Fast electron and slow hole spin relaxation in CsPbI₃ nanocrystals

Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI₃ nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI₃ NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC₆₀BM as an electron acceptor along with CsPbI₃ to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI₃:PC₆₀BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI₃ NCs. We find that the room temperature hole spin lifetime (τ_h ∼ 5 ps) is ∼13 times longer than the electron spin lifetime (τ_e ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.