Relaxation of Externally Strained Halide Perovskite Thin Layers with Neutral Ligands

In solution-processed halide perovskite thin films, an external strain can be generated owing to the anisotropy of solvent volatilization through the thin film surface and the mismatch of thermal expansion with the substrate. Furthermore, the addition of methylammonium chloride to a formamidinium lead triiodide (FAPbI₃) precursor solution to increase the crystallinity and form α-FAPbI₃ at low temperatures also induces the preferred orientation (PO) of the perovskite layer. However, excessive PO can cause anisotropic strain in the thin film, increasing defects, widening the band gap, and reducing long-term stability. As a way to alleviate this, a perovskite thin film was deposited by adding trioctylphosphine, a neutral ligand, to the FAPbI₃ precursor solution. As a result, perovskite solar cells (PSCs) based on these perovskite thin films exhibit an efficiency close to 25% with high stability, showing the highest value among PSCs using TiO₂ as an electron transport layer.