In situ and ex situ investigations on ternary strategy and co-solvent effects towards high-efficiency organic solar cells

The morphology of organic solar cells (OSCs) is a core topic for achieving the ultimate photovoltaic performance. Herein, we focused on the combination of two important morphology regulation strategies, i.e., ternary strategy and cosolvent engineering. Using an amorphous polymer acceptor, BN-T, as the third component, the PM6:BTP-eC11 and PM6:eC9 host binary systems, treated by the o-xylene and 1-phenylnaphthalene solvent system and chlorobenzene and 1,8-diiodooctane solvent system, respectively, both realized an effective improvement in the power conversion efficiencies (PCEs). However, the ex situ morphological characterization revealed that these two systems undergo different types of changes in phase segregation and molecular packing, which cannot be understood by the current popular miscibility analysis. In this work, an in situ investigation was carried out during the spin casting and thermal annealing processes. The time-resolved reflection spectroscopy technique showed that BN-T retained more PN in the ternary films during the casting process, thereby facilitating the aggregation of eC11 and enlarging its domain size. In contrast, the incorporation of BN-T did not affect the DIO content in the films, resulting in a less separated morphology for the eC9-based systems, as predicted by the miscibility study. In addition to state-of-the-art PCEs, this work provides an insightful understanding of the morphology evolution in ternary OSCs assisted by a high-boiling solvent additive via in situ investigation techniques.