Double-Layer Passivation by Common Fullerene Materials for Ultraviolet Stability of Organic Solar Cells and Modules

The ultraviolet (UV) light is the major cause of performance decay in organic solar cells (OSCs), often initiated by photochemical reactions at the ZnO/active-layer interface. Here, ZnO was passivated with C60 and phenyl-C61-butyric acid methyl ester (PCBM) as single or stacked double layers, deposited by solution processing (cylinder blade coating and spin coating) to enable module fabrication. All device variants achieved comparable initial efficiencies (13.7 ± 0.2%). Under UV aging, the double-layer structure ZnO/PCBM/C60 showed superior stability: PCBM modifies the ZnO surface and provides a solution-processable contact, while the low-solubility C60 upper layer preserves the fullerene-rich interface during active-layer deposition. After 500 h UV irradiation (50 W/m², approximating the natural sunlight UV fraction), unpassivated, single-, and double-passivated devices retained PCEs of 3.3%, 5.7%, and 6.8% (retention ratios of 23%, 37%, and 44%, respectively). Combining double-layer passivation with a low-cost UV-protective film further suppressed module degradation: after 1000 h, efficiency declined only from 13% to 12% (92% retention). These results demonstrate a practical, scalable route toward UV-stable outdoor organic modules.