Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

Perovskite solar cells (PSCs) based on organic-inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA_1-xCs_xPbI₃ and (FA_1-xCs_x)_1-yRb_yPbI₃ (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs⁺ and Rb⁺ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA_1-xCs_xPbI₃ by providing a source of I^- that fills iodine vacancy sites in the perovskite lattice, while Rb⁺ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.

Hybrid Perovskite Light-Emitting Diodes Based on Perovskite Nanocrystals with Organic-Inorganic Mixed Cations

Organic-inorganic hybrid perovskite materials with mixed cations have demonstrated tremendous advances in photovoltaics recently, by showing a significant enhancement of power conversion efficiency and improved perovskite stability. Inspired by this development, this study presents the facile synthesis of mixed-cation perovskite nanocrystals based on FA_(1-x) Cs_x PbBr₃ (FA = CH(NH₂ )₂ ). By detailed characterization of their morphological, optical, and physicochemical properties, it is found that the emission property of the perovskite, FA_(1-x) Cs_x PbBr₃ , is significantly dependent on the substitution content of the Cs cations in the perovskite composition. These mixed-cation perovskites are employed as light emitters in light-emitting diodes (LEDs). With an optimized composition of FA_0.8 Cs_0.2 PbBr₃ , the LEDs exhibit encouraging performance with a highest reported luminance of 55 005 cd m^-2 and a current efficiency of 10.09 cd A^-1 . This work provides important instructions on the future compositional optimization of mixed-cation perovskite for obtaining high-performance LEDs. The authors believe this work is a new milestone in the development of bright and efficient perovskite LEDs.

Ion-Exchange-Induced 2D-3D Conversion of HMA1-x FAx PbI3 Cl Perovskite into a High-Quality MA1-x FAx PbI3 Perovskite

High-quality phase-pure MA_1-x FA_x PbI₃ planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy-to-crystallize but unwanted δ-FAPbI₃ /MAPbI₃ and FA_x MA_1-x PbI₃ requiring rigid crystallization conditions. Here A 2D-3D conversion to transform compact 2D mixed composition HMA_1-x FA_x PbI₃ Cl perovskite precursor films into 3D MA_1-x FA_x PbI₃ (x=0.1-0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase-pure and high quality MA_1-x FA_x PbI₃ without δ-FAPbI₃ and MAPbI₃ impurity. In all, we successfully developed a facile one-step method to fabricate high quality phase-pure MA_1-x FA_x PbI₃ (x=0.1-0.9) perovskite films by 2D-3D conversion of HMA_1-x FA_x PbI₃ Cl perovskite. This 2D-3D conversion is a promising strategy for lead halide perovskite fabrication.