Effect of substituting on the transition dipole moment of the double perovskite Cs2AgInCl6.

The all-inorganic double perovskite Cs2AgInCl6with three dimensional structure has attracted much attention due to its direct bandgap property and particular luminescence mechanism, which is self-trapped exciton emission. However, it is a pity that Cs2AgInCl6exhibits low photoluminescence quantum yield, which affects its application for light-emitting devices. In this paper, the band structure and transition dipole moment of Cs2AgIn(1-x)SbxCl6(x= 0, 0.25, 0.5, 0.75) are calculated using first principle calculation. The calculated results shows that the pure material Cs2AgInCl6not only has a large band gap but also has the dipole forbidden transition, which means that the electrons cannot be excited from the valence band maximum to the conduction band minimum. However, the substituted Cs2AgIn0.75Sb0.25Cl6have a good property for the band gap about 3.066 eV and break forbidden transition at point X. The reason for its change is due to the overlap of electron and hole for charge density. Our work provides theoretical guidance for the design of more efficient light-emitting devices.

Multi-functional application potential of Ruddlesden-Popper perovskite-based heterostructure PtSe2/Cs2PbI4with tunable electronic properties.

Heterogeneous stacking based on two-dimensional Ruddlesden-Popper (RP) perovskite is a desired strategy for the reasonable combination of stability and efficiency. Constructing heterostructures with tunable optoelectronic properties further provide opportunities to design multi-functional devices. Herein, we present a first-principle research to investigate the geometric and electronic structures of RP perovskite heterostructure PtSe2/Cs2PbI4and its tunable electronic properties induced by thickness modulation and external strains. The results indicate that the heterostructure based on Cs2PbI4monolayer and PtSe2monolayer has a type-II band alignment, which is suitable for the photovoltaic applications. With the layer number of PtSe2in heterostructure increases from monolayer to bilayer, the band alignment of PtSe2/Cs2PbI4heterostructure can switch from type-II to type-I, which is beneficial for the luminescence device applications. However, when the thickness of PtSe2in heterostructure further increases to trilayer, the heterostructure exhibits metallic characteristic with a p-type Schottky barrier. In addition, we find the strain engineering is an effective knob in tuning the electronic properties of PtSe2/Cs2PbI4heterostructures with different thickness. These findings reveal the potential of PtSe2/Cs2PbI4heterostructure as a tunable hybrid material with substantial prospect in multi-functional applications.

Enhanced photocatalytic toluene oxidation performance induced by two types of cooperative fluorine doping in polymeric carbon nitride with the first-principles calculations.

Fluorine atoms doping was reported in experiment to reduce the band gap, improve the oxidation potential of hole, and polarize the electron distribution of polymeric carbon nitride (PCN). However, the relationships between different types of F doping and the roles of F doping in electronic and optical properties remain elusive. In this work, we investigate several F doping types in PCN and analyze their different roles in electronic and optical properties with the first-principles calculations. The results show that two stable and cooperative F doping types are found, one is to form the C (sp3)-F bond (Fcorner type), and the other is F atom replacing amino group -NH2 (FN3 type) forming covalent C-F bond. The Fcorner doping reduces the energy level of valence-band maximum (VBM), causes excited electron-hole distribution polarized, and increases the hole distribution on F atoms, which strengthens the capacity of photocatalytic oxidation and improves the electron-hole separation efficiency, while FN3 type doping plays the roles of reducing the bandgap and improving the light absorption. In addition, under the synergistic action of two types of F doping, the adsorption energy of toluene on F-codoped PCN is greatly enhanced, improving the ability of photocatalytic activation of toluene. Our work develops a new understanding of F doping and reveals the roles of different types of F doping, providing a rationale for designing and regulating more efficient photocatalysts and improving the properties of photocatalytic toluene oxidation.

Recognition of Water-Induced Double-Edged Sword Effects in Photocatalytic Selective Oxidation of Toluene on Titanium Dioxide Clusters with Density Functional Theory Calculations.

