Cation Dynamics in Mixed-Cation (MA)x(FA)1–xPbI3 Hybrid Perovskites from Solid-State NMR

Surface ligand manipulation enables ∼15% efficient MAPbI3 perovskite quantum dot solar cells

We reported a surface ligand manipulation strategy for hybrid MAPbI3 perovskite quantum dots (PeQDs) using methylamine iodide (MAI), methylamine thiocyanate (MASCN) and methylamine acetate (MAAc) salts. After MAI salt post-treatment, a record high efficiency of 14.98% was obtained for MAPbI3 PeQD solar cells together with enhanced ambient stability.

Hydrogen Diffusion in Hybrid Perovskites from Exchange NMR

Ion migration is an important phenomenon affecting the performance of hybrid perovskite solar cells. It is particularly challenging, however, to disentangle the contribution of H+ diffusion from that of other ions, and the atomic-scale mechanism remains unclear. Here, we use 2H exchange NMR to prove that 2H+ ions exchange between MA+ cations on the time scale of seconds for both MAPbI3 and FA0.7MA0.3PbI3 perovskites. We do this by exploiting 15N-enriched MA+ to label the cations by their 15N spin state. The exchange rates and activation energy are then calculated by performing experiments as functions of mixing time and temperature. By comparing the measured exchange rates to previously measured bulk H+ diffusivities, we demonstrate that, after dissociating, H+ ions travel through the lattice before associating to another cation rather than hopping between adjacent cations.

Quantifying Organic Cation Ratios in Metal Halide Perovskites: Insights from X-ray Photoelectron Spectroscopy and Nuclear Magnetic Resonance Spectroscopy

The employment of metal halide perovskites (MHPs) in various optoelectronic applications requires the preparation of thin films whose composition plays a crucial role. Yet, the composition of the MHP films is rarely reported in the literature, partly because quantifying the actual organic cation composition cannot be done with conventional characterization methods. For MHPs, NMR has gained popularity, but for films, tedious processes like scratching several films are needed. Here, we use mechanochemical synthesis of MA1-x FA x PbI3 powders with various MA+: FA+ ratios and combine solid-state NMR spectroscopy (ssNMR) and X-ray photoelectron spectroscopy (XPS) to provide a reference characterization protocol for the organic cations' quantification in either powder form or films. Following this, we demonstrate that organic cation ratio quantification on thin films with ssNMR can be done without scraping the film and using significantly less mass than typically needed, that is, employing a single ∼800 nm-thick MA1-x FA x PbI3 film deposited by pulsed laser deposition (PLD) onto a 1 × 1 in.2, 0.2 mm-thick quartz substrate. While background signals from the quartz substrate appear in the 1H ssNMR spectra, the MA+ and FA+ signals are easily distinguishable and can be quantified. This study highlights the importance of calibrating and quantifying the source and the thin film organic cation ratio, as key for future optimization and scalability of physical vapor deposition processes.

Organic cations promote exciton dissociation in Ruddlesden-Popper lead iodide perovskites: a theoretical study

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) are a class of quantum well (QW) materials showing large exciton binding energy owing to quantum confinement. The existence of localized edge states was proposed to accelerate exciton dissociation into long-lived charge carriers in 2D RPPs, but recent experimental reports suggested that highly efficient internal exciton dissociation is achievable in 2D RPPs despite the absence of edge states. Herein, we adopt first-principles calculations to unveil the physical origin of the high internal quantum efficiency in the bulk region of widely familiar (BA)2(MA)n-1PbnI3n+1 (BA = butylammonium; MA = methylammonium) materials. We discover that the dipolar nature of MA cations provides the driving force for the separation of photoexcited electron-hole pairs inside QWs as the inorganic layer thickens from n = 1 to n = 3. Concurrently, electronic coupling between organic spacer layers and QWs is enhanced in the energetically favorable configurations where MA cations orient with their CH3 groups towards the exterior PbI2 layers of QWs in the n = 3 structure. Consequently, hole delocalization is promoted along the out-of-plane direction of QWs, which in turn facilitates exciton dissociation into free charge carriers despite large exciton binding energy. Our simulations reveal that the hydrogen bonding between organic species (including both MA and BA cations) and iodine atoms, which is subtly interconnected, engineers the response of morphology in QWs and electronic interactions at organic-inorganic interfaces, providing novel insights for the exciton-free carrier behavior in the bulk area of 2D RPPs.

Achieving Well-Oriented FAPbI3 Perovskite Photovoltaics by Cyclohexane Modification

It is essential and challenging to develop green and cost-effective solar cells to meet the energy demands. Solar cells with a perovskite light-harvesting layer are the most promising technology to propel the world toward next-generation solar energy. Formamidinium lead tri-iodide (FAPbI3)-based perovskite solar cells (F-PSCs), with their considerable performance, offer cost-effective solar cells. One of the major issues that the PSC community is now experiencing is the stability of α-FAPbI3 at relatively low temperatures. In this study, we fabricated FAPbI3-PSCs using cyclohexane (CHX) material via a two-step deposition method. For this purpose, CHX is added to the formamidinium iodide:methylammonium chloride (FAI:MACl) solution as an additive and used to form a better FAPbI3 layer by controlling the reaction between FAI and lead iodide (PbI2). The CHX additive induces the reaction of undercoordinated Pb2+ with FAI material and produces an α-FAPbI3 layer with low charge traps and large domains. In addition, the CHX-containing FAPbI3 layers show higher carrier lifetimes and facilitate carrier transfer in F-PSCs. The CHX-modified F-PSCs yield a high champion efficiency of 22.84% with improved ambient and thermal stability behavior. This breakthrough provides valuable findings regarding the formation of a desirable FAPbI3 layer for photovoltaic applications and holds promise for the industrialization of F-PSCs.

