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device stability impedes its commercialization, mainly stemming from the chemical decomposition of regular 3D perovskites in damp environment. [3,4] Encapsulation techniques can slow down the degradation process, but the essential approach to tackle this issue is to find stable perovskite materials capable of achieving long-term stability. [5] In contrast to traditional 3D counterparts, quasi-2D layered perovskites have shown enhanced stability owing to the bulkier and hydrophobic organic molecule in the structure, and have been applied both in photovoltaics and light-emitting diodes. [6][7][8][9] Quasi-2D perovskites take the generic structural formula of L 2 S n−1 M n X 3n+1 , where n is an integer, M is a divalent metal, X is a halide anion, and L and S are organic cations with large and small sizes, respectively. [10] Layered structures are usually formed by inserting the large-sized organic cation spacers into the inorganic sheets of corner-sharing [MX 6 ] octahedra. These quasi-2D compounds can be regarded as natural formed quantum-well (QW) structures, in which the semiconducting inorganic sheets act as the wells and the organic dielectric layers correspond to the Quasi-2D layered organometal halide perovskites have recently emerged as promising candidates for solar cells, because of their intrinsic stability compared to 3D analogs. However, relatively low power conversion efficiency (PCE) limits the application of 2D layered perovskites in photovoltaics, due to large energy band gap, high exciton binding energy, and poor interlayer charge transport. Here, efficient and water-stable quasi-2D perovskite solar cells with a peak PCE of 18.20% by using 3-bromobenzylammonium iodide are demonstrated. The unencapsulated devices sustain over 82% of their initial efficiency after 2400 h under relative humidity of ≈40%, and show almost unchanged photovoltaic parameters after immersion into water for 60 s. The robust performance of perovskite solar cells results from the quasi-2D perovskite films with hydrophobic nature and a high degree of electronic order and high crystallinity, which consists of both ordered large-bandgap perovskites with the vertical growth in the bottom region and oriented smallbandgap components in the top region. Moreover, due to the suppressed nonradiative recombination, the unencapsulated photovoltaic devices can work well as light-emitting diodes (LEDs), exhibiting an external quantum efficiency of 3.85% and a long operational lifetime of ≈96 h at a high current density of 200 mA cm −2 in air.

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