Chen Weijie

Endured, low-cost, and high-performance flexible perovskite solar cells (PSCs) featuring lightweight and mechanical flexibility have attracted tremendous attention for portable power source applications. However, flexible PSCs typically use expensive and fragile indium–tin oxide as transparent anode and high-vacuum processed noble metal as cathode, resulting in dramatic performance degradation after continuous bending or thermal stress. Here, all-carbon-electrode-based flexible PSCs are fabricated employing graphene as transparent anode and carbon nanotubes as cathode. All-carbon-electrode-based flexible devices with and without spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) hole conductor achieve power conversion efficiencies (PCEs) of 11.9% and 8.4%, respectively. The flexible carbon-electrode-based solar cells demonstrate superior robustness against mechanical deformation in comparison with their counterparts fabricated on flexible indium–tin oxide substrates. Moreover, all carbon-electrode-based flexible PSCs also show significantly enhanced stability compared to the flexible devices with gold and silver cathodes under continuous light soaking or 60 °C thermal stress in air, retaining over 90% of their original PCEs after 1000 h. The promising durability and stability highlight that flexible PSCs are fully compatible with carbon materials and pave the way toward the realization of rollable and low-cost flexible perovskite photovoltaic devices.

An endurable all-carbon-electrode-based flexible perovskite solar cell is developed, employing graphene as front transparent electrode and carbon nanotubes as back electrode. All-carbon-electrode-based flexible perovskite solar cells with and without the spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) hole transport material show power conversion efficiencies of 11.8% and 8.3%, respectively. Moreover, flexible devices demonstrate outstanding bending durability and thermal stability.

A terthieno[3,2-b]thiophene (6T) based fused-ring low bandgap electron acceptor, 6TIC, is designed and synthesized for highly efficient nonfullerene solar cells. The chemical, optical, and physical properties, device characteristics, and film morphology of 6TIC are intensively studied. 6TIC shows a narrow bandgap with band edge reaching 905 nm due to the electron-rich π-conjugated 6T core and reduced resonance stabilization energy. The rigid, π-conjugated 6T also offers lower reorganization energy to facilitate very low V_OC loss in the 6TIC system. The analysis of film morphology shows that PTB7-Th and 6TIC can form crystalline domains and a bicontinuous network. These domains are enlarged when thermal annealing is applied. Consequently, the device based on PTB7-Th:6TIC exhibits a high power conversion efficiency (PCE) of 11.07% with a high J_SC > 20 mA cm⁻² and a high V_OC of 0.83 V with a relatively low V_OC loss (≈0.55 V). Moreover, a semitransparent solar cell based on PTB7-Th:6TIC exhibits a relatively high PCE (7.62%). The device can have combined high PCE and high J_SC is quite rare for organic solar cells.

Terthieno[3,2-b]thiophene (6T) based low bandgap fused-ring electron acceptor, 6TIC, is developed for highly efficient solar cells, which exhibits a high power conversion efficiency (PCE) of 11.07% with a high J_SC over 20 mA cm⁻² and a high V_OC of 0.83 V with a relatively low V_OC loss (≈0.55 V). Moreover, the semitransparent solar cell based on PTB7-Th:6TIC exhibits a very promising PCE of 7.62%.

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has now exceeded 20%; thus, research focus has shifted to establishing the foundations for commercialization. One of the pivotal themes is to curtail the overall fabrication time, to reduce unit cost, and mass-produce PSCs. Additionally, energy dissipation during the thermal annealing (TA) stage must be minimized by realizing a genuine low-temperature (LT) process. Here, tin oxide (SnO₂) thin films (TFs) are formulated at extremely high speed, within 5 min, under an almost room-temperature environment (<50 °C), using atmospheric Ar/O₂ plasma energy (P-SnO₂) and are applied as an electron transport layer of a “n–i–p”-type planar PSC. Compared with a thermally annealed SnO₂ TF (T-SnO₂), the P-SnO₂ TF yields a more even surface but also outstanding electrical conductivity with higher electron mobility and a lower number of charge trap sites, consequently achieving a superior PCE of 19.56% in P-SnO₂-based PSCs. These findings motivate the use of a plasma strategy to fabricate various metal oxide TFs using the sol–gel route.

A tin oxide (SnO₂) electron transport layer for a perovskite solar cell is successfully fabricated at extremely high speeds at a genuinely low temperature using atmospheric Ar/O₂ plasma annealing. This plasma-annealed SnO₂ (P-SnO₂) exhibits outstanding electrical conductivity and charge-extraction ability compared to thermally-annealed SnO₂, consequently achieving a superior PCE of 19.56% in P-SnO₂-based PSCs.

