Role of the Adsorption of Alkali Cations on Ultrathin n-Layers of Two-Dimensional Perovskites

Abstract

Metal halide semiconductors have great potential for real-life photovoltaic applications; however, surface defects induce several challenges to thermodynamic stability. Here, we employed density functional theory calculations within van der Waals corrections (D3) to investigate the role of monovalent cations (Li, Na, K, Rb, and Cs) in the passivation of 2D perovskite ultrathin films, namely, A 2 (MA n−1 )Pb n I 3n+1 , where n = 1 (monolayer) and n = 2 (bilayer). We found connections between the periodic trends of alkaline ions, such as the ionic radius, polarizability, and electronegativity, and their direct influence on the structural, energetic, and electronic properties of the slab. The increase in the distortion of the octahedra is affected by the ionic radius, and the smaller is the alkaline ionic radius, the greater is the out-of-phase distortion of the octahedra. Consequently, we have an increase in the band gap for systems that have greater distortions. Spin−orbit coupling effects reduce the electronic band gap, while the hybrid functional increases the band gap. Therefore, in most cases, the scissor operator band gap matches the value obtained from the PBE+D3 functional when comparing the final results, except for Cs on the monolayer. The acid−base behavior of the A site with the octahedra promotes a charge transfer from the alkaline to the iodide, which is more effective for systems with less electronegative alkaline ions.

Cubic-to-hexagonal structural phase transition in metal halide compounds: a DFT study

“…This quantum connement is similar to that present in "2D" perovskite materials, where the band gap is sensitive to the octahedral planes within the crystal, having wider band gaps in solids with a smaller number of octahedral planes. 50 Further analysis on the band gap variation was carried out in Sections 3.5.1 and 3.5.2.…”

“…† The ECN in the present case, could act as a descriptor of the octahedral deformation throughout the phase change process, since it has been proven previously that the ECN Pb av reects the octahedral deformation. 25,50,54 We have plotted the ECN Pb av and, as mentioned in Fig. 4 and in Section 2.2, there are two types of Pb atoms within each TS: those who remained xed and those who had constrained degrees of freedom within the structure throughout the Cub / Hex-PT (for which we calculated ECN Pb-x.…”