Regulating structural stability and photoelectrical properties of FAPbI₃via formamidine cation orientation

Organic cations play a significant role in the structural stability and photoelectrical properties of organic-inorganic hybrid perovskites. The orientation of organic cations impacts its interaction with inorganic octahedrons [PbI]^6-, subsequently modifying the atomic structure and electronic and optical properties of perovskite materials. However, it is still challenging to regulate the stability of perovskites with different orientations. In this work, density functional theory calculations were performed to investigate the effects of the formamidine cation (FA⁺) located at the angles of 0°, 45°, 90° and 180° (relative to the normal of the crystal plane) along the typical crystal directions ([001], [010], [110] and [111]) on the structural stability and photoelectrical properties of formamidine lead iodide (FAPbI₃). The results show that when FA⁺ is located at 45° along the [111] direction, FAPbI₃ achieves the highest stability and excellent photoelectrical properties. The energy evolution curves display that the system with the orientation of [111] has the minimum energy value, signifying stronger stability than the other orientations. Especially, when FA⁺ is located at 45° along the [111] direction. it exhibits a stronger hydrogen bond between H and I atoms, shorter Pb-I bond length and smaller [PbI]^6- octahedral tilt bond angle. The band gap in the [110] direction changes from direct to indirect while FAPbI₃ with other FA⁺ orientations still maintains the direct band structure located at the high symmetric R point. Furthermore, FA⁺ orientation drives the redshift of FAPbI₃ towards the long wavelength region in the [111] crystal direction, which enhances the light absorption coefficient. This work can offer guidance in employing molecular regulation technology for the development of stable perovskite solar cells.