In this research CdSe0.4Te0.6 NCs were synthesized in aqueous solution through a cheap, simple, and modified hot injection chemical precipitation method. The reflux time was altered in a wide range for the synthesis of these NCs with different size and bandgap energies. Then they were applied as a co-sensitizing layer together with CdS NCs film in the photoanod of quantum dot sensitized solar cells (QDSCs). It was shown that the QDSC with TiO2 NCs/CdSeTe(7 h)/CdS/ZnS photoelectrode demonstrated the maximum power conversion efficiency (PCE) of 4.2% in AM1.5 solar irradiation. The selected CdSeTe NCs prepared in 7 h of the reflux time were utilized in a second growth process to form core-shell CdSeTe-CdS NCs. The shell formation time was widely altered and NCs were applied in the photoanod of corresponding QDSCs. It was demonstrated that the cell with TiO2 NCs/CdSeTe (7 h)-CdS(100 min)/CdS/ZnS photoelectrode revealed the best efficiency of 4.6% in the experiments. This was due to the surface passivation of the sensitizing NCs and better charge collection efficiency.