How Does Polymorphism Affect the Interfacial Charge-Transfer States in Organic Photovoltaics?

The bulk heterojunction in organic photovoltaic (OPV) devices is a mixture of polymer (electron donor) and an electron acceptor material (typically functionalized fullerenes), and it is crucial for the device operation, as this is where excitons are split into electrons and holes to produce current. Nonfullerene acceptors (NFAs) are promising new materials for improving the device efficiency, and their solid-state arrangement with respect to the electron donor polymer is critical for the charge mobility and the performance of OPV devices. Although there have been numerous studies on NFAs, most of the current understanding comes from empirical considerations, with little atomistic-level interpretation of why and how the packing influences the charge-transport properties of these materials. In this work, we describe large-scale (with up to 3462 atoms) density functional theory simulations for ground and excited states on a number of polymer–NFA interfaces of realistic size, whose NFA domains consist of polymorphs of the same materials. Hence, we bridged the gap between experimental evidence and the intuitive expectation on the importance of intermolecular π–π stacking interactions in the NFA phase. We show that low connectivity leads to highly localized excitons, whereas in phases with a higher connectivity, excitons are able to delocalize over multiple directions. Remarkably, excitons with a three-dimensional delocalization were also observed, leading to isotropic mobilities, similar to fullerenes. Furthermore, a lower charge-transfer exciton binding energy and lower energy loss between the lowest excitation of the polymer and first charge-transfer state in the interface were both observed in systems characterized by a highly interconnected NFA phase. This suggests a higher probability of exciton splitting for these interfaces, which could potentially lead to higher device efficiencies.