Stabilizing the resonance structure of nonfused-ring electron acceptors a closed-loop carbazole side chain for efficient and stable organic solar cells

For commercially applicable organic solar cells (OSCs), both high efficiency and stability are essential requirements. A long-existing hidden problem of electron acceptors is the carbon-carbon double bond, which favors efficiency but harms stability. To explore a potential solution, we herein designed two nonfused-ring electron acceptors (NFREAs) with the same molecular backbone but varying side chains, a closed-loop carbazole side chain for 3TTCz and an open-loop diphenylamine side chain for 3TTDPA . It was unveiled that photodegradation proceeded via the oxidation of the carbon-carbon double bond through the quinoid resonance structure, which would be promoted by the open-loop diphenylamine unit, but hindered by the closed-loop carbazole unit. As a result, device stability was significantly improved for the 3TTCz -based OSC. In addition, the carbazole unit also allowed higher crystallinity, better molecular orientation and less charge recombination, leading to an exceptionally high fill factor (FF) of 78.78% and a good power conversion efficiency (PCE) of 14.00% for the 3TTCz -based OSC, better than those of the 3TTDPA -based one (FF = 66.93%, PCE = 13.85%). This work provides a feasible approach for stabilizing the resonance structure of NFREAs, thus enabling efficient and stable OSCs. We have unveiled that a closed-loop carbazole side chain is superior to an open-loop diphenylamine side chain in designing both efficient and stable nonfused-ring electron acceptors.