Novel material for super high efficiency multi-junction solar cells

For realizing a future four-junction solar cell InGaP/GaAs/InGaAsN/Ge with high conversion efficiency of over 40% (AM1.5G), we are developing the chemical beam epitaxy (CBE) technology to grow high-quality InGaAsN. This diluted nitride will be used as a third cell material, because it can be grown on Ge with lattice matching and a 1.0 eV band gap. However, due to the small amount of N incorporated into the GaAs (or InGaAs) crystal, the diffusion length becomes too short to fabricate the tandem solar cells with the high performance we expect. The films in the CBE process are grown using organic gas molecules as sources under high vacuum conditions (10^-2 Pa). Because of the ultra-low pressure, the reactions between the source-gas molecules in the gas phase are suppressed and the reactions only occur on the growing surface. This allows the use of active source gases that decompose at low temperatures. The CBE growth technique produces high quality GaAsN. The lower TEGa flow rate is one of the most important factors to obtain low residual impurities, higher mobility (Hall hole mobility), and longer lifetime (Photoluminescence lifetime).