The stability of perovskite solar cell materials in the natural environment is crucial to their commercial application. Understanding the detailed interaction mechanism between the air or water molecules (H2O) and the perovskite materials is a key way to explore their stabilities. In this study, we evaluate the geometric stabilities and thermodynamic stabilities of the lead-free Cs2B'BiCl6 (B' = Li, Na, K) double perovskite materials, and further explore the effect of air and H2O molecules on the stability and degradation of the Cs2B'BiCl6 double perovskite materials. The calculated results indicate that the pristine Cs2B'BiCl6 materials possess good geometric stabilities and thermodynamic stabilities. Under natural environment, the N2, O2, and CO2 gas molecules hardly penetrate into Cs2LiBiCl6 and Cs2NaBiCl6 because of positive absorption energies, while N2 and O2 can easily penetrate into Cs2KBiCl6 due to negative absorption energies. In contrast, the H2O molecules can easily penetrate into all Cs2B'BiCl6 double perovskite materials with negative absorption energies. Furthermore, the penetration of H2O molecules causes structural deformations of the double perovskites and the resultant rotation or rupture of some octahedra. Further simulations of H2O molecules adsorption on the surface of the double perovskites reveal that the adsorption energies of H2O molecules at all adsorption sites are negative, indicating that the Cs2B'BiCl6 double perovskite materials are prone to absorb H2O molecules in a humid environment. Therefore, the H2O molecules are the main factor affecting the stability and degradation of all Cs2B'BiCl6 double perovskite materials. Additionally, Cs2KBiCl6 shows much worse stability, which is responsible for the scarcely experimental reports on its synthesis and characterization.

The tin dioxide (SnO2) layer is commonly used as a traditional electron transport layer (ETL) in perovskite solar cells. However, it exists numerous defects in interior and on surface, diminishing the electron transport rate and causing energy level mismatches, thereby limiting the photoelectric conversion efficiency (PCE). In this study, cadmium p-aminobenzoate (PABACd) is synthesized using the displacement reaction and applied to modify the interface between the SnO2 ETL and CsPbI2Br film to achieve bidirectional passivation. Cd2+ effectively passivates defects in the ETL and penetrates perovskite crystals, contributing to defect passivation. Furthermore, PABA(-) stabilizes the [PbX6](4-) octahedron, enhancing the stability of CsPbI2Br perovskite solar cells (PSCs). As a result of these interactions, the PABACd-optimized device achieved a maximum PCE of 14.34 % and an outstanding open-circuit voltage of 1.27 V. Simultaneously, the PCE of the optimized CsPbI2Br PSCs remains at 92 % of the initial efficiency after 30 days of aging in an air environment with 15-20 % humidity.

The stability of perovskite solar cell materials in the natural environment is crucial to their commercial application. Understanding the detailed interaction mechanism between the air or water molecules (H2O) 2 O) and the perovskite materials is a key way to explore their stabilities. In this study, we evaluate the geometric stabilities and thermodynamic stabilities of the lead-free Cs2B'BiCl6 2 B ' BiCl 6 (B' B ' = Li, Na, K) double perovskite materials, and further explore the effect of air and H2O 2 O molecules on the stability and degradation of the Cs2B'BiCl6 2 B ' BiCl 6 double perovskite materials. The calculated results indicate that the pristine Cs2B'BiCl6 2 B ' BiCl 6 materials possess good geometric stabilities and thermodynamic stabilities. Under natural environment, the N2, 2 , O2, 2 , and CO2 2 gas molecules hardly penetrate into Cs2LiBiCl6 2 LiBiCl 6 and Cs2NaBiCl6 2 NaBiCl 6 because of positive absorption energies, while N2 2 and O2 2 can easily penetrate into Cs2KBiCl6 2 KBiCl 6 due to negative absorption energies. In contrast, the H2O 2 O molecules can easily penetrate into all Cs2B'BiCl6 2 B ' BiCl 6 double perovskite materials with negative absorption energies. Furthermore, the penetration of H2O 2 O molecules causes structural deformations of the double perovskites and the resultant rotation or rupture of some octahedra. Further simulations of H2O 2 O molecules adsorption on the surface of the double perovskites reveal that the adsorption energies of H2O 2 O molecules at all adsorption sites are negative, indicating that the Cs2B'BiCl6 2 B ' BiCl 6 double perovskite materials are prone to absorb H2O 2 O molecules in a humid environment. Therefore, the H2O 2 O molecules are the main factor affecting the stability and degradation of all Cs2B'BiCl6 2 B ' BiCl 6 double perovskite materials. Additionally, Cs2KBiCl6 2 KBiCl 6 shows much worse stability, which is responsible for the scarcely experimental reports on its synthesis and characterization.

