CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA_1-x Cs_x PbI₃ (Cl) Perovskite Solar Cells

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX₃ , formamidinium (FA)-based mixed perovskite (e.g., FA_1-x Cs_x PbI₃ ) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA_0.9 Cs_0.1 PbI₃ (Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs⁺ in the FA_0.9 Cs_0.1 PbI₃ (Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA_0.9 Cs_0.1 PbI₃ (Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI₃ (Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX₃ material system by delivering a PCE of 19.8%.