It's widely believed that moist (H 2 O) acts as the reactive source for the collapse of the inorganic framework. We therefore evaluated the environmental stabilities of 2D monolayer PEA 2 SnI 4 , PEA 2 SnBr 4 , and PEA 2 GeI 4 by calculating the adsorption energies of water molecules on their surfaces. The calculated adsorption energies are listed in Table , and the corresponding optimized configurations of H 2 O molecules adsorbed on the surface of each compound are illustrated in Figure , respectively. Compared with MAPbI 3 (-0.48 eV), 2D monolayer PEA 2 SnI 4 (-0.26 eV), PEA 2 SnBr 4 (-0.3 eV), and PEA 2 GeI 4 (-0.28 eV) exhibit smaller adsorption energies of H 2 O on the surfaces, which suggest a better moisture resistance. Further microstructural analysis reveals that (Figure ) there is no hydrogen bonding interaction between the benzene ring in PEA + and the water molecule. Moreover, the weak hygroscopicity of PEA + leads to the separation of the water molecule and the inorganic framework, which inhibits the water-induced distortions of the inorganic framework at the surface. The increment in the energy barrier of the H 2 O penetration process thereby prevents the hydration degradation of PEA-based 2D monolayer hybrid halide perovskites in practical applications.