Unlocking the Ambient Temperature Effect on FA-Based Perovskites Crystallization by In Situ Optical Method

Multiple cation-composited perovskites are demonstrated as a promising approach to improving the performance and stability of perovskite solar cells (PSCs). However, recipes developed for fabricating high-performance perovskites in laboratories are always not transferable in large-scale production, as perovskite crystallization is highly sensitive to processing conditions. Here, using an in situ optical method, the ambient temperature effect on the crystallization process in multiple cation-composited perovskites is investigated. It is found that the typical solvent-coordinated intermediate phase in methylammonium lead iodide (MAPbI3) is absent in formamidinium lead iodide (FAPbI3), and nucleation is almost completed in FAPbI3 right after spin-coating. Interestingly, it is found that there is noticeable nuclei aggregation in Formamidinium (FA)-based perovskites even during the spin-coating process, which is usually only observed during the annealing in MAPbI3. Such aggregation is further promoted at a higher ambient temperature or in higher FA content. Instead of the general belief of stress release-induced crack formation, it is proposed that the origin of the cracks in FA-based perovskites is due to the aggregation-induced solute depletion effect. This work reveals the limiting factors for achieving high-quality FA-based perovskite films and helps to unlock the existing narrow processing window for future large-scale production.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Development of Quasi-Two-Dimensional Perovskites and Their Application in Light-Emitting Diodes

Quasi-two-dimensional (quasi-2D) perovskites have attracted much attention due to their outstanding properties, such as inherent quantum-well structure, strong dielectric and quantum confinement, large exciton binding energy, and high photoluminescence quantum yield. By virtue of these superior merits, quasi-2D perovskites have shown great potential for next-generation light-emitting diodes (LEDs). Herein, this review presents an overview of the basic properties of quasi-2D perovskites and their photoluminescence modulations by large organic cation engineering, monovalent cation engineering, halogen engineering, defect passivation engineering, and dimensionality engineering. Furthermore, the strategies of charge-transport layer optimization, interfacial engineering, light-outcoupling efficiency improvement, and operating stability improvement are summarized for fabricating high-performance quasi-2D perovskite LEDs (PeLEDs). Finally, the challenges and outlook for the future development of quasi-2D PeLEDs are unambiguously proposed.

Composition-Property Relations for GAx FAy MA1- x - y PbI3 Perovskites

Halide perovskites are materials for diverse optoelectronic applications owing to a combination of factors, including their compositional flexibility. A major source of this diversity of compositions comes from the use of mixed organic cations in the A-site of such compounds to form solid solutions. Many organic cations are possible for this purpose. Although significant progress is made over years of intensive research, the determination of systematic relationships between the compositions and properties of halide perovskites is not exploited accordingly. Using the MAPbI3 prototype, a wide range of compositions substituted by formamidinium (FA+ ) and guanidinium (GA+ ) cations are studied. From a detailed collection of experimental data and results reported in the literature, heat maps correlating the composition of GAx FAy MA1- x - y PbI3 solid solutions with phase transition temperatures, dielectric permittivity, and activation energies are constructed. Considering the characteristics of organic cations, namely their sizes, dipole moments, and the number of N─H bonds, it is possible to interpret the heat maps as consequences of these characteristics. This work brings a systematization of how obtaining specific properties of halide perovskites might be possible by customizing the characteristics of the A-site organic cations.

Atomic Structure and Dynamics of Organic-Inorganic Hybrid Perovskite Formamidinium Lead Iodide

The atomic dynamic behaviors of formamidinium lead iodide [HC(NH2)2PbI3] are critical for understanding and improving photovoltaic performances. However, they remain unclear. Here, we investigate the structural phase transitions and the reorientation dynamics of the formamidinium cation [HC(NH2)2+, FA+] of FAPbI3 using neutron scattering techniques. Two structural phase transitions occur with decreasing temperature, from cubic to tetragonal phase at 285 K and then to another tetragonal at 140 K, accompanied by gradually frozen reorientation of FA cations. The nearly isotropic reorientation in the cubic phase is suppressed to reorientation motions involving a two-fold (C2) rotation along the N···N axis and a four-fold (C4) rotation along the C-H axis in the tetragonal phase, and eventually to local disordered motion as a partial C4 along the C-H axis in another tetragonal phase, thereby indicating an intimate interplay between lattice and orientation degrees of freedom in the hybrid perovskite materials. The present complete atomic structure and dynamics provide a solid standing point to understand and then improve photovoltaic properties of organic-inorganic hybrid perovskites in the future.

Cation Dynamics as Structure Explorer in Hybrid Perovskites-The Case of MAPbI3

Hybrid organic-inorganic perovskites exhibit remarkable potential as cost-effective and high-efficiency materials for photovoltaic applications. Their exceptional chemical tunability opens further routes for optimizing their optical and electronic properties through structural engineering. Nevertheless, the extraordinary softness of the lattice, stemming from its interconnected organic-inorganic composition, unveils formidable challenges in structural characterization. Here, by focusing on the quintessential methylammonium lead triiodide, MAPbI3, we combine first-principles modeling with high-resolution neutron scattering data to identify the key stationary points on its shallow potential energy landscape. This combined experimental and computational approach enables us to benchmark the performance of a collection of semilocal exchange-correlation functionals and to track the local distortions of the perovskite framework, hallmarked by the inelastic neutron scattering response of the organic cation. By conducting a thorough examination of structural distortions, we introduce the IKUR-PVP-1 structural data set. This data set contains nine mechanically stable structural models, each manifesting a distinct vibrational response. IKUR-PVP-1 constitutes a valuable resource for assessing thermal behavior in the low-temperature perovskite phase. In addition, it paves the way for the development of accurate force fields, enabling a comprehensive understanding of the interplay between the structure and dynamics in MAPbI3 and related hybrid perovskites.

