Elucidating the Primary Mechanism of Multiexciton Generation in Semiconductor Nanocrystals by Multi -Quantum Coherent Spectroscopy

Solar energy has the potential to provide a clean and inexpensive form of renewable energy, but high efficiency remains a major, unsolved problem. A promising approach is multiexciton generation (MEG) in which two or more electron-hole pairs or excitons are produced by absorption of a single photon. MEG may increase photoconversion in bulk materials but suffers from low efficiency. In contrast, MEG in quantum-confined structures such as semiconductor nanocrystals may achieve high efficiency presumably due to the inhibition of deleterious relaxation pathways. Understanding MEG processes in nanocrystals by both experiments and theory is a defining principle that will guide advances in the next generation of solar cells. Unfortunately and despite enormous effort, a complete picture of the mechanism of MEG is lacking in large part because a direct measurement of the multi-exciton states involved remains elusive. A novel experimental approach is proposed to perform the first direct measurements of MEG in semiconductor nanocrystals, providing much needed insights into the mechanism of coherent carrier relaxation processes. A unique aspect of this approach is that MEG may be tracked in space and time across the entire solar spectrum in a single laser shot, thereby avoiding artifacts and ambiguities that plague other measurement techniques.