Electron conducting buffer layers in organic photovoltaics

It is common to incorporate a cathode-side buffer layer in organic photovoltaic devices (OPVs) to mitigate damage from the evaporation of metal onto the underlying acceptor layer (e.g. C₆₀), which can lead to exciton quenching and/or a barrier to charge extraction. Additionally, these materials can act as both an optical spacer and an exciton blocking layer. One class of buffer layers consists of a wide bandgap material (e.g. bathocuproine), that transports carriers via damage-induced midgap states. A second class of buffers consists of a material such as tris(acetylacetonato) ruthenium(III) (Ru(acac)₃), which has a small highest occupied molecular orbital (HOMO) energy. In that case, electrons from the C₆₀ and holes from the cathode recombine at the C₆₀/Ru(acac)₃ interface. In this work, we introduce a third class of buffer that, due to alignment of its lowest unoccupied molecular orbital with that of the acceptor, allows for low resistance transport of electrons between the acceptor and cathode. By utilizing 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) as a buffer layer, we show improved fill factor in squaraine/C₆₀-based devices without a loss in open-circuit voltage or photocurrent, leading to a >;20% increase in power conversion efficiency. Although limited exciton transfer occurs from C₆₀ to PTCBI, the short exciton diffusion length of PTCBI, coupled with the lack of loss at the C₆₀/PTCBI interface, suggests that PTCBI also blocks excitons from quenching at metal-induced defects that are present in the absence of a buffer layer.