Solution-processed and low-temperature Sn-rich perovskites show their low bandgap of about 1.2 eV, enabling potential applications in next-generation cost-effective ultraviolet (UV)–visible (vis)–near infrared (NIR) photodetection. Particularly, the crystallization (crystallinity and orientation) and film (smooth and dense film) properties of Sn-rich perovskites are critical for efficient photodetectors, but are limitedly studied. Here, controllable crystallization for growing high-quality films with the improvements of increased crystallinity and strengthened preferred orientation through a introducing rubidium cation into the methylammonium Sn-Pb perovskite system (65% Sn) is achieved. Fundamentally, the theoretical results show that rubidium incorporation causes lower surface energy of (110) plane, facilitating growth in the dominating plane and suppressing growth of other competing planes. Consequently, the methylammonium-rubidium Sn-Pb perovskite photodetectors simultaneously achieve larger photocurrent and lower noise current. Finally, highly efficient UV–vis–NIR (300–1100 nm) photodetectors with record-high linear dynamic range of 110 and 3 dB cut-off frequency reaching 1 MHz are demonstrated. This work contributes to enriching the cation selection in Sn-Pb perovskite systems and offering a promising candidate for low-cost UV–vis–NIR photodetection.

Controllable crystallization for growing high-quality films with increased crystallinity and strengthened preferred orientation is achieved through introducing rubidium cation into the methylammonium Sn-Pb perovskite system. Methylammonium-rubidium Sn-rich perovskite-photodetectors simultaneously achieve larger photocurrent and lower noise current. Highly efficient UV–vis-near infrared (300–1100 nm) photodetectors with record-high linear dynamic range of 110 and 3 dB cut-off frequency reaching 1 MHz are demonstrated.

The sensitivity of organic–inorganic perovskites to environmental factors remains a major barrier for these materials to become commercially viable for photovoltaic applications. In this work, the degradation of formamidinium lead iodide (FAPbI₃) perovskite in a moist environment is systematically investigated. It is shown that the level of relative humidity (RH) is important for the onset of degradation processes. Below 30% RH, the black phase of the FAPbI₃ perovskite shows excellent phase stability over 90 d. Once the RH reaches 50%, degradation of the FAPbI₃ perovskite occurs rapidly. Results from a Kelvin probe force microscopy study reveal that the formation of nonperovskite phases initiates at the grain boundaries and the phase transition proceeds toward the grain interiors. Also, ion migration along the grain boundaries is greatly enhanced upon degradation. A post-thermal treatment (PTT) that removes chemical residues at the grain boundaries which effectively slows the degradation process is developed. Finally, it is demonstrated that the PTT process improves the performance and stability of the final device.

Moisture-induced degradation of FAPbI₃ perovskite is systematically investigated. A Kelvin probe force microscopy study reveals that the formation of nonperovskite phases initiates at the grain boundaries and the phase transition proceeds toward the grain interiors. A post-thermal treatment that removes chemical residues at the grain boundaries which effectively slows the degradation process is developed.

In this work, sidechain engineering on conjugated fused-ring acceptors for conformation locking is demonstrated as an effective molecular design strategy for high-performance nonfullerene organic solar cells (OSCs). A novel nonfullerene acceptor (ITC6-IC) is designed and developed by introducing long alkyl chains into the terminal electron-donating building blocks. ITC6-IC has achieved definite conformation with a planar structure and better solubility in common organic solvents. The weak electron-donating hexyl upshifts the lowest unoccupied molecular orbital level of ITC6-IC, resulting in a higher V_OC in comparison to the widely used ITIC. The OSCs based on PBDB-T:ITC6-IC reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V. The weaker π–π stacking induced by steric hindrance affords ITC6-IC with enhanced compatibility with polymer donors. The blend film treated with suitable thermal annealing exhibits a fibril crystallization feature with a good bicontinuous network morphology. The results indicate that the molecular design approach of ITC6-IC can be inspirational for future development of nonfullerene acceptors for high efficiency OSCs.

Conformation locking by introducing alkyl chains onto central electron-donating building blocks has been explored on fused-ring electron acceptor for high-performance nonfullerene organic solar cells. PBDB-T:ITC6-IC based devices treated with suitable thermal annealing reveal a promising power conversion efficiency of 11.61% and an expected high V_OC of 0.97 V with a small energy loss.

