Understanding the Chemical and Structural Properties of Multiple-Cation Mixed Halide Perovskite

Despite the excellent power conversion efficiency of multiple-cation mixed halide perovskite solar cells (PSCs), the underlying mechanisms in its efficiency improvement remain unclear. To promote the research and development of advanced PSCs, it is essential to understand the influence of mixed inorganic cations on the morphological, structural, and composition properties of perovskite materials. In this research, a detailed study is conducted to clarify the impact of Rb⁺ and Cs⁺ cations on the crystallographic structure and phase transition of Rb_0.03Cs_0.07FA_0.765MA_0.135PbI_2.55Br_0.45 hybrid perovskites. Our time-of-flight secondary-ion mass spectrometry results reveal that Rb⁺ and Cs⁺ cations were typically segregated at the grain boundary of the perovskite film as a discrete Rb- A nd Cs-rich phase. However, the Cs⁺ cation was also found to be incorporated into the perovskite structure. Our electron diffraction studies show the visibility of forbidden reflections in the electron diffraction patterns. We propose that these forbidden reflections are a direct result of the perovskite structure and attribute them to superlattice reflections. Furthermore, we show evidence for the coexistence of cubic and tetragonal phases in the diffraction patterns at room temperature. The results presented in this research offer additional insights into the cation incorporation in mixed halide perovskite materials.