Fluoroalkylated Non-fullerene Acceptor as Surface Segregated Monolayer for Controlling Molecular Orientation of Acceptor Layer in Organic Photovoltaics

A fluoroalkyl-containing electron acceptor (Y-SSM) is designed and synthesized to control the orientation of the benchmark non-fullerene acceptor Y6 in thin films. Due to the low surface energy of the two fluoroalkyl chains at the terminal part of Y-SSM, it spontaneously segregates to the film surface during spin coating, forming a monolayer of edge-on oriented Y-SSM. The Y-SSM monolayer leads to crystallization of the underlying Y6 to induce a standing-up orientation in the bulk of the films, which is strikingly different from pure Y6 films that tend to be a face-on orientation. Solid evidence for standing-up Y6 in the film is provided by two-dimensional grazing incidence wide-angle X-ray scattering and optical anisotropy measurements based on variable angle spectroscopic ellipsometry. The surface Y-SSM can be partially removed by washing with hexane without disrupting the orientation of Y6, resulting in exposure of the standing-up Y6 on the surface. Organic photovoltaics based on a planar heterojunction structure with standing-up Y6 show a significant increase in short-circuit current density, reaching 2.5 times the value compared to face-on oriented Y6, due to the improved charge generation efficiency resulting from the different relative orientation with respect to PM6.