Halide Segregation in Mixed-Halide Perovskites: Influence of A-Site Cations

All-perovskite tandem solar cells: from fundamentals to technological progress

Organic-inorganic perovskite materials have gradually progressed from single-junction solar cells to tandem (double) or even multi-junction (triple-junction) solar cells as all-perovskite tandem solar cells (APTSCs). Perovskites have numerous advantages: (1) tunable optical bandgaps, (2) low-cost, e.g. via solution-processing, inexpensive precursors, and compatibility with many thin-film processing technologies, (3) scalability and lightweight, and (4) eco-friendliness related to low CO2 emission. However, APTSCs face challenges regarding stability caused by Sn2+ oxidation in narrow bandgap perovskites, low performance due to V oc deficit in the wide bandgap range, non-standardisation of charge recombination layers, and challenging thin-film deposition as each layer must be nearly perfectly homogenous. Here, we discuss the fundamentals of APTSCs and technological progress in constructing each layer of the all-perovskite stacks. Furthermore, the theoretical power conversion efficiency (PCE) limitation of APTSCs is discussed using simulations.

Bandgap-universal passivation enables stable perovskite solar cells with low photovoltage loss

The efficiency and longevity of metal-halide perovskite solar cells are typically dictated by nonradiative defect-mediated charge recombination. In this work, we demonstrate a vapor-based amino-silane passivation that reduces photovoltage deficits to around 100 millivolts (>90% of the thermodynamic limit) in perovskite solar cells of bandgaps between 1.6 and 1.8 electron volts, which is crucial for tandem applications. A primary-, secondary-, or tertiary-amino-silane alone negatively or barely affected perovskite crystallinity and charge transport, but amino-silanes that incorporate primary and secondary amines yield up to a 60-fold increase in photoluminescence quantum yield and preserve long-range conduction. Amino-silane-treated devices retained 95% power conversion efficiency for more than 1500 hours under full-spectrum sunlight at 85°C and open-circuit conditions in ambient air with a relative humidity of 50 to 60%.

Stable and Efficient Mixed-halide Perovskite LEDs

Tailoring bandgap by mixed-halide strategy in perovskites has attracted extraordinary attention due to the flexibility of halide ion combinations and has emerged as the most direct and effective approach to precisely tune the emission wavelength throughout the entire visible light spectrum. Mixed-halide perovskites, yet, still suffered from several problems, particularly phase segregation under external stimuli because of ions migration. Understanding the essential cause and finding sound strategies, thus, remains a challenge for stable and efficient mixed-halide perovskite light-emitting diodes (PeLEDs). The review herein presents an overview of the diverse application scenarios and the profound significance associated with mixed-halide perovskites. We then summarize the challenges and potential research directions toward developing high stable and efficient mixed-halide PeLEDs. The review thus provides a systematic and timely summary for the community to deepen the understanding of mixed-halide perovskite materials and resulting PeLEDs.

Regulating Surface Defects to Achieve More Positive Light Soaking Effect in Perovskite Solar Cells

The dynamic defect tolerance under light soaking is a crucial aspect of halide perovskites. However, the underlying physics of light soaking remains elusive and is subject to debate, exhibiting both positive and negative effects. In this investigation, we demonstrated that surface defects in perovskite films significantly impact the performance and stability of perovskite solar cells, closely correlated with light soaking behaviors. Removing the top surface layer through adhesive tape, the surface defect density noticeably decreases, leading to enhanced photoluminescence (PL) efficiency, prolonged carrier lifetime, and higher conductivity. Consequently, the power conversion efficiency (PCE) of solar cells improves from 17.70% to 20.5%. Furthermore, we confirmed a positive correlation between surface defects and the light soaking effect. Perovskite films with low surface defects surprisingly exhibit a 3-fold increase in PL intensity and an 85% increase in carrier lifetime under 500 s of continuous illumination at an intensity of 100 mW/cm2. Beyond the conventional strategy of suppressing defect trapping, we propose increasing the capability of dynamic defect tolerance as an effective strategy to enhance the optoelectronic properties and performance of perovskite solar cells.

Microstress for metal halide perovskite solar cells: from source to influence and management

The power conversion efficiency of metal halide perovskite solar cells (PSCs) has increased dramatically in recent years, but there are still major bottlenecks in the commercial application of such materials, including intrinsic instability caused by external stimuli such as water, oxygen, and radiation, as well as local stress generated inside the perovskite and external stress caused by poor interlayer contact. However, some crucial sources of instability cannot be overcome by conventional encapsulation engineering. Among them, the tensile strain can weaken the chemical bonds in the perovskite lattice, thereby reducing the defects formation energy and activation energy of ion migration and accelerating the degradation rate of the perovskite crystal. This review expounds the latest in-depth understanding of microstrain in perovskite film from the thermodynamic sources and influences on the perovskite physicochemical structure and photoelectric performance. Furthermore, it also summarizes the effective strategies for strain regulation and interlayer contact performance improvement, which are conducive to the improvement of photovoltaic performance and internal stability of PSCs. Finally, we present a prospective outlook on how to achieve more stable and higher efficiency PSCs through strain engineering.

