Reversible Dimensionality Tuning of Hybrid Perovskites with Humidity: Visualization and Application to Stable Solar Cells

Hybrid perovskites have attracted much attention as a promising photovoltaic material in the past few years. Typically, these hybrid perovskites such as methyl ammonium lead halides (MAPbX(3)) undergo dimensionality reduction from three-dimensional (3D) to zero-dimensional (OD), and finally to PbX2, upon continuous moisture exposure. Our current study shows that OD-perovskite-related structures exhibit a reversible transformation from a transparent state to a colored 3D state upon exposure to humidity. Fluorescence imaging of individual microcrystals reveals that the structural phase transition could be visualized in the solid state, wherein the crystals transform into cubic crystals. The plausible reason for this transformation is proposed to be a dynamic dissolution and recrystallization of the excess methyl ammonium halide with varying humidity. The thermal and moisture stability are found to be greatly enhanced in the transformed 3D perovskite. Excellent device stability is also demonstrated when the devices are kept under moist (similar to 70% RH) conditions.