Halide perovskites have attracted tremendous attention from many researchers recently, particularly for their excellent optoelectronic properties in applications such as photovoltaic solar cells and light-emitting diodes. In recent years, the application of halide perovskites has rapidly extended into nanoelectronics, such as thermoelectric, memory, and artificial synapse applications. Halide perovskites can be synthesized easily, even at relatively low temperatures, and organic and inorganic ions can even coexist in one crystal structure. Moreover, the structural flexibility is excellent, where two- and three-dimensional crystals can be linked together. The combination of various types of halide ions not only controls the physical properties of the halide perovskite, but also facilitates control of the bandgap by varying the size of nanoparticles when they exhibit quantum effects. Halide perovskites thus provide an excellent platform for optoelectronics with interesting optical, electrical, and magnetic properties. The articles in this issue introduce the wide range of basic properties and potential applications of halide perovskites.

Perovskite solar cells are poised to be a game changer in photovoltaic technology with a current certified efficiency of 25.2%, already surpassing that for multicrystalline silicon solar cells. On the path to higher efficiencies and much needed higher stability, however, interfacial and bulk defects in the active material should be carefully engineered or passivated. Post-treatment techniques show great potential to address defect issues (e.g., by coarsening the perovskite grains or establishing an interfacial heterogeneous layer). In this article, we summarize current fundamental understanding of the major energy-loss routes in perovskite materials and devices, including bulk/interfacial defects mediated nonradiative recombination and band mismatch-induced recombination. This is followed by a survey of the important post-treatment techniques developed over the past few years to minimize energy loss in perovskite solar cells, including solvent annealing, amine halide solution dripping-induced Ostwald ripening, three-dimensional–two-dimensional interface layer from phenethylammonium iodide (PEAI) dripping, and wide bandgap interface layer engineering from n-hexyl trimethylammonium bromide washing. Finally, we provide a prospective view about further developments of post-treatment techniques.

The efficiency of halide perovskite solar cells has progressed rapidly through a series of major breakthroughs. Currently, a certified efficiency of 25.2% has been achieved for a solar cell using a polycrystalline thin film. This is the result of having reached 75% of the Shockley–Queisser limit for single-junction solar cells. However, for further improvements, new breakthrough technologies are required. This article reviews the impact of previous breakthrough technologies on the efficiency of halide perovskite solar cells, based on certified efficiencies. We clarify the current status of halide perovskite solar cells and introduce photon recycling as the next technological innovation for higher efficiencies. Photon recycling keeps the photon concentration inside the light-harvesting layer high, and consequently, leads to open-circuit voltages close to the theoretical value. Although photon recycling has not yet been implemented in real halide perovskite solar cells, three key technologies for implementing it are examined.

Metal-halide perovskite solar cells (PSCs) have become a promising candidate for photovoltaic applications. Current popular organic hole conductors for highly efficient PSCs bring cost and stability issues, which hinder the commercialization of the PSCs. Hole-conductor-free PSCs are attracting great interest because they eliminate the adverse effects of organic hole conductors by transporting holes in the perovskite itself. In this article, we summarize recent progress in conventional, inverted, and printable mesoscopic hole-conductor-free PSCs. Specifically, we emphasize the stunning stability and scale-up manufacturing of printable hole-conductor-free PSCs, discussing their potential from laboratory to market. The causes for hole-conductor-free PSCs’ current low efficiency are also discussed, and are primarily ascribed to energy-level alignment and interface recombination. We believe that the efficiencies of hole-conductor-free PSCs can be enhanced to be comparable with hole-conductor-containing PSCs by interface modification and material design.

Metal-halide perovskites, in particular their nanocrystal forms, have emerged as a new generation of light-emitting materials with exceptional optical properties, including narrow emissions covering the whole visible region with high photoluminescence quantum efficiencies of up to near-unity. Remarkable progress has been achieved over the last few years in the areas of materials development and device integration. A variety of synthetic approaches have been established to precisely control the compositions and microstructures of metal-halide perovskite nanocrystals (NCs) with tunable bandgaps and emission colors. The use of metal-halide perovskite NCs as active materials for optoelectronic devices has been extensively explored. Here, we provide a brief overview of recent advances in the development and application of metal-halide perovskite NCs. From color tuning via ion exchange and manipulation of quantum size effects, to stability enhancement via surface passivation, new chemistry for materials development is discussed. In addition, processes in optoelectronic devices based on metal-halide perovskite NCs, in particular, light-emitting diodes and radiation detectors, will be introduced. Opportunities for future research in metal-halide perovskite NCs are provided as well.

The presence of 6s2 (5s2) lone-pair electrons on the B-site Pb (Sn) in all-inorganic and hybrid halide ABX3 perovskites distinguishes these materials from the familiar tetrahedral semiconductors traditionally employed in optoelectronics and is key to many of their appealing properties. These electrons are stereochemically active, albeit often in a hidden fashion, resulting in unusual and highly anharmonic lattice dynamics that are linked to many of the special optoelectronic properties displayed by this material class. This article describes the connections between this atypical electronic configuration and the electronic structure and lattice dynamics of these compounds. We illustrate how the lone pair leads to favorable bandwidths and band alignments, mobile holes, large ionic dielectric response, large positive thermal expansion, and even possibly defect-tolerant electronic transport. Taken together, the evidence suggests that other high-performing semiconductors may be found among compounds with lone-pair-bearing cations in high symmetry environments and a high degree of connectivity between atoms.

Although halide perovskites (HaPs) are synthesized in ways that appear antithetical to those required for yielding high-quality semiconductors, the properties of the resulting materials imply, particularly for single crystals, ultralow densities of optoelectronically active defects. This article provides different views of this unusual behavior. We pose the question: Can present models of point defects in solids be used to interpret the experimental data and provide predictive power? The question arises because the measured ultralow densities refer to static defects using our present methods and models, while dynamic defect densities are ultrahigh, a result of the material being relatively soft, with a shallow electrostatic energy landscape, and with anharmonic lattice dynamics. All of these factors make the effects of dynamic defects on the materials’ optoelectronic properties minimal. We hope this article will stimulate discussions on the nontrivial question: Are HaPs, and especially the defects within them, business as usual?

Quantum computing, sensing, and communications are emerging technologies that may circumvent known limitations of their existing traditional counterparts. While the promises of these technologies are currently narrow in scope, it is possible that they will broadly impact our lives by revolutionizing the capabilities of data centers and medical diagnostics, for example. At the heart of these technologies is the use of a quantum object to contain information, called a quantum bit or qubit. Current realizations of qubits exist in a broad variety of material systems, including individual spins in semiconductors or insulators, superconducting circuits, and trapped ions. Further advancement of qubits requires significant contributions from materials science in areas of materials selection, synthesis, fabrication, simulation and characterization. Here, we discuss some of the needs and opportunities for contributions to advance the fundamental understanding of materials used in quantum information applications.

Hydrogen, the simplest of all molecules, made of the simplest of all atoms, is a material that has successfully accomplished many historic missions: it has powered the engines of space rockets and has served in the ammonia-based fertilizer revolution, the iron and steel sector, and electronics manufacturing. But the task of putting hydrogen in the center of the global energy scene has proven tantalizing, perpetually coming closer to materialization, but repeatedly being pushed to the future. Is it possible that the time has arrived for hydrogen to finally come into its own?