Two-dimensional Perovskites Research Articles

Magneto-chiral Nonlinear Optical Effect with Large Anisotropic Response in Two-Dimensional Halide Perovskite.

The chiral organic-inorganic halide perovskites (OIHPs) are vital candidates for superior nonlinear optical (NLO) effects associated with circularly polarized (CP) light. NLO in chiral materials often couples with magnetic dipole (MD) transition, as well as the conventional electric dipole (ED) transition. However, the importance of MD transition in NLO process of chiral OIHPs has not yet been well recognized. Here, the circular polarized probe analysis of second harmonic generation circular dichroism (SHG-CD) provides the direct evidence that the contribution of MD leads to a large anisotropic response to CP lights in chiral OIHPs, (R-/S-MBACl)2PbI4. The thin films exhibit great sensitivity to CP lights over a wide wavelength range, and the g-value reaches up to 1.57 at the wavelength where the contribution of MD is maximized. Furthermore, it is also effective as CP light generator, outputting CP-SHG with maximum g-factor of 1.76 upon the stimulation of linearly polarized light. This study deepens the understanding of relation between chirality and magneto-optical effect, and such an efficient discrimination and generation of CP light signal is highly applicable for chirality-based sensor and optical communication devices.

High-Quality Solution-Processed Quasi-2D Perovskite for Low-Threshold Lasers.

Spin-coated quasi-two-dimensional halide perovskite films, which exhibit superior optoelectronic properties and environmental stability, have recently been extensively studied for lasers. Crystallinity is of great importance for the laser performance. Although some parameters related to the spin-coating process have been studied, the in-depth understanding and effective control of the acceleration rate on two-dimensional perovskite crystallization during spin-coating are still unknown. Here we investigate the effect of solvent evaporation on the microstructure of the final perovskite films during the spin-coating process. The crystallization quality of the film can be significantly improved by controlling solvent evaporation. As a result, the prepared quasi-2D perovskite film exhibits a stimulated emission threshold (pump: 343 nm, 6 kHz, 290 fs) of 550 nm as low as 16.2 μJ/cm2. Transient absorption characterization shows that the radiative biexciton recombination time is reduced from 738.5 to 438.3 ps, benefiting from the improved crystallinity. The faster biexciton recombination significantly enhanced the photoluminescence efficiency, which is critical for population inversion. This work could contribute to the development of low-threshold lasers.

Research Progress on Rashba Effect in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites.

The Rashba effect appears in the semiconductors with an inversion-asymmetric structure and strong spin-orbit coupling, which splits the spin-degenerated band into two sub-bands with opposite spin states. The Rashba effect can not only be used to regulate carrier relaxations, thereby improving the performance of photoelectric devices, but also used to expand the applications of semiconductors in spintronics. In this mini-review, recent research progress on the Rashba effect of two-dimensional (2D) organic-inorganic hybrid perovskites is summarized. The origin and magnitude of Rashba spin splitting, layer-dependent Rashba band splitting of 2D perovskites, the Rashba effect in 2D perovskite quantum dots, a 2D/3D perovskite composite, and 2D-perovskites-based van der Waals heterostructures are discussed. Moreover, applications of the 2D Rashba effect in circularly polarized light detection are reviewed. Finally, future research to modulate the Rashba strength in 2D perovskites is prospected, which is conceived to promote the optoelectronic and spintronic applications of 2D perovskites.

Interface optoelectrical dynamic in 2D perovskite/InSe heterostructure

Van der Waals (vdW) heterostructure composed of two-dimensional (2D) materials and 2D perovskite has shown excellent optoelectrical properties recently, while the interfacial coupling is less investigated. Here, van der Waals InSe/(C4H9NH3)2(CH3NH3)Pb2I7 heterostructure is fabricated and systemically investigated by photoluminescence (PL) spectroscopy. An optical broadband emission is generated due to the formation of heterostructure. The first principle calculation and variable PL spectra suggest the dynamic generation of defects such as iodine vacancy and charge states. Electron–phonon coupling of the 2D perovskite is weakened from −127 meV to −90 meV in the heterostructure. The band offset causes the built-in electrical field, which strengthens the optoelectrical response of heterostructure devices. The results deepen the understanding of the interfacial optoelectrical dynamic of 2D perovskite-related vdW heterostructure and help develop superior optoelectrical vdW heterostructure devices.

Two-dimensional lead halide perovskite lateral homojunctions enabled by phase pinning

Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.

