Reveal the Humidity Effect on the Phase Pure CsPbBr3 Single Crystals Formation at Room Temperature and Its Application for Ultrahigh Sensitive X-Ray Detector

High-Throughput Growth of Armored Perovskite Single Crystal Fibers for Pixelated Arrays

The poor machinability of halide perovskite crystals severely hampered their practical applications. Here a high-throughput growth method is reported for armored perovskite single-crystal fibers (SCFs). The mold-embedded melt growth (MEG) method provides each SCF with a capillary quartz shell, thus guaranteeing their integrality when cutting and polishing. Hundreds of perovskite SCFs, exemplified by CsPbBr3, CsPbCl3, and CsPbBr2.5I0.5, with customized dimensions (inner diameters of 150-1000 µm and length of several centimeters), are grown in one batch, with all the SCFs bearing homogeneity in shape, orientation, and optical/electronic properties. Versatile assembly protocols are proposed to directly integrate the SCFs into arrays. The assembled array detectors demonstrated low-level dark currents (< 1 nA) with negligible drift, low detection limit (< 44.84 nGy s-1), and high sensitivity (61147 µC Gy-1 cm-2). Moreover, the SCFs as isolated pixels are free of signal crosstalk while showing uniform X-ray photocurrents, which is in favor of high spatial resolution X-ray imaging. As both MEG and the assembly of SCFs involve none sophisticated processes limiting the scalable fabrication, the strategy is considered to meet the preconditions of high-throughput productions.

One-Step Preparation of High-Stability CsPbX3/CsPb2X5 Composite Microplates with Tunable Emission

The core-shell structure is an effective means to improve the stability and optoelectronic properties of cesium lead halide (CsPbX3 (X = Cl, Br, I)) perovskite quantum dots (QDs). However, confined by the ionic radius differences, developing a core-shell packaging strategy suitable for the entire CsPbX3 system remains a challenge. In this study, we introduce an optimized hot-injection method for the epitaxial growth of the CsPb2X5 substrate on CsPbX3 surfaces, achieved by precisely controlling the reaction time and the ratio of lead halide precursors. The synthesized CsPbX3/CsPb2X5 composite microplates exhibit an emission light spectrum that covers the entire visible range. Crystallographic analyses and density functional theory (DFT) calculations reveal a minimal lattice mismatch between the (002) plane of CsPb2X5 and the (11 ¯ 0) plane of CsPbX3, facilitating the formation of high-quality type-I heterojunctions. Furthermore, introducing Cl- and I- significantly alters the surface energy of CsPb2X5's (110) plane, leading to an evolutionary morphological shift of grains from circular to square microplates. Benefiting from the passivation of CsPb2X5, the composites exhibit enhanced optical properties and stability. Subsequently, the white light-emitting diode prepared using the CsPbX3/CsPb2X5 composite microplates has a high luminescence efficiency of 136.76 lm/W and the PL intensity decays by only 3.6% after 24 h of continuous operation.

Diffusion-Dominated Luminescence Dynamics of CsPbBr3 Studied Using Cathodoluminescence and Microphotoluminescence Spectroscopy

Time-resolved or time-correlation measurements using cathodoluminescence (CL) reveal the electronic and optical properties of semiconductors, such as their carrier lifetimes, at the nanoscale. However, halide perovskites, which are promising optoelectronic materials, exhibit significantly different decay dynamics in their CL and photoluminescence (PL). We conducted time-correlation CL measurements of CsPbBr3 using Hanbury Brown-Twiss interferometry and compared them with time-resolved PL. The measured CL decay time was on the order of subnanoseconds and was faster than PL decay at an excited carrier density of 2.1 × 1018 cm-3. Our experiment and analytical model revealed the CL dynamics induced by individual electron incidences, which are characterized by highly localized carrier generation followed by a rapid decrease in carrier density due to diffusion. This carrier diffusion can play a dominant role in the CL decay time for undoped semiconductors, in general, when the diffusion dynamics are faster than the carrier recombination.

