Isomerization strategy on a non-fullerene guest acceptor for stable organic solar cells with over 19% efficiency

The strategy of isomerization plays a simple and effective role in optimizing the molecular configurations and improving the performance of binary organic solar cells (OSCs). However, the effect of isomerization in guest materials on ternary OSCs has rarely been reported, and their structure–property relationships are not yet clearly understood. Herein, two large π-conjugated isomers, QX-α and QX-γ, with different orientations of their fused thiophene-rings were designed and synthesized to investigate the influence of isomers in non-fullerene guest acceptors on the photovoltaic properties in a D18:N3 host system. Compared to QX-γ, QX-α demonstrated a stronger dipole moment, a more ordered stacking, and a higher surface energy due to the presence of S⋯N non-covalent interactions. As a result, the OSCs device based on D18:N3:QX-α achieved the higher efficiency of 19.33%, while the device based on D18:N3:QX-γ exhibited an efficiency of only 18.30%. Remarkably, the flexible OSC based on D18:N3:QX-α produced an outstanding PCE of 18.01%, which is a record PCE for flexible OSCs. In addition, the ternary device showed a significant increase in efficiency retention from 49% to 87% after 476 h of storage in a N2-filled glove box at 85 °C compared to the binary device. Moreover, the extrapolated T80 lifetime of the D18:N3:QX-α-based ternary device was as high as over 17 000 h in a glove box at room temperature. The results indicate that rational tuning of the atomic orientation can be an effective way to construct non-fullerene guest acceptors for achieving highly efficient and stable OSCs.