Published information on polymer composite solar cells was screened to pin down the most critical parameters for a further improvement of these photovoltaic systems towards higher efficiency and stability. Largely irreversible separation of photocharge between the polymer (e.g. PPV) and the fullerene with subsequent percolation of electrons (e.g. to the Al contact) and holes (to the ITO contact), under the symmetry breaking influence of an electric field imposed by the work function difference of the contacts (Al and ITO respectively) accounts for photocurrent generation. The energetics is determined by n-p hetero junction, which is formed between ITO and the polymer/fullerene composite. Positive polarization of the ITO contact leads to the generation of an interfacial potential drop and an increase of the effective ITO work function. This explains the generation of a photovoltage reaching more than 0.8 Volt. Parameters limiting solar cell performance are discussed.