Two-dimensional Hybrid Organic-inorganic Perovskites Research Articles

(C7H18N2)PbI4: A 2D Hybrid Perovskite Solid-State Phase Transition Material with Semiconducting Properties.

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) have gained attention as a result of their flexibility and adjustability of the structure. However, the large band gap of two-dimensional perovskites limits their application in the photoelectric field. In the present work, we report a two-dimensional organic-inorganic hybrid compound of (C7H18N2)PbI4 (1) with a narrow band gap, which consists of [PbI4]2-n layers and N-(2-aminoethyl)piperidinium cations. 1 exhibits semiconducting properties with a narrow optical band gap of ∼2.02 eV and a photoelectric response with a ratio of photocurrent to dark current of ∼100. In addition, it exhibits a reversible solid-state phase transition at 228 K. This finding should inspire research into more 2D layered OIHPs with the combination of phase transition and semiconductor properties.

Renormalization of the valence and conduction bands of (C6H5C2H4NH3)2PbI4 hybrid perovskite

Quasi two-dimensional hybrid organic-inorganic perovskites (HOIPs) have been rediscovered recently for photovoltaics due to a higher stability than other HOIPs. We focus here on the electronic structure of the 2D perovskite (C6H5C2H4NH3)2PbI4. We perform an experimental k-resolved determination of the valence and conduction bands by angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES). The experimental and theoretical dispersions are compared. The valence band width is in agreement with that of renormalized theoretical bands, while no significative renormalization is observed for the conduction band. The effect of the spin–orbit coupling in the conduction band is also experimentally observed.

In-Plane Mechanical Properties of Two-Dimensional Hybrid Organic-Inorganic Perovskite Nanosheets: Structure-Property Relationships.

In-plane strains are commonly found in two-dimensional (2D) metal halide organic-inorganic perovskites (HOIPs). The in-plane mechanical properties of 2D HOIPs are vital for mitigating the strain-induced stability issues of 2D HOIPs, yet their structure and mechanical property relationship largely remains unknown. Here, we employed atomic force microscope indentation to systematically investigate the in-plane Young's moduli E∥ of 2D lead halide Ruddlesden-Popper HOIPs with a general formula of (R-NH3)2PbX4, where the spacer molecules R-NH3+ are linear alkylammonium cations (CmH2m+1-NH3+, m = 4, 6, 8, or 12) and X = I, Br, or Cl. Fixing the spacer molecule to butylammonium, we discovered that the E∥ of 2D HOIPs generally follows the trend of Pb-X bond strength, different from the tendency found in the out-of-plane moduli E⊥, showing more prominent effects of the metal halide inorganic framework on E∥ than E⊥. E∥ exhibits nonmonotonic dependence on the chain length of the linear alkyl spacer molecules, which would first decrease and plateau but then increase again. This is likely due to the competition of the bond strength and structural distortion in the inorganic layer, the relative fraction of the soft organic spacers, and the interfacial mechanical coupling associated with the interdigitation of the alkyl chains. The mechanical anisotropy of 2D HOIPs, marked by E∥/E⊥, shows wide tunability based on structural composition, particularly for iodide-based 2D HOIPs. Our results provide valuable insights into the structure-property relationships regarding the mechanical anisotropy and in-plane mechanical behaviors of 2D HOIPs, which can guide the materials design and device optimization to achieve required mechanical performance in 2D HOIP-based applications.

Magneto-Electric Directional Anisotropy in Polar Soft Ferromagnets of Two-Dimensional Organic-Inorganic Hybrid Perovskites.

Two-dimensional organic-inorganic hybrid perovskites (2D-OIHPs) are attracting interest due to their structural tunability and rich functional characteristics, such as ferroelectricity and ferromagnetism. Here, we report the chiral-polar ferromagnetic 2D-OIHP copper chlorides with discernable electric polarization in the inorganic layers. In these systems, the magneto-electric (ME) correlation has been clearly observed by measuring a magneto-electric directional anisotropy (MEA), in which an optical absorption coefficient changes with reversal of the light propagating direction. We have found that the MEA can be induced by a low magnetic field of about 50 mT, reflecting soft magnetic nature. The present results suggest a new paradigm for designing functional ME multiferroics, which effectively couples magnetic and electric properties.

Cooperative Nature of Ferroelectricity in Two-Dimensional Hybrid Organic-Inorganic Perovskites.

