NOVELTY - Multi-junction photovoltaic device comprises: set of semiconductor material layers including first light absorbing layer of silicon germanium or silicon germanium tin material (51) with photocarrier separating p-n junction, and second light absorbing layer of gallium nitride arsenide bismide material or indium gallium nitride arsenide bismide material (52) with photocarrier separating p-n junction, where materials are lattice matched to gallium arsenide.    USE - As multi-junction photovoltaic device, such as solar cell (all claimed).    ADVANTAGE - The multi-junction photovoltaic device has more than one subcell for absorbing different parts of spectrum of incident light and contains semiconductor light absorbing layers lattice matched to gallium arsenide. The gallium nitride arsenide bismide, or indium gallium nitride arsenide bismide subcell; and silicon germanium, or silicon germanium tin subcell of device are able to provide good absorption coverage of spectral wavelengths longer than those absorbed by e.g. gallium arsenide subcell, and offer high absorption efficiency; which is in contrast of approach of such prior art devices in e.g. US2009/00140161 and US2011/0232730 which have only one such subcell, where subcell is of different material i.e. GaInNAsSb. Also, in US2011/0083729 there are GaNAs and SiGe layers that absorb in general area of spectrum but GaNAs cannot be lattice matched to GaAs which means that strain relieving buffer layer has to be used between subcells and substrate. Further this device has no subcell in 1.4 eV region that would be provided by GaAs, and none could be provided simply because it would not lattice match GaNAs. Further, InGaP subcell in this device would in order to lattice match GaNAs has larger band gap (by changing proportion of In and Ga) than InGaP subcell used in present device which is lattice matched to GaAs, thus leaving large gap in absorption spectrum of device of US2011/0083729.    DETAILED DESCRIPTION - A multi-junction photovoltaic device comprises: set of semiconductor material layers including first light absorbing layer of silicon germanium or silicon germanium tin material (51) including photocarrier separating p-n junction, and second light absorbing layer of gallium nitride arsenide bismide material or indium gallium nitride arsenide bismide material (52), including photocarrier separating p-n junction, where silicon germanium or silicon germanium tin layer, and gallium nitride arsenide bismide or indium gallium nitride arsenide bismide layer are lattice matched to gallium arsenide. The multi-junction photovoltaic device further comprises light absorbing layer of gallium arsenide material (53) including photocarrier separating p-n junction; light absorbing layer of indium gallium phosphide (54) including photocarrier separating p-n junction and is lattice matched to gallium arsenide; light absorbing layer of aluminum gallium arsenide (55) including photocarrier separating p-n junction and is lattice matched to gallium arsenide; and light absorbing layer of aluminum indium gallium phosphide including photocarrier separating p-n junction and is lattice matched to gallium arsenide. The multi-junction photovoltaic device comprises gallium arsenide substrate (50), and set of layers is on and lattice matched to substrate; or substrate that is lattice matched to gallium arsenide, and set of layers is on and lattice matched to substrate. An INDEPENDENT CLAIM is included for method for making multi-unction photovoltaic device involving: providing substrate of gallium arsenide or another material that is lattice matched to gallium arsenide, growing first light absorbing layer of silicon germanium or silicon germanium tin material including photocarrier separating p-n junction and lattice matched to substrate, and growing second light absorbing layer of gallium nitride arsenide bismide or indium gallium nitride arsenide bismide material including photocarrier separating p-n junction and lattice matched to first light absorbing layer.    DESCRIPTION OF DRAWING(S) - The figure shows subcells of the multi-junction photovoltaic device.    GaAs substrate (50)    Bottom SiGe/SiGeSn subcell having light absorbing layer (51)    GaNAsBi/InGaNAsBi subcell having light absorbing layer (52)    Upper middle GaAs subcell (53)    InGaP subcell (54)    AlGaAs/AlInGaP subcell having light absorbing layer (55)