Quantifying the Size‐Dependent Exciton‐Phonon Coupling Strength in Single Lead‐Halide Perovskite Quantum Dots

Abstract Optimizing the performance of semiconductors in both classical and quantum applications, not only requires a solid understanding of elementary excitations such as electrons, holes, or bound electron–hole pairs (excitons), but also of their interaction with the host material's vibrational states (phonons). Exciton‐phonon coupling is particularly relevant in quantum dots (QDs) of APbX 3 lead‐halide perovskite (where “A” can be Cs, formamidinium (FA), or methylammonium (MA), and X can be Cl, Br, or I), a new class of semiconductors with a soft crystal structure. Here, they quantify the strength of coupling to interband transitions for both FAPbBr 3 and CsPbBr 3 QDs, via the magnitude of phonon replicas in their photoluminescence (PL) spectra at cryogenic temperatures. CsPbBr 3 QDs exhibit weaker exciton‐phonon coupling than similarly sized FAPbBr 3 QDs. While the phonon energies are size‐independent, the exciton‐phonon coupling strength decreases with increasing QD size due to the decreased coupling of the transition to low‐energy surface‐enhanced phonon modes, consistent withab initio molecular‐dynamics (AIMD) simulations. Furthermore, within the harmonic approximation, the size‐dependent PL linewidth at room temperature can coarsely be estimated from the low‐temperature phonon replica spectrum, highlighting the crucial role of anharmonic effects. These findings contribute to realizing perovskite QD‐based devices with narrow and coherent emission for quantum technologies.