in the present study we focus on the effect of donor/acceptor composition on the conjugation and ordered structure of the donor-polymer in the most common organic solar cell based on a bulk heterojunction of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In addition, we try to illustrate the effect of composition, over a wide range, on the internal energy configuration and energy gaps of the donor polymer and acceptor fullerene in the solar cell active layer. Through the present investigations, we detected an improvement in conjugation length and ordered structure of the P3HT molecules under the increase its concentration in the solar cell active layer. This improvement is revealed through red shift of the P3HT absorption peak in optical absorption spectroscopy. In addition, we found a relation between energy gab and composition of the P3HT:PCBM combination, where the energy gaps of both P3HT and PCBM increase with increasin g P3HT concentration in the solar cell active layer. The higher concentrations of P3HT or PCBM are the responsible for determining the energy gaps of the molecules in solar cell active layer, while the large variation in the energy gap of a molecule is a r esult of its poor concentration in the solar cell active layer. The device performance parameters are investigated under the wide composition range of P3HT:PCBM solar cell active layer. The best (optimized) solar cell composition in these investigations is the 50:50 wt.% of the P3HT:PCBM binary at which the J SC, VOC, FF, and PCE are 6.3 mA/cm2, 0.59 V, 0.55, and 2.03 %, respectively.