Insight on the Stability of Thick Layers in 2D Ruddlesden–Popper and Dion–Jacobson Lead Iodide Perovskites

Structure-Emission Property Relationship of Bilayer 2D Hybrid Perovskites

Two-dimensional hybrid perovskites (2D-PVKs) have shown great promise for optoelectronic applications. However, the structure-emission property relationship of 2D-PVKs, particularly those with multiple octahedral layers in the metal-halide lattice (n > 1), is not fully understood. Here we combine experimental and theoretical studies to investigate a series of bilayer (n = 2) 2D-PVK crystals in both Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases. Our results reveal that DJ-phase crystals exhibit a higher degree of octahedral lattice distortion compared with RP-phase crystals, with this distortion scaling inversely with interlayer spacing. Such octahedral distortion leads to (1) lower formation energies for iodine vacancies that act as nonradiative recombination centers, thereby reducing light emission yields, and (2) local inversion asymmetry that impacts electronic band structure and light emission properties. Among all the studied crystals, the DJ-phase crystal based on 4-(aminomethyl)piperidinium cations demonstrates the largest intra- and interoctahedral distortions, leading to inversion asymmetry that causes significant Rashba band splitting and circular-polarization dependent photoluminescence at room temperature. Our results provide insights into the development of 2D-PVKs for future optoelectronic/spintronic applications.

Halogen Bonding Enable Improved Performance and Stability of Dion-Jacobson Perovskite Solar Cells

The undesired phase distribution and intrinsic residual lattice strain remain the bottleneck for high-performance and stable 2D or Q-2D perovskite solar cells (PSCs), especially for the Dion-Jacobson (DJ) type Q-2D PSCs. Here, we showcase employing halogen bonds to stabilize the halide anions and fine-tuning the residual lattice strain by introducing perfluorodecyl iodide (PFI). It reduces the density of iodide species defects and releases the residual tensile strain, leading to more homogenous phase distribution, suppressed carrier recombination, promoted charge transport, enhanced efficiency, and stability. Therefore, DJ PSCs maintain over 81% of initial efficiency after thermal-light aging (i.e., under 85 °C and 1 sun illumination) for 952 h, and remain over 91% of initial efficiency after under reverse bias (-2.5 V, under ISOS-V aging tests) for over 21 h, representing one of the most bias-stable Q-2D PSCs reported so far.

Stress Release of Single Crystal Arrays Bridged by SAM Interface Toward Highly Mechanically Durable Flexible Perovskite NIR Photodetector

Perovskite photodetectors with superior optoelectronic properties, lightweight, and compatibility with flexible substrates have attracted much attention in wearable electronics. However, the large bandgap, inherent brittleness, poor environmental stability, and weak interfacial adhesion interaction between perovskites and substrates hinder the application of near-infrared (NIR) wearable devices. Herein, a universal strategy to enhance the performance and mechanical stability of flexible perovskite NIR photodetector arrays is demonstrated through a combination of mussel-inspired self-assembled monolayer (SAM) bridging interface and precise modulation of the nano-array size, which enables to significantly increase interfacial adhesion, crystallinity, crystallographic orientation, and reduce mechanical stresses of perovskite single-crystal arrays. Moreover, inserting paddle-wheel metal-organic cluster ligands lead to an unprecedented small bandgap of 1.04 eV, enhanced lattice rigidity, and environmental stability for 2D perovskite. The flexible perovskite NIR photodetector arrays with superior mechanical robustness and record NIR performance are revealed with a maximum response wavelength of 1050 nm, a responsivity of 1.66 A W-1, detectivity of 6.19 × 1012 Jones, high fidelity imaging, and extra-long environmental stability. This work pioneers a new insight into the integration of high-performance and mechanically durable perovskite flexible wearable devices.

Solvent Engineering-Induced Dual Effect on Enhanced Spontaneous Polarization and Charge Transport in Ferroelectric P(VDF-TrFE)-Doped Quasi-2D Perovskites

Incorporation of ferroelectric materials into perovskite solar cells (PSCs) has been demonstrated as an effective approach to elevating the device performance. Dipole polarization of the ferroelectric material can be tuned by an external bias, facilitating the charge transport of PSCs. However, fundamental challenges still exist to utilize the ferroelectric effect in PSCs because the external bias leads to ion migration of the perovskites, deteriorating the performance and long-term stability of the devices. In this study, we report an effective method to improve the spontaneous polarization of the ferroelectric polymer incorporated in a quasi-two-dimensional (2D) perovskite without utilizing the external bias. The solvent engineering strategy effectively enhances the ferroelectric phase and crystallinity of the ferroelectric polymer, improving the local electric field and consequently enhancing charge transport. In addition, the solvent strategy offers the formation of a smooth surface as well as a favorable crystal orientation, leading to a reduced defect density in the quasi-2D perovskite films. Attributed to the dual effects, a significant improvement of the power conversion efficiency is achieved from 6.27 to 14.83%, and it is further improved to 16.65% through fine-tuning of the functional layers' thickness. We believe that the method reported offers an avenue for designing the quasi-2D perovskite without the requirement for structural modification of the PSCs.

