Unusually Strong Bi-Exciton Repulsion Detected in Quantum Confined CsPbBr3 Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy

Transient absorption measurements were conducted on pristine and on monoexciton saturatedCsPbBr3 nanocrystals varying in size within the regime of strong quantum confinement. Oncethe difference spectra were translated to absolute transient changes in absorption cross section,a single exciton is shown to completely bleach the band edge absorption peak, and induce anew absorption roughly two times weaker ~100 meV to the blue. Difference spectra obtainedduring Auger recombination of biexcitons demonstrate that addition of a second exciton, ratherthan double the effect of a first, bleaches the blue induced absorption band without producingnet stimulated emission at the band edge. Accompanied by high time resolution transientabsorption spectra pumping at the lowest exciton band, these results identify the blue inducedabsorption as the second transition to 1Se1Sh which is shifted in energy due to unusually strongand promptly rising biexciton repulsion.