Synthesis, characterization and photovoltaic properties of dithienobenzodithiophene-based conjugated polymers

Two new donor-acceptor typed conjugated polymers (P1 and P2) based on the large coplanar and more-extended donor dithieno[2,3-d:2,3-d′]benzo[1,2-b:4,5-b′]dithiphene and thienopyrroledione (TPD) units were synthesized. P1 and P2 have the same polymer backbone, but with different side chains, where P1 has 2-ethylhexyl side chains and P2 has the longer branched 4-ethyoctyl side chains. The solubility measurements show that P2 with longer branched 4-ethyloctyl side chain has better solubility in organic solvents than that of P1. The optical, electrochemical and photovoltaic properties were investigated. P1 and P2 show the very same absorption features in both solution and film states indicating an enhanced molecular planarity of the polymer chains in P1 and P2, which benefits from the large coplanar dithieno[2,3-d:2,3-d′]benzo[1,2-b:4,5-b′]dithiphene unit. The band-gaps of P1 and P2 are at 1.83 eV and 1.74 eV, respectively. The solar cells with P1 or P2 as the electron donor component and PC71BM as the electron acceptor were fabricated and measured under AM 1.5G illumination at 100 mW cm−2. The results show that the P2/PC71BM-based device showed a moderate power conversion efficiency of 2.86%, which is ∼50% improvement in comparison with P1/PC71BM-based device.