Ting Yu

We explore a series of furan‐based non‐fullerene acceptors and report their optoelectronic properties, solid‐state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red‐shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels–Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non‐fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well‐balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan‐based acceptors reported to date and all indacenodithiophene‐based acceptors. Our findings show the possibility of photostable furan‐based alternatives to the currently omnipresent thiophene‐based photovoltaic materials.