Vacancy-ordered Double Perovskites Research Articles

Vacancy Effect on the Luminescent and Water Responsive Properties of Vacancy-Ordered Double Perovskite Derivatives.

Vacancy-ordered perovskites and derivatives represent an important subclass of hybrid metal halides with promise in applications including light emitting devices and photovoltaics. Understanding the vacancy-property relationship is crucial for designing related task-specific materials, yet research in this field remains sporadic. For the first time, we use the Connolly surface to quantitatively calculate the volume of vacancy (V□, □=vacancy) in vacancy-ordered double perovskite derivatives (VDPDs). A relationship between void fraction and the structure, photoluminescent properties and humidity stability was established based on zero-dimensional (0-D) [N(alkyl)4]2Sb□Cl5□'-type VDPDs. Compared with the more commonly studied A2M(IV)X6□-type double perovskite (A=cation, M=metal ion, X=halide), [N(alkyl)4]2Sb□Cl5□' features double vacancy sites. Our results demonstrate an inverse relationship between the photoluminescent quantum yield and V□ in 0-D VDPDs. Additionally, structural transformation from A2SbCl5 to A3Sb2Cl9 was first reported, during which the novel 'gate-opening' gas adsorption phenomenon was observed in VDPDs for the first time, as evidenced by 'S'-shaped sorption isotherms for water vapor, indicating a cation-controlled water-vapor response behavior. A mixed-cation strategy was developed to modulate the humidity stability of VDPDs. Characterized by controllable water-responsive behavior and unique 'on-off-on' luminescent switching, A2M(III)□X5□'-type materials show great promise for multi-level information anti-counterfeiting applications.

Tuning the Optical Absorption Edge of Vacancy-Ordered Double Perovskites through Metal Precursor and Solvent Selection.

Vacancy-ordered double perovskites with the formula A 2 MX 6 (where A is a +1 cation, M is a +4 metal, and X is a halide ion) offer improved ambient stability over other main-group halide AMX 3 perovskites and potentially reduced toxicity compared to those containing lead. These compounds are readily formed through a number of synthetic routes; however, the manner in which the synthetic route affects the resulting structure or optoelectronic properties has not been examined. Here, we investigate the role of distinct precursors and solvents in the formation of the indirect band gap vacancy-ordered double perovskite Cs2TeBr6. While Cs2TeBr6 can be synthesized from TeBr4 or TeO2, we find that synthesis from TeBr4 is more sensitive to solvent selection, requiring a polar solvent to enable the conversion of TeBr4. Synthesis from TeO2 proceeds in all of the organic solvents tested, provided that HBr is added to solubilize TeO2 and enable the formation of [TeBr6]2-. Furthermore, the choice of metal precursor and solvent impacts the product color and optical absorption edge, which we find arises from particle size effects. The emission energy remains unaffected, consistent with the idea that emission in these zero-dimensional structures arises from the isolated [TeBr6]2- octahedra, which undergo dynamic Jahn-Teller distortion rather than band-edge recombination. Our work highlights how even minor changes in synthetic procedures can lead to variability in metrics such as the absorption edge and emission lifetime and sheds light on how the optical properties of these semiconductors can be controlled for light-emitting applications.

Excitonic and optoelectronic investigations of bromide-chloride mixed vacancy-ordered double tellurium perovskite systems

Metal-halide perovskites, including vacancy-ordered double perovskites, are versatile semiconductors with applications in photovoltaics and sensors. Here we explore the structural stability, electronic and optical properties of Cs2Te(Br1-xClx)6 series using first-principles-based on density functional theory and state-of-the-art many-body perturbation theory. Our study reveals consistent energy levels across different crystal structures obtained by varying Cl concentrations, indicating uniform structural stability. By employing hybrid density functional theory with spin-orbit coupling, we analyze the effect of Cl-site substitutions on electronic and optical properties. We observe a quasi-linear relationship between band gap and Cl concentration in Cs2Te(Br1-xClx)6 compositions, with alloying resulting in a redshift in the optical absorption coefficient. Moreover, we explore electron-hole interactions using the GW approximation and Bethe−Salpeter equation, revealing exciton binding energies and dark-bright splitting. Remarkably, exciton binding energies, reaching up to 930 meV, can be finely tuned through substitutional engineering. Finally, we investigate the thermoelectric properties of the Br/Cl mixed halide double perovskites, suggesting promising prospects for their application as thermoelectric materials.