The role of ion migration, octahedral tilt, and the A-site cation on the instability of Cs1-xFAxPbI3

Organic-inorganic hybrid perovskites are promising materials for the next generation photovoltaics and optoelectronics; however, their practical application has been hindered by poor structural stability mainly caused by ion migration and external stimuli. Understanding the mechanism(s) of ion migration and structure decomposition is thus critical. Here we observe the sequence of structural changes at the atomic level that precede structural decomposition in the technologically important Cs1-xFAxPbI3 using ultralow dose transmission electron microscopy. We find that these changes differ, depending upon the A-site composition. Initially, there is a random loss of FA+, complemented by the loss of I-. The remaining FA+ and I- ions then migrate, unit cell by unit cell, into an ordered and more stable phase with a √2 x √2 superstructure. Further ion loss is accompanied by A-site dependent octahedral tilt modes and associated tetragonal phases with different stabilities. These observations of the loss of FA+/I- ion pairs, ion migration, octahedral tilt modes, and the role of the A-cation, provide insights into the atomic-scale structural mechanisms that drive and block ion loss and ion migration, opening pathways to inhibit ion loss, migration and improve structural stability.

The role of oxygen defects in the electronic, optical and phonon dispersion of the LAGO perovskite: a density functional theory investigation

The study aims to investigate the electronic, optical and phonon dispersion properties of a pure and 2.5% O-defect induced LAGO perovskite, using density functional theory (DFT) with generalized gradient approximation (GGA) and the PBE functional. The research reveals a significant reduction in the band gap from 3.27 eV in pure LAGO to 2.18 eV in defect-induced LAGO. The defect-induced LAGO exhibits relatively strong light absorption in the visible region compared to pure LAGO. The phonon-dispersion analysis identifies one acoustic and two transverse optical mode branches. The calculated Debye temperatures for pure and defect-induced systems are 469.92 K and 463.69 K, respectively, attributed to weaker bonds in defect-induced LAGO. The findings offer fundamental insights into the impact of oxygen vacancies on the electronic, optical, and phonon properties of the LAGO perovskite that can potentially improve the electronic and optoelectronic devices operating across a wide range of spectral frequencies.

Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2-xTeO6 Solid Electrolytes with Solid-State NMR

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (23Na, 125Te, 25Mg, and 67Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na2MgxZn2-xTeO6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10-4 S cm-1 for Na2MgZnTeO6 at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the P6322 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25Mg and 67Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τNMR = 0.83 × 109 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23Na NMR, which are sensitive to different Mg/Zn ratios. The Na2MgZnTeO6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.

Lattice engineering for stabilized black FAPbI3 perovskite single crystals for high-resolution x-ray imaging at the lowest dose

Preliminary theoretical analyses indicate that lattice relaxation may be used to release lattice strain in the FAPbI3 perovskite to warrant both high x-ray detection performance and improved stability. Herein, we demonstrate stable black α-phase FAPbI3 single crystals (SCs) realized by lattice engineering via annealing in the ambient atmosphere. The engineered α-FAPbI3 SC detector shows almost all the best figures of merit including a high sensitivity of 4.15 × 105 μC Gyair-1 cm-2, a low detection limit of 1.1 nGyair s-1, a high resolution of 15.9 lp mm-1, and a short response time of 214 μs. We further demonstrate high-definition x-ray imaging at a dose rate below 10 nGyair s-1 on the FAPbI3 SC, indicating a minimal dose-area product of 0.048 mGyair cm2 to the patient for one-time posteroanterior chest diagnosis, which is more than 3000 times lower than the international reference level of 150 mGyair cm2. In addition, the robust long-term stability enables the FAPbI3 SC x-ray detector to work steadily for more than 40 years.

Enhanced Hot-Phonon Bottleneck Effect on Slowing Hot Carrier Cooling in Metal Halide Perovskite Quantum Dots with Alloyed A-Site

A deep understanding of the effect of the A-site cation cross-exchange on the hot-carrier relaxation dynamics in perovskite quantum dots (PQDs) has profound implications on the further development of disruptive photovoltaic technologies. In this study, the hot carrier cooling kinetics of pure FAPbI3 (FA+ , CH(NH2 )2 + ), MAPbI3 (MA+ , CH3 NH3 + + ), CsPbI3 (Cs+ , Cesium) and alloyed FA0.5 MA0.5 PbI3 , FA0.5 Cs0.5 PbI3 , and MA0.5 Cs0.5 PbI3 QDs are investigated using ultrafast transient absorption (TA) spectroscopy. The lifetimes of the initial fast cooling stage (<1 ps) of all the organic cation-containing PQDs are shorter than those of the CsPbI3 QDs, as verified by the electron-phonon coupling strength extracted from the temperature-dependent photoluminescence spectra. The lifetimes of the slow cooling stage of the alloyed PQDs are longer under illumination greater than 1 sun, which is ascribed to the introduction of co-vibrational optical phonon modes in the alloyed PQDs. This facilitated efficient acoustic phonon upconversion and enhanced the hot-phonon bottleneck effect, as demonstrated by first-principles calculations.

