再生エネルギー研究センターパンフレット（英語）

11/20

10●Next-generation multijunction solar cell “smart stack technology”　The “smart stack technology” using metal nanoparticle arrays has been developed, for the first time, making possible the interconnection of various solar cells based on different materials and bandgaps. It can provide the flexibility in material choice and device design because the mismatch in lattice constants, thermal expansion coefficients, etc., can be neglected in this technique. 　A GaAs/InP-based four-junction solar cell has achieved a conversion efficiency of as high as 31.6%, and a GaAs/CIGS-based three-junction solar cell has achieved a conversion efficiency of as high as 24.2% (joint research with the Research Center for Photovoltaics at AIST Tsukuba Center). We are aiming to improve and establish this technology for mass production.　The use of thin crystalline silicon as a bottom cell provides high efficiency and low cost multijunction cells. The team is developing the crystalline silicon based smart stack cells that goes beyond the theoretical limit efficiency of single junction crystalline silicon solar cells. The demonstrative GaAs/Si three-junction with a conversion efficiency of 24.7% was successfully fabricated.Smart stack technologyGaAs/Si-based three-junction smart stack cellTop cell: GaAs etc.Bottom cell: SiMetal nano particle, Pd etc.Electrode firing furnaceA furnace for forming contacts between the electrode and the diffusion layer as well as Al-BSF layersSpin etching apparatusAn apparatus that etches a single side of the wafer by spin rotation. Only one side can be etched without a protective filmIon implantation equipment An equipment which implants accelerated phosphorus or boron ions into the wafer. The diffusion profile can be precisely controlledMain Research FacilitiesActivities and Achievements①The facilities for crystalline Si solar cell and module fabrication have been installed and started operation. The FREA standard cells with Al-BSF have been fabricated with an average cell efficiency of about 19.3% equivalent to the best reported efficiencies from mass-production②The thin wafer (0.12 mm thick) slicing technique from silicon ingots has been established using diamond wires. The processing conditions close to the mass production of the wafer with the thickness of 0.12 mm have been established with a 99.8% yield.③The smart stack technology was applied to a GaAs/Si-based three-junction cell to achieve a conversion efficiency of as high as 24.7%.④0.1 mm-thick double-sided solar cells have been fabricated.⑤A diffusion layer with a uniform depth on pyramid-shape surfaces has been formed successfully by means of ion implantation, demonstrating cell efficiencies of as high as 19.1%.⑥A new module evaluation technique was developed (absolute EL method, an in-situ AC impedance measurement method). The place of failure can be identified nondestructively, and the voltage of each cell within a module can be evaluated individually.Outlook of a crystalline silicon cell by standard FREA processPhosphorus diffusion layer formed by the ion implantation