Molecular Origins of Near-Infrared Luminescence in Molybdenum and Tungsten Oxyhalide Perovskites.

Materials with near-infrared (near-IR) luminescence are desirable for applications in communications and sensing, as well as biomedical diagnostics and imaging. The most used inorganic near-IR emitters rely on precise doping of host crystal structures with select rare-earth or transition metal ions. Recently, another class of materials with intrinsic near-IR emission has been reported. The compositions of these materials were initially described as vacancy-ordered halide double perovskites Cs2MoCl6 and Cs2WCl6, but further investigation by some of us on the compound reported as Cs2WCl6 revealed an oxyhalide instead, with a composition Cs2WO x Cl6-x , where 1 < x < 2. Here we demonstrate that the Mo compounds similarly possess the composition Cs2MoO x Cl6-x or Cs2MoO x Br6-x where 1 < x < 2. Preparing the pure halide appears harder for Mo than for W, and we have not succeeded in doing so. The distinctly different composition requires the coordination environment and oxidation state for the Mo and W centers to be reconsidered from what was assumed for the pure halides. In this work, we examine the mechanism for near-IR emission in these materials given their true structures and compositions. We demonstrate that the luminescence is due to the specific d-orbital splitting caused by the presence of oxygen in the distorted [MOX5]2- octahedra (X is Cl or Br). The fine structure in the emission spectra at low temperatures has been resolved and is attributed to vibronic coupling to the Mo-O and W-O bond stretches. Understanding the true structure and composition of these interesting materials, besides explaining the near-IR luminescence, suggests how this desirable emission can be realized and manipulated.

Expanding the Family of Magnetic Vacancy-Ordered Halide Double Perovskites

Expanding the Family of Magnetic Vacancy-Ordered Halide Double Perovskites

Manipulating the Crystallization of Tin Halide Perovskites for Efficient Moisture-to-Electricity Conversion.

Manipulating the crystallization of perovskite in thin films is essential for the fabrication of any thin-film-based devices. Fabricating tin-based perovskite films from solution poses difficulties because tin tends to crystallize faster than the commonly used lead perovskite. To achieve optimal device performance in solar cells, the preferred method involves depositing tin perovskite under inert conditions using dimethyl sulfoxide (DMSO), which effectively retards the formation of the tin-bromine network, which is crucial for perovskite assembly. We found that under ambient conditions, a DMSO-based tin perovskite salt solution resulted in the formation of a two-phase system, SnBr4(DMSO)2 and MABr, whereas a dimethylformamide-based solution resulted in the formation of vacancy-ordered double perovskite MA2SnBr6. Humidity is known to solvate MABr to form the solvated ions, and so we used the two-phase system for the application in moisture to electricity conversion. The importance of the presence of the scaffold can be seen with the negligible power output from the vacancy-ordered double perovskite obtained with MA2SnBr6. We have fabricated a device with two-phase system that can generate an open-circuit potential of 520 mV and a short-circuit current density of 30.625 μA/cm2 at 85% RH. Also, the device charges a 10 μF capacitor from 150 mV at 51% RH to 500 mV at 85% RH in 6 s at a rate of 52.5 mV/s. Moreover, the output can be scaled by connecting devices in series and parallel configurations. A 527 nm green LED was powered by connecting five devices in series at 75% RH. This indicates a potential for utilizing these moisture-to-electricity conversion devices in powering low-energy requirement devices.

Vacancy-ordered CsRbGeCl6 and CsRbGeBr6 perovskites as new promising non-toxic materials for photovoltaic applications: A DFT investigation

In this study, we have proposed novel vacancy-ordered double perovskites, CsRbGeCl6 and CsRbGeBr6, and conducted computational analyses to investigate their mechanical, optoelectronic and thermoelectric properties, for the first time. All calculations are performed using the Density Functional Theory (DFT) as implemented in the Quantum Espresso package. The two materials exhibit considerable chemical and mechanical stability, with CsRbGeCl6 additionally exhibiting thermodynamic stability. Using the PBE (SCAN) functional, the calculated band gap energies for CsRbGeCl6 and CsRbGeBr6 perovskites are 2.17 eV (2.57 eV) and 0.88 eV (1.23 eV), respectively. Consequently, the perovskites, particularly CsRbGeBr6, possess the optimal band gap energy for photovoltaic applications. Their excellent electronic properties are further supported by their remarkable optical properties spanning from visible to ultraviolet region, characterized by high absorption coefficients (up to 106 cm−1) and minimal reflectivity (below 23 %). Moreover, the analysis of their thermoelectric properties revealed that they exhibit the required characteristics as thermoelectric materials.

