NOVELTY - Optoelectronic semiconductor device comprises: an absorber layer made of gallium arsenide and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the device; a heterojunction formed between the emitter layer and the absorber layer; a p-n junction formed between the emitter layer and absorber layer at a location offset from the heterojunction; and an intermediate layer located between the absorber layer and the emitter layer and providing the offset of the p-n junction from the heterojunction.    USE - The optoelectronic semiconductor device is used as solar cell (claimed) or photovoltaic device.    ADVANTAGE - The optoelectronic semiconductor device provides photovoltaic device which exhibits increased efficiency and flexibility, and fabricated at reduced costs when compared to conventional devices.    DETAILED DESCRIPTION - Optoelectronic semiconductor device comprises: an absorber layer made of gallium arsenide and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the device, where the emitter layer made of a different material than the absorber layer and having a higher band gap than the absorber layer; a heterojunction formed between the emitter layer and the absorber layer; a p-n junction formed between the emitter layer and absorber layer at a location offset from the heterojunction, where the p-n junction causes a voltage to be generated in the device in response to the device being exposed to light at a front side of the device; and an intermediate layer located between the absorber layer and the emitter layer and providing the offset of the p-n junction from the heterojunction, where the intermediate layer has the same type of doping as the absorber layer and including the different material. The intermediate layer includes a graded layer having a material gradation from gallium arsenide at a side closer to the absorber layer, to the different material of the emitter layer, and a back window layer not having the gradation and having a uniform composition of the different material. An INDEPENDENT CLAIM is also included for a solar cell comprising: the absorber layer; the emitter layer; the intermediate layer, where the intermediate layer includes a material gradation from gallium arsenide at a side closer to the absorber layer, to the different material of the emitter layer at a side closer to the emitter layer; the heterojunction; and a p-n junction formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction, where the p-n junction causes a voltage to be generated in the cell in response to the cell being exposed to light at a front side of the cell.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a photovoltaic unit.    Photovoltaic unit (90)    Growth wafer (101)    Buffer layer (102)    Sacrificial layer (104)    Gallium arsenide based cell (140)