Structural design of benzo[1,2- b:4,5- b ′]dithiophene-based 2D conjugated polymers with bithienyl and terthienyl substituents toward photovoltaic applications

In this contribution, six conjugated polymers consisting of benzo[1,2-b:4,5-b′]dithiophene-bithiophene (BDT-BT) and benzo[1,2-b:4,5-b′]dithiophene-benzothiadiazle (BDT-BTD) as building blocks in the main chain were synthesized by coupling polymerization and utilized for photovoltaic applications. By directly attaching three kinds of alkylthienyl side chains to the conjugated main chain, the resulted two-dimensional configuration revealed a broader absorption range due to the ground state electron transition of their corresponding alkylthienyl units and polymer backbone. Temperature-dependent absorbance, emission spectra, and thermal annealing further verify that the shoulder band(s) were originated from the aggregated (crystalline) species of polymers. The photovoltaic properties of the donor-acceptor polymers revealed well-defined side chain geometries, physical, and electronic structures and showed the highest power conversion efficiency of 4.25% among polymer solar cells based on two-dimensional (2-D) bithienyl- or terthienyl-substituted benzodithiophene.