The saturation current of I S is defined by (3) IS= AA*T2exp(−qΦbkBT)where A is the area of the device (7.06 mm2), A* is the effective Richadson constant, and it is 32 A cm−2 K−2 for p‐type Si.69 Based on above equations, Φ b is deduced to be 793 mV for the heterojunction device with graphene top electrode. The large bulit‐in electrical field is responsble for the effective spearation of photogenerated carriers.