|Ph.D Student||Manor Assaf|
|Subject||Photoluminescent and Thermal Spectral Shaping for Heat|
Harvesting in Photovoltaics
|Department||Department of Nanoscience and Nanotechnology||Supervisor||Professor Carmel Rotschild|
|Full Thesis text|
Sunlight is the most abundant sustainable energy source available, and will play a major role in future energy generation. Photovoltaic cells (PVs) are the most mature and widespread technology for harnessing this energy source. However, a major challenge in PVs is the enhancement of the single-junction PV efficiency above the maximal Shockley-Queisser (SQ) limit. Such an enhancement would lead to reduction in the system pay-back time, a parameter of major significance for PV installations.
The SQ limit results from the basic trade-off in broadband optical energy conversion: The PV semiconductor can only harvest photons above its bandgap energy. Thus, high bandgap PV will yield small current and high voltage, and vice versa. This trade-off can be exceeded by “tricks”: For example, the splitting of an energetic photon to two, or the merging of two low energy photons to a single high-energy photon. Throughout the years, many approaches for exceeding the SQ limit were proposed, without experimental success. These concepts always employ a spectrum-shaping function to the PV cell, so it “sees” a different light source than the sun. In this work, we begin by reviewing existing “hot” concepts that employ an absorber which modifies the solar-spectrum by the conversion to thermal radiation, requiring ultra-high temperatures (>2500 K) for efficient thermal emission and conversion - a requirement which hinders device realization. We continue with the investigation of hot photoluminescence (PL), which has significantly different properties than thermal emission. This mechanism is demonstrated in the first paper (published in Optica), which shows how thermally-enhanced photoluminescence (TEPL) is an efficient optical heat-pump, capable of generating high-energy photon emission rates, order of magnitude above thermal emission. The result is the ability to operate a novel TEPL converter, which significantly reduces the device operating temperatures. Our next step is to theoretically and experimentally demonstrate such a TEPL converter, as published in the second paper (published in Nature Communications). Here, heat is harvested by a low bandgap PL absorber that emits TEPL toward a higher bandgap PV cell, resulting in very high theoretical efficiencies (50%-70%) with decreased operating temperatures down to 1100 K. Experimentally, TEPL upconversion with white light excitation of a tailored Cr:Nd:Yb Glass absorber suggests that conversion efficiencies as high as 48% at 1500 K are in reach. In the third paper (published in ACS Photonics), a different approach to operate in non-thermal equilibrium is investigated. Here, we explore a different mechanism that yields extreme tenfold up-conversion in rates that are above thermal emission rates. We show evidence for such a process by comparing the upconverted emission resulting from a CO2 laser non-equilibrium excitation, with the emission originating from thermal-equilibrium heating in a furnace. We measure the upconversion efficiency to be 4% - a factor of 2 above thermal emission. Lastly, we show good fit of our experimental results to a recently suggested model for the emission rate of rare-earth oxides.
I am truly hopeful that this work will make an impact on PV research by enabling a new class of efficient PV devices.