Modulating the packing and photovoltaic performance of (bisthiophene)benzene-linked polymer acceptors through simple methylation engineering†

Three polymerized small molecule acceptors (PSMAs), namely PY-TP, PY-TPMe2, and PY-TPMe4, were designed and synthesized by employing (bisthiophene)benzene linkers containing various methyl-substituted phenylene groups. All the PSMAs exhibit similar absorption maxima in films as well as LUMO energy levels. However, the increased number of methyl groups on the linkers induces steric hindrance and decreases the coplanarity of the polymer backbones, which, in turn, increases intermolecular π–π stacking distances. When the three acceptors are blended with a classical polymer donor PM6, PY-TPMe4 with a highly twisted backbone has a large π–π stacking distance and excessive phase separation, whereas PY-TP and PY-TPMe2 with moderately twisted backbones demonstrate more compact π–π stacking and suitable phase separation morphology. As a result, PY-TP and PY-TPMe2 exhibit better exciton dissociation and charge transport, leading to much higher photovoltaic performance compared to PY-TPMe4. Particularly, PY-TPMe2 effectively regulates the crystallinity and achieves a more suitable phase separation morphology in the blend films. The optimal PY-TPMe2-based photovoltaic device exhibits the best exciton dissociation and charge transport performance, achieving the highest power conversion efficiency (PCE) of 8.4% among the devices based on the three PSMAs, with a high open-circuit voltage (V_OC) of 0.97 V, a short-circuit current density (J_SC) of 14.74 mA cm⁻² and a fill factor (FF) of 59.65%. These findings provide new insights into the regulation of the molecular packing and photovoltaic performance of polymer acceptors through simple methylation modification on linkers for designing novel PSMA materials.