Two-Dimensional Organic-Inorganic Hybrid Perovskites: A New Platform for Optoelectronic Applications

Molecularly Engineered Alicyclic Organic Spacers for 2D/3D Hybrid Tin-based Perovskite Solar Cells

The high defect density and inferior crystallinity remain great hurdles for developing highly efficient and stable Sn-based perovskite solar cells (PSCs). 2D/3D heterostructures show strong potential to overcome these bottlenecks; however, a limited diversity of organic spacers has hindered further improvement. Herein, a novel alicyclic organic spacer, morpholinium iodide (MPI), is reported for developing structurally stabilized 2D/3D perovskite. Introducing a secondary ammonium and ether group to alicyclic spacers in 2D perovskite enhances its rigidity, which leads to increased hydrogen bonding and intermolecular interaction within 2D perovskite. These strengthened interactions facilitate the formation of highly oriented 2D/3D perovskite with low structural disorder, which leads to effective passivation of Sn and I defects. Consequently, the MP-based PSCs achieved a power conversion efficiency (PCE) of 12.04% with superior operational and oxidative stability. This work presents new insight into the design of organic spacers for highly efficient and stable Sn-based PSCs.

Unravelling Structural, Optical, and Band Alignment Properties of Mixed Pb-Sn Metal-Halide Quasi-2D Ruddlesden-Popper Perovskites

Lead-tin (Pb-Sn) mixed-halide perovskites show potential for single-junction and tandem solar cells due to their adjustable band gaps, flexible composition, and superior environmental stability compared to three-dimensional (3D) perovskites. However, they have lower power conversion efficiencies. Understanding band alignment and charge carrier dynamics is essential for enhancing photovoltaic performance. In this view, herein we have prepared thin films of mixed Pb-Sn-based two dimensional (2D) Ruddlesden-Popper (RP) perovskites BA2FA(Pb1-xSnx)2I7 using a solution-based method. XRD study revealed the formation of orthorhombic phases for pristine (BA2FAPb2I7) and mixed Pb-Sn perovskite thin films. UV-vis analysis showed that different n = 2 and n = 3 phases are present in the pristine sample. In contrast, Pb-Sn-doped samples showed no signature of other phases with a prominent red-shift in the visible spectral region. Cyclic voltammetry showed peaks for electron transfers at the band edges. Additionally, electrochemical and optical band gap matching was observed, along with decreased peak intensity due to less reactant and altered electrolyte-perovskite interface stability. Density functional theory (DFT) calculations revealed that the reduced band gap is due to the alteration of electrostatic interactions and charge distribution within the lattice upon Sn substitution. Low-temperature PL analysis provided insights into charge carrier dynamics with Sn substitution and suggested the suppression of higher n phases and self-trapped excitons/carriers in mixed Pb-Sn quasi-2D RP perovskite thin films. This study sheds light on the electron transfer phenomena between TiO2 and SnO2 layers by estimating band offsets from valence band maximum (VBM) and conduction band minimum (CBM), which is crucial for future applications in fabricating stable and efficient 2D-Pb-Sn mixed perovskites for optoelectronic applications.

Manipulation of Chiral Nonlinear Optical Effect by Light-Matter Strong Coupling

Light-matter strong coupling (LMSC) is an intriguing state in which light and matter are hybridized inside a cavity. It is increasingly recognized as an excellent way to control material properties without any chemical modification. Here, we show that the LMSC is a powerful state for manipulating chiral nonlinear optical (NLO) effects through the investigation of second harmonic generation (SHG) circular dichroism. At the upper polariton band in LMSC, in addition to the enhancement of SHG by more than 1 order of magnitude, the responsivity to the handedness of circularly polarized light was largely modified, where sign inversion and increase of the dissymmetry factor were achieved. Quarter waveplate rotation analysis revealed that the LMSC clearly influenced the coefficients associated with chirality in the NLO process and also contributed to the enhancement of nonlinear magnetic dipole interactions. This study demonstrated that LMSC serves as a great platform for controlling chiral and magneto-optics.

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.

Methanol as an anti-solvent to improve the low open-circuit voltage of CsPbBr3 perovskite solar cells prepared with water

CsPbBr3 has received more and more attention in the field of optoelectronic devices due to its excellent stability. To address the cost and environmental concerns associated with the use of toxic methanol, water has been explored as a substitute solvent for CsBr in the preparation of CsPbBr3 perovskite solar cells (PSCs). In this study, we utilized methanol as an anti-solvent of the CsBr/H2O solution to regulate the detrimental effects of water on the CsPbBr3 film and control the crystallization process. From results of the experiment, it was found that methanol anti-solvent treatment greatly improved the crystallization of the CsPbBr3 film, increased the grain size, and reduced the defect density. After the introduction of methanol anti-solvent treatment, the power conversion efficiency (PCE) increased from 6.09% to 7.91%, while the open-circuit voltage (Voc) increased from 1.18 V to 1.39 V. Furthermore, we incorporated 2-hydroxyethylurea into the CsPbBr3 PSCs to improve the wettability of PbBr2 towards the CsBr/H2O solution and ensure the formation of pure-phase CsPbBr3 films. The introduction of 2-hydroxyethylurea resulted in an additional increase in Voc from 1.19 V to 1.42 V. The PCE further improved from 6.56% to 8.62% after methanol anti-solvent treatment. These results demonstrate that methanol treatment effectively addresses the low Voc issue observed in CsPbBr3 PSCs prepared with water as a solvent. Importantly, this approach significantly reduces the reliance on methanol compared to conventional fabrication methods for CsPbBr3 PSCs. Overall, this work presents a promising pathway for achieving high Voc and efficiency in CsPbBr3 PSCs by utilizing water as a solvent.

