Understanding the Effect of End Group Halogenation in Tuning Miscibility and Morphology of High-Performance Small Molecular Acceptors

A multitude of studies have been conducted on organic solar cells (OSCs) to understand how end group halogenation changes the property of the small molecular acceptors (SMAs) energetically, morphologically, and so on. But how the halogenation impacts the miscibility between SMAs and polymers, which is an important index to thermodynamically predict and understand the morphology of blend films, has not been systematically studied, particularly for the state-of-the-art polymer-SMA blends. Herein, three series of asymmetric or symmetric SMAs, all reported recently with high photovoltaic performances, are used to investigate the effect of halogenation on miscibility, crystallinity, and solar cell performance. Using the asymmetric SMA named TPIC, and its derivatives (TPIC-4F and TPIC-4Cl) as the focus, it is revealed that the enhancement in solar cell performance for the halogenated SMAs is to reduce the miscibility between the acceptor and donor, which leads to a more favorable morphology, enhanced charge transport, and an extended absorption range. Similarly, the halogenation-induced reduction in miscibility for the initially overmixed donor:acceptor blend is also demonstrated in the symmetric ITIC and the state-of-the-art BTP series, where attractive power conversion efficiencies (PCEs) of 16.5% and 16.8%, respectively, are achieved by the halogenated-SMA-based devices in each series.