Recent Advances in Wide Bandgap Perovskite Solar Cells: Focus on Lead-Free Materials for Tandem Structures

A tandem solar cell, which is composed of a wide bandgap (WBG) top sub-cell and a narrow bandgap (NBG) bottom subcell, harnesses maximum photons in the wide spectral range, resulting in higher efficiency than single-junction solar cells. WBG (>1.6 eV) perovskites are currently being studied a lot based on lead mixed-halide perovskites, and the power conversion efficiency of lead mixed-halide WBG perovskite solar cells (PSCs) reaches 21.1%. Despite the excellent device performance of lead WBG PSCs, their commercialization is hampered by their Pb toxicity and low stability. Hence, lead-free, less toxic WBG perovskite absorbers are needed for constructing lead-free perovskite tandem solar cells. In this review, various strategies for achieving high-efficiency WBG lead-free PSCs are discussed, drawing inspiration from prior research on WBG lead-based PSCs. The existing issues of WBG perovskites such as VOC loss are discussed, and toxicity issues associated with lead-based perovskites are also addressed. Subsequently, the natures of lead-free WBG perovskites are reviewed, and recently emerged strategies to enhance device performance are proposed. Finally, their applications in lead-free all perovskite tandem solar cells are introduced. This review presents helpful guidelines for eco-friendly and high-efficiency lead-free all perovskite tandem solar cells.

Self-Healing Behavior of the Metal Halide Perovskites and Photovoltaics

Perovskite solar cells have achieved rapid progress in the new-generation photovoltaic field, but the commercialization lags behind owing to the device stability issue under operational conditions. Ultimately, the instability issue is attributed to the soft lattice of ionic perovskite crystal. In brief, metal halide perovskite materials are susceptible to structural instability processes, including phase segregation, component loss, lattice distortion, and fatigue failure under harsh external stimuli such as high humidity, strong irradiation, wide thermal cycles, and large stress. Developing self-healing perovskites to further improve the unsatisfactory operational stability of their photoelectric devices under harsh stimuli has become a cutting-edge hotspot in this field. This self-healing behavior needs to be studied more comprehensively. Therefore, the self-healing behavior of the metal halide perovskites and photovoltaics is classified and summarized in this review. By discussing recent advances, underlying mechanisms, strategies, and existing challenges, this review provides perspectives on self-healing of perovskite solar cells in the future.

Towards Long-Term Stable Perovskite Solar Cells: Degradation Mechanisms and Stabilization Techniques

It is certain that perovskite materials must be a game-changer in the solar industry as long as their stability reaches a level comparable with the lifetime of a commercialized Si photovoltaic. However, the operational stability of perovskite solar cells and modules still remains unresolved, especially when devices operate in practical energy-harvesting modes represented by maximum power point tracking under 1 sun illumination at ambient conditions. This review article covers from fundamental aspects of perovskite instability including chemical decomposition pathways under light soaking and electrical bias, to recent advances and techniques that effectively prevent such degradation of perovskite solar cells and modules. In particular, fundamental causes for permanent degradation due to ion migration and trapped charges are overviewed and explain their interplay between ions and charges. Based on the degradation mechanism, recent advances on the strategies are discussed to slow down the degradation during operation for a practical use of perovskite-based solar devices.

Thermal Disorder-Induced Strain and Carrier Localization Activate Reverse Halide Segregation

The reversal of halide ions is studied under various conditions. However, the underlying mechanism of heat-induced reversal remains unclear. This work finds that dynamic disorder-induced localization of self-trapped polarons and thermal disorder-induced strain (TDIS) can be co-acting drivers of reverse segregation. Localization of polarons results in an order of magnitude decrease in excess carrier density (polaron population), causing a reduced impact of the light-induced strain (LIS - responsible for segregation) on the perovskite framework. Meanwhile, exposing the lattice to TDIS exceeding the LIS can eliminate the photoexcitation-induced strain gradient, as thermal fluctuations of the lattice can mask the LIS strain. Under continuous 0.1 W cm⁻2 illumination (upon segregation), the strain disorder is estimated to be 0.14%, while at 80 °C under dark conditions, the strain is 0.23%. However, in situ heating of the segregated film to 80 °C under continuous illumination (upon reversal) increases the total strain disorder to 0.25%, where TDIS is likely to have a dominant contribution. Therefore, the contribution of entropy to the system's free energy is likely to dominate, respectively. Various temperature-dependent in situ measurements and simulations further support the results. These findings highlight the importance of strain homogenization for designing stable perovskites under real-world operating conditions.

