By actually addressing the title question, we provide a comprehensive and critical review of self-healing (SH) in lead-based halide perovskites (HaPs), a phenomenon with profound implications for the stability of these materials across all applications, from photovoltaics to light emission and radiation detection. We emphasize reasoning as a guide to interpreting the dynamic balance between degradation and recovery when HaPs are exposed to light, heat, mechanical stress, or radiation. We compile and assess what are, in our view, the most relevant, available reports of damage-healing dynamics, distinguishing verified facts and observations from interpretations and unresolved questions. Key topics include damage accumulation, light soaking, and photo-brightening, as well as the mechanistic roles of lattice dynamics, halide migration, redox chemistry, and acid-base equilibria in the disappearance of defects on accessible time scales. Thus, we go beyond a conventional summary by providing a unifying framework to clarify contradictions in the literature and reveal the underlying principles of reversible damage. By consolidating results that are often scattered into a coherent picture, we strive to establish a foundation for predictive models of SH kinetics, while guiding strategies to stabilize devices. We anticipate that this critical synthesis will serve as an authoritative reference for the metal halide perovskite research field.

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of halide perovskites (HaPs) and their devices. Even though DT lacked direct experimental evidence, it became a “fact” in the field. DT in semiconductors implies that structural defects do not translate to electrical and optical effects (e.g., due to charge trapping), associated with such defects. We present pioneering direct experimental evidence for DT in Pb-HaPs by comparing the structural quality of 2-dimensional (2D), 2D-3D, and 3D Pb-iodide HaP crystals with their optoelectronic characteristics using high-sensitivity methods. Importantly, we get information from the materials’ bulk because we sample at least a few hundred nanometers, up to several micrometers, from the sample’s surface, which allows for assessing intrinsic bulk (and not only surface-) properties of HaPs. The results point to DT in 3D, 2D-3D, and 2D Pb-HaPs. Overall, our data provide an experimental basis to rationalize DT in Pb-HaPs. These experiments and findings will help the search for and design of materials with real DT.

Self-healing materials can become game changers for developing sustainable (opto)electronics. APbX3 halide (=X-) perovskites, HaPs, have shown a remarkable ability to self-heal damage. While we demonstrated self-healing in pure HaP compounds, in single crystals, and in polycrystalline thin films (as used in most devices), HaP compositions with multiple A+ (and X-) constituents are preferred for solar cells. We now show self-healing in mixed A+ HaPs. Specifically, if at least 15 atom % of the methylammonium (MA+) A cation is substituted for by guanidinium (Gua+) or acetamidinium (AA+), then the self-healing rate after damage is enhanced. In contrast, replacing MA+ with dimethylammonium (DMA+), comparable in size to Gua+ or AA+, does not alter this rate. Based on the times for self-healing, we infer that the rate-determining step involves short-range diffusion of A+ and/or Pb2+ cations and that the self-healing rate correlates with the strain in the material, the A+ cation dipole moment, and H-bonding between A+ and I-. These insights may offer clues for developing a detailed self-healing mechanism and understanding the kinetics to guide the design of self-healing materials. Fast recovery kinetics are important from the device perspective, as they allow complete recovery in devices during operation or when switched off (LEDs)/in the dark (photovoltaics).

Interest in halide perovskites (HaPs) is motivated by the combination of superior optoelectronic properties, ease of synthesis, and a surprisingly low density of electrically active defects. HaPs possess high chemical sensitivity, especially those having an organic cation at their A position (AMX(3)). X-ray photoelectron spectroscopy (XPS) is a surface technique with sensitivity that goes down to a single atomic layer and provides unique information that relates the elemental composition with the chemical and electronic states of the elements in the material. Our study focuses on XPS imaging in combination with selected small-area spectra and uses aged (3 years old) solution-grown single crystals of mixed A-cation Cs(x)MA(1-x)PbBr(3) (MA = CH3NH3+) HaPs as a candidate for investigating intracrystal heterogeneity. With XPS, we followed the variations in chemical composition by measuring crystals at different regions down to 50 mu m diameter of the samples. By comparing the surface of the crystals with their cross-section, we found significant changes in the Cs+ and Br- concentrations, which increase toward the interior of the crystal. Contrarily, concentrations of carbon and nitrogen predominate on the top surface and especially at crystal edges, which form a partial covering of the crystals beyond the visible crystal boundaries, something that is not seen by electron microscopy analysis and shows the advantage of the XPS for measurements of light elements. Besides demonstrating the utility of the XPS technique, this compositional heterogeneity within the Cs(x)MA(1-x)PbBr(3) crystals reveals novel insights into the complex chemical nature of what may be seen as uniform single crystals and brings crucial information for their understanding.