Unveiling the role of linear alkyl organic cations in 2D layered tin halide perovskite field-effect transistors

Two-dimensional (2D) tin halide perovskites are promising semiconductors for field-effect transistors (FETs) owing to their fascinating electronic properties. However, the correlation between the chemical nature of organic cations and charge carrier transport is still far from understanding. In this study, the influence of chain length of linear alkyl ammonium cations on film morphology, crystallinity, and charge transport in 2D tin halide perovskites is investigated. The carbon chain lengths of the organic spacers vary from propylammonium to heptanammonium. The increase of alkyl chain length leads to enhanced local charge carrier transport in the perovskite film with mobilities of up to 8 cm2 V-1 s-1, as confirmed by optical-pump terahertz spectroscopy. A similar improved macroscopic charge transport is also observed in FETs, only to the chain length of HA, due to the synergistic enhancement of film morphology and molecular organization. While the mobility increases with the temperature rise from 100 K to 200 K due to the thermally activated transport mechanism, the device performance decreases in the temperature range of 200 K to 295 K because of ion migration. These results provide guidelines on rational design principles of organic spacer cations for 2D tin halide perovskites and contribute to other optoelectronic applications.

Stable and Highly Efficient Photocatalysis with Two-Dimensional Organic-Inorganic Hybrid Perovskites

Two-dimensional organic-inorganic hybrid perovskites (OIHPs) have excellent photoelectric properties, such as high charge mobility and a high optical absorption coefficient, which have attracted enormous attention in the field of optoelectronic devices and photochemistry. However, the stability of 2D OIHPs in solution is deficient. In particular, the lack of stability in polar solutions hinders their application in photochemistry. In this work, (iso-BA)2PbI4 was used as a model to explore the three possibilities of the stable existence of a 2D perovskite in aqueous solution. And two of these systems that stabilize the presence of (iso-BA)2PbI4 were further investigated through electrochemical testing. Moreover, (iso-BA)2PbI4 2D hybrid perovskites exhibited an outstanding degradation rate. The chiral perovskite (R/S-MBA)2PbI4 is able to degrade a 30 mg/L methyl orange solution completely within 5 min, making it one of the fastest catalysts for this particular organic reaction. Further, based on the electron spin resonance test, a degradation mechanism by the halide perovskite was proposed. Based on the great catalytic performance as well as good reusability and stability, (R/S-MBA)2PbI4 perovskites are expected to be a new generation of catalysts, making a great impact on the application of asymmetrically catalyzed photoreactions.

Orbital Interactions in 2D Dion-Jacobson Perovskites Using Oligothiophene-Based Semiconductor Spacers Enable Efficient Solar Cells

2D Dion-Jacobson (DJ) perovskites have emerged as promising photovoltaic materials, but the insulating organic spacer has hindered the efficient charge transport. Herein, we successfully synthesized a terthiophene-based semiconductor spacer, namely, 3ThDMA, for 2D DJ perovskite. An interesting finding is that the energy levels of 3ThDMA extensively overlap with the inorganic components and directly contribute to the band formation of (3ThDMA)PbI4, leading to enhanced charge transport across the organic spacer layers, whereas no such orbital interactions were found in (UDA)PbI4, a DJ perovskite based on 1,11-undecanediaminum (UDA). The devices based on (3ThDMA)MAn-1PbnI3n+1 (nominal n = 5) obtained a champion efficiency of 15.25%, which is a record efficiency for 2D DJ perovskite solar cells using long-conjugated spacers (conjugated rings ≥ 3) and a 22.60% efficiency for 3ThDMA-treated 3D PSCs. Our findings provide an important insight into understanding the orbital interactions in 2D DJ perovskite using an organic semiconductor spacer for efficient solar cells.

Methylammonium-Based Quasi-Two-Dimensional Perovskite Single Crystals for Highly Sensitive X-ray Detection and Imaging

The strategy of introducing large organic cations into three-dimensional perovskites could reduce the dimensionality of perovskites to form quasi-two-dimensional (quasi-2D) perovskites, resulting in increased stability and reduced detection limits due to less ion migration. Herein, a quasi-2D perovskite single crystal (BDA)(MA)2Pb3Br10 (BDA = NH3C4H8NH3, MA = CH3NH3) with a layered structure was grown by the temperature-cooling solution method. The X-ray detector based on the (BDA)(MA)2Pb3Br10 single crystal has a sensitivity as high as 1984 μC Gy-1 cm-2 at 55.6 V/mm, and it could detect X-rays as low as 28.12 nGy s-1 at 22.2 V/mm. In addition, the X-ray imaging system based on the single-crystal device easily distinguishes between metals and plastics and exhibits a spatial resolution estimated as 250 μm, indicating the feasibility of (BDA)(MA)2Pb3Br10 crystals for X-ray imaging. This research offers a method for the design of quasi-2D layered perovskites and enhances photoelectronic applications in X-ray inspection and imaging.

Thickness control of organic semiconductor-incorporated perovskites

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent. The length of the conjugated semiconducting organic cations and the dimensionality (n value) of the inorganic layers can be manipulated at the same time. The energy band offsets and exciton dynamics at the organic-inorganic interfaces can therefore be precisely controlled. Furthermore, we show that longer and more planar π-conjugated organic cations induce a more rigid inorganic crystal lattice, which leads to suppressed exciton-phonon interactions and better optoelectronic properties as compared to conventional two-dimensional perovskites. As a demonstration, optically driven lasing behaviour with substantially lower lasing thresholds was realized.

Tailoring Interlayer Charge Transfer Dynamics in 2D Perovskites with Electroactive Spacer Molecules

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore the optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3 to 5, as cations in the two-dimensional (2D) perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably reveal an organic-inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i = 5 to i = 3. The charge transfer results in long-lived carriers (10-100 ns) and quenched emission, in stark contrast to the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i = 5 to i = 3. This study paves the way for the rational design of 2D perovskites with combined inorganic-organic electronic properties through the wide range of functionalities available in the world of organics.

