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Most efficient perovskite solar cells are based on polycrystalline thin films; however, substantial structural disorder and defective grain boundaries place a limit on their performance. Perovskite single crystals are free of grain boundaries, leading to significantly low defect densities and thus hold promise for high-efficiency photovoltaics. However, the surfaces of perovskite single crystals present a major performance bottleneck because they possess a higher density of traps than the bulk. Hence, it is crucial to understand and control the surface trap population to fully exploit perovskite single crystals. This perspective highlights the importance of surface-trap management in unleashing the potential of perovskite single-crystal photovoltaics and discusses strategies to take this technology beyond the proof-of-concept stage.

Despite the well-known implications in the field of III–V semiconductors, lattice strain in halide perovskite materials has been largely overlooked until recently. Here, we review the effect of lattice strain on the structural, chemical, and optoelectronic properties of metal halide perovskites to understand how strain engineering can be applied to improve device performance. We start by arguing that perovskites, like any other semiconducting material, are not immune to the negative effects of mismanaged strain. We analyze the originand detrimental consequencesof lattice strain in perovskite crystals and heterostructures. We then discuss how strain management addresses the polymorphism issue of some of the most desirable perovskite compositions, and how it prevents the harmful migration of ions in perovskites. We conclude by offering our perspective on the unexplored potential of strain engineering and argue that its controlled management can lead to untapped territories, including perovskite large-area single-crystalline thin films and electrically pumped lasers.

Lead (Pb) in conventional perovskite solar cells (PSCs) is toxic and has to be replaced. Situated in one group of the periodic table of elements, tin (Sn) has the same valence electrons' configuration as Pb (ns 2 np 2 ), promising analogous chemical properties. Hence, Sn is considered a suitable replacement to Pb. However, because of the lack of lanthanide shrinkage, Sn behaves differently: Pb is stable in Pb 2+ form, an oxidation state needed for perovskite structure, while Sn tends to lose all its valence electrons forming Sn 4+ . As a result, PSCs based on Sn are not efficient. Traces of oxygen have been conventionally discussed as a source of Sn oxidation. But recent findings point to the oxidation of Sn-based perovskites even in the absence of oxygen. This perspective summarizes recentlydiscovered unconventional oxidation pathways of Sn perovskites, including reaction with solvent molecules and disproportionation. We explain these phenomena by a Frost−Ebsworth diagram and argue that a deeper understanding of this diagram is a key toward stable and efficient Pb-free Sn-based PSCs.

Lead-free halide light-emitting diodes (LEDs) are fabricated using nontoxic and earth-abundant CsCu2I3 with a strong yellow emission at a peak wavelength of 568 nm. CsCu2I3-based host–dopant emitters are formed by vacuum thermal evaporation (VTE) film codeposition process instead of the commonly used solution-based film deposition process. Using the VTE process, extremely thin (30 nm) host–dopant emitters have successfully been formed with the CsCu2I3 dopant and various organic host molecules. A bright yellow emission with a photoluminescence quantum yield value of 84.8% is achieved in the 0.5% CsCu2I3-doped halide emitter film due to the successful spatial localization of charge carriers and excitons using an organic host with appropriate energy levels to CsCu2I3. With the further enhancement in charge balance using the cohost system, a record-breaking lead-free halide LED has been fabricated with an EQE of 7.4%. The lead-free halide LEDs are also highly stable in the device operation with LT70 of 20 h at 100 cd/m2.

The conventional approach to search for new materials is to synthesize a limited number of candidates. However, this approach might delay or prevent the discovery of better-performing materials due to the narrow composition space explored. Here, we fabricate binary alloy films with a composition gradient in a single shot in less than one minute. We apply this approach to study the stability of halide perovskites. We synthesize all possible binary compositions from MAPbI3 and MAPbBr3 and then study their optical properties, structure, and environmental stability in a high-throughput manner. We find that perovskite alloys experience three different degradation mechanisms depending on halogen content: bromine-rich perovskites degrade by hydration, iodine-rich perovskites by the loss of the organic component, and all other intermediate alloys by phase segregation. The proposed method offers an avenue for discovering new materials and processing parameters for a wide range of applications that rely on compositional engineering.