Stabilization of photoactive phases for perovskite photovoltaics

Interest in photovoltaics (PVs) based on Earth-abundant halide perovskites has increased markedly in recent years owing to the remarkable properties of these materials and their suitability for energy-efficient and scalable solution processing. Formamidinium lead triiodide (FAPbI₃)-rich perovskite absorbers have emerged as the frontrunners for commercialization, but commercial success is reliant on the stability meeting the highest industrial standards and the photoactive FAPbI₃ phase suffers from instabilities that lead to degradation - an effect that is accelerated under working conditions. Here, we critically assess the current understanding of these phase instabilities and summarize the approaches for stabilizing the desired phases, covering aspects from fundamental research to device engineering. We subsequently analyse the remaining challenges for state-of-the-art perovskite PVs and demonstrate the opportunities to enhance phase stability with ongoing materials discovery and in operando analysis. Finally, we propose future directions towards upscaling perovskite modules, multijunction PVs and other potential applications.

Dynamic Nuclear Polarization of Inorganic Halide Perovskites

The intrinsic low sensitivity of nuclear magnetic resonance (NMR) experiments limits their utility for structure determination of materials. Dynamic nuclear polarization (DNP) under magic angle spinning (MAS) has shown tremendous potential to overcome this key limitation, enabling the acquisition of highly selective and sensitive NMR spectra. However, so far, DNP methods have not been explored in the context of inorganic lead halide perovskites, which are a leading class of semiconductor materials for optoelectronic applications. In this work, we study cesium lead chloride and quantitatively compare DNP methods based on impregnation with a solution of organic biradicals with doping of high-spin metal ions (Mn²⁺) into the perovskite structure. We find that metal-ion DNP provides the highest bulk sensitivity in this case, while highly surface-selective NMR spectra can be acquired using impregnation DNP. The performance of both methods is explained in terms of the relaxation times, particle size, dopant concentration, and surface wettability. We envisage the future use of DNP NMR approaches in establishing structure-activity relationships in inorganic perovskites, especially for mass-limited samples such as thin films.

3D to 0D cesium lead bromide: A 79/81Br NMR, NQR and theoretical investigation

Inorganic metal halides offer unprecedented tunability through elemental variation of simple three-element compositions, but can exhibit complicated phase behaviour, degradation, and microscopic phenomena (disorder/dynamics) that play an integral role for the bulk-level chemical and physical properties of these materials. Understanding the halogen chemical environment in such materials is crucial to addressing many of the concerns regarding implementing these materials in commercial applications. In this study, a combined solid-state nuclear magnetic resonance, nuclear quadrupole resonance and quantum chemical computation approach is used to interrogate the Br chemical environment in a series of related inorganic lead bromide materials: CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆. The quadrupole coupling constants (C_Q) were determined to range from 61 to 114 MHz for ⁸¹Br, with CsPbBr₃ exhibiting the largest measured C_Q and Cs₄PbBr₆ the smallest. GIPAW DFT was shown to be an excellent pre-screening tool for estimating the EFG of Br materials and can increase experimental efficiency by providing good starting estimates for acquisition. Finally, the combination of theory and experiment to inform the best methods for expanding further to the other quadrupolar halogens is discussed.

Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide

Formamidinium lead triiodide (FAPbI₃) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl₂) has been used as an additive in FAPbI₃. MDA²⁺ has been reported as incorporated into the perovskite lattice alongside Cl^-. However, the precise function and role of MDA²⁺ remain uncertain. Here, we grow FAPbI₃ single crystals from a solution containing MDACl₂ (FAPbI₃-M). We demonstrate that FAPbI₃-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA²⁺ is not the direct cause of the enhanced material stability. Instead, MDA²⁺ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI₃ crystals grown from a solution containing HMTA (FAPbI₃-H) replicate the enhanced α-phase stability of FAPbI₃-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA⁺ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H⁺). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H⁺ is selectively incorporated into the bulk of both FAPbI₃-M and FAPbI₃-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites

We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH₂]₂⁺, FA) and methylammonium (CH₃NH₃⁺, MA) cations in FA_1-xMA_xPbI₃ with x = 0 and 0.4 and compare it to the dynamics in MAPbI₃. For FAPbI₃, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature (T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase (T ⩽ 140 K). For FA_0.6MA_0.4PbI₃, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI₃ and MAPbI₃ at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI₃. This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA_1-xMA_xPbI₃.

Disorder to order: how halide mixing in MAPbI3-xBrx perovskites restricts MA dynamics

Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI_3-xBr_x perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). ²⁰⁷Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI_3-xBr_x samples are cubic. The experimental ¹⁴N spectra and ¹H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb₈I_12-nBr_n "cages". Based on the experimental and simulated results, we develop a phenomenological model that correlates the ¹H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb-X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.

Hybrid Organic-Inorganic Perovskite Superstructures for Ultrapure Green Emissions

All inorganic CsPbBr₃ superstructures (SSs) have attracted much research interest due to their unique photophysical properties, such as their large emission red-shifts and super-radiant burst emissions. These properties are of particular interest in displays, lasers and photodetectors. Currently, the best-performing perovskite optoelectronic devices incorporate organic cations (methylammonium (MA), formamidinium (FA)), however, hybrid organic-inorganic perovskite SSs have not yet been investigated. This work is the first to report on the synthesis and photophysical characterization of APbBr₃ (A = MA, FA, Cs) perovskite SSs using a facile ligand-assisted reprecipitation method. At higher concentrations, the hybrid organic-inorganic MA/FAPbBr₃ nanocrystals self-assemble into SSs and produce red-shifted ultrapure green emissions, meeting the requirement of Rec. 2020 displays. We hope that this work will be seminal in advancing the exploration of perovskite SSs using mixed cation groups to further improve their optoelectronic applications.