Detecting toxic gas molecules is of great significancefor industryand public health. However, how to select and design highly sensitiveand selective gas sensors has been a great challenge. In the presentstudy, we explore the adsorption properties of monolayer PdS2 for the NH3, N2O, NO2, NO, CO,H2S, SO2, SO2F2, and SOF2 toxic gas molecules by first-principles calculations. Bycombined analyses of adsorption energy, adsorption distance, chargetransfer, electronic structure, work function, and recovery time,the selectivity and sensitivity of the monolayer PdS2 toNO(2), NO, and SO2 toxic gas molecules are identified.The adsorptions of NO2, NO, and SO2 lead toobvious changes of the electronic structure and charge transfer forthe monolayer PdS2, which are further analyzed based onthe frontier molecular orbitals theory. Additionally, NO adsorptioneffectively leads the work function of monolayer PdS2 toreduce by 17%, and the recovery time of the monolayer PdS2 sensor at room temperature is estimated to be considerably shortfor NO2, NO, and SO2 gases. The present studynot only uncovered that the monolayer PdS2 can be usedas a promising reusable sensing material featured with high selectivityand excellent sensitivity for environmentally hazardous gas moleculesbut also proposed a strategy for screening suitable sensing materialby comparing the frontier molecular orbitals of the gas moleculeswith the band edges of the candidate adsorption materials, which willattract more attention from the wide audience working in the low-dimensionalmaterial system.

As alternatives to lead-based halide perovskite materials, the lead-free halide doubleperovskite materials have many advantages but often show wide indirect band gaps and exceed the optimum band-gap range of 0.9–1.6 eV. Based on first-principles calculations, we scan a serial of halide double-perovskites Cs2B'B''X6 (B' = Li, Na, K; B'' = In, Bi; X= Cl, Br, I) and find that most of Cs2B'B''X6 double-perovskite materials have direct band gaps but larger than 1.6 eV, while the direct band gaps of Cs2LiInI6 and Cs2NaInI6 are smaller than 0.9 eV. We modulate the electronic structure of Cs2B'InI6 (B' = Li, Na, K) by an ionic doping strategy through substituting partial In ions with Bi ions and discover more halide double-perovskite solar cell materials with suitable band gap. By exploring the stability, electronic structures and optical properties of the doped double-perovskite Cs2B'In1-xBixI6 (B' = Li, Na, K, x = 0.25 and 0.75), we discover the Cs2B'In0.75Bi0.25I6 (B' = Li, Na and K) DP materials show direct band gaps within the optimal band-gap range of 0.90−1.60 eV for promising solar-cell materials. Meanwhile, the doped Cs2B'In1-xBixI6 DP materials show good carrier mobility as high as 103 cm2S-1V-1. Moreover, the predicted new materials exhibit excellent light absorption coefficients of 106 cm−1 in the visible light range. These new lead-free inorganic DP materials may offer great promise as candidates for highly efficient solar absorber materials for photovoltaic applications.

Monolayer palladium diselenide (PdSe2) has attracted increasing attention due to its outstanding physicochemical properties and puckered pentagonal layered structure. In this study, the adsorption of nitrogen-containing gases on the surface of monolayer PdSe2 has been investigated by first-principles calculations to fully explore its gas-sensing capability. It is found that NH3 and N2O show physisorption, while NO2 and NO prefer chemisorption, indicating excellent selectivity of monolayer PdSe2. Furthermore, the NH3 and N2O (NO2 and NO) adsorption exhibits a negligible (significant) amount of charge transfer, which in turn leads to a tiny (considerable) change in the electronic structure and magnetic property of the adsorbent surface. In particular, the adsorption of NO2 and NO introduces additional states around the Fermi level, which induces a dramatic decrease of the band gap and appearance of magnetic moments, and further leads to a huge change of the electrical conductivity of monolayer PdSe2. Additionally, the recovery time of the PdSe2 sensor at room temperature is estimated to be considerably short for NO2 and NO. Overall, the present work reveals that two-dimensional monolayer PdSe2 can be used as a promising reusable sensing material featured with high selectivity and excellent sensitivity for environmentally hazardous gas molecules.

