Phase transitions in a Cs2−xK1+xBiCl6 solid solution

Theoretical insight of stabilities and optoelectronic properties of double perovskite Cs2CuIrF6: Ab-initio calculations

CONTEXT

In this study, we predict the stability, elastic, electronic and optical properties of double perovskite (DP) Cs₂CuIrF₆. The detailed investigation of electronic structure and optical properties to find the suitability of DP Cs₂CuIrF₆ for device applications. From the structural optimization results, the stability of DP (Cs₂CuIrF₆) is in cubic order and belongs to the Fm-3 m space group (#225) with a nonmagnetic (NM) state. Additionally, the elastic results show that this DP is mechanically stable in a cubic and ductile manner. Further, we explain in detail the semiconducting nature of the proposed DP with the help of electronic structure and density of states (DOS). The electronic band gap of DP Cs₂CuIrF₆ is 0.72 eV (L_V-X_C). The optical part discussion, like the dielectric function ε, reflectivity R, refractive index n, absorption coefficient α and optical conductivity σ up to 13.00 eV. The studied compound is explored as a potential candidate for optoelectronic applications.

METHODS

The density functional theory (DFT) within generalized gradient approximation (GGA) scheme of Perdew, Burke and Ernzerhof (PBE) as implemented in Wien2k computational code is utilized to achieve stable structure, elastic, electronic and optical properties of this material. The dynamic stability of this material was studied using the finite displacement method implemented in the CASTEP computational code. The elastic results have been computed by the IRelast package implemented in the Wien2k computational code.

The effects of monovalent metal cations on the crystal and electronic structures of Cs2MBiCl6 (M = Ag, Cu, Na, K, Rb, and Cs) perovskites

Lead-halide perovskites have attracted much attention over the past decade, while two main issues, i.e., the lead-induced toxicity and materials' instability, limit their further practice in widespread applications. To overcome these limitations, an effective alternative is to design lead-free perovskite materials with the substitution of two divalent lead ions with a pair of monovalent and trivalent metal ions. However, fundamental physics and chemistry about how tuning material's composition affects the crystal phase, electronic band structures, and optoelectronic properties of the material have yet to be fully understood. In this work, we conducted a series of density functional theory calculations to explore the mechanism that how various monovalent metal ions influence the crystal and electronic structures of lead-free Cs₂MBiCl₆ perovskites. We found that the Cs₂MBiCl₆ (M = Ag, Cu, and Na) perovskites preferred a cubic double perovskite phase with low carrier effective masses, while the Cs₂MBiCl₆ (M = K, Rb, and Cs) perovskites favored a monoclinic phase with relatively high carrier effective masses. The different crystal phase preferences were attributed to the different radii of monovalent metal cations and the orbital hybridization between the metal and Cl ions. The calculation showed that all Cs₂MBiCl₆ perovskites studied here exhibited indirect bandgaps. Smaller bandgap energies for the perovskites with a cubic phase were calculated than those of the monoclinic phase counterparts. Charge density difference calculation and electron localization functional analysis were also conducted and revealed that the carrier mobility can be improved via changing the characteristics of metal-halide bonds through compositional and, thus, crystal structure tuning. Our study shown here sheds light on the future design and fabrication of various lead-free perovskite materials for optoelectronic applications.

Advances in lead-free double perovskite nanocrystals, engineering band-gaps and enhancing stability through composition tunability

In this topical review, we have focused on the recent advances made in the studies of lead-free perovskites in the bulk form and as nanocrystals. Substitution of lead in halide perovskites is essential to overcome the toxicity concerns and improve the relatively low stability of these materials. In lead-free double perovskites the unit cell is doubled and two divalent lead cations are replaced by mono and trivalent cations. The current main challenge with the double perovskite metal halides lies in overcoming their inherently indirect and disallowed optical transitions. In this review, we have discussed the recent discoveries made in the synthesis of these materials and highlighted how nanocrystals can serve as model systems to explore the schemes of cationic exchange, doping and alloying for engineering the electronic structure of double perovskites. In nanocrystals, the quantum confinement effects can modify the electronic structure and the resulting optical transition, thus increasing the absorption cross-section and emission, which are important properties for optoelectronic devices. Lastly, the enlarged surface to volume ratio in the nanocrystals adds a surface energy term that may enhance the stability of the metastable crystallographic phases. We have reviewed how the nanocrystal can provide information on phases that are inherently stable and investigated how the facile exchange reactions can help in achieving material compositions that are impossible to achieve by any other way. Finally, based on our recent synthetic experience, we have emphasized the similarities between lead-based and lead-free perovskite nanocrystals; we hope that our insight along with a summary of recent progress in this fast-growing field will help to expand the interest in lead-free perovskites towards a greener and brighter future.

Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation

Hybrid organic-inorganic halide perovskites with the prototype material of CH₃NH₃PbI₃ have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 20% achieved by their solar cells, two key issues-the poor device stabilities associated with their intrinsic material instability and the toxicity due to water-soluble Pb²⁺-need to be resolved before large-scale commercialization. Here, we address these issues by exploiting the strategy of cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells. The idea is to convert two divalent Pb²⁺ ions into one monovalent M⁺ and one trivalent M³⁺ ions, forming a rich class of quaternary halides in double-perovskite structure. We find through first-principles calculations this class of materials have good phase stability against decomposition and wide-range tunable optoelectronic properties. With photovoltaic-functionality-directed materials screening, we identify 11 optimal materials with intrinsic thermodynamic stability, suitable band gaps, small carrier effective masses, and low excitons binding energies as promising candidates to replace Pb-based photovoltaic absorbers in perovskite solar cells. The chemical trends of phase stabilities and electronic properties are also established for this class of materials, offering useful guidance for the development of perovskite solar cells fabricated with them.