Novel vacancy-ordered RbKGeCl6 and RbKGeBr6 double perovskites for optoelectronic and thermoelectric applications: an ab-initio DFT study

The present study examines the key characteristics of new vacancy-ordered halide double perovskites, RbKGeCl6 and RbKGeBr6, encompassing the elastic, structural, mechanical, optoelectronic, and thermoelectric properties. The Density Functional Theory (DFT) was employed to perform the calculation of the properties, facilitating the evaluation of their potential applications in optoelectronic and thermoelectric devices. The DFT calculation was conducted using the Quantum Espresso package alongside the thermo_pw tool and the BoltzTraP codes. The results revealed that the two proposed compounds possess both chemical and mechanical stability with optimized lattice constants recorded at 10.14 Å and 10.72 Å for RbKGeCl6 and RbKGeBr6, respectively. The evaluation of the elastic properties of the materials suggested reasonably high mechanical moduli of the materials. Based on the calculated electronic properties, the materials are classified as direct gap semiconductors, with energy gap values of 2.11 eV for RbKGeCl6 and 0.80 eV for RbKGeBr6 using the GGA-PBE functional. Furthermore, the use of the SCAN approximation yields more reliable energy gap of 2.51 eV and 1.08 eV for the respective compounds. The materials exhibited a high absorption coefficient and a significantly low reflectivity within the visible-ultraviolet energy spectrum. These findings strongly suggest the promising properties of the materials under study for optoelectronic applications. Furthermore, the calculated thermoelectric properties of the materials, particularly the figure of merit, revealed the materials’ potential use as thermoelectric materials. The calculated figure of merit values of RbKGeCl6 and RbKGeBr6 were found to range from 0.73 to 0.75, respectively, between 300 K and 800 K. Despite being lower, these values are comparable to those of some well-established materials including SiGe alloys (0.95), Bi2Te3 (≈0.90), and PbTe (≈0.80).

Effect of the spin-orbit coupling and Hubbard corrections in predicting the electronic and magnetic properties of vacancy-ordered double perovskites A2WCl6 (A = Rb and Cs)

The structural, elastic, electronic, and magnetic properties of the vacancy ordered double perovskite compounds A2WCl6 (A = Rb, Cs) have been investigated within the full-potential linearized augmented plane waves (FP-LAPW) method. The structural properties in the cubic phase were examined and are consistent with the available experimental results. The materials are more stable in the ferromagnetic phase. The mechanical, dynamical and the thermodynamical stability have also been verified. The electronic and magnetic properties were first calculated by GGA-WC approximation, the studied materials are found to be half-metallic. Including Hubbard correction (U), the W-d states move towards higher energy enlarging the band gap in the spin-down channel. The spin–orbit coupling (SOC) was added to GGA-WC+U which reveal that these compounds behave as semiconductors in both spin channels. The density of states showed that the valence band is mainly characterized by the W-d together with Cl-p states while the conduction band is dominated by W-d and (Cs/Rb)-d states. The calculated properties present semiconducting ferromagnetic nature that is considered as necessary factor in spintronics applications.

A spin-polarized DFT analysis of the physical attributes of vacancy ordered Rb2TcCl6 double perovskite for optoelectronic and spintronics

Due to their versatile properties, vacancy-ordered double perovskites offer a wide range of potential uses due to their versatile properties. We have thoroughly examined the physical characteristics of vacancy-ordered Rb2TcCl6 in its cubic phase by employing the spin-polarized computations. The ground state energy and optimum structural parameters are obtained in order to assess the structural stability, which is further validated using the tolerance factor and formation energy values. The ductile character of Rb2TcCl6 is revealed by a comprehensive evaluation of mechanical properties. Vickers hardness factor suggests that Rb2TcCl6 can withstand against pressure-induced disturbance more strongly. A direct bandgap for spin-up is measured up to 2.33 eV, while for spin-dn it is 2.02 eV. According to the magnetic moments of 2.69 μB, Rb2TcCl6 is an attractive material for spintronic devices. A number of optical parameters are determined. For optoelectronic applications, Rb2TcCl6 is an excellent choice due to its strong polarization in the visible and ultraviolet regions, as shown by its optical characteristics.

