Optically Addressing Spin Relaxation Mechanism in 2D Hybrid Organic–Inorganic Perovskites via Charge Transfer States Formation

The key to spintronics is to control the spin relaxation mechanism and obtain spin states with a high polarization degree and long spin polarization lifetime. Two‐dimensional (2D) hybrid organic–inorganic perovskites (HOIP) have attracted great attention for spintronic applications. Here, we investigate the spin relaxation mechanism in 2D HOIP by using temperature‐dependent circularly polarized transient absorption spectroscopy with pump photon energy in excess of the resonance energy of band‐edge excitons. At low temperature, a spin polarization lifetime as long as 200 picoseconds and even up to nanoseconds is achieved in PEA 2 PbI 4 and EOA 2 PbI 4 films, respectively. We attribute this control to the formation of charge transfer states due to the coexistence of different domains in 2D HOIP. Specifically, hot electrons can cross a potential energy barrier and transfer from the high‐energy domain to the low‐energy domain, thereby reducing the electron‐hole wave function overlap that modulates the spin relaxation mechanism from the Maialle‐Silva‐Sham mechanism to the Elliott‐Yafet mechanism. More intriguingly, owing to the spin selection rules, the spin polarization degree can be reversed by tuning the photon energy of the pump laser. Such optical‐modulated spin states with high spin polarization degree and long spin polarization lifetime provide a novel opportunity for spintronic devices.