New Los Alamos approach may be key to quantum dot solar cells with real gains in efficiency
Researchers have demonstrated an almost four-fold boost of the carrier multiplication yield with nanoengineered quantum dots.
Core/shell PbSe/CdSe quantum dots (a) and a carrier multiplication (CM) pathway (b) in these nano structures. (a) Transmission electron microscopy image of thick-shell PbSe/CdSe quantum dots developed for this study. (b) A hot hole generated in the shell via absorption of a photon collides with a core-localized valence-band electron, promoting it across the energy-gap, which generates a second electron-hole pair. In thick-shell PbSe/CdSe quantum dots this process is enhanced due to slow relaxation of shell-localized holes into the core.
To realize the effect of slowed carrier cooling LANL researchers have fabricated PbSe quantum dots with an especially thick CdSe shell. Qianglu Lin, a CASP student working on the synthesis of these materials said, “A striking feature of the thick-shell PbSe/CdSe quantum dots is fairly bright visible emission, from the shell, observed simultaneously with the infrared emission from the core. This shows that intraband cooling is slowed down dramatically, so that holes reside in the shell long enough to produce emission.”
“This slowed relaxation, which underlies the observed enhancement of carrier multiplication, likely relates to the interplay between core- versus shell-localization of valence-band states” explained Nikolay Makarov, a spectroscopist working on this project. Istvan Robel, another CASP member added “Our modeling indicates that when the shell is thick enough, the higher-energy hole states lay primarily in the shell, while lower-energy states still remain confined to the core. This separation leads to electronic decoupling of higher- from lower-energy holes states, which is responsible for the observed slowed cooling.”
IMO where it is at is gathering all the available sources of energy in one device such as spectrum, vibration, and heat. Who knows.....I have a feeling we are about to find out!!!!!
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