In this work, the pyrite crystalline phase of iron disulfide nanoparticles (FeS2) about 20 to 30 nm was obtained by a two-pot thermal method at 220 degrees C. Subsequently, different concentrations of these nanoparticles were used as a doping agent for the conjugated poly-3-hexylthiophene (P3HT). The electrical resistivity of P3HT was decreased almost three orders of magnitude while adding FeS2 nanoparticles as doping, and dichlorobenzene solvent was a determinant factor for the dispersion of polymer with nanoparticles. Doped-P3HT dichlorobenzene solution was spin coated onto the FTO/TiO2 substrate to fabricate the FTO/TiO2/P3HT:FeS2/C-Au hybrid solar cells. Moreover, the power conversion efficiency (PCE) of hybrid devices was studied as a function of pyrite FeS2 nanoparticle concentration. The highest efficiency of 0.83% was obtained at 1% concentration of FeS2 nanoparticles. Hence, the results revealed that the FeS2 nanoparticles could be considered as an alternative charge carrier to develop the bulk hybrid solar cells.