Herein, three benzodithiophene (BDT)-based conjugated polymers, PBDTT-FBT, P3TBDT-FBT, and PTBOT-FBT, with gradually increased biaxially-extended side-chain conjugation are synthesized. The increase in the side-chain conjugated plane is revealed to enhance the π-π* transition and intramolecular charge transfer (ICT) of the polymer to result in improved absorption at near-ultraviolet region. Besides, the electron-withdrawing ability of the large side-chain conjugation effectively downshifts the energy levels of the derived polymer. Through providing complementary absorption spectra and suitable cascade energy levels, PTBOT-FBT with the largest side-chain conjugation is demonstrated as an effective third photoactive component for both fullerene and non-fullerene-acceptor (NFA) binary bulk-heterojunction (BHJ) systems, which enables a respective improvement of 2.5% and 11.8% in PCE of the studied fullerene- and NFA-based ternary solar cell devices. Moreover, it is found that the ternary NFA-based BHJ film exhibits a more favorable morphology after adding PTBOT-FBT as compared to the fullerene-based film, suggesting the higher tolerance of the polymers with bulky sidechains in the NFA system. Three benzoditiophene (BDT)-based conjugated polymers with gradually increased biaxially-extended side-chain conjugation are synthesized in this study. The increase in the side-chain conjugated plane is revealed to enhance the π-π* transition and intramolecular charge transfer (ICT) of the polymer to result in improved absorption at near-ultraviolet region. Besides, the electron-withdrawing ability of the large side-chain conjugation effectively downshifts the energy levels of the derived polymer. Display omitted •PTBOT-FBT with biaxially-extended side-chains was synthesized, possessing broad UV-vis absorption/deep-lying energy levels.•PTBOT-FBT serves an efficient third component for fullerene- and non-fullerene-acceptor (NFA) bulk-heterojunction systems.•It enables a respective improvement of 2.5% and 11.8% in efficiency for the studied fullerene- and NFA-based solar cells.•Out results suggest the rigid and amorphous polymers, like PTBOT-FBT, have a better compatibility with NFA than fullerene.