Understanding the Mixing Phase Structure in Multi-length-scale Morphology to Arrest High-performance Photovoltaic Devices

The properties of the mixing phase are investigated in detail by introducing PC71BM into two typical organic photovoltaic blends J51:N2200 and PM6:Y6. It is found that the process of exciton dissociation and carrier transport in the mixing zone play a key role in determining the power conversion efficiency. In J51:N2200:PC71BM blend, the aggregation of PC71BM in the mixing zone brings in energetic barrier to inhibit the hole transfer process. In the meanwhile, the enlarged intervals in-between the double fibril network limit the effective diffusion of the split electrons and holes in the mixing zone, leading to large recombination and energy loss. While in PM6:Y6:PC71BM blend, the introduced PC71BM could homogeneously distribute in the amorphous zone, mixing well with PM6 and Y6 molecules. Meanwhile, the addition of PC71BM does not perturb the hole transfer from Y6 to PM6. Such a morphology is advantageous where electrons and holes generated at abundant donor/acceptor interface could diffuse out rapidly, and transport in the crystalline pathway towards the corresponding electrodes. Such results reveal the importance of manipulating the mixing phase structure in the multi-length-scale morphology, of high demand towards 20% efficiency in the next episode organic solar cell (OSC) development.