In this work, thermal annealing is investigated to augment photovoltaic conversion efficiency (PCE) in bulk heterojunction polymer solar cells (BHJ-PSCs). Active layer (P3HT/PC70BM) and PEDOT: PSS film of these PSCs contain copper oxide nanoparticles (CuO-NPs) and gold nanoparticles (Au-NPs) respectively. Thermal annealing of P3HT/PC70BM thin films was performed at 150ºC for 30 minutes. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) examination show an increased crystallinity of the heat-treated samples. Atomic force microscopic analysis (AFM) shows increased surface roughness after annealing, producing extra locations for crystallization of P3HT. Ultraviolet (UV)-visible as well as external quantum efficiency (EQE) spectra show enhanced self-organization, enhancing both crystallinity of P3HT and phase separation of the spin coated P3HT/PC70BM polymer films. However, ultraviolet-visible spectra do not show remarkable enhancement in light absorption with increasing gold concentration of PEDOT:PSS thin film owing to localised surface plasmonic resonance (LSPR) being distributed horizontally through the PEDOT:PSS film. PCE improved by 24% in the heat treated (Au/PEDOT: PSS)/(CuO/P3HT/PC70BM) solar cells.