Molecular aspects of organic cations affecting the humidity stability of perovskites
Two important factors in solar cells are efficiency and long-term stability. The commercialization of halide perovskite solar cells (PSCs) containing organic cations may be limited due to their low stability, in spite of their high efficiency. The molecular features of the organic cations adopted in perovksites belong to a subset of ammonium cations, such as methylammonium (MA), which are known to display poor humidity stability. Based on the molecular aspects of organic cations, the humidity stability of perovskites can be affected by their structural stability, the hydrophobicity, chemical environment near the heteroatom, and stereochemistry. Organic cations with large ionic radii produce two-dimensional (2D) structures with significantly improved stability. 2D perovskites show high humidity stability due to their increased chemical stability and hydrophobicity, but have the drawback of slow charge transport due to the insulating properties of the organic cations. Accordingly, many efforts have been devoted to form conductive inorganic layers in 2D perovskites in addition to the light-active layer of PSCs in several ways. In this perspective, we have analyzed the possible degradation initiators formed under humid conditions based on a comprehensive review of the literature, followed by practical experimental results using ammonium-driven PSCs, focusing on their humidity stability and device performance. As a strategy to enhance the wet-fastness of perovskites, we propose new under-explored sulfonium cations (R3S+), showing characteristic stereochemistry and significantly increased humidity stability of perovskites, which differ from conventional protic ammonium cations.