Ruddlesden Popper Hybrid Perovskites Research Articles

Fast rotating dipole array inducing large dielectric response in a Ruddlesden–Popper hybrid perovskite ferroelastic

A 2D RP hybrid perovskite shows dynamic rotating motion of the polar rotor which inducing ferroelasticity and a tremendous dielectric response. This study provides a new vision for the development of dynamic functional materials.

Phonon-Assisted Nonradiative Recombination Tuned by Organic Cations in Ruddlesden-Popper Hybrid Perovskites

Significant efforts have been devoted to further increasing the photoconversion efficiency of two-dimensional (2D) organic-inorganic halide perovskites, which are promising photovoltaic materials with superior stability but lower efficiency compared to their three-dimensional counterparts. One of the main factors that limit their photoconversion efficiency is the nonradiative recombination of photoexcited carriers. A widely used strategy for tuning the photoconversion efficiency of 2D perovskites is exchanging various organic cations. However, due to mixed effects contributed to by extrinsic factors, such as defect concentration and sample morphology, experiments alone are insufficient to explain the role of organic cations in phonon-assisted carrier recombination. With time-domain simulations based on first-principles nonadiabatic molecular dynamics, we investigate six prototypical 2D Ruddlesden-Popper perovskites to reveal the impact of organic cations on tuning the nonradiative recombination time through displacing inorganic ions and affecting electron-phonon coupling. The phonon-assisted band-to-band nonradiative recombination time is on the order of a few hundred nanoseconds and can be tuned up to 5 times by modifying organic cations. A distinct correlation between the pure-dephasing time and inorganic atom distortions induced by the motion of organic parts is revealed. Compared to the pure-dephasing time, nonadiabatic coupling (NAC) associated with the strength of electron-phonon coupling plays a more important role in determining the nonradiative carrier recombination time. Notably, fluorinated organic spacers increase the NAC between frontier electronic states and speed up the nonradiative recombination process.

Sub-angstrom noninvasive imaging of atomic arrangement in 2D hybrid perovskites.

Noninvasive imaging of the atomic arrangement in two-dimensional (2D) Ruddlesden-Popper hybrid perovskites (RPPs) is challenging because of the insulating nature and softness of the organic layers. Here, we demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable nonperturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. Moreover, such a joint technique also allows for the atomic-scale imaging of the electrostatic potential variation across the twin-domain walls, revealing alternating quasi-1D electron and hole channels at neighboring twin boundaries, which may influence in-plane exciton transport and dissociation.

2D Hybrid perovskite incorporating cage-confined secondary ammonium cations toward effective photodetection

A Ruddlesden–Popper hybrid perovskite was assembled by confining secondary ammonium cations into the inorganic framework, for which 2D quantum-well motifs render a fascinating photoresponse and effective photodetection properties.

First-principles study of lead-free Ge-based 2D Ruddlesden-Popper hybrid perovskites for solar cell applications.

Recently, 2D halide perovskites have attracted attention because they are excellent photo absorbing materials for perovskite solar cells. To date, the majority of 2D perovskite-based devices have been made of Pb, a material with toxic properties and environmental concerns. Thus, lead-free alternatives are essential to enable the expansion of photovoltaic systems based on perovskites. Herein, we examine the structural, electronic, optical and stability properties of Pb-free 2D Ruddlesden-Popper (RP) perovskites (BA)2(MA)n-1GenI3n+1 (BA = CH3(CH2)3NH3+; MA = CH3NH3+; n = 1-5, and ∝) by using DFT calculations and comparing the results to their Pb-based counterparts (BA)2(MA)n-1PbnI3n+1 (n = 1-5, and ∝). Theoretical analysis indicates that Pb and Ge-based 2D perovskites are significantly more thermodynamically stable than their corresponding 3D materials. A more accurate bandgap is achieved using the HSE06 + SOC scheme and compared to the findings of the PBE and PBE + SOC. These materials are direct bandgap semiconductors. Due to spin-orbit coupling, Pb-based perovskite displays higher Rashba energy splitting than Ge-based ones. The bandgap changes from 2.37 eV (n = 1) to 1.79 eV (n = 5), and from 1.92 eV (n = 1) to 1.56 eV (n = 5) for Pb and Ge-based perovskites, respectively. The bandgap of all Ge-based perovskites is lower than their corresponding Pb-based ones. We show that the 2D perovskites could serve as hole-transporting materials when they are alongside 3D perovskites. The trade-off between thermodynamic stability and absorption coefficient of the considered compounds indicates that 2D RP perovskites BA2MA4Ge5I16 are promising Pb-free halide semiconductors for solar cell applications.

Bimolecular Generation of Excitonic Luminescence from Dark Photoexcitations in Ruddlesden-Popper Hybrid Metal-Halide Perovskites.

