University of Delaware - College of Engineering


Photonics & Electromagnetics

Germanium-Based Solar Cells for Long Wavelength Sensitivity

James Kolodzey

This project addresses the development of solar cells that are optimized for photons from the low energy part of the solar spectrum in the range between the bandgaps of germanium (0.66 eV) and silicon (1.12 eV). This is the wavelength range from 1.88 to 1.1 micrometer, where silicon is transparent. This research is important, because the germanium cells could be operated underneath silicon cells in a tandem multijunction configuration, and thus collect solar power that would otherwise be lost. The amount of power in the solar spectrum between 0.66 and 1.1 eV with "1-sun" (AM 1.5) illumination is 15 %, or about 15 milliwatts–cm-2, compared to the total illumination of 100 mW–cm-2. Of the long wave solar energy that remains after conversion by conventional silicon cells, the proposed low energy cells can add a few percent of marginal efficiency, which could be leveraged over the huge number of new solar systems, which are growing at about 48 % per year worldwide. To combine excellent performance with low cost, the proposed cells will be fabricated of silicon germanium (SiGe) alloys by the epitaxial technique of chemical vapor deposition (CVD). Silicon and germanium can intermix across the whole alloy composition range, and the net bandgap energy can vary from 0.66 to 1.12 eV, depending on the composition. By optimizing solar cells that utilize low energy photons, our research should significantly improve the efficiency of solar power systems.

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