Two-dimensional Perovskites Research Articles

Bifunctional Lead–Iron Oxyfluorides, 3D Cubic PbFeO2f and 2D Layered Pb3Fe2O5F2, Driving Photoelectrochemical and Electrochemical Water-Oxidation

Introduction Photoelectrochemical water splitting has attracted attention as a method of generating solar fuel such as hydrogen gas to be useful as a renewable energy carrier and a clean fuel of an internal‐combustion engine. For water oxidation of the half reaction in the water splitting, mixed-anion compounds have appeared as good candidates of the catalyst. In the mixed-anion compounds, a metal center is coordinated by several kind of anionic species. This environment induces their wide variety of physical and chemical properties not to be realized in single anion counterparts. The mixed-anion compounds possess visible-light responsibility and can utilize sunlight efficiently, therefore, which have fascinated photocatalyst researchers. The reason why they absorb visible light is the narrow bandgap with less electronegative anion species than oxygen. However, they are not very stable for photoelectrochemical water-oxidation because photogenerated holes oxidize the anion species except for oxygen in the compounds and cause degradation. Recently, we have reported an oxyfluoride Pb2Ti2O5.4F1.2 as a stable photoanode material that absorbs visible light at wavelengths up to ~500 nm.1 Oxyfluoride containing O2− and F− anions in the same phase is an example of the mixed-anion compounds. The other anion F− is less oxidized than O2−. Thus, the Pb2Ti2O5.4F1.2 photoanode showed essential stability toward the degradation by photogenerated holes exceptionally. However, this is the sole example of a visible-light-responsive oxyfluoride photoanode. Development of a new oxyfluoride photoanode absorbing visible light is important to gain the insight for water oxidation using the oxyfluorides. In the present work, another type of lead-iron oxyfluorides, PbFeO2F and Pb3Fe2O5F2, are applied to water oxidation as not only a photoanode but also an electrocatalyst.2,3 The lead-iron oxyfluoride, PbFeO2F, drives photoelectrochemical water-oxidation in visible light up to ~600 nm. In addition, this oxyfluoride functions as the electrocatalyst for water oxidation.2 In the electrochemical water-oxidation, another Pb3Fe2O5F2 shows higher performance for water oxidation than PbFeO2F. It seems that the crystal structure of two-dimensional layered perovskite Pb3Fe2O5F2 influences the superior performance to three-dimensional cubic perovskite PbFeO2F.3 Results and Discussion PbFeO2F and Pb3Fe2O5F2 powder were synthesized through a solid-state reaction and that in the evacuated glass tube, respectively.2,3 They were deposited on a fluorine-doped tin oxide (FTO) glass substrate by using electrophoresis method. The PbFeO2F or Pb3Fe2O5F2/FTO electrode was subjected to post-necking treatment to deposit TiO2 layer (TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode), followed by electrodeposition of cobalt phosphate (Co-Pi) cocatalyst (Co-Pi/TiO2/PbFeO2F or Pb3Fe2O5F2/FTO electrode). Linear-sweep voltammetry for the as-prepared electrodes was conducted under intermittent visible-light irradiation (>400 nm) in 0.1 M K3PO4 solution (Figure 1A). The PbFeO2F/FTO electrode exhibited no photocurrent response. By contrast, a slight photocurrent response was observed for the TiO2/PbFeO2F/FTO electrode because of mainly improved interparticle conductivity in the electrode. Furthermore, a clear anodic photocurrent was observed for the Co-Pi/TiO2/PbFeO2F/FTO electrode. The Co-Pi cocatalyst, known as a water-oxidation promoter, improved the charge transfer at the electrode/water interface and charge separation in the electrode. Meanwhile, the Co-Pi/TiO2/PbFeO2F/FTO electrode exhibited an anodic photocurrent in visible light with longer wavelengths than ~600 nm. These tendencies were like those of Pb3Fe2O5F2 electrodes though the photocurrent were slightly higher than the PbFeO2F electrodes (Figure 1A, right). On contrary to the photoelectrochemical water-oxidation, a clear difference was observed when using those lead-iron oxyfluorides as an electrocatalyst. Under dark conditions, controlled-potential electrolysis with non-modified PbFeO2F and Pb3Fe2O5F2/FTO electrode was performed at +1.7 V vs. RHE (Figure 1B). The evolved O2 was detected by GC analysis in the measurement. The amount of that was almost one-fourth of electrons flowed during the electrolysis, which means the water oxidation was the major path for the electrochemical reaction. Besides, Pb3Fe2O5F2/FTO electrode showed 8 times higher activity than PbFeO2F. That reason has not been revealed but the difference is likely to be attributed to two-dimensional layered perovskite Pb3Fe2O5F2 and three-dimensional cubic perovskite PbFeO2F. References Hirayama, H. Nakata, H. Wakayama, S. Nishioka, T. Kanazawa, R. Kamata, Y. Ebato, K. Kato, H. Kumagai, A. Yamakata, K. Oka and K. Maeda, Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode, J. Am. Chem. Soc., 141, 17158 (2019).Mizuochi, K. Izumi, Y. Inaguma and K. Maeda, A bifunctional lead–iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation, RSC Adv., 11, 25616 (2021).Mizuochi, K. Oka, Y. Inaguma and K. Maeda, A two-dimensional perovskite oxyfluoride Pb3Fe2O5F2 as a catalyst for electrochemical oxidation of water to oxygen, Sustain. Energy Fuels, 6, 2423 (2022). Figure 1

