硅、锗和硅锗光电池在热光伏中的应用

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 effificiencies 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 fifingers and a rear surface mirror to reflflect 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 effificiencies 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 reflflectance of up to 82–87% for sub-bandgap radiation and a cell effificiency 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 fifirst SiGe quantum dot solar cell has achieved 12% effificiency, but still suffers from a low open circuit voltage.

The fifirst TPV systems working with Si photocells have been built. A system effificiency 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 effificiency of 1–2% is suffificient and Si photocells have the advantage of being inexpensive.

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Raue et al have described the fabrication of crystalline SiGe photocells. Polycrystalline SiGe ingots were grown. These were cut into wafers and the pn-junction was formed by P-diffusion. The short circuit current of the SiGe cells was slightly increased compared to a Si reference, but the open circuit voltage was reduced by about 150 mV. The carrier diffusion length in the SiGe substrates was drastically smaller than in Si wafers.

Said et al  have investigated the implementation of a 10 µm thick Si_0.9Ge_0.1 layer, grown by liquid phase epitaxy (LPE) and chemical vapour deposition (CVD), in the base of a Si photocell. Due to the lattice mismatch between Si and SiGe, the SiGe relaxes and forms misfifit dislocations. An attempt was made to bind these misfifit dislocations in a highly doped SiGe buffer layer. A maximum cell effificiency of 11.3% was achieved with an increase of 2 mA cm⁻² in short circuit current compared to Si. However, the open circuit voltage of the SiGe cell was below 560 mV.

Thick SiGe layers can be grown epitaxially on a virtual substrate on Si. A virtual substrate consists of a SiGe layer with linearly grading Ge concentration forming a strainrelaxed substrate for the pseudomorphic growth of SiGe structures. Virtual SiGe substrates have been investigated for the development of fast fifield-effect transistors but, so far, there have been no reports of SiGe photocells grown on to virtual substrates.