<i>In Situ</i> Interface Engineering for\nHighly Efficient Electron-Transport-Layer-Free Perovskite Solar Cells

Electron-transport-layer\nfree perovskite solar cells (ETL-free\nPSCs) have attracted great attention due to their low cost and simple\nmanufacturing process. However, an additional interface layer has\nto be introduced, and the currently achieved efficiency remains far\nfrom full-structure PSCs. Here, we report an <i>in situ</i> interface engineering strategy by the methylammonium acetate (MAAc)\nionic liquid perovskite precursor. We found that a dipole layer was <i>in situ</i> constructed through the physical adsorption of the\nresidual MAAc polar molecules on the indium tin oxide electrode, which\nis significantly different from the treatment by the interface layer\nin previous reports. This allows a decrease of the effective work\nfunction and enables <i>in situ</i> band bending in the\nperovskite semiconductor. The <i>in situ</i> band bending\nfacilitates charge collection and hinders interfacial charge recombination,\nleading to ETL-free PSCs with a maximum power conversion efficiency\nof 21.08%, which is the highest report to date.