Halide Diffusion Research Articles

Mitigating bulk halide defects in blue-emissive perovskite nanocrystals using benzoyl halides for efficient light-emitting diodes

While recent advances have demonstrated highly efficient and stable perovskite light-emitting diodes (LEDs), blue perovskite LEDs still show low performance compared to green, red, near IR counterparts. Despite perovskites being well-known for their defect tolerance, blue perovskites are highly vulnerable to defects, which introduce deep trap states. In this study, we mitigated bulk halide vacancies in perovskite nanocrystals (PNCs) via the diffusion-assisted halide repairing strategy. We confirmed halide vacancies inside as-synthesized PNCs, both in the core and on the surface, through HADDF-STEM. By treating PNCs with benzoyl halides, these halide vacancies were further mitigated due to halide diffusion via concentration gradients between the surface and the core, resulting in enhanced PLQY of the PNCs. In the fabrication of blue PNC LEDs, this approach yielded a comparable efficiency with an EQE of 5.37 % at 469 nm, demonstrating the robustness of our method for efficient blue perovskite LEDs. Furthermore, the peak wavelengths of benzoyl halide-treated PNC LEDs remained stable during operation, whereas those of pristine PNC LEDs exhibited peak shifts. We believe that our novel approach can be universally applied to various types of perovskites, reducing bulk vacancies. This advancement will be a significant breakthrough for the performance enhancement of blue perovskite LEDs in the future.

Recent Progress on Cross-Linkable Fullerene-Based Electron Transport Materials for Perovskite Solar Cells.

Fullerene-based derivatives are frequently used as electron transport materials (ETMs) and interface buffers for perovskite solar cells (PSCs) due to their excellent properties, including high electron affinity and mobility, low recombination energy, tunable energy levels, and solution processability. However, significant challenges arise because fullerene derivatives tend to aggregate and dimerize, which reduces exciton dissociation and charge transport capacity. Additionally, their chemical compatibility with perovskite absorbers facilitates halide diffusion and degradation of PSCs. This overlap causes delamination and dissolution during device fabrication, hindering the performance enhancement of fullerene-based PSCs. To address these issues, researchers have developed cross-linkable fullerene materials. These materials have been shown to not only significantly improve the power conversion efficiency (PCE) of PSCs but also effectively enhance the device stability. In this review, we summarized recent research progress on cross-linkable fullerene derivatives as ETMs for PSCs. We systematically analyze the impact of these cross-linked ETMs on device performance and long-term stability, focusing on their molecular structures and working mechanisms. Finally, we discuss the future challenges that need to be overcome to advance the application of cross-linkable fullerene materials in PSCs.

Effects of Ligand Chemistry on Ion Transport in 2D Hybrid Organic–Inorganic Perovskites

Abstract2D hybrid organic–inorganic perovskites are potentially promising materials as passivation layers that can enhance the efficiency and stability of perovskite photovoltaics. The ability to suppress ion transport is proposed as a stabilization mechanism, yet an effective characterization of relevant modes of halide diffusion in 2D perovskites is nascent. In light of this knowledge gap, molecular dynamics simulations with enhanced sampling and experimental validation to systematically characterize how ligand chemistry in seven (R‐NH3)2PbI4 systems impacts halide diffusion, particularly in the out‐of‐plane direction is combined. It is found that increasing stiffness and length of ligands generally inhibits ion transport, while increasing ligand polarization generally enhances it. Structural and energetic analyses of the migration pathways provide quantitative explanations for these trends, which reflect aspects of the disorder of the organic layer. Overall, this mechanistic analysis greatly enhances the current understanding of halide migration in 2D hybrid organic–inorganic perovskites and yields insights that can inform the design of future passivation materials.

Halide Ion Mobility in Paired 2D Halide Perovskites: Ruddlesden–Popper Versus Dion–Jacobson Phases

Abstract2D lead halide perovskites with tunable optoelectronic properties through modulating inorganic components and organic spacer ligands represent a promising candidate in perovskite optoelectronics. However, an intrinsic soft lattice of perovskites gives rise to a vacancy‐mediated halide ion migration, which further affects the long‐term stability of materials as well as devices. Here, halide ion mobility is evaluated using two different 2D perovskites yielded with butylammonium (BA) for the Ruddlesden–Popper (RP) phase and butane‐1,4‐diamine (BDA) for the Dion–Jacobson (DJ) phase, respectively. By probing the halide ion migration kinetics in the physically paired 2D bromide and iodide perovskite films, the softer lattice (RP) shows a lower activation energy of 50.9 kJ mol−1 as compared to that of more rigid lattice (DJ) of 60.8 kJ mol−1. Given similar halide diffusion pathlength and halide mixing chemical potential, the binding mode (RP and DJ) can indeed dictate the overall halide ion stability. Understanding such halide ion mobility is important in designing perovskites with increased stability and performances at material and device levels.

