Importance of 2D Conjugated Side Chains of Benzodithiophene-Based Polymers in Controlling Polymer Packing, Interfacial Ordering, and Composition Variations of All-Polymer Solar Cells

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 poly(benzo[1,2- b:4,5- b ̀]-dithiophene-thieno[3,4- c ]pyrrole-4,6-dione) (PBDTTPD) polymer 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.