Angular/linear-shaped indacenodithiophene (IDT) for donor-acceptor copolymers: Geometric shape effects on physical properties and photovoltaic performance

Angular-indacenodithiophene (a-IDT) as the analogue of linear-IDT (l-IDT), in which the central phenyl ring was linked on the β-position of the thiophene ring but fused on its α-position, was designed and synthesized through intramolecular annulation. This designed dibrominated a-IDT monomer was copolymerized with 4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) as the acceptor unit through Stille coupling reaction to furnish an alternating D-A conjugated polymer Pa-IDTDTBT. Similarly, l-IDT unit and its D-A conjugated polymer Pl-DTDTBT were also synthesized for comparison. The geometric shape of l-IDT and a-IDT subunits have tremendous influence on the conjugated backbone curvature for Pl-IDTDTBT and Pa-IDTDTBT, mainly reflects in changes observed in their photoelectric properties, hole mobilities, film morphology and photovoltaic performance. Theoretical calculation concludes that Pl-IDTDTBT possesses a more linear conjugated backbone and hence stronger intermolecular π–π interactions, leading to more red-shifted absorption spectra of Pl-IDTDTBT in the solution as well as the thin film relative to those of Pa-IDTDTBT. XRD measurements demonstrated that Pl-IDTDTBT has more ordered π-stacking than Pa-IDTDTBT counterpart as a result of the less curvature of the Pl-IDTDTBT conjugated backbone. BHJ PSCs device based on Pl-IDTDTBT/PC ₇₁ BM achieved a PCE of 5.34%, is higher than Pa-IDTDTBT (3.64%). As expected, Pa-IDTDBT with the more conjugated backbone curvature showed a high Voc of 1.02 V owing to its deeper HOMO energy level (−5.47 eV) relative to Pl-IDTDTBT, which is one of the highest Voc values of IDT-based polymers. Our work demonstrates that the geometric shape of subunits should be considered in designing the novel photovoltaic polymers and other organic optoelectronic materials.