Recently, water promotion effects in the selective oxidation of benzyl alcohol to benzaldehyde have been experimentally recognized and identified. However, the effects of water on the photocatalytic selective oxidation of toluene into benzaldehyde remain elusive. In this work, the Ti3O9H6 clusters in different solvents (water and toluene solvent) are used to study the water-induced effects in photocatalytic oxidation reactions in kinetics and thermodynamics using density functional theory (DFT) calculations. In addition, the influences of the OH groups on catalysts (Ti-OH bonds) from photocatalytic water splitting are also considered. The results clearly demonstrate the water-induced double-edged sword effects in the photocatalytic selective oxidation of toluene. We expect that our work can not only shed light on the mechanisms of photocatalytic selective oxidation of toluene into benzaldehyde and other activation reactions of sp3 C-H bonds but also design and modulate highly efficient photocatalysts.

Interface-engineering studies on the photoelectric properties and stability of the CsSnI3-SnS heterostructure.

The stability of Sn-based perovskites has always been the main obstacle to their application. Interface engineering is a very effective method for improving the stability of perovskites and the efficiency of batteries. Two-dimensional (2D) monolayer SnS is selected as a surface-covering layer for the CsSnI3 lead-free perovskite. The structure, electronic properties, and stability of the CsSnI3-SnS heterostructure are studied using density functional theory. Due to the different contact interfaces (SnI2 and CsI interfaces) of CsSnI3, the interface electronic-transmission characteristics are inconsistent in the CsSnI3-SnS heterostructure. Because of the difference in work functions, electrons flow at the interface of the heterostructure, forming a built-in electric field. The heterostructures form a type-I energy-level arrangement. Under the action of an electric field in the CsI-SnS heterostructure, electrons at the CsI interface recombine with holes at the SnS interface; however, the holes of the SnI2 interface and the electrons of the SnS interface are easily recombined in the SnI2-SnS heterostructure. Moreover, monolayer SnS can enhance the light absorption of the CsSnI3-SnS heterostructure. Monolayer SnS can inhibit the migration of iodine ions and effectively improve the structural stability of the SnI2-SnS interface heterostructure. This work provides a new theoretical basis for improving the stability of lead-free perovskites.

2D and 3D double perovskite with dimensionality-dependent optoelectronic properties: first-principle study on Cs2AgBiBr6and Cs4AgBiBr8.

Recently, the effect of dimensional control on the optoelectronic performance of two-dimensional (2D)/three-dimensional (3D) single perovskites has been confirmed. However, how the dimensional change affects the photoelectric properties of 2D/3D all-inorganic double perovskites remains unclear. In this study, we present a detailed theoretical research on a comparison between the optoelectronic properties of 3D all-inorganic double perovskite Cs2AgBiBr6and recently reported 2D all-inorganic double perovskite Cs4AgBiBr8with Ruddlesden-Popper (RP) structure based on density functional theory calculations. The results demonstrate the charge carrier mobility and absorption coefficients in the visible spectrum of Cs4AgBiBr8(2D) is poorer than Cs2AgBiBr6(3D). Moreover, the value of exciton-binding energy for 2D RP all-inorganic double perovskite Cs4AgBiBr8(720 meV) is 3 times larger than that of 3D all-inorganic double perovskite Cs2AgBiBr6(240 meV). Our works indicate that Cs4AgBiBr8(2D) is a promising material for luminescent device, while Cs2AgBiBr6(3D) may be suitable for photovoltaic applications. This study provides a theoretical guidance for the understanding of 2D RP all-inorganic double perovskite with potential applications in photo-luminescent devices.

High-performance photovoltaic application of the 2D all-inorganic Ruddlesden-Popper perovskite heterostructure Cs2PbI2Cl2/MAPbI3.

