Semi-Random Two-Acceptor Polymers: Elucidating Electronic Trends Through Multiple Acceptor Combinations

Four semi-random poly(3-hexylthiophene) (P3HT)-based donor/acceptor copolymers each containing two different acceptor monomers with a total acceptor content of 15% and a constant 1:1 acceptor ratio are synthesized (P3HTT-BTD-DPP, P3HTT-TP-DPP, P3HTT-TP-TPD, and P3HTT-BTD-TPD). All four polymers have considerably broadened absorption spectra compared with P3HT and the shape of the absorption profile and the bandgap of the two-acceptor copolymers can be predicted through a linear combination of absorption profiles of the respective one-acceptor semi-random copolymers. Important properties of P3HT, such as semicrystallinity, high hole mobility, and favorable mixing with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in solar cells at low fullerene ratios are retained for all of the two-acceptor copolymers. The highest occupied molecular orbital (HOMO) energies of two-acceptor copolymers are, in almost all cases, higher than the lowest measured value for the corresponding parent one-acceptor polymers and are in some cases higher than the values for both. The HOMO energies correlate in a generally linear fashion with the Voc measured in solar cells. Efficiencies of up to 3% emphasize the promise of this class of polymers.