Over-18%-Efficiency Quasi-2D Ruddlesden-Popper Pb-Sn Mixed Perovskite Solar Cells by Compositional Engineering

Quasi-two-dimensional (2D) Pb-Sn mixed perovskites show great potential in applications of single and tandem photovoltaic devices, but they suffer from low efficiencies due to the existence of horizontal 2D phases. Here, we obtain a record high efficiency of 18.06% based on 2D ⟨n⟩ = 5 Pb-Sn mixed perovskites (iso-BA₂MA₄(Pb_xSn_1-x)₅I₁₆, x = 0.7), by optimizing the crystal orientation through a regulation of the Pb/Sn ratio. We find that Sn-rich precursors give rise to a mixture of horizontal and vertical 2D phases. Interestingly, increasing the Pb content can not only entirely suppress the unwanted horizontal 2D phase in the film but also enhance the growth of vertical 2D phases, thus significantly improving the device performance and stability. It is suggested that an increase of the Pb content in the Pb-Sn mixed systems facilitates the incorporation of iso-butylammonium (iso-BA⁺) ligands in vertically oriented perovskites because of the reduced lattice strain and increased interaction between the organic ligands and inorganic framework. Our work sheds light on the optimal conditions for fabricating stable and efficient 2D Pb-Sn mixed perovskite solar cells.

Phase-Modulated Multidimensional Perovskites for High-Sensitivity Self-Powered UV Photodetectors

2D Ruddlesden-Popper perovskites (PVKs) have recently shown overwhelming potential in various optoelectronic devices on account of enhanced stability to their 3D counterparts. So far, regulating the phase distribution and orientation of 2D perovskite thin films remains challenging to achieve efficient charge transport. This work elucidates the balance struck between sufficient gradient sedimentation of perovskite colloids and less formation of small-n phases, which results in the layered alignment of phase compositions and thus in enhanced photoresponse. The solvent engineering strategy, together with the introduction of poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS) and PC₇₁ BM layer jointly contribute to outstanding self-powered performance of indium tin oxide/PEDOT:PSS/PVK/PC₇₁ BM/Ag device, with a photocurrent of 18.4 µA and an on/off ratio up to 2800. The as-fabricated photodetector exhibits high sensitivity characteristics with the peak responsivity of 0.22 A W^-1 and the detectivity up to 1.3 × 10¹²  Jones detected at UV-A region, outperforming most reported perovskite-based UV photodetectors and maintaining high stability over a wide spectrum ranging from UV to visible region. This discovery supplies deep insights into the control of ordered phases and crystallinity in quasi-2D perovskite films for high-performance optoelectronic devices.

Chiral-phonon-activated spin Seebeck effect

Utilization of the interaction between spin and heat currents is the central focus of the field of spin caloritronics. Chiral phonons possessing angular momentum arising from the broken symmetry of a non-magnetic material create the potential for generating spin currents at room temperature in response to a thermal gradient, precluding the need for a ferromagnetic contact. Here we show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite implanted with chiral cations when subjected to a thermal gradient. The generated spin current shows a strong dependence on the chirality of the film and external magnetic fields, of which the coefficient is orders of magnitude larger than that produced by the reported spin Seebeck effect. Our findings indicate the potential of chiral phonons for spin caloritronic applications and offer a new route towards spin generation in the absence of magnetic materials.

A room-temperature antiferroelectric in hybrid perovskite enables highly efficient energy storage at low electric fields

Molecular antiferroelectrics (AFEs) have taken a booming position in the miniaturization of energy storage devices due to their low critical electric fields. However, regarding intrinsic competitions between dipolar interaction and steric hindrance, it is a challenge to exploit room-temperature molecular AFEs with high energy storage efficiency. Here, we present a new 2D hybrid perovskite-type AFE, (i-BA)₂(FA)Pb₂Br₇ (1), which shows ultrahigh energy storage efficiencies at room temperature. Most strikingly, the typical double P-E hysteresis loops afford an ultrahigh storage efficiency up to ∼91% at low critical electric fields (E _cr = 41 kV cm^-1); this E _cr value is much lower than those of state-of-the-art AFE oxides, revealing the potential of 1 for miniaturized energy-storage devices. In terms of the energy storage mechanism, the dynamic ordering and antiparallel reorientation of organic cations trigger its AFE-type phase transition at 303 K, thus giving a large spontaneous electric polarization of ∼3.7 μC cm^-2, while the increasement of steric hindrance of the organic branched-chain i-BA⁺ spacer cations stabilizes its antipolar sublattices. To the best of our knowledge, this exploration of achieving ultrahigh energy storage efficiency at such a low critical electric field is unprecedented in the AFE family, which paves a pathway for miniaturized energy storage applications.

Strategies to Improve the Stability of Perovskite Light-Emitting Diodes: Progress and Perspective

Perovskite light-emitting diodes (PeLEDs) featuring excellent electroluminescent (EL) characteristics and facile production have been emerging as promising candidates for next-generation high-definition displays. In recent years, tremendous advances have been achieved in the EL efficiency of PeLEDs. However, their poor operational stability impedes practical applications. Particularly, the severe spectral instability of pure-blue and pure-red PeLEDs lags far behind the requirements of commercial displays. In this Perspective, the critical factors related to device degradation are first summarized, including perovskite crystal defects, unbalanced charge injection, Auger recombination, and Joule heating. Then, the recent progress in improving the operational and spectral stabilities is reviewed in categories. Considering the present achievements, we provide potential research directions for further development of stable PeLEDs.

Point Defects in Two-Dimensional Ruddlesden-Popper Perovskites Explored with Ab Initio Calculations

Two-dimensional Ruddlesden-Popper (RP) halide perovskites stand out as excellent layered materials with favorable optoelectronic properties for efficient light-emitting, spintronic, and other spin-related applications. However, properties often determined by defects are not well understood in these perovskite systems. This work investigates the ground state electronic structure of commonly formed defects in a typical RP perovskite structure by density functional theory. Our study reveals that these 2D perovskites generally retain their defect tolerance with limited perturbation of the electronic structure in the case of neutral-type point defects. In contrast, donor/acceptor defects induce deep midgap states, potentially causing harm to the material's electronic performance. To retain positive intrinsic properties, the halide vacancies and interstitial defects should be avoided. The observed strong electron localization results in trap states and consequently leads to reduced device performance. This understanding can guide experimental efforts that aim for improved 2D halide perovskite-based device performance.