All-organic infrared (IR)-to-visible upconversion organic light-emitting diodes (OLEDs) with an IR sensitivity up to 1100 nm were fabricated using a low-band-gap polymer as the organic IR sensitizing layer. A novel low-band-gap (1 eV) polymer, poly 4-(4,8-bis­(5-(2-butyloctyl)­thiophen-2-yl)­benzo­[1,2-b:4,5-b′]­dithiophen-2-yl)-6,7-diethyl-[1,2,5] thiadiazolo­[3,4-g]­quinoxaline (PBDTT-BTQ), with a strong photoresponse in near-IR wavelengths of 700–1100 nm was first synthesized using a thiadiazolo­[3,4]­quinoxaline (BTQ) and a thiophene-substituted benzo­[1,2-b:4,5-b2-b]­dithiophene (BDTT) as the electron-withdrawing and donating building blocks, respectively. The near-IR photodetector was then fabricated for evaluating a PBDTT-BTQ as the IR sensitizing layer. The PBDTT-BTQ IR photodetector showed detectivity greater than 1011 Jones in the multispectral region (300–1100 nm) and the maximum detectivity of 3.1× 1011 Jones at the wavelength of 1000 nm due to significantly reducing dark current (8.8 × 10–6 mA/cm2 at −1 V). Finally, the all-organic IR upconversion OLED with a PBDTT-BTQ IR sensitizer successfully converted invisible near-IR light of 700–1100 nm directly to visible green light with a peak emission wavelength of 520 nm. This is the first report of an all-organic IR-to-visible upconversion OLED with near-IR sensitivity up to 1100 nm.

Direct conversion X-ray detectors operate by directly converting X-ray photons into an electrical signal, while indirect conversion detectors (also called scintillators) first convert X-ray photons to visible light. Subsequently, this light is converted to an electrical signal using visible light detectors. Direct conversion detectors are simpler in configuration and offer higher spatial resolution than scintillators. [2] Among traditional semiconductors, amorphous selenium (a-Se), [3] and Cd 1−x Zn x Te (CZT, x < 20%)-based materials [4] dominate the market of commercial X-ray detectors. a-Se has a low device dark current and stable device performance; however, it possesses low X-ray absorptivity. On the other hand, CZT offers excellent absorptivity, but requires high-temperature processing conditions (>900 °C), [5] and suffers from structural imperfections [5,6] and compositional inhomogeneity. [6,7] These limitations demand the development of novel semiconductors for direct X-ray detectors.Solution-processed metal halide perovskites have recently emerged as a family of unique semiconductors for X-ray detectors. [2b,8] Due to their elemental constitution of heavy atoms such as lead and iodine, metal halide perovskites have a high X-ray attenuation coefficient. [2b,8b,9] However, conventional perovskites suffer from long-term instability, and high dark current (determines noise level). [10] Early diagnoses of diseases requires high spatial resolution, which is measured by recording the resolving power of a line-pair (lp) phantom. [11] For radiology, a minimum spatial resolution of 5.7 lp mm −1 is required for adequate resolution of small objects. [12] Imaging becomes more challenging when it comes to mammographic applications, where a resolution of 10 lp mm −1 is needed to resolve small microcalcifications. [13] Unfortunately, conventional direct conversion X-ray detectors have low resolution as they are integrated into transistor arrays with limited pixel dimensions. [14] Here we report a strategy to grow aligned orthorhombic δ-CsPbI 3 microwires offering one of the highest X-ray absorption coefficients. The crystals show a record-low dark current density of 12 pA mm −2 under 600 V mm −1 electric field. A Schottky junction with δ-CsPbI 3 is able to sense dose rates as low as 33.3 nGy air s −1 . We also show an X-ray spatial resolution of ≈12.4 lp mm −1 , which is one of the highest values reported to date (Table S1, Supporting Information). The fabricated X-ray detectors are used widely, but they suffer from short operational stability and insufficient absorptivity of X-ray photons. Here, a strategy for roomtemperature, solution-grown δ-CsPbI 3 monocrystalline microwires exhibiting one of the highest linear X-ray absorption coefficients among known semiconductors is reported. In a metal-semiconductor-metal architecture, δ-CsPbI 3 demonstrates one of the lowest detectable X-ray dose rates at 33.3 nGy air s −1 , enabled by its exceptionally low dark current density of 12 pA mm −2 . The detector remains stab...