Crystal Systems and Lattice Parameters of CH₃NH₃Pb(I_1–xBr_x)₃ Determined Using Single Crystals: Validity of Vegard’s Law

Abstract

Metal halide perovskites are promising materials for light absorbers in solar cell applications. Use of the Br/I system enables us to control band gap energy and improves the efficiency of solar cells. Precise knowledge of lattice parameters and band gap energies as functions of compositions are crucially important for developing the devices using those materials. In this study, we have determined lattice parameters and band gap energies of CH3NH3Pb­(I1–x Br x )3, one of the most intensively studied mix-halide perovskites, as functions of Br content x. We measured accurate Br contents and lattice parameters of CH3NH3Pb­(I1–x Br x )3 (0 ≤ x ≤ 1) using single-crystalline samples by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. The CH3NH3Pb­(I1–x Br x )3 crystal system is tetragonal for x ≤ 0.06 and cubic for x ≥ 0.08 at 300 K. Lattice parameters of CH3NH3Pb­(I1–x Br x )3 strictly follow Vegard’s law; i.e., they are linearly dependent on x. We give linear expressions of x of lattice parameters for the tetragonal and cubic phases of CH3NH3Pb­(I1–x Br x )3 at 300 K. We have shown that these expressions can be used for determining the Br contents of CH3NH3Pb­(I1–x Br x )3 polycrystalline thin-film samples based on XRD measurements and, in addition, demonstrated that XPS measurements on polycrystalline samples may be erroneous because of impure ingredients in the samples. Furthermore, we determined band gap energies of CH3NH3Pb­(I1–x Br x )3 (0 ≤ x ≤ 1) at room temperature using absorption spectra of polycrystalline thin films taking account of excitonic effects.

Gradient 1D/3D Perovskite Bilayer using 4‐tert‐Butylpyridinium Cation for Efficient and Stable Perovskite Solar Cells

“…The 2D–WAXS and 2D–GIWAXS patterns were collected at SPring‐8 on beamline BL19B2 with X‐ray energy of 12.39 keV ( λ = 1 Å) at an incident angle on the order of 4.0° and 0.08° through a Huber diffractometer, respectively. [ 60 ] The patterns were recorded using a 2D image detector (Pilatus 300 K). The surface morphological observations of the perovskite films were made by field‐emission SEM (FEI, Teneo SEM).…”

Strain Engineering in Halide Perovskites

“…28 In perovskites with different band gaps, large lattice misfits pose a significant challenge for heteroepitaxy; however, strain engineering could allow for the use of perovskites in similar applications to III−V semiconductors. 89 (C) Understanding of strain at the grain boundaries is needed to minimize grain boundary and defect formation. Single-crystalline films outperform polycrystalline films due to lower defect concentrations with the lack of grain boundaries.…”

“…(B) The minimal change in lattice constants between different III–V semiconductors allows for stable heteroepitaxythis, combined with the large band gap differences, led to its applications in electrically pumped lasing, with typical III–V laser heterostructures such as the cross-section shown . In perovskites with different band gaps, large lattice misfits pose a significant challenge for heteroepitaxy; however, strain engineering could allow for the use of perovskites in similar applications to III–V semiconductors . (C) Understanding of strain at the grain boundaries is needed to minimize grain boundary and defect formation…”

Progressive Polytypism and Bandgap Tuning in Azetidinium Lead Halide Perovskites

“…Varying the halide composition is a common strategy for tuning the bandgap in hybrid halide perovskites. [14][15][16] Yuiga et al 14 showed that the bandgap of 3C perovskite MAPbBrxI3-x (MA = methylammonium) varied quadratically from 1.65 to 2.38 eV with increasing Br content accompanied by a symmetry change from tetragonal to cubic. Gratia et al 16 reported a crystallization process with a progressive structural change from 2H, 4H and 6H to 3C depending on x in (FAPbI3)x(MAPbBr3)1−x (FA = formamidinium); however, DMSO solvent molecules were found to be present in the crystal lattice and it is unclear how their presence may affect the formation of the polytypes and their resulting band structure.…”