Phase‐Switchable 2D Single‐Crystalline Perovskite With Low‐Temperature Out‐of‐Plane Rashba Splitting and Room‐Temperature Out‐of‐Plane Ferroelectricity for Electrically Reconfigurable Spin‐Photonics

Two‐dimensional organic‐inorganic hybrid perovskites (2D OIHPs), with their unique soft lattice and versatile polarization tunability, offer an exceptional platform for exploring Rashba spin‐orbit coupling (SOC). While in‐plane Rashba SOC and ferroelectricity have been extensively studied, the out‐of‐plane Rashba SOC combined with out‐of‐plane ferroelectricity is more desirable for realizing low‐power, multifunctional, non‐volatile spintronic devices. Here, we synthesize single‐crystalline, phase‐switchable polar 2D OIHPs within a PDMS‐based sealed space and reveal the transition process between the two polar structures. The complex polar structure enables the sequential emergence of out‐of‐plane Rashba spin polarization and out‐of‐plane ferroelectric order. By manipulating the anomalous Rashba splitting with electric and magnetic fields in the LT phase, we directly reveal the dominant role of momentum offset in Rashba exciton recombination. Furthermore, we develop a self‐powered photodetector exhibiting chiral‐light selectivity and achieve reversible electrical control over the electron spin texture orientation in the RT phase. These findings establish a new material paradigm for electrically tunable spin‐photonics and low‐power spintronic devices.