The three-dimensional (3D) organic-inorganic halide perovskite MAPbI3 has excellent light-harvesting properties but is unstable. However, the newly synthesized two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs2PbI2Cl2 has superior stability but adverse photoelectric properties. Therefore, constructing a 2D Cs2PbI2Cl2/3D MAPbI3 heterostructure is expected to combine the superstability of the 2D material and the high efficiency of the 3D one. The photoelectric properties and charge transfer of 2D Cs2PbI2Cl2/3D MAPbI3 heterostructures are investigated using density functional theory, where MAPbI3 has two kinds of contacting interfaces, i.e., MAI and PbI interfaces. The band gaps of 2D/MAI and 2D/PbI heterostructures are 1.52 eV and 1.40 eV, smaller than those of the free-standing materials (2D ∼ 2.50 eV, MAI ∼ 1.77 eV, and PbI ∼ 1.73 eV), which can broaden the light absorption spectrum. Moreover, the 2D/3D heterostructures are typical type-II heterostructures, which is beneficial to facilitate the separation of carriers for increasing the photoelectric conversion. Interestingly, due to the work function difference (2D ∼ 4.97 eV, MAI ∼ 3.57 eV, and PbI ∼ 5.49 eV), the charge transfer directions of the 2D/MAI and 2D/PbI heterostructures are completely opposite, which shows that interface engineering to impose a consistent interface termination is needed to obtain good performance for solar cells. These results demonstrate that constructing 2D Cs2PbI2Cl2 and 3D MAPbI3 heterostructures by interfacial engineering is a potential strategy to improve the performance of perovskite solar cells (PSCs).

Enhanced photocatalytic activity of the direct Z-scheme black phosphorus/BiOX (X = Cl, Br, I) heterostructures.

Bismuth oxyhalides (BiOX), as a typical photocatalytic material, have attracted much attention due to their unique layered structure, non-toxicity and excellent stability. However, the photocatalytic performance of BiOX is limited by their weak light absorption ability and rapid recombination of photo-generated carriers. In the present work, first-principles calculations have been performed to comprehensively explore the structural, electronic and optical properties of black phosphorus (BP)/BiOX (X = Cl, Br, I) heterostructures, revealing the inherent reasons for their enhanced photocatalytic performance. By combining band structures and work function analysis, the migration paths of photo-generated electrons and holes are obtained, proving a direct Z-scheme photocatalytic mechanism in BP/BiOX heterostructures. Moreover, the BP/BiOX heterostructures have decent band edge positions, which are suitable for photocatalytic overall water splitting. Compared with single BiOX, the light absorption performance of BP/BiOX heterostructures is significantly improved, in which BP/BiOI exhibits the highest optical absorption coefficient among the BP/BiOX heterostructures. Meanwhile, the better carrier migration performance of the BP/BiOX heterostructures is attributed to the reduction in effective mass. The present work offers theoretical insight into the application of BP/BiOX heterostructures as prominent photocatalysts for water splitting.

Effects of doping on photocatalytic water splitting activities of PtS2/SnS2 van der Waals heterostructures.

Photocatalytic water splitting is a promising technology to solve serious energy and environmental problems. The PtS2 monolayer has been previously predicted to be a water splitting photocatalyst. But the high efficiency of carrier recombination in the monolayer results in poor photocatalytic performance. It is well known that the construction of van der Waals (vdW) heterojunctions can improve the photocatalytic performance of a monolayer. In this investigation, we constructed a PtS2/SnS2 vdW heterojunction and systematically investigated the influence of the doping position and doping ratio on its performance using density functional theory calculations. Interestingly, the band alignment transforms from Type-II to Type-I and from Type-I to Type-II when the S in SnS2 is replaced with Se in the PtS2/SnS2 vdW heterojunction and the S in PtS2 is replaced with Se in the PtS2/SnSe2 vdW heterojunction, respectively. More importantly, from the PtS2/SnS2 to PtSe2/SnSe2 vdW heterojunction, the decomposition of water also changes from semi-decomposed water to fully decomposed water. Furthermore, the results show that the direct Z-scheme photocatalytic mechanism exists in the PtSSe/SnSe2 vdW heterojunction by analysis of the migration paths of photoinduced electrons and holes. And compared with the PtS2/SnS2, the PtSSe/SnSe2 heterostructure exhibits better photocatalytic water splitting activities. These results can provide a direction that doping can improve the photocatalytic water splitting performance of heterojunction photocatalysts.

