Effects of the number of methoxy groups in the thiophene π-bridges on the photovoltaic performance of the A–π–A type quasi-macromolecular acceptors†

Abstract

A–π–A type quasi-macromolecular (QM) acceptors have garnered significant research attention owing to their well-defined structural characteristics and excellent long-term stability. The π-bridge, serving as the core structural motif, plays a critical role in determining both the optoelectronic properties of the acceptor and the overall performance of the device. This work proposes a strategy of introducing methoxy groups into thiophene π-bridges to improve molecular planarity through non-covalent interactions, while leveraging its electron-donating capacity to enhance the open-circuit voltage (V_OC) and minimize voltage losses in devices, ultimately achieving excellent photovoltaic performance. Three A–π–A type QM acceptors, QM-T, QM-OT and QM-DOT were constructed by varying the number of methoxy groups in the thiophene π-bridge. Theoretical calculations and experiments revealed distinct geometry and structural features among the three molecules. Notably, QM-OT, featuring a 3-methoxythiophene π-bridge, showed optimal molecular planarity, which facilitated favorable π–π stacking and demonstrated superior compatibility with the polymer donor PM6. Therefore, the PM6:QM-OT-based active layer formed a nanoscale morphology with optimal phase separation, effectively mitigating voltage and charge recombination losses while promoting efficient exciton dissociation and charge transport. As a result, the PM6:QM-OT-based device achieved an impressive power conversion efficiency (PCE) of 18.15% with the highest V_OC, short-circuit current density (J_SC) and fill factor (FF). This work provides some guidance for further improving the performance of organic solar cells.