Thermalization and Spin Relaxation Dynamics of Localized Photocarriers in the Band Tails of Nanocrystalline MAPbBr3 Films

Abstract

We have studied the spin properties of localized photocarriers in the band tails (BT) of polycrystalline MAPbBr₃ films having a nanometer crystal size using circularly polarized photoluminescence (PL) induced by a magnetic field up to 17.5 T at cryogenic temperatures as well as time-of-flight (TOF) transient photocurrent. The absorption spectrum of these films reveals BT states caused by structural and energetic disorders, having a broadly distributed Urbach edge ranging from 28 to 120 meV. This is corroborated by dispersive transport of photogenerated electrons and holes observed via TOF, where the photocarriers thermalize with time deeply in the BT, giving rise to time-dependent mobility. Consequently, the PL emission spectrum in these films originates from radiative recombination of the localized electron and hole pairs in the BT states. Upon applying a magnetic field in the Faraday configuration, field-induced circular polarized PL has been observed, from which an effective Landé g-factor of the localized e–h pairs, g_e–h, was extracted to be 2.5 ± 0.2, in good agreement with the g-factor of free excitons measured using magnetic circular dichroism spectroscopy. In addition, we also found that the spin relaxation time for the e–h pairs in the BT states is ∼26 ns at 5 K and ∼10 ns at 80 K, indicating that nanocrystalline MAPbBr₃ could be a good candidate for applications in spintronics and quantum computing.