Cation Tuning of Polaron Barriers in Layered Perovskites for Optical Spin Lifetime Control

Layered metal-halide perovskites (L-MHPs) form self-assembled quantum wells with strongly bound excitons and electron-phonon interactions that promote polaron formation. Due to spin-orbit coupling and Rashba-type spin-splitting of the electronic bands, spin-polarized excitons can be photoexcited with circularly polarized light, making these materials promising in opto-spintronics. Recently, we have shown that photoexcitation with excess energy extends spin-lifetimes in (BA)2FAPb2I7 by over 2 orders of magnitude compared to resonant excitation and attributed this to polaron formation. Here, we study spin-lifetimes in L-MHPs with different A-site cations: (Hexa)2MAPb2I7, (Hexa)2FAPb2I7, (Hexa)2CsPb2I7 (Hexa: hexylammonium, MA: methylammonium, FA: formamidinium, Cs: cesium). We find that all studied materials exhibit vastly extended spin-lifetimes under excess-energy excitation, but that the polaron formation barrier is reduced with increasing polarity of the A-site cations. First-principles calculations show that (Hexa)2MAPb2I7 has the most stable polarons and (Hexa)2CsPb2I7, the least. Our findings demonstrate tuning of optically controlled exciton spin-lifetimes in L-MHPs through composition engineering, providing a pathway toward optimized materials for spintronics.