Two-dimensional (2D) ferroelectric (FE) hybrid organic-inorganic perovskites (HOIPs) are promising for potential applications as miniaturized flexible ferroelectric/piezoelectric devices. Recently, several 2D HOIPs [e.g., Ruddlensden-Popper type HOIP BA2PbCl4 (BA = C6H5CH2NH3+)] were reported to possess room-temperature ferroelectricity. However, the underlying microscopic mechanisms for ferroelectricity in 2D HOIPs remain elusive. Here, by performing first-principles calculations and symmetry mode analysis, we demonstrate that there exists a cooperative coupling between A-site organic molecules and B-site inorganic Pb2+ ions that is essential to the ferroelectricity in 2D BA2PbCl4. The nonpolar ground state of the closely related compounds BA2PbBr4 and BA2PbI4 can also be explained in terms of the weakened cooperative coupling. We further predict that 2D BA2PbF4 displays in-plane ferroelectricity with a higher Curie temperature and larger electric polarization. Our work not only reveals the unusual FE mechanism in 2D HOIPs but also provides a solid theoretical basis for the rational design of 2D multifunctional materials.

Characterization of the Ammonium Bending Vibrations in Two-Dimensional Hybrid Lead-Halide Perovskites from Raman Spectroscopy and First-Principles Calculations

The facile synthesis and electronic properties of two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) make these self-assembled systems an important class of energy materials. The basic building blocks of these materials include inorganic lattice frameworks that often consist of lead-halide octahedra and organic molecules possessing ammonium functional groups. Understanding the coupling between the inorganic and organic layers is key to unraveling how the electronic properties of 2D HOIPs relate to their structures. In this work, we leverage Raman spectroscopy measurements and first-principles calculations to characterize the Raman-active modes in four 2D HOIPs: hexylammonium lead iodide [(HA)$_2$PbI$_4$, HA = C$_6$H$_{13}$NH$_3^+$], hexylammonium lead bromide [(HA)$_2$PbBr$_4$], butylammonium lead iodide [(BA)$_2$PbI$_4$, BA = C$_4$H$_9$NH$_3^+$], and benzylammonium lead iodide [(BNA)$_2$PbI$_4$, BNA = C$_6$H$_5$CH$_2$NH$_3^+$]. We focus on the 1400-1600 cm$^{-1}$ range where the Raman intensity of the molecular constituents is the strongest, and assign the major peaks observed in experiments as ammonium bending vibrations. We employ a combination of density functional perturbation theory based on the local density approximation and the frozen-phonon approach based on the vdw-DF-cx functional to find quantitative agreement between experimental and calculated Raman spectra. Furthermore, by comparing the vibrational spectra of isolated molecular cations with those near lead-halide clusters, we show how the inorganic lattice framework modulates the vibrational properties of the organic cations. We conclude that the properties of the Raman-active ammonium bending modes could effectively probe the local microscopic structure of the inorganic lattice framework in 2D HOIPs.

Spin-Dependent Photovoltaic and Photogalvanic Responses of Optoelectronic Devices Based on Chiral Two-Dimensional Hybrid Organic-Inorganic Perovskites.

Two-dimensional hybrid organic-inorganic perovskites (2D-HOIPs) that form natural multiple quantum wells have attracted increased research interest due to their interesting physics and potential applications in optoelectronic devices. Recent studies have shown that spintronics applications can also be introduced to 2D-HOIPs upon integrating chiral organic ligands into the organic layers. Here we report spin-dependent photovoltaic and photogalvanic responses of optoelectronic devices based on chiral 2D-HOIPs, namely, (R-MBA)2PbI4 and (S-MBA)2PbI4. The out-of-plane photocurrent response in vertical photovoltaic devices exhibits ∼10% difference upon right and left circularly polarized light (CPL) excitation, which originates from selective spin transport through the chiral multilayers. In contrast, the in-plane photocurrent response generated by CPL excitation of planar photoconductive devices shows a typical response of chirality-induced circular photogalvanic effect that originates from the Rashba splitting in the electronic bands of these compounds. Our studies may lead to potential applications of chiral 2D-HOIPs in optoelectronic devices that are sensitive to the light helicity.

A two-dimensional organic-inorganic lead iodide perovskite: poly[bis(3-fluorocyclobutylammonium) [di-μ-iodido-diiodidoplumbate(VI)]

In recent years, great technological advances have been achieved in the growth of hybrid organic-inorganic perovskites (HOIPs) and these have attracted extensive attention due to their optoelectronic properties, structural tunability and stability. We present here a new two-dimensional hybrid organic-inorganic perovskite, namely, poly[bis(3-fluorocyclobutylammonium) [di-μ-iodido-diiodidoplumbate(VI)]], {(C4H9FN)2[PbI4]}n, showing a two-dimensional reticular layer with the organic cations in the middle of the meshes. The calculated experimental band gap is 2.44 eV and the band gap is calculated as 2.20 eV theoretically, which further suggests the potential of this compound as a semiconductor.