Automated Graph Neural Networks Accelerate the Screening of Optoelectronic Properties of Metal-Organic Frameworks

The numerous organic and inorganic components of metal-organic framework (MOF) materials provide intriguing optoelectronic properties. Accurately predicting the electronic structural properties of MOFs has become the main focus. This work establishes two graph neural network models, crystal graph convolutional neural networks and a materials graph network, for predicting the band gaps of more than 10 000 MOF structures and promotes to improve the prediction accuracy through automatic hyperparameter tuning algorithms. Subsequently, for exploring machine learning-assisted screening of MOFs for the broader electronic properties, the screened copper-based MOFs are compared with lead-based MAPbI₃ solar cells with respect to the band gaps, densities of states, and charge density distributions, and the results have demonstrated that the overlap of the wave functions between the initial and final states of MOFs is weakened, which is conducive to the improvement of photoelectric performance. The chlorine doping strategy further enhances the advantage. The tuning of the machine learning model and hyperparameters and the doping strategy of halogen elements furnish empirical rules for the design of MOFs with excellent optoelectronic properties.

Light-Induced Structural Dynamics and Charge Transport in Layered Halide Perovskite Thin Films

The dynamic nature of the metal halide perovskite lattice upon photoexcitation plays a vital role in their properties. Here we report an observation of light-induced structure dynamics in quasi-2D Ruddlesden-Popper phase perovskite thin films and its impact on the carrier transport properties. By a time-resolved X-ray scattering technique, we observe a rapid lattice expansion upon photoexcitation, followed by a slow relaxation over the course of 100 ns in the dark. Theoretical modeling suggests that the expansion originates from the lattice's thermal fluctuations caused by photon energy deposition. Power dependent optical spectroscopy and photoconductivity indicate that high laser powers triggered a strong local structural disorder, which increased the charge dissociation activation energy that results in localized transport. Our study investigates the impact of laser energy deposition on the lattices and the subsequent carrier transport properties, that are relevant to device operations.

Synthetic approaches for perovskite thin films and single-crystals

Halide perovskites are compelling candidates for the next generation of photovoltaic technologies owing to an unprecedented increase in power conversion efficiency and their low cost, facile fabrication and outstanding semiconductor properties.

A Complete Picture of Cation Dynamics in Hybrid Perovskite Materials from Solid-State NMR Spectroscopy

The organic cations in hybrid organic-inorganic perovskites rotate rapidly inside the cuboctahedral cavities formed by the inorganic lattice, influencing optoelectronic properties. Here, we provide a complete quantitative picture of cation dynamics for formamidinium-based perovskites and mixed-cation compositions, which are the most widely used and promising absorber layers for perovskite solar cells today. We use ²H and ¹⁴N quadrupolar solid-state NMR relaxometry under magic-angle spinning to determine the activation energy (E_a) and correlation time (τ_c) at room temperature for rotation about each principal axis of a series of organic cations. Specifically, we investigate methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GUA⁺) cations in current state-of-the-art single- and multi-cation perovskite compositions. We find that MA⁺, FA⁺, and GUA⁺ all have at least one component of rotation that occurs on the picosecond timescale at room temperature, with MA⁺ and GUA⁺ also exhibiting faster and slower components, respectively. The cation dynamics depend on the symmetry of the inorganic lattice but are found to be insensitive to the degree of cation substitution. In particular, the FA⁺ rotation is invariant across all compositions studied here, when sufficiently above the phase transition temperature. We further identify an unusual relaxation mechanism for the ²H of MA⁺ in mechanosynthesized FA_xMA_1-xPbI₃, which was found to result from physical diffusion to paramagnetic defects. This precise picture of cation dynamics will enable better understanding of the relationship between the organic cations and the optoelectronic properties of perovskites, guiding the design principles for more efficient perovskite solar cells in the future.

The effect of caesium alloying on the ultrafast structural dynamics of hybrid organic-inorganic halide perovskites

Hybrid inorganic-organic perovskites have attracted considerable attention over recent years as promising processable electronic materials. In particular, the rich structural dynamics of these 'soft' materials has become a subject of investigation and debate due to their direct influence on the perovskites' optoelectronic properties. Significant effort has focused on understanding the role and behaviour of the organic cations within the perovskite, as their rotational dynamics may be linked to material stability, heterogeneity and performance in (opto)electronic devices. To this end, we use two-dimensional IR spectroscopy (2DIR) to understand the effect of partial caesium alloying on the rotational dynamics of the methylammonium cation in the archetypal hybrid perovskite CH₃NH₃PbI₃. We find that caesium incorporation primarily inhibits the slower 'reorientational jump' modes of the organic cation, whilst a smaller effect on the fast 'wobbling time' may be due to distortions and rigidisation of the inorganic cuboctahedral cage. 2DIR centre-line-slope analysis further reveals that while static disorder increases with caesium substitution, the dynamic disorder (reflected in the phase memory of the N-H stretching mode of methylammonium) is largely independent of caesium addition. Our results contribute to the development of a unified model of cation dynamics within organohalide perovskites.

