Exciton States in MAPbI3 Probed by Spin-dependent Pump-probe Spectroscopy

The solution-processed hybrid organic-inorganic perovskites (HOIPs) have shown remarkable optoelectronic properties, because deep defects have high formation energies and shallow defects are close to band edges. This also implies easy formation of bound excitons and complicated exciton physics. Therefore, it is crucial to understand the exciton states in HOIPs, as they affect light absorption, exciton dissociation, and light emission. We study the exciton states with ultrafast pump-probe spectroscopy based on Faraday rotation, and utilize differences in spin dynamics and optical spectra of Faraday rotation to distinguish different photoexcited species. In polycrystalline MAPbI₃ film samples with sufficiently long spin lifetimes, four quantum beating frequencies among exciton states show up as oscillations in time-resolved Faraday rotation in Voigt geometry. In magnetic field range of 0 to 350 mT, all four frequencies are linearly proportional to the field, implying four different g-factors, which can be easily grouped into a fast pair and a slow pair. We will also present the spin dynamics and optical spectra associated with four frequencies, and their correlation with absorption and photoluminescence, and infer useful information about the exciton states.