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.
©2011 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. Parisa Sehati, Slawomir Braun, Linda Lindell, Xianjie Liu, Lars Mattias Andersson and Mats Fahlman, Energy-Level Alignment at Metal-Organic and Organic-Organic Interfaces in Bulk-Heterojunction Solar Cells, 2010, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, (16), 6, 1718-1724. http://dx.doi.org/10.1109/JSTQE.2010.2042684