Inhibition of Ion Migration for Highly Efficient and Stable Perovskite Solar Cells

In recent years, organic-inorganic halide perovskites are now emerging as the most attractive alternatives for next-generation photovoltaic devices, due to their excellent optoelectronic characteristics and low manufacturing cost. However, the resultant perovskite solar cells (PVSCs) are intrinsically unstable owing to ion migration, which severely impedes performance enhancement, even with device encapsulation. There is no doubt that the investigation of ion migration and the summarization of recent advances in inhibition strategies are necessary to develop "state-of-the-art" PVSCs with high intrinsic stability for accelerated commercialization. This review systematically elaborates on the generation and fundamental mechanisms of ion migration in PVSCs, the impact of ion migration on hysteresis, phase segregation, and operational stability, and the characterizations for ion migration in PVSCs. Then, many related works on the strategies for inhibiting ion migration toward highly efficient and stable PVSCs are summarized. Finally, the perspectives on the current obstacles and prospective strategies for inhibition of ion migration in PVSCs to boost operational stability and meet all of the requirements for commercialization success are summarized.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Potential and perspectives of halide perovskites in light emitting devices

Light emitting diodes (LEDs) have become part of numerous electrical and electronic systems such as lighting, displays, status indicator lamps and wearable electronics. Owing to their excellent optoelectronic properties and deposition via simple solution process, metal halide perovskites possess unique potential for developing halide perovskite-based LEDs (PeLEDs) with superior photoluminescence efficiencies leading to external quantum efficiencies beyond 20% for PeLEDS. However, the limited durability, high operative voltages, and challenges of scale-up are persisting barriers in achieving required technology readiness levels. To build up the existing knowledge and raise the device performance this review provides a state-of-the-art study on the properties, film and device fabrication, efficiency, and stability of PeLEDs. In terms of commercialization, PeLEDs need to overcome materials and device challenges including stability, ion migration, phase segregation, and joule heating, which are discussed in this review. We hope, discussions about the strategies to overcome the stability issues and enhancement the materials intrinsic properties towards development more stable and efficient optoelectronic devices can pave the way for scalability and cost-effective production of PeLEDs.

Unraveling Size-Dependent Ion-Migration for Stable Mixed-Halide Perovskite Light-Emitting Diodes

Mixed-halide perovskites show tunable emission wavelength across the visible-light range, with optimum control of the light color. However, color stability remains limited due to the notorious halide segregation under illumination or an electric field. Here, a versatile path toward high-quality mixed-halide perovskites with high emission properties and resistance to halide segregation is presented. Through systematic in and ex situ characterizations, key features for this advancement are proposed: a slowed and controllable crystallization process can promote achievement of halide homogeneity, which in turn ensures thermodynamic stability; meanwhile, downsizing perovskite nanoparticle to nanometer-scale dimensions can enhance their resistance to external stimuli, strengthening the phase stability. Leveraging this strategy, devices are developed based on CsPbCl1.5 Br1.5 perovskite that achieves a champion external quantum efficiency (EQE) of 9.8% at 464 nm, making it one of the most efficient deep-blue mixed-halide perovskite light-emitting diodes (PeLEDs) to date. Particularly, the device demonstrates excellent spectral stability, maintaining a constant emission profile and position for over 60 min of continuous operation. The versatility of this approach with CsPbBr1.5 I1.5 PeLEDs is further showcased, achieving an impressive EQE of 12.7% at 576 nm.

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.

Sterically Suppressed Phase Segregation in 3D Hollow Mixed-Halide Wide Band Gap Perovskites

Band gap tuning in mixed-halide perovskites enables efficient multijunction solar cells and LEDs. However, these wide band gap perovskites, which contain a mixture of iodide and bromide ions, are known to phase segregate under illumination, introducing voltage losses that limit stability. Previous studies have employed inorganic perovskites, halide alloys, and grain/interface passivation to minimize halide segregation, yet photostability can be further advanced. By focusing on the role of halide vacancies in anion migration, one expects to be able to erect local barriers to ion migration. To achieve this, we employ a 3D "hollow" perovskite structure, wherein a molecule that is otherwise too large for the perovskite lattice is incorporated. The amount of hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA), varies the density of the hollow sites. Photoluminescence measurements reveal that 1% EDA in the perovskite bulk can stabilize a 40% bromine mixed-halide perovskite at 1 sun illumination intensity. These, along with capacitance-frequency measurements, suggest that hollow sites limit the mobility of the halide vacancies.

Surface Passivation Suppresses Local Ion Motion in Halide Perovskites

We use scanning probe microscopy to study ion migration in formamidinium (FA)-containing halide perovskite semiconductor Cs_0.22FA_0.78Pb(I_0.85Br_0.15)₃ in the presence and absence of chemical surface passivation. We measure the evolving contact potential difference (CPD) using scanning Kelvin probe microscopy (SKPM) following voltage poling. We find that ion migration leads to a ∼100 mV shift in the CPD of control films after poling with 3 V for only a few seconds. Moreover, we find that ion migration is heterogeneous, with domain interfaces leading to a larger CPD shift than domain interiors. Application of (3-aminopropyl)trimethoxysilane (APTMS) as a surface passivator further leads to 5-fold reduction in the CPD shift from ∼100 to ∼20 mV. We use hyperspectral microscopy to confirm that APTMS-treated perovskite films undergo less photoinduced halide migration than control films. We interpret these results as due to a reduction in the halide vacancy concentration after APTMS passivation.

