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

Abstract

The photovoltaic performance of polymer solar cells (PSCs) depends largely on the ordering structures (i.e., backbone orientation and crystallinity) of the active-layer molecules. It is crucial to make the molecules adopt a face-on orientation for efficient vertical carrier transport. However, the driving force that directs face-on or edge-on orientation is not yet clear, and the control of molecular orientation by side-chain engineering remains challenging. Here, we develop two ADA-type nonfullerene acceptors (M3 and M32) by using an electron-rich core that is free of sp3 carbon-bonded side chains. Through tailoring the bulkiness of the neighboring side chains, molecular orientation of the acceptors can be altered from edge-on (M32) to face-on (M3), thereby leading to an improved vertical charge transport and enhanced efficiency. The best performing PSC based on M3 shows an outstanding efficiency of 16.66%, which is the highest among all ADA-type nonfullerene acceptors.