Thickness-induced band-gap engineering in lead-free double perovskite Cs2AgBiBr6 for highly efficient photocatalysis.

In recent years, two-dimensional (2D) lead-free double perovskites have been attracting much attention because of their unique performance in photovoltaic solar cells and photocatalysis. Nonetheless, how thickness affects the photoelectric properties of lead-free double perovskite remains unclear. In this work, by means of density functional theory (DFT) with a spin orbit coupling (SOC) effect, we have investigated the electronic and optical properties systemically, including band structures, carrier mobility, optical absorption spectra, exciton-binding energies, band edges alignment and molecule adsorption performance of Cs2AgBiBr6 with different thicknesses. The calculated results revealed the thickness-induced band gap and optical performance for Cs2AgBiBr6. It shows a low band gap and outstanding optical absorption of visible and ultraviolet light. When the thickness is reduced to a monolayer, Cs2AgBiBr6 moves from an indirect band gap to a direct band gap. Moreover, the carrier mobility of Cs2AgBiBr6 is excellent and the exciton-binding energy increases with the decreased thickness. Importantly, an analysis of molecule adsorption and band edge alignment indicates that Cs2AgBiBr6 is prone to H2O adsorption and H2 desorption theoretically, which is conducive to the photocatalytic water splitting for hydrogen generation and other photovatalytic reactions. Our work suggests that Cs2AgBiBr6 is a potential candidate as a solar cell or a photocatalyst, and we provide theoretical explorations into reducing the layers of lead-free double perovskite materials to 2D atomic thickness for a better photocatalytic application, which can serve as guidelines for the design of excellent photocatalysts.

Transition of the Type of Band Alignments for All-Inorganic Perovskite van der Waals Heterostructures CsSnBr3/WS2(1-x)Se2x.

In general, two-dimensional semiconductor-based van der Waals heterostructures (vdWHs) can be modulated to achieve the transition of band alignments (type-I, type-II, and type-III), which can be applied in different applications. However, it is rare in three-dimensional perovskite-based vdWHs, and it is challenging to achieve the tunable band alignments for a single perovskite-based heterostructure. Here, we systematically investigate the electronic and optical properties of all-inorganic perovskite vdWHs CsSnBr3/WS2(1-x)Se2x based on density functional theory (DFT) calculation. The calculated results show that the transitions of band alignment from type-II to type-I and type-III to type-II are achieved by modulating the doping ratio of the Se atom in the WS2(1-x)Se2x monolayer for SnBr2/WS2(1-x)Se2x and CsBr/WS2(1-x)Se2x heterostructures, respectively, in which the CsBr and SnBr2 represent two different terminated surfaces of CsSnBr3. The change of band alignments can be attributed to the conduction band minimum (CBM) transforming from the W 5d to Sn 5p orbital in SnBr2/WS2(1-x)Se2x vdWHs, and the valence band maximum (VBM) and CBM change from an overlapped state to a separated one in CsBr/WS2(1-x)Se2x vdWHs. This work can provide a theoretical basis for the dynamic modulation of band alignments in perovskite-based vdWHs.

Theoretical study on the tunable electronic band structure of Cs2PbI2Cl2/CsPbBr3 halide perovskite heterostructure driven by ferroelectric polarization modulation.

