Thiophene-benzothiadiazole based D-A₁-D-A₂ type alternating copolymers for polymer solar cells

A series of D-A₁-D-A₂ type regioregular copolymers based on difluorobenzothiadiazole (DFBT) and dialkoxybenzothiadiazole (ROBT) structures was synthesized. The copolymers were prepared with different alkyl chain lengths and branching point positions. The weaker acceptor, ROBT, was incorporated as a solubilizing moiety, while the stronger acceptor, DFBT, was used to enhance intramolecular charge transfer interactions with low-lying frontier orbital levels. The design of DFBT-ROBT copolymers ensures good planarity via intrachain noncovalent F⋯S, S⋯O, and F⋯H coulombic interactions. Changing the alkyl chain branching point and length had pronounced effects on the interchain packing and charge carrier transport/recombination characteristics of the resultant polymers, which in turn influenced their photovoltaic performances. P2 (with 3-hexylundecyloxy) showed tight π-π stacking, high charge mobility, reduced bimolecular charge recombination, and an optimal nanoscale morphology compared to P1 (with 2-hexyldecyloxy). We prepared photovoltaic devices containing a blend of the copolymers with [6,6]-phenyl-C₇₁-butyric acid methyl ester, and the resultant devices showed high power conversion efficiencies, 8.27% for P2, which is higher than that of P1 (6.87%). Furthermore, the alkyl side-chain length in P2 was varied systematically to study the correlation between the alkyl chain length in the interchain packing and photovoltaic performances. The variation in the alkyl chain branching was effective to modulate intermolecular packing to improve the photovoltaic performances. The optimum side chain length should be determined by carefully considering the solubility and interchain packing interactions.