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.