Conjugated side-chains engineering of polymer donor enabling improved efficiency for polymer solar cells
Generally, molecular optimization is widely used to fine-tune the absorption features and energy levels of photovoltaic materials to improve their photovoltaic performance for polymer solar cells (PSCs). In this work, we demonstrate an example that the morphological properties can be effectively optimized by conjugated side-chains engineering on benzo[1,2-b:4,5-b']dithiophene (BDT) unit. The polymer donors PBNT-S with alkylthio-thienyl substitution and PBNP-S with alkylthio-phenyl substitution have identical absorption spectra and energy levels, while exhibit significantly different morphological properties when blended with nonfullerene acceptor Y6. The PBNT-S:Y6 blend shows obviously over crystallinity with excessive domain sizes, while the PBNP-S:Y6 blend realizes better nanoscale phase separation. As a result, a notable power conversion efficiency (PCE) of 14.31% with a high fill factor (FF) of 0.694 is achieved in the PBNP-S:Y6-based device, while the PBNT-S:Y6-based device yields a moderate PCE of 11.10% and a relatively low FF of 0.605. Additionally, PBNP-S shows promising potential in semitransparent PSCs, that the PBNP-S:Y6-based semitransparent PSC achieves an outstanding PCE of 11.86%, with an average visible transmittance of 26.4%. The results demonstrate a feasible strategy to manipulate the morphological properties of blend film via rational molecular optimization to improve the photovoltaic performance.