Efficient and Robust Organic Solar Cells Enabled by Nanofibrillar Electron Transport Layer via Seed-Initiated Epitaxial Growth.

In organic solar cells (OSCs), the morphology of charge transport layers (CTLs) is as critical as that of the active layer, directly governing charge transport and collection efficiency as well as overall device performance. However, current CTL design predominantly focuses on molecular structure engineering, while systematic morphological control of CTLs remains largely underexplored. Herein, we report a seed-initiated epitaxial growth strategy to construct well-ordered fibrillar electron transport layers (ETLs) with tunable fiber lengths and narrow dispersity. This nanofiber-textured ETL provides dedicated pathways for efficient electron transport and extraction, while its enlarged contact area strengthens interlayer adhesion, improving both interfacial charge transport and mechanical integrity. Using this ETL, power conversion efficiencies of 20.6% in rigid binary OSCs and 19.1% in flexible ones are achieved. The nanofibrillar ETL also exhibits broad active layer compatibility, excellent thickness tolerance, and outstanding mechanical stability. This work provides new insights into ETL morphological engineering as a strategy to overcome interfacial bottlenecks, unlocking significant efficiency gains in OSCs.