Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.

Sn-based perovskites are promising Pb-free photovoltaic materials with an ideal 1.3 eV bandgap.However, to date, Sn-based thin film perovskite solar cells have yielded relatively low power conversion efficiencies (PCEs). This is traced to their poor photophysical properties (i.e., short diffusion lengths (<30 nm) and two orders of magnitude higher defect densities) than Pb-based systems. Herein we reveal that melt-synthesized cesium tin iodide (CsSnI 3 ) ingots containing high quality large single crystal (SC) grains transcend these fundamental limitations. Through detailed optical spectroscopy, we uncover their inherently superior properties, with bulk carrier lifetimes reaching 6.6 ns, doping concentrations of around 4.5 × 10 17 cm -3 , and minority-carrier diffusion lengths approaching 1 µm, as compared to their polycrystalline counterparts: ~54 ps, >9.2 × 10 18 cm -3 , and 16 nm, respectively. CsSnI 3 SCs also exhibit very low surface recombination velocity of <2 × 10 3 cm.s -1 similar to Pb-based perovskites. Importantly, these key parameters are comparable to highperformance p-type photovoltaic materials (e.g., InP crystals). Our findings predict a PCE of ~23% for optimized CsSnI 3 SCs solar cells, highlighting their great potential.

Ion migration, one of the causes of hysteresis effects, is the bane of halide perovskite optoelectronics. Herein, we leverage this unwelcome trait to unlock new opportunities for resistive switching using layered Ruddlesdsen-Popper perovskites (RPP) and explicate the underlying mechanisms. The ON/OFF ratio of RPP-based devices is strongly dependent on the layers and peaks at = 5 demonstrating the highest ON/OFF ratio of ~10 4 and minimal operation voltage in 1.0 mm 2 devices. A long data retention even in 60% relative humidity and stable write/erase capabilities exemplifies their potential for memory applications. Activation studies during impedance spectroscopy measurements reveal that the high resistive state is related to impeded charge injection from the contacts. A chemical reaction between migrating ions and the external contacts modify the charge transfer barrier at the interface. The interplay between perovskite compositions and external electrodes confirm that ions population, migration and their reactivity with external contacts are essential for resistive switching. Our findings uncover the origin of resistive switching and a new family of facile materials and devices for switching and memory applications.

The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar-cell devices is inspired by solution-processed organic-inorganic lead halide perovskites that exhibit unprecedented light-to-electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light-emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic-inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite-metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon-metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.

The role of anti-solvent treatment on the morphological and optoelectronic properties of multiple-cation and mixed-halide perovskite solar cells have been studied.

In recent years, two-dimensional (2D) Ruddlesden-Popper perovskites have emerged as promising candidates for environmentally stable solar cells, highly efficient light-emitting diodes, and resistive memory devices. The remarkable existence of self-assembled quantum well (QW) structures in solution-processed 2D perovskites offers a diverse range of optoelectronic properties, which remain largely unexplored. Here, we experimentally observe ultrafast relaxation of free carriers in 20 ps due to the quantum confinement of free carriers in a self-assembled QW structures that form excitons. Furthermore, hybridizing the 2D perovskites with metamaterials on a rigid and a flexible substrate enables modulation of terahertz fields at 50-GHz modulating speed, which is the fastest for a solution-processed semiconductor-based photonic device. Hence, an exciton-based ultrafast response of 2D perovskites opens up large avenues for a wide range of scalable dynamic photonic devices with potential applications in flexible photonics, ultrafast wavefront control, and short-range wireless terahertz communications.

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Excitonic effects underpin the fascinating optoelectronic properties of 2D perovskites that are highly favorable for photovoltaics and light‐emitting devices. Analogous to switching in transistors, manipulating these excitonic properties in 2D perovskites using coherent phonons could unlock new applications. Presently, a detailed understanding of this underlying mechanism remains modest. Herein, the origins of the carrier‐phonon coupling in 2D perovskites using transient absorption (TA) spectroscopy are explicated. The exciton fine structure is modulated by coherent optical phonons dominated by the vibrational motion of the PbI6 octahedra via deformation potential. Originating from impulsive stimulated Raman scattering, these coherent vibrations manifest as oscillations in the TA spectrum comprising of the generation and detection processes of coherent phonons. This two‐step process leads to a unique pump‐ and probe‐energy dependence of the phonon modulation determined by the imaginary part of the refractive index and its derivative, respectively. The phonon frequency and lattice displacement of the inorganic octahedra are highly dependent on the organic cation. This study injects fresh insights into the exciton–phonon coupling of 2D perovskites relevant for emergent optoelectronics development.