Various approaches to synthesize water-stable halide PeNCs Avijit Das,† Arup Ghorai,† Kundan Saha, Arka Chatterjee and Unyong Jeong * The “halide perovskite fever” is ongoing in material-based research due to the extraordinary properties of halide perovskites like high absorption coefficient, tunable band gap (throughout the visible range), near- unity emission quantum yield, large carrier diffusion length (exceeding 1 mm), and a long recombination time (∼ms order). However, the water instability of halide perovskites is an Achilles' heel that must be overcome. Recently, some approaches have been adopted to improve the water stability of ABX 3 perovskites, including the substitution of A cations, ligand exchange, encapsulation in porous frameworks, passivation with inorganic or organic layers, and encapsulation in hydrophobic polymers and glass matrices. This review briefly introduces the degradation mechanisms according to the RH and summarizes various approaches to stabilize halide perovskites. An outlook for research directions of halide perovskites is also suggested. 1. Introduction Solution-processed halide PeNCs have already set benchmarks in the research on optical devices 1–4 and optoelectronic devices, 5–7 especially solar cells 8–11 (making them competitors to Si-based solar cells in terms of efficiency) and solid-state light- emitting diodes (providing outstanding colour purity and high luminescence efficiency). 12–17 Despite the relatively short period of research, perovskite nanocrystals (PeNCs) achieved almost ∼30% power conversion efficiency (PCE), 10 near-unity photo- luminescence quantum yield (PLQY), 14,18 large carrier lifetime ($1 ms), 19 and long carrier diffusion length (in the range of mm). 20 However, instability in the presence of water impedes large-scale commercialization or daily use of products. Conventional emitters like inorganic chalcogenide nanocrystals and organic emitters exceed 10 6 hours of lifetime (LT50 = initial luminance dropping to 50% of the original value) with very low thermal stability. 21 The PeNCs should meet the air/water stability criterion for commercialization, which is over 10 000 hours of lifetime at about 5000 cd m -2 needed for outdoor display applications. 22 The instability is attributed largely to their low formation energy (0.1–0.6 eV) and the intrinsic ionic nature, 23,24 making the PeNCs unstable in various external stimuli (water, light, and electric eld). 25,26 Another big concern in the case of lead halide perovskites is toxicity. 27 These obsta- cles have brought extensive research on underwater stabiliza- tion of PeNCs and the reduction of toxicity, especially by encapsulating or replacing toxic Pb with Sn, Ag or Bi for industrial standards. A variety of synthetic strategies have been explored to stabi- lize PeNCs under ambient conditions. Initially, the replacement of organic cations by inorganic cations like Cs + , Rb + , and Bi 3+ was found to improve the thermal and water stabilities by many times even though it was not enough for commercialization. 28,29 The hybrid perovskites may immediately degrade to their non- perovskite phase even under low humidity conditions because water molecules can form hydrogen bonding with the organic cations. 26,30 Another important parameter to determine the stability of perovskites is the Goldschmidt tolerance factor (s). It can be dened by s ¼ r A þ r B O2ðr B þ r X Þ , where r A , r B , and r X are the effective ionic radii of the A, B, and X site atoms of the general form of the ABX 3 perovskite. In a simple way, it indicates how the A cation can be tted within the structural cage. 31 Stability increases as the s value approaches unity. Especially, since iodine-based perovskites have low s values in the range of ∼0.85–0.9, they are prone to degradation compared to the bromide- or chloride-based perovskites (s $ 0.9). It is oen re- ported that inorganic perovskites can transform to 0D (Cs 4 PbI 6 ), 3D (CsPbI 3 ), and 2D (CsPb 2 X 5 ) crystals at low relative humidity (RH) without much degradation in their optical properties; 26 however, they degrade to the non-perovskite d-phase in an aqueous solution (excess water). Now that commercial products are required to be stable in the water medium, this approach needs to be modied. Research has evolved to bi- or tri-cation substitutions to further enhance the stability under humid conditions. 32,33 Also, the 2D/3D hybrid perovskite strategies are oen used to utilize both exceptional optical properties of 3D perovskites and water immunity by 2D perovskites. 34,35 Recently, Department of Materials Science and Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Republic of Korea. E-mail: ujeong@postech.ac.kr † They contributed equally. Cite this: J. Mater. Chem. A, 2023, 11, 6796 Received 29th November 2022 Accepted 19th February 2023 DOI: 10.1039/d2ta09286g rsc.li/materials-a 6796 | J. Mater. Chem. A, 2023, 11, 6796–6813 This journal is © The Royal Society of Chemistry 2023 Journal of Materials Chemistry A REVIEW Published on 22 February 2023. Downloaded by POSTECH Library on 5/4/2023 4:04:52 AM. View Article Online View Journal | View Issue