Flexibility potential of Cs2BX6 (B = Hf, Sn, Pt, Zr, Ti; X = I, Br, Cl) with application in photovoltaic devices and radiation detectors

As interest in double perovskites is growing, especially in applications like photovoltaic devices, understanding their mechanical properties is vital for device durability. Despite extensive exploration of structure and optical properties, research on mechanical aspects is limited. This article builds a vacancy-ordered double perovskite model, employing first-principles calculations to analyze mechanical, bonding, electronic, and optical properties. Results show Cs2HfI6, Cs2SnBr6, Cs2SnI6, and Cs2PtBr6 have Young's moduli below 13 GPa, indicating flexibility. Geometric parameters explain flexibility variations with the changes of B and X site composition. Bonding characteristic exploration reveals the influence of B and X site electronegativity on mechanical strength. Cs2SnBr6 and Cs2PtBr6 are suitable for solar cells, while Cs2HfI6 and Cs2TiCl6 show potential for semi-transparent solar cells. Optical property calculations highlight the high light absorption coefficients of up to 3.5×105 cm−1 for Cs2HfI6 and Cs2TiCl6. Solar cell simulation shows Cs2PtBr6 achieves 22.4% of conversion efficiency. Cs2ZrCl6 holds promise for ionizing radiation detection with its 3.68 eV bandgap and high absorption coefficient. Vacancy-ordered double perovskites offer superior flexibility, providing valuable insights for designing stable and flexible devices. This understanding enhances the development of functional devices based on these perovskites, especially for applications requiring high stability and flexibility.

Electronic, optical, and thermoelectric properties of vacancy-ordered double perovskite K2SnX6 (X = Cl, Br, I) from first-principle calculations

Abstract The present study explores the structural, optoelectronic, and thermoelectric properties of potassium tin halide vacancy-ordered double perovskites K2SnX6 (X = Cl, Br, and I) in their stable monoclinic phase. Our study uses first-principles calculations based on density functional theory (DFT). Electronic band structures reveal direct band gaps for K2SnCl6 and K2SnBr6, while K2SnI6 exhibits an indirect band gap. Theoretical computations utilising the modified Becke-Johnson potential (mBJ-GGA) demonstrate that the optical band gaps of K2SnCl6, K2SnBr6, and K2SnI6 decrease in the following order: 2.581 eV, 1.707 eV, and 4.126 eV, respectively. These values render the materials suitable for photovoltaic applications. Analysis of dielectric functions, absorption coefficients, and refractive indices demonstrates their potential as light-absorbing materials. We evaluate the thermoelectric properties, including electronic and lattice thermal conductivities, Seebeck coefficients, and power factors, which lead to favorable thermoelectric performance. The maximum figure of merit (ZT) values of 0.58, 0.69, and 0.50 are achieved for K2SnCl6, K2SnBr6, and K2SnI6, respectively, at 500 K. These findings highlight the potential of these materials for applications in solar cells and thermoelectric devices, emphasising their effectiveness at elevated temperatures.

Physical properties of vacancy-ordered double perovskites K2TcZ6 (Z = Cl, Br) for spintronics applications: DFT calculations.

