Disorder in Lead Halide Perovskites
Scheme of the NMR crystallographic approach which is developed to understand ion disorder and dynamics in the quaternary phase diagram MAPbI3-MAPbr3-FAPbI3-FAPbBr3.
Hybrid lead halide perovskites are currently considered one of the most promising semiconductor materials for the next generation of solar cells. Due to their structural flexibility to incorporate mixtures of different organic and inorganic cations, and halides, a wide compositional range is accessible allowing for e.g. tuning of the band-gap energy. However, highly alloyed perovskite compositions introduce a high degree of disorder, statically and dynamically. Differing microstructures, for example in the form of compositional fluctuations within the perovskite particles, has a strong impact on the opto-electronic properties. As such, it is essential to gain insights in the ion (dis)order on the local and global scale, as well as its influence on the ion dynamics. For this purpose, we use a combination of solid-state NMR and NQR spectroscopy, powder X-ray diffraction and molecular dynamic simulations.
In recent projects we explored the local and global ion disorder and dynamics in mechanochemically synthesized MAPbI3, as well as mixed cation MA1‑xFAxPbI3 and mixed halide MAPbI3-xBrx perovskite systems. Our observations for mixed perovskite compositions show that local, short-range ordering of ions occurs in mixed systems, and that mixing cations and/or halides in the perovskite structure has significant effects on the reorientational dynamics of the A cations, MA and FA. These local inhomogeneities in the perovskite’s composition and changes in A cation dynamics are expected to impact the opto-electronic properties, especially charge carrier lifetimes and diffusion lengths. Furthermore, these studies represent the basis for an investigation of structural changes of the perovskites upon illumination and their impact on phase stability, opto-electronic properties and performance with respect to an application in solar cells.