Future development of InGaP/(In)GaAs based multijunction solar cells

Although technologies for the InGaP/InGaAs/Ge cell have been matured, there is still room for improvement of the InGaP/(In)GaAs/Ge cell in practical level. Band gap of the top cell should be increased a little to get higher V_OC. Thinning the Ge substrate is thought to be effective to increase a power per weight even for rigid panel. For concentrator application, grid pitch, cell size and current matching design should be optimized with taking account of the spectrum of concentrated light. The InGaP/(In)GaAs based solar cells shall be cornerstone in high efficiency multijunction solar cells in future. High efficiency cell consisted of 1 eV lattice-match material such as InGaAsN is strongly desired for high efficiency 4-junction or 6-junction cell. Wafer bonding and layer transfer techniques might be sophisticated to make solar cells. Paper-like InGaP/GaAs solar cells with efficiency of 29.4% on flexible metal film developed by SHARP Corp. are newly reported. Material cost of the cell is basically very low, because it has only very thin layers of III-V compounds and cheap metal film. Thin film technology shall be a hint for future cells.