Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells

Ultraviolet photoelectron spectroscopy measurements in combination with the integer charge transfer (ICT) model is used to obtain the energy-level alignment diagrams for two common types of bulk-heterojunction solar cell devices based on poly(3-hexylthiophene) or poly(2-methoxy-5-(3',7' -dimethyloctyloxy)- 1,4-phenylene vinylene) as the donor polymer and (6,6)phenyl- C61-butric-acid as the acceptor molecule. A ground-state interface dipole at the donor/acceptor heterojunction is present for both systems, but the origin of the interface dipole differs, quadrupole-induced in the case of poly(2-methoxy-5-(3',7'-dimethyl-octyloxy)-1,4-phenylene vinylene), and ICT state based for poly(3-hexylthiophene). The presence of bound electron-hole charge carriers (CT states) and/or interface dipoles are expected to enhance exciton dissociation into free charge carriers, thus reducing the probability that charges become trapped by Coulomb forces at the interface followed by recombination.