A Roundabout Approach to Control Morphological Orientation and Solar-Cell Performance by Modulating Side-Chain Branching Position in Benzodithiophene-Based Polymers

To be meaningful to guide the rational design of novel high‐performance conjugated semiconductors, we prepared three benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based polymers by systematically moving the branching point of the alkyl chain. The effect of side‐chain engineering was thoroughly investigated by a range of techniques. We demonstrate that a subtle change in the branching position in the BDT core can have a critical impact on polymer packing and preferential backbone orientation in thin films; copolymers made from BDT and thieno[3,4‐c]pyrrole‐4,6‐dione units (TPD) adopt more of a face‐on orientation as the branching point is shifted closer to the backbone, which can be correlated with a dramatic difference in solar‐cells performance. The high short‐circuit current density (11.6 mA cm−2) for the copolymer with one carbon atom between the alkoxylated oxygen atom and the branching point results from its predominantly face‐on orientation and smoother surface in thin films, which results in power conversion efficiencies as high as 4.56 %. Branching out: The authors systemically investigate the structure–property relationships that exist between the control of morphological orientation and macroscopic solar‐cell efficacy through engineering the side chains of the polymer, especially the branching point of the alkyl chain, of high‐performance conjugated semiconductors. PBDTTPD=poly(benzo[1,2‐b:4,5‐b′]dithiophene‐thieno[3,4‐c]pyrrole‐4,6‐dione).