Effects of conjugated structure on electronic and transport properties in organic-inorganic hybrid superlattices Cd2Se2(C2H4N2)1/2

By inducingπ-conjugated organic molecule C₂H₄N₂in group II-VI based CdSe network structure materials, the band structures and carrier transport of organic-inorganic hybrid superlattices Cd₂Se₂(C₂H₄N₂)_1/2were investigated via first-principles calculations based on the density functional theory. With different stacking patterns, it is found that the carrier mobility can be modulated by 5-6 orders of magnitude. The physical mechanism of the high carrier mobility in the hybrid structures has been revealed, which means dipole organic layers realize electron delocalization via electrostatic potential difference and build-in electric field. Our calculations shown that the dipole organic layers originate from asymmetricπ-conjugated organic molecules and the charges movement between molecules, while symmetric organic molecules tend to electrostatic balance. And although the electronic transport properties were highly restrained by the flat bands of organic layers around Fermi energy in most structures, we found that the collective electrostatic effect can lead to very high electron mobility in AA1 and AA2 stacking systems, which might be attributed to the superposition of molecule electrostatic potential along with electrons transfer between molecules. Furthermore, it is also found that the anisotropy of electron mobility can be modulated via the difference directions of dipole layers.

Insight into the photoelectrical properties of metal adsorption on a two-dimensional organic–inorganic hybrid perovskite surface: theoretical and experimental research

In order to study the photoelectric properties of the adsorption of different metal atoms on a two-dimensional (2D) perovskite surface, in this article, we built many models of Ag, Au, and Bi atoms adsorbed on 2D perovskite. We studied the rules influencing 2D perovskite adsorbing metal atoms with different n values (the n value is the number of inorganic layers of 2D perovskite; here n = 1, 2, and 3). Based on n = 2 2D perovskite, we successively used Ag, Au, and Bi metal atoms to adsorb on the 2D perovskite surface. Firstly, we calculated their adsorption energies. Based on the lowest energy principle, we found that Bi atom adsorption on the 2D perovskite surface gave the most stable structure among the three metal adsorptions because the energy of the Bi adsorption system was the smallest. Secondly, the electron transport process takes place from the s to the p orbital when Au and Ag atoms adsorb on the 2D perovskite surface, but in the Bi atom adsorption, the electron transport process takes place from the p to the p orbital, because the p–p orbital transport energy is lower than that of the s–p orbital. Therefore, Bi atom adsorption on the 2D perovskite surface can improve charge carrier transfer. Thirdly, we calculated the bond angles and bond energies of different metal adsorptions on 2D perovskite. Bi adsorption has greater interaction with the surface atoms of 2D perovskite than Ag or Au atom adsorption, which effectively enhances the surface polarization effects, and enhances the photoelectric properties of 2D perovskite. The light absorption spectrum further confirms that Bi atom adsorption has a greater impact on the 2D perovskite than the action of Ag or Au adsorption. Finally, in an experiment, we fabricated a 2D perovskite solar cell with an ITO/PEDOT:PSS/2D perovskite/PEI/Ag (Au, Bi) structure. The Bi electrode solar cell achieves the highest photoelectric conversion efficiency (PCE) of 15.16% among the three cells with forward scanning, which is consistent with the theoretical analysis. We believe that the adsorption of metals like Bi on a 2D perovskite surface as an electrode is conducive to improving the charge transport performance.

Bi atom adsorption on a 2D perovskite surface structure has the minimum adsorption energy. When it uses on the solar cell electrode, the 2D perovskite solar cell of ITO/PEDOT:PSS/2D perovskite/PEI/Bi structure exhibits the highest photoelectric conversion efficiency (PCE) of 15.16%.

High temperature molecular-based phase transition compounds with tunable and switchable dielectric properties

Molecular-based dielectric switching compounds [ClEt-Dabco][ReO4]2 and [BrEt-Dabco][ReO4]2 with high-temperature phase transition.

A novel 2-(aminomethyl)pyridineH2PO4 crystal with second-order nonlinear optical performance

Single-crystal structure diagram and SHG response of (2-Ampy)H2PO4 crystal.

Spacer Cation Alloying in Ruddlesden-Popper Perovskites for Efficient Red Light-Emitting Diodes with Precisely Tunable Wavelengths

Perovskite light-emitting diodes (PeLEDs) have recently shown significant progress with external quantum efficiencies (EQEs) exceeding 20%. However, PeLEDs with pure-red (620-660 nm) light emission, an essential part for full-color displays, remain a great challenge. Herein, a general approach of spacer cation alloying is employed in Ruddlesden-Popper perovskites (RPPs) for efficient red PeLEDs with precisely tunable wavelengths. By simply tuning the alloying ratio of dual spacer cations, the thickness distribution of quantum wells in the RPP films can be precisely modulated without deteriorating their charge-transport ability and energy funneling processes. Consequently, efficient PeLEDs with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak EQEs up to 11.5%, representing the highest values among RPP-based pure-red PeLEDs. This work opens a new route for color tuning, which will spur future developments of pure-red or even pure-blue PeLEDs with high performance.

