Ambient-air fabrication of bar-coated ionic liquid-assisted perovskite films with micrometer-scale grain sizes for stable solar cells

Fabricating perovskite solar cells (PSCs) in ambient air may lower production costs and enhance the feasibility of commercialization of this cutting-edge technology. However, ambient-air fabrication of perovskite films often results in rough surfaces, pinholes, and non-uniform coverage, which increase non-radiative recombination in PSCs. In this work, we fabricated uniform pinhole-free perovskite films with large crystal domains of several tens of micrometers under ambient conditions (40 % relative humidity) by bar coating using 1-hexyl-3-methylimidazolium chloride as an ionic liquid (IL). The IL molecules in the perovskite solution protected the perovskite from interacting with moisture, facilitating the fabrication of high-quality perovskite films under ambient conditions. PSCs with bar-coated IL-assisted perovskite films exhibited over 90 % of their initial solar output after storage for 1000 h in the dark under ambient conditions (relative humidity of 40 %–50 %), illustrating their higher long-term moisture stability than that of devices with conventional spin-coated perovskite films. This high moisture stability is attributed to the dense perovskite grains with sizes of one to several tens of micrometers obtained by bar coating rather than spin coating.