Polaron-Driven Spin Funneling through Rashba-Split Bands in Mixed-Phase Quasi-Two-Dimensional Ruddlesden-Popper Perovskites
Metal halide perovskites (MHPs) exhibit pronounced spin-orbit coupling (SOC) as a result of their heavy metal constituents, leading to distinctive electronic properties such as Rashba type band splitting which make them promising candidates for next generation spintronic applications. Here, using circularly polarized luminescence (CPL) and polarization dependent pump-probe spectroscopy, we found that spin polarization is present across all phases of our two-dimensional (2D) Ruddlesden-Popper (RP) mixed-phase perovskites, (C6H7SNH3)2 (CH3NH3)n-1PbnI3n+1 (n=1-4), irrespective of the number of inorganic layers. The origin of these spin polarized bands is attributed to the Rashba effect. Interestingly, the highly disordered nature of this system facilitates remarkably efficient ultrafast funneling of photoexcited spin-polarized excitons from the pure 2D phase (n=1) to higher-n phases at room temperature. We demonstrate that significant polaron formation due to the inherent soft crystal lattice and higher exciton-phonon interaction is responsible for the observed spin funneling effect in mixed-phase 2D RP perovskites. Polaron act as a protective mechanism for spin-polarized excitons, preserving their spin information through the screening of omnipresent phonon-induced momentum scattering. These findings not only offer valuable guidance for the design of 2D RP perovskites with pronounced Rashba effects but also unveil a compelling class of solution-processed perovskites capable of efficient spin-preserving energy transport at room temperature.