Organophosphorus Derivatives as Cathode Interfacial-Layer Materials for Highly Efficient Fullerene-Free Polymer Solar Cells

Roles of cathode interfacial layer (CIL) for low work function metal cathode, which influences significantly the electron extraction and transport processes, are in current trends for improvement in the organic solar cell (OSC) performance. Two organophosphorus derivatives tetraphenylphosphonium bromide (QPhPBr) and ((2-(1,3-dioxan-2-yl)ethyl)triphenylphosphonium bromide) (TPhPEtBr) as CILs individually and with mixed binary layer with N719 were demonstrated. Tremendous improvement in photovoltaic performance with QPhPBr with an average power conversion efficiency, PCE, of 11.08% and TPhPEtBr with PCE of 10.20% as well as their binary layers with 11.61 and 10.74%, respectively, has been achieved using the PBDBT:ITIC blend active layer, in comparison to that of the bare Al cathode (7.37%). The maximum PCE of 12.0% is achieved with QPhPBr:N719 as the CIL, which is the highest value reported in the literature to date for PBDB-T:ITIC-based single junction binary fullerene-free OSCs, suggesting the potential of ionic organophosphorus derivatives and their binary blended mixtures with an ionic n-type organic semiconductor such as N719 used as CILs for realizing high-efficiency fullerene-free OSCs. Their efficient performance would be helpful for potential selection of CILs in OSCs.