Photovoltaic Performance of FAPbI3 Perovskite Is Hampered by Intrinsic Quantum Confinement

Formamidinium lead trioiodide (FAPbI₃) is a promising perovskite for single-junction solar cells. However, FAPbI₃ is metastable at room temperature and can cause intrinsic quantum confinement effects apparent through a series of above-bandgap absorption peaks. Here, we explore three common solution-based film-fabrication methods, neat N,N-dimethylformamide (DMF)-dimethyl sulfoxide (DMSO) solvent, DMF-DMSO with methylammonium chloride, and a sequential deposition approach. The latter two offer enhanced nucleation and crystallization control and suppress such quantum confinement effects. We show that elimination of these absorption features yields increased power conversion efficiencies (PCEs) and short-circuit currents, suggesting that quantum confinement hinders charge extraction. A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI₃ films corroborates our findings, indicating that PCEs rarely exceed a 20% threshold when such absorption features are present. Accordingly, ensuring the absence of these absorption features should be the first assessment when designing fabrication approaches for high-efficiency FAPbI₃ solar cells.

Probing the Local Electronic Structure in Metal Halide Perovskites through Cobalt Substitution

Owing to the unique chemical and electronic properties arising from 3d-electrons, substitution with transition metal ions is one of the key routes for engineering new functionalities into materials. While this approach has been used extensively in complex metal oxide perovskites, metal halide perovskites have largely resisted facile isovalent substitution. In this work, it is demonstrated that the substitution of Co²⁺ into the lattice of methylammonium lead triiodide imparts magnetic behavior to the material while maintaining photovoltaic performance at low concentrations. In addition to comprehensively characterizing its magnetic properties, the Co²⁺ ions themselves are utilized as probes to sense the local electronic environment of Pb in the perovskite, thereby revealing the nature of their incorporation into the material. A comprehensive understanding of the effect of transition metal incorporation is provided, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.

Mixed-Cation Halide Perovskite Doped with Rb+ for Highly Efficient Photodetector

Photodetectors are widely employed as fundamental devices in optical communication, automatic control, image sensors, night vision, missile guidance, and many other industrial or military fields. Mixed-cation perovskites have emerged as promising optoelectronic materials for application in photodetectors due to their superior compositional flexibility and photovoltaic performance. However, their application involves obstacles such as phase segregation and poor-quality crystallization, which introduce defects in perovskite films and adversely affect devices' optoelectronic performance. The application prospects of mixed-cation perovskite technology are significantly constrained by these challenges. Therefore, it is necessary to investigate strategies that combine crystallinity control and defect passivation to obtain high-quality thin films. In this study, we incorporated different Rb⁺ ratios in triple-cation (CsMAFA) perovskite precursor solutions and studied their effects on crystal growth. Our results show that a small amount of Rb⁺ was enough to induce the crystallization of the α-FAPbI₃ phase and suppress the formation of the yellow non-photoactive phase; the grain size increased, and the product of the carrier mobility and the lifetime (μτ) improved. As a result, the fabricated photodetector exhibited a broad photo-response region, from ultraviolet to near-infrared, with maximum responsivity (R) up to 11.8 mA W^-1 and excellent detectivity (D*) values up to 5.33 × 10¹¹ Jones. This work provides a feasible strategy to improve photodetectors' performance via additive engineering.

Temperature-Dependent Reversal of Phase Segregation in Mixed-Halide Perovskites

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH₃ NH₃ Pb(Br_0.4 I_0.6 )₃ first accelerates toward ≈290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.

Competition of Carrier Kinetics Contributes to Amplified Spontaneous Emission in Quasi-2D/3D (PBA)2MAn-1PbnBr3n+1 Thin Films under Strip Light Mode

Quasi-2D halide perovskites have potential in lasing due to their amplified spontaneous emission (ASE) properties. The ASE of (PBA)₂MA_n-1Pb_nBr_3n+1 thin films has been confirmed by photoluminescence (PL) testing using stripe light excitation (SLE). The ASE threshold decreases with decreasing environmental temperature (T_E) or increasing number of inorganic layers (n). Using the transient absorption technique, the Auger recombination and the cooling process of the high-activity carrier are accelerated with the decrease of n or T_E. A new ASE mechanism is proposed where high-activity carriers directly emit photons under photon perturbation from adjacent sites, leading to the accumulation and amplification of emitted photons only in the SLE region for ASE to occur. In addition, the reduction of n promotes light scattering between nano-thin layers, which supports a rapid increase in the ASE signal after the ASE threshold is crossed.