(Invited) Synthesis and Properties of Novel PL-Active 2D Lead-Free Double Perovskites

In this work, novel two-dimensional lead-free double perovskites ((R)4MaMbMcX8, a+b+c=2) were synthesized to achieve novel compositional and dimensional tunability. Moreover, the samples were tuned to be PL-emissive under room temperature via a manganese alloying strategy. Bandgap and PL emission turnability were explored through halogen substitution (chlorine and iodine). An enhanced PL emission was observed for mixed halogen phases. Three emission pathways are possible to be investigated including band edge emission, emission stemmed from self-trapped excitons and the unique energy transfer process of alloyed Mn2+ ions, which will be discussed.

Halide Ion Migration in 2D Metal Halide Perovskites

Two-dimensional (2D) halide perovskites have drawn attention for their unique optoelectronic properties. Interestingly, Ruddlesden-Popper 2D mixed-halide perovskite films with spacer cations such as butylammonium and Dion-Jacobson 2D mixed-halide perovskite films with 1,4-phenylenedimethanammonium undergo halide segregation under irradiation. Anions become mobile under steady state irradiation to form I-rich and Br-rich domains. Upon ceasing illumination, film reverts to their original, mixed-halide compositions, establishing the reversibility of the phase segregation phenomenon. However, if films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to 2D films. The sequential disappearance of I- and Br- from pristine films (BA2Pb2Br4, and BA2Pb2I4) under photoirradiation, as observed from variances in expulsion rates, reflect differences in halide ion mobilities in these films. The impact of anion migration, leading to I- and Br- expulsion into solvents, within the context of 2D perovskite photostability will be discussed.

Electronic and optical properties of two-dimensional perovskite materials in DJ and RP phases: density functional theory approach

Electronic and optical properties of two-dimensional perovskite materials in DJ and RP phases: density functional theory approach

Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites.

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic-inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals. Here, we employ one- and two-quantum two-dimensional white-light microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA2MAn-1PbnI3n+1 (n = 2-4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness (n). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and two-quantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.

Built-in Electric Field in Quasi-2D CsPbI3 Perovskites Using High-Polarized Zwitterionic Spacer for Enhanced Charge Separation/Transport.