Organic Spacers Modulated Low Dose X-Ray Detection in Hybrid Halide 2D Perovskites: Unveiling Exciton Dynamics

In this study, we investigate how modulating organic spacers in perovskites influences their X-ray detection performance and reveal the mechanism of low-dose detection with high sensitivity using femtosecond-transient absorption spectroscopy (fs-TAS). Particularly, we employ N,N,N',N'-tetramethyl-1,4-phenylenediammonium (TMPDA) and N,N-dimethylphenylene-p-diammonium (DPDA) as organic spacers to synthesize 2D perovskite single crystals (SCs). We find that DPDA-based SCs exhibit reduced interplanar spacing between inorganic layers, leading to increased lattice packing. Density functional theory (DFT) results indicate the reduced effective mass and lower lattice distortion in (DPDA)PbBr4 suppressing the formation of self-trapped exciton (STEs) and electron-phonon coupling and enhancing carrier delocalization in these SCs. Further, X-ray detection measurements reveal that (DPDA)PbBr4 demonstrates higher sensitivity than (TMPDA)PbBr4, attributed to its enhanced carrier delocalization, and higher mobility-lifetime product. The limit of detection (LoD) for (DPDA)PbBr4 is determined to be 13 nGy/s, significantly lower than both commercial detectors and state-of-the-art perovskite-based X-ray detectors. Furthermore, fs-TAS study reveals that (DPDA)PbBr4 crystals exhibit prolonged hot STE cooling and decay lifetimes, which directly correlate with their higher sensitivity. This study highlights the impact of organic spacers on X-ray detection performance, providing a framework for designing ultra-low LoD detectors essential for health and security applications.

Stability and reversibility of organic molecule modifications of CVD-synthesized monolayer MoS2

We investigated the stability of monolayer MoS2samples synthesized using chemical vapor deposition and subsequently modified with organic molecules under ambient conditions. By analyzing the optical signatures of the samples using photoluminescence spectroscopy, Raman spectroscopy, and surface quality using atomic force microscopy, we observed that this modification of monolayer MoS2with organic molecules is stable and retains its optical signature over time under ambient conditions. Furthermore, we show the reversibility of the effects induced by the organic molecules, as heating the modified samples restores their original optical signatures, indicating the re-establishment of the optical properties of the pristine monolayer MoS2.

Reconstruction and Solidification of Dion-Jacobson Perovskite Top and Buried Interfaces for Efficient and Stable Solar Cells

Quasi-two-dimensional (Q-2D) perovskites show great potential in the field of photonic and optoelectronic device applications. However, defects and local lattice dislocation still limit performance and stability improvement by nonradiative recombination, unpreferred phase distribution, and unbonded amines. Here, a low-temperature synergistic strategy for both reconstructing and solidifying the perovskite top and buried interface is developed. By post-treating the 1,4-phenylenedimethanammonium (PDMA) based (PDMA)MA4Pb5I16 films with cesium acetate (CsAc) before thermal annealing, a condensation reaction between R-COO- and -NH2 and ion exchange between Cs+ and MA+ occur. It converts the unbonded amines to amides and passivates uncoordinated Pb2+. Meanwhile, it adjusts film composition and improves the phase distribution without changing the out-of-plane grain orientation. Consequently, performance of 18.1% and much-enhanced stability (e.g., stability for photo-oxygen increased over 10 times, light-thermal for T90 over 4 times, and reverse bias over 3 times) of (PDMA)MA4Pb5I16 perovskite solar cells are demonstrated.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

Advances and challenges in molecular engineering of 2D/3D perovskite heterostructures

Organic-inorganic hybrid perovskites have been intensively studied in past decades due to their outstanding performance in solar cells and other optoelectronic devices. Recently, the emergence of two-dimensional/three-dimensional (2D/3D) heterojunctions have enabled many solar cell devices with >25% power conversion efficiency, driven by advances in our understanding of the structural and photophysical properties of the heterojunctions and our ability to control these properties through organic cation configuration in 2D perovskites. In this feature article, we discuss a fundamental understanding of structural characteristics and the carrier dynamics in the 2D/3D heterojunctions and their impact factors. We further elaborate the design strategies for the molecular configuration of organic cations to achieve thorough management of these properties. Finally, recent advances in 2D/3D heterostructures in solar cells, light-emitting devices and photodetectors are highlighted, which translate fundamental understandings to device applications and also reveal the remaining challenges in ligand design for the next generation of stable devices. Future development prospects and related challenges are also provided, with wide perspectives and insightful thoughts.

Ultrastable and efficient slight-interlayer-displacement 2D Dion-Jacobson perovskite solar cells

Stability has been a long-standing concern for solution-processed perovskite photovoltaics and their practical applications. However, stable perovskite materials for photovoltaic remain insufficient to date. Here we demonstrate a series of ultrastable Dion-Jacobson (DJ) perovskites (1,4-cyclohexanedimethanammonium)(methylammonium)n-1PbnI3n+1 (n ≥ 1) for photovoltaic applications. The scalable technology by blade-coated solar cells for the designed DJ perovskites (nominal n = 5) achieves a maximum stabilized power conversion efficiency (PCE) of 19.11% under an environmental atmosphere. Un-encapsulated cells by blade-coated technology retain 92% of their initial efficiencies for over 4000 hours under ~90% relative humidity (RH) aging conditions. More importantly, these cells also exhibit remarkable thermal (85 °C) and operational stability, which shows negligible efficiency loss after exceeding 5000-hour heat treatment or after operation at maximum power point (MPP) exceeding 6000 hours at 45 °C under a 100 mW cm-2 continuous light illumination.

