Yin, Jian, & Roberto Paiell (2010). Multiple-Junction Quantum Cascade Photodetectors for Thermophotovoltaic Energy Conversion. Department of Electrical and Computer Engineering and Photonics Center, Boston University. Boston, Massachusettes USA.
DIGEST (Walter Baltzley): This paper explains how Quantum Cascade Photodetectors (QCP)–a kind of Intersubband (ISB) photovoltaic detector–can be used to design thermo-photovoltaic (TPV) cells with high theoretical efficiencies. This is achieved by building many layers (junctions)–each tuned to a specific wavelength.
Unlike traditional solar cells, QCPs make use of the same physical materials, meaning that many layers can be grown one on top of one another–each tuned to a different frequency of light. Current Photovoltaic Technologies require that each frequency use a different material with a different crystalline structure, and when layered these materials tend to pull apart and break. Thus, current multi-junction (layer) solar cells are limited to only three or four layers–making them less efficient.
ISB Photovoltaic detectors can only absorb polarized light, meaning that a special waveguide or filter will have to be included for it to work efficiently. Fortunately, these are relatively easy to fabricate.
Each layer of QCPs can theoretically achieve complete absorption of its frequency, and thus with enough layers can aborb 100% of the entire light spectrum–up to a maximum of 1.4 W/cm^2 at a temperature of 1,880 F. This is a substantial improvement over and TPV cells made of Indium-Gallium Arsenide which produce 0.8 W/cm^2 at the same temperature.
Achieving these temperatures using solar power requires us to concentrate the incoming light by roughly 10 times–making it highly efficient, considering that current TPV systems require sunlight concentrated 500-1,000 times. While QCPs could work well enough alone, they can also be combined with existing Indium Gallium Arsenide TPVs for substantial gains in efficiency.
ABSTRACT: The use of intersubband transitions in quantum cascade structures
for thermophotovoltaic energy conversion is investigated numerically. The
intrinsic cascading scheme, spectral agility, and design flexibility of these
structures make them ideally suited to the development of high efficiency
multiple-junction thermophotovoltaic detectors. A specific implementation
of this device concept is designed, based on bound-to-continuum
intersubband transitions in large-conduction-band-offset
In0.7Ga0.3As/AlAs0.8Sb0.2 quantum wells. The device electrical
characteristics in the presence of thermal radiation from a blackbody source
at 1300 K are calculated, from which a maximum extracted power density
of 1.4 W/cm2 is determined. This value compares favorably with the present
state-of-the-art in interband thermophotovoltaic energy conversion,
indicating that quantum cascade photodetectors may provide a promising
approach to improve energy extraction from thermal sources.
©2010 Optical Society of America
OCIS codes: (250.5590) Quantum-well, -wire and -dot devices; (040.5350) Photovoltaic.