Quantum Confinement Theory of Auger-Assisted Biexciton Recombination Dynamics in Type‑I and Quasi Type-II Quantum Dots

Use of quantum confinement as a tool to control biexciton recombination in quantum dots is investigated theoretically for a series of quasi-Type-II and Type-I CdSe/CdS core/shell spherical quantum dots. Recent experimental measurements show that in such nanostructures, the CdS shell may act as a type of an efficient retarder for biexciton recombination in the quasi-Type-II regime, but not as efficient in the Type-I regime, and that this phenomenon is achieved by a strong charge separation in the former. These findings are interpreted on the basis of Auger-assisted biexciton decay using a quantum confinement theory. We perform single-band effective mass calculations simulating the quasi-Type-II regime by a CdSe 2.4 nm core + CdS shell and the Type-I by a CdSe 3.8 nm core + CdS shell. The calculations reveal a tightly confined hole for both types of regimes. The key difference occurs in the behavior of the electron, whose wavefunction is progressively delocalized into the growing shell region in the quasi-Type-II regime but remains partially confined in the core in the Type-I regime despite the increasing shell thickness. This behavior yields the calculated Auger transition amplitudes and consequently the lifetimes, which closely correlate with the measured ones. The calculations expose further details of biexciton Auger recombination dynamics, such as the dominance of the hot electron channel over the hot hole channel, and the fact that the cold-to-hot electron (and cold-to-hot hole) transitions are qualitatively correctly described by the single-band effective mass approximation, giving this primitive theory a useful validation for treating high-energy excitonic transitions in Type-II/Type-I core/shell quantum dot structures.