Modulation of bulk heterojunction morphology through small π-bridge changes for polymer solar cells with enhanced performance

Abstract

Investigations on the relationships among the chemical structures, morphology and photovoltaic properties of conjugated polymers are crucial in designing high-efficiency semiconducting polymers. Here, two novel copolymers, PIBTO-T and PIBTO-TT, were designed and synthesized to demonstrate the improvement in photovoltaic performance of conjugated polymers by a small change in their chemical structures. PIBTO-TT with thieno[3,2-b]thiophene π-bridges has a more linear backbone conformation, thereby resulting in enhanced intermolecular π–π interactions compared to PIBTO-T with thiophene π-bridges. Benefiting from the closer intermolecular π–π stacking, PIBTO-TT:PC₇₁BM exhibits a higher hole mobility than PIBTO-T:PC₇₁BM. Morphological studies reveal that the miscibility of PC₇₁BM in PIBTO-TT is better than that in PIBTO-T. This enhanced miscibility could shorten the distances between adjacent fullerenes and improve electron transportation in the miscible region. Meanwhile, PIBTO-TT:PC₇₁BM blends have larger donor/acceptor interfacial areas than PIBTO-T:PC₇₁BM samples. All these factors contribute to the better photovoltaic performance of PIBTO-TT-based devices. This study clearly shows that the morphological characteristics and photovoltaic properties of conjugated polymers are closely related to their molecular structures, and the manipulation of backbone conformation through π-bridge modulation is a promising molecular engineering approach to improve the photovoltaic properties of conjugated polymers.