Solid-State Nuclear Magnetic Resonance of Triple-Cation Mixed-Halide Perovskites

Mixed-cation lead mixed-halide perovskites are the best candidates for perovskite-based photovoltaics, thanks to their higher efficiency and stability compared to the single-cation single-halide parent compounds. TripleMix (Cs_0.05MA_0.14FA_0.81PbI_2.55Br_0.45 with FA = formamidinium and MA = methylammonium) is one of the most efficient and stable mixed perovskites for single-junction solar cells. The microscopic reasons why triple-cation perovskites perform so well are still under debate. In this work, we investigated the structure and dynamics of TripleMix by exploiting multinuclear solid-state nuclear magnetic resonance (SSNMR), which can provide this information at a level of detail not accessible by other techniques. ¹³³Cs, ¹³C, ¹H, and ²⁰⁷Pb SSNMR spectra confirmed the inclusion of all ions in the perovskite, without phase segregation. Complementary measurements showed a peculiar longitudinal relaxation behavior for the ¹H and ²⁰⁷Pb nuclei in TripleMix with respect to single-cation single-halide perovskites, suggesting slower dynamics of both organic cations and halide anions, possibly related to the high photovoltaic performances.

Chemical exchange of labile protons by deuterium enables selective detection of pharmaceuticals in solid formulations

Chemically assisted swapping of labile protons by deuterons is presented for amino acids, polysaccharides, pharmaceutical compounds, and their solid formulations. Solid-state packing interactions in these compounds are elucidated by ¹H-²H isotope correlation NMR spectroscopy (iCOSY). A minuscule concentration of dopamine, 5 wt% or ∼100 μg, in a solid formulation can be detected by ²H NMR at 28.2 T (¹H, 1200 MHz) in under a minute.

The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration.

Dynamic Nuclear Polarization Enables NMR of Surface Passivating Agents on Hybrid Perovskite Thin Films

Surface and bulk molecular modulators are the key to improving the efficiency and stability of hybrid perovskite solar cells. However, due to their low concentration, heterogeneous environments, and low sample mass, it remains challenging to characterize their structure and dynamics at the atomic level, as required to establish structure–activity relationships. Nuclear magnetic resonance (NMR) spectroscopy has revealed a wealth of information on the atomic-level structure of hybrid perovskites, but the inherent insensitivity of NMR severely limits its utility to characterize thin-film samples. Dynamic nuclear polarization (DNP) can enhance NMR sensitivity by orders of magnitude, but DNP methods for perovskite materials have so far been limited. Here, we determined the factors that limit the efficiency of DNP NMR for perovskite samples by systematically studying layered hybrid perovskite analogues. We find that the fast-relaxing dynamic cation is the major impediment to higher DNP efficiency, while microwave absorption and particle morphology play a secondary role. We then show that the former can be mitigated by deuteration, enabling ¹H DNP enhancement factors of up to 100, which can be harnessed to enhance signals from dopants or additives present in very low concentrations. Specifically, using this new DNP methodology at a high magnetic field and with small sample volumes, we have recorded the NMR spectrum of the 20 nm (6 μg) passivating layer on a single perovskite thin film, revealing a two-dimensional (2D) layered perovskite structure at the surface that resembles the n = 1 homologue but which has greater disorder than in bulk layered perovskites.

Direct investigation of the reorientational dynamics of A-site cations in 2D organic-inorganic hybrid perovskite by solid-state NMR

Limited methods are available for investigating the reorientational dynamics of A-site cations in two-dimensional organic-inorganic hybrid perovskites (2D OIHPs), which play a pivotal role in determining their physical properties. Here, we describe an approach to study the dynamics of A-site cations using solid-state NMR and stable isotope labelling. 2H NMR of 2D OIHPs incorporating methyl-d3-ammonium cations (d3-MA) reveals the existence of multiple modes of reorientational motions of MA. Rotational-echo double resonance (REDOR) NMR of 2D OIHPs incorporating 15N- and ¹³C-labeled methylammonium cations (13C,15N-MA) reflects the averaged dipolar coupling between the C and N nuclei undergoing different modes of motions. Our study reveals the interplay between the A-site cation dynamics and the structural rigidity of the organic spacers, so providing a molecular-level insight into the design of 2D OIHPs.

Perovskite Quantum Dots in Solar Cells

Perovskite quantum dots (PQDs) have captured a host of researchers' attention due to their unique properties, which have been introduced to lots of optoelectronics areas, such as light-emitting diodes, lasers, photodetectors, and solar cells. Herein, the authors aim at reviewing the achievements of PQDs applied to solar cells in recent years. The engineering concerning surface ligands, additives, and hybrid composition for PQDSCs is outlined first, followed by analyzing the reasons of undesired performance of PQDSCs. Subsequently, a novel overview that PQDs are utilized to improve the photovoltaic performance of various kinds of solar cells, is provided. Finally, this review is summarized and some challenges and perspectives concerning PQDs are also discussed.

Exploring Structural Nuances in Germanium Halide Perovskites Using Solid-State 73Ge and 133Cs NMR Spectroscopy

Metal halide perovskites remain top candidates for higher-performance photovoltaic devices, but concerns about leading lead-based materials remain. Ge perovskites remain understudied for use in solar cells compared to their Sn-based counterparts. In this work, we undertake a combined ⁷³Ge and ¹³³Cs solid-state Nuclear Magnetic Resonance (NMR) spectroscopy and density functional theory (DFT) study of the bulk CsGeX₃ (X = Cl, Br, or I) series. We show how seemingly small structural variations within germanium halide perovskites have major effects on their ⁷³Ge and ¹³³Cs NMR signatures and reveal a near-cubic phase at room temperature for CsGeCl₃ with severe local Ge polyhedral distortion. Quantum chemical computations are effective at predicting the structural impact on NMR parameters for ⁷³Ge and ¹³³Cs. This study demonstrates the value of a combined solid-state NMR and DFT approach for investigating promising materials for energy applications, providing information that is out of reach with conventional characterization methods, and adds the challenging ⁷³Ge nucleus to the NMR toolkit.

