A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells

of 0.62 eV. Furthermore, side-chain engineering demonstrated that subtle side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. With the benefits of the stronger interchain π-π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory-Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT-4F blend, a high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of the DTBDT-EF unit for future organic photovoltaic applications.