Vacancy-ordered Double Perovskites Research Articles

First principles study of electronic, optical, and thermoelectric properties of K2Pd (Cl/Br)6 for solar cells and renewable energy

The vacancy-ordered double perovskites are extensively studied for solar cells and thermoelectric characteristics. Herein, the electronic, mechanical, optical, and transport characteristics of K2Pd(Cl/Br)6 are addressed comprehensively. The structural, and dynamic stabilities are confirmed by tolerance factors (0.93, 0.95) and the positive frequencies of phonon band structures. The criteria for mechanical stability and elastic parameters ensure their stable, anisotropic, and ductile nature. The computation of band structures (BS) and density of states (DOS) show band gaps 2.0 eV, and 1.3 eV for K2PdCl6, and K2PdBr6, respectively. The optical properties are calculated by dielectric constants, absorption coefficient, and their dependent parameters. The K2PdBr6 has an ideal absorption band for visible light solar cells. The thermoelectric performance has been analyzed by figure of merit (ZT). The positive Seebeck coefficient p-type nature with significantly high ZT (0.73, 0.74) at room temperature.

Exploring the structural, electronic and optical properties of vacancy-ordered double perovskites Cs2TlAsX6 (X = I, Br, Cl) based on first-principles

In this work, we have been systematically studied the structural stability, electronic and optical properties of Cs2TlAsX6 (X = I, Br, Cl) by adopting first-principles calculations based on density functional theory (DFT). The mechanical and thermodynamic stability of Cs2TlAsX6 are confirmed by the calculated elastic constants, negative binding energies and formation energies. The calculated Pugh and Poisson's ratio indicate that Cs2TlAsI6 is brittle, and Cs2TlAsBr6 and Cs2TlAsCl6 are ductile. Moreover, the calculated results reveal that Cs2TlAsX6 (X = I, Br, Cl) are direct bandgap semiconductors with values of 1.476 eV, 1.874 eV, and 2.19 eV, respectively. Furthermore, the effective mass of electrons of Cs2TlAsX6 is much smaller than that of many perovskite materials. In addition, all the materials exhibit significant absorption capacity of visible and near ultraviolet light. Perovskite Cs2TlAsX6 is very suitable for solar cell applications and other optoelectronic devices because of its good light absorption performance and narrow band gap.

Ab-Inito Simulation of the Structural, Electronic and Optical Properties for the Vacancy-Ordered Double Perovskites A2tii6 (a = Cs and Nh4); a Time-Dependent Density Functional Theory Study

Ab-Inito Simulation of the Structural, Electronic and Optical Properties for the Vacancy-Ordered Double Perovskites A2tii6 (a = Cs and Nh4); a Time-Dependent Density Functional Theory Study

Density functional theory investigation of the mechanical, electronic and optical properties of Pb-free vacancy-ordered double perovskites K2PdCl6 and K2PdBr6

The current work has investigated the mechanical, electronic and optical properties of Pb-free vacancy-ordered double perovskites K2PdCl6 and K2PdBr6 by using first-principles calculations based on the framework of density functional theory (DFT). The calculated lattice constants of K2PdCl6 and K2PdBr6 are close to the experimental data. It is determined by calculating the Goldschmidt’s tolerance factors and elastic constants of K2PdCl6 and K2PdBr6 that they can be stabilized into 3D cubic crystal structures. The calculated Poisson and Pugh’s ratios indicate that K2PdCl6 is a brittle material, while K2PdBr6 exhibits ductile behavior. Both K2PdCl6 and K2PdBr6 are indirect band gap semiconductors, which show suitable band gaps of 2.151 eV and 1.368 eV for optoelectronic devices, respectively. In addition, the optical properties of K2PdCl6 and K2PdBr6 in the photon energy range of 0–6 eV further reveal the application potential of these compounds in single-junction and tandem solar cells as well as other optoelectronic devices.