Two-dimensional (2D) halide perovskites are promising candidates for the fabrication of stable and high-efficiency solar cells. However, the low power conversion efficiency (PCE) of cell devices using 2D perovskites is attributed to reduced charge transport caused by poor organic barrier conductivity. In this study, we propose the use of a high-polarized organic zwitterionic spacer, p-aminobenzoic acid (PABA), to construct novel quasi-2D perovskite structures with enhanced self-driven charge separation and transfer. The NH3+ and COO- groups in PABA generate an aligned electric field, promoting carrier separation and aggregation on the opposite edges of the inorganic layer. This enables efficient in-plane transportation along the inorganic layer. Additionally, PABA intercalated quasi-2D perovskite exhibits improved stability compared with counterparts with diamine cation spacers due to the strong interaction between -COO- and inorganic layers. Our findings suggest that high-polarized organic zwitterionic spacers, with NH3+ and COO- functionality, hold promise for stable and efficient quasi-2D perovskite solar cells.

Thermodynamic stability and electronic properties of 2D all-inorganic halogen perovskites

Two-dimensional (2D) all-inorganic metal halide perovskites capture more attentions due to its superior stability compared with three-dimensional (3D) perovskites. However, the power conversion efficiency of 2D all-inorganic perovskite is needed to be largely improved and few theoretical research focuses on its structural stability and optoelectronic properties in depth. Herein, we exploit the electronic structure, thermodynamically stable phase, effective masses, and optical properties of Ruddlesden-Popper (RP) 2D inorganic perovskites Csn+1MnIn+1Cl2n (M = Pb or Sn, n = 1–4 and ∞) by employing density functional theory calculations. It is found that the most stable configuration is closely related to the specific position of the halogen atoms. Moreover, the thermodynamic stability of Pb-based 2D perovskites is further calculated based on the phase diagram, which is notably more stable than that of Cs2SnI2Cl2. In addition, our calculated results show that the band gaps decrease as the number of layers increase in both Sn- and Pb-based perovskites. Moreover, Sn-based 2D perovskites can be adjusted to superior electronic properties with the optimal bandgap ranging from 0.9 to 1.6 eV and small effective mass of carriers considering its future application. This systematic theoretical study provides a significant methodology for fabricating stable, new-type 2D inorganic photoelectric materials according to a reasonable stoichiometric ratio.

Two-dimensional perovskite heterostructures for single crystal semiconductor devices

Two-dimensional (2D) perovskites have gained much attention lately owing to their excellent optoelectronic properties, chemical tunability, and environmental stability. Multiple methods have been devised to synthesize high quality 2D perovskite single crystals, and recent progress in fabricating its heterostructures is notable as well. In particular, with growing interest in 2D van der Waals heterostructures, 2D perovskites have become a strong candidate as a new building block for heterostructures to reveal unique physical properties across different interfaces. Until now, various heterostructure devices of 2D perovskite single crystals with other types of 2D materials such as transition metal dichalcogenides (TMDs) and graphene have been studied, which have shown intriguing results including interlayer excitons and enhanced electronic properties. Here, we introduce various synthetic approaches to realize 2D perovskite single crystals and unique characteristics of their single crystal heterostructures fabricated with precision, possessing sharp interfaces. Moreover, recent studies of semiconductor devices based on 2D perovskite single crystal heterostructures are discussed in-depth. New perspectives to further the horizon in the field of 2D perovskite heterostructures are suggested in this work including the consideration of metal–2D material van der Waals contact, application of dry transfer techniques, electric bias driven ion diffusion studies, and nanocrystal array fabrication. 2D perovskite heterostructure single crystal devices factoring in these novel perspectives will further uncover the true potential of these materials for highly efficient and stable semiconductor devices.

Spatially Correlated Chirality in Chiral Two-Dimensional Perovskites Revealed by Second-Harmonic-Generation Circular Dichroism Microscopy.

