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Pengfei Li — 2013-14 Fellow

Project Title: Thermophotovoltaic Solar Cells: Towards Fundamental Improvements of Solar-to-Electrical Conversion Efficiency

Past technological developments in converting solar energy to electricity have focused on photovoltaic (PV) solar cells, in which photons directly excite electron-hole pairs that are extracted at separate electrodes to generate power. Traditional PV cells, such as silicon-based cells, can only utilize photons with energies larger than the bandgap, and hence a large fraction (e.g., 25% for Si-based PV cells) of useful energy with photons below the bandgap is wasted. Furthermore, only a fraction of the photon energy above the bandgap is useful, while the remaining is lost to the crystal lattice as heat. The combination of these loss mechanisms results in the Shockley-Queisser limit for single junction solar cells, which is 32 % in the absence of concentration.

The concept of solar thermophotovoltaics (TPV) is appealing because it enables one to overcome the Shockley-Queisser limit with a single junction cell by utilizing nearly the entire solar spectrum. In a solar TPV system, one converts the sunlight into heat through a broad-band absorber. The heat is then used to generate narrow-band thermal radiation from an emitter. High theoretical efficiency (> 80 %) can be achieved if the emitter generates narrow-band radiation that is well matched in wavelength to the band gap of a single-junction PV cell.

Successful development of this transformative technology can be achieved only through resolving fundamental challenges in materials design and transport processes. For solar TPV, the key is to control the absorption of solar photons and their re-emission of near-bandgap photons, while simultaneously suppressing emission in other wavelength ranges. To realize high efficiency solar TPV systems, we plan to resolve the following technical challenges:

  1. Developments of large scale and inexpensive absorber and emitter structures that operate at high temperature. A high efficiency solar TPV system requires the intermediate absorber/emitter to operate at a high temperature (1000-2000K). This places severe constraints on the materials and structures that can be used in the absorber/emitter. In this project, we will overcome this challenge by exploring large scale metallic nanostructures that can provide excellent spectral control and enable high temperature operation, and by developing novel nanocomposites with narrow band emission.

  2. Thermal engineering and design of overall system. A solar TPV system places very substantial challenges on thermal engineering and design. In a solar TPV system, the intermediate absorber/emitter needs to be maintained at a high temperature, whereas the PV cells need to be kept relatively cool. This, in turn, places significant difficulties in reducing thermal leakage. In this project, we will develop techniques that accurately characterize the performance of various devices in the specific environment of solar TPV systems, and examine design issues that will arise in overall system integration.

By utilizing clean and unlimited solar energy, solar TPV provides us with a promising pathway towards substantially reducing global greenhouse gas emissions and therefore significantly contributes to the environment and sustainable cities. Solar TPV is particularly attractive as it has the potential to provide a practical and large scale approach that breaks some of the fundamental efficiency limitations in single junction solar cells.