Direct Observation of the Exciton Self-Trapping Process in CsCu <subscript>2</subscript> I <subscript>3</subscript> Thin Films.

Low-dimensional copper halides, such as CsCu 2 I 3 , have emerged as promising LED materials featuring strongly Stokes-shifted photoluminescence with high quantum yield. Previous calculations suggest an exciton self-trapping mechanism; however, direct experimental evidence for this process is still lacking. Here, we present femtosecond UV-vis transient absorption experiments of CsCu 2 I 3 thin films. The films were analyzed by SEM, XRD, and 133 Cs/ 63 Cu NMR for crystallinity and defects. Unique spectral dynamics is observed. The band gap absorption exhibits a characteristic double-peak structure arising from the 130 meV spin-orbit splitting of the copper d electrons. Emission at the direct band gap disappears because of the formation of the lowest-energy self-trapped exciton state. We determined the time constant of 12 ps for the trapping process of thermally relaxed free excitons, with an energy barrier of at least 60 meV. The data are successfully modeled by global kinetic analysis, providing also accurate time constants for charge carrier cooling processes.