To achieve high-performance large-area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal-annealing-free process and thickness-insensitive photovoltaic properties. Here, an n-type self-doping fullerene electrolyte, named PCBB-3N-3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB-3N-3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB-3N-3I ETL also functions as light-harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB-3N-3I ETL-based inverted PSCs with a PTB7-Th:PC₇₁BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB-3N-3I ETL, yielding a PCE of 9.32% for the device with thick PCBB-3N-3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL-based PSCs reported so far. Importantly, the flexible PSCs with PCBB-3N-3I ETL also show robust bending durability that could pave the way for the future development of high-performance flexible solar cells.

An n-type doping fullerene electrolyte (PCBB-3N-3I) with high-content doping groups, resulting in high conductivity and well-matched energy levels, is synthesized. The inverted polymer solar cells with PCBB-3N-3I electron transport layer show a record efficiency in the flexible polymer solar cells with an extremely high bending durability and thickness-insensitive photovoltaic behavior.

A major breakthrough in the field of organic photovoltaics (OPVs) was the development of the donor/acceptor heterojunction that aids in separating Coulombically bound excitons that are generated upon photoabsorption. Additionally, bound charge transfer (CT) states that result from the exchange of charge carriers across the donor/acceptor interface are believed to play an important role in charge generation. Though organic thin films are often disordered, enhancements to the local structural order at the donor/acceptor interface have recently been shown to greatly influence CT state energetics and the charge generation process. In this progress report, recent efforts to understand the role that donor/acceptor morphology plays in the behavior of CT states and the resulting implications on OPV function are presented. It is aimed to provide a survey of different experimental approaches and to present a balanced examination of current interpretations of key results, and to offer best practices for the fabrication and study of morphologically tunable donor/acceptor CT states.

The local morphology at organic donor/acceptor interfaces has recently been shown to greatly influence charge transfer state energies and dynamics. This progress report summarizes some of the key discussions in recent literature concerning the role of local morphology in the charge transfer and charge generation process in organic photovoltaics, as well as the potential impact on device performance.

Nonfullerene polymer solar cells (PSCs) are fabricated by using one wide bandgap donor PBDB-T and one ultranarrow bandgap acceptor IEICO-4F as the active layers. One medium bandgap donor PTB7-Th is selected as the third component due to the similar highest occupied molecular orbital level compared to that of PBDB-T and their complementary absorption spectra. The champion power conversion efficiency (PCE) of PSCs is increased from 10.25% to 11.62% via incorporating 20 wt% PTB7-Th in donors, with enhanced short-circuit current (J_SC) of 24.14 mA cm⁻² and fill factor (FF) of 65.03%. The 11.62% PCE should be the highest value for ternary nonfullerene PSCs. The main contribution of PTB7-Th can be summarized as the improved photon harvesting and enhanced exciton utilization of PBDB-T due to the efficient energy transfer from PBDB-T to PTB7-Th. Meanwhile, PTB7-Th can also act as a regulator to adjust PBDB-T molecular arrangement for optimizing charge transport, resulting in the enhanced FF of ternary PSCs. This experimental result may provide new insight for developing high-performance ternary nonfullerene PSCs by selecting two well-compatible donors with different bandgap and one ultranarrow bandgap acceptor.

Highly efficient ternary nonfullerene polymer solar cells (PSCs) are fabricated by employing two well-compatible donors with complementary absorption spectra and one ultranarrow bandgap acceptor. The power conversion efficiency and short-circuit current density of ternary PSCs are simultaneously increased to 11.62% and 24.14 mA cm⁻² by incorporating 20 wt% PTB7-Th due to the enhanced photon harvesting and optimized film morphology.

Organic–inorganic perovskites have demonstrated an impressive potential for the design of the next generation of solar cells. Perovskite solar cells (PSCs) are currently considered for scaling up and commercialization. Many of the lab-scale preparation methods are however difficult to scale up or are environmentally unfriendly. The highest efficient PSCs are currently prepared using the antisolvent method, which utilizes a significant amount of an organic solvent to induce perovskite crystallization in a thin film. An antisolvent-free method is developed in this work using flash infrared annealing (FIRA) to prepare methylammonium lead iodide (MAPbI₃) PSCs with a record stabilized power conversion efficiency of 18.3%. With an irradiation time of fewer than 2 s, FIRA enables the coating of glass and plastic substrates with pinhole-free perovskite films that exhibit micrometer-size crystalline domains. This work discusses the FIRA-induced crystallization mechanism and unveils the main parameters controlling the film morphology. The replacement of the antisolvent method and the larger crystalline domains resulting from flash annealing make FIRA a highly promising method for the scale-up of PSC manufacture.

