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

Abstract

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] pyrrol e-4,6-dione) (PBDTTPD) polymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDTTPD (P1, 100% alkoxy side chain), PBDTT0.29-TPD (P2), PBDTT039-TPD (P3), PBDTT0.76-TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1 P5 polymer donors produced similar PCEs of similar to 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 (Jsc) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the PD/PA 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.