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.

*Corresponding author. Email: wfei@zzu.edu.cn; kaiwang@jlu.edu.cn; hguo@zzu.edu.cn; This PDF file includes： Experimental Section Figure S1. Pressure-dependent Raman and IR spectra of (NH 4 ) 2 SeBr 6 Figure S2. IR spectra of (NH 4 ) 2 SeBr 6 at different pressures and the frequency shifts of these modes. Figure S3. Refinement lattice parameters and refinement statistics at ambient pressure and 14.26 GPa of (NH 4 ) 2 SeBr 6 . Figure S4. Rietveld refinements of (NH 4 ) 2 SeBr 6 at different pressures. Figure S5. The lattice parameters and unit cell volume of (NH 4 ) 2 SeBr 6 at different pressures Figure S6. Schematic illustrations of the band gap evolution of (NH 4  1mL of 57% (w/w) HBr, which was stirred for 30 min. This solution was slowly cooled to room temperature and placed in a freezer overnight. The resulted yellow powder was washed three times with hydrobromic acid and vacuum-dried overnight. [1] High pressure experiments were performed with a symmetric diamond anvil cell (DAC). The culet diameter of the diamond anvils was 400 μm.
The sample was loaded into a 150 μm diameter hole of the T301 steel gasket, which was preindented to a thickness of 40 μm. A small ruby chip was inserted into the sample compartment for in situ pressure calibration, utilizing the R1 ruby fluorescence method. Silicon oil was utilized as the pressure transmitting medium (PTM) for optical absorption and XRD experiments, while the argon was employed as PTM for Raman measurements. High-pressure absorption spectra were measured in the exciton absorption band region by a deuterium-halogen light source and recorded with an optical fiber spectrometer (Ocean Optics, QE65000). The transmission spectrum of silicon oil around the sample was subtracted as the background.
High-pressure Raman spectra were recorded using a spectrometer equipped with liquid nitrogen cooled CCD (iHR 550, Symphony II, Horiba Jobin Yvon). A 532 nm single-mode DPSS laser was utilized to excite the sample, and the output power was 10 mW. The resolution of the system was 1 cm -1 .
High-pressure IR measurements were conducted using a Nicolet iN10 FT-IR micro-spectrometer.
Computational Methodology. The first-principles calculations based on density functional theory (DFT) were performed using the plane-wave pseudopotential as implemented in the VASP code.
The electron-core interactions were described with the frozen-core projector-augmented wave pseudopotentials. The generalized gradient approximation formulated by Perdew, Burke, and Ernzerhof (PBE) as the exchange correlation functional with cutoff energies of 400 eV was chosen in all of our calculations. A reciprocal space sampling with a 6 6 6 and 8 8 6 k-points is set in the Brillouin zone for cubic and tetragonal phase. The total energy convergence criteria of 1.0 10 -5 eV and the force on each atom converge to 0.01 eV Å -1 while optimizing the geometric structure. Figure S1. Pressure-dependent Raman (a) and IR spectra (b) of (NH 4 ) 2 SeBr 6 . The interaction between halides and organic cation inside the inorganic cage is well known to influence the lattice parameters and structural memory effect. To effectively explore the interplay between the NH 4 + group and the electronegative Br atoms, we further perform in situ high pressure Raman and IR experiments. Under ambient condition, only three bands are observed. All bands pertain to Se-Br vibrations in the [SeBr 6 ] 2octahedra. The strong peaks at 157 (E g ) and 179 cm -1 (A 1g ) can be assigned to the symmetric Br-Se-Br stretching and asymmetric stretching modes, respectively. The low frequency peak at 92 cm -1 (F 2g ) can be assigned to the Se-Br asymmetric bending modes. [2,3] Moreover, all vibrational modes are found to shift to higher frequency with increased pressure, indicating the hardening of the chemical bonding during compression. Above 0.59 GPa, the intensity of the F 2g mode increases gradually. With increased pressure up to approximately 11.05 GPa, except for the frequency shifts, additional splitting of the Se-Br bending and stretching modes is also observed, as indicated by the arrows. Such observation can be assigned to the rotation and distortion of [SeBr 6 ] 2octahedra, directly supporting the phase transition of (NH 4 ) 2 SeBr 6 from cubic to tetragonal phase. Except for the phase transition, all Raman bands exhibit normal blue shift. The IR spectra of (NH 4 ) 2 SeBr 6 show one broad band in 1394 cm -1 and three discernable bands between 3050 and 3400 cm -1 , which can be assigned to the N-H bending mode and N-H stretching mode, respectively. [4] Figure S2. (a-b) IR spectra of (NH 4 ) 2 SeBr 6 at different pressures and (c) the frequency shifts of these modes. Most of IR modes are found to continuously red shift at different rates. The red shift suggests that the N-H distance is extended, resulting in the strengthening of N-H…Br hydrogen bond under high pressure. With increased pressure, the N-H bending mode split into two modes at 11.98 GPa, leading to a distortion and rotation angle for the [SeBr 6 ] 2octahedron due to the hydrogen bond interaction. [5,6] Hence, the organic cation has a strong influence on the rotation and distortion of [SeBr 6 ] 2octahedron and in turn affects the stability of (NH 4 Figure S5. The lattice parameters and unit cell volume of (NH 4 ) 2 SeBr 6 at different pressures. Figure S6. Schematic illustrations of the band gap evolution of (NH 4 ) 2 SeBr 6 upon compression.
The bandgap of (NH 4 ) 2 SeBr 6 is highly variable based on the overlap of electronic wave functions between metal Se and halide Br ions. As for the Se-Br bond lengths and the intra-octahedral Br−Br distances, they are contracted as the [SeBr 6 ] 2octahedra compression are contracted with the increase in pressure, resulting in a broadening of valence and conduction bands due to their antibonding characteristics. However, the shortest Br-Br bonds of the cubic phase will definitely change from the intra-to inter-octahedra under mild pressure, which mainly influence the band gap of defect perovskites. Given the stiffness of the octahedra, the Se-Br bond is considerably stronger than the Br-Br bond either on the surface of the octahedra or between the isolated octahedra. Hence, VBM rises up greater that of the CBM, and consequently the bandgap is narrowed.