Machine Learning Enables Prediction of Halide Perovskites' Optical Behavior with >90% Accuracy

The composition-dependent degradation of hybrid organic-inorganic perovskites (HOIPs) due to environmental stressors still precludes their commercialization. It is very difficult to quantify their behavior upon exposure to each stressor by exclusively using trial-and-error methods due to the high-dimensional parameter space involved. We implement machine learning (ML) models using high-throughput, in situ photoluminescence (PL) to predict the response of Cs _y FA_1-y Pb(Br _x I_1-x )₃ while exposed to relative humidity cycles. We quantitatively compare three ML models while generating forecasts of environment-dependent PL responses: linear regression, echo state network, and seasonal autoregressive integrated moving average with exogenous regressor algorithms. We achieve accuracy of >90% for the latter, while tracking PL changes over a 50 h window. Samples with 17% of Cs content consistently showed a PL increase as a function of cycle. Our precise time-series forecasts can be extended to other HOIP families, illustrating the potential of data-centric approaches to accelerate material development for clean-energy devices.

Morphology-Dependent Carrier Accumulation Dynamics in Mixed Halide Perovskite Thin Films Caused by Phase Segregation

The phase segregation in mixed halide perovskites is recently found to improve the photoluminescence quantum yield (PLQY) of the perovskites by concentrating the carriers. However, how phase segregation affects the photoinduced carrier dynamics is unclear. Herein, we find that the phase segregation in CH₃NH₃PbBr_xI_3-x mixed halide perovskite thin film is morphology-dependent by showing I-rich domains mainly along the grain boundaries. Ultrafast transient absorption (TA) and photoluminescence upconversion (PL-UC) spectroscopy measurements uncover that the carrier accumulation in the low energy I-rich domains includes two carrier transfer pathways. Carrier transfer from the Br-rich domain and the mixed phase to the I-rich domain is realized by consecutive hole (∼0.5 ps) and electron (<12.4 ps) transfer and energy transfer (<12.4 ps), respectively. The finding reveals the carrier funneling dynamic mechanism in phase-segregated halide perovskite films and provides a guideline for the applications of mixed halide perovskites in color-conversion devices or high-efficiency LEDs.

In Situ and Operando Characterizations of Metal Halide Perovskite and Solar Cells: Insights from Lab-Sized Devices to Upscaling Processes

The performance and stability of metal halide perovskite solar cells strongly depend on precursor materials and deposition methods adopted during the perovskite layer preparation. There are often a number of different formation pathways available when preparing perovskite films. Since the precise pathway and intermediary mechanisms affect the resulting properties of the cells, in situ studies have been conducted to unravel the mechanisms involved in the formation and evolution of perovskite phases. These studies contributed to the development of procedures to improve the structural, morphological, and optoelectronic properties of the films and to move beyond spin-coating, with the use of scalable techniques. To explore the performance and degradation of devices, operando studies have been conducted on solar cells subjected to normal operating conditions, or stressed with humidity, high temperatures, and light radiation. This review presents an update of studies conducted in situ using a wide range of structural, imaging, and spectroscopic techniques, involving the formation/degradation of halide perovskites. Operando studies are also addressed, emphasizing the latest degradation results for perovskite solar cells. These works demonstrate the importance of in situ and operando studies to achieve the level of stability required for scale-up and consequent commercial deployment of these cells.

From Heterostructures to Solid-Solutions: Structural Tunability in Mixed Halide Perovskites

The stability, reliability, and performance of halide-perovskite-based devices depend upon the structure, composition, and particle size of the device-enabling materials. Indeed, the degree of ion mixing in multicomponent perovskite crystals, although challenging to control, is a key factor in determining properties. Herein, an emerging method termed evaporation-crystallization polymer pen lithography is used to synthesize and systematically study the degree of ionic mixing of Cs_0.5 FA_0.5 PbX₃ (FA = formamidinium; X = halide anion, ABX₃ ) crystals, as a function of size, temperature, and composition. These experiments have led to the discovery of a heterostructure morphology where the A-site cations, Cs and FA, are segregated into the core and edge layers, respectively. Simulation and experimental results indicate that the heterostructures form as a consequence of a combination of both differences in solubility of the two ions in solution and the enthalpic preference for Cs-FA ion segregation. This preference for segregation can be overcome to form a solid-solution by decreasing crystal size (<60 nm) or increasing temperature. Finally, these tools are utilized to identify and synthesize solid-solution nanocrystals of Cs_0.5 FA_0.5 Pb(Br/I)₃ that significantly suppress photoinduced anion migration compared to their bulk counterparts, offering a route to deliberately designed photostable optoelectronic materials.

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.

A Roadmap for Efficient and Stable All-Perovskite Tandem Solar Cells from a Chemistry Perspective

Multijunction tandem solar cells offer a promising route to surpass the efficiency limit of single-junction solar cells. All-perovskite tandem solar cells are particularly attractive due to their high power conversion efficiency, now reaching 28% despite being made with relatively easy fabrication methods. In this review, we summarize the progress in all-perovskite tandem solar cells. We then discuss the scientific and engineering challenges associated with both absorbers and functional layers and offer strategies for improving the efficiency and stability of all-perovskite tandem solar cells from the perspective of chemistry.

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.

