Assaf Manor

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

Conservation of photon rate in endothermic photoluminescence and its transition to thermal emission

Photoluminescence (PL) is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Conversely, in optical refrigeration, the absorption of a low-energy photon is followed by endothermic PL of an energetic photon. These two quantum processes are, in contrast to thermal emission, governed by photon-rate conservation. Thus far, both aspects of PL have been studied under thermal population that is far weaker than the photonic excitation, hiding the role of rate conservation when thermal excitation is significant. Here we theoretically and experimentally study endothermic PL at high temperatures. In contrast to thermal emission, we find that the PL photon rate is conserved with temperature increase, while each photon is blueshifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrate 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. This opens the way for a proposed novel device that harvests thermal losses in photovoltaics with record efficiency.

Thermally enhanced photoluminescence for heat harvesting in photovoltaics.

The maximal Shockley–Queisser efficiency limit of 41% for single-junction photovoltaics is primarily caused by heat dissipation following energetic-photon absorption. Solar-thermophotovoltaics concepts attempt to harvest this heat loss, but the required high temperatures (T>2,000 K) hinder device realization. Conversely, we have recently demonstrated how thermally enhanced photoluminescence is an efficient optical heat-pump that operates in comparably low temperatures. Here we theoretically and experimentally demonstrate such a thermally enhanced photoluminescence based solar-energy converter. Here heat is harvested by a low bandgap photoluminescent absorber that emits thermally enhanced photoluminescence towards a higher bandgap photovoltaic cell, resulting in a maximum theoretical efficiency of 70% at a temperature of 1,140 K. We experimentally demonstrate the key feature of sub-bandgap photon thermal upconversion with an efficiency of 1.4% at only 600 K. Experiments on white light excitation of a tailored Cr:Nd:Yb glass absorber suggest that conversion efficiencies as high as 48% at 1,500 K are in reach.

Efficient 10-Fold Upconversion through Steady-State Non-Thermal-Equilibrium Excitation

Frequency upconversion of a few low-energy photons into a single high-energy photon contributes to imaging, light sources, and detection. However, the upconverting of many photons exhibits negligible efficiency. Upconversion through laser heating is an efficient means to generate energetic photons; yet the spectrally broad thermal emission and the challenge of operating at high temperatures limit its practicality. Heating specific modes can potentially generate narrow upconverted emission; however, so far such “hot-carriers” have been observed only in downconversion processes and as having negligible lifetime, due to fast thermalization. Here we experimentally demonstrate upconversion by excitation of a steady-state non-thermal-equilibrium population, which induces steady, narrow emission at a practical bulk temperature. Specifically, we used a 10.6 μm laser to generate 980 nm narrow emission with 4% total efficiency and upconverted radiance that far exceeds the device’s possible blackbody radiation. This...

Optical refrigeration for ultra-efficient photovoltaics

The Shockley-Queisser (SQ) efficiency limit for single-junction photovoltaic cell (PV) is to a great extent due to inherent heat dissipation accompanying the quantum process of electro-chemical potential generation. Concepts such as solar thermophotovoltaics1,2,3 (STPV) and thermo-photonics4 aim to harness this dissipated heat, claiming very high theoretical limit. In practice, none of these concepts have been experimentally proven to overcome the SQ limit, mainly due to the very high operating temperatures, which significantly challenge electro-optical devices. In contrast to the above concepts for harnessing thermal emission at thermal equilibrium, Photoluminescence (PL) is a fundamental light-matter interaction under non-thermal equilibrium, which conventionally involves the absorption of energetic photon, thermalization and the emission of a red-shifted photon. Conversely, in optical-refrigeration the absorption of low energy photon is followed by endothermic-PL of energetic photon5,6. Both aspects were mainly studied where thermal population is far weaker than photonic excitation, obscuring the generalization of PL and thermal emissions. Here we experimentally study endothermic-PL at high temperatures7. In accordance with theory, we show how PL photon rate is conserved with temperature increase, while each photon is blue shifted. Further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also show how endothermic-PL generates orders of magnitude more energetic photons than thermal emission at similar temperatures. Relying on these observations, we propose and study thermally enhanced PL (TEPL) for highly efficient solar-energy conversion. Here, solar radiation is absorbed by a low-bandgap PL material. The dissipated heat is emitted by endothermic PL, and harvested by a higher-bandgap photovoltaic cell. While such device operates at much lower temperatures than STPV, the theoretical efficiencies approach 70%, bringing its realization into reach.

