Material and Device Characterizations: The thickness and crystal structures of MoSe2 film were determined by atomic force microscopy (AFM, Veeco) and X‐ray diffraction (XRD, PANalytical Empyrean), respectively. Raman spectra of the MoSe2 film were measured in a LabRAMHR800 Raman microscopy system using 514 nm laser as excitation source. XPS measurements were performed using a monochromatic Al Kα source (1486.6 eV) produced by the XPS system (Kratos AXIS UltraDLD). The XPS data were calibrated by comparing with C 1s peak to assure the correction. The absorption spectra of MoSe2 on quartz glass, Si substrate, and MoSe2/Si heterojunction were detected by UV–vis spectrometer (Perkin‐Elmer LAMBDA 750) equipped with an integrating sphere. To observe the interfacial structure of heterojunction, the cross‐section structure, element mapping, and line‐scan EDX were characterized by HRTEM (FEI Tecnai G2). The photoresponse of the photodetectors was detected using a semiconductor parameter analyzer (Keithley 4200‐SCS). The light sources were produced by a Xe lamp (CEL‐HXF300) with tunable output power and a monochromator (Zolix Instruments, Omni‐nx I). To measure the NIR photoresponse, 808, 1064, and 1310 nm lasers were also used as light sources (Changchun Laser Optoelectronics Technology, MW‐GX‐808, MW‐GX‐1064, and MW‐GX‐1310). Light intensity is measured by an optical power meter (CEL‐NP2000). The response speed of the photodetector was measured by using a LD (650 nm) driven by a function signal generator (Shengpu, F‐40), and a digital oscilloscope (Tektronix TDS 2012C) was used to monitor the variation of photovoltage.