Stirred, Not Stacked: Co‐Evaporated C ₆₀ :BCP Electron Transport Layers for Efficient and Robust Metal‐Halide Perovskite Solar Cells

Electron transport layers (ETL) based on sequentially deposited C 60 and BCP are widely used in perovskite solar cells but suffer from parasitic optical absorption and interfacial limitations. We investigate co‐evaporated C 60 :BCP layers as an alternative interfacial design strategy. By comparing sequentially deposited and co‐evaporated ETLs while keeping other device components constant, we isolate the influence of the ETL architecture on optical, optoelectronic, and mechanical properties. We show that co‐evaporation enables dilution of C 60 within a BCP matrix. In combination with a substantial reduction of the amount of C 60 used, we achieve a reduction of parasitic absorption below 600 nm without compromising charge extraction. Single‐junction devices employing co‐evaporated ETLs exhibit efficiencies comparable to, and in some cases exceeding, those of devices with conventional, sequentially deposited C 60 |BCP stacks, alongside reduced recombination losses and higher implied open‐circuit voltages, indicating improved interfacial electronic quality. Qualitative adhesion tests further suggest enhanced interfacial cohesion at the perovskite/ETL interface. Reproducibility is confirmed by an independent device batch achieving efficiencies up to 23.5%. These results establish co‐evaporated C 60 :BCP as a robust and reproducible alternative to sequential C 60 |BCP layers, with particular relevance for perovskite tandem solar cells, where reduced parasitic absorption and high‐quality interfaces are critical.