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Bulk heterojunction organic solar cells have recently drawn tremendous attention because of their technological advantages for actualization of large-area and cost effective fabrication. Two important criteria of these cells are efficiency and cost. The research in this dissertation focuses on the enhancement of these criteria with two different approaches. In the first approach, power conversion efficiency of organic photovoltaic devices is enhanced by introducing Deoxyribonucleic acids DNA into the device structure. DNA provide exciting opportunities as templates in self assembled architectures and functionality in terms of optical and electronic properties. In the first method, we investigate the effects of DNA and metalized DNA sequences in polymer fullerene bulk-heterojunction (BHJ) solar cells. These effects are characterized via optical, quantum efficiency and current-voltage measurements. We demonstrate that by placing on the hole collection side of the active layer, DNA and Pt-DNA sequences lead to an increase in the power conversion efficiency (PCE) by %16 and %30, respectively. Furthermore, we studied the electrical charge characteristics of our DNA layer by using capacitance-voltage (C-V) measurements to explain the increase in hole collection which shows that spray coated DNA formed a negative layer which can increase the hole collection in the cathode side. In the second approach, device cost is tried to reduce by replacing the most expansive material, indium thin oxide (ITO) thin films, with graphene thin films. Large area graphene films were grown with chemical vapor deposition (CVD) method. It is observed that, its pristine form, the electrical and surface properties of these films are not sufficient enough for the organic photovoltaic applications. These properties are enhanced with a surface treatment of Argon (Ar) plasma and nitric acid bath. The results of these treatments show that the surface becomes hydrophilic and surface resistance can be decreased by %25. Then, it is demonstrated that the PCE of the graphene based solar cells can be reached up to one tenth of the ITO based devices. The research conducted in this dissertation offers promising potential of bulk heterojunction organic solar cells as a clean and affordable source of energy source in the near future.