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. Yb 2 O 3 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 Er 2 O 3 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 cutoff 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.

Solar thermophotovoltaics: reshaping the solar spectrum

“…The availability of inexpensive, earth-abundant, and high-quality cells made c-Si historically the first choice for TPV and STPV applications [119,120]. However, the band gap energy of cSi is about 1.1 eV, which indicates that the emission peak of a blackbody emitter would never overlap with the band edge of the material, unless the emitter temperature goes above 2500 K. To date, no thermal emitter in the literature has been demonstrated to operate efficiently and sustainably at this temperature range.…”

Assessment of the radiative properties of some semi-conductors for applications in thermophotovoltaic and thermophotonic conversion systems

“…Most research groups focus on cells from Gallium Antimonide (GaSb), Silicon (Si), and Gallium Arsenide Indium (InGa-As). The germanium (Ge) is also cost effective [6]. The main element in the TPV and TPX conversion system is the selective emitter i.e.…”

Electrical characterisation of ruthenium Schottky contacts on n-Ge (100)

“…The bandgap of Ge of 0.66 eV is located close to the radiation maximum of a blackbody. Therefore, Ge seems suitable to convert the radiation of a broadband thermophotovoltaics emitter with a temperature below 1 650°C [17]. A lot of research work has been performed on the electrical properties of defects introduced during high-energy, electron and proton irradiation of Ge [18,19,20,21,22,23].…”