To evaluate the performance of the Gr/MoSe2/Si heterojunction for broadband detection, UV light (365 nm), green light (500 nm), and red light (650 nm) were chosen as exciting light sources. From I–V curves of the device measured at different light wavelengths (Figure 4 a), it is observed that the device shows a remarkable photocurrent upon incident light illumination at reverse bias. The high sensitivity at reverse bias is due to the fact that the photogenerated electron‐hole pairs significantly change the concentration of minority carriers, which dominates the photocurrent under a reverse bias.70 From the time‐dependent photocurrent excited by pulsed UV light (365 nm, 15 mW cm−2) at different reverse bias voltages, Figure 4b, we note that the device exhibits a high sensitivity to the UV light with a large I on/I off ratio of 140 and 130 at −1 V and −2 V, respectively. The fast response and recovery speed can be deduced by the steep rise and fall edges of the response curves, indicating the effective generation and separation of electron–hole pairs in the Gr/MoSe2/Si heterojunction. In addition, excellent stability and reproducibility can also be verified by the unchanged response current of the device illuminated with a pulsed light with about 16 s per cycle. To quantify the performance of the Gr/MoSe2/Si photodetector, two key figure‐of‐merit parameters, i.e., responsivity (R) and detectivity (D*) that indicate the efficiency of a detector responding to optical signals and the ability of a detector to detect weak optical signals, respectively, are calculated by following equations71 (4)R =IphPin (5) D*=A1/2R(2qId)1/2 =R(2qJd)1/2where I ph, P in, I d, and J d represent the photocurrent, incident light power, dark current, and dark current density, respectively. Figure 4c plots both the responsivity and detectivity at different light wavelengths as a function of applied reverse bias voltage. It is seen that the responsivity at different wavelengths increases, whereas the detectivity first increases and then decreases, with increasing reverse bias voltage. The higher reverse bias can result in a larger photocurrent under the same incident light intensity since more carriers can pass through the junction, thereby contributing to an increased responsivity. However, the dark current will increase at the meantime, ultimately resulting in the decrease of detectivity at high reverse bias. Based on Equations (4) and (5), the Gr/MoSe2/Si photodetector shows optimal R and D* values of 270 mA W−1 and 7.13 × 1010 Jones (Jones = cm Hz1/2 W−1) at 650 nm, 93 mA W−1 and 2.55 × 1010 Jones at 500 nm, and 182 mA W−1 and 6.22 × 1010 Jones at 365 nm, respectively. Figure 4d depicts the absorption spectrum of the MoSe2/Si heterojunction, with the spectra of both of MoSe2 film on quartz substrate and bare Si substrate for comparison. The MoSe2 film possesses stronger light absorption at both short wavelength direction (<800 nm) and long wavelength direction (>1060 nm) as compared to the bare Si substrate. Therefore, the combination of MoSe2 film with Si substrate makes the system capable of absorption of the broadband light ranging from UV, visible, to NIR light. In addition, it is noted that there is an absorption valley around 500 nm. This may be responsible for the relatively lower responsivity and detectivity at that wavelength.