Plasma Chemistry Meets Photovoltaics: Methane‐Engineered Titanium Dioxide Layers for Efficient and Robust Perovskite Solar Cells

The performance and long‐term stability of perovskite solar cells (PSCs) are critically dependent on the quality of the electron transport layer (ETL), particularly its defect states and charge transport characteristics. In this work, we report a plasma‐assisted magnetron sputtering approach that incorporates methane (CH 4 ) as a reactive gas to precisely tailor the oxygen vacancy ( O vac ) distribution in the titanium dioxide (TiO 2 ) ETL. By introducing varying CH 4 concentrations (0%–20%) into an argon (Ar) plasma environment, we achieved a controlled O vac generation, favoring subsurface oxygen depletion while minimizing surface defects. To isolate the effect of CH 4 ‐derived carbon species, a comparative sample (SP‐10‐H 2 ) was also fabricated by introducing 10% H 2 instead of CH 4 during sputtering. The CH 4 molecules undergo partial oxidation in the plasma phase, forming volatile CO and CO 2 and enabling selective oxygen removal. Comprehensive structural, morphological, and electrical characterizations reveal that CH 4 concentration of 10% yields TiO 2 films with optimal surface uniformity, reduced trap‐density, and enhanced carrier mobility. Planar‐type PSCs employing these CH 4 ‐modified ETLs demonstrated a power conversion efficiency (PCE) of 22.3%, surpassing those fabricated with conventional spray‐coated TiO 2 (20.4%) and CH 4 ‐free sputtered TiO 2 (19.2%). Moreover, the optimized devices retained over 90% of their initial efficiency after 800 h. These findings establish CH 4 ‐assisted reactive sputtering as an effective, scalable strategy for defect engineering in oxide ETLs, offering a promising pathway toward high‐efficiency, durable perovskite photovoltaics.