This Conference Presentation, Thermally enhanced photoluminescence and fundamental upper limit of luminescence: theoretical study was recorded at Photonics West 2020 held in San Francisco, California United States.
Conversion efficiency of broad-band sunlight in single-junction photovoltaics (PV's) is limited due to heat dissipation to less than 32%. Overcoming this requires inventive techniques, where their viability is compared to cheap and abundant silicon photovoltaics on one hand; or the efficient and costly multi-junction cells on the other.
The recently proposed Thermally Enhanced Photoluminescence (TEPL) conversion device may take the place in-between, and potentially present high conversion efficiencies – with a single-junction solar cell. A PV cell is placed adjacent to a thermally insulated photo-luminescent (PL) absorber. The absorber is excited and heated by concentrated sun-light, consequently radiating blue-shifted PL emission toward the PV cell, resulting in higher conversion efficiencies compared to direct illumination or Thermal-PV at similar temperature. Spectral measurements based calculations show that efficiencies over 46% may be reached using a GaAs PV, and absorber working temperatures below 1500°C.
TEPL prototype faces two major design challenges: absorber material, and photon management. Broad absorption, together with high PL external quantum efficiency (EQE), must be maintained at high temperatures. Here we demonstrate our achievements toward a TEPL converter. Using Cr, Ce & Nd, co-doped in YAG, we reach over 85% EQE with full absorption of sunlight up to 1.1µm. For photon recycling, paramount to maintain high chemical potential of the solar radiation, highly reflective surfaced and dichroic mirrors surround the absorber; reflecting photons not used by the PV cell to be reabsorbed in the absorber. We demonstrate a TEPL conversion device predicted to support conversion efficiencies over 15% under these conditions.
While single-junction photovoltaics (PV's) are considered limited in conversion efficiency according to the Shockley-Queisser limit, concepts such as solar thermo-photovoltaics aim to harness lost heat and overcome this barrier. We claim the novel concept of Thermally Enhanced Photoluminescence (TEPL) as an easier route to achieve this goal.
Here we present a practical TEPL device where a thermally insulated photo-luminescent (PL) absorber, acts as a mediator between a photovoltaic cell and the sun. This high temperature absorber emits blue-shifted PL at constant flux, then coupled to a high band gap PV cell. This scheme promotes PV conversion efficiencies, under ideal conditions, higher than 62% at temperatures lower than 1300K. Moreover, for a PV and absorber band-gaps of 1.45eV (GaAs PV's) and 1.1eV respectively, under practical conditions, solar concentration of 1000 suns, and moderate thermal insulation; the conversion efficiencies potentially exceed 46%.
Some of these practical conditions belong to the realm of optical design; including high photon recycling (PR) and absorber external quantum efficiency (EQE). High EQE values, a product of the internal QE of the active PL materials and the extraction efficiency of each photon (determined by the absorber geometry and interfaces), have successfully been reached by experts in laser cooling technology. PR is the part of emitted low energy photons (in relation to the PV band-gap) that are reabsorbed and consequently reemitted with above band-gap energies. PV back-reflector reflectivity, also successfully achieved by those who design the cutting edge high efficiency PV cells, plays a major role here.
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