Unraveling the Impact of Organic Cation Dipole Moment on Rashba Spin‐splitting in 2D Hybrid Organic‐Inorganic Perovskites

Rashba spin‐splitting in hybrid organic‐inorganic perovskites (HOIPs) holds great promise for spintronic applications. However, strategies for controlling Rashba spin‐splitting in these materials remain underexplored. Here, we find organic cation dipole moments play a pivotal role in modulating Rashba effects. By comparing p‐FPEA2PbI4 and PEA2PbI4, we demonstrate that larger dipole moments amplify lattice distortion and local symmetry breaking, as confirmed by second‐harmonic generation. This structural asymmetry doubles the Rashba spin‐splitting energy, quantified through circular photogalvanic effect and circularly polarized light excited photoluminescence. Circular dichroism and magnetic circular dichroism reveal a Rashba‐induced effective magnetic field, while spin pumping resolves a ~50% higher Rashba coefficient in p‐FPEA2PbI4. These results establish a direct correlation between dipole strength and spin splitting in 2D HOIPs, a trend that extends to quasi‐2D and 3D HOIPs. Our work shows dipole engineering as a universal design rule for tailoring Rashba effects in HOIPs, paving the way for high‐efficiency spin‐based optoelectronics and quantum devices.