An optoelectronic semiconductor device comprising: an absorber layer made of gallium arsenide (GaAs) and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the optoelectronic semiconductor device , the emitter layer made of a different material than the absorber layer and having a higher bandgap than a bandgap of the absorber layer , the optoelectronic semiconductor device being a photovoltaic device or a solar cell; a heterojunction formed between the emitter layer and the absorber layer ; a p-n junction formed between the emitter layer and the absorber layer at a location offset from the heterojunction , the p-n junction causing a voltage to be generated in the optoelectronic semiconductor device in response to the optoelectronic semiconductor device being exposed to light at a front side of the optoelectronic semiconductor 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 , the intermediate layer having the same type of doping as the absorber layer and including the different material, wherein the intermediate layer includes: a graded layer having a material gradation from GaAs at a side closer to the absorber layer , to the different material of the emitter layer at a side closer to the emitter layer , and a back window layer not having the material gradation and having an approximately uniform composition of the different material. an absorber layer made of gallium arsenide (GaAs) and having only one type of doping; an emitter layer located closer than the absorber layer to a back side of the optoelectronic semiconductor device , the emitter layer made of a different material than the absorber layer and having a higher bandgap than a bandgap of the absorber layer , the optoelectronic semiconductor device being a photovoltaic device or a solar cell; a heterojunction formed between the emitter layer and the absorber layer ; a p-n junction formed between the emitter layer and the absorber layer at a location offset from the heterojunction , the p-n junction causing a voltage to be generated in the optoelectronic semiconductor device in response to the optoelectronic semiconductor device being exposed to light at a front side of the optoelectronic semiconductor 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 , the intermediate layer having the same type of doping as the absorber layer and including the different material, wherein the intermediate layer includes: a graded layer having a material gradation from GaAs at a side closer to the absorber layer , to the different material of the emitter layer at a side closer to the emitter layer , and a back window layer not having the material gradation and having an approximately uniform composition of the different material. an intermediate layer located between the absorber layer and the emitter layer and providing the offset of the p-n junction from the heterojunction , the intermediate layer having the same type of doping as the absorber layer and including the different material, wherein the intermediate layer includes: a graded layer having a material gradation from GaAs at a side closer to the absorber layer , to the different material of the emitter layer at a side closer to the emitter layer , and a back window layer not having the material gradation and having an approximately uniform composition of the different material. a graded layer having a material gradation from GaAs at a side closer to the absorber layer , to the different material of the emitter layer at a side closer to the emitter layer , and a back window layer not having the material gradation and having an approximately uniform composition of the different material. <EOS>