Effect of mono alkoxy-carboxylate-functionalized benzothiadiazole-based donor polymers for non-fullerene solar cells

Structural modification of benzo[c]-1,2,5-thiadiazole (BT) has been proved to be the prominent way to fine-tune the frontier energy levels and the intermolecular and intramolecular interactions in organic conjugated materials. In this study, a new acceptor unit, alkyl benzo[c][1,2,5]thiadiazole-5-carboxylate (BT-Est), was designed and synthesized by drafting mono alkoxy-carboxylate substituent on 5-position of BT core. Its compatibility in the conjugated system was investigated by co-polymerizing BT-Est with well-known benzo[1,2-b:4,5-b']dithiophene monomers containing either 2-(2-ethylhexyl)thienyl or 2((2-ethylhexyl)thio)thienyl side chains to form two new polymers, P1 and P2, respectively. The BT-Est yielded polymers with good solubility, medium bandgap (similar to 1.71 eV), and deep highest occupied molecular orbital energy levels (-5.48 to -5.54 eV). Among the polymers, P1 exhibited broader absorption, compact molecular packing, high charge carrier mobility, and effective exciton dissociation, despite of the torsion angle caused by the free rotation of the carboxylate group in the polymer backbone. Consequently, the best power-conversion efficiency of 6.9%, with a J(SC) of 14.6 mA cm(-2), V-OC of 0.9 V, and FF of 52.5% were obtained for P1-based devices with the well-known non-fullerene acceptor ITIC. We systematically expounded the structure-property relationship of the BT-Est polymers using diverse characterization methods. Our results demonstrated that the mono carboxylate-substitution on the BT core can be used as the alternate strategy to modulate the optoelectronic properties and control the aggregation in the conjugated polymers. Thus, BT-Est has the potential to produce new donor-acceptor conjugated polymers and small molecules for application in organic electronics.