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Nanoengineering quadruples carrier multiplication in quantum dots

Technology News |
By eeNews Europe


Carrier multiplication, which is when a single photon can excite multiple electrons, is inefficient in the bulk solids used in ordinary solar cells but is appreciably enhanced in ultrasmall semiconductor particles such as called quantum dots.  In conventional quantum dots, however, carrier multiplication is not efficient enough to boost the power output of practical devices.

“Typical solar cells absorb a wide portion of the solar spectrum, but because of the rapid cooling of energetic (or ‘hot’) charge carriers, the extra energy of blue and ultraviolet solar photons is wasted in producing heat,” said Victor Klimov, director of the Center for Advanced Solar Photophysics (CASP) at Los Alamos National Laboratory.

“In principle, this lost energy can be recovered by converting it into additional photocurrent via carrier multiplication. In that case, collision of a hot carrier with a valence-band electron excites it across the energy gap,” explained Klimov. “In this way, absorption of a single photon from the high-energy end of the solar spectrum produces not just one but two electron-hole pairs, which in terms of power output means getting two for the price of one.”

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.

A new study conducted within the Center for Advanced Solar Photophysics demonstrates that appropriately engineered core/shell nanostructures made of lead selenide and cadmium selenide (PbSe and CdSe) can increase the carrier multiplication yield four-fold compared with simple PbSe quantum dots.

“This strong enhancement is derived primarily from the unusually slow phonon relaxation of hot holes that become trapped in high-energy states within the thick CdSe shell. The long lifetime of these energetic holes facilitates an alternative relaxation mechanism via collisions with core-localized valence band electron which leads to highly efficient carrier multiplication,” explained Klimov.

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 the 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.”

While the present CASP work is based on PbSe/CdSe quantum dots, the concept of ‘carrier-multiplication engineering’ through control of intraband cooling is general, and should be realizable with other combinations of materials and/or nanostructure geometries.

Jeff Pietryga, lead CASP chemist commented: “Further enhancement in carrier multiplication should be possible by combining this new approach with other demonstrated means for increasing multicarrier yields, such as by using shape-control (as in nanorods) and/or materials in which cooling is already naturally slower, like PbTe.”

Applied together, the strategies might provide a practical route to nanostructures exhibiting carrier multiplication performance approaching the limits imposed by energy conservation.

Related articles and links:

https://lanl.gov

News articles:

Los Alamos researchers point the way to more efficient harvesting of solar radiation

Quantum dot solar concentrator opens energy harvesting window


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