Triplet State Suppression for Energy Loss Reduction in 20% Nonhalogenated Solvent Processed Binary Organic Solar Cells

Abstract Boosting power conversion efficiency (PCE) of organic solar cells (OSCs) has been restricted by its undesirably high energy loss, especially for those nonhalogenated solvent‐processed ones. Here,a dichloro‐methoxylated terminal group in an asymmetric small molecular acceptor design, which realizes a significantly reduced non‐radiative energy loss (0.179 eV) compared to its symmetric counterpart (0.202 eV), is reported. Consequently, the device efficiency is improved by up to 20% for PM6:BTP‐eC9‐4ClO, without sacrificing the photon harvest or charge transport ability of the control system PM6:BTP‐eC9. Further characterizations reveal the asymmetric acceptor BTP‐eC9‐4ClO's blend film demonstrates a suppressed triplet state formation, enabled by an enhanced electron delocalization. In addition, the asymmetric BTP‐eC9‐4ClO is found to be thermally stabler than BTP‐eC9, and thus providing an improved device stability, whose T80 value reaches > 7800 h under 80 °C anneal in N 2 via linear extrapolation. This work represents state‐of‐the‐art device performance for nonhalogenated solvent‐processed binary OSCs with certified results (19.45%).