Reduced Energy Loss Enabled by a Chlorinated Thiophene‐Fused Ending‐Group Small Molecular Acceptor for Efficient Nonfullerene Organic Solar Cells with 13.6% Efficiency

Abstract Generally, highly efficient organic solar cells require both a high open‐circuit voltage ( V OC ) and a high short‐circuit current density ( J SC ). Reducing the energy loss ( E loss ) is an effective way to achieve a high V OC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower E loss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with E loss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with E loss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with E loss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower E loss . These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.