Polaronic Biexcitons in Metal Halide Perovskite Quantum Dots

Strong exciton-phonon interactions in the soft-polar lattice of metal halide perovskites result in large polaron formation, which profoundly dominates the optical and electronic properties. Quantum confinement further fosters the exciton-phonon coupling and leads to the exciton-polaron formation in the low dimension structures. Here, we study the exciton-exciton interactions in the background of lattice coupling in the CsPbBr₃ quantum dots (QDs) using temperature and polarization-dependent transient absorption spectroscopy. We find that intervalley polaron pairing of charge carriers derives an unusually strong exciton-exciton interaction and leads to the occurrence of polaronic biexciton, a strongly coupled system of biexciton and polarons. It is observed that the ratio of biexciton to exciton binding energy (Haynes factor) is near unity for the CsPbBr₃ QDs, which is one order larger than the bulk semiconductor and more than five times compared to 2D and quantum well semiconductors. It will pave the way for the creation of room-temperature quantum droplets and Bose-Einstein condensates of biexcitons.