Intriguing ferroelectric (FE) polarization has been reported in various materials, where intrinsic symmetry-breaking characteristics and spin-orbit coupling (SOC) effect play a significant role. However, the mechanism underlying of the Rashba SOC manipulation ferroelectric properties under Br atomic displacement remains unclear. In this study, we employ hybrid functional and ab initio nonadiabatic molecular dynamics with SOC to elucidate the impact of Rashba SOC on the ferroelectricity and carrier spin dynamics in the FE $\mathrm{C}{\mathrm{s}}_3\mathrm{B}{\mathrm{i}}_2\mathrm{B}{\mathrm{r}}_9/\alpha \text{−}\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3\downarrow$ heterostructure with atomic displacement. The results reveal that the ferroelectricity can be significantly enhanced by Rashba SOC due to increased symmetry breaking. Remarkably, Br atomic displacement modulation leads to enhancement in both ferroelectric polarization $\left(0.69\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2\right)$ and spin-splitting strength (${\alpha }_{\mathrm{R}}=1.18\mathrm{eV}/\AA$). Spin splitting occurs at the conduction band minimum of heterostructure, where carriers in same-spin channels relax primarily through electron-phonon coupling. The relaxation process is dominated by spin-down carriers, with electron and hole relaxation time differences being five times larger than spin-up carriers (twice times). Notably, interfacial electron-hole recombination time accelerates to 122.78 fs under $\mathrm{\Delta }{d}_{\mathrm{Br}\text{−}\mathrm{Bi}}=0.008\AA$, significantly improving carriers' separation efficiency. Furthermore, we demonstrate that the spin photocurrent in the $\mathrm{C}{\mathrm{s}}_3\mathrm{B}{\mathrm{i}}_2\mathrm{B}{\mathrm{r}}_9/\alpha \text{−}\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3\downarrow$ heterostructure also can be effectively modulated by ferroelectric polarization. These findings advance the fundamental understanding of ferroelectric perovskite materials.

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• Ferroelectricity
• Photocurrent
• Spin dynamics
• Spin-orbit coupling
• 2-dimensional systems
• Ferroelectrics
• Semiconductors
• Density functional theory
• Green's function methods
• Molecular dynamics