Flexible organic solar cells were successfully prepared using spin-coating technique on PET substrates covered with a thick layer of ITO. The goal of this study is to identify the effects of the LiF layer and ZnO nanoparticles mixing in PEDOT:PSS solution and to compare the photovoltaic properties of these „customized” cells PET/ITO/PEDOT:PSS + ZnO nanoparticles/P3HT:PCBM(1:1)/Al, PET/ITO/PEDOT:PSS + ZnO nanoparticles/P3HT:PCBM(1:1)/LiF/Al, with ones of the “conventional” structures, PET/ITO/PEDOT:PSS/P3HT:PCBM(1:1)/Al. The thickness of the LiF layer, deposited by thermal vacuum evaporation, was two nanometers. Composite samples were prepared by adding ZnO nanoparticles into PEDOT:PSS solution, followed by ultrasonication. I-V characteristics were measured in dark and under A.M.1.5 conditions for all samples immediately and after one month from the preparation date. The action spectra measurements revealed that the structures which have a nanometric LiF layer are more stable than those without. Photoelectrical measurements indicate that the ZnO nanoparticles have a positive influence on the conversion efficiency and also to reduce the serial resistance of the structure.