Design of Low Bandgap Tin-Lead Halide Perovskite Solar Cells to Achieve Thermal, Atmospheric and Operational Stability

Low bandgap tin–lead iodide perovskites are key components of all-perovskite tandem solar cells, but can be unstable because tin is prone to oxidation. Here, to avoid a reaction with the most popular hole contact, we eliminated polyethylenedioxythiophene:polystyrenesulfonate as a hole transport layer and instead used an upward band offset at an indium tin oxide–perovskite heterojunction to extract holes. To suppress oxidative degradation, we improved the morphology to create a compact and large-grained film. The tin content was kept at or below 50% and the device capped with a sputtered indium zinc oxide electrode. These advances resulted in a substantially improved thermal and environmental stability in a low bandgap perovskite solar cell without compromising the efficiency. The solar cells retained 95% of their initial efficiency after 1,000 h at 85 °C in air in the dark with no encapsulation and in a damp heat test (85 °C with 85% relative humidity) with encapsulation. The full initial efficiency was maintained under operation near the maximum power point and near 1 sun illumination for over 1,000 h.