Putting the Squeeze on Lead Iodide Perovskites: Pressure-Induced Effects To Tune Their Structural and Optoelectronic Behavior

Abstract

Lattice compression through hydrostatic pressure has emerged as an effective means of tuning the structural and optoelectronic properties of hybrid halide perovskites. In addition to external pressure, the local strain present in solution-processed thin films also causes significant heterogeneity in their photophysical properties. However, an atomistic understanding of structural changes of hybrid perovskites under pressure and their effects on the electronic landscape is required. Here, we use high level ab initio simulation techniques to explore the effect of lattice compression on the formamidinium (FA) lead iodide compound, FA1–x Cs x PbI3 (x = 0, 0.25). We show that, in response to applied pressure, the Pb–I bonds shorten, the PbI6 octahedra tilt anisotropically, and the rotational dynamics of the FA+ molecular cation are partially suppressed. Because of these structural distortions, the compressed perovskites exhibit band gaps that are narrower (red-shifted) and indirect with spin-split band edges. Furthermore, the shallow defect levels of intrinsic iodide defects transform to deep-level states with lattice compression. This work highlights the use of hydrostatic pressure as a powerful tool for systematically modifying the photovoltaic performance of halide perovskites.

Polymorphism in metal halide perovskites

“…Pressure-induced compression of M-X-M bonds results in a decrease in the bandgap due to increased overlap between Pb 6s and I 5p orbitals, increasing the valence band energy and decreasing the overall bandgap. 171,211 A linear positive correlation between Pb-I-Pb bond length and the bandgap, for example, has been reported for MAPbI 3 . 172 On the other hand, octahedral tilting reduces this overlap, resulting in an increase in bandgap at higher pressures.…”

Importance of tailoring lattice strain in halide perovskite crystals

“…The chemical pressure induced by employing foreign ions is represented as lattice strain in crystals 56,57 . The inhomogeneity in a mixed perovskite film occasionally leads to a vertical gradient in residual strain 35 .…”

Switchable Rashba anisotropy in layered hybrid organic–inorganic perovskite by hybrid improper ferroelectricity

“…In recent years, there has been an intense research effort surrounding the conversion of solar energy into electric power through photovoltaic cells [1][2][3] . Hybrid organic-inorganic perovskites (HOIPs) with composition ABX 3 (A = organic cation, such as [CH 3 NH 3 ] + ; B = divalent metal cation, such as Sn 2+ , Pb 2+ ; X = halogen anion) are proposed as a next generation of materials for solar cells and light-emitting diodes [4][5][6][7][8] . The increase in structural degrees of freedom from the inclusion of organic cations into a BX 3 inorganic framework enables structural distortions and columnar shifts which are otherwise forbidden in standard inorganic perovskites.…”