Revealing the role of thiocyanate anion in layered hybrid halide perovskite (CH3NH3)2Pb(SCN)2I2

Abstract

The effect of the SCN ion on the structural, electronic, optical, and mechanical properties of the layered (MA)Pb(SCN)I (MA=CHNH) perovskite is investigated by using first-principles calculations. Our results suggest that the introduction of SCN ions at the apical positions gives rise to shorter Pb-S bond lengths, more distorted octahedra, and more hydrogen bonds, which have important effects on the electronic, optical, mechanical, and piezoelectric properties in (MA)Pb(SCN)I. Furthermore, a strong relativistic Rashba splitting is induced due to the breaking of the inversion symmetry, which helps to suppress the carrier recombination and enhance the carrier lifetime. The analysis of mechanical properties reveals that the incorporation of SCN ions is beneficial to strengthen Young's modulus of the perovskite materials and it enhances the piezoelectric properties. Our investigation suggests that doping SCN ions into the perovskite materials could be a promising strategy to improve the stability and mechanical properties of organic-inorganic hybrid halide perovskite compounds.

On the True Band Gap of the (CH3NH3)2Pb(SCN)2I2 Hybrid Perovskite: An Interesting Solar‐Cell Material

“…It is clearly seen that the SCN À anion dominates the low energy zone (À5 to À2 eV), and that I 5p, Pb 6s, and S 3p states populate the valence band zone also; Pb 6p x and 6p y states populate the conduction band. This results are in concordance with similar reports, [6,8] and also demonstrates that substitution of I atoms in axial positions by SCN À anions does not affect the electronic transport properties of MAPSI by eventually depopulating the conduction band zone (there is not a significant SCN À anion-Pb interaction that could cause a decrease in the density of Pb states in conduction band); the valence band, in contrast with the MAPI compound, shows a major SCN À contribution suggesting that the SCN À anion participates in Van Der Waals interactions between the 2D-layers in the MAPSI thin film. The PDOS for MAPSI taking into account SOC are shown in Figure S7, Supporting Information with the main change in DOS comparing to the non-relativistic approach is a noticeable decrease in density of states in higher conduction bands, which does not affect significantly the electronic conduction in MAPSI; apart from the expected band gap reduction when spin-orbit coupling is included in DFTbased calculations.…”

“…, who confirmed this observations by PL measurements for a SCN − ‐doped MAPI material. The decreased optical absorption is explained as reported by Tang, Yang, and Stroppa, who affirm that the optical absorption is weakened in MAPSI due to a decrease in the imaginary part of the complex dielectric function for MAPSI, as calculated by the authors using the VASP package software. Further analysis on this point can be read on the Supporting Information file.…”

“…One interesting key result from the XRD data is that the refined crystal structure presents a 2D‐layered type material [layers of successive Pb‐I octahedra (Pb‐atom center, I‐atoms equatorial) with SCN − ions both in top and in bottom positions (axial positions), see Figure ], also as reported by Xiao . This reveals that the expected semiconductor band gap energy value must not be as low as in the original report of Ganose et al (1.57 eV), taking into account that a 2D‐layered structure material is expected to have a quantum confinement effect widening the band gap compared to similar 3D structures . MAPI, for example, has a 3D‐type structure and a band gap of 1.59 eV (confirmed by state‐of‐the‐art DFT calculations), so MAPSI must be of higher value for band gap.…”

“…(1.57 eV), taking into account that a 2D-layered structure material is expected to have a quantum confinement effect widening the band gap compared to similar 3D structures. [6,8] MAPI, for example, has a 3D-type structure and a band gap of 1.59 eV (confirmed by state-of-the-art DFT calculations [9] ), so MAPSI must be of higher value for band gap.…”

Influence of Sn/Ge Cation Exchange on Vacancy‐Ordered Double Perovskite Cs2Sn(1−x)GexI6: A First‐Principles Theoretical Study

“…Doping is a useful strategy to improve the properties of CH 3 NH 3 PbI 3 perovskite . Doping strategy has also been used in low dimensional perovskites to tune their properties . Recently, Cheng et al have successfully synthesized a series of 2D mixed Ge‐Sn perovskites, which shows that doping Ge into the Sn‐based perovskite is suitable.…”