Fig. 5.

Degradation process of the MAPbI₃ thin film. a MIM images over 1 week with 35% relative humidity at 23 °C. Zoom-in AFM images on Days 0, 3, and 6 are shown to the left. Note that the fine features on large grains on Day 0 (inside dashed circles) are associated with crystal edges or terraces, which are common for polycrystalline thin films. The sample was only illuminated by the 532-nm laser during the MIM imaging for about 30 min at each frame. All scale bars are 1 μm. b Selected XRD and c TRPL data on MAPbI₃ films prepared by the same method and exposed to the same conditions. The XRD peak at 2θ = 12.66° is associated with PbI₂. d Schematic of the aging process due to the diffusion of water molecules through the PMMA layer, which drives the decomposition of MAPbI₃ into CH₃NH₂ and HI in the gas phase and PbI₂ in the solid phase