Unraveling the Spin Relaxation Mechanism in Hybrid Organic–Inorganic Perovskites

Abstract

The hybrid organic−inorganic perovskite (HOIP) CH 3 NH 3 PbI 3 exhibits a long spin coherence despite its strong spin−orbit coupling. Elucidation of the spin relaxation mechanism is a prerequisite to harness this spin coherence for spintronic applications. Here, we show that exciton spin relaxation behaviors can be quantitatively accounted for by the Rashba splitting in the exciton bands together with piezoelectric (PE) coupling, both arising from structural inversion asymmetry of HOIPs. A large exciton size and Debye screening influence piezoelectric scattering and result in the unusual temperature and photoexcitation dependences in spin relaxation. Our results suggest that the strong PE coupling helps retain spin coherence in HOIPs with strong spin−orbit coupling and Rashba splittings.

Transient circular dichroism and exciton spin dynamics in all-inorganic halide perovskites

“…Meanwhile, nanosecond exciton spin coherence was observed in polycrystalline CH 3 NH 3 PbCl x I 3−x at low temperatures by using TRFR 19,49 . This comes from an interplay between the Rashba effect and distinct piezoelectric coupling, which overall contributes to a strongly suppressed DP mechanism 19,49 . Furthermore, the nanosecond spin coherence of localized electrons and holes was observed in solution-grown CsPbBr 3 crystals by using TRFR 21 .…”

“…Generally, there are three types of spin relaxation mechanisms in semiconductors known as Elliott-Yafet (EY), D'yakonov-Perel (DP), BAP mechanism 36 . For MHPs, spin depolarization mechanisms are complicated and strongly depend on sample quality 18 , organic or inorganic cations 19,21 , structural phases 14,49 , dimensionality 15,16,50 , lattice temperature 18,19,21 , and so on. For instance, the EY mechanism through impurities or grain boundaries scattering plays a major role in solution-processed MHPs.…”

Observation of long spin lifetime in MAPbBr3 single crystals at room temperature

“…However, at the surface or interface this inversion symmetry is necessarily broken and therefore allows for the formation of a surface Rashba state. Indeed, it has been proposed that the giant Rashba splitting previously observed in MAPbBr3 stems from this surface state and not the bulk [27,65,66] and recently experimentally proven in the MAPbrBr3 polycrystalline thin film [62]. It is thus tremendously advantageous to utilize single crystals in the analysis of the surface Rashba state, where a single material surface may be isolated and probed and the effects of defects such as grain boundaries is minimized.…”

“…We are unaware of any other reports on the spin lifetime in MAPbBr 3 single crystals at room temperature with which we could compare our experimentally obtained value of τ sf =189 ps. However, there have been some reports on polycrystalline MAPbBr 3 thin films measured at low temperature as well as several recent theoretical studies on MAPbBr 3 and MAPbI 3 single crystals [27,65,66,75,76]. Wang et al used partial circularly polarized photoluminescence (CPL) to measure spin lifetime via the optical Hanle effect in MAPbBr 3 thin films at low temperature, with measured relaxation times of τ sf =240 ps at 77 K that reduces by an order of magnitude at 110 K [76].…”

“…In addition, they measured electrical Hanle responses for the thin film in a spin valve configuration to obtain a spin relaxation time of τ sf =936 ps at 10 K [75]. The large reduction in spin lifetime with increasing temperature has also been considered theoretically [66], where it is shown that τ sf may be reduced by over two orders of magnitude between 10 and 300 K. Thus, our obtained value of τ sf =189 ps for MAPbBr 3 at 300 K is eminently remarkable. Please note this estimated value could be larger since the dynamical spin injection method used in this paper, the spin lifetime may be reduced by coupling of the spins at the surface of the MAPbBr 3 single crystal to the adjacent NiFe layer [77,78].…”