Branched molecules slow moisture damage in perovskite powders
A change in molecular shape reduced moisture degradation sixfold in tested perovskite powders. The single peer-reviewed materials study linked stability to the alternative crystals that water can help form. Researchers compared organic spacers separating the material’s layers, identifying how their geometry changes possible breakdown routes in compositions investigated for solar cells.
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Moisture opens routes to different crystals
Isaac Metcalf and colleagues investigated why some layered perovskites withstand moisture better than others. These materials, studied for solar cells, contain lead-and-iodide layers separated by organic molecules. In the single peer-reviewed materials study, changing a straight molecular spacer to a branched one reduced moisture degradation sixfold in the tested powders. Researchers compared butylammonium with isobutylammonium, organic cations with different shapes.[1]
Molecular shape determines which phases fit
The experiments at Rice University and collaborating institutions examined crystals containing the organic spacer, lead iodide and water. Phase diagrams mapped which solid structures can coexist at a given composition. Crystal growth and X-ray scattering identified the products. As water entered, the composition shifted toward water-rich and lead-rich conditions, opening routes to structures with different properties from the original perovskite.[1]
Linear, branched, aromatic and cyclic spacers were compared. Their shape affected whether they could enter water-containing crystal phases. Low-energy hydrates, crystals incorporating water, provided particularly favorable breakdown routes in less stable examples.[1]
Mixed films also respond to secondary phases
Some stable layered perovskites could coexist with water at equilibrium, while hydrate formation prevented that coexistence in less stable compositions. Films built from both three-dimensional and layered components were also vulnerable to secondary crystals containing mixed organic cations. The findings concern particular powders and films. A finished solar cell operates under combined light, heat and environmental exposure, beyond the moisture comparison tested here.[1]
