Texture Formation in Polycrystalline Thin Films of All-Inorganic Lead Halide Perovskite

TL;DR

This study investigates how grain orientations in all-inorganic lead halide perovskite thin films are influenced by internal lattice distortions and external substrate effects, revealing that bromine content controls texture formation and structure.

Contribution

It uncovers the mechanisms of texture formation in inorganic perovskite films, highlighting the role of lattice distortions and bromine incorporation in controlling grain orientation.

Findings

Tetragonal distortions trigger preferential grain alignment during cooling.

Br incorporation reduces orthorhombic distortion and influences texture formation.

Substrate choice has minimal impact compared to composition and internal structure.

Abstract

Controlling grain orientations within polycrystalline all-inorganic halide perovskite solar cells can help increase conversion efficiencies toward their thermodynamic limits, however the forces governing texture formation are ambiguous. Using synchrotron X-ray diffraction, we report meso-structure formation within polycrystalline CsPbI2.85Br0.15 powders as they cool from a high-temperature cubic perovskite ({\alpha}-phase). Tetragonal distortions (\b{eta}-phase) trigger preferential crystallographic alignment within polycrystalline ensembles, a feature we suggest is coordinated across multiple neighboring grains via interfacial forces that select for certain lattice distortions over others. External anisotropy is then imposed on polycrystalline thin films of orthorhombic ({\gamma}-phase) CsPbI3-xBrx perovskite via substrate clamping, revealing two fundamental uniaxial texture formations; (i) I-rich films possess orthorhombic-like texture (<100> out-of-plane; <010> and <001> in-plane), while (ii) Br-rich films form tetragonal-like texture (<110> out-of-plane; <1-10> and <001> in-plane). In contrast to relatively uninfluential factors like the choice of substrate, film thickness and annealing temperature, Br incorporation modifies the {\gamma}-CsPbI3-xBrx crystal structure by reducing the orthorhombic lattice distortion (making it more tetragonal-like) and governs the formation of the different, energetically favored textures within polycrystalline thin films.