As an exitonic photovoltaic device, organic solar cells (OSCs) consist of electron donating and accepting components in their photoactive layer, in which the molecular interactions between donor and acceptor can significantly affect the nanoscale morphology as well as the photovoltaic performance of OSCs. In this work, by diluting electron donor with electron acceptor having opposite electrostatic potentials to promote the structural order via strengthened intermolecular interactions, this study shows that polymeric diluent is more effective due to its long-ranged conjugated backbone compared with small molecular diluent. The ternary heterojunction made of C5-16:L8-BO binary acceptors diluted with D18 shows the strongest structural order, benefiting from the strong interactions between L8-BO and C5-16. The enhanced structural order within the photoactive layer prepared by layer-by-layer deposition of the diluted p-type and n-type heterojunctions contributes to enhanced light absorption, improved charge transport, and inhibited charge recombination. As the result, OSC based on D18 (PY-IT diluted)/L8-BO:C5-16 (D18 diluted) having donor and acceptor dual fibrils obtains an unprecedented power conversion efficiency of 21.0% (certified value of 20.25%), which is one of the highest certified PCE up to date.