The nature of photoexcitations in Ruddlesden-Popper (RP) hybrid metal halide perovskites is still under debate. While the high exciton binding energy in the hundreds of millielectronvolts indicates excitons as the primary photoexcitations, recent reports found evidence for dark, Coulombically screened populations, which form via strong coupling of excitons and the atomic lattice. Here, we use time-resolved mid-infrared spectroscopy to gain insights into the nature and recombination of such dark excited states in (BA)2(MA)n-1PbnI3n+1 (n = 1,2,3) via their intraband electronic absorption. In stark contrast to results in the bulk perovskites, all samples exhibit a broad, unstructured mid-IR photoinduced absorbance with no infrared activated modes, independent of excitonic confinement. Further, the recombination dynamics are dominated by a bimolecular process. In combination with steady-state photoluminescence experiments, we conclude that screened, dark photoexcitations act as a population reservoir in the RP hybrid perovskites, from which nongeminate formation of bright excitons precedes generation of photoluminescence.

Structural and Optoelectronic Properties of Two-Dimensional Ruddlesden-Popper Hybrid Perovskite CsSnBr3.

Ultrathin inorganic halogenated perovskites have attracted attention owing to their excellent photoelectric properties. In this work, we designed two types of Ruddlesden–Popper hybrid perovskites, Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2, and studied their band structures and band gaps as a function of the number of layers (n = 1–5). The calculation results show that Csn+1SnnBr3n+1 has a direct bandgap while the bandgap of CsnSnn+1Br3n+2 can be altered from indirect to direct, induced by the 5p-Sn state. As the layers increased from 1 to 5, the bandgap energies of Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2 decreased from 1.209 to 0.797 eV and 1.310 to 1.013 eV, respectively. In addition, the optical absorption of Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2 was blue-shifted as the structure changed from bulk to nanolayer. Compared with that of Csn+1SnnBr3n+1, the optical absorption of CsnSnn+1Br3n+2 was sensitive to the layers along the z direction, which exhibited anisotropy induced by the SnBr2-terminated surface.

Cage-incorporation of secondary amine in Ruddlesden–Popper 2D hybrid perovskite with strong photoconductivity and polarization response

Through the mixed-cation alloying strategy, we have successfully introduced secondary DMA+ cations in perovskite cages to design a new 2D Ruddlesden–Popper hybrid perovskite, showing great potential in photodetection and polarization detection.

Edge States Drive Exciton Dissociation in Ruddlesden–Popper Lead Halide Perovskite Thin Films

Efficient photovoltaic cells based on thin films of solution-processed 2D Ruddlesden–Popper hybrid perovskites (RPPs) represent an exciting breakthrough due to their enhanced tunability and chemica...

Modulation of Broadband Emissions in Two-Dimensional ⟨100⟩-Oriented Ruddlesden–Popper Hybrid Perovskites

Two-dimensional (2D) Ruddlesden–Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions,...

Spontaneous Formation of 2D/3D Heterostructures on the Edges of 2D Ruddlesden–Popper Hybrid Perovskite Crystals

The observation of low energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden-Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modellings and observations also do not seem to converge. Using 2D (BA)2(MA)2Pb3Br10 as an example, we show that 3D MAPbBr3 is formed due to the loss of BA on the edge. This self-formed MAPbBr3 can explain the reported edge emission under various conditions, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by submicron infrared spectroscopy, the emergence of XRD signature from freezer-milled nanometer-sized 2D perovskite and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach to engineering 2D perovskites for high-performance optoelectronic devices.

Direct assessment of structural order and evidence for stacking faults in layered hybrid perovskite films from X-ray scattering measurements

The in-plane (0k0) reflections in highly textured Ruddlesden–Popper hybrid perovskite films are probed using synchrotron-based X-ray scattering measurements, providing direct evidence for the presence of structurally ordered n-phases.

Origin of Broad-Band Emission and Impact of Structural Dimensionality in Tin-Alloyed Ruddlesden–Popper Hybrid Lead Iodide Perovskites

Hybrid organic–inorganic lead halide perovskites have shown promising results as active layers in light-emitting diodes, typically utilizing the near-monochromatic, free exciton emission. Some perovskite compounds, however, show broad-band emission that is more intense than the free exciton counterpart. In this study, we show that the light emission properties of Ruddlesden–Popper hybrid perovskites PEA2MAn–1PbnI3n+1 (PEA = phenethylammonium, MA = methylammonium) can be tuned by Sn alloying and are highly sensitive to Sn %. With increasing dimensionality, the broad-band emission quantum yield decreases drastically, from 23% in n = 1 to <1% for the n = 3 compound. Using density functional theory calculations and transient reflectance spectroscopy, the broad emission is identified as originating from self-trapped excitons. A dynamic picture of the formation process is also presented, for which ultrafast (<5 ps) hole-trapping at the Sn site is the first step, followed by electron localization from Coulombic ...