Drop-Casting Method to Screen Ruddlesden-Popper Perovskite Formulations for Use in Solar Cells

Small-area metal-halide perovskite solar cells (PSCs) having power-conversion efficiencies (PCEs) of greater than 25% can be prepared by using a spin-coated perovskite layer, but this technique is not readily transferrable to large-scale manufacturing. Drop-casting is a simple alternative method for film formation that is more closely aligned to industry-relevant coating processes. In the present work, drop-casting was used to prepare films for screening two-dimensional Ruddlesden-Popper (2DRP) metal-halide perovskite formulations for potential utility in PSCs, without additional processing steps such as inert-gas blowing or application of antisolvent. The composition of the 2DRP formulation used for drop-casting was found to have a profound effect on optical, spectroscopic, morphological, and phase-distribution properties of the films as well as the photovoltaic performance of related PSC devices. This facile method for screening film quality greatly assists in speeding up the identification of perovskite formulations of interest. The optimal 2DRP perovskite formulation identified from screening was utilized for industry-relevant one-step roll-to-roll slot-die coating on a flexible plastic substrate, producing PSCs having PCEs of up to 8.8%. A mechanism describing film formation and phase distribution in the films is also proposed.

A Systematic Review of Metal Halide Perovskite Crystallization and Film Formation Mechanism Unveiled by In Situ GIWAXS

Metal halide perovskites are of fundamental interest in the research of modern thin-film optoelectronic devices, owing to their widely tunable optoelectronic properties and solution processability. To obtain high-quality perovskite films and ultimately high-performance perovskite devices, it is crucial to understand the film formation mechanisms, which, however, remains a great challenge, due to the complexity of perovskite composition, dimensionality, and processing conditions. Nevertheless, the state-of-the-art in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) technique enables one to bridge the complex information with device performance by revealing the crystallization pathways during the perovskite film formation process. In this review, the authors illustrate how to obtain and understand in situ GIWAXS data, summarize and assess recent results of in situ GIWAXS studies on versatile perovskite photovoltaic systems, aiming at elucidating the distinct features and common ground of film formation mechanisms, and shedding light on future opportunities of employing in situ GIWAXS to study the fundamental working mechanisms of highly efficient and stable perovskite solar cells toward mass production.

Alkyl-Aryl Cation Mixing in Chiral 2D Perovskites

We report 2D hybrid perovskites comprising a blend of chiral arylammonium and achiral alkylammonium spacer cations (1:1 mole ratio). These new perovskites feature an unprecedented combination of chirality and alkyl-aryl functionality alongside noncovalent intermolecular interactions (e.g., CH···π interactions), determined by their crystal structures. The mixed-cation perovskites exhibit a circular dichroism that is markedly different from the purely chiral cation analogues, offering new avenues to tune the chiroptical properties of known chiral perovskites, instead of solely relying on otherwise complex chemical syntheses of new useable chiral cations. Further, the ability to dilute the density of chiral cations by mixing with achiral cations may offer a potential way to tailor the spin-based properties in 2D hybrid perovskites, such as Rashba-Dresselhaus spin splitting and chirality-induced spin selectivity and magnetization effects.

Advances of Nonlinear Photonics in Low-Dimensional Halide Perovskites

Hybrid halide perovskites emerging as a highly promising class of functional materials for semiconductor optoelectronic applications have drawn great attention from worldwide researchers. In the past few years, prominent nonlinear optical properties have been demonstrated in perovskite bulk structures indicating their bright prospect in the field of nonlinear optics (NLO). Following the surge of 3D perovskites, more recently, the low-dimensional perovskites (LDPs) materials ranging from two-, one-, to zero-dimension such as quantum-wells or colloidal nanostructures have displayed unexpectedly attractive NLO response due to the strong quantum confinement, remarkable exciton effect, and structural diversity. In this perspective, the current state of the art is reviewed in the field of NLO for LDP materials. The relationship between confinement effect and NLO is analyzed systematically to give a comprehensive understanding of the function of dimension reduction. Furthermore, future directions and challenges toward the improvement of the NLO in LDP materials are discussed to provide an outlook in this rapidly developing field.

Mechanism of ultrafast energy transfer between the organic-inorganic layers in multiple-ring aromatic spacers for 2D perovskites

Lead halide based perovskite semiconductors self-assemble with distinct organic cations in natural multi-quantum-well structures. The emerging electronic properties of these two-dimensional (2D) materials can be controlled by the combination of the halide content and choice of chromophore in the organic layer. Understanding the photophysics of the perovskite semiconductor materials is critical for the optimization of stable and efficient optoelectronic devices. We use femtosecond transient absorption spectroscopy (fs-TAS) to study the mechanism of energy transfer between the organic and inorganic layers in a series of three lead-based mixed-halide perovskites such as benzylammonium (BA), 1-naphthylmethylammonium (NMA), and 1-pyrenemethylammonium (PMA) cations in 2D-lead-based perovskite thin films under similar experimental conditions. After optical excitation of the 2D-confined exciton in the lead halide layer, ultrafast energy transfer is observed to organic singlet and triplet states of the incorporated chromophores. This is explained by an effective Dexter energy transfer, which operates via a correlated electron exchange between the donating 2D-confined exciton and the accepting chromophore under spin conservation.

Monolayer-to-Multilayer Dimensionality Reconstruction in a Hybrid Perovskite for Exploring the Bulk Photovoltaic Effect Enables Passive X-ray Detection

Halide hybrid perovskites are attracting considerable attention as highly promising candidates for directly sensing X-ray radiation, but it is challenging to realize passive X-ray detection without an external power supply. However, the bulk photovoltaic effect (BPVE) in ferroelectrics promotes the independent separation of photoexcited carriers. Herein, by dimensionality reconstruction of a pure-two-dimensional (P-2D) monolayered perovskite (CH₃ OC₃ H₉ N)₂ PbBr₄ , we obtained a quasi-two-dimensional (Q-2D) ferroelectric (CH₃ OC₃ H₉ N)₂ CsPb₂ Br₇ . Converting P-2D into Q-2D perovskite stimulates a significant BPVE associated with robust ferroelectricity, as well as an enhanced mobility lifetime product. These features show the potential of the first passive X-ray detector based on ferroelectrics with an impressive sensitivity up to 410 μC Gy^-1  cm^-2 at zero bias, which is even superior to the value of the state-of-the-art α-Se detector operated at relatively high bias.

