Compositional Analysis of Metal Halide Perovskites: Insight into the Coevaporation Process via Nuclear Magnetic Resonance and X‑ray Fluorescence Spectroscopy

Lead halide perovskites have emerged as a highly promising class of materials for solar cells. Nevertheless, suitable procedures to analyze the exact composition of the perovskite films are rare and are not generally applicable. Here we show a simple and fast method to determine the absolute composition of mixed metal halide perovskites through a combination of nuclear magnetic resonance and total reflection X-ray fluorescence spectroscopy. The method is validated on powder mixtures and applied to thin film samples prepared by coevaporation of formamidinium iodide (FAI), CsI, PbI2 and PbBr2, allowing correlation between film composition and solar cell performance. We show that the rates of the individual precursors do not always translate linearly into film composition and that the preparation of the hole transport layer MeO-2PACz strongly influences the incorporation of FAI. Our work highlights the importance of compositional analysis and demonstrates how the developed method provides new insights into the complex coevaporation process in perovskite solar cells.