Perovskite solar cells (PSCs) using organic-inorganic halides as a light-absorbing layer exhibit high power conversion efficiencies (PCEs; 25.2%) in small-area solar cells [1]. The film quality dramatically affects the performance of PSCs and depends on the fine structure of the perovskite layers, including the crystallinity, surface coverage, and roughness. Varied fabrication methods have been employed including a one-step and, a sequential two-step process which can be either solution or vacuum processed for perovskite thin-film preparation [2]. Currently, mainly one-step spin-coating with an antisolvent is used to precipitate perovskite thin films. This method can fabricate relatively uniform and compact perovskite layers that lead to significantly increased PCEs of the PSCs [3,4], but this method requires technical skill, and a large volume of antisolvent such as toluene and chlorobenzene have an enormous environmental impact. In this conference, we report the fabrication of methylammonium-free FA⁺ and Cs⁺ mixed cation perovskite thin films with micrometer-sized grains using a simple one-step spin-coating method with the use of two additives and without an antisolvent [5,6].

 Furthermore, this organic-inorganic perovskite thin films were treated with a fluorinated ammonium iodide solution, and the thin fluorinated two-dimensional layer was formed on the surface, resulting in a significant improvement in photoluminescence intensity. When applied to PSCs, untreated PSCs exhibited PCEs of 18.6% and 17.0% during the reverse and forward scanning, respectively. The PSCs treated with the optimal concentration of fluorinated ammonium iodide showed improved open-circuit voltage, fill factor, and PCEs of 20.6% and 18.5% during reverse and forward scanning, respectively.

 Also, we report the fabrication of perovskite thin films with a fluorinated polymer (FP) additive via a one-step spin coating method for improving the perovskite-layer passivation quality and the crystalline-phase grain boundary. Here, the FP self-organizes and forms a very thin overlayer atop the perovskite layer surface during film formation upon using an FP-added precursor solvent, which suppresses the surface roughness (measured as the RMS value) of the perovskite layer. Further, the PCE increases from 17.2% for FP-free PSCs (control) to 19.1% for FP-added PSCs operating in the reverse scan (RS) mode due to the enhanced interfacial passivation capability upon addition of the surface-segregated FP.