2D–3D Mixed Organic–Inorganic Perovskite Layers for Solar Cells with Enhanced Efficiency and Stability Induced by <i>n</i>-Propylammonium Iodide Additives

Device instability has become an obstacle for the industrial application of organic-inorganic metal halide perovskite solar cells that has already demonstrated over 23% laboratory power conversion efficiency (PCE). It has been discovered that the sliding of A-site cations in the perovskite compound through and out of the three-dimensional [PbI₆]^4- crystal frame is one of the main reasons that are responsible for decomposition of the perovskite compound. Herein, we report an effective method to enhance the stability of the FA_0.79MA_0.16Cs_0.05PbI_2.5Br_0.5 perovskite film through the incorporation of <i>n</i>-propylammonium iodide (PAI). Both density functional theory calculation and the X-ray diffraction patterns have confirmed the formation of two-dimensional (PA)₂PbI₄ with the Ruddlesden-Popper perovskite as a result of the reaction between PAI and PbI₂ in the perovskite film. X-ray photoelectron spectroscopy reveals less -COOH (carboxyl) groups on the surface of the perovskite film containing (PA)₂PbI₄, which indicates the suppressed penetration of oxygen and moisture into the perovskite material. This is further confirmed by the surface water wettability test of the (PA)₂PbI₄ film that exhibits excellent hydrophobic property with over 110° contact angle. Ultraviolet photoelectron spectroscopy demonstrates the introduction of PAI additives that resulted in the upshift of the conduction band minimum of the perovskite by 160 meV, leading to a more favorable energy alignment with an adjacent electron transporting material. As a consequence, enhanced 17.23% PCE with suppressed hysteresis was obtained with the 5% PAI additive (molar ratio) in perovskite solar cells that retained nearly 50% of the initial efficiency after 2000 h in air without encapsulation under 45% average relative humidity.