Based on first-principles calculations, we explored the interplay between stacking effect and electron-electron correlation in the layered vdW material of bulk 1T-NbS2 with a 2D charge density wave (CDW) order. Without considering the Coulomb correlation, two energetically favorable out-of-plane stacking configurations are identified: one is a metallic phase with a single-layer stacking pattern, another is a band insulator with a paired-bilayer stacking configuration. Even though the Coulomb correlation is taken into account, the two energetic favorable stacking orders are still far more stable than other stacking orders. Furthermore, increasing the Coulomb interaction, the paired-bilayer stacking configuration transforms from nonmagnetic band insulator to antiferromagnetic insulator, while the single-layer stacking undergoes a Slater-Mott metal-insulator transition, which indicates the non-negligible role of electron-electron correlation interactions. In addition, the electronic structure and magnetic ground state change drastically among different stacking configurations, providing a platform to tune the electronic structures and interlayer magnetic interactions by altering the stacking order. In contrast to the widely accepted scenario of Mott localization as the driving force behind the gap formation in the CDW phase of layered transition metal dichalcogenides, our results not only highlight the crucial role of stacking order in the electronic structures of 1T-NbS2, but also shed fresh light on the distinct effects of Coulomb interaction in different stacking arrangements.

Although lead-based perovskite solar cells have achieved more than 25％power conversion efficiency,the toxicity of lead and instability are still urgent problems faced in industrial application.Lead-free halide double perovskite(DP)materials are promising candidates to resolve these issues.Based on the density functional theory,we explore the geomet-ric stability,thermodynamic stability,mechanical stability,electronic structures,and optical properties of the Cs2B'BiI6(B'=Li,Na and K)DP materials.By analyzing the tolerance factor and octahedral factor,we find the geometric stabilities of Cs2NaBiI6 and Cs2KBiI6 DPs are better than Cs2LiBiI6.By calculating the total energy,formation energy and decom-position energy,we propose that the most favorable structure of Cs2B'BiI6 is the orthorhombic phase,and Cs2LiBiI6 is less stable relative to the other two counterparts from an energetic viewpoint.Mechanical stability evaluations reveal that the orthorhombic Cs2LiBiI6 material is less stable relative to the isostructural Cs2NaBiI6 and Cs2KBiI6 DPs.The mechani-cal property calculations indicate that the Cs2B'BiI6 DPs possess good ductility,which can be used as flexible materials.Electronic structures and optical property calculations show that the orthorhombic Cs2B'BiI6 DPs have suitable band gap values,weaker exciton binding energies,and excellent optical absorption performance in the visible-light range.Based on the above comprehensive assessments,we can conclude that the orthorhombic Cs2NaBiI6 and Cs2KBiI6 DPs with good stability are promising candidates for solar cell applications.

A new bismuth-based halide double perovskite Cs2LiBiCl6 was isolated successfully using solid-state reactions. The crystal structure was investigated using X-ray diffraction and complemented by Li-7 solid-state nuclear magnetic resonance spectros-copy, which indicated the highly ordered nature of Li and Bi ions in the B sublattice of double perovskite. Compared with the Na analogue, the more moisture-sensitive Cs2LiBiCl6 has a smaller indirect band gap of 3.15(2) eV and red-shift luminescence of around 612 nm as well as stronger intensities at Mn-2(+) doping. The stability mechanism of Cs2LiBiCl6 was discussed together with that of Cs2NaBiCl6 and their inaccessible Br counterparts based on the tolerance and octahedral factors and density functional theory (DFT) calculations. Owing to the small size of Li+ cations, Cs2LiBiCl6 has low tolerance and octahedral factors close to the boundary between the stable perovskite and nonperovskite compounds, which possess a small decomposition energy and thus easily coexist with other ternary phases. The Brbased compositions of Cs2LiBiBr6 and Cs2NaBiBr6 have even smaller octahedral factors and negative decomposition energies, and therefore their double perovskite polymorphs are destabilized. DFT simulations indicate that the moisture sensitivity of Cs2LiBiCl6 is related to the coordination preference of Li+ due to its smaller size and the steric effect from water molecules, in contrast with the larger Na+ showing a more stable octahedral configuration under the water incorporation. These mechanisms combining the geometry factors and decomposition energies provide new insights into the stability of halide double perovskites.