Preaggregation in Solution Producing Multiple Crystal Forms of Y6 Corresponding to a Variation of Miscibility in PM6-Based Ternary Solar Cells

The mechanism of how the preaggregation of components induced by the solvent type affects the crystallization and miscibility of Y6-based blends remains to be explored. In this article, PDCBT-2F and N2200 serving as polymer additives were incorporated into the high-performance PM6:Y6 system to investigate morphological evolution as well as photovoltaic performance. It is found that both PDCBT-2F and N2200 could enhance the power conversion efficiency (PCE) of PM6:Y6 solar cells when processed with chloroform (CF). Unfortunately, the PCE value sharply reduced when processed with chlorobenzene (CB), which further decreased upon the incorporation of PDCBT-2F. In contrast, N2200 could still improve the performance of solar cells due to the formation of interconnected fibrils, although Y6 formed large-scale aggregates as observed using a transmission electron microscope (TEM). According to the UV spectra in solution and surface energy analysis, the components show close free energy in CF, indicating preaggregation matching of components. In contrast, the free energies of Y6 and PDCBT-2F in CB are different from those of PM6 and N2200, and the mismatching of preaggregation of the components contributes to the poor performance of solar cells. Based on the differential scanning calorimetry (DSC) analysis, the multiple melting peaks of Y6 corresponding to the face-on and edge-on crystals have been distinguished. The Flory–Huggins interaction parameters in Y6-based blends were highly influenced by the solvent type. N2200 shows higher miscibility with Y6 than PDCBT-2F, and N2200 induces the crystallization of Y6. The variation of miscibility induced by the preaggregation in different solvents could provide theoretical guidelines to optimize or control the morphology of the active layer as well as the resulting performance of organic solar cells.