Synthesis and absorption spectra of poly(3-(phenylenevinyl)thiophene)s with conjugated side chains

Six soluble polythiophene derivatives with phenylene-vinylene conjugated side chains were synthesized by GRIM or Stille methods. The absorption spectra of the polymers show two peaks located in the UV and visible region, where the visible absorption peak is attributed to the pi-pi* transition of the conjugated main chains and the UV absorption peak results from the conjugated side chains. By doubling the conjugation length of the phenylene-vinylene side chains and reducing the concentration of the conjugated side chains, we synthesized PT4, which possesses a strong and broad absorption in the visible region from 380 to 650 nm and is promising for the application in polymer solar cells. For the polymers of PEHPVT and PMEHPVT where every thiophene unit is attached with a conjugated side chain, the absorption spectra of their films red-shifted by 30 and 50 nm, respectively, in comparison with their solution. After thermal annealing at 130 degrees C for 10 min, the absorption of the films red-shifted further, and the band gap of the polymers was reduced. XRD results reveal that the ordering structure of the two polymers was enhanced and their interlayer d-spacing expanded after the annealing. Cyclic voltammograms display that the p-doping/dedoping and n-doping/dedoping processes of all the six polymers are reversible. The HOMO and LUMO energy levels and energy gap of the polymers were calculated from the electrochemical measurements.