Cesium lead bromide (CsPbBr 3) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr 3 (MA=CH 3 NH 3 +). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e. low-temperature solution-based growth, are needed. We show here two simple ways: anti-solvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr 3 , to grow single crystals of CsPbBr 3 from a precursorsolution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr 3 perovskite crystals, avoiding a CsBr-rich (or PbBr 2rich) composition, which is often present alongside the perovskite phase. Further growth has been demonstrated to be possible with crystal-seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar solvent (DMSO), which is similar to those used to grow hybrid organic-inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals.

The remarkable optoelectronic, and especially photovoltaic performance of hybrid-organicinorganic perovskite (HOIP) materials drives efforts to connect materials properties to this performance. From nano-indentation experiments on solution-grown single crystals we obtain elastic modulus and nano-hardness values of APbX 3 (A=Cs, CH 3 NH 3 ; X=I, Br). The Young's moduli are ~14, 19.5 and 16 GPa, for CH 3 NH 3 PbI 3 , CH 3 NH 3 PbBr 3 and CsPbBr 3 , respectively, lending credence to theoretically calculated values. We discuss possible relevance of our results to suggested 'self-healing', ion diffusion and ease of manufacturing. Using our results, together with literature data on elastic moduli, we classified HOIPs amongst relevant materials groups, based on their elasto-mechanical properties.

Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI 3 ). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI 3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI 3 (unlike MAPbBr 3 ) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material's relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity's hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI 3 , we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material's noncentrosymmetry. We note that the material's ferroelectric nature, can, but need not be important in a PV cell at room temperature.halide perovskites | photovoltaics | semiconductors | ferroelectricity | pyroelectricity N ew optoelectronic materials are of interest for producing solar cells with higher power and voltage efficiencies, lower costs, and improved long-term reliability. A very recent entry is the family of halide perovskites (HaPs), in particular those based on methylammonium (MA) lead iodide (MAPbI 3 ), MAPbBr 3 , and its inorganic analog CsPbBr 3 . Devices based on these perform remarkably well as solar cells (1, 2), as well as in other optoelectronic applications, such as LEDs and electromagnetic radiation detectors (3-5). Understanding possible unique characteristics of HaPs may show the way to other materials with similar key features.The ABX 3 (X = I, Br, Cl) HaP semiconductors (SCs), that is, with perovskite or perovskite-like structures, reach, via a steep absorption edge, a high optical absorption coefficient (∼10 5 cm −1 ) (6, 7), long charge carrier lifetimes (∼0.1-1 μs) (8), and reasonable carrier mobilities (less than or equal to ∼100 cm 2 ·V −1 ·s −1 ) (9), and have a low exciton binding energy (10). With these characteristics, the thickness of the optical absorber layer can be ≤0.5 μm, which allows the charge carriers (separated electrons and holes) to diffuse/d...

Self-healing, where a modification in some parameter is reversed with time without any external intervention, is one of the particularly interesting properties of halide perovskites. While there are a number of studies showing such self-healing in perovskites, they all are carried out on thin films, where the interface between the perovskite and another phase (including the ambient) is often a dominating and interfering factor in the process. Here, self-healing in perovskite (methylammonium, formamidinium, and cesium lead bromide (MAPbBr , FAPbBr , and CsPbBr )) single crystals is reported, using two-photon microscopy to create damage (photobleaching) ≈110 µm inside the crystals and to monitor the recovery of photoluminescence after the damage. Self-healing occurs in all three perovskites with FAPbBr the fastest (≈1 h) and CsPbBr the slowest (tens of hours) to recover. This behavior, different from surface-dominated stability trends, is typical of the bulk and is strongly dependent on the localization of degradation products not far from the site of the damage. The mechanism of self-healing is discussed with the possible participation of polybromide species. It provides a closed chemical cycle and does not necessarily involve defect or ion migration phenomena that are often proposed to explain reversible phenomena in halide perovskites.

The realization of high-quality optoelectronic properties in halide perovskite semiconductors through low-temperature, low energy processing is unprecedented. Understanding the unique aspects of the formation chemistry of these semiconductors is a critical step toward understanding the genesis of high quality material via simple preparation procedures. The toolbox of preparation procedures for halide perovskites grows rapidly. The prototypical reaction is that between lead iodide (PbI2) and methylammonium iodide (CH3NH3I, abbr. MAI) to form the perovskite CH3NH3PbI3 (MAPbI3), which we discuss in this work. We investigate the conversion of small, single-crystalline PbI2 crystallites to MAPbI3 by two commonly used synthesis processes: reaction with MAI in solution or as a vapor. The single crystal nature of the PbI2 precursor allows definitive conclusions to be made about the relationship between the precursors and the final product, illuminating previously unobserved aspects of the reaction process. From in situ photoluminescence microscopy, we find that the reaction in solution begins via isolated nucleation events followed by growth from the nuclei. We observe via X-ray diffraction and morphological characterization that there is a strong orientational and structural relationship between the final stage of the solution-reacted MAPbI3 product and the initial PbI2 crystallite. In all these measurements, we find that the reaction does not proceed below a certain MAI threshold concentration, which allows the first experimental determination of a free energy of formation for a widely used synthetic procedure of ∼0.1 eV. From these conclusions, we present a more detailed hypothesis about the reaction pathway than has yet been proposed: Our results suggest that the reaction in solution begins with a topotactic nucleation event followed by grain growth by dissolution–reconstruction. By similar techniques, we find the reaction via vapor phase produces material lacking a preferred orientation, suggesting the transformation is dominated by a deconstruction–reconstruction process due to the higher thermal energy involved. We also find that the crystal lattice structure of the vapor-reacted material is clearly different from that of the solution-phase reaction due to the temperature conditions of the synthesis.