A Thermal-Photovoltaic Device Based on Thermally Enhanced Photoluminescence

Abstract: Single-junction photovoltaic cells are considered to be efficient solar energy converters, but even ideal cells cannot exceed the their fundamental thermodynamic efficiency limit, first analysed by Shockley and Queisser (SQ). For moderated irradiation levels, the efficiency limit ranges between 30%-40%. The efficiency loss is, to a great extent, due to the inherent heat-dissipation accompanying the process of electro-chemical potential generation. Concepts such as solar thermo-photovoltaics (STPV) and thermo-photonics4 aim to harness this dissipated heat, yet exceeding the SQ limit has not been achieved, mainly due to the very high operating temperatures needed. Recently, we demonstrated that in high-temperature endothermic-photoluminescence (PL), the photon rate is conserved with temperature increase, while each photon is blue shifted. We also demonstrated how endothermic-PL generates orders of magnitude more energetic-photons than thermal emission at similar temperatures. These new findings show that endothermic-PL is an ideal optical heat-pump. Here, we propose and thermodynamically analyse a novel device based on Thermally Enhanced Photo Luminescence (TEPL). In such a device, solar radiation is harvested by a low-bandgap PL material. In addition to the PL excitation, the otherwise lost heat raises the temperature and allows the TEPL emission to be coupled to a higher bandgap solar cell. The excessive thermal energy is then converted to electrical work at high voltage and enhanced efficiency. Our results show that such a TEPL based device can reach theoretical maximal efficiencies of 70%, as high as in STPV, while the significantly lowered operating temperatures are below 1000C. In addition to the theoretical analysis, we experimentally demonstrated enhanced photo-current in TEPL device pumped by sub-bandgap radiation. This opens the way for a new direction in photovoltaics.