Narrow-bandgap acceptors with low energetic disorder achieve over 21% efficiency in organic solar cells

Narrow-bandgap acceptors are the basis for achieving high short-circuit current density in organic solar cells; however, the lack of effective strategies to reduce energy loss under narrow-bandgap systems makes it challenging to solve the trade-off of open-circuit voltage and short-circuit current density. Here an acceptor Qx-Se-NF, featuring quinoxaline (Qx) central moiety, naphthyl-based terminal group (NF), and selenium (Se)-substituted central core, is synthesized, reaching a narrow bandgap of 1.31 eV. Theoretical calculations show that Qx-Se-NF exhibits low energetic disorder, which is beneficial for reducing energy loss. Furthermore, its strong aggregation properties tend to form a unique vertically segregated alloy structure in ternary systems, which is beneficial for increasing the short-circuit current density without sacrificing the open-circuit voltage. As a result, the ternary system achieved a certified power conversion efficiency of 21.01% with a low energy loss of 0.486 eV, providing a deep insight into the design of narrow-bandgap acceptors and their ternary systems.