Excitation Intensity- and Size-Dependent Halide Photosegregation in CsPb(I0.5Br0.5)3 Perovskite Nanocrystals

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. Here, we quantitatively compare experimental, CsPb(I_0.5Br_0.5)₃ nanocrystal (NC) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate local photogenerated carrier densities. We observe that mixed-anion perovskite NCs exhibit significant robustness to photosegregation, with photosegregation propensity decreasing with decreasing NC size. Observed size- and excitation intensity-dependent photosegregation data agree with model predicted size- and excitation intensity-dependent terminal halide stoichiometries. Established correspondence between experiment and theory, in turn, suggests that mixed-halide perovskite photostabilities can be predicted a priori using local gradients of (empirical) Vegard's law expressions of composition-dependent band gaps.

Stabilization of Perovskite Solar Cells: Recent Developments and Future Perspectives

Exceptional power conversion efficiency (PCE) of 25.7% in perovskite solar cells (PSCs) has been achieved, which is comparable with their traditional rivals (Si-based solar cells). However, commercialization-worthy efficiency and long-term stability remain a challenge. In this regard, there are increasing studies focusing on the interface engineering in PSC devices to overcome their poor technical readiness. Herein, the roles of electrode materials and interfaces in PSCs are discussed in terms of their PCEs and perovskite stability. All the current knowledge on the factors responsible for the rapid intrinsic and external degradation of PSCs is presented. Then, the roles of carbonaceous materials as substitutes for noble metals are focused on, along with the recent research progress in carbon-based PSCs. Furthermore, a sub-category of PSCs, that is, flexible PSCs, is considered as a type of exceptional power source due to their high power-to-weight ratios and figures of merit for next-generation wearable electronics. Last, the future perspectives and directions for research in PSCs are discussed, with an emphasis on their commercialization.

Colloidal FAPbBr3 perovskite nanocrystals for light emission: what's going on?

Semiconducting nanomaterials have been widely explored in diverse optoelectronic applications. Colloidal lead halide perovskite nanocrystals (NCs) have recently been an excellent addition to the field of nanomaterials, promising an enticing building block in the field of light emission. In addition to the notable optoelectronic properties of perovskites, the colloidal NCs exhibit unique size-dependent optical properties due to the quantum size effect, which makes them highly attractive for light-emitting diodes (LEDs). In the past few years, perovskite-based LEDs (PeLEDs) have demonstrated a meteoritic rise in their external quantum efficiency (EQE) values, reaching over 20% so far. Among various halide perovskite compositions, FAPbBr3 and its variants remain one of the most interesting and sought-after compounds for green light emission. This review focuses on recent progress in the design and synthesis protocols of colloidal FAPbBr3 NCs and the emerging concepts in tailoring their surface chemistry. The structural and physicochemical features of lead halide perovskites along with a comprehensive discussion on their defect-tolerant properties are briefly outlined. Later, the prevalent synthesis, ligand, and compositional engineering strategies to boost the stability and photoluminescence quantum yield (PLQY) of FAPbBr3 NCs are extensively discussed. Finally, the fundamental concepts and recent progress on FAPbBr3-based LEDs, followed by a discussion of the challenges and prospects that are on the table for this enticing class of perovskites, are reviewed.

Suppressing Halide Segregation in Wide-Band-Gap Mixed-Halide Perovskite Layers through Post-Hot Pressing

Mixed-halide perovskites (MHPs) have attracted attention as suitable wide-band-gap candidate materials for tandem applications owing to their facile band-gap tuning. However, when smaller bromide ions are incorporated into iodides to tune the band gap, photoinduced halide segregation occurs, which leads to voltage deficit and photoinstability. Here, we propose an original post-hot pressing (PHP) treatment that suppresses halide segregation in MHPs with a band gap of 2.0 eV. The PHP treatment reconstructs open-structured grain boundaries (GBs) as compact GBs through constrained grain growth in the in-plane direction, resulting in the inhibition of defect-mediated ion migration in GBs. The PHP-treated wide-band-gap (2.0 eV) MHP solar cells showed a high efficiency of over 11%, achieving an open-circuit voltage (V_oc) of 1.35 V and improving the maintenance of the initial efficiency under the working condition at AM 1.5G. The results reveal that the management of GBs is necessary to secure the stability of wide-band-gap MHP devices in terms of halide segregation.

Optoelectronic Properties of Mixed Iodide-Bromide Perovskites from First-Principles Computational Modeling and Experiment

Halogen mixing in lead-halide perovskites is an effective route for tuning the band gap in light emission and multijunction solar cell applications. Here we report the effect of halogen mixing on the optoelectronic properties of lead-halide perovskites from theory and experiment. We applied the virtual crystal approximation within density functional theory, the GW approximation, and the Bethe-Salpeter equation to calculate structural, vibrational, and optoelectronic properties for a series of mixed halide perovskites. We separately perform spectroscopic measurements of these properties and analyze the impact of halogen mixing on quasiparticle band gaps, effective masses, absorption coefficients, charge-carrier mobilities, and exciton binding energies. Our joint theoretical-experimental study demonstrates that iodide-bromide mixed-halide perovskites can be modeled as homovalent alloys, and local structural distortions do not play a significant role for the properties of these mixed species. Our study outlines a general theoretical-experimental framework for future investigations of novel chemically mixed systems.

