Moisture, heat, and light instabilities of halide perovskites (HPs) represent a serious Achilles' heel that must be overcome, to enable future advancements in perovskite‐based optoelectronic devices such as solar cells and light‐emitting diodes. The instabilities are attributed to the unavoidable fragile ionic bonding between cationic and anionic parts of HPs during their formation. Surface passivation of HPs by various surface‐passivating materials has proven to be an attractive approach to stabilize perovskites against moisture, heat, and light, keeping intact their structural integrity and ionic bonding. Herein, the experimental and theoretical background for degradation mechanisms of HPs is reviewed along with various surface passivating materials to stabilize HPs. Finally, the existing challenges associated with thin‐film and device fabrication and an outlook for improving the stability of perovskites in optoelectronics are presented

The all-inorganic perovskite CsPbI3 has emerged as an alternative photovoltaic material to organic–inorganic hybrid perovskites due to its non-volatile composition and comparable photovoltaic performance.

Low‐temperature α‐phase stabilization using HI or zwitterions in cesium lead iodide (CsPbI3) endures the metastable phase properties but is thermally unstable. Doping with a small amount of heterovalent metals (i.e., Bi3+, Sb3+) in CsPbI3 has been assumed to stabilize the α‐phase, while here this assumption is challenged. It is demonstrated that heterovalent metal ion doping stabilizes β‐CsPbI3 at low temperatures without replacing the Pb2+ cations, while divalent cations (i.e., Ba2+, Sr2+, and Sn2+) doping stabilizes the α‐CsPbI3 by replacing the Pb2+ cations. This finding is demonstrated by both theoretical and experimental results. It is also found that the divalent cations stabilize α‐CsPbI3 films, making thermally stable at high temperatures, whereas heterovalent metal‐doping stabilizes β‐CsPbI3 films, making metastable. The doping influence on crystal grains and the chemical composition of thin films is discussed. In particular, the charge dissociation kinetics for the Sr doped thin film are much enhanced than α‐CsPbI3 and Ba doped thin films, also the initial results of the fabricated perovskite red‐light‐emmiting diode suggests that the Sr‐doped thin films would be more suitable for the device fabrication. These findings will guide a way for further development in thermally and air‐stable optoelectronic devices.

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Additive-assisted organic−inorganic perovskite materials have attracted substantial attention as photovoltaic light absorbers which lead to outstanding power conversion efficiency. Here we report an easy and effective fabrication of cubic-phase perovskite with an inorganic molecule additive like hydrazinium chloride (N 2 H 5 Cl, to be denoted as HZCl). We predict that this inorganic cation of N 2 H 5 + , which can substitute for the organic A-site in the perovskite structure, can tune Froḧlich polaron properties by controlling the interaction strength and the number of proton coordinations to halide. This prediction is experimentally demonstrated with an optimized perovskite device with 2% N 2 H 5 Cl additive, which exhibits an unprecedented 85% fill factor (FF) with the highest value close to the Shockley−Queisser limit. An extra power conversion efficiency (PCE) of 2.3% and a fill factor (FF) efficiency of 14% are boosted. These optimized performances by additive effects lead to a new approach based on the theoretical calculation toward an improved performance of the perovskite solar cell.

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