Dion-Jacobson Type Lead-Free Double Perovskite with Ultra-Narrow Aromatic Interlayer Spacing for Highly Sensitive and Stable X-ray Detection

The low-toxic and environmentally friendly 2D lead-free perovskite has made significant progress in the exploration of "green" X-ray detectors. However, the gap in detection performance between them and their lead-based analogues remains a matter of concern that cannot be ignored. To reduce this gap, shortening the interlayer spacing to accelerate the migration and collection of X-ray carriers is a promising strategy. Herein, a Dion-Jacobson (DJ) lead-free double perovskite (4-AP)2AgBiBr8 (1, 4-AP = 4-amidinopyridine) with an ultra-narrow interlayer spacing of 3.0 Å, is constructed by utilizing π-conjugated aromatic spacers. Strikingly, the subsequent enhanced carrier transport and increased crystal density lead to X-ray detectors based on bulk single crystals of 1 with a high sensitivity of 1117.3 µC Gy-1 cm-2, superior to the vast majority of similar double perovskites. In particular, the tight connection of the inorganic layers by the divalent cations enhances structural rigidity and stability, further endowing 1 detector with ultralow dark current drift (3.06 × 10-8 nA cm-1 s-1 V-1, 80 V), excellent multiple cycles switching X-ray irradiation stability, as well as long-term environmental stability (maintains over 94% photoresponse after 90 days). This work brings lead-free double perovskites one step closer to realizing efficient practical green applications.

Interfacial Rivet to Fill Structural Defects: A Spacer Engineering Gift for 3D Solar Cells

The combination of 2D and 3D perovskites to passivate surfaces or interfaces with a high concentration of defects shows great promise for improving the efficiency of perovskite solar cells (PSCs). Constructing high-quality perovskite film systems by precisely modulating 2D perovskites with good morphologies and growth sites on 3D perovskite films remains a formidable challenge due to the complexity of spacer-engineered surface reactions. In this study, phase-pure 2D (HA)2(MA)n-1PbnI3n+1 perovskites with a controlled number of layers (n) are separated on a large scale and exploited as interface rivets to optimize 3D perovskite films, resulting in tunable film structural defects and grain boundaries. The optimized PSCs system benefits from a reduction in non-radiative recombination, resulting in improved optical performance, higher mobility, and lower trap density. The corresponding device achieves a champion power conversion efficiency (PCE) of more than 25%, especially for voltage (VOC) and fill factor (FF). The quality and uniformity of the perovskite films are further confirmed using large-area devices with an active area of 14 cm2, which exhibits a PCE of more than 21.24%. The high-quality thin-film system based on the 2D perovskites presented herein provides a new perspective for improving the efficiency and stability of PSCs.

Large-n quasi-phase-pure two-dimensional halide perovskite: A toolbox from materials to devices

Despite their excellent environmental stability, low defect density, and high carrier mobility, large-n quasi-two-dimensional halide perovskites (quasi-2DHPs) feature a limited application scope because of the formation of self-assembled multiple quantum wells (QWs) due to the similar thermal stabilities of large-n phases. However, large-n quasi-phase-pure 2DHPs (quasi-PP-2DHPs) can solve this problem perfectly. This review discusses the structures, formation mechanisms, and photoelectronic and physical properties of quasi-PP-2DHPs, summarises the corresponding single crystals, thin films, and heterojunction preparation methods, and presents the related advances. Moreover, we focus on applications of large-n quasi-PP-2DHPs in solar cells, photodetectors, lasers, light-emitting diodes, and field-effect transistors, discuss the challenges and prospects of these emerging photoelectronic materials, and review the potential technological developments in this area.

Diammonium spacer-induced stable zigzag type 2D Dion-Jacobson lead/tin-based perovskite solar cells

Hybrid halide two dimensional (2D) perovskites have attracted considerable attention because they exhibit an improvement in perovskite solar cells compared with their 3D analogs. However, their bulky organic space group, leading to higher bandgaps and exciton binding energy, limits the charge transport in solar cells. Herein, the 3-(aminomethyl)pyridinium (3API2, C6H10N2I2) dication is incorporated into FA(Pb0.5Sn0.5)I3 to develop zigzag type 2D Dion-Jacobson-phase perovskites, which have low band gaps in the range of 1.44 to 1.53 eV for concentrations from 5 mol% to 20 mol% due to structural distortion. The introduction of the 3API2 cation increases the carrier conductivity and produces a high-quality perovskite film with no pinhole and connected grains, which is favorable for efficient carrier transport. Consequently, solar cells employing FA(Pb0.5Sn0.5)I3 with 10 mol% 3API2 added as a light absorber achieve a power conversion efficiency of 5.46% with an open-circuit voltage of 0.47 V, a fill factor of 58.07% and a short-circuit current density of 20.18 mA cm-2. With this class of new 2D Dion-Jacobson perovskite compositions, this work suggests potential future directions for improving the performance and device stability of perovskite solar cells.

The rise of quasi-2D Dion-Jacobson perovskites for photovoltaics

With the advance of nanotechnology, the past couple of years have witnessed the fast development of quasi two-dimensional (2D) halide perovskites, which exhibit outstanding long-term stability against moisture and heat, compared with their three-dimensional (3D) counterparts. As one of the most common structures in 2D halide perovskites, quasi-2D Dion-Jacobson (DJ) perovskites show multiple-quantum-well structures with n layers of [BX6]4- octahedral inorganic sheets sandwiched by two layers of diammonium spacers, thus exhibiting superior structural stability due to the elimination of van der Waals gaps. Thanks to the achievement of high power conversion efficiency accompanied by impressive stability, quasi-2D DJ perovskite solar cells (PSCs) have recently drawn extensive attention in the field. This review first introduces the fundamental understanding of quasi-2D DJ halide perovskites, including their superior stability, high exciton binding energy, and compositional flexibility and tunable properties. We then summarize detailed strategies to prepare high-quality quasi-2D DJ perovskites for PSCs, encompassing compositional engineering, solvent engineering, additive addition, and annealing processes. Moreover, the surface/interface modification and 2D-3D hybrid perovskite heterojunction are also discussed, for providing strategies to optimize the fabrication of quasi-2D DJ PSCs. Lastly, current challenges and perspectives toward the future development of quasi-2D DJ perovskites for photovoltaics are outlined.

