 2 I 7 and ( BA ) 2 MASn 2 I 7 ?

Highlights: • (BA)₂MAGe₂I₇ and (BA)₂MASn₂I₇ exhibit low elastic modulus. • Atomic structure is easy to produce distortion in electric field. • The optical phonon with low energy limits carrier mobility. • Carrier effective masses increase sharply with the biaxial strain. • The hole mobility for (BA)₂MASn₂I₇ is higher than that for (BA)₂MAGe₂I₇. 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 ${\left(\mathrm{BA}\right)}_2{\mathrm{MAGe}}_2{I}_7$ and ${\left(\mathrm{BA}\right)}_2{\mathrm{MASn}}_2{I}_7$. 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 ($\Delta ∊$). 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 $\Delta ∊$. 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.