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 saturated
CsPbBr3 nanocrystals varying in size within the regime of strong quantum confinement. Once
the 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 a
new absorption roughly two times weaker ~100 meV to the blue. Difference spectra obtained
during Auger recombination of biexcitons demonstrate that addition of a second exciton, rather
than double the effect of a first, bleaches the blue induced absorption band without producing
net stimulated emission at the band edge. Accompanied by high time resolution transient
absorption spectra pumping at the lowest exciton band, these results identify the blue induced
absorption as the second transition to 1Se1Sh which is shifted in energy due to unusually strong
and promptly rising biexciton repulsion.