Exploring CsPbI3 – FAI alloys: Introducing low-dimensional Cs2FAPb2I7 absorber for efficient and stable perovskite solar cells

Abstract

Among inorganic lead halide perovskites, cesium lead triiodide (CsPbI3) is considered the most promising for photovoltaic applications. The material is a good absorber due to its narrow bandgap enabling efficient light harvesting; however, the low stability of the “black” photoactive phases of CsPbI3 at low temperatures represents a major challenge for the practical application of this material. Herein, we report a new family of promising perovskite-like 2D absorber materials produced by alloying CsPbI3 with formamidinium iodide (FAI). Using a set of complementary experimental techniques in combination with the density functional theory calculations we have found that the introduction of FAI as overstoichiometric additive to CsPbI3 favors the formation of perovskite-like phases with reduced dimensionality and improved stability. The optimal CsPbI3⋅xFAI (x = 0.5, 0.6) (~Cs2FAPb2I7) material formulations do not undergo any phase transitions and do not show any signs of degradation upon continuous exposure to light for more than 1200 h. The observed high phase stability of the optimal CsPbI3-FAI alloys has been explained through DFT calculations by using criteria for thermodynamic stability. The perovskite solar cells based on Cs2FAPb2I7 absorber material delivered light power conversion efficiency of ca. 13% and retained ~ 80% of the initial efficiency after 1200 h of continuous light soaking. The obtained results set new efficiency and stability records for the Cs-rich low dimensional perovskite-like absorber materials. Further exploration of the newly emerging family of 2D materials stemming from the CsPbI3-FAI and similar alloys will pave the way to the development of a new generation of efficient and stable perovskite solar cells.