Double Electron Transport Layers Toward 20% Efficient Perovskite Solar Cells

We present planar perovskite solar cells incorporating thin SnO ₂ /Al ₂ O ₃ double electron transport layers between the perovskite and an indium tin oxide bottom electrode. When measured under 1 sun illumination, we obtained a maximum power conversion efficiency (PCE) of 20.1% and a steady state efficiency of 17.8% for the best cell. These values were ~20%-30% higher in relative terms than those of cells with SnO ₂ only (i.e., a maximum PCE of 15.3% and a steady state PCE of 14.9%). Insertion of the thin UV-irradiated solution-processed nanoparticle Al ₂ O ₃ interlayer effectively enhanced the wettability of the electron transport layer, provided enhanced interface area, as well as a lower work function, leading to improved charge extraction. Incorporation of an Al ₂ O ₃ layer between the perovskite and SnO ₂ layers also improved the rectification ratios of the diodes as well as both series and shunt resistances. Our devices are fabricated using fully solution-processed transport and active semiconducting layers processed at low temperatures (≤150 °C).