Methanol as an anti-solvent to improve the low open-circuit voltage of CsPbBr3 perovskite solar cells prepared with water

CsPbBr3 has received more and more attention in the field of optoelectronic devices due to its excellent stability. To address the cost and environmental concerns associated with the use of toxic methanol, water has been explored as a substitute solvent for CsBr in the preparation of CsPbBr3 perovskite solar cells (PSCs). In this study, we utilized methanol as an anti-solvent of the CsBr/H2O solution to regulate the detrimental effects of water on the CsPbBr3 film and control the crystallization process. From results of the experiment, it was found that methanol anti-solvent treatment greatly improved the crystallization of the CsPbBr3 film, increased the grain size, and reduced the defect density. After the introduction of methanol anti-solvent treatment, the power conversion efficiency (PCE) increased from 6.09% to 7.91%, while the open-circuit voltage (Voc) increased from 1.18 V to 1.39 V. Furthermore, we incorporated 2-hydroxyethylurea into the CsPbBr3 PSCs to improve the wettability of PbBr2 towards the CsBr/H2O solution and ensure the formation of pure-phase CsPbBr3 films. The introduction of 2-hydroxyethylurea resulted in an additional increase in Voc from 1.19 V to 1.42 V. The PCE further improved from 6.56% to 8.62% after methanol anti-solvent treatment. These results demonstrate that methanol treatment effectively addresses the low Voc issue observed in CsPbBr3 PSCs prepared with water as a solvent. Importantly, this approach significantly reduces the reliance on methanol compared to conventional fabrication methods for CsPbBr3 PSCs. Overall, this work presents a promising pathway for achieving high Voc and efficiency in CsPbBr3 PSCs by utilizing water as a solvent.

Three-Dimensional Polar Perovskites for Highly Sensitive Self-Driven X-Ray Detection

Three-dimensional (3D) organic-inorganic hybrid perovskites (OIHPs) have achieved tremendous success in direct X-ray detection due to their high absorption coefficient and excellent carrier transport. However, owing to the centrosymmetry of classic 3D structures, these reported X-ray detectors mostly require external electrical fields to run, resulting in bulky overall circuitry, high energy consumption, and operational instability. Herein, we first report the unprecedented radiation photovoltage in 3D OIHP for efficient self-driven X-ray detection. Specifically, the 3D polar OIHP MhyPbBr3 (1, Mhy=methylhydrazine) shows an intrinsic radiation photovoltage (0.47 V) and large mobility-lifetime product (1.1×10-3  cm2  V-1 ) under X-ray irradiation. Strikingly, these excellent physical characteristics endow 1 with sensitive self-driven X-ray detection performance, showing a considerable sensitivity of 220 μC Gy-1  cm-2 , which surpasses those of most self-driven X-ray detectors. This work first explores highly sensitive self-driven X-ray detection in 3D polar OIHPs, shedding light on future practical applications.

Origin of Water-Stable CsPbX3 Quantum Dots Assisted by Zwitterionic Ligands and Sequential Strategies for Enhanced Luminescence Based on Crystal Evolution

Water stability is a crucial issue always addressed for commercial practical application of perovskite quantum dots (QDs). Recent advances in ligand engineering for in situ synthesis of water-stable perovskite QDs have attracted growing interest. However, the exact mechanism remains unclear. Here, the function of 4-bromobutyric acid (BBA) and oleylamine (OLA) is systematically studied in water-stable CsPbX3 (X = Br and I) QDs and confirms that the zwitterionic ligands generated in situ by BBA and OLA are anchored on the QDs surface, thus preventing water from penetrating into the QDs. Cs4PbBr6 intermediate crystal found in the crystal structure evolution process of CsPbX3 QD further reveals a complete crystallization process: PbX2 + CsX + Br- → Cs4PbBr6 crystals + X-→ CsPbX3 QDs + Br-. Furthermore, it is found that the solvent coordination of the precursor solution has a significant effect on the crystallinity of Cs4PbBr6 intermediate crystal, while the Rb+ doping can effectively passivate the surface defects of CsPbX3 QDs, thereby jointly achieving photoluminescence quantum yields (PLQY) of 94.6% for CsPbBr3 QDs (88.2% for CsPbI3 QDs). This work provides new insights and guiding ideas for the green synthesis of high-quality and water-stable perovskite QDs.

