Over 19% Efficiency in Ternary Organic Solar Cells Enabled by n-Type Dopants

Molecular doping has become a valuable technique for enhancing the efficiency of high-performance organic photovoltaic systems (OPVs). However, the number of known dopant molecules, especially n-type ones, that enhance the PCE of OPVs remains limited. In this study, two n-type dopants, ethyl viologen (EV) and methyl viologen (MV), are synthesized and incorporated into ternary PM6:BTP-eC9:PC71BM bulk heterojunction (BHJ) OPVs. Both dopants are found to enhance the OPV performance, yielding maximum PCE values of 19.03% and 18.61%, respectively. We show that EV and MV function as n-type dopants and microstructure modifiers, enhancing π–π stacking while increasing the absorption coefficient of the BHJs. Moreover, the n-doping balances the carrier mobility while increasing the carrier lifetime and reducing bimolecular recombination. Our results demonstrate the potential of EV and MV to improve the performance of highly efficient OPVs to levels beyond those achievable by the pristine BHJ.