(Invited) Graphene Ion Blocking Layers for Robust Metal-Halide Perovskite Heterostructures

Despite being only one atomic layer thick, a continuous monolayer of graphene should be completely impermeable to atoms and ions. As such, graphene monolayers could potentially be employed as ion blocking layers in numerous technologies where charge and/or energy transport across a key interface is desirable, but ionic or atomic transport is not. As a key example, the development of metal-halide perovskite/perovskite heterostructures is hindered by rapid interfacial halide diffusion leading to mixed alloys rather than sharp interfaces. This halide diffusion hinders the creation of tailored interfaces that funnel energy or charge in specified directions within energy technologies such as photovoltaic solar cells or light-emitting diodes. To circumvent this issue, we developed an ion-blocking layer consisting of single layer graphene (SLG) deposited between the metal-halide perovskite layers and demonstrate that it effectively blocks anion diffusion in perovskite/perovskite heterostructures. We study these interfaces with a suite of spectroscopy and elemental analysis tools to demonstrate that halides do not diffuse across the graphene-modified interface, whereas control samples without the SLG show rapid homogenization of the halides and loss of the sharp interface. The SLG has little electronic impact on the band alignment and optical properties of the individual semiconductors, as supported by density functional theory calculations, facilitating photoinduced energy transfer across the abrupt heterointerface and light-emitting diodes with no evidence of ion-diffusion during operation. Our results demonstrate that monolayer graphene can be used as an ion blocking layer in a variety of different heterostructures for advanced opto-electronic and electrochemical applications. Further, it establishes a route towards robust perovskite/perovskite heterostructures that are difficult to achieve by other means.

Fabrication and Characterization of 2D Layered Perovskites with a Gradient Band Gap.

Vertical gradient band-gap heterostructures of two-dimensional (2D) layered perovskites have attracted considerable research interest due to their superior optoelectronic properties and demonstrated potential for use in optical devices. However, its fabrication has been challenging. In this investigation, 2D Ruddlesden-Popper mixed halide perovskite single crystals with a vertical gradient band gap were synthesized by using a solid-state halide diffusion process. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements after diffusion confirm that the crystalline and morphology remain intact. The transmittance and photoluminescence (PL) spectra show the formation of a vertical gradient band gap that is ascribed to gradient halide distribution through halide intermixing. The mixed halide crystal exhibits high stability with completely suppressed phase segregation in the time-dependent PL measurement. The time-resolved photoluminescence (TRPL) spectra prove that the mixed halide sample has an enhanced carrier transport due to the Förster resonance energy transfer (FRET) effect. Besides, the halide diffusion behavior is found to be different from the previously proposed "layer-by-layer" diffusion model in exfoliated crystals. The gradient band-gap structure is critical for various applications in which vertical carrier transport is demanded.

Immobilizing Surface Halide in Perovskite Solar Cells via Calix[4]pyrrole.

Halide diffusion across the charge-transporting layer followed by a reaction with metal electrode represents a critical factor limiting the long-term stability of perovskite solar cells (PSCs). In this work, a supramolecular strategy with surface anion complexation is reported for enhancing the light and thermal stability of perovskite films, as well as devices. Calix[4]pyrrole (C[4]P) is demonstrated as a unique anion-binding agent for stabilizing the structure of perovskite by anchoring surface halides, which increases the activation energy for halide migration, thus effectively suppressing the halide-metal electrode reactions. The C[4]P-stabilized perovskite films preserve their initial morphology after ageing at 85°C or under 1 sun illumination in humid air over 50 h, significantly outperforming the control samples. This strategy radically tackles the halide outward-diffusion issue without sacrificing charge extraction. Inverted-structured PSCs based on C[4]P modified formamidinium-cesium perovskite exhibit a champion power conversion efficiency of over 23%. The lifespans of unsealed PSCs are unprecedentedly prolonged from dozens of hours to over 2000 h under operation (ISOS-L-1) and 85°C ageing (ISOS-D-2). When subjected to a harsher protocol of ISOS-L-2 with both light and thermal stresses, the C[4]P-based PSCs maintain 87% of original efficiency after ageing for 500 h.

A multiscale ion diffusion framework sheds light on the diffusion-stability-hysteresis nexus in metal halide perovskites.

