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| a) Schematic shows the band structure of the semiconducting HfS2/HfO2 when under strain, and the consequent charge funnelling. b) The strain is induced in the semiconductor by creating a region of oxide using intense laser light. c) A photocurrent map of the device; the photoresponse drastically increases when a region (dashed circle, bottom) is oxidized, compared with the same device before oxidation (top), a sign of the charge funnelling effect. Figure reproduced with permission from the authors and Nature Communications. |
Topics: Green Energy, Green Tech, Laser, Nanotechnology, Semiconductor Technology, Solar Power
Note: Radiant solar energy = 1.1 x 10^18 kilowatt hours/year; 3.013 x 10^15 kilowatt hours/day. We're literally "leaving money on the table"... for fossil fuel greed.
Source: United Nations World Energy Assessment: Energy and the Challenge of Sustainability
Funnels are efficient tools for channelling liquids into containers with narrow openings. Now, researchers in Exeter have demonstrated the first funnel for electrical charges on a chip. The discovery builds on the ability to oxidize the atomically thin semiconductor, hafnium disulphide (HfS2), with a high-intensity UV laser. The non-uniform strain between oxidized and non-oxidized regions, and the subsequent band-gap modulation, generates electric fields, which effectively funnel the charges in the semiconductor flakes to areas where they can be more easily collected. This concept could enable a new generation of solar cells with 60% efficiency (currently around 21%), thanks to the increased efficiency in collecting photo-excited charges and the potential for hot-carrier extraction.
Intense laser light means oxidation, oxidation means strain
In general, bulk semiconductors can only sustain strains up to 0.4% before breaking. However, a layer of semiconductor that is only a few atoms thick can support strains of up to 25%. This amount of strain changes the band gap in the energy dispersion by up to 1 eV. In this work, Saverio Russo and his group at the University of Exeter, induce the strain in the HfS2 using a 375 nm laser to remove sulphur atoms, which are then replaced by oxygen atoms. According to calculations performed using density functional theory, the hafnium atoms have different separations in HfS2 and HfO2. This produces a 2.7% strain at the boundary between the oxidized and non-oxidized regions. Electrical contacts anchor the material to a substrate, so a strain gradient is present across the whole flake, shifting asymmetrically the conduction and valence bands to higher energies, and opening the band gap by 30 meV.
Funneling charges to boost solar-cell efficiency, Lauren Barr, PhD, network contributor for nanotechweb.org
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