Organic bulk heterojunction solar cell devices based on blend of poly(3-rnhexylthiophene) (P3HT) and [6, 6] phenyl C61 butyric acid methyl ester (PCBM) withrndifferent concentration of propylthiouracil (PTU) additive have been fabricated andrncharacterized in an open air. The current density-voltage characteristics werernmeasured in the dark and under white light illumination at illumination intensity of 80rnmW/cm2. The incident monochromatic photon to current conversion efficiencyrn(IPCE), and consistency of optical absorption and IPCE of the device without additivernand with propylthiouracil have been compared. Optimizations were made by varyingrnthe concentration of propylthiouracil from 0.75% (w/w) to 1.5% (w/w). It was foundrnthat a system with 1% (w/w) of propylthiouracil showed the best photovoltaicrnperformance with a short circuit current density, JSC of 5.69 mA/cm2, open circuitrnvoltage, VOC of 0.648 V, fill factor, FF of 40%, and power conversion efficiencyrn(PCE), of 1.8%, as compared to the system without propylthiouracil and with otherrnconcentrations of propylthiouracil. The best device showed IPCE of 62% at 480 nm.rnIt was also observed clearly that IPCE spectra of each solar cell match therncorresponding optical absorption. Our results in general show that addition of 1%rn(w/w) Propylthiouracil additive to P3HT:PCBM bulk heterojunction solar cellrnimproves efficiency of the device from 1% to 1.8%.