Interface Engineering with BPhen:Cs₂CO₃ for High‐Performance and Stable Inverted Nonfullerene Organic Solar Cells

The widely used ZnO electron transport layer in inverted nonfullerene organic solar cells (nf‐OSCs) offers advantages such as excellent electron mobility and optical transparency. However, challenges arise from surface defects in solution‐processed ZnO, where oxygen‐containing defects can penetrate the photoactive layer, leading to photocatalytic reactions with nonfullerene acceptors under UV light, thereby compromising device stability. Another challenge is that most recent high‐efficiency nf‐OSCs employ conventional structures, while inverted structures exhibit comparatively lower performance. To develop stable and high‐performance inverted nf‐OSCs, interface modification is essential to mitigate photocatalytic issues and enhance the relatively lower power conversion efficiency (PCE). To overcome these limitations, we introduce bathophenanthroline (BPhen) doped with Cs 2 CO 3 . The BPhen:Cs 2 CO 3 layer creates suitable energy levels, enhancing electron transport and reducing charge recombination. This approach significantly improves current density and fill factor, resulting in a notable enhancement in the PCE of pristine ZnO devices from 15.54% to 17.09% in PM6:Y6 inverted nf‐OSCs. Furthermore, ZnO/BPhen:Cs 2 CO 3 devices exhibit excellent stability, retaining ~83% of their initial efficiency even after 1000 h without encapsulation, showcasing superior stability compared to pristine ZnO‐based devices.