Effects of brominated terminal groups on the performance of fused-ring electron acceptors in organic solar cells

Abstract We synthesize three fused-ring electron acceptors (m-ITBr, o-ITBr and IT-2Br) based on indacenodithieno[3,2-b]thiophene as the core unit and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile with 1 or 2 bromine substituents as the end-capped unit. The effects of bromine atom number and substituted position on molecular film absorption, energy level, photovoltaic performance, charge transport and film morphology are systematically carried out. Compared with ITIC, m-ITBr, o-ITBr and IT-2Br all show red-shifted film absorption (550–750 nm) and downshifted highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The PM6:IT-2Br based devices exhibit power conversion efficiency (PCE) of up to 12.92%, which is much higher than the reference devices based on ITIC (8.98%). In addition, all three brominated devices achieve high efficiency over 11% (11.84% for m-ITBr devices and 11.14% for o-ITBr devices). Herein, it is also concluded that bromination on terminal groups plays a vital role in enhancing the photovoltaic performance of IDTT-based FREAs.