Tuning band gap and enhancing optical functions of AGeF3 (A = K, Rb) under pressure for improved optoelectronic applications

High-entropy alloy screening for halide perovskites

As the concept of high-entropy alloying (HEA) extends beyond metals, new materials screening methods are needed. Halide perovskites (HP) are a prime case study because greater stability is needed for photovoltaics applications, and there are 322 experimentally observed HP end-members, which leads to more than 1057 potential alloys. We screen HEAHP by first calculating the configurational entropy of 106 equimolar alloys with experimentally observed end-members. To estimate enthalpy at low computational cost, we turn to the delta-lattice parameter approach, a well-known method for predicting III-V alloy miscibility. To generalize the approach for non-cubic crystals, we introduce the parameter of unit cell volume coefficient of variation (UCV), which does a good job of predicting the experimental HP miscibility data. We use plots of entropy stabilization versus UCV to screen promising alloys and identify 102 HEAHP of interest.

Exploring the influence of pressure-induced semiconductor-to-metal transition on the physical properties of cubic perovskites FrXCl3 (X = Ge and Sn)

Even though lead halide perovskites have outstanding physiochemical properties and improved power conversion efficiency, most of these compounds threaten their future commercialization because of their instability and highly toxic nature. Thus, it is preferable to use stable alternative elements rather than lead to make environmentally friendly perovskite material that will have comparable optical and electronic properties to those constructed from Pb-based perovskites. However, devices constructed from lead-free perovskites typically display a lower power conversion efficiency. Applying hydrostatic pressure could be deemed an effective method to alter the physical properties of these compounds. This not only improves their performance in application but also reveals significant correlations between structure and properties. This work uses DFT to investigate the structural, electronic, optical, and elastic properties of non-toxic, francium-based halide perovskites FrXCl3 (X = Ge, Sn) at different levels of hydrostatic pressures that vary from 0 to 10 GPa. The estimated structural parameter's strong correlation with the data from earlier studies ensures the accuracy of the current findings. Pressure causes the Fr-Cl and Ge (Sn)-Cl bonds to shorten and become stronger. The electronic property calculations demonstrated that both compounds are direct band-gap semiconductors. The application of pressure leads to a linear reduction in the band gap (semiconducting to metallic state) and raises the electronic density of states around the Fermi level by forcing the valence band electrons upward, indicating that the optoelectronic device's performance can be tuned and improved. The values of the dielectric constant, absorptivity and reflectivity showed an increasing tendency with pressure. As the pressure applied to the compounds increases, the absorption spectra show a redshift. These findings suggested that the FrXCl3 (X = Ge and Sn) compound becomes more appropriate for usage in optoelectronic applications under pressure. Furthermore, our examination of the mechanical properties indicates that both FrGeCl3 and FrSnCl3 exhibit mechanically stability, and ductility. Interestingly, we observe an increase in ductility as pressure levels rise.

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.

A DFT investigation of lead-free TlSnX3 (X = Cl, Br, or I) perovskites for potential applications in solar cells and thermoelectric devices

In the present study, the Density Functional Theory (DFT) was employed to computationally investigate the potential application of newly developed lead-free perovskites with the formula of TlSnX3 (X = Cl, Br, or I) as absorbers in the perovskite solar cells and as thermoelectric materials. The Quantum Espresso code was implemented to optimize the structural configuration of the perovskites and to compute a range of their properties, including their elasticity, electronic behavior, optical characteristics, and thermoelectric attributes. The findings indicated that these perovskite materials exhibit both chemical and structural stability and that TlSnBr3 and TlSnI3 perovskites possess high dynamic stability. The findings additionally revealed direct (R → R) band gap energy values of 0.87 eV for TlSnCl3, 0.52 eV for TlSnBr3, and 0.28 eV for TlSnI3 using the GGA-PBE functional. Further analysis of their elastic properties suggested that these materials are mechanically stable and displayed overall ductile behaviour. They also demonstrated remarkable optical properties, particularly a high absorption coefficient, ranging from 105 cm-1 to 106 cm-1. Consequently, it is reasonable to infer that these materials exhibit considerable potential for utilization in solar cells. Finally, the evaluation of their thermoelectric properties has revealed the highly promising potential of these materials to be employed in thermoelectric applications.

A Thorough Investigation of Electronic, Optical, Mechanical, and Thermodynamic Properties of Stable Glasslike Sodium Germanate under Compressive Hydrostatic Pressure: Ab Initio Study

In this paper, we have tried to elucidate the variation of structural, electronic, and thermodynamic properties of glasslike Na₂GeO₃ under compressive isotropic pressure within a framework of density functional theory (DFT). The result shows stable structural (orthorhombic → tetragonal) and electronic (indirect → direct) phase transitions at P ∼ 20 GPa. The electronic band gap transition plays a key role in the enhancement of optical properties. The results of the thermodynamic properties have shown that Na₂GeO₃ follows Debye's low-temperature specific heat law and the classical thermodynamic of the Dulong-Petit law at high temperature. The pressure sensitivity of the electronic properties led us to compute the piezoelectric tensor (both in relaxed and clamped ions). We have observed significant electric responses in the form of a piezoelectric coefficient under applied pressure. This property suggested that Na₂GeO₃ could be a potential material for energy harvest in future energy-efficient devices. As expected, Na₂GeO₃ becomes harder and harder under compressive pressure up to the phase transition pressure (∼20 GPa) which can be read from Pugh's ratio (k_H) > 1.75, however, at pressures above 20 GPa k_H < 1.75, which may be due to the formation of fractures at high pressure.

Molecular engineering of several butterfly-shaped hole transport materials containing dibenzo[b,d]thiophene core for perovskite photovoltaics

Several butterfly-shaped materials composed of dibenzo[b,d]thiophene (DBT) and dibenzo-dithiophene (DBT5) cores were designed as hole transporting materials (HTMs) and their properties were studied by density functional theory (DFT) computations for usage in mesoscopic n-i-p perovskite solar cells (PSCs). To choose suitable HTMs, it was displayed that both of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies of molecules were located higher than those of CH₃NH₃PbI₃ (MAPbI₃) perovskite as they were able to transfer holes from the MAPbI₃ toward Ag cathode. Negative solvation energy (ΔE_solvation) values for all HTMs (within the range of - 5.185 to - 18.140 kcal/mol) revealed their high solubility and stability within CH₂Cl₂ solvent. The DBT5-COMe demonstrated the lowest values of band gap (E_g = 3.544) and hardness (η = 1.772 eV) (the greatest chemical activity) and DBT5-CF₃ displayed the biggest η = 1.953 eV (maximum stability) that were predominantly valuable for effective HTMs. All HTMs presented appropriately high LHEs from 0.8793 to 0.9406. In addition, the DBT5 and DBT5-SH depicted the lowest exciton binding energy (E_b) values of 0.881 and 0.880 eV which confirmed they could produce satisfactory results for the PSCs assembled using these materials. The DBT5-SH and DBT5-H had maximum hole mobility (μ_h) values of 6.031 × 10^-2 and 1.140 × 10^-2 which were greater than those measured for the reference DBT5 molecule (μ_h = 3.984 × 10^-4 cm²/V/s) and about 10 and 100 times superior to the calculated and experimental μ_h values for well-known Spiro-OMeTAD. The DBT5-COOH illustrated the biggest open circuit voltage (V_OC), fill factor (FF) and power conversion efficiency (PCE) values of 1.166 eV, 0.896 and 23.707%, respectively, establishing it could be as the best HTM candidate for high performance PSCs.