Disentangling Static and Dynamic Contributions to the Urbach Energy in Mixed Sn–Pb Perovskites

Sn–Pb perovskites are promising low-bandgap absorbers for photovoltaics, but their efficiency is limited by the low open-circuit voltage, VOC. The VOC deficit, WOC, is related to energetic disorder, characterized by the Urbach energy, EU, comprising static and dynamic contributions. For Cs0.25FA0.75SnxPb1–xI3, these contributions were determined using time-resolved microwave conductivity (TRMC) and photoluminescence (PL) measurements between 180–350 K. For Pb-based films, both techniques provided similar EU values. However, for Sn-containing compositions (x = 0.2 and 0.5), TRMC-derived EU were twice higher than those found by PL. These differences arise from compositional heterogeneity: TRMC integrates absorption over the entire thickness while the PL-derived absorption originates from low-bandgap domains, likely from the Sn-rich top surface as revealed by X-ray photoelectron spectroscopy (XPS). The PL-derived EU trend corresponds to the bandgap-normalized WOC, which is smallest for x = 0.5 (∼9.2 meV), attributed to the low dynamic contribution originating from the lower mass of Sn.