Graphene has unique characteristic of zero band-gap. Its electrical properties can be widely tuned by varying the number of layers or by doping as p or n type with different materials. Due to high optical transmittance and zero band-gap, graphene can be used as both, electron, and hole extraction layer. We have simulated n-graphene/Silicon hetero-structure Ag/ITO/n-Graphene/a-Si:H/c-Si(p)/Ag using AFORS (Automat FOR Simulation of HETero structures) software under AM1.5 illumination and power density of 100 mW/cm2. Here, the thickness of a-Si:H i-layer from 0 to 20 nm, the n-type doping concentration of Graphene from 1 × 1010cm−3 to 1 × 1020cm−3 to get the optimized results. Corresponding to that, we studied the band diagram and hence barrier height, Quantum efficiency etc. The best power conversion efficiency obtained is 18.59% where the Voc is 611.8 mV, Jsc is 37.04 mA/cm2 and FF is 82.03% corresponding to i-layer thickness 5 nm, single layer graphene and n-type doping concentration of 1 × 1020cm−3.