The War on the Bulk Heterojunction: A Battle for the Future of Organic Solar Cells

Abstract

Organic photovoltaics (OPVs) promise cheap and flexible solar energy. Whereas light generates free charges in silicon photovoltaics, excitons are normally formed in organic semiconductors due to their low dielectric constants, and thus OPVs require molecular heterojunctions to split excitons into charges. This invention of the ‘bulk heterojunction’ has helped progress OPV efficiencies over the last 30 years. However, it has also held the field back from achieving its full potential. The bulk heterojunction adds significant complexity that makes rational device and materials design extremely difficult, and introduces fundamental limits to device efficiencies. In this talk I will summarise our efforts over the last years to overcome the limitations of the bulk heterojunction. I will initially outline our attempts to enhance exciton diffusion length(1), and to make use of the exceptional properties of new small molecule ‘fused ring electron acceptors’(FREAs) (2). Then I will discuss work undertaken in the last two years at VUW that shows a new direction of OPV development is possible, thanks to our discovery(3) of intrinsic free charge photogeneration in the highly efficient molecule known as ‘Y6’. This discovery paves the way for a new type of OPV, based on a ‘doped p-n homojunction’ design, and sets us on a path for reimagining the future of organic electronics.