Flash infrared annealing (FIRA) is demonstrated as an antisolvent-free method to prepare methylammonium lead iodide perovskite solar cells with over 18% efficiency. FIRA enables the preparation of pinhole-free perovskite films with micrometer-size crystalline domains over a large area of both glass and plastic substrates. FIRA, as a rapid and environmentally friendly method to scale up perovskite solar cells is proposed.

Solution-processed organic–inorganic lead halide perovskite solar cells (PSCs) are considered as one of the most promising photovoltaic technologies thanks to both high performance and low manufacturing cost. However, a key challenge of this technology is the lack of ambient stability over prolonged solar irradiation under continuous operating conditions. In fact, only a few studies (carried out in inert atmosphere) already approach the industrial standards. Here, it is shown how the introduction of MoS₂ flakes as a hole transport interlayer in inverted planar PSCs results in a power conversion efficiency (PCE) of ≈17%, overcoming the one of the standard reference devices. Furthermore, this approach allows the realization of ultrastable PSCs, stressed in ambient conditions and working at continuous maximum power point. In particular, the photovoltaic performances of the proposed PSCs represent the current state-of-the-art in terms of lifetime, retaining 80% of their initial performance after 568 h of continuous stress test, thus approaching the industrial stability standards. Moreover, it is further demonstrated the feasibility of this approach by fabricating large-area PSCs (0.5 cm² active area) with MoS₂ as the interlayer. These large-area PSCs show improved performance (i.e., PCE = 13.17%) when compared with the standard devices (PCE = 10.64%).

Highly stable and efficient inverted planar perovskite solar cells are fabricated based on a molybdenum disulfide (MoS₂) hole extraction interlayer. The performance improvement is attributed to improved hole extraction, while the enhancement in the long-term stability is attributed to the stabilization of the hole transporting materials/perovskite interface, inhibiting the bulk degradation process of the MAPbI₃ structure itself.

In this work, four donor (D)–acceptor (A) copolymers based on benzodithiophene (BDT) and benzothiadiazole (BT) with different alkylthiolated and/or fluorinated side chains are developed for efficient fullerene and nonfullerene polymer solar cells (PSCs). The synergistic effect of sulfuration and fluorination on the optical absorption, energy level, crystallinity, carrier mobility, blend morphology, and photovoltaic performance is investigated systematically. By incorporating sulfur atoms onto the side chains, a little blueshifted but significantly increased absorption can be obtained for PBDTS-FBT compared to PBDT-FBT. On the other side, a little more blueshifted but much stronger absorption and much lower-lying highest occupied molecular orbital (HOMO) level can be realized for PBDTF-FBT when introducing fluorine atoms instead of sulfur atoms. With the combination of both fluorination and sulfuration strategies, PBDTS-FBT exhibits the best absorption ability, lowest HOMO energy level, and highest crystallinity, which make PBDTSF-FBT devices show the highest power conversion efficiency (PCE) of 10.69% in fullerene PSCs and 11.66% in nonfullerene PSCs. The PCE of 11.66% is the best value for PSCs based on BT-containing copolymer donors reported so far. The results indicate that fluorination and sulfuration have a synergistically positive effect on the performance of D–A photovoltaic copolymers and their solar cell devices.

With the combination of fluorination and sulfuration strategies, new benzodithiophene (BDT)–benzothiadiazole (BT) copolymer donors are developed for improving the optical absorption, energy level, carrier mobility, and blend morphology. The fabricated fullerene and nonfullerene polymer solar cells exhibit high power conversion efficiencies of 10.69% and 11.66%.

Carrier collection in conventional n-type Si (n-Si)/organic hybrid heterojunction solar cells (HHSCs) is mainly limited by the nonoptimized top grid-electrode and inadequate work function (WF) of the PH1000-type poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Here, a novel modified metal polymer nanocomposite top electrode (M-MPNTE) is designed to achieve a full-area carrier collection in n-Si/PEDOT:PSS HHSCs. The carrier collection in both lateral and vertical directions is significantly improved by the introduction of an ultrathin Au/MoO_x modified layer between 6 nm ultrathin Ag film and AI4083-type PEDOT:PSS layer. In addition, the carrier separation is boosted by the enhanced built-in potential owing to a high WF of M-MPNTE, which also suppresses the carrier recombination at the surface of n-Si. Due to these collaborative improvements, a record fill factor of 80.21% is obtained, which is even comparable to the best value of the traditional Si-based solar cells. With the addition of a MoO_x antireflective coating layer on top of M-MPNTE, the short-circuit current density and open-circuit voltage are finally increased to 23.13 mA cm⁻² and 621.07 mV, respectively, yielding a power conversion efficiency of 10.82%. The finding suggests a novel strategy for the development of highly efficient HHSCs with ideal carrier transport mechanism.

