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 Cs₂CO₃. The BPhen:Cs₂CO₃ 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:Cs₂CO₃ 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.