III-V compound multi-junction (MJ) solar cells have great potential for space and terrestrial applications because they have high efficiency potential of more than 50% and superior radiation-resistance. Recently, more than 40% efficiency cells were reported by Fraunhofer ISE, Spectrolab, Sharp and others. Concentrator 4-junction or 5-junction solar cells have great potential for realizing super high-efficiency of over 50%. In order to realize super high-efficiency of more than 50%, it is substantially important to understand and reduce several losses of solar cells. This paper reviews loss mechanism for III-V compound solar cells and MJ solar cells. In addition, recent results under the EU-Japan Collaborative Research on Concentrator Photovoltaics are also presented. The conversion efficiency of inverted epitaxially grown InGaP/GaAs/InGaAs triple-junction solar cells has been improved to 37.9% (1-sun, AM1.5G) and 44.4% (250- 300 suns) as a result of proposing double-hetero structure wide-band-gap tunnel junctions, and inverted epitaxial growth.
While single-junction solar cells may be capable of attaining AM1.5 efficiencies of up to 29%, multi-junction (MJ,
Tandem) III-V compound solar cells appear capable of realistic efficiencies of up to 50% and are promising for space
and terrestrial applications. In fact, the InGaP/GaAs/Ge triple-junction solar cells have been widely used for space since
1997. In addition, industrialization of concentrator solar cell modules using III-V compound MJ solar cells have been
announced by some companies. This paper presents principles and key issues for realizing high-efficiency MJ solar cells,
issues relating to development and manufacturing, and applications for space and terrestrial uses.