Effect of branched conjugation structure on the optical, Electrochemical, hole mobility, and photovoltaic properties of polythiophenes

Four branched polythiophenes (PTs) with different ratios of conjugated terthiophene-vinylene side chains, PT-TThV10 to PT-TThV40, were synthesized by Stille coupling reaction. The polymers exhibited reversible p-doping/dedoping (oxidation/re-reduction) and n-doping/dedoping (reduction/reoxidation) processes. The absorption spectra, hole mobility, and photovoltaic properties of the polymers were much improved in comparison with the PT derivative without the terthiophene-vinylene side chain and were influenced by the content of the conjugated side chains. With the increase of the content of the conjugated side chains, the absorption peak of the branched PTs enhanced and blue-shifted. The maximum hole mobility reached 6.35 × 10^-4 cm ²/V·s (SCLC method) and the maximum power conversion efficiency of the polymer solar cell reached 1.91% under the illumination of AM 1.5, 100 mW/cm², for the polymer with 20% terthiophene-vinylene side chains. The results indicate that the branched PTs with suitable content of the terthiophene-vinylene side chains could be promising photovoltaic materials.