light-emitting diodes, [5][6][7] and photodetectors, [8,9] because of their remarkable structural flexibility, tunability, and excellent stability compared with their 3D perovskite counterparts. [10][11][12] 2D perovskites are generally a class of quantum wells (QWs), including Ruddlesden-Popper (RP), [13][14][15] Dion-Jacobson (DJ), [16][17][18] and alternating cations in the interlayer space (ACI) perovskites. [19,20] The RP and DJ families adopt the general formulas A′ 2 A n−1 M n X 3n+1 and BA n−1 M n X 3n+1 , respectively, where A is a univalent organic cation: methylammonium (MA + ) or formamidinium (FA + ), A′ is a large univalent organic spacer cation like phenylethylammonium (PEA + ), [8,21] or butylammonium (BA + ), etc., [3,22] B is a divalent organic cation like 3-(aminomethyl)piperidinium (3AMP 2+ ) [10] or 1,3-propanediamine. [23] Much work on 2D perovskites to date has focused on the RP and DJ families. For example, it was found that the incorporation of large organic spacer cations leads to the formation of a QW structure with strong quantum confinement, which leads to a higher bandgap and large exciton binding energy. [14,[24][25][26][27] The RP and DJ perovskites feature poor charge dissociation and transportation within the bulk polycrystalline film, which significantly limits the power conversion efficiency (PCE) of solar cells. [28][29][30] To address the issue, a hot-casting strategy was developed to achieve preferential outof-plane alignment of RP QWs. [3] A systematic understanding of how RP perovskites are formed as well as the charge transfer between QWs were also demonstrated, which guided development of the dynamic control of the phase transformation during QWs growth for better compositional and orientation control. [31][32][33][34][35][36] Compositional and solvent engineering were also developed to fabricate high-quality DJ films with significantly improved charge transport. [23,37] For the RP and DJ families, much has been achieved toward a deep understanding of the relationships between molecular chemistry, crystal structure, film quality and optoelectronic properties, leading to outstanding PCEs of 15.42% and 13.3% for PR and DJ perovskite solar cells, respectively. [23,38] The hybrid halide ACI perovskites, which are derived from the oxide perovskite family, are a very new entry in the class 2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge-transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA) n Pb n I 3n+1 (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive ena...

All-inorganic halide perovskites hold promise for emerging thin-film photovoltaics due to their excellent thermal stability. Unfortunately, it has been challenging to achieve high-quality thin films over large areas using scalable methods under realistic ambient conditions. Here, we provide important lessons on controlling the solidification and crystallization of CsPbI2Br perovskite inks during ambient scalable fabrication, with results of superior thin-film quality and device performance compared to lab-scale processes.

efficiency of laboratory-scale all-inorganic solar cells has been improved to 20.37%, [5] approaching ~70% of the efficiency limit based on the Shockley-Queisser (S-Q) theory. [9][10][11] Among all the high-performance all-inorganic perovskite devices, the photocurrent and fill factor have exceeded 95% and 90% of their theoretical S-Q limits, respectively, while the open-circuit voltage (V oc ) falls at ≈80% of the limit. Therefore, there is relatively larger room for improvement in the V oc for yielding higher power conversion efficiency (PCE). [5,[12][13][14][15] The V oc depends on the dynamics of charge carrier recombination, which is connected to the non-radiative recombination processes. [16][17][18][19] Defects usually scatter the carriers as non-radiative recombination reaction centers. [20][21][22][23] Therefore, preparing high-quality perovskite films with low-defect density is a prerequisite for high-performance photovoltaic devices. The quality of the perovskite active layer can be manipulated by the crystallization processes, thus dynamic control of crystallization appears to be very important. In a common solvent crystallization process, the perovskite grain growth is mainly completed in the initial solvent evaporation process because: 1) solvent evaporation leads to the super-saturation of the solute, producing a mass of seed crystals; 2) solvent evaporation drives the migration of the perovskite precursor colloids, which is beneficial to the grain growth. Subsequent extended annealing has faint effect on the grain growth. We assumed that if the "reactive solvent medium" can remain for a longer time, the colloids will react completely and the grains can merge better. Inspired by this idea, we consider molten salt (MS) synthesis, a classical method for preparing functional materials owing to its simplicity, speed, large-scale compatibility, and low cost. [24][25][26][27][28] The MS melts can exhibit homogeneous heat and mass transfer to carry out chemical transformation even at low temperature. [24,29] MS methods have been widely used in industrial production. For example, they are regarded as ideal reactors for the closed Th-U cycle due to their lower fissile inventory [30] and are applied in lignite pyrolysis to affect the products. [31] The MS processes have been applied not only in chemical reactions but also in material morphology manipulation. [32][33][34] So far, MS can be classified into two types: solid high-temperature MS, [27,34,35] and low-temperature MS (ionic liquids [ILs]) [32,36,37] depending on their states within a certain temperature range. The solid MSs show high ionic conductivity only in the liquid state, and thereby should be used at high temperatures above their melting points. ILs are considered Dynamic manipulation of crystallization is pivotal to the quality of polycrystalline films. A molten-salt-assisted crystallization (MSAC) strategy is presented to improve grain growth of the all-inorganic perovskite films. Compared with the traditional solvent annealing, MSAC enables...