  Perovskite solar cells have shown remarkable efficiencies beyond 22%, through
organic and inorganic cation alloying. However, the role of alkali-metal
cations is not well-understood. By using synchrotron-based nano-X-ray
fluorescence and complementary measurements, we show that when adding RbI
and/or CsI the halide distribution becomes homogenous. This homogenization
translates into long-lived charge carrier decays, spatially homogenous carrier
dynamics visualized by ultrafast microscopy, as well as improved photovoltaic
device performance. We find that Rb and K phase-segregate in highly
concentrated aggregates. Synchrotron-based X-ray-beam-induced current and
electron-beam-induced current of solar cells show that Rb clusters do not
contribute to the current and are recombination active. Our findings bring
light to the beneficial effects of alkali metal halides in perovskites, and
point at areas of weakness in the elemental composition of these complex
perovskites, paving the way to improved performance in this rapidly growing
family of materials for solar cell applications.
