Monitoring Electron–Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy

Lead-halide perovskite semiconductors have low-frequency phonon modes within the lead-halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting opto-electronic properties. Electron–phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics. Directly observing the interplay between free-charge carriers and phonons can provide details on how phonons impact these properties, e.g. exciton populations and other collective modes. Here, we observe a delicate interplay among carriers, phonons and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of charge carriers and phonons in the time-resolved terahertz photoconductivity spectra. We are able to observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free-charge carriers when excitons interact with phonons and dissociate, and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures. Our studies shed light on how charge carriers interact with the low-energy phonons and discuss implications.