Understanding the chiral mechanism of chiral hybrid perovskites is a prerequisite for developing relevant chiroptoelectronic applications. Although conventional circular dichroism (CD) spectroscopy can be used to characterize chirality in chiral perovskites, it has a low signal-to-noise ratio and can provide only information about macroscopic chirality. Herein, with the aim of revealing the microscopic chiral mechanism in chiral perovskites, we utilize a spacer cation alloying strategy to construct chiral two-dimensional perovskites. For the first time, we demonstrate second-harmonic-generation CD microarea imaging in chiral perovskite thin films to unveil their spatially correlated chirality. In combination with theoretical calculations, it is revealed that the spatially correlated chirality is caused by localized out-of-plane supramolecular orientations. This work will not only advance the understanding of the mechanism of chiroptical activity in chiral perovskites but also provide inspiration for the rational design and synthesis of perovskites for chirality-related nonlinear optoelectronic devices.

First-principles study of electronic properties of two-dimensional metal mixed perovskites solar cell and water resistance performance

Two-Dimensional (2D) layered hybrid organic-inorganic perovskite (HOIP) materials have been considered as potential photoconductive materials for solar cells owing to extraordinary conversion efficiency and easy preparation process. In this study, we perform first-principles design on metal mixed 2D HOIPs (BA2(Cs1-x Rb x )2Sn1.5Pb1.5I10 with different proportions of composition (x = 0, 0.5 and 1)) to investigate their electronic properties and stability against moisture. We find that the band gap and effective mass of BA2(Cs1-x Rb x )2Sn1.5Pb1.5I10 increases as the concentrations of mixed Rb atoms increases. Moreover, the result of calculated adsorption energy indicates that the metal mixed 2D HOIPs with different composition ratios of Rb atom exhibit better capacity on the resistance of water than the pure one (BA2MA2Sn3I10). The metal mixed 2D HOIPs shows great potential to be a new generation of solar cell with low-cost, suitable band gap, high thermal stability, and desirable power conversion efficiency.

Two-dimensional layered CsPb2Br5 perovskite as high-performance electrode material for supercapacitor application

Two-dimensional layered CsPb2Br5 perovskite as high-performance electrode material for supercapacitor application

A Two-Dimensional Hybrid Lead Bromide Ferroelectric Semiconductor with an Out-of-Plane Polarization.

A two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) material with out-of-plane ferroelectricity is the key to the miniaturization of vertical-sandwich-type ferroelectric optoelectronic devices. However, 2D OIHP ferroelectrics with out-of-plane polarization are still scarce, and effective design strategies are lacking. Herein, we report a novel 2D Dion-Jacobson perovskite ferroelectric semiconductor synthesized by a rigid-to-flexible cationic tailoring strategy, achieving an out-of-plane polarization of 1.7 μC/cm2 and high photoresponse. Integrating out-of-plane ferroelectricity with excellent photoelectric properties affords a promising platform to investigate ferroelectricity-related effects in vertical optoelectronic devices.

Suppressing halide phase segregation in wide-bandgap perovskite film by co-doping strategy for high-performance and stable perovskite solar cells

Wide-bandgap perovskites formed by tuning the iodine/bromine ratio can be combined with narrow-bandgap silicon materials to construct tandem solar cells, thus overcoming the Shockley-Queissel limit. However, light-induced halide ion migration causes poor perovskite film quality and detrimental halide phase segregation in it. Herein, a facile co-doping strategy for wide-bandgap (FA0.65MA0.2Cs0.15)Pb(I0·8Br0.2)3 film is demonstrated, that is, Pb(SCN)2 and PEACl are synergistically incorporated into perovskite film to break through this hurdle. The proposed strategy not only enables high-quality perovskite films with larger grains, fewer grain boundaries, and high crystallinity, but also the two-dimensional (2D) perovskite formed at grain boundaries effectively suppresses halide phase separation and improves the hydrophobicity of the films. As a result, we achieved a power conversion efficiency (PCE) of 19.92% for the small-area (0.07 cm2) perovskite solar cells (PSCs) and 17.70% for modules with an active area of 10.00 cm2. And, the operational and long-term storage stability of the modules are improved, maintaining 76% of their initial efficiency after 150 h of exposure to an environment with a relative humidity of ∼40% and a temperature of ∼20 °C. Furthermore, a semitransparent device with a PCE of 19.51% and a four-terminal (4T) perovskite/silicon tandem solar cell with a PCE of 30.53%, were successfully obtained.