Alkali-Metal-Assisted Green-Solvent Synthesis for In Situ Growth of Perovskite Nanocrystals in Porous Materials

Inorganic metal halide perovskite CsPbX3 (X = I, Br, and Cl) nanocrystals (NCs) are rapidly developed due to their excellent photophysical properties and potential applications in lighting, lasers, and scintillators. However, the materials for growing perovskite NCs are insoluble or hydrolyzed in most green solvents, limiting their further development. Based on rational chemical analysis, an alkali-metal-assisted green-solvent synthesis method for in situ growth of CsPbBr3 NCs within SAPO-34 zeolite with bright luminescence is developed. Water is the only solvent used in the whole process. Surprisingly, by the synergistic effect of the channel structure of SAPO-34 and alkali-metal ions crystallization regulation, the CsPbBr3 NCs embedded in SAPO-34 assisted by Na+ emit bright blue light under ultraviolet illumination, with a 30 nm blue shift comparing to the CsPbBr3 NCs assisted by K+. Moreover, CsPbBr3 NCs can also be grown in mesoporous SiO2 SBA-15 and zeolites including ZSM-5, AlPO-5, and SOD, indicating that the method is universal for in situ growth of luminescent perovskite NCs in porous materials. This alkali-metal-assisted green-solvent synthesis provides a new strategy for developing high-quantum-yield, tunable-emission, and stable perovskite luminescent materials.

Reconfigurable perovskite X-ray detector for intelligent imaging

X-ray detection is widely used in various applications. However, to meet the demand for high image quality and high accuracy diagnosis, the raw data increases and imposes challenges for conventional X-ray detection hardware regarding data transmission and power consumption. To tackle these issues, we present a scheme of in-X-ray-detector computing based on CsPbBr3 single-crystal detector with convenient polarity reconfigurability, good linear dynamic range, and robust stability. The detector features a stable trap-free device structure and achieves a high linear dynamic range of 106 dB. As a result, the detector could achieve edge extraction imaging with a data compression ratio of ~50%, and could also be programmed and trained to perform pattern recognition tasks with a high accuracy of 100%. Our research shows that in-X-ray-detector computing can be used in flexible and complex scenarios, making it a promising platform for intelligent X-ray imaging.

Enhanced performance of BiI3-incorporated CsPbBr3 solar cells

CsPbBr3 all-inorganic perovskite solar cells (PSCs) have been extensively investigated due to their remarkable stability. However, their limited film quality and wide bandgap result in a low photoelectric conversion efficiency (PCE). In this study, BiI3 was incorporated into CsPbBr3 films to synergistically enhance light absorption and film quality. It was found that the partial substitution of Pb2+ and Br- with Bi3+ and I- in CsPbBr3 improved film quality, enhanced light absorption, and facilitated charge transfer and extraction. The device incorporating BiI3-incorporated CsPbBr3 as a light absorbing layer achieved an efficiency of 9.54%, exhibiting a significant enhancement of 19.4% compared to the undoped device. This work provides a new incorporating strategy that collaboratively improves light absorption and film quality.

Tellurium-Doped 0D Organic-Inorganic Hybrid Lead-Free Perovskite for X-ray Imaging

The application of X-ray imaging in military, industrial flaw detection, and medical examination is inseparable from the wide application of scintillator materials. In order to substitute for lead, lower costs, and reduce self-absorption, organic-inorganic hybrid lead-free perovskite scintillators are emerging as a new option. In this work, novel (TEA)2Zr1-xTexCl6 perovskite microcrystals (MCs) were successfully synthesized by a hydrothermal method, with Te4+ doping, which leads to yellow triplet-state self-trapped excitons emission. The emission peak of (TEA)2Zr1-xTexCl6 located at 605 nm under X-ray excitation, which was applied to X-ray imaging, shows a clear wiring structure inside the USB connector. The detection limit (DL) of 820 nGyair/s for (TEA)2Zr0.9Te0.1Cl6 is well below the dose rate corresponding to a standard medical X-ray diagnosis is 5.5 μGyair/s. This work opens up a new path for organic-inorganic hybrid lead-free scintillators.

Lead-Free Halide Perovskites for Direct X-Ray Detectors

Lead halide perovskites have made remarkable progress in the field of radiation detection owing to the excellent and unique optoelectronic properties. However, the instability and the toxicity of lead-based perovskites have greatly hindered its practical applications. Alternatively, lead-free perovskites with high stability and environmental friendliness thus have fascinated significant research attention for direct X-ray detection. In this review, the current research progress of X-ray detectors based on lead-free halide perovskites is focused. First, the synthesis methods of lead-free perovskites including single crystals and films are discussed. In addition, the properties of these materials and the detectors, which can provide a better understanding and designing satisfactory devices are also presented. Finally, the challenge and outlook for developing high-performance lead-free perovskite X-ray detectors are also provided.

