Nanoparticles enable novel approaches to light converter devices that may allow preventing the energy loss related to spectral mismatch, by enlarging the absorption range from UV to IR. Broadband absorption in solar cell is crucial to harvest solar energy in the wide spectral range which increases the efficiency of light converter devices. Quantum dot (QD) inserted as the thin film layer inside the device structure is particularly attractive candidate to make broadband absorption due to the QD size adjustable band gap values. The broadband absorption and photon harvesting in the wide range by using QD layer are the main aim of this study. Lead sulfide quantum dot (PbS QD) layer is introduced in the Ag/n-Si/p-CIGS/In heterojunction structure to study the effects of QD layer on the performance of the device. The device performance of heterojunction diode structure was significantly improved by incorporating PbS QD thin film in between CIGS thin film layer and In metal contact layer. Here, we discussed the characterization of CIGS thin film deposited by thermal evaporation method by using XRD, Raman spectroscopy, absorption spectra measurements and the contributions of QDs layer on the Ag/n-Si/p-CIGS/In heterojunction diode structure by the temperature dependent photoconductivity and current-voltage measurements performed both in dark and under illumination. The results showed that the QD thin film layer in the device structure exhibited significant improvements of the device parameters and the solar cell properties as well.