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Layered hybrid halide perovskites are known to be more environmentally stable than their 3D analogues. The enhanced stability is particularly relevant for Dion-Jacobson-type layered perovskites due to their promising photovoltaic performances. However, in contrast to the expected resilience to moisture, we reveal that the structure of Dion-Jacobson perovskite phases based on a 1,4-phenylenedimethanammonium spacer is disrupted in humid conditions using X-ray diffraction, UV-vis spectroscopy, thermogravimetric analysis, and solid-state NMR spectroscopy. The process takes place at >= 65 +/- 5% relative humidity, with a time scale on the order of minutes. The original layered structure can be restored upon annealing and the hydration can be suppressed by postsynthetic annealing in air, which is attributed to the generation of a self-protective layer of PbI2. This study thereby reveals a unique behavior of layered perovskites in humid environments, which is critical to their stabilizing role in perovskite devices.

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)A(n)(-1)Pb(n)I(3)(n)(+1) composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.