Ferroelectric polarizationhas been considered to be an key factor to tune the structural and photoelectric properties of perovskites and their heterostructures. While there has been growing researches made in the novel phenomena originating from interface formed between oxide perovskites, the effects of ferroelectric polarization on the electronic properties of halide perovskites and their heterostructures are rarely studied. Herein, by using first-principles calculations, all-inorganic halide perovskite heterostructure composed of 3D perovskite tetragonal CsPbBr3 and 2D Ruddlesden-Popper (RP) perovskite Cs2PbI2Cl2 is constructed for disclosing the relationship between the intrinsic polarization of tetragonal CsPbBr3 and electronic band structure of heterostructure. Cs atoms and Pb atoms of tetragonal CsPbBr3 in heterostructure are artificially moved away from the equivalent centers to simulate increased polarization. Our results show that with the spontaneous polarization of tetragonal CsPbBr3 increasing, the bandgap of heterostructure decreases, and the band alignment switches from staggered type-II to broken-gap type-III. Moreover, large cation-anion displacements along z-direction in tetragonal CsPbBr3 can be observed when tensile strains (≥5%) are applied, indicating a increased ferroelectric polarization, which also facilitates the decreasing of bandgap in heterostructure and the type-II-type-III transition of band alignment. Our study suggests that control over the polarization of ferroelectric materials is of great importance to tune the photoelectric properties of perovskite-based devices.

First-principles study on photovoltaic properties of 2D Cs2PbI4-black phosphorus heterojunctions.

Both 2D perovskite Cs2PbI4 and phosphorus are significant optoelectronic semiconductor materials, the optical-electrical characters between both contact interfaces are interesting topics. In present work, we demonstrate comparative investigation of optoelectronic properties for two kinds of electrical contact interfaces. i.e. Pb-I and Cs-I interfaces with black phosphorus contacts. The carrier transport, charge transferring and optical properties for both cases are investigated by using first principle calculation. Both contact interfaces exhibit type II band alignment with direct band gap. Charge carrier migration from Cs-I interface to black phosphorus is more strong than that of Pb-I interface by considering differential charge density and bader charge between distinct electrical contact interfaces. Besides, electron-hole effective masses of heterojunctions for both cases along different direction are investigated. Optical absorption coefficients of both cases are compared with those of free-standing Cs2PbI4 and black phosphorus in the visible spectrum. We systematically compared advantages and disadvantages of two kinds of contact interfaces for photovoltaic application, and the results reveal interfacial engineering of 2D heterojunction plays a important role in tuning optoelectronic properties.

Theoretical study on the effect of the optical properties and electronic structure for the Bi-doped CsPbBr3.

Metal doping, including Bi, Yb, Eu, Sb and so on, are important means to improve the photoelectric properties and stability of metal halide perovskite materials. Among these works, Bi-doped CsPbBr3 especially has attracted much attention for both experimental and theoretical investigation. But there are still some arguments to be solved. One view thinks that Bi doping in CsPbBr3 not only influences the band structure, but also improves the charge transfer (Raihana et al 2017 J. Am. Chem. Soc. 139 731-7). The other supported the points that there are no changes in the valence band structure of Bi-doped CsPbBr3 and the concept of the band-gap engineering in Bi-doped CsPbBr3 halide perovskite is not valid (Olga et al 2018 J. Phys. Chem. Lett. 9 5408-11). They also have different opinions for the reason of the red-shift phenomenon caused by Bi-doped CsPbBr3. In this work, the density functional theory (DFT) based first-principles methods is adopted to investigate the effect of the optical properties and electronic structure for Bi doping CsPbBr3. The calculated results clarify that the red-shift phenomenon is caused by the slight reduction of band gap and the transition levels of Bii and BiPb defects. The values of red-shift also were estimated about 150 meV for Bii defects, which is close the experimental value of about 140 meV. Moreover, our studies also show that the Bi doping does not affect the valence bands, but Bii defects change the electron distribution of the conduction band. Our work and experimental results support and confirm each other, which provides a useful reference for the study of Sb-doped CsPbBr3, Eu-doped CsPbBr3 and so on.

