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Formamidinium (FA)-based perovskite solar cells (PSC) show enhanced stability compared to their methylammonium (MA)-based counterparts. However, their stability needs to be further enhanced for the potential commercial applications. We demonstrate here that the high-quality thin film can be obtained for the FA-based perovskite with remarkably enhanced moisture stability using a confined-pressure annealing (CPA) processing and controlling the amount of Cs introduced into the FAPbI 3 perovskite precursor solution. Without additional 2D cation additives, surface passivation, or encapsulation, no phase degradation was observed for the Cs 0.1 FA 0.9 PbI 3 perovskite under a humidity of 70% RH for over 500 h. The unencapsulated device maintained over 70% performance for 500 h compared to <80 h of the conventional processed devices, which is comparable to the outstanding moisture stability of the 2D/3D perovskites. The FAbased perovskite grain size was promoted to over 1 μm with a high static water contact angle up to 112°. The surface cation composition was varied with the concentrated FA cation species at the surface while homogenized Cs/FA distribution inside the film. The trapped-state density and carrier recombination rate were also reduced with a high fill factor of 82% and an efficiency over 20.23%. This work suggests that the intrinsic stability of the 3D perovskite could be further enhanced by adjusting the surface compositions during the crystallization process.

Organic-inorganic hybrid perovskite solar cells have generated wide interest due to the rapid development of their photovoltaic conversion efficiencies. However, the majority of the reported devices have been fabricated via spin coating with a device area of < 1 cm 2. In this study, we fabricated a wide-bandgap formamidinium lead bromide (FAPbBr 3) film using a cost-effective, high-yielding doctor-blade-coating process. The effects of different surfactants, such as l-α-phosphatidylcholine, polyoxyethylene sorbitan monooleate, sodium lauryl sulfonate, and hexadecyl trimethyl ammonium bromide, were studied during the printing process. Accompanying the optimization of the blading temperature, crystal sizes of over 10 mm and large-area perovskite films of 5 cm  5 cm were obtained using this method. The printed FAPbBr 3 solar cells exhibited a short-circuit current density of 8.22 mA/cm 2 , an open-circuit voltage of 1.175 V, and an efficiency of 7.29%. Subsequently, we replaced the gold with silver nanowires as the top electrode to prepare a semitransparent perovskite solar cell with an average transmittance (400-800 nm) of 25.42%, achieving a highpower efficiency of 5.11%. This study demonstrates efficient doctor-blading printing for preparing large-area FAPbBr 3 films that possess high potential for applications in building integrated photovoltaics.

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