Lattice Dynamics and Optoelectronic Properties of Vacancy-Ordered Double Perovskite Cs2TeX6 (X = Cl–, Br–, I–) Single Crystals

The soft, dynamic lattice of inorganic lead halide perovskite CsPbX3 (X = Cl-, Br-, I-) leads to the emergence of many interesting photophysical and optoelectronic phenomena. However, probing their lattice dynamics with vibrational spectroscopy remains challenging. The influence of the fundamental octahedral building block in the perovskite lattice can be better resolved in zero-dimensional (0D) vacancy-ordered double perovskites of form A2BX6. Here we study Cs2TeX6 (X = Cl-, Br-, I-) single crystals to yield detailed insight into the fundamental octahedral building block and to explore the effect that its isolation in the crystal structure has on structural and electronic properties. The isolated [TeX6]2- octahedral units serve as the vibrational, absorbing, and emitting centers within the crystal. Serving as the vibrational centers, the isolated octahedra inform the likelihood of a random distribution of 10 octahedral symmetries within the mixed-halide spaces, as well as the presence of strong exciton-phonon coupling and anharmonic lattice dynamics. Serving as the absorbing and emitting centers, the isolated octahedra exhibit compositionally tunable absorption (1.50-3.15 eV) and emission (1.31-2.11 eV) energies. Due to greater molecular orbital overlap between neighboring octahedra with increasing halide anion size, there is a transition from a more molecule-like electronic structure in Cs2TeCl6 and Cs2TeBr6-as expected from the effective 0D nature of these single crystals-to a dispersive electronic structure in Cs2TeI6, typical of three-dimensional (3D) bulk single crystals.

The effect of the A-site cation on the stability and physical properties of vacancy-ordered double perovskites A2PtI6 (A = Tl, K, Rb, and Cs)

In this article, a theoretical study on the structural, mechanical, electronic and optical properties of vacancy-ordered double perovskites A2PtI6 (A ​= ​Tl, K, Rb, and Cs) is conducted for the first time. The calculated lattice constants of each compound are consistent well with the reported experimental data. The tetragonal phase is adopted for Tl2PtI6 and K2PtI6, while Rb2PtI6 and Cs2PtI6 show the cubic phase. The results suggest that four compounds show good stability and the stability order is Cs2PtI6 ​> ​Rb2PtI6 ​> ​K2PtI6 ​> ​Tl2PtI6. The mechanical stability and ductile nature of each compound are confirmed. It is found that A2PtI6 (A ​= ​Tl, K) and A2PtI6 (A ​= ​Rb, Cs) are indirect and direct bandgap semiconductors, respectively. The predicted indirect and direct band gaps of four compounds are between 1.2 and 1.6 ​eV based on the HSE06 method. Rb2PtI6 and Cs2PtI6 have similar absorption curves. Moreover, Tl2PtI6 and K2PtI6 show stronger light absorption capacity. These four compounds are potential candidates for photovoltaic applications.

Theoretical prediction of the structural, electronic and optical properties of vacancy-ordered double perovskites Tl2TiX6 (X = Cl, Br, I)

Recently, more and more vacancy-ordered double perovskites have been proposed as functional materials in optoelectronic devices. In this paper, a simulation work on the structural, electronic and optical properties of vacancy-ordered double perovskites Tl2TiX6 (X ​= ​Cl, Br, I) has been carried out by using first-principles calculations to reveal their potential application in optoelectronic devices. The Goldschmidt's tolerance factors of these compounds are very close to the ideal value of 1 for cubic perovskites, which confirm their structural stability. Moreover, the formation energies of these compounds were also calculated to evaluate their thermodynamic stability. The theoretically predicted lattice constants of Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 are 10.060 ​Å, 10.553 ​Å and 11.320 ​Å, respectively, which can provide references for future experimental research. The calculated electronic properties indicate that Tl2TiCl6, Tl2TiBr6 and Tl2TiI6 have direct bandgaps of 3.39 ​eV, 2.57 ​eV and 1.58 ​eV, respectively, showing ideal bandgap nature for optoelectronic applications. Compared with many vacancy-ordered double perovskites, the three Tl2TiX6 compounds all show the advantage of small effective mass of holes. In addition, the investigation of optical properties, such as reflectivity, absorption coefficient and energy loss function in the photon energy range of 0–25 ​eV further confirms the potential utilization of these Tl2TiX6 compounds in optoelectronic devices.

