Silicon, germanium and silicon/germanium photocells for thermophotovoltaics applications

Abstract

Silicon (Si) and germanium (Ge) are semiconducting materials, which are industrially used for the large-scale production of various electronic devices. Solar cells are commonly manufactured from Si. For thermophotovoltaics (TPV) Si has the disadvantage of a high bandgap of 1.1 eV, which requires the use of a spectrally matched selective emitter. Yb2O3 is widely used as an emitter material to illuminate Si photocells. Si concentrator solar cells have been investigated for TPV applications, because they have a high performance at a typical illumination density of 1 W cm−2 in a TPV system. Non-concentrator solar cells achieve lower efficiencies under TPV conditions, but up to now they are more cost effective than concentrator cells. A new Si photocell optimized for TPV has a front side textured with rectangular grooves with vertically evaporated contact fingers and a rear surface mirror to reflect sub-bandgap radiation back to the emitter.Ge photocells have a bandgap of 0.66 eV and can effectively be illuminated by a selective Er2O3 emitter. Their efficiencies are lower than those of photocells from low bandgap III/V materials, such as GaSb. But, due to low free carrier absorption in Ge, an effective rear surface mirror can be formed. A reflectance of up to 82–87% for sub-bandgap radiation and a cell efficiency of 13% for solar air mass 0 (AM0) radiation with a cut-off for wavelengths smaller than 900 nm have been achieved with a Ge TPV cell.SiGe photocells allow the variation of the bandgap as a function of the Ge content. In principle, a SiGe photocell can be matched to a given selective emitter spectrum. A first SiGe quantum dot solar cell has achieved 12% efficiency, but still suffers from a low open circuit voltage.The first TPV systems working with Si photocells have been built. A system efficiency of 2.4% can be achieved. The most promising application of Si-based TPV is likely to be an electrically self-powered residential heating system. For a self-powered operation, a TPV system efficiency of 1–2% is sufficient and Si photocells have the advantage of being inexpensive.