This paper represents a model to increase efficiency beyond the theoretical limit for a single junction solar cell.Conventionally, p-i-n junction solar cell is used with addition layer p+ and n+ to increase efficiency.The conventional solar cell used classical silicon which has less efficiency. Another disadvantages is that the high surface reflection from the surface.Moreover, some associated problems like low conversion efficiency and materials with the appropriate band gaps that can perfectly match the broad range solar radiation remain.Therefore, this proposal consists of GaP as a window layer due to overcome conventional limitations by using AlGaAs as a window layer.GaP is binary alloy material which is easier to prepare.Moreover, GaP has 2-colour capability at room temperature and GaP has outstanding thermal shock resistance for rapid heating environment.