20.5 % efficient ternary organic photovoltaics using an asymmetric small-molecular acceptor to manipulate intermolecular packing and reduce energy losses

Oligomeric acceptors are increasingly recognized as promising n-type materials for organic photovoltaics (OPVs) due to their precise molecular structures, long-term stability, and high efficiency. However, inferior molecular packing and high energy losses have hindered their further use. Here, we overcome these challenges by developing an asymmetric small molecular acceptor (SMA), BTP-J17, and applying it as the second acceptor component in OPVs composed of PM6:DIBP3F-Se:BTP-J17 (refer to our recent work on dimeric acceptor DIBP3F-Se). The BTP-J17 is very miscible with the DIBP3F-Se and appears to diffuse into the host donor-acceptor interface. The ensuing ternary cells exhibit enhanced exciton dissociation, improved carrier mobility, and more efficient charge extraction. Optimised OPVs based on PM6:DIBP3F-Se:BTP-J17 show enhanced open-circuit voltage (V_OC) while maintaining the high short-circuit current (J_SC) from the binary blends, boosting the power conversion efficiency (PCE) from 18.40 % to 19.60 %. By integrating MgF₂ as an antireflection coating and n-doping the ternary BHJ with ethyl viologen (EV), we were able to further boost the PCE to 20.5 % (uncertified) and simultaneously extended the outdoor stability to seven weeks. Our findings highlight the crucial role of asymmetric SMA as an additional component for boosting the performance and stability of OPVs.