The performance of state-of-the-art perovskite solar cells is currently limited by defectinduced recombination at interfaces between the perovskite and the electron and hole transport layers. These defects, most likely under-coordinated Pb and halide ions, must either be removed or passivated if cell efficiencies are to approach their theoretical limit. In this work, we introduce a universal double-side polymer passivation approach using ultrathin poly(methyl methacrylate) (PMMA) films. We demonstrate very high-efficiency (~20.8%) perovskite cells with some of the highest open circuit voltages (1.22 V) reported for the same 1.6 eV bandgap. Photoluminescence imaging and transient spectroscopic measurements confirm a significant reduction in non-radiative recombination in the passivated cells, consistent with the voltage increase. Analysis of the molecular interactions between Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

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With record power conversion efficiencies (PCE) close to 24%, perovskite solar cells (PSCs) hold great promise to combine high photovoltaic performance with low processing cost. [1][2][3][4] This combination can be ascribed to several factors, such as the remarkable optoelectronic properties of perovskites-evidenced by a sharp absorption onset, a small Urbach energy, and a low exciton binding energy-as well as the fact that perovskites use earth-abundant elements. [5][6][7] The organic-inorganic metal halide perovskites used in such devices feature the characteristic AMX 3 structure, where A is an organic or inorganic cation, most often methylammonium (CH 3 NH 3 + , MA + ), formamidinium (HC(NH 2 ) 2 + , FA + ), or cesium (Cs + ), M is a metal cation, such as Pb 2+ or Sn 2+ , and X is a monovalent anion (halide ion Cl, Br, or I). [8][9][10] Various deposition techniques have been developed to obtain polycrystalline perovskite films, including spin-, dip-, and blade-coating, as well as vacuumbased processes. [11,12] To date, one-step spin coating remains the most popular and facile implementation to fabricate high-quality hybrid perovskite thin films from solution. The device efficiency and stability are strongly dictated by the electronic properties and trap state densities in the perovskite film. These, in turn, are influenced by the film crystallization, its morphology, and the presence of defects and imperfections. [13,14] However, despite the simplicity of the one-step deposition, the exact mechanisms underlying the growth process, from precursor solution to solid-state film, and the crystallization mechanism within the sol-gel state, are still poorly understood. This lack of information is partially caused by the heavy reliance on post-deposition ex situ characterization methods to date. [15,16] Recently, in situ time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) has emerged as a powerful technique to study the microstructural evolution from the solution-precursor to the solid state, revealing the crystallization behavior and formation of crystalline intermediates and byproducts. [17,18] Using in situ GIWAXS methods, the classic MAPbI 3 perovskite material has already been investigated; Gong and coworkers identified the Perovskite solar cells increasingly feature mixed-halide mixed-cation compounds (FA 1−x−y MA x Cs y PbI 3−z Br z ) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti-solvent processing window for the fabrication of high-quality films and efficient solar cells. This window widens from seconds, in the case of single cation/ halide systems (e.g., MAPbI 3 , FAPbI 3 , and FAPbBr 3 ), to several minutes for mixed systems. In situ X-ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol-gel and to the number of crystalline byprodu...