Category Archives: featuredposts

Temperature dependent photoluminescence

The beauty of Planck’s law, describing thermal radiation, lies in its generality. Knowing the emissivity and temperature uniquely defines the spectral radiance. On the other hand, nonthermal emission, such as photoluminescence, is a fundamental light–matter quantum interaction that is associated with the chemical potential and is not bound by Planck’s law. In this work, we study the upper limit for non-thermal emission. The study supports recent experiments on photoluminescence at high temperatures, which may result in a reduction of photon emission with increased temperature. We also present the maximal possible rate for any emission, and the inherent dependency between quantum efficiency (QE) and emissivity, which leads to a universal point defined by the pump rate and the temperature where the emitted rate is shared by any material, independent of the QE. This generalization sets an upper limit for any luminescence, which can be useful in lighting and energy conversion systems.

We solve detailed balance rate-equation model that includes both radiative and nonradiative, stimulated and spontaneous interactions. The model describe for the first time experimental observations and predicts many  new observables.

a) A typical temperature-dependent PL and thermal photon rates. b) Ne3+ emission spectrum for various temperatures. c) Theoretical simulation showing similar temperature dependent rate.

Ideal Light Source

Ideal Light Source – De-coherence without time averaging for high radiance uniform light source

Imaging of microscopic structures, photography, and excitation of materials benefit from a uniform and high-power light source. Since lasers are high radiance light sources, they are good candidates for these purposes. However, their high coherence results in speckles; a nonuniform intensity.

Spectral Radiance is the thermodynamic quantity describing illumination and is defined as the power of the radiation per wavelength per solid angle per area.

Looking on this definition we see the inherent tradeoff between high radiance and uniformity. High radiance source requires a narrow spectral and angular width, which results in high coherence and speckles.  Conventionally, speckles reduction is done by a time average of a fast moving element which is time-consuming (For example a rotating diffuser combined with a slow detector).

Our aim is to reduce the coherence instantaneously, thereby allowing fast and uniform illumination.

Our solution is taken from the diffusion process, described by a “Random walk” behavior. Consider an ensemble of random walkers starting at a point location and time. In time, they spread in a diffusive form. More important, looking at meeting events between different walkers, as time evolves the average time difference between walkers increases.  Assuming short memory (Drunk random walkers), after some time of evolution, the meetings occur between walkers that do not recognize each other. Going back to photonics, coherent light propagating in a diffused media acquires Optical Path Difference (OPD) that increases in time. After sufficient propagation, the average OPD exceeds the coherence length, which results in incoherent interaction and the elimination of speckles.

Specifically, we design a multi-mode fiber which contains pre-designed static scattering centers along its propagation axis. Propagation through the fiber volume, filled with these centers, results in different OPD acquired by different components of light fields. By engineering the centers, so that the standard deviation of the OPD is proportional to the coherence length of the light source, we achieve orders of magnitude reduction in the coherence of light, while maintaining its high radiance.

 

Luminescent Solar Power

The challenge in solar energy today is not the cost of photovoltaics (PVs) electricity generation, already competing with fossil fuel prices, but rather utility-scale energy storage costs. Alternatively, low-cost thermal energy storage (TES) exists but relies on expensive concentrated solar power (CSP). A technology, able to unify PV conversion and TES, may usher in the era of efficient base-load renewable power plants. Spectral splitting, where inefficient photons for PV conversion are redirected and thermally utilized, is economically limited by the low yield of each generator. Operating PVs at high temperatures while utilizing the thermalization induced heat for the thermal cycle is another possibility; yet, while conceptually supporting full utilization of solar energy, it is limited by PV efficiency reduction with temperature increase. My group recently introduced the concept of Luminescence Solar Power (LSP), where sunlight is absorbed in a photoluminescent (PL) absorber, followed by red-shifted PL emission matched to an adjacent PV band-edge. The PL absorber temperature rises due to thermalization, allowing spatial separation between heat and free-energy, for maximal harvesting of both. We solved the material challenge by demonstrating tailored PL with an efficiency of up to 90% while operating at 550oC. At such temperatures, LSP efficiency is 200% higher than conventional CSP and may lead to a reduction in the levelized cost of electricity (LCOE) to below 3¢/kWh

Conceptually, if PV efficiency would tolerate high temperatures, as high as 550°C, for example, it would be beneficial to concentrate solar radiation onto PV cells, harvesting the available free energy while in parallel harvesting high-quality thermal energy through a conventional steam cycle such as exists in CSP. Such a method can potentially surpass any spectral splitting method, where part of the solar spectrum is channeled to the PV while the other is channeled to a heat cycle which falls short by sacrificing heat utilization for PV efficiency or vice versa. Unfortunately, operating PVs at a high temperature cannot be done, as PV efficiency decreases sharply with temperature. Nevertheless, what cannot be done with electrons can be done all-optically. Recently, my group presented and experimentally demonstrated a new concept, Luminescence Solar Power (LSP), where solar radiation is focused onto a photoluminescence (PL) absorber having a high EQE while operating at 550°C. The PL has a narrow line shape emission that can match the band-edge absorption of single or dual-junction PVs, offering concentrated-PV (CPV) above 30% efficiency with minimal heating of the PV. The high-quality heat at the PL-absorber is collected by a heat transfer fluid (HTF) and converted into electricity at a turbine efficiency of 40%. The concept of using PL to separate free energy and high-temperature heat in this manner has never been explored before, even though each component of the system, namely the CPV cells, CSP, and the PL-absorber, rely on well-established technologies. Externally LSP and CSP installations appear the same. The figure above depicts the internal mechanism where the PL-absorber is placed at the focal point of a solar field similar to what is done in CSP. The light is absorbed and re-emitted towards a single or dual-junction PV cell at the back side. A preferred directional emission is achieved by  AR-HR coatings. HTF maintains the PL-absorber’s temperature at 550°C while transferring the heat to the turbine operating at 40% efficincy.

  • The outcome:
    • PV cell operates more efficient – high concentration & low temperature (direct electrical energy)
    • The absorber is heated – act as a thermal source for a steam turbine (stored energy)
  • Absorber material:
    • Rare-earth emitters (Nd3+, Yb3+…) → Narrow emission to match the PV band-edge, stable External quantum efficiency (EQE) at high temperatures.
    • Adding Cr3+ → enhanced solar absorption, efficient energy transfer to the rare-earth emitters.
    • YAG/Silica… → Transparent, thermally stable, efficient host for rare-earth emitters.
  • Experimental results:
    • High absorption up to 650 nm
    • EQE of 90% @ 600°C
  • Total device efficiency of above 40% with 30% storage is in reach.

Current status:

The group is building a 50KW technology demonstrator to be completed by 2020. Further investment toward commercialization will be followed.