Tailoring the optoelectronic properties of monolayer BA2PbI4/MoS2 heterojunction through terminal variations and defect engineering

In light of the inherent challenge posed by the simultaneous pursuit of structural stability and favorable optoelectronic characteristics in two-dimensional organic-inorganic halide perovskites, the construction of heterojunctions emerges as a promising strategy for mitigating electronic deficiencies and enhancing overall performance. Furthermore, defect engineering stands as a commonly employed technique to fine-tune the photoelectric properties of materials. In this study, we systematically investigate the pivotal vacancy defects, namely VBA and VI, within the monolayer BA2PbI4/MoS2 heterojunction system, employing first-principles calculations. Our computational findings illuminate the conductivity of MoS2 can be modulated when it interfaces with distinct surface terminations of 2D BA2PbI4, thereby exerting influence over electron flow within MoS2. Additionally, we observe a progressive transformation of p-type doped MoS2 into a heavily doped material as the concentration of BA vacancy defects increases. To further optimize the optoelectronic properties, we strategically introduce the I vacancy (Pb-Pb dimer) defect, effectively transitioning the indirect bandgap heterojunction into a direct one. Our study underscores the profound impact of different surface terminations and defect configurations on the physical attributes of the heterojunction, offering valuable insights for the fabrication of high-performance photoelectric materials.

Interlayer Carrier Dynamics in Two-Dimensional Perovskites Determined by the Length of Conjugated Organic Cations.

Unlocking the restricted interlayer carrier transfer in a two-dimensional perovskite is a crucial means to achieve the harmonization of efficiency and stability in perovskite solar cells. In this work, the effects of conjugated organic molecules on the interlayer carrier dynamics of 2D perovskites were investigated through nonadiabatic molecular dynamics simulations. We found that elongated conjugated organic cations contributed significantly to the accelerated interlayer carrier dynamics, originating from lowered transport barrier and boosted π-p coupling between organic and inorganic layers. Utilizing conjugated molecules of moderate length as spacer cations can yield both superior efficiency and exceptional stability simultaneously. However, conjugated chains that are too long lead to structural instability and stronger carrier recombination. The potential of conjugated chain-like molecules as spacer cations in 2D perovskites has been demonstrated in our work, offering valuable insights for the development of high-performance perovskite solar cells.

Exciton engineering of 2D Ruddlesden–Popper perovskites by synergistically tuning the intra and interlayer structures

Designing two-dimensional halide perovskites for high-performance optoelectronic applications requires deep understanding of the structure-property relationship that governs their excitonic behaviors. However, a design framework that considers both intra and interlayer structures modified by the A-site and spacer cations, respectively, has not been developed. Here, we use pressure to synergistically tune the intra and interlayer structures and uncover the structural modulations that result in improved optoelectronic performance. Under applied pressure, (BA)2(GA)Pb2I7 exhibits a 72-fold boost of photoluminescence and 10-fold increase of photoconductivity. Based on the observed structural change, we introduce a structural descriptor χ that describes both the intra and interlayer characteristics and establish a general quantitative relationship between χ and photoluminescence quantum yield: smaller χ correlates with minimized trapped excitons and more efficient emission from free excitons. Building on this principle, we design a perovskite (CMA)2(FA)Pb2I7 that exhibits a small χ and an impressive photoluminescence quantum yield of 59.3%.

Water-Oil Interfacial Synthesis of Two-Dimensional Colloidal Lead-Iodide Perovskites with Enhanced Efficiency and Stability.

Two-dimensional Ruddlesden-Popper perovskites L2PbI4 (L = alkylammonium cation) (RPPs) have attracted significant attention due to their unique excitonic characteristics. However, their ultrafast reaction dynamics exacerbates the structural distortion of the inorganic framework, leading to severe deterioration in photoluminescence. Here, we propose a water-oil interfacial synthesis approach to achieve controlled growth of the RPPs nanosheets. By segregating Pb and I precursors in two immiscible solvents, the nucleation and growth of RPPs are prolonged to the minute level. L2PbI4 nanosheets terminated with various alkylammonium are synthesized, and the factors influencing the growth kinetics of RPPs nanosheets are investigated. The resulting (PEA)2PbI4 nanosheets exhibit a 3.6-time enhancement in quantum efficiency and 3.2-time improvement photostability compared to those synthesized using the classical recrystallization method. A white light-emitting diode based on (HDA)2PbI4 nanosheets is fabricated, achieving a color gamut of 119.7% of the NTSC display standards. This innovative approach offers a new method for the controlled growth of 2D RPPs with improved optical quality and stability.

High-performance quasi-2D perovskite solar cells enabled by using π-conjugated spacer

The excellent photoelectric properties and low fabrication cost of perovskite solar cells have attracted extensive research attention. Despite this, long-term stability issues associated with perovskite solar cells continue to pose a significant barrier to commercialization. A new generation of two-dimensional (2D) and quasi-2D perovskites and newly introduced members of the 2D material family, have attracted growing attention due to their excellent stability and physical properties in contrast to their three-dimensional (3D) counterparts. Herein, we have presented the feasibility of using 4,4[Formula: see text]-dipyridyl as an additive material for quasi-2D perovskite solar cells. As a result of the addition of 4,4[Formula: see text]-dipyridyl, the solar cell device achieves a power conversion efficiency of 17.92% and a fill factor of 76.5%. Additionally, this strategy can be expanded to quasi-2D perovskite solar cells with an open-circuit voltage of 1.05[Formula: see text]V.

