We report on the power conversion efficiency (PCE) enhancement for organic solar cells (OSCs) based on several approaches. A standard cell composed of an indium tin oxide (ITO) anode, P3HT/ PCBM active layer, PEDOT:PSS hole transport layer and an aluminum cathode is used as a reference. We investigate the effects of the following three modifications. We first incorporate CdSe quantum dots (QDs) in the photo-optically active P3HT/PCBM blend in order to enhance the optical absorption. In opposite to other studies, QDs are not used here to replace the donor material (PCBM), and we always measured an enhanced PCE compared to the standard cell with a QDs:P3HTPCBM volume ratio up to 1:5. As a second modification, NaYF4:Yb,Er up conversion (UC) microcrystals are incorporated into a TiO2-x sol-gel to form an additional layer used to convert IR photons to blue and green photons. Again, OSCs with UC layer showed an improved PCE compared to the reference cell. The PCE enhancement is both attributed to the IR light absorption and to a better electron transport between the active layer and the cathode due to the electron transport layer capabilities of TiO2-x. Finally, MoO3 layer is used to replace the PEDOT:PSS layer as hole transport layer (HTL). This layer is deposed either by thermal evaporation or by spin coating from a sol-gel solution. We found evaporation better in terms of thickness control and reproducibility. It has been demonstrated that the PEDOT:PSS HTL can be replaced by MoO3, and the thickness of this MoO3 layer strongly affects the PCE of the cell. The maximum PCE was obtained with a thickness of 40nm, and again is better that the reference cell.