Compared with all‐small‐molecule (ASM) and other types of organic solar cells (OSCs), the small molecule donor:polymer acceptor (SMD:PA) OSCs develop much slower due to the lack of material matching rules. Herein, by changing the end‐cap substituent of the small molecule donor from ethyl (MPhS‐C2) to benzyl (MPhS‐Ph), the different selection rules of donor properties and thermal annealing (TA) treatment between the ASM and the SMD:PA system under tetrahydrofuran processing are thoroughly investigated. Therefore, MPhS‐Ph exhibits more ordered molecular packing, leading to better adaptability in the SMD:PA system without TA; while the inferior molecular packing of MPhS‐C2 after spin‐coating performs better in the ASM system with TA. Whether spin‐coating or TA process dominates morphological optimization also dominates their energy loss. Therefore, the MPhS‐Ph:PYF‐T‐o and MPhS‐C2:BTP‐eC9 based devices achieve the highest power conversion efficiency (PCE) of 12.1% and 15.7%, respectively, both of which are cutting‐edge PCEs in their own type of OSCs fabricated by non‐halogen solvent. This result suggests that intrinsic strong crystallization independent of the thermal drive is hoped in SMD:PA‐OSCs, while high miscibility after spin‐coating and proper assembly under thermal drive is expected in ASM‐OSCs, providing deep understanding and guidance in highly efficient materials design rules in both ASM‐OSCs and SMD:PA‐OSCs.