Molecularly thin, two-dimensional all-organic perovskites.

Recently, the emergence of all-organic perovskites with three-dimensional (3D) structures has expanded the potential applications of perovskite materials. However, the synthesis and utilization of all-organic perovskites in 2D form remain largely unexplored because the design principle has not been developed. We present the successful synthesis of a metal-free 2D layered perovskite, denoted as the Choi-Loh van der Waals phase (CL-v phase), with the chemical formula A2B2X4, where A represents a larger-sized cation compared to B and X denotes an anion. The CL-v phase exhibits a van der Waals gap enabled by interlayer hydrogen bonding and can be exfoliated or grown as molecularly thin 2D organic crystals. The dielectric constants of the CL-v phase range from 4.8 to 5.5 and we demonstrate their potential as gate dielectrics for thin-film transistors.

Modulating Dimensionality of 2D Perovskite Layers for Efficient and Stable 2D/3D Perovskite Photodetectors.

Two-dimensional (2D) perovskites have been widely adopted for improving the performance and stability of three-dimensional (3D) metal halide perovskite devices. However, rational manipulation of the phase composition of 2D perovskites for suitable energy level alignment in 2D/3D perovskite photodetectors (PDs) has been rarely explored. Herein, we precisely controlled the dimensionality of the 2D perovskite on CsPbI2Br films by tuning the polarity of the n-butylammonium iodide (BAI)-based solvents. In comparison to the pure n = 1 2D perovskite (ACN-BAI) formed by acetonitrile treatment, a mixture of n = 1 and n = 2 phases (IPA-BAI) generated by isopropanol (IPA) treatment guaranteed more robust defect passivation and favorable energy level alignment at the perovskite/hole transport layer interface. Consequently, the IPA-BAI PD exhibited a responsivity of 0.41 A W-1, a detectivity of 1.01 × 1013 Jones, and a linear dynamic range of 120 dB. Furthermore, the mixed-phase 2D layer effectively shielded the 3D perovskite from moisture. The IPA-BAI device retained 76% of its initial responsivity after 500 h of nonencapsulated storage at 10% relative humidity. This research provides valuable insights into the dimensional modulation of 2D perovskites for further enhancing the performance of 2D/3D perovskite PDs.

Boosting MA-based two-dimensional Ruddlesden-Popper perovskite solar cells by incorporating a binary spacer

Two-dimensional Ruddlesden-Popper (2DRP) perovskite exhibits excellent stability in perovskite solar cells (PSCs) due to introducing hydrophobic long-chain organic spacers. However, the poor charge transporting property of bulky organic cation spacers limits the performance of 2DRP PSCs. Inspired by the A-site cation alloying strategy in 3D perovskites, 2DRP perovskites with a binary spacer can promote charge transporting compared to the unary spacer counterparts. Herein, the superior MA-based 2DRP perovskite films with a binary spacer, including 3-guanidinopropanoic acid (GPA) and 4-fluorophenethylamine (FPEA) are realized. These films (GPA0.85FPEA0.15)2MA4Pb5I16 show good morphology, large grain size, decreased trap state density, and preferential orientation of the as-prepared film. Accordingly, the present 2DRP-based PSC with the binary spacer achieves a remarkable efficiency of 18.37% with a VOC of 1.15 V, a JSC of 20.13 mA cm−2, and an FF of 79.23%. To our knowledge, the PCE value should be the highest for binary spacer MA-based 2DRP (n ≤ 5) PSCs to date. Importantly, owing to the hydrophobic fluorine group of FPEA and the enhanced interlayer interaction by FPEA, the unencapsulated 2DRP PSCs based on binary spacers exhibit much excellent humidity stability and thermal stability than the unary spacer counterparts.

Electrically induced directional ion migration in two-dimensional perovskite heterostructures

Electrically induced directional ion migration in two-dimensional perovskite heterostructures

Boosting the internal electron transfer of two-dimensional hybrid perovskites via fluorination of organic cation towards enhanced photocatalytic hydrogen production

Two-dimensional (2D) organic-inorganic hybrid perovskites have emerged as promising candidates for photocatalytic hydrogen production due to their unique optoelectronic properties. However, the organic cations in the interlayer of 2D perovskites generally retard the internal charge transfer due to the large interlayer van der Waals barrier for electron transfer, which defines the imperative to engineering the organic entity. Herein, taking phenethylammonium lead iodide (PEA2PbI4) as a typical example, we show that by substituting the hydrogen with fluorine atom in the para-position of the phenyl group, the internal electron transfer was promoted remarkably. The improved electron transfer is ascribed to the introduction of fluorine with strong electronegativity, which leads to interlayer polarization and almost zero-potential barrier between the organic layers. Consequently, 4-FPEA2PbI4 exhibits ca. 8.2 times higher photocatalytic H2 evolution activity than PEA2PbI4. This work establishes a paradigm for optimizing the photocatalytic properties of 2D hybrid perovskites by engineering the organic cations.

