Morphological Control of Y6 Thin Films Reveals Charge Transfer Is Facilitated by Co-facial Interactions.

The organic semiconductor Y6 has been extensively used as an acceptor in organic photovoltaic devices, yielding high efficiencies. Its unique properties include a high refractive index, intrinsic exciton dissociation, and barrierless charge generation in bulk heterojunctions. However, the direct impact of the crystal packing morphology on the photophysics of Y6 has remained elusive, hindering further development of heterojunction and homojunction devices. Herein, we study the photogenerated species in multiple distinct Y6 crystal packing geometries via transient absorption spectroscopy and photovoltaic measurements of the corresponding single-component devices. Our results reveal that "co-facial" interactions drive the generation of charge-transfer states in neat films of Y6 and that exciton dissociation can be switched on and off by controlling these interactions. Additionally, we find that a combination of long-range order and more co-facial packing interactions accelerates the charge-transfer generation process and increases the exciton to charge-transfer conversion efficiency. These insights provide valuable structure-property relationships for optimizing device performance.