Metal-free halide perovskites (MFHaPs) have garnered significant attention in recent years due to their desirable properties, such as low toxicity, light weight, chemical versatility, and potential for optoelectronics. MFHaPs with the formula A2+ B+ X-3 (where A is a large organic divalent cation, B+ is typically NH4 + , and X is a halide) have been studied extensively, but few studies have examined alternative cations at the B position. This paper reports the synthesis of three MFHaP-related single crystals, DABCO-N2 H5 -X3 (DABCO = N-N-diazabicyclo[2.2.2]octonium, X = Br and I) and (DABCO)3 -N2 H5 (NH4 )2 Cl9 , which feature hydrazinium (N2 H5 ) at the B position. The crystals have a perovskite-like, one-dimensional, edge-connected structure and exhibit optical and band structure properties. The crystals were then tested as X-ray detectors, where they showed excellent photoresponsivity, stability, and low background noise, owing to the large semi-gap that dictates long lifetimes. The detectors exhibited sensitivity as high as 1143 ± 10 µC Gyair -1 cm-2 and a low detection limit of 2.68 µGy s-1 at 10 V. The researchers suggest that the stronger hydrogen bonding in N2 H5 + compounds compared to NH4 + MFHaPs may contribute to the detectors' enhanced stability.

In terms of sustainable use, halide perovskite (HaP) semiconductors have a strong advantage over most other classes of materials for (opto)electronics, as they can self-heal (SH) from photodamage. While there is considerable literature on SH in devices, where it may not be clear exactly where damage and SH occur, there is much less on the HaP material itself. Here we perform "fluorescence recovery after photobleaching" (FRAP) measurements to study SH on polycrystalline thin films for which encapsulation is critical to achieving complete and fast self-healing. We compare SH in three photoactive APbI3 perovskite films by varying the A-site cation ranging from (relatively) small inorganic Cs through medium-sized MA to large FA (the last two are organic cations). While the A cation is often considered electronically relatively inactive, it significantly affects both SH kinetics and the threshold for photodamage. The SH kinetics are markedly faster for γ-CsPbI3 and α-FAPbI3 than for MAPbI3. Furthermore, γ-CsPbI3 exhibits an intricate interplay between photoinduced darkening and brightening. We suggest possible explanations for the observed differences in SH behavior. This study's results are essential for identifying absorber materials that can regain intrinsic, insolation-induced photodamage-linked efficiency loss during its rest cycles, thus enabling applications such as autonomously sustainable electronics.

The future of halide perovskites (HaPs) is beclouded by limited understanding of their long‐term stability. While HaPs can be altered by radiation that induces multiple processes, they can also return to their original state by “self‐healing.” Here two‐photon (2P) absorption is used to effect light‐induced modifications within MAPbI 3 single crystals. Then the changes in the photodamaged region are followed by measuring the photoluminescence, from 2P absorption with 2.5 orders of magnitude lower intensity than that used for photodamaging the MAPbI 3 . After photodamage, two brightening and one darkening process are found, all of which recover but on different timescales. The first two are attributed to trap‐filling (the fastest) and to proton‐amine‐related chemistry (the slowest), while photodamage is attributed to the lead‐iodide sublattice. Surprisingly, while after 2P‐irradiation of crystals that are stored in dry, inert ambient, photobrightening (or “light‐soaking”) occurs, mostly photodarkening is seen after photodamage in humid ambient, showing an important connection between the self‐healing of a HaP and the presence of H 2 O, for long‐term steady‐state illumination, practically no difference remains between samples kept in dry or humid environments. This result suggests that photobrightening requires a chemical‐reservoir that is sensitive to the presence of H 2 O, or possibly other proton‐related, particularly amine, chemistry.

Adding a 2D character to halide perovskite (HaP) active layers in ambient-protected cells can improve their stability drastically, which is not obvious from the hydrophobicity of the large cations that force the HaP into a 2D structure. Results of two-photon confocal microscopy are reported to study inherent photo-stability of 2D Pb iodide HaPs in the interior of single crystals. Compared to 3D HaP crystals, 2D ones have higher photo-stability and, under a few sun-equivalent conditions, self-heal efficiently after photo-damage. Using both photoluminescence (PL) intensities (as function of time after photo-damage) and spectra, self-healing dynamics of 2D HaP (C4H9NH3)(2)PbI4, 2D/3D (C4H9NH3)(2)(CH3NH3)(2)Pb3I10 and 3D MAPbI(3) are compared. Differences in response to photo-damage and self-healing ability from different degrees of photo-damage are found between these HaPs. Based on the findings, a possible chemical mechanism for photo-damage and self-healing of the 2D HaPs is suggested: the layered lattice arrangement limits out-diffusion of degradation products, facilitating damage reversal, leading to better 2D HaP photo-stability and self-healing uniformity than for 2D/3D HaPs. One implication of the layered structures' resilience to photo-damage is transfer of their increased stability to devices made with them, such as photovoltaic solar cells and light-emitting diodes.