A novel modified nanocomposite electrode is designed to achieve a full-area carrier collection in Si/organic heterojunction solar cells. The carrier collection in both lateral and vertical directions as well as the carrier separation are boosted, resulting in a record fill factor of 80.21%. The finding suggests a strategy for the development of highly efficient solar cells with ideal carrier transport route.

In article number 1703282, Shang-Wei Chou, Yun Chi, Hung-Lung Chou, Pi-Tai Chou, and co-workers describe PtCoFe nanowires with rich {111} facets exhibiting superior I⁻₃ reduction activity in DSSCs (dye-sensitized solar cells). The PRT-22 DSSC using Pt₄₉Co₂₃Fe₂₈ nanowire as cathode shows a certified power conversion efficiency of 12.0%, which surpasses the previous power conversion efficiency (PCE) record of the DSSCs using Ru(II)-based dyes.

Significant development has been achieved in nonfullerene organic solar cells. However, most of the high-efficiency nonfullerene systems are composed of polymer donors and fused-ring acceptors, and only a few small molecule donors can work well. Herein, a new A–D–A small molecule donor named NDTSR with naphtho[1,2-b:5,6-b′]dithiophene (NDT) as building blocks is synthesized. Two energy levels well-matched fused-ring acceptors ITIC and IDIC are chosen to construct all-small-molecule solar cells with NDTSR, respectively. When mixed with IDIC, a high power conversion efficiency (PCE) of 8.05% is achieved, which is the highest efficiency for NDT-based small molecule donor. However, the NDTSR:ITIC system only exhibits a low PCE of 1.77%. The big difference in the performance of these two systems should be attributed to the different morphology and phase separation resulting from the crystallinity and aggregation ability of the acceptors. The results demonstrate that NDT-based small molecule is a promising candidate donor for all-small-molecule systems, while the crystallinity of fused-ring acceptors is a critical factor for optimizing the phase separation in the active layer.

An all-small-molecule nonfullerene solar cell is constructed with a novel small molecule NDTSR as donor, and ITIC and IDIC as acceptor, respectively. Through enhancing the crystallinity of acceptors, a high power conversion efficiency of 8.05% is obtained, which indicates that the crystallinity of the acceptor is a key factor for the performance of all-small-molecule solar cells.

In article number 1701659, Jinsong Huang, Jiarong Lian, and co-workers propose a simple hot-substrate deposition method to prepare a thin film with higher coverage and improved uniformity. The hot substrate improves the adhesion of the solvent on the substrate and speeds its drying process to avoid the aggregation of the upmost molecules, so that both reduced current leakage and series resistance are simultaneously realized in perovskite solar cells.

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂Br absorber and polythiophene hole-acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (V_OC) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss.

The interfacial properties between CsPbI₂Br absorber and poly(3-hexylthiophene) (P3HT) hole-acceptor are improved by passivating the surface defects of CsPbI₂Br and reducing the energy disorder of P3HT. Consequently, a stable inorganic perovskite solar cell with high power conversion efficiency of 12.02% and minimal energy loss of 0.50 eV is obtained.

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (J_SC) of 18.41 mA cm⁻², significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm⁻²). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h/μ_e, higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

A novel small-molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits higher lowest unoccupied molecular orbital (LUMO) level, tighter molecular packing, and better crystallite quality. MeIC-based polymer solar cells with J71 as donor achieve high power conversion efficiency up to 12.54%, significantly higher than that of the device of ITCPTC.

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 (MAPbBr₃, FAPbBr₃, and CsPbBr₃)) 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 FAPbBr₃ the fastest (≈1 h) and CsPbBr₃ 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.

Direct proof of self-healing inside lead bromide perovskite crystals is provided. Two-photon excitation, from high-intensity sub-bandgap (800 nm) pulsed illumination, allows damage to and monitoring deep (>100 μm) inside bromide perovskite single crystals via photoluminescence. Complete or partial recovery of photodamage is observed in minutes to hours. A complete chemical mechanism involving ABr₃ species is proposed to explain the phenomenon.