Importance of Optimal Crystallinity and Hole Mobility of BDT‐Based Polymer Donor for Simultaneous Enhancements of Voc, Jsc, and FF in Efficient Nonfullerene Organic Solar Cells

In this work, a new series of polymer donors consisting of thienothiophene π‐bridged N ‐octylthieno[3,4‐ c ]pyrrole‐4,6‐dione (8ttTPD) and benzo[1,2‐ b :4,5‐ b ']dithiophene (BDT) units for producing highly efficient organic solar cells (OSCs) paired with a Y6 acceptor is developed. The incorporation of the highly planar 8ttTPD unit enhances crystalline properties as well as hole mobilities of the BDT‐based polymers that typically have amorphous features. Further, the 2D side chains with halogen atoms (fluorine and chlorine) are designed as another handle to control the crystallinity and energy levels of the BDT‐based polymer donors: PBDT‐X (X = H, F, or Cl). Synergistic effects of incorporated 8ttTPD unit and the halogenated 2D side chain generate significantly enhanced charge transport and recombination properties of the OSCs, which is mainly attributed to optimized crystallinity and hole mobility of the polymer donors. Therefore, the PBDT‐Cl:Y6‐based OSCs exhibit the highest power conversion efficiency (PCE) of 15.63% with simultaneous improvements of open‐circuit voltage, short‐circuit current density, and fill factor, which outperforms the PCEs of PBDT‐H:Y6 (11.84%) and PBDT‐F:Y6 (14.86%).