Random and block conjugated polymers: a comparative study of properties

The synthesis and comparative study of four conjugated terpolymers with different arrangements for application in polymer solar cells (PSC’s) as electron donor materials is reported. The synthesized polymers were characterized by NMR, GPC, UV–Vis, TGA, FTIR, Cyclic Voltammetry (CV) and AFM. The photovoltaic devices were characterized by comparison between J(V) curves in dark and standard illumination conditions (100 µW/cm²). All materials synthesized showed good solubility and thermal stability, indicating that they are suitable for PSC's. Terpolymers (random and block) were synthesized by the Suzuki–Miyaura route varying proportion of carbazole and fluorene monomers, showing high yields. The optical and electronic characterizations showed that increasing carbazole amount decreases the polymer energy band gap, a strategic issue in order to improve photovoltaic device efficiency. The block and the random terpolymer presented similar maximum absorbance λmax, HOMO, LUMO, and band gap. Therefore, the light absorption is more influenced by composition than by differences in microstructure between random and block arrangements. Additionally, the low roughness obtained from AFM indicated a good mixture between polymers and fullerene derivative PCBM, also a key factor in the PSCs efficiency. However, the device efficiency presented not only a strong dependence on the fluorene/carbazole ratio but also on polymer structure.