Chiral Cation Doping for Modulating Structural Symmetry of 2D Perovskites.

Cation mixing in two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) structures represents an important degree of freedom for modifying organic templating effects and tailoring inorganic structures. However, the limited number of known cation-mixed 2D HOIP systems generally employ a 1:1 cation ratio for stabilizing the 2D perovskite structure. Here, we demonstrate a chiral-chiral mixed-cation system wherein a controlled small amount (<10%) of chiral cation S-2-MeBA (S-2-MeBA = (S)-(-)-2-methylbutylammonium) can be doped into (S-BrMBA)2PbI4 (S-BrMBA = (S)-(-)-4-bromo-α-methylbenzylammonium), modulating the structural symmetry from a higher symmetry (C2) to the lowest symmetry state (P1). This structural change occurs when the concentration of S-2-MeBA, measured by solution nuclear magnetic resonance, exceeds a critical level─specifically, for 1.4 ± 0.6%, the structure remains as C2, whereas 3.9 ± 1.4% substitution induces the structure change to P1 (this structure is stable to ∼7% substitution). Atomic occupancy analysis suggests that one specific S-BrMBA cation site is preferentially substituted by S-2-MeBA in the unit cell. Density functional theory calculations indicate that the spin splitting along different k-paths can be modulated by cation doping. A true circular dichroism band at the exciton energy of the 3.9% doping phase shows polarity inversion and a ∼45 meV blue shift of the Cotton-effect-type line-shape relative to (S-BrMBA)2PbI4. A trend toward suppressed melting temperature with higher doping concentration is also noted. The chiral cation doping system and the associated doping-concentration-induced structural transition provide a material design strategy for modulating and enhancing those emergent properties that are sensitive to different types of symmetry breaking.

Recent advances in bioinspired vision sensor arrays based on advanced optoelectronic materials

Animals can learn about the outside world in many ways, and the visual organ is a key organ for acquiring information about the outside world. With the continuous development of intelligent technology, artificial vision techniques are becoming easier and more automated; however, the rigidity, process complexity, and complicated optical components of traditional commercial photodetectors have hindered their development in bionic vision. In recent years, a new generation of optoelectronic materials has attracted extensive research due to their simple preparation process, continuously tunable bandgap, and excellent optoelectronic properties. Two-dimensional optoelectronic materials and perovskites have become the most promising and effective optoelectronic materials for next-generation optoelectronic devices. Based on the excellent properties of next-generation optoelectronic materials, they have also triggered intensive exploration by researchers in the field of visual bionics. This paper highlights a review of the latest research progress of next-generation optoelectronic materials, including their preparation methods, working mechanisms, structural designs, and advances in the field of imaging. The applications of new generation optoelectronic materials in visual bionics by simulating biological visual structures are also described. Finally, the prospects and challenges for the development of next-generation optoelectronic materials in the emerging field of bionic vision are discussed.

Bose glass and Fermi glass

It is known that two-dimensional superconducting materials undergo a quantum phase transition from a localized state to superconductivity. When the disordered samples are cooled, bosons (Cooper pairs) are generated from Fermi glass and reach superconductivity through Bose glass. However, there has been no universal expression representing the transition from Fermi glass to Bose glass. Here, we discovered an experimental renormalization group flow from Fermi glass to Bose glass in terms of simple beta-function analysis. To discuss the universality of this flow, we analyzed manifestly different systems, namely a Nd-based two-dimensional layered perovskite and an ultrathin Pb film. We find that all our experimental data for Fermi glass fall beautifully into the conventional self-consistent beta-function. Surprisingly, however, flows perpendicular to the conventional beta-function are observed in the weakly localized regime of both systems, where localization becomes even weaker. Consequently, we propose a universal transition from Bose glass to Fermi glass with the new two-dimensional critical sheet resistance close to R_Box = h/e^{2}.