Arene and functionalized arene based two dimensional organic-inorganic hybrid perovskites for photovoltaic applications

Recently, two-dimensional organic-inorganic hybrid perovskites have attracted great attention for their outstanding performances in solar energy conversion devices. By using first principles calculations, we explored the structural, electronic and optical properties of recently synthesized (PEA)₂ PbI₄ and (PEA)₂ SnI₄ organic-inorganic hybrid perovskites to understand the photovoltaic performances of these systems. Our study reveals that both the perovskites are direct band gap semiconductors and possess desirable band gap for solar energy absorption. We have further extended our study to fluoro-, chloro-, and bromo-functionalized phenethylammonium (PEA) cations based [X(X = F, Cl, Br)PEA]₂ A(A = Pb, Sn)I₄ perovskite materials. The halogenated benzene moiety confers an ultrahydrophobic character and protects the perovskites from ambient moisture. The halogen functionalized perovskites remain direct band gap semiconductors and all the perovskites show very strong optical absorption (∼7 × 10⁵  cm^-1 ) across UV-visible region. We have further calculated the photo-conversion efficiency (PCE) of both arene and functionalized arene based perovskites. The halogen-functionalized PEA-based perovskites also exhibit high PCE as like pristine ones and finally achieve high PCE of up to 24.30%, making them competitive with other previously reported perovskite-based photovoltaic devices.

(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.

Dimensional structure regulation of organic-inorganic hybrid perovskite and its application in thin film transistors

The effects of dimensional structure on the properties of lead iodide perovskite (C₈H₉NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1were investigated. Furthermore, perovskite thin films with different dimensionalities were applied as the channel layer of thin film transistors (TFT). The electrical performance and stability of TFT devices were significantly improved through the regulation of dimensional microstructure of the perovskites. As a result, the quasi-2D (n = 6) perovskite TFTs achieved a field-effect mobility (μ_FE) of 3.90 cm²V^-1s^-1, with 10⁴on-off current ratio and -1.85 V threshold voltage, which can be maintained well after 4 days without degradation at 30% ambient humidity. Moreover, the electrical performance of the TFTs based on Pure-2D and Quasi-2D perovskite also exhibited a good bias stability.

High-Performance Flexible Self-Powered Photodetectors Utilizing Spontaneous Electron and Hole Separation in Quasi-2D Halide Perovskites

Although there are recent advances in many areas of quasi-2D halide perovskites, photodetectors based on these materials still cannot achieve satisfactory performance for practical applications where high responsivity, fast response, self-powered nature, and excellent mechanical flexibility are urgently desired. Herein, utilizing one-step spin-coating method, self-assemble quasi-2D perovskite films with graded phase distribution in the order of increasing number of metal halide octahedral layers are successfully prepared. Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained and then employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibits impressive performance with the responsivity up to 444 mA W^-1 and ultrashort response time down to 52 µs. With a bias voltage of 1.5 V, the device responsivity becomes 3463 mA W^-1 with the response speed as fast as 24 µs. Importantly, the device's mechanical flexibility is greatly enhanced since the photocurrent prefers flowing through the metal halide octahedral layers between the top and bottom contact electrodes in the vertical device structure, being more tolerant to film damage. These results evidently indicate the potential of graded quasi-2D perovskite phases for next-generation optoelectronic devices.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

Strong Self-Trapped Exciton Emissions in Two-Dimensional Na-In Halide Perovskites Triggered by Antimony Doping

Soft lattice and strong exciton-phonon coupling have been demonstrated in layered double perovskites (LDPs) recently; therefore, LDPs represents a promising class of compounds as excellent self-trapped exciton (STE) emitters for applications in solid-state lighting. However, few LDPs with outstanding STE emissions have been discovered, and their optoelectronic properties are still unclear. Based on the three-dimensional (3D) Cs₂ NaInCl₆ , we synthesized two 2D derivatives (PEA)₄ NaInCl₈ :Sb (PEA=phenethylamine) and (PEA)₂ CsNaInCl₇ :Sb with monolayer and bilayer inorganic sheets by a combination of dimensional reduction and Sb-doping. Bright broadband emissions were obtained for the first time under ambient temperature and pressure, with photoluminescence quantum efficiency (PLQE) of 48.7 % (monolayer) and 29.3 % (bilayer), superior to current known LDPs. Spectroscopic characterizations and first-principles calculations of excited state indicate the broadband emissions originate from STEs trapped at the introduced [SbCl₆ ]^3- octahedron.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

Low-Dimensional Metal Halide Perovskite Photodetectors

Metal halide perovskites (MHPs) have been a hot research topic due to their facile synthesis, excellent optical and optoelectronic properties, and record-breaking efficiency of corresponding optoelectronic devices. Nowadays, the development of miniaturized high-performance photodetectors (PDs) has been fueling the demand for novel photoactive materials, among which low-dimensional MHPs have attracted burgeoning research interest. In this report, the synthesis, properties, photodetection performance, and stability of low-dimensional MHPs, including 0D, 1D, 2D layered and nonlayered nanostructures, as well as their heterostructures are reviewed. Recent advances in the synthesis approaches of low-dimensional MHPs are summarized and the key concepts for understanding the optical and optoelectronic properties related to the PD applications of low-dimensional MHPs are introduced. More importantly, recent progress in novel PDs based on low-dimensional MHPs is presented, and strategies for improving the performance and stability of perovskite PDs are highlighted. By discussing recent advances, strategies, and existing challenges, this progress report provides perspectives on low-dimensional MHP-based PDs in the future.