Two-dimensional hybrid organic–inorganic perovskites as emergent ferroelectric materials

Hybrid organic–inorganic perovskite (HOIP) materials have attracted significant attention in photovoltaics, light emission, photodetection, etc. Based on the prototype metal halide perovskite crystal, there is a huge space for tuning the composition and crystal structure of this material, which would provide great potential to render multiple physical properties beyond the ongoing emphasis on the optoelectronic property. Recently, the two-dimensional (2D) HOIPs have emerged as a potential candidate for a new class of ferroelectrics with high Curie temperature and spontaneous polarization. Room-temperature solution-processability further makes HOIP a promising alternative to traditional oxide ferroelectrics such as BaTiO3 and PbTiO3. In this perspective, we focus on the molecular aspects of 2D HOIPs, their correlation with macroscopic properties, as well as the material design rules assisted by advanced simulation tools (e.g., machine learning and atomistic modeling techniques). The perspective provides a comprehensive discussion on the structural origin of ferroelectricity, current progress in the design of new materials, and potential opportunities and challenges with emerging materials. We expect that this perspective will provide inspiration for innovation in 2D HOIP ferroelectrics.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum-well thickness, ranging from 0.06 ± 0.03 to 0.34 ± 0.03 cm2 s−1, which leads to long-range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long-range exciton transport and slow annihilation highlights the unique attributes of two-dimensional perovskites as an exciting class of optoelectronic materials.

Advances in two-dimensional organic–inorganic hybrid perovskites

Recent achievements of 2D perovskites for various optoelectronic applications along with their basic properties and future opportunities are discussed.

Thin film transistor based on two-dimensional organic-inorganic hybrid perovskite

Despite the fact that three-dimensional organic-inorganic hybrid perovskite is regarded as a promising material in the field of optoelectronics and microelectronics due to its excellent photoelectric properties, however, the instability under the moisture environment and the gate-voltage screening effect associated with ionic transport are still serious, which restricts the development of perovskite devices. Here in this work, the lead iodide perovskite (PEA)<sub>2</sub>(MA)<i><sub>n–</sub></i><sub>1</sub>Pb<sub>n</sub>I<sub>3<i>n</i>+1</sub> series are successfully prepared by one-step solution method, including pure-two-dimensional (pure-2D), quasi-two-dimensional (quasi-2D) and traditional three-dimensional (3D) perovskite materials. The dimension and microstructure of the perovskites are regulated, and the effects of dimensions on the performance of organic-inorganic hybrid perovskite materials are investigated firstly. The crystallization of the 2D perovskites and 3D perovskite films are observed obviously. Moreover, the surface of pure-2D perovskite film with discoid, regular and micron-sized grains is smoother than that of 3D perovskite film. And also, the unapparent grain boundary is exhibited in the quasi-2D perovskites. A uniform perovskite film with full coverage and inconspicuous grain boundaries facilitates the transmission capacity of the charge carriers in the channel layer due to the reduction of defects caused by the grain boundaries. And benefited from the high-quality films with inconspicuous grain boundary as demonstrated, the quasi-2D hybrid perovskite film exhibits a longer carrier lifetime (<i>τ</i><sub>ns</sub>) than traditional 3D MAPbI<sub>3</sub> perovskite film, revealing that the layered 2D structure is more favorable for carrier transport due to the fewer defects in it. In addition, under the condition of the same environment humidity, the 2D perovskite materials show better moisture stability. Then, to investigate the influences of dimensional structure on the perovskite field-effect devices, we fabricate the bottom-gate and top-contact thin film transistors (TFTs) based on the perovskite materials with different dimensions. As a result, the instability and ion migration effect for each of the devices are suppressed effectively due to the distinct 2D layer-structure and quantum confinement effect, which leads the device performance to be further improved. The device based on quasi-2D (<i>n</i> = 6) channel TFT achieves a hole mobility (<i>μ</i><sub>hole</sub>) of 3.9 cm<sup>2</sup>/(V·s), an on-off current ratio of 10<sup>4</sup> and more, and a 1.85V turn-on voltage of 1.85 V. The first application of quasi-2D organic and inorganic hybrid perovskite materials to thin film transistors provides a new idea for preparing the high-performance and stable thin film transistor devices.

Aryl-Perfluoroaryl Interaction in Two-Dimensional Organic–Inorganic Hybrid Perovskites Boosts Stability and Photovoltaic Efficiency

Two-dimensional (2D) organic–inorganic hybrid perovskites (OIHPs) have showed impressive stability, compared to their three-dimensional (3D) counterparts. However, tuning the chemical structure of ...

Impact of Grain Sizes on Programmable Memory Characteristics in Two-Dimensional Organic-Inorganic Hybrid Perovskite Memory.

