Tuning the Surface Electronic Landscape of Ultrathin (Al,Sn)Ox 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, we interrogate the surface electronic landscape of the ITO and np-SnO2 layers and examine the impact of introducing an ultrathin aluminum tin oxide ((Al,Sn)Ox) interlayer with various cation ratios in high power conversion efficiency (PCE) cells. 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)Ox films deposited on ITO. Optimum 31% Al (Al,Sn)Ox films increases the mean work function by approximately 100 meV, promoting a remarkable increase in PCE from 22.7 to 24.6%. Devices incorporating (Al,Sn)Ox maintain 90% of their initial performance after 1,200 hours 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.