Improving X-ray Scintillating Merits of Zero-Dimensional Organic-Manganese(II) Halide Hybrids via Enhancing the Ligand Polarizability for High-Resolution Imaging

Luminescent metal halides have been exploited as a new class of X-ray scintillators for security checks, nondestructive inspection, and medical imaging. However, the charge traps and hydrolysis vulnerability are always detrimental to the three-dimensional ionic structural scintillators. Here, the two zero-dimensional organic-manganese(II) halide coordination complexes 1-Cl and 2-Br were synthesized for improvements in X-ray scintillation. The introduction of a polarized phosphine oxide can help to increase the stabilities, especially the self-absorption-free merits of these Mn-based hybrids. The X-ray dosage rate detection limits reached up to 3.90 and 0.81 μGyair/s for 1-Cl and 2-Br, respectively, superior to the medical diagnostic standard of 5.50 μGyair/s. The fabricated scintillation films were applied to radioactive imaging with high spatial resolutions of 8.0 and 10.0 lp/mm, respectively, holding promise for use in diagnostic X-ray medical imaging.

Two-Dimensional Metal Halides for X-Ray Detection Applications

Metal halide perovskites have recently emerged as promising candidates for the next generation of X-ray detectors due to their excellent optoelectronic properties. Especially, two-dimensional (2D) perovskites afford many distinct properties, including remarkable structural diversity, high generation energy, and balanced large exciton binding energy. With the advantages of 2D materials and perovskites, it successfully reduces the decomposition and phase transition of perovskite and effectively suppresses ion migration. Meanwhile, the existence of a high hydrophobic spacer can block water molecules, thus making 2D perovskite obtain excellent stability. All of these advantages have attracted much attention in the field of X-ray detection. This review introduces the classification of 2D halide perovskites, summarizes the synthesis technology and performance characteristics of 2D perovskite X-ray direct detector, and briefly discusses the application of 2D perovskite in scintillators. Finally, this review also emphasizes the key challenges faced by 2D perovskite X-ray detectors in practical application and presents our views on its future development.

Ultrahigh-Flux X-ray Detection by a Solution-Grown Perovskite CsPbBr3 Single-Crystal Semiconductor Detector

Solution-processed perovskites are promising for hard X-ray and gamma-ray detection, but there are limited reports on their performance under extremely intense X-rays. Here, a solution-grown all-inorganic perovskite CsPbBr₃ single-crystal semiconductor detector capable of operating at ultrahigh X-ray flux of 10¹⁰ photons s^-1 mm^-2 is reported. High-quality solution-grown CsPbBr₃ single crystals are fabricated into detectors with a Schottky diode structure of eutectic gallium indium/CsPbBr₃ /Au. A high reverse-bias voltage of 1000 V (435 V mm^{- 1} ) can be applied with a small and stable dark current of ≈60-70 nA (≈9-10 nA mm^{- 2} ), which enables a high sensitivity larger than 10 000 µC Gy_air ^-1 cm^{- 2} and a simultaneous low detection limit of 22 nGy_air s^{- 1} . The CsPbBr₃ semiconductor detector shows an excellent photocurrent linearity and reproducibility under 58.61 keV synchrotron X-rays with flux from 10⁶ to 10¹⁰ photons s^{- 1} mm^{- 2} . Defect characterization by thermally stimulated current spectroscopy shows a similar low defect density of a synchrotron X-ray and a lab X-ray irradiated device. Solid-state nuclear magnetic resonance spectroscopy suggests that the excellent performance of the solution-grown CsPbBr₃ single crystal may be associated with its good short-range order, comparable to the spectrometer-grade melt-grown CsPbBr₃ .

CsPbBr3-DMSO merged perovskite micro-bricks for efficient X-ray detection

Inorganic perovskite wafers with good stability and adjustable sizes are promising in X-ray detection but the high synthetic temperature is a hindrance. Herein, dimethyl sulfoxide (DMSO) is used to prepare the CsPbBr3 micro-bricks powder at room temperature. The CsPbBr3 powder has a cubic shape with few crystal defects, small charge trap density, and high crystallinity. A trace amount of DMSO attaches to the surface of the CsPbBr3 micro-bricks via Pb-O bonding, forming the CsPbBr3-DMSO adduct. During hot isostatic processing, the released DMSO vapor merges the CsPbBr3 micro-bricks, producing a compact and dense CsPbBr3 wafer with minimized grain boundaries and excellent charge transport properties. The CsPbBr3 wafer shows a large mobility-lifetime (μτ) product of 5.16 × 10-4 cm2·V-1, high sensitivity of 14,430 μC·Gyair-1·cm-2, low detection limit of 564 nGyair·s-1, as well as robust stability in X-ray detection. The results reveal a novel strategy with immense practical potential pertaining to high-contrast X-ray detection.

