Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites

TL;DR

This study demonstrates that alloying perovskites with RbI and CsI leads to more uniform halide distribution and improved charge carrier dynamics, enhancing solar cell efficiency and revealing the effects of alkali-metal cations.

Contribution

It provides new insights into how alkali-metal cation alloying affects halide distribution and carrier dynamics in perovskites, using advanced synchrotron-based imaging techniques.

Findings

Homogenized halide distribution with RbI and CsI addition.

Longer charge carrier decay times in alloyed perovskites.

Rb clusters are inactive in current conduction and act as recombination centers.

Abstract

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.