Tuning Energy Levels of Low Bandgap Semi-Random Two Acceptor Copolymers

A series of low bandgap semi-random copolymers incorporating various ratios of two acceptor units-thienothiadiazole and benzothiadiazole-were synthesized by Pd-catalyzed Stille coupling. The polymer films exhibited broad and intense absorption spectra, covering the spectral range from 350 nm up to 1240 nm. The optical bandgaps and HOMO levels of the polymers were calculated from ultraviolet visible spectroscopy and cyclic voltammetry measurements, respectively. By changing the ratio of the two acceptor monomers, the HOMO levels of the polymers were tuned from -4.42 to -5.28 eV and the optical bandgaps were varied from 1.00 to 1.14 eV. The results indicate our approach could be applied to the design and preparation of conjugated polymers with specifically desired energy levels and bandgaps for photovoltaic applications.