DFT calculation on electronic properties of vacancy-ordered double perovskites Cs2(Ti, Zr, Hf)X6 and their alloys: Potential as light absorbers in solar cells

Electronic properties of Cs2(Ti, Zr, Hf)X6, where X = I, Br, and Cl in vacancy-ordered double perovskite (VODP) structure were studied by using the density functional theory (DFT) calculation with the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional and spin–orbit coupling (SOC). The “quasi-direct bandgap” of these semiconductors was found, indicating the significantly small difference between indirect (Γ-X) and direct (Γ- Γ) bandgaps. The bandgaps of VODP are in the range of 1.08–3.51 eV for Cs2TiX6, 2.63–5.21 eV for Cs2ZrX6, and 3.04–5.65 eV for Cs2HfX6, which could be suitable for a wide range of applications such as solar cells, optoelectronics, and photodetectors. Moreover, the electron contributions to the valence band maximum (VBM) and conduction band minimum (CBM) of VODP are dominated from p-orbital of X-site atoms (I, Br, Cl) and d-orbital of B-site atoms (Ti, Zr, Hf), opening up the opportunity to tune the bandgap by mixing X- or B-site atoms. In this study, we also examined two alloy-systems by mixing B-site atoms, which possess lattice mismatches of less than 1%. We found that by mixing Hf and Zr more than 25% (x greater than 0.25) into Cs2TiI6, the bandgaps of Cs2Ti1-xZrxI6 and Cs2Ti1-xHfxI6 change to the direct type. A fraction of Hf and Zr less than 25% could provide the suitable bandgaps for the light-absorber layer of single junction solar cells, while higher fractions of Zr (25–75%) and Hf (25–50%) could achieve the appropriate bandgap for higher-bandgap materials in tandem solar cells. Additionally, when the relationship between bandgaps and mixing concentration (x) was extracted using Vegard’s law, we found that this relationship aligned well with the x-dependent bowing parameter, b(x), rather than the constant b. Our investigation proved that Cs2(Ti, Zr, Hf)X6 could provide greater benefit than the conventional perovskite (CP) CsBX3 due to their stability, lattice matches, and the wide range of their bandgaps.

Exploring the electronic and optical properties of vacancy-ordered double perovskites Cs2PtX6 (X = Cl, Br, I)

We have comprehensively investigated the structural stability, mechanical, electronic and optical properties of vacancy-ordered double perovskites Cs2PtX6 (X ​= ​Cl, Br, I) through the first-principles calculations in this article. The stability of three compounds are proved by calculating their elastic constants and phonon dispersion. The results suggest that three Pt-based double perovskites are direct bandgap semiconductors. The band gaps of Cs2PtCl6, Cs2PtBr6, and Cs2PtI6 are 3.45, 2.45, and 1.41 ​eV, respectively. The optical properties of these compounds in the visible light region have been explored by the HSE06 method. The highest optical absorption coefficient of Cs2PtBr6 exceeds 5 ​× ​105 ​cm−1, and the strong optical absorption of Cs2PtI6 is in the range of 300–600 ​nm. The excellent physical properties make lead-free Cs2PtI6 suitable for single-junction solar cells.

