Temperature-Dependent Electromodulation Spectroscopy of Excitons in Perovskite Solar Cells

Temperature-dependent electromodulation spectroscopy is applied to perovskite solar cells for the determination of excitonic properties. Measurements have been performed on semitransparent (in transmission and reflection) and non-transparent devices (in reflection) as well as on cells with different absorber materials. The comparison with Elliott fits of standard absorption spectra enables a distinction between bound and continuum excitonic states and points towards bound electron-hole pairs as origin of the optical transitions observed in electromodulation spectra. The excitonic Sommerfeld enhancement leads to a significantly increased absorption in perovskites. Additionally, a comparison of multiple-cation mixed-halide perovskites with standard methylammonium lead iodide indicates a suppression of the phase transition at low temperatures.