Harnessing heat in solar cells with thermionics

Scientists in Canada evaluated the potential of a lesser-known approach to boosting solar generation efficiency. Thermionics uses heat from the sun to generate electricity, and could be combined with photovoltaics to create devices with better than 40% efficiency from a single junction. In their evaluation, the scientists find promising pathways for further research, despite a mountain of challenges that will need to be overcome.

August 11, 2021 Mark Hutchins

Aerial view of the University of British Columbia campus in Vancouver. Scientists at the university evaluated a novel way to reduce the harmful effects of heat on a PV cell.

Gord Mckenna/Flickr

From pv magazine Global

Energy lost as heat is one of the fundamental, and largely unavoidable, challenges to improving solar cell efficiencies. Heat generated in the device limits the conversion of light into electricity, and also causes degradation of the device, negatively affecting both performance and lifetime.

Thermionics (TI) is an alternate approach that uses heat from the sun to generate electricity, rather than light as in photovoltaics. TI has been known by scientists for a long time, but has not proved itself as practical as PV as a way to generate electricity. However, recently scientists have begun to explore TI effects in solar PV cell materials, and the possibility of combining the two to both increase efficiency and reduce the negative effects of heat on a device – an approach known as photon enhanced thermionic emission (PETE).

The potential of this approach is evaluated in a new paper published in nature communications, Semiconductor thermionics for next generation solar cells: photon enhanced or pure thermionic. The research, led by scientists at Canada’s University of British Columbia, notes that previous work in this area has simplified certain aspects to focus on others – an approach which “does not enable a realistic analysis of these photothermal phenomena and devices based on them,” according to the group.

Early stages

The group concludes that while both PETE and thermionics alone are promising pathways to addressing the thermal limitations of PV, there is a wide range of challenges to be overcome before it could make any meaningful contributions. These include ensuring temperature stability of the various materials that make up a solar PV cell, increasing the likelihood of thermally excited electrons being emitted and collected, minimising recombination, and many more. “The ultimate challenge is to combine all the desired properties into a single material or heterostructure,” the group states.

The group sought to improve understanding of TI/PETE, and hopes to foster further interest in the approach. “Overall, the concept of semiconductor thermionics is still in early stages and much remains to be investigated on the experimental front,” they conclude, noting that many of the challenges have been identified already. “We point out these issues and possible solutions to both fundamental and practical challenges, in order to provide a broader perspective as well as to motivate further research into semiconductor thermionics.”

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