Giant Bandgap Renormalization and Exciton–Phonon Scattering in Perovskite Nanocrystals

Understanding the interactions between photoexcited charge carriers (electrons and holes) with lattice vibrations (phonons) in quantum confined semiconductor nanocrystals (NCs) is of fundamental interest and a prerequisite for their use in fabricating high‐performance optoelectronic devices. Such interactions have a significant impact on their optoelectronic properties including their charge carrier mobility and photoluminescence. Here, these interactions are investigated in cesium lead halide (CsPbX₃, where X is Cl, Br, or I) NC perovskites. It is shown that a wide broadening of the excitonic linewidth in these NCs arises from strong exciton–phonon coupling, which is substantially dominated by longitudinal optical phonons via the Fröhlich interaction. Unlike the behavior of conventional semiconductors these NCs display a general redshift of their emission energy peak with reducing temperature. Interestingly, the CsPbCl₃ NCs also display an initial blueshift and undergo at structural phase transition at ≈175–200 K. The anomalous redshift observed is modeled and analyzed using a Bose–Einstein two‐oscillator model to interpret the interaction of excitons with acoustic and optical phonons which induce a renormalization of the bandgap. The net renormalization due to zero point motion (T = 0 K) is found to be ≈41.6 and ≈94.9 meV for CsPbBr₃ and CsPbI₃ NCs, respectively.