Giant-shell CdSe/CdS core/shell nanocrystals have attracted much attention due to their unique quasi-type-II band alignment, where a large valence band offset confines holes strongly to the core but electrons are delocalized due to a small conduction band offset. Here, we report the observation of the relative enhancement in the radiative recombination rate of a biexciton compared to that of an exciton in giant-shell CdSe/CdS nanocrystals. We found a clear correlation between the shell thickness of the CdSe/CdS nanocrystals and the ratio between the radiative recombination rates of the biexciton and exciton. Our finding can be explained by a picture in which the biexciton emission efficiency is enhanced through electron localization around the core due to the strong Coulomb potential of the two holes confined in the core.

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