Today, BHJ (bulk heterojunction) OSCs (organic solar cells) dominate modern age research in renewable solar energy. In recent developments, the ultimate goal is to improve the general performance of the BHJ-OSCs to enable them to compete on an equal footing basis with conventional silicon photovoltaic cells. In this presentation, we review the fundamental parameters that have been reported to improve the general performance of the BHJ-OSC devices. These parameters include, among other things, the use of ZnO nanoparticles ETL (electron transport layer) inserted between the top electrode and the photoactive layer, annealing procedure and device geometry. The BHJ-OSC devices constructed in this study comprised of successive (bottom up) layers of (3,4ethylenedioxythiophene):poly (styrenesulfonate) or PEDOT:PSS, a blend of poly (3-hexylthiophene) or P3HT and [6,6]-phenyl butyric acid methyl ester or PCBM, zinc oxide or ZnO nanoparticles and aluminum (Al) metal top electrode. These layers were deposited on ITO (indium tin oxide) coated glass substrates. The device construction was also inverted (top down) in order to evaluate the effect of inversion on the power conversion efficiency and the general performance of the devices. The devices were annealed at 155 o C either before (pre-annealed) or after (post-annealed) the deposition of the Al top electrode. Post-annealed devices showed improved PV (photovoltaic) characteristics when compared to pre-annealed devices. Furthermore, we discuss the performance of inverted geometry in comparison to ZnO nanoparticles and nanoflakes as buffer layers.