The energy band engineering for the high-performance infrared photodetectors constructed by CdTe/MoS2 heterojunction.

Recently, the traditional infrared photodetectors (PDs) shows limited application in various areas, due to the narrow band-gap, high cost and even complex manufacturing process. In this situation, scientist have paid much attention to achieve the ultra broadband PDs from the deep ultraviolet to the near infrared. The energy band engineering for two-dimensional (2D) van der Waals heterojunction with free chemical dangling bonds is an effective method to fabricate High-performance Photodetectors. In this work, we employ density functional calculation to construct a type-II CdTe/MoS2 heterostructure and calculate its electronic properties. The results reveal that the CdTe/MoS2 has the narrow band gap of 0.64 eV and electrons transfer from the CdTe to MoS2 layer, which promotes the separation of photogenerated carriers and enhance the photoelectron conversion efficiency. Driven by the smaller band gap, it can respond to near infrared, visible and ultraviolet light, demonstrating it the promising application for solar cell. Furthermore, the analysis of molecules adsorption and band edge alignment indicates that the CdTe/MoS2 is prone to capture H2O and release the H2 molecules, which is conductive to the photocatalytic water splitting for hydrogen generation. Our work suggests that the CdTe/MoS2 heterostructure is a potential candidate as a solar cell and even photocatalyst, and also provides a new sight for experimental and theoretical research to design a highly efficient device.

The influence of electrode for electroluminescence devices based on all-inorganic halide perovskite CsPbBr3.

Electroluminescence devices based on all-inorganic halide perovskite material with excellent luminescence performance have been studied extensively in recent years. However, the important role for the electrodes of electroluminescence devices is payed few attention by theoretical and experimental studies. Appropriate electrodes can reduce the Schottky barrier height to decrease the energy loss, and prevent the metal impurities from diffusing into the perovskite material to generate deep traps levels, which improves the luminous efficiency and lifetime of devices. In this paper, not only the interface effects between CsPbBr3 and common metal electrode (Ag, Au, Ni, Cu and Pt) are studied by first-principle calculations, but also the diffusion effects of metal electrode atom into the CsPbBr3 layer are also explored by nudged elastic band calculations. The calculated results show the metal Ag is more suitable for the cathode for CsPbBr3 electroluminescence devices, while the metal Pt is more applicable for the anode. Based on the overall consideration about the interface effects and diffusion effects of the CsPbBr3-metal electrode junctions, the essential principle is analyzed. The work provides theoretical guidance for how to select the right electrode for the electroluminescence performance of all-inorganic halide perovskite. The critical factor of Schottky barrier height between the electrode and the light-emitting semiconductor, and transition level generated by metal impurities also provide a valuable reference how to select the suitable electrodes for other electroluminescence devices.

Band alignment engineering: ultrabroadband photodetection with SnX2 (X = S, Se)/ZnS heterostructures.

Ultrabroadband mid-frequency infrared photodetectors have important applications in surveillance, medical diagnosis, bioimaging and navigation aids. Thus, researchers hope to detect mid-infrared radiation with larger wavelength. However, due to the limitation of room temperature, it is difficult for these detectors to detect mid-infrared with 4 µm or larger wavelength. Therefore, at room temperature, how to realize mid-infrared detection in a wide range has become an urgent problem to be solved. In this paper, the band structure of SnX2 (X = S,Se)/ZnS and SnS2(1-ŋ)Se2ŋ /ZnS was studied by the density functional theory based first-principles methods. Under the specific stacking procedure, changing the [Formula: see text] of SnS2(1-ŋ)Se2ŋ , the band gap of heterojunctions can be continuously tuned from 0 to 0.97 eV. Amazingly, the band structure maintains the characteristics of a type-II heterojunction. The photodetection in our work is estimated for wavelengths from 1.2 µm to 10 µm, covering a wide wavelength range of mid-infrared. Such a wide range is considerable in current research. The characteristics of type-II band structure and the wide detection range imply that SnX2 /ZnS has great potential in mid-frequency infrared detection. Our work may provide some breakthroughs for the research of multiband photodetectors at room temperature.

