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"Ternary blending" and "random terpolymerization" strategies have both proven effective for enhancing the performance of organic solar cells (OSCs). However, reports on the combination of the two strategies remain rare. Here, a terpolymer PM6-Si30 was constructed by inserting chlorine and alkylsilyl-substituted benzodithiophene (BDT) unit (0.3 equivalent) into the state-of-the-art polymer PM6. The terpolymer exhibitsadeep highest-occupied-molecular-orbital energy and good miscibility with both PM6 and BTP-eC9 (C9) and enables its use as a third component into PM6:PM6-Si30:C9 bulk-heterojunction for OSCs. The resulting cells exhibit maximum power conversion efficiency (PCE) of 18.27%, which is higher than that obtained for the optimized control binary PM6:C9-based OSC (17.38%). The enhanced performance of the PM6:PM6-Si30:C9 cells is attributed to improved charge transport, favorable molecular arrangement, reduced energy loss and suppressed bimolecular recombination. The work demonstrates the potential of random terpolymer as a third component in OSCs and highlights a new strategy for the construction of a ternary system with improved photovoltaic performance. A terpolymer PM6-Si30 was developed by inserting chlorine and alkylsilyl-substituted BDT unit (0.3 equivalent) into the state-of-the-art polymer PM6. The terpolymer shows deep highest-occupied-molecular-orbital and good compatibility with PM6 and C9. Thus, used as a third component in the PM6:PM6-Si30:BTP-eC9 ternary solar cells, leading to a high efficiency of 18.27%. [Display omitted] • A terpolymer PM6-Si30 with Cl and alkylsilyl side chains was synthesized. • The ternary PM6:PM6-Si30:C9 solar cells present enhanced efficiency of 18.27%. • Terpolymer has structural similarity to host polymer and thus good compatibility. • Terpolymer can be a promising third component for ternary OSCs. [ABSTRACT FROM AUTHOR]