Tuning the Surface Electronic Landscape of Ultrathin (Al,Sn)O _x Electron Extraction Layers in Perovskite Solar Cells

Abstract Power conversion losses at the electron transport layer (ETL) are key limiting factors in high‐performance perovskite solar cells (PSC) and silicon‐PSC tandem systems. Solution‐processed nanoparticle tin dioxide ( np ‐SnO ₂ ) films typically serve as ETLs in n ‐ i ‐ p devices with ITO substrates. Surface‐active molecules boost electron extraction, but assessing surface electronic properties remains challenging. In this study, the surface electronic landscape of the ITO and np ‐SnO ₂ layers is interrogated and the impact of introducing an ultrathin aluminum tin oxide ((Al,Sn)O _x ) interlayer with various cation ratios in high power conversion efficiency (PCE) cells is examined. Energy‐filtered photoemission electron microscopy (EF‐PEEM) reveals evidence of chemical disorder in np ‐SnO ₂ , with a broad local work function distribution across the surface, in stark contrast with the (Al,Sn)O _x films deposited on ITO. Optimum 31% Al (Al,Sn)O _x films increase the mean work function by ≈100 meV, promoting a remarkable increase in PCE from 22.7% to 24.6%. Devices incorporating (Al,Sn)O _x maintain 90% of their initial performance after 1200 h at 85 °C, 85% humidity under 1 SUN illumination. This study highlights the importance of tailoring ETL interfaces to improve both efficiency and long‐term stability in PSC devices.