Yanqi Luo1 Juan-Pablo Correa-Baena2 Thomas Brenner1 Jordan Snaider3 Shijing Sun2 Xueying Li1 Mallory Jensen2 Sarah Wieghold2 Jeremy Poindexter2 Shen Wang1 Y. Shirley Meng1 Ti Wang3 Barry Lai4 Moungi Bawendi2 Libai Huang3 David Fenning1 Tonio Buonassisi2

1, University of California, San Diego, La Jolla, California, United States
2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
3, Purdue University, West Lafayette, Indiana, United States
4, Argonne National Laboratory, Lemont, Illinois, United States

Abstract. Organic and inorganic cation alloying on the A-site in mixed halide perovskites solar cells has enabled remarkable improvements in efficiency and environmental stability. However, the added compositional complexity may lead to undesired phase segregation that detracts from optoelectronic performance. To clarify this balancing act, we assess the nanoscale chemical and electronic impacts of alkali-metal cation addition by means of synchrotron-based nanoprobe X-ray fluorescence and induced current. We find that the halide distribution homogenizes upon the addition of CsI and RbI precursors for films prepared with stoichiometric or excess lead halide. The halide homogenization coincides with long-lived charge carrier decays and spatially homogenous carrier dynamics visualized by ultrafast microscopy. At the same time, we observe Rb-rich clusters that phase separate within the film. We identify these Rb aggregates as recombination active sites using X-ray and E-beam induced current microscopy. Based on our microscopy investigations upon careful tuning of precursor stoichiometry, we will share quantitative precursor design strategies for preparing high-performance alloyed perovskite films that take advantage of the beneficial effects of alkali cations on homogenizing the lead-halide electronic backbone while minimizing secondary phase formation. These insights provide a more comprehensive understanding of the role of alkali cations in alloyed perovskite and indicate directions to further improve the performance and stability of perovskite optoelectronic devices.