Stability and current-voltage hysteresis stand as major obstacles to the commercialization of metal halide perovskites. Both phenomena have been associated with ion migration, with anecdotal evidence that stable devices yield low hysteresis. However, the underlying mechanisms of the complex stability-hysteresis link remain elusive. Here we present a multiscale diffusion framework that describes vacancy-mediated halide diffusion in polycrystalline metal halide perovskites, differentiating fast grain boundary diffusivity from volume diffusivity that is two to four orders of magnitude slower. Our results reveal an inverse relationship between the activation energies of grain boundary and volume diffusions, such that stable metal halide perovskites exhibiting smaller volume diffusivities are associated with larger grain boundary diffusivities and reduced hysteresis. The elucidation of multiscale halide diffusion in metal halide perovskites reveals complex inner couplings between ion migration in the volume of grains versus grain boundaries, which in turn can predict the stability and hysteresis of metal halide perovskites, providing a clearer path to addressing the outstanding challenges of the field.

Metal Halide Perovskite Heterostructures: Blocking Anion Diffusion with Single-Layer Graphene

The development of metal halide perovskite/perovskite heterostructures is hindered by rapid interfacial halide diffusion leading to mixed alloys rather than sharp interfaces. To circumvent this outcome, we developed an ion-blocking layer consisting of single-layer graphene (SLG) deposited between the metal halide perovskite layers and demonstrated that it effectively blocks anion diffusion in a CsPbBr3/SLG/CsPbI3 heterostructure. Spatially resolved elemental analysis and spectroscopic measurements demonstrate the halides do not diffuse across the interface, whereas control samples without the SLG show rapid homogenization of the halides and loss of the sharp interface. Ultraviolet photoelectron spectroscopy, DFT calculations, and transient absorbance spectroscopy indicate the SLG has little electronic impact on the individual semiconductors. In the CsPbBr3/SLG/CsPbI3, we find a type I band alignment that supports transfer of photogenerated carriers across the heterointerface. Light-emitting diodes (LEDs) show electroluminescence from both the CsPbBr3 and CsPbI3 layers with no evidence of ion diffusion during operation. Our approach provides opportunities to design novel all-perovskite heterostructures to facilitate the control of charge and light in optoelectronic applications.

Halide Diffusion Equilibrium and Its Impact on Efficiency Evolution of Perovskite Solar Cells

AbstractUnderstanding the degradation mechanism of perovskite solar cells (PSCs) is of particular importance to solve their instability issue, which is one of the major hindrances toward commercialization. Here, it is shown that a halide diffusion equilibrium exists at the heterointerface of perovskite devices, which strongly impacts the evolution of device performance. The combined experimental and theoretical studies reveal that halide components diffuse from perovskite to fullerene layers in a p‐i‐n PSC device and equilibrate with an iodine density of 1018–1019 cm−3 within 80 h under dark aging condition. It is found that there is a strong correction between the device efficiency and halide diffusion equilibrium of PSCs, as the diffused halides can chemically dope the transport layer and result in the nonstoichiometric perovskite surface, leading to both initial enhancement and long‐term loss of the photovoltaic efficiency of solar cells. In response to this issue, a predoping strategy is developed to attain the halide diffusion equilibrium once the device is fabricated, thereby avoiding the further halide migration and initial efficiency variations. As a result, the as‐prepared PSC achieved an efficiency of 23.13% as well as stable power output under continuous one sun illumination.

Sequential vacuum-evaporated perovskite solar cells with more than 24% efficiency.

Vacuum evaporation is promising for the high-throughput fabrication of perovskite solar cells (PSCs) because of its solvent-free characteristic, precise control of film thickness, and compatibility with large-scale production. Nevertheless, the power conversion efficiency (PCE) of PSCs fabricated by vacuum evaporation lags behind that of solution-processed PSCs. Here, we report a Cl-containing alloy-mediated sequential vacuum evaporation approach to fabricate perovskite films. The presence of Cl in the alloy facilitates organic ammonium halide diffusion and the subsequent perovskite conversion reaction, leading to homogeneous pinhole-free perovskite films with few defects. The resulting PSCs yield a PCE of 24.42%, 23.44% (certified 22.6%), and 19.87% for 0.1, 1.0, and 14.4 square centimeters (mini-module, aperture area), respectively. The unencapsulated PSCs show good stability with negligible decline in performance after storage in dry air for more than 4000 hours. Our method provides a reproducible approach for scalable fabrication of large-area, high-efficiency PSCs and other perovskite-based optoelectronics.

Anion diffusion in two-dimensional halide perovskites

Commercialization of halide perovskites in the semiconductor industry is hindered by their short-term stability. The instability of perovskites is closely interlinked with ionic diffusion. Historically, attempts to study diffusion in 2D perovskites mostly utilized electrical characterizations, but these characterizations pose a challenge in deconvoluting the impact of device architecture, interlayers, and ionic species. In this Perspective, we focus our attention on simple optical characterizations employed in the literature to investigate halide diffusion in 2D perovskites using lateral and vertical heterostructure platforms. We review the various synthesis techniques used for fabrication of halide perovskite heterostructures and discuss the qualitative and quantitative diffusion studies performed using these platforms. We discuss the numerical methods used to validate and supplement the experimental halide diffusion kinetics. Finally, we highlight the need to conduct further research on the impact of device operating conditions, lattice structure, and vacancy concentration on halide diffusion. Through this Perspective, we aim to emphasize the need of developing a comprehensive understanding of halide diffusion in perovskites for their successful deployment in optoelectronics.

