Present and Future of High Efficiency Multi-Junction Solar Cells

This paper reviews our research activities for III-V compound multi-junction (MJ) solar cells and recent R&D topics on concentrator MJ solar cells under the NEDO's R&D Project on Innovative Solar Cells initiated since 2008. The conversion efficiency of InGaP/(In) GaAs/InGaAs triple-junction solar cells has been improved to 35.8% (1-sun, AM1.5G) and 42.1% (230-suns) as a result of proposing double-hetero wide-band-gap tunnel junctions, understanding dislocation behavior in the lattice mismatched system and inverted epitaxial layer structure. We have contributed to industrialization of InGaP-based multi-junction solar cell for space use in Japan. The authors have also shown great potential of concentrator PV for terrestrial applications by developing concentrator 3-junction solar cell modules (area of 5,500cm(2) for 550suns use) with an efficiency of 31.5%. Now, we are challenging higher efficiency toward 50% under the NEDO's Innovative R&D Project.