Tuning Hot Carrier Cooling Dynamics by Dielectric Confinement in Two-Dimensional Hybrid Perovskite Crystals

Hot carrier (HC) cooling is a critical photophysical process that significantly influences the optoelectronic performance of hybrid perovskite-based devices. The hot carrier extraction at the device interface is very challenging because of its ultrashort lifetime. Here, ultrafast transient reflectance spectroscopy measurements and time-domain ab initio calculations show how the dielectric constant of the organic spacers can control and slow the HC cooling dynamics in single-crystal 2D Ruddlesden-Popper hybrid perovskites. We find that (EA)2PbI4 (EA = HOC2H4NH3+) that correspond to a high dielectric constant organic spacer has a longer HC cooling time compared to that of (AP)2PbI4 (AP = HOC3H6NH3+) and (PEA)2PbI4 (PEA = C6H5C2H4NH3+). The slow HC relaxation process in the former case can be ascribed to a stronger screening of the Coulomb interactions, a small nonradiative internal conversion within the conduction bands, as well as a weak electron-phonon coupling. Our findings provide a strategy to prolong the hot carrier cooling time in low-dimensional hybrid perovskite materials by using organic spacers with reduced dielectric confinement.

General Synthesis Principles for Ruddlesden–Popper Hybrid Perovskite Halides from a Dynamic Equilibrium

Two-dimensional (2D) Ruddlesden–Popper hybrid perovskites are a homologous series of compounds with the formula A′2An–1BnX3n+1 (A′ = bulky organic cation; A = small organic cation; B = Pb2+ or Sn2+, X = Cl–, Br–, I–; n is an integer) composed of inorganic octahedral layers separated by organic spacer cations. The octahedral layer thickness is modified by the stoichiometry of the A-site cation, but limited methods exist for controlled and discriminating synthesis for all compositions. We report a general synthesis method and its principles that yield phase-pure 2D hybrid perovskites; the chemistry operates within a dynamic equilibrium established by the molar solubility of the compounds within the homologous series. A solvent–antisolvent (HI–acetic acid) pair and the common-ion effect provide selective control over the molar solubility to precipitate phase-pure compounds, as is supported by simple and predictive calculations. Here, this approach is demonstrated in detail with A′ = n-butylammonium, A = meth...

Tin and germanium based two-dimensional Ruddlesden-Popper hybrid perovskites for potential lead-free photovoltaic and photoelectronic applications.

Despite their high power conversion efficiency, the commercial applications of hybrid organic-inorganic lead (Pb) halide perovskite based solar cells are still hampered by concerns about the toxicity of Pb and the structural stability in open air. Herein, based on density-functional theory computation, we show that lead-free tin (Sn) and germanium (Ge) based two-dimensional (2D) Ruddlesden-Popper hybrid organic-inorganic perovskites with a thickness of a few unit-cells, BA2MAn-1MnI3n+1 (M = Sn or Ge, n = 2-4), possess desirable electronic, excitonic and light absorption properties, thereby showing promise for photovoltaic and/or photoelectronic applications. In particular, we show that by increasing the layer thickness of the Sn-based 2D perovskites, the bandgap can be lowered towards the optimal range (0.9-1.6 eV) for solar cells. Meanwhile, the exciton binding energy is reduced to a more optimal value. In addition, theoretical assessment indicates that the thermodynamic stability of Sn-/Ge-based 2D perovskites is notably enhanced compared to that of their 3D analogues. These features render the Sn-/Ge-based 2D hybrid perovskites with a thickness of a few tens of unit cells promising lead-free perovskites with much improved structural stabilities for photovoltaic and/or photoelectronic applications.

Two Regimes of Bandgap Red Shift and Partial Ambient Retention in Pressure-Treated Two-Dimensional Perovskites

The discovery of elevated environmental stability in two-dimensional (2D) Ruddlesden–Popper hybrid perovskites represents a significant advance in low-cost, high-efficiency light absorbers. In comparison to 3D counterparts, 2D perovskites of organo-lead-halides exhibit wider, quantum-confined optical bandgaps that reduce the wavelength range of light absorption. Here, we characterize the structural and optical properties of 2D hybrid perovskites as a function of hydrostatic pressure. We observe bandgap narrowing with pressure of 633 meV that is partially retained following pressure release due to an atomic reconfiguration mechanism. We identify two distinct regimes of compression dominated by the softer organic and less compressible inorganic sublattices. Our findings, which also include PL enhancement, correlate well with density functional theory calculations and establish structure–property relationships at the atomic scale. These concepts can be expanded into other hybrid perovskites and suggest that ...