Bathocuproine (BCP) or tris(acetylacetonato)ruthenium(in) (Ru(acaC)(3)) is used as the exciton-blocking layer (EBL) in photovoltaic cells. The difference in thickness-dependent efficiency characteristics between the blockers (see Figure) is that the Ru(acac)3 energy-level alignment allows for the transport of holes from the cathode to the C-60 acceptor level, whereas BCP relies on metal-deposition-induced damage for charge transport.