Two-Dimensionally Extended π-Conjugation of Donor–Acceptor Copolymers via Oligothienyl Side Chains for Efficient Polymer Solar Cells

A series of two-dimensional conjugated polymers containing π-conjugated oligothienyl side chains, namely PBDT2FBT-T1, PBDT2FBT-T2, PBDT2FBT-T3, and PBDT2FBT-T4, was designed and synthesized to investigate the effect of two-dimensionally extended π-conjugation on the polymer solar cell (PSC) performance. The oligothienyl units introduced into the side chains significantly affect the optoelectronic properties of the parent polymers as well as the performances of the resulting solar cell devices by altering the molecular arrangement and packing, crystalline behavior, and microstructure of the polymer:PC71BM blend films. The crystallinity and blend morphology of the polymers can be systematically controlled by tuning the π-conjugation length of side chains; PBDT2FBT-T3 exhibited the most extended UV/vis light absorption band and the highest charge mobility, leading to a high short-circuit current density up to 12.5 mA cm–2 in the relevant PSCs. The PBDT2FBT-T3:PC71BM-based PSC exhibited the best power conversion efficiency of 6.48% among this series of polymers prepared without the use of processing additives or post-treatments. These results provide a new possibility and valuable insight into the development of efficient medium-bandgap polymers for use in organic solar cells.