Development of formamidinium lead iodide-based perovskite solar cells: efficiency and stability

Perovskite materials have been particularly eye-catching by virtue of their excellent properties such as high light absorption coefficient, long carrier lifetime, low exciton binding energy and ambipolar transmission (perovskites have the characteristics of transporting both electrons and holes). Limited by the wider band gap (1.55 eV), worse thermal stability and more defect states, the first widely used methylammonium lead iodide has been gradually replaced by formamidinium lead iodide (FAPbI3) with a narrower band gap of 1.48 eV and better thermal stability. However, FAPbI3 is stabilized as the yellow non-perovskite active phase at low temperatures, and the required black phase (α-FAPbI3) can only be obtained at high temperatures. In this perspective, we summarize the current efforts to stabilize α-FAPbI3, and propose that pure α-FAPbI3 is an ideal material for single-junction cells, and a triple-layer mesoporous architecture could help to stabilize pure α-FAPbI3. Furthermore, reducing the band gap and using tandem solar cells may ulteriorly approach the Shockley-Queisser limit efficiency. We also make a prospect that the enhancement of industrial applications as well as the lifetime of devices may help achieve commercialization of PSCs in the future.

Carrier recombination in CH3NH3PbI3: why is it a slow process?

In methylammonium lead iodide (MAPbI₃), a slow recombination process of photogenerated carriers has often been considered to be the most intriguing property of the material resulting in high-efficiency perovskite solar cells. In spite of intense research over a decade or so, a complete understanding of carrier recombination dynamics in MAPbI₃has remained inconclusive. In this regard, several microscopic processes have been proposed so far in order to explain the slow recombination pathways (both radiative and non-radiative), such as the existence of shallow defects, a weak electron-phonon coupling, presence of ferroelectric domains, screening of band-edge charges through the formation of polarons, occurrence of the Rashba splitting in the band(s), and photon-recycling in the material. Based on the up-to-date findings, we have critically assessed each of these proposals/models to shed light on the origin of a slow recombination process in MAPbI₃. In this review, we have presented the interplay between the mechanisms and our views/perspectives in determining the likely processes, which may dictate the recombination dynamics in the material. We have also deliberated on their interdependences in decoupling contributions of different recombination processes.

Multi-Length Scale Structure of 2D/3D Dion-Jacobson Hybrid Perovskites Based on an Aromatic Diammonium Spacer

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)A_n-1 Pb_n I_3n+1 composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.

Research progress of ABX3-type lead-free perovskites for optoelectronic applications: materials and devices

We summarize the development and application of ABX3-type lead-free halide perovskite materials, especially in optoelectronic devices.

Surface Chemistry Engineering of Perovskite Quantum Dots: Strategies, Applications, and Perspectives

The presence of surface ligands not only plays a key role in keeping the colloidal integrity and non-defective surface of metal halide perovskite quantum dots (PQDs), but also serves as a knob to tune their optoelectronic properties for a variety of exciting applications including solar cells and light-emitting diodes. However, these indispensable surface ligands may also deteriorate the stability and key properties of PQDs due to their highly dynamic binding and insulating nature. To address these issues, a number of innovative surface chemistry engineering approaches have been developed in the past few years. Based on an in-depth fundamental understanding of the surface atomistic structure and surface defect formation mechanism in the tiny nanoparticles, a critical overview focusing on the surface chemistry engineering of PQDs including advanced colloidal synthesis, in-situ surface passivation, and solution-phase/solid-state ligand exchange is presented, after which their unprecedented achievements in photovoltaics and other optoelectronics are presented. The practical hurdles and future directions are critically discussed to inspire more rational design of PQD surface chemistry toward practical applications.

Unveiling the Nature of Light-Triggered Hole Traps in Lead Halide Perovskites: A Study with Time-Dependent Density Functional Theory

Structural variations of lead halide perovskites (LHPs) upon light illumination play an important role in their photovoltaic applications. However, density functional theory (DFT)-based simulations have often been performed to unveil the nature of defects in LHPs without light illumination. So far, the nature of traps in LHPs triggered by the light remains largely unexplored. In this work, hole traps induced by the halogen interstitial in LHPs are studied by combining DFT and time-dependent DFT approaches, the latter of which treats electron-hole and electron-nuclei interactions on the same footing. Both a semilocal exchange functional and hybrid functional are adopted to relax the ground-state and excited-state geometries followed by the calculations of energy levels of hole traps. The effect of the self-interaction corrections on the light-triggered geometric deformation and the electronic structure of hole traps is analyzed. Relaxation energies that correspond to the light-triggered geometric deformation are also calculated with different functionals. The relationship between the hole traps and light-triggered geometric variations are then explored.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of ¹H, ¹³C, ¹⁵N, ¹⁴N, ¹³³Cs, ⁸⁷Rb, ³⁹K, ²⁰⁷Pb, ¹¹⁹Sn, ¹¹³Cd, ²⁰⁹Bi, ¹¹⁵In, ¹⁹F and ²H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

The Contribution of NMR Spectroscopy in Understanding Perovskite Stabilization Phenomena

Although it has been exploited since the late 1900s to study hybrid perovskite materials, nuclear magnetic resonance (NMR) spectroscopy has only recently received extraordinary research attention in this field. This very powerful technique allows the study of the physico-chemical and structural properties of molecules by observing the quantum mechanical magnetic properties of an atomic nucleus, in solution as well as in solid state. Its versatility makes it a promising technique either for the atomic and molecular characterization of perovskite precursors in colloidal solution or for the study of the geometry and phase transitions of the obtained perovskite crystals, commonly used as a reference material compared with thin films prepared for applications in optoelectronic devices. This review will explore beyond the current focus on the stability of perovskites (3D in bulk and nanocrystals) investigated via NMR spectroscopy, in order to highlight the chemical flexibility of perovskites and the role of interactions for thermodynamic and moisture stabilization. The exceptional potential of the vast NMR tool set in perovskite structural characterization will be discussed, aimed at choosing the most stable material for optoelectronic applications. The concept of a double-sided characterization in solution and in solid state, in which the organic and inorganic structural components provide unique interactions with each other and with the external components (solvents, additives, etc.), for material solutions processed in thin films, denotes a significant contemporary target.

