Photogeneration and the bulk quantum efficiency of organic photovoltaics

We introduce a method to analyze the performance of bulk heterojunction (BHJ) organic photovoltaics (OPVs) by calculating its “bulk quantum efficiency” (BQE), a quantity related to the recombination losses within the BHJ, but not in the surrounding device layers. By applying the method to both vacuum- and solution-processed OPVs with various BHJ, buffer layers and interface layer compositions, we show that measurements of the BQE isolates the properties of the BHJ from other device layers and interfaces. We use measurements of the BQE to study various mechanisms in OPV degradation and find that for solution-processed OPVs with a ZnO cathode buffer layer, the BHJ undergoes degradation due primarily to the ZnO. By inserting a self-assembled monolayer at the interface between the buffer and the BHJ, the stability of the OPV is significantly improved.