Relaxation of externally strained halide perovskite thin layers with neutral ligands

The preferred orientation (PO) and non-uniform strain of the perovskite layer coated on the substrate can cause unwanted blue shifts in the band gap and several defects inside and on the grain boundaries. Therefore, suppressing the anisotropic preferred growth of halide perovskite crystals while satisfying the trap passivation of grain boundaries is important to the fabrication of efficient and stable perovskite solar cells (PSCs). In this study, the properties of spin-coated perovskite thin films were investigated by adding an appropriate amount of trioctylphosphine (TOP) as a neutral ligand to a formamidinium lead triiodide (FAPbI3) precursor solution. The TOP in the FAPbI3 precursor solution containing MACl significantly alleviated the PO by MACl and further induced a redshift of the band gap through the reduction of microstrain. Furthermore, TOP contributed to the passivation of defects via the surface termination of grain boundaries. PSCs based on these perovskite thin films exhibited an efficiency close to 25%.