From Photoluminescence to Thermal Emission: Thermally Enhanced PL (TEPL) for efficient PV
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The Shockley-Queisser efficiency limit of ~40% for single-junction photovoltaic (PV) cells is mainly caused by the heat dissipation accompanying the process of electro-chemical potential generation. Concepts such as solar thermo-photovoltaics (STPV) aim to harvest this heat loss by the use of a primary absorber which acts as a mediator between the sun and the PV, spectrally shaping the light impinging on the cell. However, this approach is challenging to realize due to the high operating temperatures required in order to generate high thermal emission fluxes (>2000K). After over thirty years of STPV research, the record conversion efficiency for STPV device stands at 3.2% at 1285K.
In contrast, we demonstrate how thermally-enhanced photoluminescence (TEPL) is an optical heat-pump, in which photoluminescence is thermally blue-shifted upon heating while the number of emitted photons is conserved. This process generates energetic photon-rates which are comparable to thermal emission in significantly reduced temperatures, opening the way for a TEPL based energy converter. In such a device, a photoluminescent low bandgap absorber replaces the STPV thermal absorber. The thermalization heat induces a temperature rise and a blue-shifted emission, which is efficiently harvested by a higher bandgap PV. We show that such an approach can yield ideal efficiencies of 70% at 1140K, and realistic efficiencies of almost 50% at moderate concentration levels. We experimentally demonstrate 1.4% efficient TEPL energy conversion of an Nd3+ system coupled to a GaAs cell, at 600K.
Assaf is finishing his finishing his Ph. D. studies at the "Excitonics" lab in the Tehcnion (Israel) under the supervision of Dr. Carmel Rotshild. He is researching the thermodynamics of a novel photo-thermal device based on Thermally-Enhanced PL. He did his undergraduate studies in EE and physics, also at the Technion, and his M. Sc. studies under the supervision of Prof. Eugene Katz in the Institute of desert research, where he studies the physics of organic-PV's under concentrated sunlight.