Front-electrode design for efficient near-field thermophotovoltaics

Abstract

© 2019 SPIE. In near-field TPV cells, the effects of the necessary front electrode are considered and shown to be of great importance. The electrode tradeoff between required high dc conductivity but low photonic absorption becomes detrimental for the efficiency in the very near field, as the thermal-emitter evanescent waves decay fast and are absorbed inside the electrode without penetrating sufficiently into the semiconductor. Therefore, near-field cells fail to deliver ultra-high power efficiently, as hoped. Still, efficient mid-power conversion is possible, and we compare the performance of several tunable-by-doping conducting-electrode materials. Moreover, novel phenomena arise in the near field, such as the inability to use thick transparent electrodes, while instead the feasibility of ultra-thin 'opaque' ones. The metallic-grid fingers exhibit an 'anomalous' shading loss, significantly smaller than predicted by geometry, by suppressing the thermal emission in the emitter regions across them.