Resolving mobile halide diffusion in perovskite films with frequency-domain photoluminescence.

Intensity-modulated photoluminescence spectroscopy (IMPLS) is a frequency-domain, fully optical technique that probes ionic processes in halide perovskites. Here, IMPLS is expanded to resolve photoluminescence (PL) spectra, instead of only the emission intensity, as a function of excitation modulation frequency. For a triple-cation, mixed-halide perovskite, the PL peak energy and bandwidth are tracked between 2 mHz and 1 Hz, revealing distinct spectral responses. To identify the halide species governing these responses, IMPLS is further measured on four simplified compositions, MAPb(I x Br1-x )3 where x = 1, 0.8, 0.5, 0. The results show that the pure-bromide sample exhibits a distinctly different response, with an onset at a lower frequency. The onset frequency differences between the pure-bromide and pure-iodide samples align with diffusion coefficient differences reported in literature. These findings demonstrate the benefits of spectrally resolving IMPLS - providing chemical insight into reactive species and better interpretation through combined spectral and intensity analysis.