Two-Dimensional Double Perovskites in the Dion–Jacobson Phase Alleviate Parity Forbidden Transitions for Photovoltaic Applications

Two-Dimensional Double Perovskites in the Dion–Jacobson Phase Alleviate Parity Forbidden Transitions for Photovoltaic Applications

Directed Assembly of Ordered Mixed-Spacer Quasi-2D Halide Perovskites through Homomeric Chains of Intermolecular Bonds.

Two-dimensional (2D) halide perovskites (HPs) are of significant interest to researchers because of their natural structural frameworks and intriguing optoelectronic properties. However, the direct fabrication of ordered mixed-spacer quasi-2D HPs remains challenging. Herein, a synthetic strategy inspired by the principle of supramolecular synthons is employed for the self-assembly of a series of ordered mixed-spacer bilayered HPs. The key innovation involves the introduction of intermolecular hydrogen bonds using a bifunctional 3-aminopropionitrile cation. Three homogeneous n = 2 structures are obtained, with a subtly ordered perovskite connected by two distinct types of organic cation layers, resulting in a recurrent ABAB' stacking sequence. These three compounds exhibit attractive semiconducting properties. Moderate bandgaps in the range of 2.70 to 2.76eV with an absorption wavelength range of 448-459nm exhibit excellent photoelectric response. Moreover, the ordered structures facilitate excellent polarization-sensitive photodetection, with an impressive on/off ratio of 103. The response speed ranged from 298 to 381 µs, and the out-of-plane polarization-related dichroism ratio is determined to be 1.19. Such ordered mixed-spacer bilayered perovskites have not been reported. These results enrich the HPs system and play a significant role in the direct assembly of novel perovskites with ordered structures.

Large exchange-driven intrinsic circular dichroism of a chiral 2D hybrid perovskite

In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.

Impact of large A-site cations on electron-vibrational interactions in 2D halide perovskites: Ab initio quantum dynamics.

Using abinitio nonadiabatic molecular dynamics, we study the effect of large A-site cations on nonradiative electron-hole recombination in two-dimensional Ruddlesden-Popper perovskites HA2APb2I7, HA = n-hexylammonium, A = methylammonium (MA), or guanidinium (GA). The steric hindrance created by large GA cations distorts and stiffens the inorganic Pb-I lattice, reduces thermal structural fluctuations, and maintains the delocalization of electrons and holes at ambient and elevated temperatures. The delocalized charges interact more strongly in the GA system than in the MA system, and the charge recombination is accelerated. In contrast, replacement of only some MA cations with GA enhances disorder and increases charge lifetime, as seen in three-dimensional perovskites. This study highlights the key influence of structural fluctuations and disorder on the properties of charge carriers in metal halide perovskites, providing guidance for tuning materials' optoelectronic performance.

Perovskite-Supramolecular Co-Assembly for Chiral Optoelectronics.

Hybrid inorganic-organic perovskites with chiral response and outstanding optoelectronic characteristics are promising materials for next-generation spin-optoelectronics. In particular, two-dimensional (2D) perovskites are promising chiroptical candidates due to their unique ability to incorporate chiral organic cations into their crystal structure, which imparts chirality. To enable their practical applications in chiral optoelectronic devices, it is essential to achieve an anisotropy factor (gCD ∼ 2) in chiral 2D perovskites. Currently, chiral 2D perovskites exhibit a relatively low gCD of 3.1 × 10-3. Several approaches have been explored to improve the chiral response of chiral 2D perovskites, including tailoring the molecular structure of chiral cations and increasing the degree of octahedral tilting in the perovskite lattice. However, current methods for chiral amplification have only achieved a moderate enhancement of gCD by 2-fold and are often accompanied by undesirable shifts or inversion in the circular dichroism spectra. There is a need for a more efficient approach to enhancing the chirality in 2D perovskites. Here, we report an innovative coassembly process that allows us to seamlessly grow chiral 2D perovskites on supramolecular helical structures. We discover that the interactions between perovskites and chiral supramolecular structures promote crystal lattice distortion in perovskites, which improves the chirality of 2D perovskites. Additionally, the obtained hierarchical coassembly can effectively harness the structural chirality of the supramolecular helices. The multilevel chiral enhancement leads to an enhancement in gCD by 2.7-fold without compromising the circular dichroism spectra of 2D perovskites.