Efficient Organic Solar Cells from Molecular Orientation Control of M-Series Acceptors

Summary

A "face-on" molecular orientation is essential for photovoltaic materials with efficient vertical carrier transport, but understanding how the molecular structures control their orientations remains challenging. Based on a ladder-type fused-ring core without sp³-hybridized bridging atoms, novel acceptor-donor-acceptor (A-D-A or ADA)-type nonfullerene acceptors (M3 and M32) are developed, and their molecular orientation behaviors are successfully controlled by the neighboring side chains. With linear n-cetyl chains on the nitrogen atoms, the acceptor (M32) tends to adopt an "edge-on" molecular orientation, whereas with bulky branched 2-hexyldecyl chains, the acceptor (M3) has a preferred "face-on" orientation. Blended with a donor polymer of PM6, M3 shows optimal phase separation and dramatically improved electron transport, consequently leading to a much higher device performance than M32. Further optimization of the M3-based devices yields an outstanding efficiency of 16.66%. The strategy of molecular orientation control proposed here will inspire many other innovative designs and syntheses of high-performance nonfullerene acceptors.