Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots

We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We
employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies.
Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are
four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption
energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time
transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present
experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical
values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent
superposition of multiple excitonic states.