We will present detailed analysis of bulk heterojunction organic solar cells which are based on thiophene polymer and C60 molecules. We measure intensity dependent quantum efficiency as well as photocurrent efficiency under 1 sun illumination and dark current and analyze it using a detailed model developed for these materials.[1] The above allows us to decipher the systems energetics and to conclude, contrary to common belief, that up to 0.3V may be lost at the contact region.