Small molecule donor/polymer acceptor (SMD/PA) solar cells demonstrate high stability and notable performance advantages due to reduced molecular weight distribution variability, indicating potential breakthroughs in power conversion efficiency (PCE). However, research in this area is limited. This manuscript synthesizes two novel small donor molecules, DTBDT-C1-D6 and DTBDT-C3-D6 (DTBDT represents dithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene, C3 denotes a three-carbon spacer between the alkyl chain’s branching point and the core linkage site, C1 denotes a one-carbon spacer between the alkyl chain’s branching point and the core linkage site, and D6 represent π bridge has two alkyl chains with six carbon atoms each), combined additives of chloronaphthalene (CN), to investigate their effects on packing properties, film formation dynamics, and device performance. Interestingly, the CN significantly impact the packing modes and ability of the donors, and ultimately the intermolecular interaction and the dynamics of film forming, making the device performance fluctuate wildly with the CN ratio. The DTBDT-C3-D6 molecule, with alkyl chains branching away from the donor core, with 1% CN in volume, forms an interpenetrating framework by the proper hetero/homo molecular interaction, promoting a PCE of 13.4%, significantly exceeding the 5.65% of the DTBDT-C1-D6 blend and also other CN volume ratios. This PCE is the highest reported for SMD/PA-type organic solar cells (OSCs). The findings highlight the importance of alkyl side chain branching and additives in modulating intermolecular interactions and film dynamics, offering insights into morphology control in OSCs.