Conjugated side-chain engineering of polymer donors 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-chain engineering on the benzo[1,2-b:4,5-b ']dithiophene (BDT) unit. The polymer donorsPBNT-Swith an alkylthio-thienyl substituent andPBNP-Swith an alkylthio-phenyl substituent have identical absorption spectra and energy levels, while exhibiting significantly different morphological properties when blended with nonfullerene acceptorY6. ThePBNT-S:Y6blend shows obviously excessive crystallinity with excessive domain sizes, while thePBNP-S:Y6blend 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 thePBNP-S:Y6-based device, while thePBNT-S:Y6-based device yields a moderate PCE of 11.10% and a relatively low FF of 0.605. Additionally,PBNP-Sshows great potential in semitransparent PSCs, and thePBNP-S:Y6-based semitransparent PSC achieves an outstanding PCE of 11.86%, with an average visible transmittance of 24.3%. The results demonstrate a feasible strategy to manipulate the morphological properties of blend filmsviarational molecular optimization to improve the photovoltaic performance.