Thermochemistry of hybrid materials

This paper is based on a lecture Navrotsky gave honouring the memory of Paul McMillan. It summarizes our recent findings in the thermodynamics of hybrid materials including metal organic frameworks (MOFs), polymer-derived ceramics (PDCs) and ionic organic-inorganic compounds. This work describes the main structure types and their corresponding thermodynamic stability, obtained from calorimetric measurements in our laboratory. The effects of linker substituent and framework topology on the thermodynamic stability of isostructural zeolitic imidazolate frameworks and other MOFs are discussed. The paper documents the effects of interdomain interaction and bonding speciation on the thermodynamic stability of various PDC compositions, including SiC, SiOC and SiCN systems. The paper further describes effects of different cations on the thermodynamic stability of selected ionic organic-inorganic compounds. Similarities and differences among these materials are emphasized. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Intrinsic Ferromagnetic Semiconductors with High Saturation Magnetization from Hybrid Perovskites

Ferromagnetic semiconductors (FMS) enable simultaneous control of both charge and spin transport of charge carriers, and they have emerged as a class of highly desirable but rare materials for applications in spin field-effect transistors and quantum computing. Organic-inorganic hybrid perovskites with high compositional adjustability and structural versatility can offer unique benefits in the design of FMS but has not been fully explored. Here, a series of molecular FMSs based on the 2D organic-inorganic hybrid perovskite structure, namely (2ampy)CuCl4 , (3ampy)CuCl4 , and (4ampy)CuCl4 , is demonstrated, which exhibits high saturation magnetization, dramatic temperature-dependent conductivity change, and tunable ferromagnetic resonance. Magnetic measurements reveal a high saturation magnetization up to 18.56 emu g-1 for (4ampy)CuCl4 , which is one of the highest value among reported hybrid FMSs to date. Conductivity studies of the three FMSs demonstrate that the smaller adjacent octahedron distance in the 2D layer results in higher conductivity. Systematic ferromagnetic resonance investigation shows that the gyromagnetic ratio and Landau factor values are strongly dependent on the types of organic cations used. This work demonstrates that 2D hybrid perovskite materials can simultaneously possess both tunable long-range ferromagnetic ordering and semiconductivity, providing a straightforward strategy for designing and synthesizing high-performance intrinsic FMSs.

Synergy of 3D and 2D Perovskites for Durable, Efficient Solar Cells and Beyond

Three-dimensional (3D) organic-inorganic lead halide perovskites have emerged in the past few years as a promising material for low-cost, high-efficiency optoelectronic devices. Spurred by this recent interest, several subclasses of halide perovskites such as two-dimensional (2D) halide perovskites have begun to play a significant role in advancing the fundamental understanding of the structural, chemical, and physical properties of halide perovskites, which are technologically relevant. While the chemistry of these 2D materials is similar to that of the 3D halide perovskites, their layered structure with a hybrid organic-inorganic interface induces new emergent properties that can significantly or sometimes subtly be important. Synergistic properties can be realized in systems that combine different materials exhibiting different dimensionalities by exploiting their intrinsic compatibility. In many cases, the weaknesses of each material can be alleviated in heteroarchitectures. For example, 3D-2D halide perovskites can demonstrate novel behavior that neither material would be capable of separately. This review describes how the structural differences between 3D halide perovskites and 2D halide perovskites give rise to their disparate materials properties, discusses strategies for realizing mixed-dimensional systems of various architectures through solution-processing techniques, and presents a comprehensive outlook for the use of 3D-2D systems in solar cells. Finally, we investigate applications of 3D-2D systems beyond photovoltaics and offer our perspective on mixed-dimensional perovskite systems as semiconductor materials with unrivaled tunability, efficiency, and technologically relevant durability.

Defect Regulation of Efficient Dion-Jacobson Quasi-2D Perovskite Solar Cells via a Polyaspartic Acid Interlayer

Interfacial modification is a promising strategy to fabricate highly efficient perovskite solar cells (PSCs). Nevertheless, research studies about optimization for the performance of Dion-Jacobson (DJ)-phase quasi-2D PSCs by underlying surface modification are rarely reported. The relevant influence of interfacial modification on defect regulation in the bulk and at the interface for PSCs is still unexplored. Herein, an interlayer of polyaspartic acid (PASP) was introduced at the interface of a hole transporting layer and a perovskite absorber to regulate both the film quality and interface property for BDA-based DJ quasi-2D PSCs (n = 5). The PASP interlayer suppressed the charge recombination, restricted the interfacial charge accumulation, and promoted the charge transport in devices and therefore improved the power conversion efficiency of PSCs from 15.03 to 17.34%. Moreover, through device simulation, it was concluded that the increase of open-circuit voltage (Voc) was mainly attributed to the suppression of interface defects, while the increase of short-circuit current (Jsc) was ascribed to the restriction of interface defects and perovskite bulk defects. The improvement of both Voc and Jsc originated from the passivation of shallow defect states. The present work provides a promising route for the fabrication of efficient quasi-2D PSCs and enriches the fundamental understanding of defect regulation on photovoltaic performance.