Tetrafluoroborate-Induced Reduction in Defect Density in Hybrid Perovskites through Halide Management

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF₄ ^- , introduced via methylammonium tetrafluoroborate (MABF₄ ) in a surface treatment for MAPbI₃ perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 10⁶ to 2.5 × 10⁵ s^-1 in BF₄ ^- -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). ¹⁹ F, ¹¹ B, and ¹⁴ N solid-state NMR is used to elucidate the atomic-level mechanism of the BF₄ ^- additive-induced improvements, revealing that the BF₄ ^- acts as a scavenger of excess MAI by forming MAI-MABF₄ cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF₄ ^- .

Formation and Near-Infrared Emission of CsPbI3 Nanoparticles Embedded in Cs4PbI6 Crystals

Cs₄PbI₆, as a rarely investigated member of the Cs₄PbX₆ (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI₂, CoI₂, and ZnI₂, as additives, play an important role in enhancing the formation of the Cs₄PbI₆ microcrystals. ZnI₂ with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI₃ nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs₄PbI₆ crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs₄PbI₆ crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.

Layered Hybrid Formamidinium Lead Iodide Perovskites: Challenges and Opportunities

ConspectusHybrid halide perovskite materials have become one of the leading candidates for various optoelectronic applications. They are based on organic-inorganic structures defined by the AMX₃ composition, were A is the central cation that can be either organic (e.g., methylammonium, formamidinium (FA)) or inorganic (e.g., Cs⁺), M is a divalent metal ion (e.g., Pb²⁺ or Sn²⁺), and X is a halide anion (I^-, Br^-, or Cl^-). In particular, FAPbI₃ perovskites have shown remarkable optoelectronic properties and thermal stabilities. However, the photoactive α-FAPbI₃ (black) perovskite phase is not thermodynamically stable at ambient temperature and forms the δ-FAPbI₃ (yellow) phase that is not suitable for optoelectronic applications. This has stimulated intense research efforts to stabilize and realize the potential of the α-FAPbI₃ perovskite phase. In addition, hybrid perovskites were proven to be unstable against the external environmental conditions (air and moisture) and under device operating conditions (voltage and light), which is related to various degradation mechanisms. One of the strategies to overcome these instabilities has been based on low-dimensional hybrid perovskite materials, in particular layered two-dimensional (2D) perovskite phases composed of organic layers separating hybrid perovskite slabs, which were found to be more stable toward ambient conditions and ion migration. These materials are mostly based on S_xA_n-1Pb_nX_3n+1 composition with various mono- (x = 1) or bifunctional (x = 2) organic spacer cations that template hybrid perovskite slabs and commonly form either Ruddlesden-Popper (RP) or Dion-Jacobson (DJ) phases. These materials behave as natural quantum wells since charge carriers are confined to the inorganic slabs, featuring a gradual decrease in the band gap as the number of inorganic layers (n) increases from n = 1 (2D) to n = ∞ (3D). While various layered 2D perovskites have been developed, their FA-based analogues remain under-represented to date. Over the past few years, several research advances enabled the realization of FA-based layered perovskites, which have also demonstrated a unique templating effect in stabilizing the α-FAPbI₃ phase. This, for instance, involved the archetypical n-butylammonium and 2-phenylethylammonium organic spacers as well as guanidinium, 5-ammonium valeric acid, iso-butylammonium, benzylammonium, n-pentylammonium, 2-thiophenemethylammonium, 2-(perfluorophenyl)ethylammonium, 1-adamantylmethanammonium, and 1,4-phenylenedimethanammonium. FAPbBr₃-based layered perovskites have also demonstrated potential in various optoelectronic applications, yet the opportunities associated with FAPbI₃-based perovskites have attracted particular attention in photovoltaics, stimulating further developments. This Account provides an overview of some of these recent developments, with a particular focus on FAPbI₃-based layered perovskites and their utility in photovoltaics, while outlining challenges and opportunities for these hybrid materials in the future.

Cation Dynamics and Structural Stabilization in Formamidinium Lead Iodide Perovskites

The vibrational dynamics of pure and methylammonium-doped formamidinium lead iodide perovskites (FAPbI₃) has been investigated by high-resolution neutron spectroscopy. For the first time, we provide an exhaustive and accurate analysis of the cation vibrations and underlying local structure around the organic moiety in these materials using first-principles electronic-structure calculations validated by the neutron data. Inelastic neutron scattering experiments on FAPbI₃ provide direct evidence of the formation of a low-temperature orientational glass, unveiling the physicochemical origin of phase metastability in the tetragonal structure. Further analysis of these data provides a suitable starting point to explore and understand the stabilization of the perovskite framework via doping with small amounts of organic cations. In particular, we find that the hydrogen-bonding interactions around the formamidinium cations are strengthened as a result of cage deformation. This synergistic effect across perovskite cages is accompanied by a concomitant weakening of the methylammonium interactions with the surrounding framework.