Comparative Study of the Optical and Electronic Properties of Y6 Derivatives: A Theoretical Study.

A series of Y-series nonfullerene acceptors (Y-NFAs) including symmetric acceptors (Y6 and TTY6) as well as asymmetric acceptors (KY6, TY6, and KTY6) have been constructed, and the electronic structure, electronic properties, and excited-state properties have been comparatively studied. The optoelectronic properties, interfacial charge-transfer (CT) mechanism, and interfacial CT rate for the solar cells composed of PM6 as the donor and Y6 derivatives as the acceptors are investigated further. We show that asymmetric Y6 derivatives have high molecular planarity, strong and wide absorption spectra, and large intramolecular charge transfer (ICT). For the solar cells, the complexes of Y6 derivatives show increased open-circuit voltage, larger fill factor, and smaller energy loss compared to Y6. In addition, the complexes of Y6 derivatives have more charge-transfer states than Y6 in the low-energy region, such that there are multiple ways for CT generations, such as hot excitation, intermolecular electric field (IEF), and direct excitation. The detailed CT mechanism as well as interfacial CT rate depends on the type of complexes, and all Y6 derivatives have a similar magnitude of charge-transfer rate to the one of Y6. This work not only reveals the differences in performance between symmetric and asymmetric NFA but also reveals that proper terminal tuning is an effective way to improve photovoltaic properties.