The mechanical flexibility of perovskite solar cells as well as high power conversion efficiency is attracting increasing attention. In addition to existing empirical approaches, such as cyclic bending tests, in this study we report the tensile properties of the perovskite materials themselves. Measuring the tensile properties of free-standing perovskite materials is critical because (1) tensile properties represent the realistic mechanical properties of the film-type perovskite layer in the solar cells including the effects of various defects, and (2) deformation behavior of the perovskite layer at any deformed state of the solar cells can be analyzed using solid mechanics with the tensile properties as input. Critical bending radius of MAPbI 3 -based flexible solar cells is found to be between 0.5 and 1.0 mm by the decrease in power conversion efficiency during cyclic bending deformation. This finding agrees well with the critical bending radius of 0.66 mm determined based on the elastic deformation limit of 1.17% for MAPbI 3 found by in situ tensile testing. Scanning electron microscopy observations and hole-nanoindentation tests suggest that the formation of coarse cracks in the perovskite layers is the primary cause of the decrease in power conversion efficiency observed in flexible perovskite solar cells.

Protein phosphorylation is one of the most studied posttranslational modifications (PTMs). Despite the remarkable advances in phosphoproteomics, a chemically less-stable subset of the phosphosites, which we call the crypto-phosphopro-teome, has remained underexplored due to technological challenges. In this Viewpoint, we briefly summarize the current understanding of these elusive protein phosphorylations and identify the missing pieces for future studies.

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