Quantification of Spin Relaxation Rates for Chiral Lead Halide Perovskites

Sammanfattning

Chirality is the property of an object that cannot be superimposed onto its mirror image. Chiral materials exhibit circular dichroism (CD) which can selectively populate one spin state over the other upon optical excitation, creating a spin-polarized carrier population. In chiral hybrid organic-inorganic perovskites, chirality is transferred from the organic spacer to the inorganic framework, making these materials strong candidates for spintronic applications where spin polarization can be generated optically without external magnetic fields. The spin lifetimes of these materials depends on the measurement technique and the standard helicity-difference method carries implicit assumptions that limits its physical transparency. Here we investigate the spin dynamics of (R/S/RAC-MBA)2PbI4 with circularly polarized transient absorption ( CP-TA ) and obtain the lifetimes by the hellicity-difference method and we also propose a new kinetic model to obtain the spin rate constant. Thin films were characterized by steady-state absorption and circular dichroism spectroscopy and CP-TA with 480 nm pump and 8μJ/cm2 fluence for different pump-probe polarization configurations was used to study the carrier dynamics. The spin lifetimes were obtained by the helicity-difference method and for a coupled two-population kinetic model that separates the carrier decay into its different contributions including excitonic recombination (kr), Auger-Meitner recombination (kA), and spin relaxation (ks). Both methods confirm polarization-dependent dynamics and faster spin relaxation in the racemic sample than in the pure enantiomers, consistent with the chiral organic molecule prolonging spin polarization through the chiral induced spin selectivity ( CISS ) effect. The kinetic model yields spin lifetimes of 43-249 ps, shorter than the 136-751 ps extracted by the helicity-difference method, confirming that spin lifetime is a model-dependent quantity. These results establish new insights into the quantification of spin relaxation rates in chiral hybrid perovskites and highlight the need for standardized approaches in this rapidly growing field.