Chemical Behavior and Local Structure of the Ruddlesden-Popper and Dion-Jacobson Alloyed Pb/Sn Bromide 2D Perovskites

The alloyed lead/tin (Pb/Sn) halide perovskites have gained significant attention in the development of tandem solar cells and other optoelectronic devices due to their widely tunable absorption edge. To gain a better understanding of the intriguing properties of Pb/Sn perovskites, such as their anomalous bandgap's dependence on stoichiometry, it is important to deepen the understanding of their chemical behavior and local structure. Herein, we investigate a series of two-dimensional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phase alloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium (3AMPY) as the spacer cations: (BA)₂(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) and (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) through a solution-based approach. Our results show that the ratio and site preference of Pb/Sn atoms are influenced by the layer thickness (n) and spacer cations (A'), as determined by single-crystal X-ray diffraction. Solid-state ¹H, ¹¹⁹Sn, and ²⁰⁷Pb NMR spectroscopy analysis shows that the Pb atoms prefer the outer layers in n = 3 members: (BA)₂(MA)Pb_xSn_n-xBr₁₀ and (3AMPY)(MA)Pb_xSn_n-xBr₁₀. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)_n-1Pb_xSn_n-xBr_3n+1 (n = 1-3) demonstrate much narrower optical band gaps, lower energy PL emission peaks, and longer carrier lifetimes compared to those of RP analogs. Density functional theory calculations suggest that Pb-rich alloys (Pb:Sn ∼4:1) for n = 1 compounds are thermodynamically favored over 50:50 (Pb:Sn ∼1:1) compositions. From grazing-incidence wide-angle X-ray scattering (GIWAXS), we see that films in the RP phase orient parallel to the substrate, whereas for DJ cases, random orientations are observed relative to the substrate.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Self-assembly formation of CuI hybrid micron phosphors with tunable emission for multifunctional applications

Low-cost and eco-friendly CuI hybrid compounds with various structures have recently attracted increasing attention due to their excellent optical properties and promising phosphor applications. However, the poor solubility and solution processability of bulk powders with agglomerated particle limited their practical applications greatly. In this work, we reported the self-assembly formation of CuI hybrid micron phosphors via the aqueous PVP micelle-assisted assembly route. Seven CuI hybrid micron phosphors with the emission from blue 450 nm to red 636 nm have been successfully synthesized. Among them, CuI-pyridine hybrid micron phosphors can be obtained via the reaction of CuI with various pyridines. PVP limits the size growth of the phosphors efficiently and it also plays an important role in controlling the distinct crystal phase formation. Whereas, micron phosphors based on bidentate ligands including 2-propylpyrazine, 5-bromopyrimidine or 4,4'-bipyridine need to be prepared via ligand exchange reaction. The micron phosphors present excellent stability in water and can be dispersed in the aqueous solution of PVP or PVA to form homogenous luminescent composites. The luminescent composites based on PVP are easy to use for fabricating anti-counterfeiting patterns via brush-painting or screen-printing. On the other hand, PVA composites can be applied for preparing free standing monochromatic or multichromatic emitting films as color convertor for display backlight. The PVA composites also exhibit the promising phosphor application for light-emitting diode (LED). Especially, the white LED can be directly realized via optimizing the mixing ratio of blue and orange phosphors.

Color-stable blue light-emitting diodes with defect management by sulfonate

Utilizing bromine-based quasi-two-dimensional (quasi-2D) perovskite is a feasible strategy to achieve efficient and stable blue perovskite light-emitting diodes (PeLEDs). However, dimension discretization is prone to emerge due to the irregular phase distribution and massive defects in the perovskite system. Here, we introduce alkali-salt to modulate the phase distribution for reducing the n = 1 phase and propose a novel Lewis base to serve as a passivating agent to decrease defects. This revealed that the external quantum efficiency (EQE) dramatically improves due to the suppression of severe non-radiative recombination losses. Consequently, efficient blue PeLEDs were obtained with a peak EQE of 3.82% located at 487 nm.

Recent progress in layered metal halide perovskites for solar cells, photodetectors, and field-effect transistors

Metal halide perovskite materials demonstrate immense potential for photovoltaic and electronic applications. In particular, two-dimensional (2D) layered metal halide perovskites have advantages over their 3D counterparts in optoelectronic applications due to their outstanding stability, structural flexibility with a tunable bandgap, and electronic confinement effect. This review article first analyzes the crystallography of different 2D perovskite phases [the Ruddlesden-Popper (RP) phase, the Dion-Jacobson (DJ) phase, and the alternating cations in the interlayer space (ACI) phase] at the molecular level and compares their common electronic properties, such as out-of-plane conductivity, crucial to vertical devices. This paper then critically reviews the recent development of optoelectronic devices, namely solar cells, photodetectors and field effect transistors, based on layered 2D perovskite materials and points out their limitations and potential compared to their 3D counterparts. It also identifies the important application-specific future research directions for different optoelectronic devices providing a comprehensive view guiding new research directions in this field.

Asymmetric Diammonium Directed In-Plane Charge Transport Enhancement in Two-Dimensional Lead Bromide Perovskite for Weak-Light Detection

Two-dimensional (2D) Dion-Jacobson (DJ) perovskites are drawing significant attention in optoelectronic fields because of their enhanced out-of-plane electron coupling and improved structure stability. However, the structural effects of organic cations on the in-plane charge transport properties of 2D DJ lead bromide perovskites have remained less explored. Herein, we adopt asymmetric 3-(dimethylamino)-1-propylammonium (DMPD) and symmetric butane-1,4-diammonium (BDA) to systematically investigate the influence of organic cations on the structural, optical, and in-plane charge transport properties of 2D lead bromide perovskites. The large penetration depth of DMPD²⁺ induces a decreased perovskite layer distortion and a lower bandgap in DMPDPbBr₄, compared with that of BDAPbBr₄. Moreover, DMPDPbBr₄ is shown to possess a low exciton binding energy, a low defect density, and a low ion migration activation energy, thereby yielding a more efficient in-plane charge collection efficiency than BDAPbBr₄. Density functional theory calculations suggest that the improved in-plane charge transport can be traced to the enlarged antibonding coupling between Pb-6s and Br-4p orbitals that enables a high band dispersion and a low carrier effective mass in the in-plane direction of DMPDPbBr₄. Finally, the planar Ag/DMPDPbBr₄/Ag photodetector delivers a satisfying detectivity of 1.73 × 10¹² Jones under an incident power intensity of 0.16 mW cm^-2 and a high on/off ratio of 5.3 × 10³. The above findings offer novel insight for the design of 2D DJ lead bromide perovskites for optoelectronic devices.

