10.1117/2.1201701.006744 Thermally enhanced photoluminescence for ultra- high-efficiency photovoltaics Assaf Manor, Nimrod Kruger, Tamilarasan Sabapathy, and Carmel Rotschild Enhanced rates of energetic photons are generated above the thermal emission limit in a hybrid thermal-photoluminescent device, which has an efficiency of about 70% at moderate operating temperatures. The efficiency of single-junction photovoltaic (PV) cells is thermodynamically restricted (to about 40% under maximally concentrated sunlight) by the Shockley-Queisser (SQ) limit. 1 In turn, the SQ limit is set by the inherent trade-off of broadband energy harvesting, i.e., between heat loss (thermalization) and sub-bandgap photon losses. That is, for a specific PV bandgap, energy is lost because of sub-bandgap photons (to which the PV is transparent) or because of hot electron thermalization after en- ergetic photon absorption. Although concepts such as solar ther- mophotovoltaics (STPVs) have been developed to deal with the second of these energy-loss mechanisms, this approach requires extremely high operating temperatures to generate high thermal emission fluxes. In STPVs, the incoming solar spectrum is first harvested by a primary absorber (i.e., which acts as a mediator between the sun and the PV) and then thermally converted and reshaped to spectrally fit the PV bandgap. For this purpose, the absorber must be thermally coupled to a material that emits thermal ra- diation and that peaks spectrally near the PV bandgap. During this process—see Figure 1(a)—any ‘memory’ of the number of absorbed photons is lost. Furthermore, the emitted radiation is characterized by an enhanced rate (which leads to higher pho- tocurrent). This is possible for thermal radiation because the chemical potential () is zero and no conservation law for the photon rate applies. There is a thermodynamic penalty, how- ever, for operation at  D 0 (i.e., the high temperatures that are required for effective thermal conversion and emission within the desired energy interval). Indeed, after more than 30 years of STPV research, the record conversion efficiency for an STPV Figure 1. Illustrations of the (a) solar thermophotovoltaic (STPV) and (b) thermally enhanced photoluminescence (TEPL) approaches to solar energy conversion. In (a) the absorbed solar light is first converted to heat and then to thermal emission. In (b), however, the absorbed solar light is directly converted to high-temperature PL, which conserves the photon rate. device is only 3.2% (at an operating temperature of 1285K). 2 Moreover, low-bandgap PVs—such as indium gallium arsenide (InGaAs) or germanium (Ge)—are required for device operation Continued on next page