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

Abstract

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.