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 (CH4) as a reactive gas to precisely tailor the oxygen vacancy (Ovac) distribution in the titanium dioxide (TiO2) ETL. By introducing varying CH4 concentrations (0%–20%) into an argon (Ar) plasma environment, we achieved a controlled Ovac generation, favoring subsurface oxygen depletion while minimizing surface defects. To isolate the effect of CH4‐derived carbon species, a comparative sample (SP‐10‐H2) was also fabricated by introducing 10% H2 instead of CH4 during sputtering. The CH4 molecules undergo partial oxidation in the plasma phase, forming volatile CO and CO2 and enabling selective oxygen removal. Comprehensive structural, morphological, and electrical characterizations reveal that CH4 concentration of 10% yields TiO2 films with optimal surface uniformity, reduced trap‐density, and enhanced carrier mobility. Planar‐type PSCs employing these CH4‐modified ETLs demonstrated a power conversion efficiency (PCE) of 22.3%, surpassing those fabricated with conventional spray‐coated TiO2 (20.4%) and CH4‐free sputtered TiO2 (19.2%). Moreover, the optimized devices retained over 90% of their initial efficiency after 800 h. These findings establish CH4‐assisted reactive sputtering as an effective, scalable strategy for defect engineering in oxide ETLs, offering a promising pathway toward high‐efficiency, durable perovskite photovoltaics.