Due to their outstanding power conversion efficiency (PCE), lightweight, flexibility, and low manufacturing cost, perovskite solar cells (PSCs) attract significant attention as the most promising candidate for next generation solar cells. However, the issue of poor intrinsic stability of the absorber materials and operational stability of devices remains unsolved. In this study, first-principle calculations were performed on the CsPbX3 (X=Br or I) perovskites to provide insight into the structural, mechanical, vibrational, and electronic properties of CsPbBr3 and CsPbI3 for their applications as active layers of solar cells. We found that the calculated lattice parameters are in good agreement with the experimental results. CsPbBr3 and CsPbI3 have negative energy of formation, which implies that the materials are thermodynamically stable. The calculated band structures indicate that CsPbBr3 and CsPbI3 are semiconductors with direct bandgaps along R-symmetry point. Furthermore, cluster expansion and simulations of Monte-Carlo were performed to identify new possible CsPbBr1-xIx structures and evaluate the effect of temperature on the system. The ground-state search generated 26 new multi-component CsPbBr1-xIx structures, and the temperature-profile showed that the system mixes well at ~800K. Thus, the phase stability insight is crucial for the development of these promising perovskite solar cells.
The oxide garnet Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for Li-based batteries due to its high Li-ion conductivity and chemical stability with respect to Li metal anode. However, at room temperature, it crystallizes into a poorly Li-ion conductive tetragonal phase. To this end, supervalent cation doping has been an effective way to stabilize the highly conductive cubic phase and enhance the ionic conductivity of the tetragonal phase at room temperature, through the creation of lithium vacancies. Yet, the fundamental aspects regarding this supervalent substitution remain poorly understood. In this study, we have employed the first-principle calculations to offer a better understanding of the stabilization of tetragonal Li7La3Zr2O12 phases by determining the structural, mechanical, and electronic properties for high-conductivity LLZO composition. We find that the structural properties calculated are in good agreement which is within a 2% error of the experimentally measured results. The negative energy of formation for t-LLZO shows that the material is thermodynamically stable. The calculated Young’s modulus is in good agreement with the experimental observations, which indicates that the material is mechanically stable. Owing to its wide electrochemical stability, the calculated band structure of t-LLZO shows that the material is a wide and indirect magnetic separator with a g-symmetry point band gap which is in good agreement with the experimental observations. Therefore, the structural, mechanical, and electronic stability of t-LLZO provides better insight about the stability of the material and this capacitates further investigations associated with ionic conductivity of the pure and supervalently doped LLZO.The oxide garnet Li7La3Zr2O12(LLZO) is a promising solid electrolyte for Li-based batteries due to its high Li-ion conductivity and chemical stability with respect to Li metal anode. However, at room temperature, it crystallizes into a poorly Li-ion conductive tetragonal phase. To this end, supervalent cation doping has been an effective way to stabilize the highly conductive cubic phase and enhance the ionic conductivity of the tetragonal phase at room temperature, through the creation of lithium vacancies. Yet, the fundamental aspects regarding this supervalent substitution remain poorly understood. In this study, we have employed the first-principle calculations to offer a better understanding of the stabilization of tetragonal Li7La3Zr2O12 phases by determining the structural, mechanical, and electronic properties for high-conductivity LLZO composition. We find that the structural properties calculated are in good agreement which is within a 2% error of the experimentally measured results. The negative energy of formation for t-LLZO shows that the material is thermodynamically stable. The calculated Young’s modulus is in good agreement with the experimental observations, which indicates that the material is mechanically stable. Owing to its wide electrochemical stability, the calculated band structure of t-LLZO shows that the material is a wide and indirect magnetic separator with a g-symmetry point band gap which is in good agreement with the experimental observations. Therefore, the structural, mechanical, and electronic stability of t LLZO provides better insight about the stability of the material and this capacitates further investigations associated with ionic conductivity of the pure and supervalently doped LLZO.
Using first-principles calculations and cluster expansion, this study generates new possible stable phases of t-LLZO containing a supervalent cation, Ta, on the Zr-site as a way to enhance the conductivity of the tetragonal crystal structure. The Monte-Carlo simulation was then used to provide further insight into the behaviour of the Ta-doped phases as a function of temperature under the canonical ensemble. The cluster expansion binary ground-state diagram generated 28 new multi-component Li5La3Zr2-xTaxO12 structures that are miscible. It is found that all the structures are thermodynamically stable with negative enthalpy of formation. The Monte-Carlo temperature profile shows no phase separation, and the system mixes well at ~900K. Further density functional theory calculations were performed on the most stable generated Ta-doped LLZO structures to determine the structural properties of the structures for their application as active solid-state electrolytes. It is found that the generated structures exhibit good structural stability due to the smooth decrease in calculated lattice parameters, which indicates that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. Therefore, the findings provide a better understanding of the phase stability of the generated Ta-doped LLZO structures, which paves the way for further analysis of the rate of lithium-ion diffusion and the mobility of the lanthanum, zirconium, tantalum, and oxygen ions in the systems at high temperatures.
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