Leveraging rhodamine-terminated small molecule acceptors for high open-circuit voltage and improved efficiency in organic solar cells

The asymmetric molecular design strategy has been proven to be an effective method to improve the photovoltaic performance of organic solar cells. In this work, to extend the pool of asymmetric non-fullerene acceptors, we introduced rhodamine as a terminal group into Y-series small molecule acceptors (SMAs) and synthesized three asymmetric SMAs (BTP-Rh2F, BTP-Rh2Cl, and BTP-Rh2Br) with different halogen substitutions. As a result, shallow lowest unoccupied molecular orbital energy levels of these SMAs contribute to high open-circuit voltage (V_OC) values exceeding 0.95 V in corresponding devices. However, insufficient exciton dissociation and inefficient charge carrier mobility limit the power conversion efficiencies (PCEs). Notably, the introduction of BTP-Rh2Cl as a third component into the PM6:BTP-eC9 binary host system effectively reduces non-radiative recombination, ultimately yielding an enhanced PCE of 18.03 % along with improved V_OC of 0.863 V. This work highlights the potential of rhodamine-modified SMAs in enhanced V_OC and high-efficiency ternary solar cells.