Pressure‐Induced Structural Evolution and Bandgap Optimization of Lead‐Free Halide Double Perovskite (NH₄)₂SeBr₆

Abstract

Lead‐free halide double perovskites (HDPs) are promising candidates for high‐performance solar cells because of their environmentally‐friendly property and chemical stability in air. The power conversion efficiency of HDPs‐based solar cells needs to be further improved before their commercialization in the market. It requires a thoughtful understanding of the correlation between their specific structure and property. Here, the structural and optical properties of an important HDP‐based (NH4)2SeBr6 are investigated under high pressure. A dramatic piezochromism is found with the increase in pressure. Optical absorption spectra reveal the pressure‐induced red‐shift in bandgap with two distinct anomalies at 6.57 and 11.18 GPa, and the energy tunability reaches 360 meV within 20.02 GPa. Combined with structural characterizations, Raman and infrared spectra, and theoretical calculations using density functional theory, results reveal that, the first anomaly is caused by the formation of a Br‐Br bond among the [SeBr6]2− octahedra, and the latter is attributed to a cubic‐to‐tetragonal phase transition. These results provide a clear correlation between the chemical bonding and optical properties of (NH4)2SeBr6. It is believed that the proposed strategy paves the way to optimize the optoelectronic properties of HDPs and further stimulate the development of next‐generation clear energy based on HDPs solar cells.

Semiconductor to metallic transition under induced pressure in Cs2AgBiBr6 double halide perovskite: a theoretical DFT study for photovoltaic and optoelectronic applications

“…[13][14][15] L. Wang et al reported on the inuence of lead halide perovskite CH(NH 2 ) 2 PbBr 3 and found that the structural phase is changed at 2.2 GPa. 16 Pressuredependent samples undergo band-gap energy shrinkage and the electron orbits move toward the electric eld. As a result, the bonding energy is changed within the octahedral state, which mostly affects the boundary conditions of the electronic wave functions and brings about a reduced band gap energy.…”

Strain Engineering of Metal Halide Perovskites on Coupling Anisotropic Behaviors

“…Similarly, Wang et al. [ 100 ] and Liang et al. [ 82 ] conducted DFT calculations and observed a distinct tendency of the bond length shortening in the (NH 4 ) 2 SeBr 6 and CsPbI 3 structures in response to pressure.…”

“…All experimental data are extracted from refs. [ 34–37,40–42,49,51,52,55,57,58,60,61,63–66,68,70,72,74,80,82,84,87–92,96–100 ] .…”

Pressure Effects on Lead‐Free Metal Halide Perovskites: a Route to Design Optimized Materials for Photovoltaics

“…[115] Selenium has also been considered as an alternative to lead, in particular in the vacancy ordered ðNH 4 Þ 2 SeBr 6 perovskite. [116] At room conditions the compound is cubic with space group Fm3m, and transforms to a tetragonal P42 phase above 11 GPa as a result of the rotation of the ½SeBr 6 2À octahedra. [116] The cubic and the tetragonal phases have bulk moduli of 22.28 and 131.91 GPa, respectively, suggesting that the tetragonal phase has a more robust structure that could contribute to the stability of the solar cell.…”

“…[116] The cubic and the tetragonal phases have bulk moduli of 22.28 and 131.91 GPa, respectively, suggesting that the tetragonal phase has a more robust structure that could contribute to the stability of the solar cell. [116] 3. From Fundamental Studies to Applications Despite the promising and interesting results obtained in situ, there is still a gap between fundamental studies and actual applications in operating devices.…”