Energy structure of CdSe/CdTe type II colloidal quantum dots—Do phonon bottlenecks remain for thick shells?

The electronic structures of CdSe/CdTe type II colloidal quantum dots are predicted using a model based on k·pk·p theory and the many-particle configuration interaction method. The separation of energy levels in the conduction band is examined and used to identify phonon bottlenecks, and how these evolve as the shell thickness is increased. Bottlenecks are found to persist both above and below the threshold for multiple exciton generation for all the shell thicknesses investigated. The overall electron cooling rate is thus expected to fall as the shell thickness is increased and Auger cooling suppressed, and this is confirmed experimentally using ultrafast transient absorption measurements. A reduced overall rate of electron cooling will enhance the quantum yield of multiple exciton generation with which it competes. Using a detailed-balance model, we have thus calculated that with proper design of core/and shell structures the efficiency of a solar cell based on CdSe/CdTe quantum dots can be enhanced to 36.5% by multiple exciton generation