Revealing the Crystalline Packing Structure of Y6 in the Active Layer of Organic Solar Cells: The Critical Role of Solvent Additives

The bulk heterojunction morphology of photovoltaic materials for organic solar cells (OSCs) is crucial to the fundamental optoelectronic properties. However, in the photoactive layer, the intrinsic crystalline packing structure of Y6, currently the hallmark molecule among the Y-series non-fullerene acceptors (NFAs), is still unclear. Here, employing grazing-incidence wide-angle x-ray scattering (GIWAXS), we resolved the crystalline packing structure of Y6 in the active layer thin film obtained under actual device fabrication conditions. It is found that the film with solvent additive 1-chloronaphthalene (CN) presents a more ordered crystalline packing of Y6. Atomistic molecular dynamics simulations indicate that there exist additional π-π interactions between CN molecules and Y6 end groups, which provide more space for Y6 molecules to relax and form more ordered structures. Our work reveals the critical role of CN in the crystalline packing of Y6 in the device active layer and provides direct correlations between this crystalline packing and the device characteristics and photophysical properties.