Generation of Multiple Excitons in PbSe and CdSe Quantum Dots by Direct Photoexcitation: First-Principles Calculations on Small PbSe and CdSe Clusters

摘要

We demonstrate for the first time using a combination of the Hartree—Fock approximation and the symmetry adapted cluster theory with configuration interaction (SAC-CI) that multiple excitons (ME) in PbSe and CdSe quantum dots (QD) can be generated directly upon photoexcitation. At energies 2.5—3 times the lowest excitation, almost all optically excited states in Pb4Se4 become MEs, while both single excitons and MEs are seen in Cd6Se6. We analyze the high-level SAC-CI results of the small clusters based on the band structure and then extend our band structure analysis to Pb_(68)Se_(68), Pb_(180)Se_(180), Cd_(33)Se_(33), and Cd_(111)Se_(111). Our results explain the ultrafast generation of MEs without the need for a phonon relaxation bottleneck and clarify why PbSe is particularly suitable for generation of MEs. Efficient exciton multiplication can be used to considerably increase the efficiency of QD-based solar cells.