Carrier recombination in CH3NH3PbI3: why is it a slow process?

Research Progress on Rashba Effect in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites

The Rashba effect appears in the semiconductors with an inversion-asymmetric structure and strong spin-orbit coupling, which splits the spin-degenerated band into two sub-bands with opposite spin states. The Rashba effect can not only be used to regulate carrier relaxations, thereby improving the performance of photoelectric devices, but also used to expand the applications of semiconductors in spintronics. In this mini-review, recent research progress on the Rashba effect of two-dimensional (2D) organic-inorganic hybrid perovskites is summarized. The origin and magnitude of Rashba spin splitting, layer-dependent Rashba band splitting of 2D perovskites, the Rashba effect in 2D perovskite quantum dots, a 2D/3D perovskite composite, and 2D-perovskites-based van der Waals heterostructures are discussed. Moreover, applications of the 2D Rashba effect in circularly polarized light detection are reviewed. Finally, future research to modulate the Rashba strength in 2D perovskites is prospected, which is conceived to promote the optoelectronic and spintronic applications of 2D perovskites.

Stabilizing Polar Domains in MAPbBr3 via the Hydrostatic Pressure-Induced Liquid Crystal-like Transition

Pressure-induced phases of MAPbBr₃ were investigated at room temperature in the range of 0-2.8 GPa by ab initio molecular dynamics. Two structural transitions at 0.7 GPa (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involved both the inorganic host (lead bromide) and the organic guest (MA). MA dipoles behave like a liquid crystal undergoing isotropic → isotropic and isotropic → oblate nematic transitions as pressure confines their orientational fluctuations to a crystal plane. Beyond 1.1 GPa, the MA ions lie alternately along two orthogonal directions in the plane forming stacks perpendicular to it. However, the molecular dipoles are statically disordered, leading to stable polar and antipolar MA domains in each stack. H-Bond interactions, which primarily mediate host-guest coupling, facilitate the static disordering of MA dipoles. Interestingly, high pressures suppress CH₃ torsional motion, emphasizing the role of C-H···Br bonds in the transitions.

Lattice disorder influences the photocarrier dynamics in lead halide perovskites

Revealing the structural impact of lead halide perovskites on photocarrier dynamics is essential for the associated solar cells but deficient in experimental visualization. In this study, with femtosecond spectroscopy, we for the first time explored the contribution of the disorder of the distorted PbX₆ octahedrons and A-site cations on the carrier behaviours. It was found that photoinduced carriers recombine almost twice slower and diffuse 20% faster in the disordered, β-phased samples than in the ordered, γ-phased ones. Yet within the same phase, with a similar PbX₆ orientation but various A-site mobility, the carrier diffusion and recombination have no apparent difference. Thus we firmly conclude that lattice disorder effectively influences the carrier dynamics and therein the Pb-X sublattice is worth more than A-site cations, which should inspire future lead halide perovskite design and applications.

How cation nature controls the bandgap and bulk Rashba splitting of halide perovskites

Because of instability issues presented by metal halide perovskites based on methylammonium (MA), its replacement to Cs $$ \mathrm{Cs} $$ has emerged as an alternative to improve the materials' durability. However, the impact of this replacement on electronic properties, especially gap energy and bulk Rashba splitting remains unclear since electrostatic interactions from organic cations can play a crucial role. Through first-principles calculations, we investigated how organic/inorganic cations impact the electronic properties of MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ and CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ perovskites. Although at high temperatures the organic cation can assume spherical-like configurations due to its rotation into the cages, our results provide a complete electronic mechanism to show, from a chemical perspective based on ab initio calculations at 0 K $$ 0\ \mathrm{K} $$ , how the MA $$ \mathrm{MA} $$ dipoles suppression can reduce the MAPbI 3 $$ {\mathrm{MAPbI}}_3 $$ gap energy by promoting a degeneracy breaking in the electronic states from the PbI 3 $$ {\mathrm{PbI}}_3 $$ framework, while the dipole moment reinforcement is crucial to align theory ↔ $$ \leftrightarrow $$ experiment, increasing the bulk Rashba splitting through higher Pb $$ \mathrm{Pb} $$ off-centering motifs. The lack of permanent dipole moment in Cs $$ \mathrm{Cs} $$ results in CsPbI 3 $$ {\mathrm{CsPbI}}_3 $$ polymorphs with a pronounced Pb $$ \mathrm{Pb} $$ on-centering-like feature, which causes suppression in their respective bulk Rashba effect.

IR Spectroscopic Degradation Study of Thin Organometal Halide Perovskite Films

The advantages of IR spectroscopy include relatively fast analysis and sensitivity, which facilitate its wide application in the pharmaceutical, chemical and polymer sectors. Thus, IR spectroscopy provides an excellent opportunity to monitor the degradation and concomitant evolution of the molecular structure within a perovskite layer. As is well-known, one of the main limitations preventing the industrialization of perovskite solar cells is the relatively low resistance to various degradation factors. The aim of this work was to study the degradation of the surface of a perovskite thin film CH₃NH₃PbI_3-xCl_x caused by atmosphere and light. To study the surface of CH₃NH₃PbI_3-xCl_x, a scanning electron microscope, infrared (IR) spectroscopy and optical absorption were used. It is shown that the degradation of the functional layer of perovskite proceeds differently depending on the acting factor present in the surrounding atmosphere, whilst the chemical bonds are maintained within the perovskite crystal structure under nitrogen. However, when exposed to an ambient atmosphere, an expansion of the NH₃⁺ band is observed, which is accompanied by a shift in the N-H stretching mode toward higher frequencies; this can be explained by the degradation of the perovskite surface due to hydration. This paper shows that the dissociation of H₂O molecules under the influence of sunlight can adversely affect the efficiency and stability of the absorbing layer. This work presents an approach to the study of perovskite structural stability with the aim of developing alternative concepts to the fabrication of stable and sustainable perovskite solar cells.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ (MAPbI₃) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI₃. Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI₃ polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI₃ and are indispensable for optoelectronic device development with better performance.