Structural engineering of benzothiadiazole-based photovoltaic materials via 2D conjugation extension and 3D configuration control

Three novel small-molecule donors (SMDs), namely PTZBT, PTZBBT and PTZSSBT, were designed and synthesized. Initially, a D-A-D type small molecule, PTZBT, was constructed using phenothiazine (PTZ) unit and benzothiadiazole (BT) unit. To overcome the challenges posed by the high crystallinity and poor solubility of the BT structure, a 3D molecule, PTZBBT, was further developed by incorporating a 5,5'-biphenylthiadiazole (BBT) unit. It was demonstrated that the introduction of the BBT significantly enhanced the performance of photovoltaic devices. Subsequently, a D-π-A-π-D type 2D planar molecule, PTZSSBT, was synthesized by introducing bithiophene as the π-bridge. This extended conjugated system not only enhances the planarity of the molecule but also broadens the absorption spectrum.Therefore, the devices based onPTZSSBT material device achieved excellent photovoltaic performance with a PCE of 4.38% and a JSC of 13.51 mA cm⁻².This study highlights that the dual strategy of optimizing 3D configurations and extending 2D conjugation can synergistically enhance the performance of photovoltaic materials.