Ruddlesden-Popper-Phase Hybrid Halide Perovskite/Small-Molecule Organic Blend Memory Transistors

Controlling the morphology of metal halide perovskite layers during processing is critical for the manufacturing of optoelectronics. Here, a strategy to control the microstructure of solution-processed layered Ruddlesden-Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)₂ PbBr₄ ) is reported. The method relies on the addition of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C₈ -BTBT) into the perovskite formulation, where it facilitates the formation of large, near-single-crystalline-quality platelet-like (PEA)₂ PbBr₄ domains overlaid by a ≈5-nm-thin C₈ -BTBT layer. Transistors with (PEA)₂ PbBr₄ /C₈ -BTBT channels exhibit an unexpectedly large hysteresis window between forward and return bias sweeps. Material and device analysis combined with theoretical calculations suggest that the C₈ -BTBT-rich phase acts as the hole-transporting channel, while the quantum wells in (PEA)₂ PbBr₄ act as the charge storage element where carriers from the channel are injected, stored, or extracted via tunneling. When tested as a non-volatile memory, the devices exhibit a record memory window (>180 V), a high erase/write channel current ratio (10⁴ ), good data retention, and high endurance (>10⁴ cycles). The results here highlight a new memory device concept for application in large-area electronics, while the growth technique can potentially be exploited for the development of other optoelectronic devices including solar cells, photodetectors, and light-emitting diodes.

Multicolor Output from 2D Hybrid Perovskites with Wide Band Gap: Highly Efficient White Emission, Dual-Color Afterglow, and Switch between Fluorescence and Phosphorescence

Herein, an organic fluorophore termed NLAC is introduced into 2D hybrid perovskites with wide band gap (>3.54 eV) to give a green emission with quantum yield up to 81%. The highly efficient luminescence is ascribed to avoiding the aggregation of NLAC and formation of an inorganic free exciton which is easy to thermally quench. On this basis, a new strategy to generate efficient white emission with afterglow has been proposed by codoping a short-wavelength fluorophore and long-wavelength phosphor into 2D organic-inorganic hybrid perovskites (OIHPs). As a result, a single-component white-light-emitting material PEPC-3N based on NLAC with CIE of (0.33, 0.36) and quantum yield up to 43% can be obtained. Interestingly, PEPC-3N shows a dual-color organic afterglow and excitation-wavelength-dependent emission, consequently forming a switch between green fluorescence and yellow afterglow. This unique performance indicates PEPC-3N has huge potential in afterglow WLEDs and information storage.

Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS₂) monolayer/2D perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of -0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS₂/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS₂ region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 μA/W and 5.7 × 10^-5, respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

2D Perovskite Single Crystals with Suppressed Ion Migration for High-Performance Planar-Type Photodetectors

2D Dion-Jacobson (DJ) perovskites have recently drawn much attention owing to their superior charge transport and high environmental stability. Unfortunately, their intrinsic optoelectronic properties are largely unexplored due to the lack of high-quality single crystals available for accurate measurements. Herein, a reactive, low-temperature-gradient crystallization process to grow high-quality 2D perovskite single crystal (BDAPbI₄ ) using symmetric straight chain 1,4-butanediammonium as a short-chain insulating spacer is developed. It is found that the BDAPbI₄ single crystal exhibits a direct bandgap with effective charge collection (μτ = 1.45 × 10^-3 cm² V^-1 ). In particular, the BDAPbI₄ single crystal shows a high ion migration activation energy (0.88 eV) that is desired for stability. Moreover, the planar-type photodetectors made of BDAPbI₄ single crystals demonstrate excellent photoresponse performance, including large dynamic linear range (150 dB), high responsivity (927 mA W^-1 ), fast response speed, and exceptional stability. Theoretical calculations reveal that the enhanced carrier transport properties are mainly attributed to the interlayer-induced coupling of the inorganic layers. Furthermore, the rigid structure of BDAPbI₄ , resulting from the strong hydrogen bonds between the organic cations and the inorganic framework, will result in both low defect density and low ion migration, leading to superior carrier transport properties and detection performance.

Superior Carrier Lifetimes Exceeding 6 µs in Polycrystalline Halide Perovskites

Lead halide perovskite films have witnessed rapid progress in optoelectronic devices, whereas polycrystalline heterogeneities and serious native defects in films are still responsible for undesired recombination pathways, causing insufficient utilization of photon-generated charge carriers. Here, radiation-enhanced polycrystalline perovskite films with ultralong carrier lifetimes exceeding 6 μs and single-crystal-like electron-hole diffusion lengths of more than 5 μm are achieved. Prolongation of charge-carrier activities is attributed to the electronic structure regulation and the defect elimination at crystal boundaries in the perovskite with the introduction of phenylmethylammonium iodide. The introduced electron-rich anchor molecules around the host crystals prefer to fill the halide/organic vacancies at the boundaries, rather than form low-dimensional phases or be inserted into the original lattice. The weakening of the electron-phonon coupling and the excitonic features of the photogenerated carriers in the optimized films, which together contribute to the enhancement of carrier separation and transportation, are further confirmed. Finally the resultant perovskite films in fully operating solar cells with champion efficiency of 23.32% are validated and a minimum voltage deficit of 0.39 V is realized.

Bication-Mediated Quasi-2D Halide Perovskites for High-Performance Flexible Photodetectors: From Ruddlesden-Popper Type to Dion-Jacobson Type

Quasi-2D halide perovskites, especially the Ruddlesden-Popper perovskites (RPPs), have attracted great attention because of their promising properties for optoelectronics; however, there are still serious drawbacks, such as inefficient charge transport, poor stability, and unsatisfactory mechanical flexibility, restricting further utilization in advanced technologies. Herein, high-quality quasi-2D halide perovskite thin films are successfully synthesized with the introduction of the unique bication ethylenediammonium (EDA) via a one-step spin-coating method. This bication EDA, with short alkyl chain length, can not only substitute the typically bulky and weakly van der Waals-interacted organic bilayer spacer cations forming the novel Dion-Jacobson phase to enhance the mechanical flexibility of the quasi-2D perovskite (e.g., EDA(MA)_n-1Pb_nI_3n+1; MA = CH₃NH₃⁺) but also serve as a normal cation to achieve the more intact films (e.g., (iBA)₂(MA)_3-2x(EDA)_xPb₄I₁₃). When fabricated into photodetectors, these optimized EDA-based perovskites deliver an excellent responsivity of 125 mA/W and a fast response time down to 380 μs under 532 nm irradiation. More importantly, the device with the Dion-Jacobson phase perovskite can be bent down to a radius of 2 mm and processed with 10,000 cycles of the bending test without any noticeable performance degradation because of its superior structure to RPPs. Besides, these films do not exhibit any material deterioration after ambient storage for 30 days. All these performance parameters are already comparable or even better than those of the state-of-the-art RPPs recently reported. This work provides valuable design guidelines of the quasi-2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronics.

