The orientation design of high-polarity ligand dipole CF₃-PEA for enhancing the surface stability and optoelectronic properties of the FAPbI₃ perovskite

Ligand dipoles are introduced onto perovskite surfaces to enhance the optoelectronic properties and stability of organic-inorganic hybrid perovskite solar cells (PSCs), achieving diverse spatial distributions and orientational variations due to their interaction. However, the impacts of ligand dipole orientation on the performance of PSCs remain unclear. In this work, we investigate the adsorption of high-polarity ligand dipole 4-trifluoromethyl-phenethylammonium (CF₃-PEA) with orientation design on the PbI₂-terminated surface of the formamidinium perovskite (FAPbI₃), revealing the impact of CF₃-PEA orientation on the atomic structure, stability, and optoelectronic properties of the FAPbI₃ surface. The results indicate that on the FAPbI₃ surface along with the (001) crystal plane, the optimal crystal orientation of CF₃-PEA is [2, 4, -1], where the strong -NH₃/π⋯I interaction results in the parallel alignment of CF₃-PEA with the FAPbI₃ surface. Under the optimal [2, 4, -1] orientation, the surface adsorption system exhibits the lowest relative total energy difference (0 eV), formation energy (-0.746 eV) and adsorption energy (-4.14 eV) among all adsorption systems, with the formation energy being 1.54 times those of bare FAPbI₃, revealing its highest stability. Moreover, the adsorption of CF₃-PEA with [2, 4, -1] orientation results in a decrease of the work function from 6.764 eV of bare FAPbI₃ to 6.743 eV due to the strong -NH₃/π⋯I interaction with significant charge transfer. Furthermore, the appearance of sub-bandgap states in density of states leads to an obvious redshift in the light absorption coefficient attributed to structural distortions, which indicates that the optoelectronic properties of FAPbI₃ are improved. This work provides insights into optimizing the performance of PSCs by regulating the orientation of ligand dipoles.