Dual function of a high-contrast hydrophobic–hydrophilic coating for enhanced stability of perovskite solar cells in extremely humid environments

In spite of a continuous increase in their power conversion efficiency (PCE) and an economically viable fabrication process, organic–inorganic perovskite solar cells (PSCs) pose a significant problem when used in practical applications: They show fast degradation of the PCE when exposed to very humid environments. In this study, the stability of PSCs under very humid conditions is greatly enhanced by coating the surface of the PSC devices with a multi-layer film consisting of ultrahydrophobic and relatively hydrophilic layers. A hydrophobic composite of poly(methyl methacrylate) (PMMA), polyurethane (PU), and SiO2 nanoparticles successfully retards the water molecules from very humid surroundings. Also, the hydrophilic layer with moderately PMMA captures the residual moisture within the perovskite layer; subsequently, the perovskite layer recovers. This dual function of the coating film keeps the PCE of PSCs at 17.3% for 180 min when exposed to over 95% humidity.