Structure and property tunability in monolayer halide lead-free double hybrid perovskites: effects of Rashba and biaxial strain

Abstract

Organic–inorganic hybrid double perovskites (OIHDPs) are playing increasingly important roles in the development of stable and lead-free perovskite materials. However, most of them are indirect-band semiconductors with large band gaps which severely limit their development for practical applications. Here we theoretically investigate the intrinsic structures/properties and biaxial strain-induced changes for the monolayer lead-free OIHDP, MA₄[AgBi]Br₈ (MA = CH₃NH₃⁺), and propose to use strain to tune its electronic structures and so optimize its properties. Our results reveal that pristine MA₄[AgBi]Br₈ has an indirect band gap originated in the inequality of spin–orbit coupling (SOC) effects on its valence and conduction band extrema and on two directions around the high symmetry point of the Brillouin zone as well, and that the strain tunes the band gap and the Rashba effect, and as a result it mediates the band structure. The band gap of MA₄[AgBi]Br₈ varies linearly with the strain ratio and a ≥2% stretching can turn it into a complete direct-band semiconductor. MA₄[AgBi]Br₈ shows different properties under various strain conditions. Compression strengthens the giant SOC-related characteristics including the SOC-induced indirect band gap and Rashba band-splitting, while moderate stretching not only lessens the band gap but also can enhance the carrier mobility and exciton separation. These results indicate that MA₄[AgBi]Br₈ has the potential to be a stable and non-toxic multifunctional perovskite material. This work also provides some full-scale insights into the Rashba effect on band structures and the correlation between structures/properties and the strain ratio.