Figure 3 a,b depicts the current versus voltage (I–V) characteristics of the MoSe2/Si heterojunctions in the dark and under light illumination, respectively. Both of the devices with and without graphene transparent electrode were measured to ascertain the role of graphene in determining the device performance. From the dark I–V curves, we note that the dark current at reverse bias direction (−5 V) decreases from 9.35 μA to 2.82 μA, whereas the dark current at forward bias direction (+5 V) increases from 2.6 mA to14.9 mA, after the use of three‐layer graphene transparent electrode. Correspondingly, the rectification ratio improves significantly from 278 for the device without graphene to 5284 for the device with graphene within ±5 V. As a result of the improved diode characteristic, under white light illumination (23 mW cm−2), the photocurrent at −5 V increases remarkably from 202 μA to 263 μA after the use of graphene transparent electrode. Figure 3b depicts the magnified I–V curves around the zero point under light illumination, revealing an open circuit voltage (V oc) of 100 mV for the device with graphene electrode, in contrast to the much lower V oc of 40 mV for the device counterpart without graphene electrode. Ideality factor (n) of the MoSe2/Si heterojunctions were deduced from the slops of the semilog I–V curves at the forward bias direction (Figure 3c), according to the following equation67 (1)n =qkBT dVdln Iwhere q is the unit charge, k B is the Boltzmann's constant, and T is the absolute temperature. The ideality factor of the MoSe2/Si heterojunction without graphene electrode exhibits a larger value of 2.43, compared to that of 1.79 for the device with graphene electrode. The above results collectively demonstrate that the graphene electrode plays a crucial role in enhancing the device performance of MoSe2/Si heterojunction. To understand the superior performance of MoSe2/Si heterojunction with graphene transparent electrode, Figure 3d,e illustrates the energy band alignments of the MoSe2/Si heterojunctions without and with graphene electrode, respectively. The n‐type MoSe2 film will form type II heterojunction with p‐type Si substrate, allowing the efficient separation of photogenerated electron‐hole pairs at junction interface. Under light illumination, electrons inject into the n‐type MoSe2 film, while holes inject into p‐type Si, forming the photocurrent. Notably, the introduction of graphene at MoSe2 side can greatly enhance the separation of photogenerated carriers. Moreover, the high conductivity of graphene can ensure the fast transport of electrons to the outside electrical circuit. As a result, the accumulation of electrons in MoSe2 film is avoided. This further reduces the electron–hole recombination and contributes to the improved photocurrent of the MoSe2/Si device. The zero bias barrier of the heterojunction, Φ b, was calculated based on following equation68 (2)I =IS [exp(eVnkBT)−1]