APbBr3 (A = Cs, CH3NH3) are prototype halide perovskites having bandgaps of 2.30–2.35 eV at room temperature, rendering their apparent color nearly identical (bright orange but opaque). Upon optical excitation, they emit bright photoluminescence (PL) arising from carrier recombination whose spectral features are also similar. At 10 K, however, the apparent color of CsPbBr3 becomes transparent yellow, whereas that of CH3NH3PbBr3 does not change significantly due to the presence of an indirect Rashba gap. With increasing the excitation level, evolution of the PL spectra, which are excitonic at 10 K, reveals the emergence of P-band emission arising from inelastic exciton–exciton scattering. Based on the spectral location of the P-band, exciton binding energies are determined to be 21.6 ± 2.0 and 38.3 ± 3.0 meV for CsPbBr3 and CH3NH3PbBr3, respectively. Intriguingly, upon further increase in the exciton density, electron–hole plasma appears in CsPbBr3 as evidenced by both red-shift and broadening of the PL. This phase, however, does not occur in CH3NH3PbBr3 presumably due to polaronic effects. Although the A-site cation is believed not to directly impact optical properties of APbBr3, our results underscore its critical role, which destines different high-density phases and apparent color at low temperatures.

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr3 and organic-inorganic CH3NH3PbBr3 (MAPbBr3) perovskite single crystals by employing temperature-and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. But the effect vanishes in CsPbBr3 at low temperatures (< 50 K), accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation, compared with the polar MA cation in MAPbBr3. We also show that static Rashba effect occurs only in MAPbBr3 below 90 K due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with poly methyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

In this study, a facile approach has been successfully applied to synthesize a W-doped Fe 2 O 3 /MoS 2 core−shell electrode with unique nanostructure modifications for photoelectrochemical performance. A two-dimensional (2D) structure of molybdenum disulfide (MoS 2 ) and tungsten (W)-doped hematite (W:α-Fe 2 O 3 ) overcomes the drawbacks of the α-Fe 2 O 3 and MoS 2 semiconductor through simple and facile processes to improve the photoelectrochemical (PEC) performance. The highest photocurrent density of the 0.5W:α-Fe 2 O 3 /MoS 2 photoanode is 1.83 mA•cm −2 at 1.23 V vs reversible hydrogen electrode (RHE) under 100 mW•cm 2 illumination, which is higher than those of 0.5W:α-Fe 2 O 3 and pure α-Fe 2 O 3 electrodes. The overall water splitting was evaluated by measuring the H 2 and O 2 evolution, which after 2 h of irradiation for 0.5W:α-Fe 2 O 3 /MoS 2 was determined to be 49 and 23.8 μmol.cm −2 , respectively. The optimized combination of the heterojunction and metal doping on pure α-Fe 2 O 3 (0.5W:α-Fe 2 O 3 /MoS 2 photoanode) showed an incident photon-to-electron conversion efficiency (IPCE) of 37% and an applied bias photon-to-current efficiency (ABPE) of 26%, which are around 5.2 and 13 times higher than those of 0.5W:α-Fe 2 O 3 , respectively. Moreover, the facile fabrication strategy can be easily extended to design other oxide/carbon-sulfide/oxide core−shell materials for extensive applications.

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