Two-Dimensional van der Waals Heterostructures Constructed via Perovskite (C4H9NH3)2XBr4 and Black Phosphorus.

Heterogeneous stacking of two-dimensional (2D) perovskites with other 2D materials is a very effective strategy for designing low-cost and high-performance photovoltaic and optoelectronic devices. The structural, electronic, and optical properties of distinctive all-2D M2XBr4-black phosphorus (BP) [M = (C4H9NH3)+; X = Pb2+, Sn2+, Ge2+] van der Waals (vdW) heterostructures have been studied by first-principle calculations. The M2SnBr4-BP and M2GeBr4-BP heterostructures show type-II band arrangement; however, the M2PbBr4-BP heterostructure exhibits type-I band arrangement. The energy level shift is ascribed to the difference of work function between M2XBr4 monolayer and BP monolayer, driving the movement of carriers spontaneously. Furthermore, the BP layers can enhance the light absorption of the total heterostructures, especially the M2GeBr4-BP heterostructure. These results indicate the all-2D perovskite and BP vdW heterostructures are competitive candidates for low-dimensional photovoltaic and optoelectronic applications.

High transport and excellent optical property of a two-dimensional single-layered hybrid perovskite (C4H9NH3)2PbBr4: a theoretical study.

Organic-inorganic hybrid perovskites are developed to pursue high charge carrier mobility and light absorption coefficient. In this study, we present a detailed comparative research of the atomic and electronic structures of single-layered perovskites (C4H9NH3)2PbBr4 with two-dimensional/three-dimensional (2D/3D) spatial arrangement to predict the in plane charge carrier mobility along with the charge effective mass, elastic constant, and deformation potential. The calculated results reveal that the intrinsic in plane carrier mobilities of 2D single-layered hybrid perovskite (C4H9NH3)2PbBr4 along the 100 and 010 directions are superior to those of the 3D structure. Furthermore, the optical properties are calculated from the electronic structure; it is found that the light absorption spectrum of 2D single-layered perovskite (C4H9NH3)2PbBr4 with a high absorption coefficient is wider than that of the 3D phase. We speculate that the superior mobility and wider absorption spectrum of the 2D mono-layered perovskite are due to high charge density and ferroelectricity originating from structure distortion upon 3D-to-2D structure transformation. These results indicate that the 2D single-layered hybrid perovskite (C4H9NH3)2PbBr4 is a potential candidate for application in the optoelectronic and photovoltaic fields.

Pressure-induced strong ferroelectric polarization in tetra-phase perovskite CsPbBr3.

Ab initio simulations combined with the Berry phase method are employed to investigate ferroelectric polarization of tetragonal CsPbBr3 crystals by applying hydrostatic pressure varying from 0 to 19 GPa; we find that the object research belongs to the P4mm space group. The calculated results show that the materials undergo a paraelectric-ferroelectric phase transition when the pressure increases to a critical value 15 GPa. The polarization is strongly enhanced and attains a high value of about 23 µC cm-2, owing to the increase in the ionic and electric contributions to the polarization under compressive strain. We present a detailed theoretical investigation to analyze the origin of polarization. The ionic polarization is mainly ascribed to the central displacements of Pb2+ cations and Br- anions induced by a highly distorted octahedral PbBr6- framework. Electronic structure calculations suggest that asymmetric hopping p orbital electrons of Br(3) ions are responsible for the enhancement in electric polarization. These discoveries suggest that tetragonal CsPbBr3 has significant potential in future ferroelectric applications, and this can broaden the application field from optoelectronics to ferroelectrics.