Akaike, K.; Koch, N.; Heimel, G.; Oehzelt, M.: The Impact of Disorder on the Energy Level Alignment at Molecular Donor-Acceptor Interfaces. Advanced Materials Interfaces 2 (2015), p. 1500232/1-6
10.1002/admi.201500232

Abstract:
Introductory paragraph: Organic photovoltaic cells (OPVCs) have attracted considerable interest in the last three decades, because of their advantages over conventional inorganic photovoltaics in terms of easy processability (roll-to-roll), low cost of production, and the possibility of fabricating ultra-thin and fl exible devices. [ 1 ] At the heart of modern OPVCs is a type II heterojunction that splits photogenerated electron–hole pairs and transports the resulting mobile charge carriers to their respective contacts. [ 2 ] This interface, consisting of electron donor- and acceptor-type molecular semiconductors, is therefore key to the photocurrent generation in an OPVC. [ 3 ] By optimizing the optical bandgaps of the organic semiconductors and the positions of the electronic levels with respect to each other, power conversion effi ciencies of 10% can be reached, even for single-junction OPVCs. [ 4 ] Therefore, precisely knowing the offsets between the solidstate values for the energies of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) is essential for effi cient devices. In addition, a thorough understanding of the electronic processes occurring at these donor–acceptor interfaces is crucial to establish rational design principles of, e.g., interface morphology and, ultimately, to improve future organic materials. [ 5 ]