Cupric oxide (CuO) and selenium (Se) are old photoelectric materials, and their photovoltaic properties have always received attention. However, CuO has a high melting point (1026 degrees C) and will decompose near its melting point, so it is difficult to prepare high crystallinity CuO films. Selenium (Se) can not efficiently absorb sunlight because of its large optical band gap (1.9 eV). Here, a CuO:Se composite film was prepared by radio frequency magnetron co-sputtering of CuO and Se and low temperature annealing (200 degrees C). The experimental results show that this composite material film has the advantages of CuO and Se, and overcome their disadvantages. It has strong absorption of light in the entire solar spectral range (250-2500 nm, average absorptance similar to 80 %). A prototype solar cell based on this composite film was fabricated and its power conversion efficiency (PCE) is up to 2.05 %, which is much higher than that of the pure CuO (0.007 %) or pure Se (1.05 %) film solar cells. This material should be promising for photovoltaic and other photo-electrical devices application.

Direct detection of intrinsic defects in halide perovskites (HaPs) by standard methods utilizing optical excitation is quite challenging, due to the low density of defects in most samples of this family of materials (<= 10(15) cm(-3) in polycrystalline thin films and <= 10(11) cm(-3) in single crystals, except melt-grown ones). While several electrical methods can detect defect densities <10(15) cm-3, such as deep level transient spectroscopy (DLTS) or thermally stimulated current (TSC), they require preparation of ohmic and/or rectifying electrical contacts to the sample, which not only poses a challenge by itself in the case of HaPs but also may create defects at the contact-HaP interface and introduce extrinsic defects into the HaP. Here, we show that low-energy photoelectron spectroscopy measurements can be used to obtain directly the energy position of gap states in Br-based wide-bandgap (E-g > 2 eV) HaPs. By measuring HaP layers on both hole- and electron-contact layers, as well as single crystals without contacts, we conclude that the observed deep defects are intrinsic to the Br-based HaP, and we propose a passivation route via the incorporation of a 2D-forming ligand into the precursor solution.

A method using vacuum evaporation and hot-pressing to prepare Cu2Se thin films on Cu substrates is developed. The effects of the heating temperature, heating time, and pressure on the crystallinities of the Cu2Se thin films were studied. The structure, morphology, and composition properties of the Cu2Se thin films were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results showed that the crystallinities of the Cu2Se thin films improved as the heating temperature was increased from 200 degrees C to 280 degrees C. A low pressure of 2 MPa and a short time of 5 min were sufficient to obtain highly crystalline Cu2Se thin films at 280 degrees C. The fabricated Cu2Se/Cu structure could directly convert heat to electricity with a thermoelectric potential of 12.6 mV at 400 K and 17.8 mV at 500 K. Compared with conventional hot-pressing of bulk materials, this method requires lower temperatures and smaller pressures.

The neutral hydrogen evolution reaction (HER) is vital in the chemical industry, and its efficiency depends on the interior character of the catalyst. Herein, work function (WF) engineering is intro-duced via 3d metal (Fe, Co, Ni, and Cu) doping for modulating the Fermi energy level of Mo2C. The defective energy level facilitates the free water molecule adsorption and, subsequently, promotes the neutral HER efficiency. Specifically, at a current density of 10 mA/cm2, Cu-Mo2C exhibits the best HER performance with an overpotential of 78 mV, followed by Ni-Mo2C, Co-Mo2C, Fe-Mo2C, and bare Mo2C with 90, 95, 100, and 173 mV, respectively, and the corresponding Tafel slope values are 40, 43, 42, 56, and 102 mV/dec. The modified WF can also lead to an enhanced photocatalytic efficiency owing to the lowered Schottky barrier and excellent carrier transition across the electrocata-lyst–solution interface. When coupling the metal-doped Mo2C samples with TiO2, enhanced photo-catalytic neutral HER rates are obtained in comparison to the case with bare TiO2. Typically, the HER rates are 521, 404, 275, 224, 147, and 112 μmol/h for Cu, Ni, Co, Fe, bare Mo2C, and bare TiO2, respectively. Time-resolved photoluminescence spectroscopy (TRPS) and ultrafast transient ab-sorption (TA) measurements are carried out to confirm the recombination and migration of the photogenerated carriers. The fitted τ values from the TRPS curves are 22.6, 20.5, 10.1, 4.7, 4.0, 2.5, and 1.9 ns for TiO2, TiO2-Mo2C, TiO2-Fe-Mo2C, TiO2-Fe-Mo2C, TiO2-Fe-Mo2C, TiO2-Fe-Mo2C, and TiO2-Pt, respectively. Additionally, the fitted τ values from the TA results are 31, 73, and 105 ps for the TiO2-Mo2C, TiO2-Cu-Mo2C, and TiO2-Pt samples, respectively. This work provides in-depth in-sights into the WF modulation of an electrocatalyst for improving the HER performance.

