Size-Dependent Biexciton Spectrum in CsPbBr₃ Perovskite Nanocrystals

We characterize exciton–exciton interactions in weakly confined CsPbBr₃ nanocrystals by combining fluence-dependent transient absorption spectroscopy with a robust spectral deconvolution method. This data-driven approach allows for the extraction of overlapping transient absorption spectra of exciton and biexciton states while making no assumptions about the spectral line shape. The ensemble spectrum of the biexciton state is found to be broader and blue-shifted from the spectrum of the exciton state, with both effects becoming more prominent as nanocrystal size decreases. We conclude that exciton–exciton interactions in CsPbBr₃ are net repulsive at room temperature, but red-shifted optical gain from the biexciton state is still possible because the spectral broadening exceeds the blue-shifting. Finally, we extract size-dependent biexciton lifetimes of 35–200 ps, increasing with nanocrystal size. In contrast to the attractive exciton–exciton interactions found in most conventional semiconductor nanocrystals, repulsive exciton–exciton interactions may arise from polaron formation in perovskites. These observations provide important insight into lasing, high-flux emission, and multicarrier interactions in this class of materials.