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 Materials “Self-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.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.