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

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-SnO₂) 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-SnO₂ 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-SnO₂, 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.