Infrared (IR) technology has been widely used in biomedical imaging, non-destructive inspection, environmental monitoring and optical communication. The important mid-far-IR photodetectors are mainly limited to compound semiconductors that normally requires intricate crystal growth process and operation at cryogenic cooling, which results in bulky and expensive system. The emergence of two-dimensional (2D) transition metal dicharcogenides (TMDCs) semiconductors offers new opportunities for optoelectronic applications for their strong quantum confinement and the easiness in forming heterostructures enabled by the out-of-plane van der Waals bonding. The interlayer excitons formed in a TMDC heterostructure possess the inherent large exciton binding from their parent materials and the flexibility in exciton energy tuning. This offers opportunity to realize excitonic devices operable at room temperature at mid- to far-IR range, which are challenging for intraband exciton based 2D devices. This paper will introduce photodetection in mid-IR range by manipulating interlayer excitons generated between two specifically selected TMDCs with appropriate band alignment. The unique band structure in the heterostructure allows the absorption band to be tuned and extended to 20μm under a modest electric field, far beyond the cutoff wavelength of 2D black phosphorous or 2D black arsenic phosphorous. The ab initio simulation suggests the sizeable charge delocalization and accumulation at interface result in greatly enhanced oscillator strength of interlayer excitons and high responsivity of the photodetector. The results provide a promising platform for realizing robust tunable room temperature operating IR photodetectors.