Vibrational and Structural Properties of Two-Dimensional Tin Mixed-Halide Perovskites

The emergence of two-dimensional (2D) hybrid metal-halide perovskites has garnered significant attentions for optoelectronic devices and light-emitting applications. Since the toxicity of lead-based perovskites could potentially be harmful to the environment, several works have attempted to change the active metal to tin (Sn). Here, we investigate the characterization of (PEA)2SnBrxI4-x mixed halide perovskites using X-ray fluorescence (XRF), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Qualitative XRF analysis suggests the presence of tin, bromine and iodine emissions under the mid-Z and high-Z ranges. In mid-Z range, Br-Kα peak appeared on 11.96 keV and Br-Kβ was detected on 13.3 keV. Meanwhile Sn-Kα, I-Kα, I-Kβ1, and I-Kβ2 peaks were detected in high-Z range on 25.24 keV, 28.6 keV, 32.35 keV and 33.11 keV, respectively. Thus, the elemental composition of mixed halide components exhibits an indicative control that bromine-rich or iodine-rich can be synthesized via rational chemical design. XRD pattern display a systematic progression at the peak 5.18° (corresponds to (002) plane), which unambiguously demonstrated the feasibility to tune halide composition in tin-based hybrid perovskite. It also confirms that (2D) hybrid metal-halide with tunable halide have identical structure for both bromine-rich and iodine-rich composition. Furthermore, the 2θ peaks slightly shifted to lower angle with increasing bromine composition. The presence of C−I bonding on ~500 cm-1 and C-Br bond on ~600 cm-1 in FTIR spectra highlights the functional group of organic cations. These experimental results promote a foundation to implement compositional engineering on 2D-tin mixed-halide perovskites for optoelectronics and scintillators.

Reduction of Hole Carriers by van der Waals Contact for Enhanced Photoluminescence Quantum Yield in Two-Dimensional Tin Halide Perovskite

Reduction of Hole Carriers by van der Waals Contact for Enhanced Photoluminescence Quantum Yield in Two-Dimensional Tin Halide Perovskite

Excellent Switchable Properties, Broad-Band Emission, Ferroelectricity, and High Tc in a Two-Dimensional Hybrid Perovskite: (4,4-DCA)2PbBr4 Exploited by H/F Substitution.

Switchable materials have gained significant attention due to their potential applications in data storage, sensors, and switching devices. Two-dimensional (2D) hybrid perovskites have demonstrated promising prospects for designing switchable materials, where the dynamic motion of the organic components coupled with the distortion of the inorganic framework provides the driving force for triggering multifunctional switchable properties. Herein, through the H/F substitution strategy, we report a polar 2D hybrid lead-based perovskite, (4,4-DCA)2PbBr4 (4,4-DCA = 4,4-difluorocyclohexylammonium) (1), which exhibits dual-stable behavior in a dielectric and second harmonic generation (SHG) response during the reversible phase transition process near the high Curie temperature Tc ∼ 409 K. The phase transition temperature is significantly increased by 41 K compared to the corresponding non-fluorinated (CHA)2PbBr4 (CHA = cyclohexylammonium). Remarkably, the material shows rare broad-band yellow emission under UV excitation, attributed to the induction of self-trapped exciton emission by the distortion of the [PbBr6]4- octahedra, as confirmed by the first-principles analysis. 1 also exhibited ferroelectricity with a saturation polarization value and a small coercive field. This study provides a new insight into the modification of multifunctional switchable materials through the H/F substitution strategy.

Continuous direct band-gap narrowing and piezochromism of lead-free two-dimensional perovskite nanocrystals under high pressure

Continuous direct band-gap narrowing and piezochromism of lead-free two-dimensional perovskite nanocrystals under high pressure