Improving the performance of near infrared binary polymer solar cells by adding a second non-fullerene intermediate band-gap acceptor

The strategy of using ternary blends for the active layer becomes increasingly important in single-junction organic solar cells (OSCs), particularly for boosting the performance of high-performance non-fullerene-based devices. In this work, a non-fullerene small molecule named IT-4F with a moderate bandgap (1.58 eV) is introduced as the third component into the PBDB-T-2Cl:IXIC-4Cl binary blend, where IXIC-4Cl is a low-bandgap acceptor with absorption up to ∼1000 nm. Compared to the 11.99% and 13.47% efficiencies for the binary devices based on PBDB-T-2Cl:IXIC-4Cl and PDBD-T-2Cl:IT-4F, respectively, the ternary OSCs achieve a power conversion efficiency (PCE) of 14.96% when the weight ratio of PBDB-T-2Cl:IXIC-4Cl:IT-4F is 1.0 : 0.3 : 0.7. Through device and film characterization, IT-4F is shown to be an effective third component with functionalities including modulating the electronic properties, adjusting the frontier orbital energy levels, complementing the absorption spectrum and improving the active-layer morphology, which lead to significantly increased open-circuit voltage (VOC), short-circuit current (JSC) and fill factor (FF). Our method could be employed as a general approach to enhance the performance of nonfullerene OSCs based on low-bandgap small molecule acceptors.