Structural, electronic, and optical properties of Pt-based vacancy-ordered double perovskites A2PtX6 (A = K, Rb, Cs; X = Cl, Br, I) in tetragonal P4/mnc polymorph

We study the structural, electronic, and optical properties of the Pt-based vacancy-ordered double perovskites A 2 Pt X 6 ( A = K, Rb, Cs; X = Cl, Br, I) in the tetragonal P 4/ mnc polymorph using the relativistic all-electron calculations based on the density functional theory. It is found that the rotation angles θ of the Pt X 6 octahedra are non-zero for all the compounds. For a given A -site cation, θ increases from Cl to Br to I while for a given halide anion, θ decreases from K to Rb to Cs. As a result, the largest θ is found for K 2 PtI 6 . Taking the three iodides, K 2 PtI 6 , Rb 2 PtI 6 and Cs 2 PtI 6 , as representatives, it is found that K 2 PtI 6 is a direct-band-gap semiconductor and that Rb 2 PtI 6 and Cs 2 PtI 6 are quasi-direct-band-gap semiconductors with the small differences between the indirect and direct band gaps. The band gap in the tetragonal P 4/ mnc polymorph is larger than the corresponding one in the cubic F m 3 ̄ m polymorph. Employing the spectroscopically limited maximum efficiency as a metric for quantifying the photovoltaic performance, K 2 PtI 6 , Rb 2 PtI 6 , and Cs 2 PtI 6 are found to be promising absorber materials for thin-film solar cells. • Pt-based vacancy-ordered double perovskites A 2 Pt X 6 in tetragonal P 4/ mnc polymorph were studied theoretically. • Non-zero rotation angles of Pt X 6 octahedra were found for all the compounds. • The electronic and optical properties of K 2 PtI 6 , Rb 2 PtI 6 , and Cs 2 PtI 6 were examined as representatives.

Ns2-containing vacancy-ordered double perovskites for optoelectronic applications: A first-principles investigation

Vacancy-ordered double perovskites have attracted broad attention due to the thermostability and excellent optoelectronic properties. Here, we report first-principles investigations on optoelectronic properties of a series of ns2-containing vacancy-ordered double perovskites by substituting monovalent Cs with divalent Ba. These perovskites are predicted to show indirect bandgaps similar to the bismuth-based double perovskites. Unlike the regular metal halide perovskites, the 6s orbital of A-site Ba cation in these ns2-containing compounds composes lower conduction bands, and calculated dipole matrix elements demonstrate the influence of A-site Ba 6s-orbital on the optical absorption. Moreover, decomposition energies, electronic bandgaps, and effective masses of holes and electrons are calculated to assess the potential optoelectronic application, which could be a theoretical guide for exploring novel lead-free perovskite materials.

Appealing perspectives of structural, electronic, mechanical, and thermoelectric properties of Tl2(Se, Te)Cl6 vacancy-ordered double perovskites

We present thermoelectric properties such as the Seebeck coefficient, thermal conductivity, electrical conductivity, figure of merit, and power factor of Tl2(Se, Te)Cl6 vacancy-ordered double perovskites obtained with use of the BoltzTraP code. For input files, structural and electronic properties were calculated with the full potential linearized augmented plane wave method within the Wien2k computational code. The mechanical and thermal parameters were computed for calculation of the lattice thermal conductivity. Both compounds were found to have indirect band gaps (2.976 eV for Tl2SeCl6 and 3.037 eV for Tl2TeCl6). The valence bands were observed to be flat, which results in large values of the Seebeck coefficient (S) of 183.50 μV/K for Tl2SeCl6 and 185.49 μV/K for Tl2TeCl6 at room temperature. In addition, positive values of S determined the p-type conductivity in these compounds. The power factor, figure of merit, and electrical conductivity were found to increase with temperature, while the thermal conductivity was found to decrease, hence confirming the semiconductor nature of Tl2(Se, Te)Cl6 compounds as predicted by their electronic properties. The figure of merit of these compounds was found to be higher than that of conventional metal halide perovskites. This makes the compounds investigated potential candidates for thermoelectric applications involving halide perovskites.

Lead-Free Cesium Titanium Bromide Double Perovskite Nanocrystals.

