Formation of CsPbI3 γ‐Phase at 80 °C by Europium‐Assisted Snowplow Effect

Herein, the convenient interplay between Eu incorporation and morphology to form the perovskite (PSK) γ-phase of CsPbI3 at 80 °C (low temperature [LT] γ-phase) is unveiled. In contrast, CsPbI3 without Eu exists in a mixture of γ-phase and non-PSK δ-phase at 65 °C or in a fully δ-phase at 80 °C. Based on experimental and theoretical findings, an argument about a double beneficial role of Eu is presented. On one hand, it assists in the formation of the γ-phase either by substituting Pb or by occupying interstitial positions in the CsPbI3 lattice. On the other hand, it indirectly promotes the formation of a fine-grained layer at LT wherein the high surface-to-volume ratio makes the establishment of the δ-phase unfavorable. Strain accommodation in the fine-grained matrix and the formation of a gluing intergrain-self-material during the kinetics of reaction (snowplow effect) cooperate in extending the lifetime of the LT γ-phase to ≈40 h at 65 °C compared to only ≈10–15 min in the sample without Eu for the complete phase transformation. The disclosed phenomena draw a method for the stabilization of PSK-CsPbI3 layers based on a self-generated thin frame of exceeding material filling the gaps between small-sized grains that can be used hereinafter to further extend the PSK lifetime.