Electronic Supplementary Material

Supplementary material (further details of the characterization, SEM images, AFM images, KPFM images, schematic illustration, XRD patterns, XPS spectra, FTIR spectra, UPS spectra, and stability tests) is available in the online version of this article at 10.1007/s12274-023-5487-3.

Solvent Evaporation Induced Large-Scale Synthesis of Cs4PbBr6 and CsPbBr3 Microcrystals: Optical Properties and Backlight Application for LEDs

The contemporary work focuses on embossing the emissive nature of lead halide perovskite materials, specifically Cs₄PbBr₆ microcrystal powder prepared via single step bulk recrystallization method followed by the solvent evaporation route from gram to kilogram scale. The X-ray diffraction pattern confirms the formation of phase pure Cs₄PbBr₆ with a goodness of fit value of 1.51 calculated from Rietveld refinement and the fluorophore powder manifesting an intrinsic band gap of 3.76 eV. The experimental yield of 99.4% indicates the absence of any unreacted precursors. The fabricated flexible, free-standing Cs₄PbBr₆@PMMA film encompassed better moisture stability without undergoing phase transitions for 400 days. The temperature-dependent photoluminescence spectra denote that 51% of the intensity was retained when cooled back to room temperature after heating it till 180 °C. Moisture studies at two extreme humidity conditions also reveal the appreciable stability of the fluorophore film against moisture. The stability studies with respect to UV irradiation substantiate that the film retained its stability even after exposing it continuously to UV radiation for seven days. The outstanding optical properties of these microcrystals, owing to the higher exciton binding energy, make them a promising candidate as excellent fluorophores for color conversion, backlight, and light-emitting applications. The Cs₄PbBr₆@PMMA film was employed as the top cover of a commercial blue LED, producing a robust green emission which revealed its possible application as a phosphor material.

Recent Development of Halide Perovskite Materials and Devices for Ionizing Radiation Detection

Ionizing radiation such as X-rays and γ-rays has been extensively studied and used in various fields such as medical imaging, radiographic nondestructive testing, nuclear defense, homeland security, and scientific research. Therefore, the detection of such high-energy radiation with high-sensitivity and low-cost-based materials and devices is highly important and desirable. Halide perovskites have emerged as promising candidates for radiation detection due to the large light absorption coefficient, large resistivity, low leakage current, high mobility, and simplicity in synthesis and processing as compared with commercial silicon (Si) and amorphous selenium (a-Se). In this review, we provide an extensive overview of current progress in terms of materials development and corresponding device architectures for radiation detection. We discuss the properties of a plethora of reported compounds involving organic-inorganic hybrid, all-inorganic, all-organic perovskite and antiperovskite structures, as well as the continuous breakthroughs in device architectures, performance, and environmental stability. We focus on the critical advancements of the field in the past few years and we provide valuable insight for the development of next-generation materials and devices for radiation detection and imaging applications.

Synergistic Effect of Surface p-Doping and Passivation Improves the Efficiency, Stability, and Reduces Lead Leakage in All-Inorganic CsPbIBr2 -Based Perovskite Solar Cells

Wide-bandgap inorganic cesium lead halide CsPbIBr₂ is a popular optoelectronic material that researchers are interested in because of the character that balances the power conversion efficiency and stability of solar cells. It also has great potential in semitransparent solar cells, indoor photovoltaics, and as a subcell for tandem solar cells. Although CsPbIBr₂ -based devices have achieved good performance, the open-circuit voltage (V_oc ) of CsPbIBr₂ -based perovskite solar cells (PSCs) is still lower, and it is critical to further reduce large energy losses (E_loss ). Herein, a strategy is proposed for achieving surface p-type doping for CsPbIBr₂ -based perovskite for the first time, using 1,5-Diaminopentane dihydroiodide at the perovskite surface to improve hole extraction efficiency. Meanwhile, the adjusted energy levels reduce E_loss and improve V_oc of the CsPbIBr₂ PSCs. Furthermore, the Cs- and Br-vacancies at the interface are filled, reducing structural disorder and defect states and thus improving the quality of the perovskite film. As a result, the target device achieves a high efficiency of 11.02% with a V_oc of 1.33 V, which is among the best values. In addition to the improved performance, the stability of the target device under various conditions is enhanced, and the lead leakage is effectively suppressed.

