Copolymers of Fluorene and Thiophene with Conjugated Side Chain for Polymer Solar Cells: Effect of Pendant Acceptors

Two copolymers of fluorene and thiophene with conjugated side-chain pending acceptor end group of cyanoacetate (P2) and malononitrile (P3) were synthesized. Both polymers exhibit good thermal stability and low highest occupied molecular orbital level (-5.5 eV). In comparison with P2, P3 exhibits stronger UV-vis absorption and higher hole mobility. Polymer solar cells based on P3:PC71BM exhibits a power conversion efficiency of 1.33% under AM 1.5, 100 mW/cm(2), which is three times of that based on P2:PC71BM. The higher efficiency is attributed to better absorption, higher hole mobility, and the reduced phase separation scale in P3:PC71BM blend. The aggregate domain size in P3:PC71BM blend is 50 nm, much smaller than that in P2:PC71BM blend (200 nm). Tiny difference in the end groups on side chains of P2 and P3 leads to great difference in phase separation scale, charge transport, and efficiency of their photovoltaic devices. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 14621470, 2011