Vacancy-ordered double perovskites (DPs) are emerging materials for spintronics due to their stable structures and non-toxic properties. In this study, we conducted a comprehensive investigation into the role of 4d electrons in Tc to understand their impact on the ferromagnetic properties of K2TcY6 (Y = Cl, Br). We have employed a modified Back and Johnson potential to assess electronic and magnetic characteristics and utilized the BoltzTraP code to investigate thermoelectric effects. Experimental lattice constants confirmed the presence of stable structures and formation energy estimates affirmed their thermodynamic stability. The Heisenberg model and density of electron states (DOS) at the Fermi level provides insights into Curie temperature and spin polarization. The presence of ferromagnetism is evident in the density of states, reflecting the transition of electron spins that support the exchange mechanism. The study delves into how electron functionality influences the control of ferromagnetism, considering exchange constants, exchange energies, hybridization process and the crystal field energies. Moreover, the exploitation of magnetic moments from Tc to K and Cl/Br sites takes precedence in driving ferromagnetism by exchanging electron spins rather than forming magnetic clusters. Additionally, to explore the optical characteristics of the compounds, we investigated their optical absorption, dielectric constants and refractive index within the energy range of 0-10 eV, ensuring absorption across both the visible and ultraviolet regions. Finally, we delve into the impact of the thermoelectric effect on both thermoelectric performance and spin functionality, taking into account factors such as the Seebeck coefficient, power factor, and electronic conductivity.

A universal supramolecular assembly strategy for achieving efficient tunable white emission and anti-counterfeiting in antimony doped tin(iv)-based vacancy-ordered double perovskites

Three compounds with high PLQY, (AC)2SnCl6:Sb3+ (A = K, Rb, and Cs), were developed using a supramolecular assembly strategy. The samples doped with Sb3+ were used to achieve tunable WLEDs, anti-counterfeiting, and information encryption.

Exploring half-metallic ferromagnetism and thermoelectric properties of Tl2WX6 (X = Cl and Br) double perovskites.

Half-metallic semiconductors typically exhibit 100% spin polarization at the Fermi level which makes them desired materials for spintronic applications. In this study, we reported a half-metallic ferromagnetic nature in vacancy-ordered double perovskites Tl2WX6 (X = Cl and Br). The magnetic, electronic, and thermoelectric properties of the material are studied by the use of density functional theory (DFT). For the calculations of exchange-correlation potential, PBE-sol is employed while more accurate electronic band structure and density of states (DOS) are calculated by the mBJ potential. Both materials exhibited structural stability in the cubic structure with Fm3̄m space-group. The mechanical stability is confirmed by their computed elastic constants while their thermodynamic stability is attested by negative formation energy. The spin-based volume optimization suggested the ferromagnetic nature of the materials which is further confirmed by the negative value of the exchange energy Δ x(pd). Moreover, computed magnetic moment value for Tl2WCl6 and Tl2WBr6 is 2 μB and the majority of this comes from W. The spin-polarized band structure and DOS confirmed that both materials are half-metallic and at the Fermi level they exhibit 100% spin polarization. Furthermore, in the spin-down state, materials behave as semiconductors with wide bandgaps. Lastly, the thermoelectric properties are evaluated by the BoltzTrap code. The thermoelectric parameters which include the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit (ZT) are investigated in the range of temperatures from 200 to 800 K. The half-metallic ferromagnetic and thermoelectric characteristics make these materials desired for spintronics and thermoelectric applications.

Scalable and room-temperature preparation of Cs2HfCl6 double perovskites with recorded photoluminescence efficiency and robust stability

Vacancy-ordered double perovskites (DPs) are promising candidates for modern white light-emitting-diodes (WLEDs) due to their low toxicity and unique optical properties. However, the grand challenge is that DPs synthesized at room temperature (RT) typically have a low photoluminescence quantum yield (PLQY), most of which reported below 51 %, making them impractical for applications. Here, we present a facile strategy for the mass production of Bi3+ and Te4+ co-doped Cs2HfCl6 (CHC) DPs at RT, to realize single-component dual-band emission with a high color rendering index. It is witnessed that the rationally designed ligand [Hmim]Cl facilitates a strong chemical interaction between the ligand and DPs. This interaction allows [Hmim]Cl to coordinate with undercoordinated Hf4+ ions and provide excess chloride ions to the surface of the DPs. Consequently, it effectively controls the crystallization of DPs, protects them against moisture, reduces surface defects, and increases the migration barrier of halogen ions. As a result, the DPs exhibit excellent overall performance with robust photostability and a superior PLQY of 96.4 %, which is ∼ 2 times the highest reported value for similar DPs synthesized at RT. Current work presents a potential pathway for the mass production of high-quality DPs under mild conditions, leading to the development of signal-component WLEDs.

