Spin Dynamics of Excitons and Carriers in Mixed‐Cation MA x FA 1– x PbI 3 Perovskite Crystals: Alloy Fluctuations Probed by Optical Orientation

ABSTRACT Optical spin orientation measured by time‐resolved photoluminescence provides a powerful tool to probe the spin dynamics of excitons and charge carriers in perovskite semiconductors. The impact of alloy fluctuations on the spin dynamics of mixed‐cation perovskite single crystals is studied here experimentally. The optical orientation is measured under nonresonant excitation for crystals with , 0.4, and 0.8 at cryogenic temperatures and compared with data on crystals. The high degree of exciton optical orientation of % for and 0.8 reduces to about 60% for . A similar trend is observed for the optical orientation of carrier spins. This behavior is attributed to enhanced scattering of free excitons and carriers in the alloys with increased compositional and structural disorder. The optical orientation remains robust for excitation detunings up to 0.3 eV, indicating the absence of the Dyakonov–Perel spin relaxation mechanism. From the Larmor spin precession measured from spin dynamics in an external magnetic field applied in Voigt geometry, the electron and hole ‐factors are evaluated. Their dependence on the bandgap energy in crystals follows the universal trend previously established for lead halide perovskites.