Solvent Effect on Film Formation and Trap States of Two-Dimensional Dion-Jacobson Perovskite

Two dimensional Dion-Jacobson (2D DJ) perovskite has emerged as a potential photovoltaic material because of its unique optoelectronic characteristics. However, due to its low structural flexibility and high formation energy, extra assistance is needed during crystallization. Herein, we study the solvent effect on film formation and trap states of 2D DJ perovskite. It is found that the nucleation process of 2D DJ perovskite can be retarded by extra coordination, which is proved by in situ optical spectra. As a benefit, out-of-plane oriented crystallization and ordered phase distribution are realized. Finally, in 1,5-pentanediammonium (PeDA) based 2D DJ perovskite solar cells (PSCs), one of the highest reported open-circuit voltage (V_OC) values of 1.25 V with state-of-the-art efficiency of 18.41% is obtained due to greatly shallowed trap states and suppressed nonradiative recombination. The device also exhibits excellent heat tolerance, which maintains 80% of its initial efficiency after being kept under 85 °C after 3000 h.

Performance optimization strategies of halide perovskite-based mechanical energy harvesters

Halide perovskites, possessing unique electronic and photovoltaic properties, have been intensively investigated over the past decade. The excellent polarization, piezoelectricity, dielectricity and photoelectricity of halide perovskites provide new opportunities for the applications of mechanical energy harvesting. Although various studies have been conducted to develop halide perovskite-based triboelectric and piezoelectric nanogenerators, strategies for their electrical performance optimization are rarely mentioned. In this review, we systematically introduce the recent research progress of halide perovskite-based mechanical energy harvesters and summarize the different optimization strategies for improving both the piezoelectric and triboelectric output of the devices, bringing some inspiration to guide future material and structure design for halide perovskite-based energy devices. A summary of the current challenges and future perspectives is also presented, offering some possible directions for development in this emerging field.

Roles of Organic Ligands in Ambient Stability of Layered Halide Perovskites

The combination of organic ligands and inorganic Pb-I frameworks in layered perovskites has bestowed upon them high structural tunability and stability, while their microscopic degradation mechanism remains unclear. Here, we found the key role of ligands in intrinsic structural stability and the consequent morphological evolution in layered perovskites during long-term ambient aging based on (GA)(MA)nPbnI3n+1 (GA = guanidinium, = 4) and (BDA)(MA)n-1PbnI3n+1 (BDA = 1,4-butanediammonium, < n > = 4) perovskites. The BDA-based perovskites have a low intrinsic stability due to high crystal formation energy (ΔH), which are prone to hydration during ambient aging. We overserved changed crystal orientation from perpendicular to parallel, a delayed charge populating time from <1 ps to >50 ps, an inhibited carrier transfer kinetics between quantum wells, an increase of 0.9 μs of charge carrier transport time and a decrease of 1.2 μs of charge carrier lifetime in the BDA-based film during ambient aging, which accounts for a large power-conversion efficiency (PCE) loss (14.2% vs 11.2%). By contrast, the GA ligand increases the intrinsic structural stability of perovskites, which not only yields an initial PCE as high as 20.0% but also helps retain excellent optoelectronic properties during aging. Therefore, only a slight PCE loss (20.0% vs 19.1%) was observed. Our work reveals the key role of organic-inorganic interaction affecting the intrinsic structural stability and optoelectronic properties, and provides a theoretical basis for the future design of stable and efficient optoelectronic devices.

Entropy Stabilization Effects and Ion Migration in 3D "Hollow" Halide Perovskites

A recently discovered new family of 3D halide perovskites with the general formula (A)_1-x(en)_x(Pb)_1-0.7x(X)_3-0.4x (A = MA, FA; X = Br, I; MA = methylammonium, FA = formamidinium, en = ethylenediammonium) is referred to as "hollow" perovskites owing to extensive Pb and X vacancies created on incorporation of en cations in the 3D network. The "hollow" motif allows fine tuning of optical, electronic, and transport properties and bestowing good environmental stability proportional to en loading. To shed light on the origin of the apparent stability of these materials, we performed detailed thermochemical studies, using room temperature solution calorimetry combined with density functional theory simulations on three different families of "hollow" perovskites namely en/FAPbI₃, en/MAPbI₃, and en/FAPbBr₃. We found that the bromide perovskites are more energetically stable compared to iodide perovskites in the FA-based hollow compounds, as shown by the measured enthalpies of formation and the calculated formation energies. The least stable FAPbI₃ gains stability on incorporation of the en cation, whereas FAPbBr₃ becomes less stable with en loading. This behavior is attributed to the difference in the 3D cage size in the bromide and iodide perovskites. Configurational entropy, which arises from randomly distributed cation and anion vacancies, plays a significant role in stabilizing these "hollow" perovskite structures despite small differences in their formation enthalpies. With the increased vacancy defect population, we have also examined halide ion migration in the FA-based "hollow" perovskites and found that the migration energy barriers become smaller with the increasing en content.