Pressure induced electronic and optical responses of vacancy-ordered double perovskites Cs2BX6 (B = Zr, Pd, Sn; X = Cl, Br, I)

Abstract Due to the toxicity and instability of lead-containing perovskites, high-performance lead-free perovskite attracts considerable attention. Lead-free vacancy-ordered double perovskites (VODP) emerge as environmentally friendly and efficient solutions as lead-containing solar cell substitutes. In this study, electronic properties of vacancy-ordered double perovskites Cs2BX6 (B = Zr, Pd, Sn; X = Cl, Br, I) under high pressure are investigated using first-principles methods. Semiconductors with bandgaps between 1.1 to 1.6 eV are considered for application. Our results show Cs2PdCl6 giving 1.60 and 1.32 eV bandgaps at 5 and 10 GPa, Cs2PdBr6 yielding 1.22 eV at ambient pressure, Cs2SnBr6 having 1.31 and 1.03 eV bandgap at 5 and 10 GPa, and Cs2ZrI6 showing 1.52 and 1.28 eV bandgap at 15 and 20 GPa. Furthermore, we considered the absorption coefficient and spectrum to ensure the materials’ optical performance. Cs2PdCl6, Cs2PdBr6, and Cs2ZrI6 display competent absorbance in the visible light range and proved these vacancy-ordered double perovskites as promising lead-free solar cell materials.

Is Rb2PtX6 (X = Cl, Br, and I) a promising Pb-free vacancy-ordered double perovskites for photoelectrochemical water splitting applications?

Is Rb2PtX6 (X = Cl, Br, and I) a promising Pb-free vacancy-ordered double perovskites for photoelectrochemical water splitting applications?

Investigation of Vacancy-Ordered Double Perovskite Halides A2Sn1-xTixY6 (A = K, Rb, Cs; Y = Cl, Br, I): Promising Materials for Photovoltaic Applications.

In the present study, the structural, mechanical, electronic and optical properties of all-inorganic vacancy-ordered double perovskites A2Sn1-xTixY6 (A = K, Rb, Cs; Y = Cl, Br, I) are explored by density functional theory. The structural and thermodynamic stabilities are confirmed by the tolerance factor and negative formation energy. Moreover, by doping Ti ions into vacancy-ordered double perovskite A2SnY6, the effect of Ti doping on the electronic and optical properties was investigated in detail. Then, according to the requirement of practical applications in photovoltaics, the optimal concentration of Ti ions and the most suitable halide element are determined to screen the right compositions. In addition, the mechanical, electronic and optical properties of the selected compositions are discussed, exhibiting the maximum optical absorption both in the visible and ultraviolet energy ranges; thus, the selected compositions can be considered as promising materials for application in solar photovoltaics. The results suggest a great potential of A2Sn1-xTixY6 (A = K, Rb, Cs; Y = Cl, Br, I) for further theoretical research as well as experimental research on the photovoltaic performance of stable and toxic-free perovskite solar cells.

Recent Advances in the Synthesis and Application of Vacancy-Ordered Halide Double Perovskite Materials for Solar Cells: A Promising Alternative to Lead-Based Perovskites.

Lead-based halide perovskite materials are being developed as efficient light-absorbing materials for use in perovskite solar cells (PSCs). PSCs have shown remarkable progress in power conversion efficiency, increasing from 3.80% to more than 25% within a decade, showcasing their potential as a promising renewable energy technology. Although PSCs have many benefits, including a high light absorption coefficient, the ability to tune band gap, and a long charge diffusion length, the poor stability and the toxicity of lead represent a significant disadvantage for commercialization. To address this issue, research has focused on developing stable and nontoxic halide perovskites for use in solar cells. A potential substitute is halide double perovskites (HDPs), particularly vacancy-ordered HDPs, as they offer greater promise because they can be processed using a solution-based method. This review provides a structural analysis of HDPs, the various synthesis methods for vacancy-ordered HDPs, and their impact on material properties. Recent advances in vacancy-ordered HDPs are also discussed, including their role in active and transport layers of solar cells. Furthermore, valuable insights for developing high-performance vacancy-ordered HDP solar cells are reported from the detailed information presented in recent simulation studies. Finally, the potential of vacancy-ordered HDPs as a substitute for lead-based perovskites is outlined. Overall, the ability to tune optical and electronic properties and the high stability and nontoxicity of HDPs have positioned them as a promising candidate for use in photovoltaic applications.