Compared to conjugated polymers, small-molecule organic semiconductors present negligible batch-to-batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small-molecular organic solar cells (SM-OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small-molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open-circuit voltage with IDIC-4F as acceptor. Due to kinetic and thermodynamic reasons, H13-based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14-based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14-based SM-OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM-OSCs using IDIC-4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine-tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.