A‐Site Management for Highly Crystalline Perovskites

Abstract

An in-depth understanding and effective suppression of nonradiative recombination pathways in perovskites are crucial to their crystallization process, in which supersaturation discrepancies at different time scales between CH₃ NH₃ I (MAI, methylammonium iodide) and PbI₂ remain a key issue. Here, an A-site management strategy via the introduction of an A-site placeholder cation, NH₄ ⁺ , to offset the deficient MA⁺ precipitation by occupying the cavity of Pb-I framework, is proposed. The temporarily remaining NH₄ ⁺ is substituted by subsequently precipitated MA⁺ . The temperature-dependent crystallization process with the generation and consumption of a transient phase is sufficiently demonstrated by the dynamic changes in crystal structure characteristic peaks through in situ grazing-incidence X-ray diffraction and the surface potential difference evolution through temperature-dependent Kelvin probe force microscopy. A highly crystalline perovskite is consequently acquired, indicated by the enlarged grain size, lowered nonradiative defect density, prolonged carrier lifetime, and fluorescence lifetime imaging. Most importantly, it is identified that the A-site I_MA defect is responsible for such crystal quality optimization based on theoretical calculations, transient absorption, and deep-level transient spectroscopy. Furthermore, the universality of the proposed A-site management strategy is demonstrated with other mixed-cation perovskite systems, indicating that this methodology successfully provides guidance for synthesis route design of highly crystalline perovskites.