Double perovskites are a promising family of lead-free materials that not only replace lead but also enable new optoelectronic applications beyond photovoltaics. Recently, a titanium (Ti)-based vacancy-ordered double perovskite, Cs2TiBr6, has been reported as an example of truly sustainable and earth-abundant perovskite with controversial results in terms of photoluminescence and environmental stability. Our work looks at this material from a new perspective, i.e., at the nanoscale. We demonstrate the first colloidal synthesis of Cs2TiX6 nanocrystals (X = Br, Cl) and observe tunable morphology and size of the nanocrystals according to the set reaction temperature. The Cs2TiBr6 nanocrystals synthesized at 185 °C show a bandgap of 1.9 eV and are relatively stable up to 8 weeks in suspensions. However, they do not display notable photoluminescence. The centrosymmetric crystal structure of Cs2TiBr6 suggests that this material could enable third-harmonic generation (THG) responses. Indeed, we provide a clear evidence of THG signals detected by the THG microscopy technique. As only a few THG-active halide perovskite materials are known to date and they are all lead-based, our findings promote future research on Cs2TiBr6 as well as on other lead-free double perovskites, with stronger focus on currently unexplored nonlinear optical applications.

Temperature effects on the vibrational properties of the Cs2SnX6 ‘defect’ perovskites (X = I, Br, Cl)

Air-stable, vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) have been attracting particular interest due their potential applications in solar cells and thermoelectrics based on their favorable structural, optical and transport properties. In this work, a comprehensive Raman investigation of Cs2SnX6 perovskites is performed over an extended temperature range of 83–433 K in order to provide insight to their lattice dynamics. Substitution of the lighter and more electronegative Cl halogens with the heavier bromide and iodide ones resulted in the decrease of the frequencies for all Raman modes comprising the internal vibrations in the SnX6 octahedra and the νL(F2g) mode due to Cs vibrations against SnX6 in the rigid lattice. Blue shift and narrowing of the SnX6 stretching and bending bands was invariably observed for all halogen analogues with temperature decrease, evidencing the presence of lattice anharmonicity. More importantly, the elusive νL(F2g) phonon was only observed for Cs2SnCl6 for all temperatures, complying with the predictions of Hirshfeld surface analysis for the enhanced Cs–SnCl6 interactions. The latter mode emerged for Cs2SnI6 only above room temperature, indicating that combination of anharmonic lattice dynamics with the increased Cs–SnI6 oscillation energy upon increasing temperature may activate this specific “lattice mode”, which in the limit of very weak Cs–I interaction approaches the “rattling’’ Cs vibrations in the cuboctahedral cage. Strong lattice anharmonicity for the chemically stable Cs2SnX6 compounds can be accordingly inferred, which is a key aspect for their performance in future optoelectronic and thermoelectric applications.

Electronic and optical properties of vacancy ordered double perovskites A2BX6 (A\u2009=\u2009Rb, Cs; B\u2009=\u2009Sn, Pd, Pt; and X\u2009=\u2009Cl, Br, I): a first principles study

The highly successful PBE functional and the modified Becke–Johnson exchange potential were used to calculate the structural, electronic, and optical properties of the vacancy-ordered double perovskites A2BX6 (A = Rb, Cs; B = Sn, Pd, Pt; X = Cl, Br, and I) using the density functional theory, a first principles approach. The convex hull approach was used to check the thermodynamic stability of the compounds. The calculated parameters (lattice constants, band gap, and bond lengths) are in tune with the available experimental and theoretical results. The compounds, Rb2PdBr6 and Cs2PtI6, exhibit band gaps within the optimal range of 0.9–1.6 eV, required for the single-junction photovoltaic applications. The photovoltaic efficiency of the studied materials was assessed using the spectroscopic-limited-maximum-efficiency (SLME) metric as well as the optical properties. The ideal band gap, high dielectric constants, and optimum light absorption of these perovskites make them suitable for high performance single and multi-junction perovskite solar cells.

