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Quantum Dots Demonstrate a New Wrinkle in Enabling High-Efficiency Photovoltaics

Quantum dots self assemble on nanowires in precise location to maximize photoluminescence

2 min read
Quantum Dots Demonstrate a New Wrinkle in Enabling High-Efficiency Photovoltaics

Quantum dots have attracted a lot of interest for researchers in photovoltaics because of their claimed ability to achieve extraordinary conversion efficiencies.

Last year researchers at the University of Buffalo said they could reach 45-percent conversion efficiency with solar cells enabled by quantum dots. And for nearly a decade now quantum dots have even been proposed as a way to achieve electron multiplication or to create so-called “hot carrier” cells for reaching higher conversion rates. However, this line of research has earned some skeptics of late who dismiss the possibility that more than one electron-hole pair can be generated from one photon.

Now researchers at the National Renewable Energy Laboratory (NREL) in conjunction with an international team have demonstrated that quantum dots can self assemble onto nanowires in a way that once again promises improved conversion efficiencies for photovoltaics.

Among their key discoveries, which were published in the journal Nature MaterialsSelf-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” was that the quantum dots self assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Further the quantum dots can be positioned precisely relative to the center of the nanowire. When this precise positioning is combined with quantum dots’ ability to confine both the electrons and the holes, the possibilities for this approach look encouraging.

In high-energy materials, the electrons and holes would typically locate themselves at the lowest energy position. But because the quantum dots can create this quantum confinement the electrons and holes overlap so that they are confined within the quantum dot, which stays located at the high-energy position. The high-energy position for this material is the gallium-arsenide core. This location results in the quantum dots being extraordinarily bright while maintaining a narrow spectral range.

While Swiss scientists had proposed this quantum confinement previously, no one quite believed them, according to Jun-Wei Luo, one of the co-authors of the study. This disbelief set Luo onto developing the quantum-dot-in-nanowire system that validated the previous research. While using NREL’s supercomputer he determined that despite the fact that the band edges were formed by the gallium Arsenide core, the aluminum-rich edges provided the quantum confinement that is observed.

In addition to applications in photovoltaics, this development should impact any area in which the detection of electric and magnetic fields are involved.

Images: NREL

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3 Ways 3D Chip Tech Is Upending Computing

AMD, Graphcore, and Intel show why the industry’s leading edge is going vertical

8 min read
A stack of 3 images.  One of a chip, another is a group of chips and a single grey chip.
Intel; Graphcore; AMD

A crop of high-performance processors is showing that the new direction for continuing Moore’s Law is all about up. Each generation of processor needs to perform better than the last, and, at its most basic, that means integrating more logic onto the silicon. But there are two problems: One is that our ability to shrink transistors and the logic and memory blocks they make up is slowing down. The other is that chips have reached their size limits. Photolithography tools can pattern only an area of about 850 square millimeters, which is about the size of a top-of-the-line Nvidia GPU.

For a few years now, developers of systems-on-chips have begun to break up their ever-larger designs into smaller chiplets and link them together inside the same package to effectively increase the silicon area, among other advantages. In CPUs, these links have mostly been so-called 2.5D, where the chiplets are set beside each other and connected using short, dense interconnects. Momentum for this type of integration will likely only grow now that most of the major manufacturers have agreed on a 2.5D chiplet-to-chiplet communications standard.

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