We delineate the important role of 2D conjugated alkylthiophene side chains of polymers in manipulating the molecular orientation and ordering at the polymer donor/polymer acceptor (P D /P A ) interface as well as the composition variations in the blend active layer of all-polymer solar cells (all-PSCs). To systematically investigate the impact of 2D conjugated side chains on the performance of all-PSCs, we synthesized a series of polypolymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDT-TPD (P1, 100% alkoxy side chain), PBDTT 0.29 -TPD (P2), PBDTT 0.59 -TPD (P3), PBDTT 0.76 -TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1−P5 polymer donors produced similar PCEs of ∼6% in fullerene-based PSCs. In contrast, for the all-PSC systems, the changes in the side chain composition of the polymers induced a strong increasing trend in the power conversion efficiencies (PCEs), from 2.82% (P1), to 3.16% (P2), to 4.41% (P3), to 5.32% (P4), and to 6.60% (P5). The significant increase in the PCEs of the all-PSCs was attributed mainly to improvements in the short-circuit current density (J SC ) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the P D /P A interfaces and improved domain purity, which led to enhanced exciton dissociation and charge transport characteristics of the all-PSCs. Our observations highlight the advantage of incorporating 2D conjugated side chains into polymer donors for producing high-performance all-PSC systems.