Low Trap Density Para-F Substituted 2D PEA2PbX4 (X = Cl, Br, I) Single Crystals with Tunable Optoelectrical Properties and High Sensitive X-Ray Detector Performance

Exploring halogen engineering is of great significance for reducing the density of defect states in crystals of organic-inorganic hybrid perovskites and hence improving the crystal quality. Herein, high-quality single crystals of PEA₂PbX₄ (X = Cl, Br, I) and their para-F (p-F) substitution analogs are prepared using the facile solution method to study the effects of both p-F substitution and halogen anion engineering. After p-F substitution, the triclinic PEA₂PbX₄ (X = Cl, Br) and cubic PEA₂PbX₄ (X = I) crystals unifies to monoclinic crystal structure for p-F-PEA₂PbX₄ (X = Cl, Br, I) crystals. The p-F substitution and halogen engineering, together with crystal structure variation, enable the tunability of optoelectrical properties. Experimentally, after the p-F substitution, the energy levels are lowered with increased Fermi levels, and the bandgaps of p-F-PEA₂PbX₄ (X = Cl, Br, I) are slightly reduced. Benefitting from the enhancement of the charge transfer and the reduced trap density by p-F substitution and halogen anion engineering, the average carrier lifetime of the p-F-PEA₂PbX₄ is obviously reduced. Compared with PEA₂PbI₄, the X-ray detector based on p-F-PEA₂PbI₄ perovskite single-crystal has a higher sensitivity of 119.79 μC Gy_air⁻¹·cm⁻². Moreover, the X-ray detector based on p-F-PEA₂PbI₄ single crystals exhibits higher radiation stability under high-dose X-ray irradiation, implying long-term operando stability.

Competing Energy Transfer-Modulated Dual Emission in Mn2+-Doped Cs2NaTbCl6 Rare-Earth Double Perovskites

A₂B^IB^IIIX₆ double perovskites are promising materials due to their outstanding photoelectronic properties and excellent stability in the environment. Herein, we synthesized Mn²⁺:Cs₂NaTbCl₆ with dual emission through a solvothermal method for the first time. Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibit excellent environmental stability and high photoluminescence quantum yields (PLQYs). The Cs₂NaTbCl₆ was successfully doped with Mn²⁺ in two modes: at Mn-feeding concentrations below 1%, Mn²⁺ first tend to insert into the interstitial void, but if the Mn-feeding concentration exceeds 1%, Mn²⁺ will further substitute Na⁺ site of the Cs₂NaTbCl₆ lattice and thus both two doping modes coexist. After Mn²⁺ doping, efficient energy transfer from the ⁵D₄ level of Tb³⁺ ions to the ⁴T₁ level of Mn²⁺ ions occurs, resulting in tunable dual emission from the Tb³⁺⁵D₄ → ⁷F_J=6,5,4,3 transition and Mn²⁺⁴T₁ → ⁶A₁ transition. Further, LED based on the Mn²⁺:Cs₂NaTbCl₆ double perovskites exhibits excellent performance and stability. This work demonstrates a strategy to achieve novel lanthanide-based double perovskites with potential applications in photonics.

What Happens When Halide Perovskites Meet with Water?

Halide perovskites are considered to be next-generation semiconductor materials with bright prospects to advance the technology of photonics and optoelectronics. Because of the intrinsic ionic feature, the interactions between perovskites and water induce serious stability issues, which has been one of the fundamental problems hindering the practical application of perovskites. The degradation of halide perovskites upon water exposure has been intensively studied, resulting in chemical insights into key processes, including hydration, phase transformation, decomposition, and dissolution. In this Perspective, we try to illustrate what happens when halide perovskites meet with water. We summarize the research progress regarding the understanding of these processes and discuss the principle of strategy design toward improved stability against water. In addition to the instability-related interactions, we also discuss the aqueous solution of perovskite precursors for fabricating perovskite-based functional materials. Hopefully, this Perspective can inspire more fundamental studies on the interactions between perovskites and water, such as spectroscopy and simulation, crystal structure and material characterizations, and solution chemistry and crystallization.