LAUSR

The environment humidity effects on performance of halide perovskites (HaPs), especially MAPbI3, are known. Nevertheless, it is hard to find direct experimental evidence of H2O in the bulk materials at the levels lower than that of Monohydrate (MAPbI3.H2O). Here, for the first time, direct experimental evidence of water being released from bulk (µm‐s deep) of MAPbI3 single crystal is reported. The thermogravimetric analysis coupled with mass spectrometry (TGA‐MS) of evolved gases is used to detect the MS signal of H2O from the penetrable depth and correlate it with the TGA mass loss due to H2O leaving the material. These measurements yield an estimate of the average H2O content of 1 H2O molecule per three MAPbI3 formula units (MAPbI3.0.33H2O). Under the relatively low temperature conditions no other evolved gases that can correspond to MAPbI3 decomposition products, are observed in the MS. In addition to being direct evidence that there is H2O inside MAPbI3, the data show that H2O diffuses into it. With this article, a solid basis is proved for further studies on the mechanisms through which water modifies the properties of MAPbI3 and all the other halide perovskites.