Recently, great attention has been paid to IV-VI colloidal quantum dots (CQDs) for their high photosensitivity, solution processability and low cost. Also, metal halide perovskites are very promising materials to realize the high-performance solution-processed visible-light photodetectors due to their cost-effective manufacturing, tunable absorption and photoluminescence in whole visible spectrum. In this paper, we present solution-processed CQDs-based tandem broadband photodetectors with low dark-current and high-sensitivity by inserting dielectric Polymethyl methacrylate (PMMA) interlayer between two sub-detectors. Our experimental data showed that the tandem broadband photodetector ITO/PEDOT:PSS/CsPbBr3:PbS0.4Se0.6/ZnO/PVK/CsPbBr3:PbS0.4Se0.6/Au showed a maximum specific detectivity of 6.8 x 10(13) Jones with a responsivity of 27 A W-1 under 57.8 mu W cm(-2) 980 nm illumination. The device performance can be further enhanced by inserting a 50 nm dielectric PMMA layer between the two sub-photodetectors. As the result, the tandem photodetector ITO/PEDOT:PSS/CsPbBr3:PbS0.4Se0.6/ZnO/PMMA(50 nm)/PVK/CsPbBr3:PbS0.4Se0.6/ZnO/Au exhibits a maximum specific detectivity of 1.32 x 10(14) Jones with a responsivity of 27.72 A W-1 under 57.8 mu W cm(-2) of 980 nm laser. Further, the physical mechanisms for the enhanced performance are discussed in detail.