Side-chain engineering of C-shaped ortho-benzodipyrrole-based A–D–A acceptors for energy loss suppression in organic photovoltaics

Abstract

The elimination of the electron-deficient thiadiazole unit from Y-series materials has led to the development of a new class of ortho-benzodipyrrole-based A–D–A-type NFAs. In this study, we designed and synthesized two CB-BO and CB-HD acceptors substituted with 2-butyloctyl and 2-hexyldecyl alkyl chains at the β-position of the thiophene units, respectively. The side-chain engineering significantly influences molecular packing and thin-film morphology. Compared to the linear side-chain counterpart, CB-BO and CB-HD with branched side chains exhibit blue-shifted absorption and enhanced intermolecular interactions. Single-crystal structure analysis revealed that CB-BO forms a more compact 3D kaleidoscope-like packing network, whereas CB-HD shows looser intermolecular packing due to its larger steric hindrance. Both PM6:CB-BO and PM6:CB-HD devices exhibited impressive V_oc values of 0.95 V and 0.96 V, respectively, attributed to their larger energy bandgap and suppressed non-radiative energy loss. Y6-16, with a relatively red-shifted absorption, was incorporated as a secondary acceptor. The ternary PM6:Y6-16:CB-BO device achieved a PCE of 17.34%, with an increased V_oc of 0.90 V and J_sc of 26.00 mA cm⁻², representing one of the highest performance values in inverted OPVs.