Thick-Layer Lead Iodide Perovskites with Bifunctional Organic Spacers Allylammonium and Iodopropylammonium Exhibiting Trap-State Emission

The nature of the organic cation in two-dimensional (2D) hybrid lead iodide perovskites tailors the structural and technological features of the resultant material. Herein, we present three new homologous series of (100) lead iodide perovskites with the organic cations allylammonium (AA) containing an unsaturated C═C group and iodopropylammonium (IdPA) containing iodine on the organic chain: (AA)₂MA_n-1Pb_nI_3n+1 (n = 3-4), [(AA)_x(IdPA)_1-x]₂MA_n-1Pb_nI_3n+1 (n = 1-4), and (IdPA)₂MA_n-1Pb_nI_3n+1 (n = 1-4), as well as their perovskite-related substructures. We report the in situ transformation of AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D perovskite structure. Single-crystal X-ray diffraction shows that (AA)₂MA₂Pb₃I₁₀ crystallizes in the space group P2₁/c with a unique inorganic layer offset (0, <1/2), comprising the first example of n = 3 halide perovskite with a monoammonium cation that deviates from the Ruddlesden-Popper (RP) halide structure type. (IdPA)₂MA₂Pb₃I₁₀ and the alloyed [(AA)_x(IdPA)_1-x]₂MA₂Pb₃I₁₀ crystallize in the RP structure, both in space group P2₁/c. The adjacent I···I interlayer distance in (AA)₂MA₂Pb₃I₁₀ is ∼5.6 Å, drawing the [Pb₃I₁₀]^4- layers closer together among all reported n = 3 RP lead iodides. (AA)₂MA₂Pb₃I₁₀ presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is slightly red-shifted in comparison to (IdPA)₂MA₂Pb₃I₁₀. The band structure calculations suggest that both (AA)₂MA₂Pb₃I₁₀ and (IdPA)₂MA₂Pb₃I₁₀ have in-plane effective masses around 0.04m₀ and 0.08m₀, respectively. IdPA cations have a greater dielectric contribution than AA. The excited-state dynamics investigated by transient absorption (TA) spectroscopy reveal a long-lived (∼100 ps) trap state ensemble with broad-band emission; our evidence suggests that these states appear due to lattice distortions induced by the incorporation of IdPA cations.

Synthesis, Crystal Structure, and Broadband Emission of (CH3)3SSnCl3

We report here on the synthesis, crystal structure, optoelectronic and vibrational properties, as well as the DFT calculations of the novel trimethylsulfonium tin trichloride (CH₃)₃SSnCl₃. The air-stable compound is prepared by reacting the (CH₃)₃SCl and SnCl₂ solid precursors in evacuated silica tubes at 100 °C. According to powder X-ray diffraction and Rietveld refinement, it crystallizes at room temperature in the orthorhombic space group Pbca (No. 61) with isolated pyramids of [SnCl₃]^- and (CH₃)₃S⁺ units. UV-vis reflectance and photoluminescence spectroscopies reveal a direct energy band gap of 3.85 eV, accompanied by a broad Stokes-shifted luminescence signal. Photoexcitation of the compound at room temperature and at -196 °C results in broadband luminescence with weak magenta emission centered at 400 nm using an excitation at 250 nm. First principal calculations provide insight into the physical properties through the electron and phonon density of states. Multitemperature Raman spectroscopy and differential scanning calorimetry reveal a reversible phase transition at ca. 70 °C that affects the vibrational modes of the [SnCl₃]^-. By dissolving (CH₃)₃SSnCl₃ in dimethylformamide in ambient air for a week, oxidation of tin occurs in the "defect" perovskite ((CH₃)₃S)₂SnCl₆. The crystal structure of ((CH₃)₃S)₂SnCl₆ is also determined with high accuracy via single-crystal X-ray diffraction (cubic space group Pa-3 (No. 205)) and compared with (CH₃)₃SSnCl₃ via Hirshfeld surface analysis.

Organic Spacer Cation Assisted Modulation of the Structure and Properties of Bismuth Halide Perovskites

ConspectusLead halide perovskites are under the spotlight of current research due to their potential for efficient and cost-effective next-generation optoelectronic devices. The unique photonic and electronic properties of these solution-processable materials brought them to the forefront of materials science. However, the toxicity and instability of lead-based perovskites are the major hurdles for their commercialization. These issues initiated an effort towards the development of environmentally friendly, lead-free perovskites. In this context, bismuth halide perovskites (BHPs) were ideal rivals for lead-based congeners due to their excellent chemical stability, lower toxicity, and structural versatility. Understanding the crystal structure and optoelectronic properties of BHPs is crucial for designing them for specific, tailor-made applications. This Account aims to review our recent research progress on the role of functional organic spacer cations in modulating the electronic confinements, optical properties, and photoconductivity of BHPs. We have employed a comprehensive experimental and theoretical investigation to probe the intriguing optical and electronic properties of these materials. Our findings on the structure-optoelectronic property correlations will be valuable guidelines for the rational selection of organic spacer cations in designing BHPs featuring low exciton binding energy, narrow optical bandgap, enhanced visible light absorption, and high photoconductivity. One of our key findings is that by increasing the electron affinity of the organic spacer ligands, photoconductivity and visible light absorption of BHPs could be significantly enhanced. We hope that the fundamental level understanding of the photophysical properties discussed in this Account will lead to new design rules for developing high-performance BHP materials.