First-principles study on the structural, electronic and optical properties of vacancy-ordered double perovskites Cs2PtI6 and Rb2PtI6

In this paper, the structural, electronic and optical properties of vacancy-ordered double perovskites Cs2PtI6 and Rb2PtI6 were studied based on first-principles calculations. The stability of studied compounds is ensured by the negative formation enthalpy. The calculated results show that both Cs2PtI6 and Rb2PtI6 are indirect bandgap semiconductors. The bandgap values of Cs2PtI6 and Rb2PtI6 are 1.29 and 1.15 eV, respectively, which are suitable for photosensitive materials of solar cells. For both Cs2PtI6 and Rb2PtI6, the valence band maximum (VBM) is dominated by the I-5p orbitals and the conduction band minimum (CBM) is mainly composed of the I-5p orbitals along with the Pt-5d orbitals. The optical properties of Cs2PtI6 and Rb2PtI6 in the photon energy range from 0 to 12 eV have been comprehensively studied by the HSE06 method. The results show that these two compounds exhibit excellent light absorption, especially in the ultraviolet range. Due to the suitable bandgaps and excellent light absorption, the vacancy-ordered double perovskites Cs2PtI6 and Rb2PtI6 can be effective candidates for lead-free photovoltaic materials.

Chemical Control of Spin-Orbit Coupling and Charge Transfer in Vacancy-Ordered Ruthenium(IV) Halide Perovskites.

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of RuIV halides are presented; A2 RuCl6 and A2 RuBr6 , where A is K, NH4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with Jeff =0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.

Achieving Ultrahigh Piezoelectricity in Organic–Inorganic Vacancy-Ordered Halide Double Perovskites for Mechanical Energy Harvesting

Piezoelectric charge coefficient (d33) and piezoelectric voltage coefficient (g33) are the two most critical parameters that define output performance of piezoelectric nanogenerators (PNGs). Herein, we propose a vacancy-ordered double perovskite of TMCM2SnCl6 (where TMCM is trimethylchloromethylammonium) with a large d33 of 137 pC/N and g33 of 980 × 10–3 V·m/N. The piezoelectric coefficients are considered from the halogen-bonding-mediated synergistic movements of atomic displacement in inorganic [SnCl6]2– octahedrons, as well as the molecular rotation of organic TMCM+, which is revealed by a combined density functional theory (DFT) and experimental study. The TMCM2SnCl6 possesses a high saturated polarization (Ps) of 8.7 μC/cm2, a high Curie temperature (Tc) of 365 K, and a low coercive field (Ec) of 0.6 kV/cm. The output voltage (Voc) and current (Isc) of the PNGs are 81 V and 2 μA at an applied mechanical excitation of (4.9 N, 40 Hz). We hope this work will provide guidance in energy harvesting by innovatively designing highly piezoelectric perovskites for the PNGs.

Density functional study on electronic properties of transition metal-based vacancy-ordered halide perovskites

Replacing toxic Pb of halide perovskites is a serious challenge in the current investigation of perovskite solar cells. Vacancy-ordered double halide perovskites have attracted considerable research interest due to their excellent stability and optoelectronic properties. A series of vacancy-ordered double perovskites based on transition metal cations are comprehensively investigated in this work through first-principles calculations. Various electronic configurations of tetravalent transition metals are considered in spin-polarized calculation. Our calculations reveal electronic properties of vacancy-ordered double halide perovskites and provide a theoretical basis for the exploration of novel lead-free perovskites.

Ferroelastic Phase Transition in Formamidinium Tin(IV) Iodide Driven by Organic-Inorganic Coupling.

Hybrid perovskites are a technologically relevant family of materials, with potential applications in photovoltaics, solid-state lighting, and radiation detection. Interactions between the inorganic octahedral framework and the organic sublattice have been implicated in the structure and optoelectronic properties, but characterization of these interactions has been challenging, because of competition between organic-inorganic coupling and intraoctahedral interactions. Owing to their decreased octahedral connectivity, vacancy-ordered double perovskites present an ideal case study to examine organic-inorganic coupling in hybrid perovskites and their derivatives. Here, we describe the low-temperature, hysteretic phase transition of formamidinium tin(IV) iodide from the high-symmetry cubic phase to a lower-symmetry monoclinic phase. We propose that the hysteresis stems from organic-inorganic coupling mediated by local and spontaneous strain from the orientations of the formamidinium cations, which result in a ferroelastic phase transition.