Abstract
The dynamic and static Rashba effects in hybrid methylammonium (MA) lead halide perovskites have recently been theoretically predicted. However, only the static effect was experimentally confirmed so far. Here, we report on the dynamic (sub-picosecond/picosecond timescale) and static (nanosecond/microsecond timescale) Rashba effects observed in a fully encapsulated layer with various thicknesses (ranging from ∼40 nm to ∼100 nm) of ∼20-nm-sized 3D MAPbBr3 nanocrystals (NCs) using transient absorption (TA) spectroscopy. The effect appears as a splitting of the corresponding peaks in TA spectra. We argue that the physical reason for the Rashba effect to be observed is fundamentally determined by configurational entropy loss in NCs possessing a strong spin asymmetry. Specifically, owing to an enhanced flexibility of the NC lattice, a built-in electric field initially induced by an ultrashort (100 fs) pumping pulse through the photo-Dember effect and subsequently developed due to dynamic charge separation throughout NCs is able to initiate the order–disorder transition associated with the MA cation reorientations, the process that efficiently breaks structural inversion symmetry and hence induces the Rashba spin–orbit interaction. The dynamic Rashba effect is found to be strongly dependent on photoexcited carrier density (pumping power), whereas it weakens sharply upon increasing the NC layer thickness up to ∼80 nm due to the NC stacking effect. The integrated intensities of the corresponding spin-split subbands demonstrate a photon-helicity-dependent asymmetry, thus proving the Rashba-type spin-splitting. The magnitudes of the Rashba and Fröhlich polaron effects and the methods of controlling the dynamic Rashba effect are discussed.
Highlights
Owing to an enhanced flexibility of the NC lattice, a built-in electric field initially induced by an ultrashort (100 fs) pumping pulse through the photo-Dember effect and subsequently developed due to dynamic charge separation throughout NCs is able to initiate the order–disorder transition associated with the MA cation reorientations, the process that efficiently breaks structural inversion symmetry and induces the Rashba spin–orbit interaction
We argue that the physical reason for the Rashba effect to be observed in a thin layer of MAPbBr3 NCs is fundamentally determined by configurational entropy loss in MAPbBr3 NCs,13,19–21 resulting in an enhanced flexibility of the crystal lattice containing heavy atoms with strong spin–orbit coupling (SOC)
This article provides direct experimental evidence for the dynamic and static Rashba effects induced by the built-in electric field initially photoexcited in a thin layer of 3D MAPbBr3 NCs by an ultrashort (∼100 fs) laser pulse through the photo-Dember effect and developed due to dynamic charge separation throughout NCs
Summary
Methylammonium (MA) lead halide perovskites appear to be one of the most promising photovoltaic semiconductors for solar cell development. numerous technological issues to commercialize this hybrid system are yet to be resolved, its unique properties, such as long-lived photoexcited carriers and extremely long carrier diffusion lengths, have special fundamental interest. We argue that the physical reason for the Rashba effect to be observed in a thin layer of MAPbBr3 NCs is fundamentally determined by configurational entropy loss in MAPbBr3 NCs, resulting in an enhanced flexibility of the crystal lattice containing heavy atoms with strong SOC This effect allows for reorientations of MA cations, which can be achieved when a built-in electric field initially induced by an ultrashort (100 fs) pumping pulse through the photo-Dember effect and subsequently developed due to dynamic charge separation throughout NCs initiates an order–disorder transition efficiently breaking structural inversion symmetry and inducing the Rashba spin–orbit interaction.. The latter behavior in combination with strong dependences on pumping power and the NC layer thickness suggest a way to control the magnitude of the dynamic Rashba effect in optoelectronic devices
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