In-situ ellipsometry measurements on the halide phase segregation of mixed halide lead perovskites

Methylammonium lead iodide bromides MAPb(Br_xI_1‐x)₃ are a class of mixed halide lead perovskites, materials that offer high‐power conversion efficiencies and bandgap tunability. For these reasons, they are a promising absorber material for future solar cells, although their use is still limited due to several factors. The reversible phase segregation under even low light intensities is one of them, lowering the effective bandgap due to local segregation into iodide‐rich and bromide‐rich phases. While several studies have been done to illuminate the mechanism and suppression of phase segregation, challenges remain to understand its kinetics. We obtained dynamic ellipsometric measurements from x=0.5 mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light with a power density of ∼11 W/cm². Time constants between 1.7(±0.7)×10⁻³ s⁻¹ for the segregation and 1.5(±0.6)×10⁻⁴ s⁻¹ for recovery were calculated. The phase segregation rate constants are surprisingly two orders of magnitude slower than and the recovery rate is consistent with those measured using photoluminescence methods under similar conditions. These results confirm a concern in the literature about the complexity in the phase separation kinetics measured from photoluminescence. We expect ellipsometry to serve as a complementary technique to other spectroscopies in studying mixed‐halide lead perovskites phase segregation in the future.

Tuning the optical properties of 2D monolayer silver-bismuth bromide double perovskite by halide substitution

Silver-bismuth double perovskites are promising replacement materials for lead-based ones in photovoltaic (PV) devices due to the lower toxicity and enhanced stability to environmental factors. In addition, they might even be more suitable for indoor PV, due to the size of their bandgap better matching white LEDs emission. Unfortunately, their optoelectronic performance does not reach that of the lead-based counterparts, because of the indirect nature of the band gap and the high exciton binding energy. One strategy to improve the electronic properties is the dimensional reduction from the 3D to the 2D perovskite structure, which features a direct band gap, as it has been reported for 2D monolayer derivates of Cs₂AgBiBr₆obtained by substituting Cs⁺cations with bulky alkylammonium cations. However, a similar dimensional reduction also brings to a band gap opening, limiting light absorption in the visible. In this work, we report on the achievement of a bathochromic shift in the absorption features of a butylammonium-based silver-bismuth bromide monolayer double perovskite through doping with iodide and study the optical properties and stability of the resulting thin films in environmental conditions. These species might constitute the starting point to design future sustainable materials to implement as active components in indoor photovoltaic devices used to power the IoT.

Halide Segregation in Mixed Halide Perovskites: Visualization and Mechanisms

Photoinduced halide segregation in mixed halide perovskites is an intriguing phenomenon and simultaneously a stability issue. In-depth probing this effect and unveiling the underpinning mechanisms are of great interest and significance. This article reviews the progress in visualized investigation of halide segregation, especially light-induced, by means of spatially-resolved imaging techniques. Furthermore, the current understanding of photoinduced phase separation based on several possible mechanisms is summarized and discussed. Finally, the remained open questions and future outlook in this field are outlined.

Toward stable lead halide perovskite solar cells: A knob on the A/X sites components

Hybrid lead halide ABX3 perovskite solar cells (PSCs) have emerged as a strong competitor to the traditional solar cells with a certified power conversion efficiency beyond 25% and other remarkable features such as light weight, solution processability, and low manufacturing cost. Further development on the efficiency and stability brings forth increasing attention in the component regulation, such as partial or entire substitution of A/B/X sites by alternative elements with similar size. However, the relationships between composition, property, and performance are poorly understood. Here, the instability of PSCs from the photon-, moisture-, thermal-, and mechanical-induced degradation was first summarized and discussed. In addition, the component regulation from the A/X sites is highlighted from the aspects of band level alignment, charge-carrier dynamics, ion migration, crystallization behavior, residual strain, stoichiometry, and dimensionality control. Finally, the perspectives and future outlooks are highlighted to guide the rational design and practical application of PSCs.

