Chlorinated Narrow Bandgap Polymer Suppresses Non‐Radiative Recombination Energy Loss Enabling Perylene Diimides‐Based Organic Solar Cells Exceeding 10% Efficiency

Abstract The scarcity of narrow bandgap donor polymers matched with perylene diimides (PDI)‐based nonfullerene acceptors (NFAs) hinders improvement of the power conversion efficiency (PCE) value of organic solar cells (OSCs). Here, it is reported that a narrow bandgap donor polymer PDX, the chlorinated derivative of the famous polymer donor PTB7‐Th, blended with PDI‐based NFA boosts the PCE value exceeding 10%. The electroluminescent quantum efficiency of PDX‐based OSCs is two orders of magnitude higher than that of PTB7‐Th‐based OSCs;therefore, the nonradiative energy loss is 0.103 eV lower. This is the highest PCE value for OSCs with the lowest energy loss using the blend of PTB7‐Th derivatives and PDI‐based NFAs as the active layer. Besides, PDX‐based devices showed larger phase separation, faster charge mobilities, higher exciton dissociation probability, suppressed charge recombination, elevated charge transfer state, and decreased energetic disorder compared with the PTB7‐Th‐based OSCs. All these factors contribute to the simultaneously improved short circuit current density, open circuit voltage, and fill factor, thus significantly improving PCE. These results prove that chlorinated conjugated side thienyl groups can efficiently suppress the non‐radiative energy loss and highlight the importance of fine‐modifying or developing novel narrow bandgap polymers to further elevate the PCE value of PDI‐based OSCs.