Entropy driven multi-photon frequency up-conversion

We experimentally demonstrate a fundamentally new, entropy driven photon up-conversion mechanism, where 10.6μ photons are tenfold up-converted to 1μ with efficiency of 10%. This work opens novel ways of converting thermal-radiation to electricity.

Thermally enhanced photoluminescence for energy harvesting: from fundamentals to engineering optimization

The radiance of thermal emission, as described by Plancks law, depends only on the emissivity and temperature of a body, and increases monotonically with the temperature rise at any emitted wavelength. Non-thermal radiation, such as photoluminescence (PL), is a fundamental light–matter interaction that conventionally involves the absorption of an energetic photon, thermalization, and the emission of a redshifted photon. Such a quantum process is governed by rate conservation, which is contingent on the quantum efficiency. In the past, the role of rate conservation for significant thermal excitation had not been studied. Recently, we presented the theory and an experimental demonstration that showed, in contrast to thermal emission, that the PL rate is conserved when the temperature increases while each photon is blueshifted. A further rise in temperature leads to an abrupt transition to thermal emission where the photon rate increases sharply. We also demonstrated how such thermally enhanced PL (TEPL) generates orders of magnitude more energetic photons than thermal emission at similar temperatures. These findings show that TEPL is an ideal optical heat pump that can harvest thermal losses in photovoltaics with a maximal theoretical efficiency of 70%, and practical concepts potentially reaching 45% efficiency. Here we move the TEPL concept onto the engineering level and present Cr:Nd:YAG as device grade PL material, absorbing solar radiation up to 1 μm wavelength and heated by thermalization of energetic photons. Its blueshifted emission, which can match GaAs cells, is 20% of the absorbed power. Based on a detailed balance simulation, such a material coupled with proper photonic management can reach 34% power conversion efficiency. These results raise confidence in the potential of TEPL becoming a disruptive technology in photovoltaics.

Thermally enhanced photoluminescence for ultra-high-efficiency photovoltaics

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 energetic photon absorption. Although concepts such as solar thermophotovoltaics (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 radiation 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 photocurrent). 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, however, 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.

Practical considerations for solar energy thermally enhanced photo-luminescence (TEPL) (Conference Presentation)

While single-junction photovoltaics (PVs) 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 PVs) 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.

Efficient extreme up-conversion through steady-state non-thermal-equilibrium excitation (Conference Presentation)

Frequency up-conversion is a technique for the generation of high energy photon from two or more lower energy photons. Although many up-conversion techniques have been demonstrated such as parametric up-conversion or multi-photon absorption, their conversion efficiencies become negligible for high-order up-conversion. Alternatively, in thermal emission very high temperatures are needed for reasonable efficiencies and the emission is spectrally broad rendering this up-conversion method impractical for most applications. We present a new efficient extreme up-conversion method for generating NIR and visible wavelengths using CW LWIR laser by non-thermal-equilibrium excitation through spontaneous reduction of the chemical potential. In this method we exploit the high chemical potential of the pump specific modes to excite the vibronic states of the host, subsequently transferring the energy to chosen emitters, resulting in narrow non-thermal steady-state emission. All while, only residual energy contributes to materials temperature, thus keeping it at a comparably low temperature. We experimentally demonstrate 7, 10, 13, 16, and 20-fold up-conversion at external efficiency of up to 4%, exceeding black-body radiation of the bulk temperature. Furthermore, we present energy transfer between emitters, a phenomenon in contrast to thermal emission, showing the photoluminescence behavior of this method. We use CW CO2 laser (10.6m) to excite silica vibronic states and transfer the energy to rare-earth emitters at the NIR and visible spectrum. This new outlook on up-conversion via energy transfer paves the way for developing new light sources and new methods of imaging and detection with high efficiencies.