Recently, organic-inorganic hybrid perovskites (OIHPs) have been used in resistive switching memory applications because of current-voltage hysteresis that originated from ion migration in the perovskite film. As the density of the memory devices continues to increase, the size of the devices approaches that of the individual grains of the polycrystalline films. Thus, the effects of the grain boundary and the grain size will become important to investigate the influence on the switching behaviors. Here, we report the effects of grain sizes on the resistive switching property of (C4H9NH3)2PbBr4 (BA2PbBr4) films. The BA2PbBr4 films were formed by using sequential vapor deposition. First, a lead bromide (PbBr2) film was deposited by thermal evaporation, and then the film was exposed to organic vapor to form BA2PbBr4 films. The grain sizes were controlled by changing the transformation temperatures ( TT = 100, 150, and 200 °C). When the TT values were 100, 150, and 200 °C, the grain sizes of BA2PbBr4 were ∼180 nm, ∼1, and ∼30 μm, respectively. In the memory device based on BA2PbBr4, the off current decreased from ∼10-4 to ∼10-8 A as the grain size increased from ∼180 nm to ∼30 μm. This method to synthesize BA2PbBr4 films provides a simple way to control the grain sizes, and understanding of the effects of grain sizes on memory characteristics will provide an insight to improve the reliability of the OIHP-based memory as the electronic devices are scaled down to the sizes of grains.

Excitonic states and structural stability in two-dimensional hybrid organic-inorganic perovskites

Abstract Two-dimensional (2D) perovskites are a new class of functional materials that may find applications in various technologically important areas. Due to the better moisture and illumination stability, layered perovskites can be the next generation of materials for solar light-harvesting applications, as well as for light emitting diodes (LEDs). Besides, extended chemical engineering possibilities allow obtaining advanced perovskite materials with desirable functional properties, such as tunable band gap, strong exciton-phonon coupling, white light emission, spin-related effects, etc. A full understanding of the fundamental properties is essential for developing new 2D perovskite-based technologies. In this paper, recent reports on 2D perovskites are reviewed, including the synthesis methods of single crystals, nanosheets and films; the crystal and electronic structures; the excitonic states and interactions; the properties of the materials under low temperature and high pressure; and a brief discussion on the challenges in understanding the fundamental properties of the layered perovskites.

Discovery of an Above-Room-Temperature Antiferroelectric in Two-Dimensional Hybrid Perovskite.

Antiferroelectric materials have been regarded as a promising candidate for electronic energy storage devices, due to their natural double polarization versus electric field ( P- E) hysteresis loops. Currently, two-dimensional organic-inorganic hybrid perovskites with structural diversity and tunability, have received blooming interests, whereas above-room-temperature antiferroelectrics are still unreported in this perovskite system. Herein, for the first time, we successfully acquire a two-dimensional Ruddlesden-Popper hybrid perovskite antiferroelectric, ((CH3)2CHCH2NH3)2CsPb2Br7 (1), which shows an above-room-temperature Curie temperature at 353 K, trigging by the synergistic dynamic motion of inorganic Cs atoms and organic isobutylammonium cations. Intriguingly, the antiferroelectricity of 1 existing over a wide temperature range of 298-353 K are revealed by the distinct double P- E hysteresis loops. Besides, 1 possesses remarkable energy storage efficiency up to 69%, comparable to those of some reported inorganic antiferroelectric ceramics, promoting 1 potential application in energy storage devices. This work provides an avenue to construct novel antiferroelectric materials for high-performance electronic device applications.

Two-dimensional organic–inorganic hybrid perovskite field-effect transistors with polymers as bottom-gate dielectrics

High-performance bottom-gate 2D-layered (PEA)2SnI4 field-effect transistors have been fabricated using PVA/CL-PVP as gate dielectric layers.

Aryl-Perfluoroaryl Interaction in Two-Dimensional Organic-Inorganic Hybrid Perovskites Boosts Stability and Photovoltaic Efficiency.

Aryl-Perfluoroaryl Interaction in Two-Dimensional Organic-Inorganic Hybrid Perovskites Boosts Stability and Photovoltaic Efficiency.

Two-dimensional organic-inorganic hybrid perovskite: from material properties to device applications

The two-dimensional (2D) perovskite (including pure-2D and quasi-2D) is formed by introducing large-group ammonium halides into conventional bulk perovskite. In the past twenty years, 2D perovskite materials were widely developed with the enriched species and advanced physical-knowledge in material characteristics as well as optoelectronic device applications. To review achievments in 2D perovskite, the fundamental mechanism and properties of 2D perovskite are introduced to offer insight into device performance. Moreover, the preparation methods of 2D perovskite films are summarized and compared. The latest successful applications of the 2D perovskite in the solar cells and light-emitting diodes fields, especially the advanced stability of 2D perovskite solar cells (PeSCs) and the efficient 2D perovskite light-emitting diodes (PeLEDs), are also achieved. Furthermore, the challenges and outlook of 2D perovskite materials are proposed.

Exciton-polaron spectral structures in two-dimensional hybrid lead-halide perovskites

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. We present a phenomenological, yet quantitative framework to simulate excitonic absorption lineshapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by $35\pm5$\,meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the lineshape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.