Synergy effect of intraband transition and impact-Auger in optical hot-carrier solar cells

Abstract

An optical hot-carrier solar cell (opt-HC-SC) is a variant of hot-carrier solar cells (HC-SCs). Hot luminescence in a specific narrow energy range is enhanced and extracted from the absorber through an optical energy-selective contact (opt-ESC) and illuminates a neighboring photovoltaic cell of a conventional type. Thus, this concept eliminates the requisites for carrier transport to achieve high conversion efficiency. In the present study, I clarified the issues for high efficiency using an improved detailed-balance model and determined the solutions. The serious issue is that the enhancement effect of the opt-ESC on the hot luminescence is considerably lower in practice than the optimal value, and this necessitates a high carrier density in the absorber for sufficient photon extraction and results in significant energy loss originating from carrier thermalization. When impact ionization occurs immediately, the average carrier energy lowers, the carrier density can decrease, and hence the thermalization energy loss significantly reduces. This, in turn, enables absorption of sub-bandgap photons arising from the intraband transition to improve the efficiency. Consequently, the synergy effect of both the aforementioned phenomena notably improves the efficiency to 53.6% under the practical upper limit of the enhancement effect, the carrier thermalization time of 1 ns, and 1000-times concentrated solar illumination, from 45.1% for no intraband transition or impact-Auger. By contrast, the individual effects are marginal. This is a unique feature of opt-HC-SCs and is not found in HC-SCs (carrier extraction). The present finding serves as a guide to material design to realize highly efficient opt-HC-SCs.