School of Chemistry and Chemical-Engineering, Northwestern Polytechnical University, Xi’an, 710129 Shaanxi, China Dongguan Sanhang Civil-Military Integration Innovation Institute, Dongguan, 52300 Guangdong, China Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology; Institute for Advanced Energy Materials; School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China School of Aeronautics, Northwestern Polytechnical University, Xi’an, 710072 Shaanxi, China Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Key Lab of Micro/Nano Systems for Aerospace, Ministry of Education, Northwestern Polytechnical University, Xi’an, 710129 Shaanxi, China University of Queensland, Australian Institute for Bioengineering & Nanotechnology, Nanomaterials Centre, St. Lucia, Qld, Australia School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an, 710129 Shaanxi, China Institute of Flexible Electronics, Northwestern Polytechnical University, Xi’an, 710129 Shaanxi, China

Buffeteau et al. note that the proton diffusion coefficient in MAPbI3 that is deduced (by the authors) from results, obtained by a suite of complementary techniques, on a large number of single crystals (Adv. Mater. 2020, 32, 2002467) is 5 orders of magnitude higher than what is estimated (by them) in J. Am. Chem. Soc. 2020, 142, 10431, from infrared spectroscopy on ultrathin MAPbI3 films; use of (deuterium/hydrogen) D/H isotope substitution is common to both studies. Buffeteau et al. speculated that proton diffusion in halide perovskite single crystals is dominated by 1D defects, which will somehow not be present in thin films, as those are made up of small‐sized crystallites. It is shown here that the idea of a 1D defect is not supported by the body of experimental data gathered on these crystals, that the statistical analysis employed in to Buffeteau et al. to support the criticism is problematic, and it is concluded that the source of the difference must lie elsewhere. Constructive suggestions for this difference are provided and experiments to discern between possible reasons for it are proposed.

We report a novel Mn-Co-Ni-O (MCN) nanocomposite in which the p-type semiconductivity of Mn-Co-Ni-O can be manipulated by addition of graphene. With an increase of graphene content, the semiconductivity of the nanocomposite can be tuned from p-type through electrically neutral to n-type. The very low effective mass of electrons in graphene facilitates electron tunneling into the MCN, neutralizing holes in the MCN nanoparticles. XPS analysis shows that the multivalent manganese ions in the MCN nanoparticles are chemically reduced by the graphene electrons to lower-valent states. Unlike traditional semiconductor devices, electrons are excited from the filled graphite band into the empty band at the Dirac points from where they move freely in the graphene and tunnel into the MCN. The new composite film demonstrates inherent flexibility, high mobility, short carrier lifetime, and high carrier concentration. This work is useful not only in manufacturing flexible transistors, FETs, and thermosensitive and thermoelectric devices with unique properties but also in providing a new method for future development of 2D-based semiconductors.

We find significant differences between degradation and healing at the surface or in the bulk for each of the different APbBr3 single crystals (A=CH3NH3+, methylammonium (MA); HC(NH2)2+, formamidinium (FA); and cesium, Cs+). Using 1- and 2-photon microscopy and photobleaching we conclude that kinetics dominate the surface, and thermodynamics the bulk stability. Fluorescence-lifetime imaging microscopy, as well as results from several other methods, relate the (damaged) state of the halide perovskite (HaP) after photobleaching to its modified optical and electronic properties. The A cation type strongly influences both the kinetics and the thermodynamics of recovery and degradation: FA heals best the bulk material with faster self-healing; Cs+ protects the surface best, being the least volatile of the A cations and possibly through O-passivation; MA passivates defects via methylamine from photo-dissociation, which binds to Pb2+. DFT simulations not only provide insight into the latter conclusion, but also show the importance and stability of the Br3- defect. These results rationalize the use of mixed A-cation materials for optimizing both solar cell stability and overall performance of HaP-based devices, and provide a basis for designing new HaP variants.

Even though the metal-halide perovskites are attracting ever-increasing interest for their breakthrough efficiencies in photovoltaics, light-emitting diodes (LED), X-ray imaging, and general optoelectronics, it was not until very recently that metal-free halide perovskites become recognized, not only for their good optoelectronic performance, but also for their wide chemical diversity, tunability, lightweight, mechanical flexibility, and eco-friendly processability. The community is turning their attention to these lightweight semiconductors, and promising results have been achieved in the initial evaluation of crystal structure and a range of properties including ferroelectric and optoelectronic properties. In this review, the crystal structure and synthesis of these materials are discussed together with the various properties that have been studied for these materials. Future prospects are further discussed for chemical diversity, structure tunability, synthetic process, potential properties, optoelectronic, and energy-related applications.