Spin Polarization Dynamics of Free Charge Carriers in CsPbI₃ Nanocrystals

Abstract

Lead halide perovskites (LHPs) exhibit large spin–orbit coupling (SOC), leading to only twofold-degenerate valence and conduction bands and therefore allowing for efficient optical orientation. This makes them ideal materials to study charge carrier spins. With this study we elucidate the spin dynamics of photoexcited charge carriers and the underlying spin relaxation mechanisms in CsPbI3 nanocrystals by employing time-resolved differential transmission spectroscopy (DTS). We find that the photoinduced spin polarization significantly diminishes during thermalization and cooling toward the energetically favorable band edge. Temperature-dependent DTS reveals a decay in spin polarization that is more than 1 order of magnitude faster at room temperature (3 ps) than at cryogenic temperatures (32 ps). We propose that spin relaxation of free charge carriers in large-SOC materials like LHPs occurs as a result of carrier–phonon scattering, as described by the Elliott–Yafet mechanism.

Ultrafast Pump‐Probe Spectroscopy—A Powerful Tool for Tracking Spin‐Quantum Dynamics in Metal Halide Perovskites

“…For PA2, the initially stronger negative signal for probe originates from transition from |+1〉 to |0〉 (namely, Δ m J = −1), followed by spin relaxation to |−1〉, and then made |−1〉 to |0〉 transition happen, which eventually detected as positive signal for probe. This ultrafast exciton spin relaxation is due to exchange Coulomb interaction, that is, due to simultaneous monomolecular spin flip of the exciton's constituents, [ 47,61 ] which can be described by the Bir–Aronov–Pikus (BAP) model, [ 34,43,62 ] where N p denotes hole concentration, α B is exciton Bohr radius, ∆ represents the exchange coupling, and E B stands for exciton binding energy. For 2D perovskites system, the exchange coupling is relatively strong.…”

“…Strohmair et al. [ 43 ] observed that the spin relaxation times of CsPbI 3 nanocrystals (cubic morphology, average edge length of 13 ± 3 nm) at cryogenic temperatures (32 ps) increase more than 1 order of magnitude compared with that at room temperature (3 ps), predominantly due to carrier‐longitudinal optical (LO) phonon scattering via E–Y mechanism. With further deceasing the dimension of perovskites down to QDs level, the spin relaxation dynamics show strong dependence on size and component, and the predominant E–Y mechanism in bulk perovskites is not applicable for QDs.…”

“…However, fundamental understanding of spin‐dependent photo‐physics in perovskites, in particular of, hyperfine description of spin relaxation, magnetic spin effect, and spin polarization processes, which is plausible for manipulation of spin electrons by spin photons leveraging circularly‐polarized pump‐probe technique, is still at its nascent stage. [ 30–32,34–37,40–48 ] Moreover, up to date, few review papers have been reported on systematic, complete, and insightful summary and discussion of such emerging ultrafast spin‐quantum dynamics and phenomena in various perovskites from the aspect of study via this technique. [ 23,49,50 ] Hence, it is highly desirable for providing a systemic review on this rapidly developing field in recent few years, to evoke extraordinary research on spintronic properties in perovskites for truly practical spintronic devices in future.…”

Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy

“…27,28,29 However consensus is growing that in QC MLHP NCs the conduction and valence band edges consist of a single level significantly isolated in energy space which is only spin degenerate. 30,31 In such a scenario expectations are for a symmetric state filling effect from both bands, with a linear increase of the BE bleach for the first 2 excitons, transforming the sample from absorption to stimulated emission of the same intensity. Together with the unusual appearance of A2, these observations demonstrate that much still remains to be understood concerning electronic structure of QC MLHP NCs.…”

Perspectives of Organic and Perovskite‐Based Spintronics

“…[159,166] Leveraging this excitonic structure, many studies reported the successful injection of spin-polarized excitons and carriers into perovskite NCs for a wide variety of sizes and compositions. [159,[167][168][169] Analogously to bulk HPs, spin lifetimes in NCs were reported to be in the ps-regime (see Figure 4d).…”