Interelectrode space optimization in photon-enhanced thermionic emission solar cells with near-field radiative transfer and bidirectional space-charge effects

This study investigates the often-overlooked bidirectional space-charge effect that arises from the simultaneous electron emission of both electrodes during the optimization of interelectrode spacing in photon-enhanced thermionic emission (PETE) devices. By incorporating both near-field radiative transfer and the bidirectional space-charge effect into our analysis, we demonstrate that the bidirectional discharge model predicts lower efficiency at elevated anode temperatures and smaller optimal interelectrode gaps width for peak efficiency compared to models considering only forward discharge. When the anode temperature ranges from 600 to 1000 K, the optimal gap width for the space-charge forward-discharge (SCBD) model falls within the interval of 1.275 to 0.901 μm. In contrast, for the SCBD model, this range is smaller, spanning from 1.275 to 0.683 μm. In addition, the cathode temperature generally rises as the gap width d increases. However, it exhibits a slight decline within the range of d≈0.8∼2 μm, attributed to changes in the transport conditions of the heat fluxes. These findings underscore the necessity of including bidirectional space-charge effects in the performance analysis and design optimization of PETE devices.