Recent experimental and computational studies suggest that biexcitons (BX) confined in large ${\mathrm{CsPbBr}}_3$ nanocrystals experience reduced dielectric screening as compared to excitons (X) and trions $\left({\mathrm{X}}^{*}\right)$. Here we provide a physical rationale to explain such a behavior. A characteristic frequency is introduced, which describes the internal dynamics of an exciton within the excitonic complex. By means of effective mass–variational Quantum Monte Carlo simulations, we show that in large nanocrystals, the frequency is similar for X and ${\mathrm{X}}^{*}$, but smaller for BX. Because the frequencies exceed that of the bulk longitudinal optical phonon, this leads to a reduced dielectric constant for BX, which is in contrast to the behavior of strongly confined nanocrystals.

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• Biexcitons
• Dielectric properties
• Electric polarization
• Electronic structure
• Excitons
• Optoelectronics
• Phonons