Characterizing the morphologies of nonfullerene acceptors (NFAs)/donor polymer layers with molecular resolution is crucially important for organic solar cells (OSCs), but remains challenging and insufficient currently. Herein, the morphologies of NFA Yn (n=5-7)/donor polymer PM6 heterogeneous films were revealed with molecular resolution through scanning tunnelling microscopy and analyzed through density functional theory (DFT) calculations. Y6 forms an interdigitated, ladder-like structure with PM6 which is verified as a kinetic product by the DFT calculations. After annealing, the ladder-like structure coexists with the energy-favorable phase separation one. The difference of the forming energy between the phase separation and ladder-like structure in Y7/PM6 and Y5/PM6 is obviously larger than the one in Y6/PM6, therefore, only the phase separation structure was observed before and after annealing in Y7/PM6 and Y5/PM6. Yn (n=5-7) shows rather different stacking in its neat domain. Y6 forms a face-to-face, dense packing pair with the lateral shift around half of a molecule. Y7 is a dense packing was well, but the stacking is the T type. Y5 forms an exact face-to-face style while its packing is not the dense one. The morphology differences of Yn (n=5-7)/PM6 films are closely related to the alterations of the ending groups in the backbones of Yn (n=5-7) molecules. Annealing reduce the surface-occupied ratio of Y5 and Y7 domains but does not change their stacking. The results are very valuable for the accurate morphology characterizations and the researches of related OSCs and optoelectronics.
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