PLB is recipient of an Australian Research Council Federation Fellowship (Project number FF0668728). PM is a Vice Chancellor's Senior Research Fellow. We acknowledge funding from the University of Queensland (Strategic Initiative-Centre for Organic Photonics and Electronics), the Queensland Government (National and International Research Alliances Program-'Queensland Organic Solar Cell Alliance'). This work was performed in part at the Queensland node of the Australian National Fabrication Facility (ANFF-Q). AMN and NK acknowledge funding from 'Molecularly Engineered Energy Materials', an Energy Frontier Research Center funded by the U.S. Department of Energy, Offi ce of Science, Offi ce of Basic Energy Sciences under Contract Number DE-SC0001342:001.