Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

The high-temperature, all-inorganic CsPbI3 perovskite black phase is metastable relative to its yellow, nonperovskite phase at room temperature. Because only the black phase is optically active, this represents an impediment for the use of CsPbI3 in optoelectronic devices. We report the use of substrate clamping and biaxial strain to render black-phase CsPbI3 thin films stable at room temperature. We used synchrotron-based, grazing incidence, wide-angle x-ray scattering to track the introduction of crystal distortions and strain-driven texture formation within black CsPbI3 thin films when they were cooled after annealing at 330°C. The thermal stability of black CsPbI3 thin films is vastly improved by the strained interface, a response verified by ab initio thermodynamic modeling.

The development of green, sustainable, and economical chemical processes represents a cornerstone challenge within chemistry today. Semiconductor heterogeneous photocatalysis is currently utilized within a wide variety of societally impactful processes, spanning reactions such as hydrogen production and CO2 conversion, to the organic transformation of raw materials for value-added chemicals. Metal halide perovskites (MHPs) have recently emerged as a new promising class of cheap and easy to make photocatalytic semiconductors, though their unstable ionically bound crystal structure has thus far restricted widespread application. In this Review, we examine the issues hampering MHP-based photocatalysis and highlight the general approaches being taken to achieve promising and stable photocatalytic reaction environments. Specifically, we outline the adoption of (1) halogen acid solutions (i.e., HX; X = I or Br) for hydrogen evolution reactions, (2) relatively low-polarity solvents for CO2 photoreduction and organic transformations, and (3) the encapsulation of perovskites for CO2 reduction and water splitting. Further, we detail the measures being taken to arrive at intrinsically stable photocatalytic materials, removing the need for atypical environments. With each technology offering unique sets of benefits and challenges, we conclude by outlining potentially promising opportunities and directions for metal halide perovskite-based photocatalysis research moving forward.

The recent surge of scientific interest for lead halide perovskite semiconductors and optoelectronic devices has seen a mix of materials science sub-fields converge on the same “magical” crystal structure.

The room-temperature charge carrier mobility and excitation-emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite CsAgBiBr via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons- via the Fröhlich interaction-dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two "single" and "double" perovskite materials branches.

The impressive optoelectronic performance and low production cost of metal halide perovskites have inspired applications well beyond efficient solar cells. Herein, we widen the materials engineering options available for the efficient and selective photocatalytic oxidation of benzylic alcohols, an industrially significant reaction, using formamidinium lead bromide (FAPbBr3) and other perovskite-based materials. The best performance was obtained using a FAPbBr3/TiO2 hybrid photocatalyst under simulated solar illumination. Detailed optical studies reveal the synergetic photophysical pathways arising in FAPbBr3/TiO2 composites. An experimentally supported model rationalizing the large conversion enhancement over the pure constituents shows that this strategy offers new prospects for metal halide perovskites in photocatalytic applications.

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.

The sensitive detection of X-rays embodies an important area of research, being motivated by a common desire to minimize the doses of ionising radiation required for detection. Amongst metal halide perovskites, the double perovskite Cs 2 AgBiBr 6 system has recently emerged as a highly promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity (105 µC.Gy −1 cm −2 ). Herein, we detail the important photophysical pathways in single crystal Cs 2 AgBiBr 6 at both room and liquid nitrogen temperatures, with emphasis made toward understanding the carrier dynamics which influence X-ray sensitivity. Our study draws upon a combination of optical probes and we develop a room temperature excitation model which is far from optimal, being plagued by a large trap density and fast recombination pathways above the conduction band minimum. We find that substantially improved operating conditions result at 77 K, due to a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. We characterise the temperature dependence of a single crystal Cs 2 AgBiBr 6 X-ray detecting device and reveal a strong and monotonic enhancement to the X-ray sensitivity upon cooling, moving from 316 µC.Gy −1 cm −2 at room temperature to 988 µC.Gy −1 cm −2 near liquid nitrogen temperatures. We conclude that even modest cooling -via thermoelectric Peltier device -will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.