Organic-Salt-Assisted Crystal Growth and Orientation of Quasi-2D Ruddlesden-Popper Perovskites for Solar Cells with Efficiency over 19

Quasi-2D Ruddlesden-Popper (RP) perovskite solar cells (PSCs) have drawn significant attention due to their appealing environmental stability compared to their 3D counterparts. However, the relatively low power conversion efficiency (PCE) greatly limits their applications. Here, high photovoltaic performance is demonstrated for quasi-2D RP PSCs using 2-thiophenemethylammonium as spacer with nominal n-value of 5, which is based on the stoichiometry of the precursors. The incorporation of formamidinium (FA) in quasi-2D RP perovskites reduces the bandgap and improves the light absorption ability, resulting in enlarged photocurrent and an increased PCE of 16.18%, which is higher than that of reported analogous methylammonium (MA)-based quasi-2D PSC (≈15%). A record high PCE of 19.06% is further demonstrated by using an organic salt, namely, 4-(trifluoromethyl)benzylammonium iodide, assisted crystal growth (OACG) technique, which can induce the crystal growth and orientation, tune the surface energy levels, and suppress the charge recombination losses. More importantly, the devices based on OACG-processed quasi-2D RP perovskites show remarkable environmental stability and thermal stability, for example, the PCE retaining ≈96% of its initial value after storage at 80 °C for 576 h, while only ≈37% of the original efficiency left for FAPbI₃ -based 3D PSCs.

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3 Bi2 I6 Cl3 Perovskite

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs₃ Bi₂ I₆ Cl₃ , by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.

Graded 2D/3D Perovskite Heterostructure for Efficient and Operationally Stable MA-Free Perovskite Solar Cells

Almost all highly efficient perovskite solar cells (PVSCs) with power conversion efficiencies (PCEs) of greater than 22% currently contain the thermally unstable methylammonium (MA) molecule. MA-free perovskites are an intrinsically more stable optoelectronic material for use in solar cells but compromise the performance of PVSCs with relatively large energy loss. Here, the open-circuit voltage (V_oc ) deficit is circumvented by the incorporation of β-guanidinopropionic acid (β-GUA) molecules into an MA-free bulk perovskite, which facilitates the formation of quasi-2D structure with face-on orientation. The 2D/3D hybrid perovskites embed at the grain boundaries of the 3D bulk perovskites and are distributed through half the thickness of the film, which effectively passivates defects and minimizes energy loss of the PVSCs through reduced charge recombination rates and enhanced charge extraction efficiencies. A PCE of 22.2% (certified efficiency of 21.5%) is achieved and the operational stability of the MA-free PVSCs is improved.

Confinement-Driven Ferroelectricity in a Two-Dimensional Hybrid Lead Iodide Perovskite

Organic-inorganic hybrid perovskites (OIHPs) hold a great potential for scientific and technological endeavors in the field of ferroelectrics, solar cells, and electroluminescent devices, because of their structural diversity, low cost of manufacture, and ease of fabrication. However, lead iodide perovskite ferroelectrics with narrow band gap have rarely been reported. Here, we present a new two-dimensional (2D) layered lead iodide perovskite ferroelectric, (4,4-DFHHA)₂PbI₄ (4,4-DFHHA = 4,4-difluorohexahydroazepine), with a spontaneous polarization (P_s) of 1.1 μC/cm² at room temperature, a direct bandgap of 2.32 eV, and a high Curie temperature T_c of 454 K (beyond that of BaTiO₃, 393 K). On the basis of the nonferroelectrics (HHA)I, (4-FHHA)I, and (4,4-DFHHA)I (HHA = hexahydroazepine, 4-FHHA = 4-fluorohexahydroazepine), we assembled them with PbI₂ to form lead iodide perovskites. Because the space between adjacent one-dimensional (1D) chains is relatively large and the confinement effect is not obvious, the cations are still in a disordered state, and 1D OIHPs (HHA)PbI₃ and (4-FHHA)PbI₃ are also nonferroelectrics at room temperature. In the confined environment of the 2D PbI₄^2- framework for (4,4-DFHHA)₂PbI₄, the 4,4-DFHHA cations become ordered, and their asymmetric distribution leads to the spontaneous polarization. This work offers an efficient strategy for enriching the family of lead iodide perovskite ferroelectrics through the confinement effect and should inspire further exploration of the interplay between ferroelectricity and photovoltaics.

Unveiling Mn2+ Dopant States in Two-Dimensional Halide Perovskite toward Highly Efficient Photoluminescence

Doping is able to create novel optoelectronic properties of halide perovskites, and the involved mechanism of efficient emission is still a challenge. Herein Mn²⁺ substitution into 2D layered perovskites (C₈H₂₀N₂)PbBr₄ was investigated, demonstrating broad-band orange-red emission originating from the ⁴T₁ → ⁶A₁ transition of Mn²⁺ dopant. The photoluminescence quantum yield (PLQY) of Mn²⁺ emission is up to 60.8% related to the energy transfer in coupled states. We verify that an actual Mn²⁺ dopant as low as 0.476% reaches a high PLQY, whereas the nominal adding amount is 0.8 as the Mn²⁺/Pb²⁺ ratio. The small activation energy (∼6.72 meV) between the Mn²⁺ d state and the trap state accounts for this highly efficient energy transfer and photoluminescence. The proposed luminescence mechanism in Mn²⁺-doped 2D halide perovskites would provide unique insights into the doping design toward high-performance luminescence materials.