Chemical Control of the Dimensionality of the Octahedral Network of Solar Absorbers from the CuI-AgI-BiI3 Phase Space by Synthesis of 3D CuAgBiI5

A newly reported compound, CuAgBiI₅, is synthesized as powder, crystals, and thin films. The structure consists of a 3D octahedral Ag⁺/Bi³⁺ network as in spinel, but occupancy of the tetrahedral interstitials by Cu⁺ differs from those in spinel. The 3D octahedral network of CuAgBiI₅ allows us to identify a relationship between octahedral site occupancy (composition) and octahedral motif (structure) across the whole CuI–AgI–BiI₃ phase field, giving the ability to chemically control structural dimensionality. To investigate composition–structure–property relationships, we compare the basic optoelectronic properties of CuAgBiI₅ with those of Cu₂AgBiI₆ (which has a 2D octahedral network) and reveal a surprisingly low sensitivity to the dimensionality of the octahedral network. The absorption onset of CuAgBiI₅ (2.02 eV) barely changes compared with that of Cu₂AgBiI₆ (2.06 eV) indicating no obvious signs of an increase in charge confinement. Such behavior contrasts with that for lead halide perovskites which show clear confinement effects upon lowering dimensionality of the octahedral network from 3D to 2D. Changes in photoluminescence spectra and lifetimes between the two compounds mostly derive from the difference in extrinsic defect densities rather than intrinsic effects. While both materials show good stability, bulk CuAgBiI₅ powder samples are found to be more sensitive to degradation under solar irradiation compared to Cu₂AgBiI₆.

We describe a way to chemically control the octahedral network of potentially useful photovoltaic solar absorbers in the CuI−AgI−BiI₃ phase space by the synthesis of CuAgBiI₅ with a 3D octahedral network. We compare the photostability of CuAgBiI₅ bulk samples and the absorption coefficient and photoluminescence of solution processed thin films with those of Cu₂AgBiI₆, which has a 2D octahedral network. This helps to understand structure−property relationships to direct further materials optimization.

Spectral Stable Blue-Light-Emitting Diodes via Asymmetric Organic Diamine Based Dion-Jacobson Perovskites

The spectral instability issue is a challenge in blue perovskite light-emitting diodes (PeLEDs). Dion–Jacobson (DJ) phase perovskites are promising alternatives to achieve high-quality blue PeLEDs. However, the current exploration of DJ phase perovskites is focused on symmetric divalent cations, and the corresponding efficiency of blue PeLEDs is still inferior to that of green and red ones. In this work, we report a new type of DJ phase CsPb(Br/Cl)₃ perovskite via introduction of an asymmetric molecular configuration as the organic spacer cation in perovskites. The primary and tertiary ammonium groups on the asymmetric cations bridge with the lead halide octahedra forming the DJ phase structures. Stable photoluminescence spectra were demonstrated in perovskite films owing to the suppressed halide segregation. Meanwhile, the radiative recombination efficiency of charges is improved significantly as a result of the confinement effects and passivation of charge traps. Finally, we achieved an external quantum efficiency of 2.65% in blue PeLEDs with stable spectra emission under applied bias voltages. To our best knowledge, this is the first report of asymmetric cations used in PeLEDs, which provides a facile solution to the halide segregation issue in PeLEDs.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Phase segregation in mixed halide perovskite is known to alter the optoelectronic properties, but how it affects charge carriers is not clear. Here, the authors use THz spectroscopy to reveal that high carrier mobilities are well preserved, while recombination dynamics is affected by charge funnelling upon segregation.

Design Principles of Large Cation Incorporation in Halide Perovskites

Perovskites have stood out as excellent photoactive materials with high efficiencies and stabilities, achieved via cation mixing techniques. Overcoming challenges to the stabilization of Perovskite solar cells calls for the development of design principles of large cation incorporation in halide perovskite to accelerate the discovery of optimal stable compositions. Large fluorinated organic cations incorporation is an attractive method for enhancing the intrinsic stability of halide perovskites due to their high dipole moment and moisture-resistant nature. However, a fluorinated cation has a larger ionic size than its non-fluorinated counterpart, falling within the upper boundary of the mixed-cation incorporation. Here, we report on the intrinsic stability of mixed Methylammonium (MA) lead halides at different concentrations of large cation incorporation, namely, ehtylammonium (EA; [CH₃CH₂NH₃]⁺) and 2-fluoroethylammonium (FEA; [CH₂FCH₂NH₃]⁺). Density functional theory (DFT) calculations of the enthalpy of the mixing and analysis of the perovskite structural features enable us to narrow down the compositional search domain for EA and FEA cations around concentrations that preserve the perovskite structure while pointing towards the maximal stability. This work paves the way to developing design principles of a large cation mixture guided by data analysis of DFT data. Finally, we present the automated search of the minimum enthalpy of mixing by implementing Bayesian optimization over the compositional search domain. We introduce and validate an automated workflow designed to accelerate the compositional search, enabling researchers to cut down the computational expense and bias to search for optimal compositions.

Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation

The feasibility of mixed-cation mixed-halogen perovskites of formula AxA’1−xPbXyX’zX”3−y−z is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.

Multication perovskite 2D/3D interfaces form via progressive dimensional reduction

Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability – but it remains unclear how the conversion of 3D-to-2D perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to n = 3 → 2 → 1 when we expose (MAPbBr₃)_0.05(FAPbI₃)_0.95 perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics.

Many best-performing perovskite photovoltaics use 2D/3D interfaces to improve efficiency and stability, yet the mechanism of interface assembly is unclear. Here, Proppe et al. use in-situ GIWAXS to resolve this transformation, observing progressive dimensional reduction from 3D to 2D perovskites.