Enhancing the Luminance Efficiency of Formamidinium-Based Dion-Jacobson Perovskite Light-Emitting Diodes via Compositional Engineering

In this paper, the phase formation mechanism of formamidinium (FA)-based Dion-Jacobson (DJ) perovskites is uncovered for the first time, which includes the formation of the n₁ domain (n = 1 perovskite phase) and a trace amount of the large n domain in the spin-coating process and the growth of the n₂ domain and the large n domain during the thermal annealing stage. The phase formation mechanism clearly reveals the different phase distributions between FA-based Ruddlesden-Popper (RP) and FA-based DJ perovskite films at different stages of film preparation due to the larger formation energy of the DJ perovskites. According to this phase formation mechanism, we put forward an effective strategy of the small A-site cation compositional engineering. Then, excess perovskitizer cations (FA⁺) are introduced to increase the crystallinity, modulate the phase distribution and passivate defects without disturbing the structure of DJ perovskites simultaneously. The final green-light DJ PeLED devices show a maximum luminance of 41 520 cd/m² and a maximum current efficiency of 31.1 cd/A (EQE: 8.5%), which are the record values so far. The final DJ PeLEDs show an improved operational lifetime of ca. 15 min at an initial luminance of ca. 800 cd/m². Our results suggest that DJ perovskites can be promising for light-emitting applications.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

m-Phenylenediammonium as a New Spacer for Dion-Jacobson Two-Dimensional Perovskites

Two-dimensional (2D) halide perovskites have several distinct structural classes and exhibit great tunability, stability, and high potential for photovoltaic applications. Here, we report a new series of hybrid 2D perovskites in the Dion-Jacobson (DJ) class based on aromatic m-phenylenediammonium (mPDA) dications. The crystal structures of the DJ perovskite materials (mPDA)MA_n-1Pb_nI_3n+1 (n = 1-3) were solved and refined using single-crystal X-ray crystallography. The results indicate a short I···I interlayer distance of 4.00-4.04 Å for the (mPDA)MA_n-1Pb_nI_3n+1 (n = 2 and 3) structures, which is the shortest among DJ perovskites. However, Pb-I-Pb angles are as small as 158-160°, reflecting the large distortion of the inorganic framework, which results in larger band gaps for these materials than those in other DJ analogues. Density functional theory calculations suggest appreciable dispersion in the stacking direction, unlike the band structures of the Ruddlesden-Popper phases, which exhibit flat bands along the stacking direction. This is a consequence of the short interlayer I···I distances that can lead to interlayer electronic coupling across the layers. The solution-deposited films (nominal (mPDA)MA_n-1Pb_nI_3n+1 compositions of n = 1-6) reveal improved surface coverage with increasing nominal n value with the higher n films being composed of a mixture of n = 1 and bulk three-dimensional MAPbI₃ perovskites. The films made from solutions of these materials behave differently from those of other 2D iodide perovskites, and their solar cells have a mixture of n = 1 DJ and MAPbI₃ as light-absorbing semiconductors.

Multiple-Noncovalent-Interaction-Stabilized Layered Dion-Jacobson Perovskite for Efficient Solar Cells

Two-dimensional Dion-Jacobson (DJ) perovskites have shown improved structure stability in comparison with Ruddlesden-Popper (RP) perovskites. However, the mechanism behind the improved stability is still largely unexplored. Here a multifluorinated aromatic spacer, namely, 4F-PhDMA, has been successfully developed for 2D DJ perovskites. It is found that the 2D DJ perovskite with a 4F-PhDMA spacer exhibits a high dissociation energy due to the multiple noncovalent interactions. The optimized 2D DJ device based on the 4F-PhDMA spacer (n = 4) exhibits a champion efficiency of 16.62% with much improved light and thermal stability. This efficiency is much higher than that of the control device using an unfluorinated spacer (n = 4, PCE = 10.11%) and is among the highest efficiencies in aromatic-spacer-based 2D DJ perovskite solar cells (PSCs). Our work highlights the importance of incorporating multiple noncovalent interactions in the 2D DJ perovskite by employing a multifluorinated aromatic spacer to achieve DJ PSCs with both high efficiency and high stability.

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 (C_mH_2m+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.

2D Lead Iodide Perovskite with Mercaptan-Containing Amine and Its Exceptional Water Stability

Two dimensional (2D) hybrid perovskites have attracted a great deal of interest because of their appropriate photovoltaic efficiency and environmental stability. Although some 2D hybrid perovskites with sulfur-containing amines have been reported, the cation having the mercaptan group has not been well explored yet. In this work, cysteamine (Cya, HS(CH₂)₂NH₂), a mercaptan-containing amine, was introduced into 2D hybrid perovskite. Two 2D lead iodides with different structures, (HCya)₂PbI₄ (1) and (HCya)₇Pb₄I₁₅ (2), were isolated as a red low-temperature phase and a yellow high-temperature phase, respectively. X-ray single-crystal structural analysis showed that the red phase 1 is a single layered corner-shared perovskite and that the yellow phase 2 is a corner/edge-shared quasi-2D perovskite. A thermo-induced reversible 1 to 2 phase transition was found in this synthetic system. The configuration of HCya cation greatly influences the crystallization equilibrium, generating different structures of the lead halides. The single-crystal structure of 1 is discussed in comparison with that of (HAE)₂PbI₄ (AE = HO(CH₂)₂NH₂), an analogue of 1. The different effects of OH and SH groups on the 2D frameworks are studied based on their hydrogen bonding properties. More remarkably, although the two perovskites have similar structures, the (HCya)₂PbI₄ (1) has an intrinsic water stability that is much more stable than (HAE)₂PbI₄, which should be attributed to the affinity of the SH group with lead on the surface of the lead halide.