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Photon-enhanced thermionic emission for solar concentrator systems

Abstract

Solar-energy conversion usually takes one of two forms: the ‘quantum’ approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the ‘thermal’ approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 °C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

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Figure 1: The PETE process.
Figure 2: Three regimes of electron emission.
Figure 3: Temperature-dependent measurements of [Cs]GaN.
Figure 4: Theoretical PETE efficiency.
Figure 5: Theoretical tandem efficiency.

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Acknowledgements

J.W.S., D.C.R. and S.J.R. were supported by the Global Climate and Energy Project, and J.W.S. was also partially supported by the US Department of Energy, Division of Materials Sciences, under Award DE-AC02-76SF00515. I.B. was partially supported by Robert Bosch Palo Alto Research and Technology Center and DARPA through the Center on Interfacial Engineering in Microelectromechanical Systems. Portions of this research were carried out at the SSRL, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences. The authors would like to thank Z. Hussain, J. Pepper, V. K. Narasimhan and K. Sahasrabuddhe for discussions.

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J.W.S., D.C.R., Z-X.S. and N.A.M. designed experiments. Sample preparation was carried out by J.W.S. and Y.S. J.W.S., D.C.R. and Y.S. made energy-resolved measurements, and J.W.S. and D.C.R. made yield measurements with the help of F.S. All authors discussed the results and analysed data. N.A.M., Z-X.S, R.T.H. and P.P. supervised the project. J.W.S and N.A.M. carried out the simulations with the help of I.B. and D.C.R. J.W.S., I.B. and N.A.M. wrote the paper with editing from D.C.R., S.J.R. and B.E.H.

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Correspondence to Nicholas A. Melosh.

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The authors declare no competing financial interests.

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Schwede, J., Bargatin, I., Riley, D. et al. Photon-enhanced thermionic emission for solar concentrator systems. Nature Mater 9, 762–767 (2010). https://doi.org/10.1038/nmat2814

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