Crystal Systems and Lattice Parameters of CH3NH3Pb(I1–xBrx)3 Determined Using Single Crystals: Validity of Vegard’s Law

Band Gap Alteration of Halide Mixing in Hybrid Perovskites: A First-Principles Study with Statistical Analysis

Despite numerous studies on the band gap of three-dimensional halide perovskites using the first-principles calculations, there are still significant discrepancies between theoretical and experimental values. Various solutions have been proposed, such as employing a system-specific hybrid functional with varying degrees of exact exchange and explicitly incorporating spin-orbit coupling effects. Our research involved a comprehensive investigation of three typical lead-containing three-dimensional perovskites MAPbI3, MAPbBr3, and MAPbCl3 (MA = CH3NH3). Through a statistical analysis comparing mean absolute error (MAE) with experimental results, we demonstrated that the nonlocal van der Waals (vdW) density functional corrections (i.e., optB86b) yielded the most approximate lattice parameters in comparison to experimental values. Furthermore, based on these lattice parameters, the HSE06 hybrid functional is the optimal estimation of the band gap among all the options. Moreover, we investigated three sets of mixed three-dimensional halide perovskites by varying the halide component. This exploration contributes to the identification of MAPb(Br0.333I0.667)3 and MAPb(Cl0.333I0.667)3 as exhibiting the smallest band gap of 1.315 (1.867) eV and 1.313 (1.885) eV for PBE (HSE06), respectively. These band gaps were determined using the HSE06 method with the optimized lattice by PBE considering the optB86b corrections. The approach employed in this work produced a band gap trend closely aligned with experimental observations, underscoring the importance of adopting a reliable and material-independent computational strategy when screening new halide perovskite materials for optoelectronic applications.

Progressive Polytypism and Bandgap Tuning in Azetidinium Lead Halide Perovskites

Mixed halide azetidinium lead perovskites AzPbBr_3-xX_x (X = Cl or I) were obtained by mechanosynthesis. With varying halide composition from Cl^- to Br^- to I^-, the chloride and bromide analogues both form in the hexagonal 6H polytype while the iodide adopts the 9R polytype. An intermediate 4H polytype is observed for mixed Br/I compositions. Overall, the structure progresses from 6H to 4H to 9R perovskite polytype with varying halide composition. Rietveld refinement of the powder X-ray diffraction patterns revealed a linear variation in unit cell volume as a function of the average radius of the anion, which not only is observed within the solid solution of each polytype (according to Vegard's law) but also extends uniformly across all three polytypes. This is correlated to a progressive (linear) tuning of the bandgap from 3.43 to 2.00 eV. Regardless of halide, the family of azetidinium halide perovskite polytypes are highly stable, with no discernible change in properties over more than 6 months under ambient conditions.

Visualization of halide perovskite crystal growth processes by in situ heating WAXS measurements

We observed the crystallization dynamics of halide perovskite crystals (CH3NH3PbI3) by in situ heating wide-angle X-ray scattering measurements. As a result, we revealed that crystal growth occurs during the conversion of complexes to perovskite crystals.

Synthesis and first-principles study of structural, electronic and optical properties of tetragonal hybrid halobismuthathes [Py2(XK)]2[Bi2Br10−xIx]

Tuning the optical properties of solid solutions by variation of the halogen composition.

Control of Molecular Orientation of Spiro-OMeTAD on Substrates

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is utilized as a p-type semiconductor layer in perovskite solar cells and solid-state dye-sensitized solar cells. Spiro-OMeTAD has been known to have a spiro center, leading to a random orientation. Although the molecular orientation of organic semiconductor materials influences the conductivity, which is directly related to semiconductor device characteristics, the molecular orientation of spiro-OMeTAD has not been fully discussed. In this study, we prepared spiro-OMeTAD layers on various substrates and investigated their orientation by grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure (NEXAFS). Additionally, we demonstrated that the molecular orientation of spiro-OMeTAD could be controlled by changing their surface energies by changing the substrate materials. Consequently, we could improve the electrical conductivity by improving its molecular orientation. The results of this study provide a guideline for the preparation of organic semiconductor material layers using the wet-coating method.