As one of the most promising materials for two-dimensional transition metal chalcogenides (2D TMDs), molybdenum diselenide (MoSe₂) has great potential in photodetectors due to its excellent properties like tunable bandgap, high carrier mobility, and excellent air stability. Although 2D MoSe₂-based photodetectors have been reported to exhibit admired performance, the large-area 2D MoSe₂ layers are difficult to be achieved via conventional synthesis methods, which severely impedes its future applications. Here, we present the controllable growth of large-area 2D MoSe₂ layers over 3.5-inch with excellent homogeneity by a simple post-selenization route. Further, a high-quality n-MoSe₂/p-Si van der Waals (vdW) heterojunction device is in-situ fabricated by directly growing 2D n-MoSe₂ layers on the patterned p-Si substrate, which shows a self-driven broadband photoresponse ranging from ultraviolet to mid-wave infrared with an impressive responsivity of 720.5 mA·W⁻¹, a high specific detectivity of 10¹³ Jones, and a fast response time to follow nanosecond pulsed optical signal. In addition, thanks to the inch-level 2D MoSe₂ layers, a 4 × 4 integrated heterojunction device array is achieved, which has demonstrated good uniformity and satisfying imaging capability. The large-area 2D MoSe₂ layer and its heterojunction device array have great promise for high-performance photodetection and imaging applications in integrated optoelectronic systems.