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This material is partially based on work supported as part of the ''Solid State Solar-Thermal Energy Conversion Center (S3TEC)'', an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577 (G.C. and Z.F.R.) and MIT-Masdar program (G.C. and M.C.).
Daniel Kraemer and Bed Poudel: These authors contributed equally to this work
Z.F.R. and G.C. are co-founders of GMZ Energy.
Supplementary Information (PDF 751 kb)
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DOI: https://doi /10.1038/nmat3013
Our findings show that the device performance, while highly promising, is limited by the various tradeoffs in terms of material properties and device physics. Moreover, we reveal that the PETE mode is not guaranteed in a semiconductor thermionic solar cell under optimal operation, nor is it necessary for achieving a performance comparable to photovoltaics. This work sheds light on the issues and challenges in semiconductor thermionic solar conversion that need to be overcome when considering a complete device-level operation.
The data are shown at the maximum power point for a Si and b GaAs for different p-type doping levels in the emitter and a solar concentration ratio of 100.
The data are shown as a function of the interelectrode distance (also referred to as gap width) for a Si and b GaAs at MPP under a solar concentration ratio of 100. The symbols (starting from the top) represent the interelectrode radiative and thermionic exchange, emitter thermal radiation loss to the ambient, net heat conduction through the lead and non-equilibrium radiative recombination loss, respectively.
The corresponding efficiency and electrode temperature trends are shown in Fig. 4. The variations of these quantities'' gap width dependence with solar concentration are shown in the Supplementary Figs. 1–3. These strong dependencies of the energy exchange channels, electrode temperatures and conversion efficiency on gap width and solar concentration level demonstrate the importance of a complete account of the complex interdependencies of materials properties and device physics.
The data are shown for a Si and b GaAs at MPP under a solar concentration ratio of 100. The symbols with subscript E and C represent the emitter and collector temperatures, respectively.
We now consider how the above tradeoffs translate to relevant performance metrics at the MPP for a wide range of solar concentrations (Fig. 5). Figure 5a shows the trend in the device current and the photon enhancement factor (n/neq). The latter is a measure of the amount of optical upshift in the electron Fermi-level due to illumination, which can be written as
a MPP current density and the corresponding photon enhancement factor (n/neq) for different semiconductor materials as a function of solar concentration ratio. n and neq represent the conduction band electron density at the emitter surface under steady-state and equilibrium conditions, respectively. b MPP output power density and solar conversion efficiency for different semiconductor materials as a function of solar concentration ratio. The performance of a tungsten device is also shown for comparison between metal and semiconductor thermionics.
The non-monotonic, semi-plateau-like behavior of the photon enhancement effect in GaAs in Fig. 5a may be understood as follows. As the solar concentration is raised, the emitter temperature tends to rise, leading to a higher thermionic emission current and thus a stronger space charge effect. This is countered by a reduction in the optimal gap size, which in turn strengthens the near-field radiative coupling and thus opposes the rise in emitter temperature. Since the carrier densities depend on temperature, a signature of this behavior is also observed in the photon enhancement factor.
a Conduction band electron density (equilibrium (neq) and steady-state (n)) and the associated photon enhancement factor (n/neq) as a function of interelectrode gap width. The data are shown at MPP and for a solar concentration ratio of 100. b Steady-state and excess carrier density ((delta n)) in the conduction band for different semiconductor materials as a function of solar concentration ratio at MPP. Photon enhancement factor for c Si and d GaAs as a function of interelectrode gap width and solar concentration ratio. The data are shown at MPP and the dash-dotted line indicates the boundary between the PETE and pure thermionic regimes.
At each solar concentration ratio, the data are shown at MPP, and the optimal interelectrode gap width was chosen for the micro-gap devices. For macro-gap devices, the interelectrode gap width is of the order of a millimeter.
Based on the above results and analyses, we conclude that while thermionic conversion using semiconducting emitters is a promising path and can address the thermal limitation of photovoltaics, its overall performance is still directly limited by materials and physics-related challenges. Moreover, beyond those fundamental issues, additional difficulties need to be overcome for thermionics to compete with photovoltaics. Here 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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