The role of unfused-ring-based small-molecule acceptors as the third component in ternary organic photovoltaics

Abstract

The intricate blend morphology of ternary systems and its correlation with organic photovoltaic (OPV) performance remain incompletely understood. Here, three unfused-ring small-molecule acceptors were utilized as guest acceptors of the benchmark active layer PM6:Y6 to probe nanophase separation and energy loss phenomena. Those acceptors feature a napththobistrazole (NTz) bridging two cyclopenta[2,1-b,3,4-b′]dithiophene (CPDT) groups as the central unit which is equipping 2-(3-oxo-2,3dihydro-1H-inden-1-ylidene)malononitrile (ID), 2-(5,6-difluoro3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2F), and malononitrile (CN) groups as terminal groups respectively. The blend morphology of ternary active layers along with their optoelectronic properties were fully analyzed. Grazing-incidence wide-angle X-ray scattering (GIWAXS) unveiled that the end group of the guest acceptor influences nanophase separation within the ternary blend, affecting energy losses, particularly non-radiative recombination in the corresponding OPV. Among them, NTzCPDT2F, sharing the same end group as Y6, exhibited the highest open-circuit voltage (V_OC) and power conversion efficiency. This can be attributed to favorable end-to-end molecular packing between NTzCPDT2F and Y6, promoting nanophase separation. Our demonstration offers a deeper understanding of how blend nanophase separation governs the energy loss and V_OC of OPVs, providing a new design principle for acceptors.