Improvement of stability of perovskite solar cells with PbS buffer layer formed by solution process

To commercialize perovskite solar cells (PSCs), it is essential to ensure their stability against heat and humidity. However, organometal halide perovskites, which are employed as light absorbing materials in PSCs, have the disadvantage of being easily decomposed by moisture. Moreover, iodine ions in the perovskite are highly mobile, reducing the stability of PSCs by reacting with metal electrodes after moving to other layers when exposed to heat or light. Introducing a firm buffer layer on the perovskite surface can be an effective solution to increase stability by preventing the perovskite from such degradation. In this work, using a solution process in which Na2S solution was spin-coated on a perovskite surface and then heat-treated, we formed a buffer layer with PbS. The PbS buffer layer prevented the perovskite from being in direct contact with moisture and suppressed ion migration from the perovskite. With the PbS buffer layer, the PSCs exhibited significantly improved long-term stability, without any encapsulation 1) under continuous illumination of AM1.5G - 1 SUN light in ambient air, 2) at high temperature of 85 degrees C in nitrogen, and 3) in high humidity of 36.4 +/- 5 % RH at elevated temperature of 60 degrees C. In addition, the PbS buffer layer enhanced the extraction of holes from the perovskite, thereby improving the power conversion efficiency.