Beyond Plasmonics: Au Nanoparticles as Electron Sinks in TiO ₂ for Interface Passivation Enhancement in Planar Perovskite Solar Cells

Improving the interface passivation and charge selectivity of electron transport layers (ETLs) is essential to enhance both performance and operational stability in perovskite solar cells (PSCs). In this work, we introduce a sputtered double compact TiO₂ ETL incorporating monodisperse Au nanoparticles (NPs) as an electronically active interlayer. The sputtering process ensures conformal encapsulation of the NPs and precise control of TiO₂ sublayer thickness, providing a highly controlled platform to disentangle structural, optical, and electronic effects of embedded metal NPs. This architecture enables, for the first time, a systematic investigation of an electron-sink-induced field-effect modulation of the AuNPs@TiO₂/perovskite interface. By precisely controlling NP size (15 and 55 nm), loading (0.15-1.20 wt %), and TiO₂ encapsulation thickness (5-20 nm), we identified an optimized architecture that delivers a 1.3% absolute gain in light-to-power conversion efficiency, primarily through increased short-circuit current density. Structural and optical analyses confirm that NPs are uniformly embedded without modifying the TiO₂ crystallinity and electronic structure. The results indicate that embedded Au NPs may passivate interface traps and enhance ETL selectivity, while their electron-sink behavior transiently captures charge and modulates the interface potential. At high NP concentrations (>0.30 wt %), overlapping depletion regions lead to transport constrictions and increased transport resistance. Furthermore, the Au NPs appear to suppress the UV-driven photocatalytic activity of TiO₂, improving device operational stability. These findings reveal a new electron-sink mechanism in metal-oxide/metal-NP systems and establish sputtered AuNPs@TiO₂ ETLs as a scalable route toward more selective and stable perovskite photovoltaics.