The post-production thermal annealing process of the polymer photoactive layer of organic solar cells at ideal temperatures is one of the crucial factors for solar cell performance since the photovoltaic performance of polymer-based organic solar cells is strongly affected by thermal annealing of the photoactive layer. In this study, inverted organic solar cells with enhanced efficiency using a blended P3HT and PCBM subjected to post-production thermal annealing have been reported. The confocal Raman and AFM measurements were performed to obtain the information about the material properties and surface morphology of electron selective layer (ESL) of ZnO and polymer photoactive layer of P3HT:PCBM. To find out the ideal temperature for annealing, the post-production heat treatment procedure was applied to the polymer photoactive layer in between 90 and 210 °C by 20 °C steps. The current density versus voltage (J-V) measurements of Glass/ITO/ZnO/Polymer/MoO3/Ag devices shown that the application of post-production heat treatment to the polymer photoactive layer with a temperature of around 150 ℃ significantly enhanced the solar cell parameters of devices. The PCE values were calculated as 1.90 and 3.91 % for the reference device without annealing and the annealed device at 150 ℃, respectively. Additionally, it was observed that the performance of the solar cell increased due to the decrease in RS value and the increase in the roughness of the polymer surface with annealing process.