Unravelling Anion Solvation in Water‐Alcohol Mixtures by Single Entity Electrochemistry

AbstractSingle entity electrochemistry is employed to gain insights into ion solvation in solvent mixtures. To this end, the time required for the oxidation of individual indicator nanoparticles to sparingly soluble products is used to probe ionic diffusion, and hence gain new insights into the solvation properties of solvent mixtures. Herein, water‐ethanol or water‐methanol mixtures of different compositions are analyzed following this new approach, using silver nanoparticle oxidation in the presence of chloride and iodide as a complementary indicator reaction. For increasing concentrations of the bulkier alcohol molecules in the mixtures with water, an increasing content of alcohol molecules in the halide's solvation shell is detected by the observation of hindered halide diffusion. The extent of this solvent replacement is shown to scale with the charge density of the ions and the experimental results are rationalized with respect to literature‐derived thermodynamic data, highlighting the ability of single entity electrochemistry to explore solvation in solvent mixtures.

Quantifying Anionic Diffusion in 2D Halide Perovskite Lateral Heterostructures

AbstractAnionic diffusion strongly impacts the stability of halide perovskite materials, but it is still not well understood. Here, a quantitative investigation of in‐plane thermally driven anionic inter‐diffusion in a series of novel 2D and quasi‐2D halide perovskites lateral heterostructures is reported. The calculated diffusion coefficients (D) reveal the inhibition of Br–I inter‐diffusion with bulky π‐conjugated organic cations compared with short‐chain aliphatic organic cations. Furthermore, halide diffusion is found to be faster in quasi‐2D (n > 1) than 2D perovskites (n = 1). The increment becomes less apparent as the “n” number increases, akin to the quantum confinement effect observed for band gaps. These trends are rationalized by molecular dynamics simulations of free energy barriers for halide diffusion that reveal mechanisms for suppressing diffusion. This work provides important fundamental insights on the anionic migration and diffusion process in halide perovskite materials.

Halide Ion Mobility of Mixed Halide Perovskites in Light and Dark

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in metal halide perovskites, which plays an important role in determining the performance of perovskite solar cells. Photoinduced phase segregation seen in mixed halide perovskite films under steady state irradiation offers a convenient way to visualize halide ion segregation. Interestingly, upon storage in dark, the process is reversed and the original mixed halide composition gets restored. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, the opposite trend observed during photoirradiation remains an intriguing phenomenon. The threshold energy of incident light to observe halide segregation increases with increasing temperature. The diffusion of these halide species, which is tracked through changes in the absorption spectra at different temperatures, offers a direct measurement of thermally activated halide diffusion in perovskite films. The thermally activated halide exchange shows the challenges of stabilizing metal halide perovskites with varying bandgap in tandem solar cells.

Interfacial stabilization for inverted perovskite solar cells with long-term stability

Perovskite solar cells (PSCs) commonly exhibit significant performance degradation due to ion migration through the top charge transport layer and ultimately metal electrode corrosion. Here, we demonstrate an interfacial management strategy using a boron chloride subphthalocyanine (Cl6SubPc)/fullerene electron-transport layer, which not only passivates the interfacial defects in the perovskite, but also suppresses halide diffusion as evidenced by multiple techniques, including visual element mapping by electron energy loss spectroscopy. As a result, we obtain inverted PSCs with an efficiency of 22.0% (21.3% certified), shelf life of 7000 h, T80 of 816 h under damp heat stress (compared to less than 20 h without Cl6SubPc), and initial performance retention of 98% after 2000 h at 80 °C in inert environment, 90% after 2034 h of illumination and maximum power point tracking in ambient for encapsulated devices and 95% after 1272 h outdoor testing ISOS-O-1. Our strategy and results pave a new way to move PSCs forward to their potential commercialization solidly.

Layer-by-layer anionic diffusion in two-dimensional halide perovskite vertical heterostructures.

Anionic diffusion in a soft crystal lattice of hybrid halide perovskites affects their stability, optoelectronic properties and the resulting device performance. The use of two-dimensional (2D) halide perovskites improves the chemical stability of perovskites and suppresses the intrinsic anionic diffusion in solid-state devices. Based on this strategy, devices with an enhanced stability and reduced hysteresis have been achieved. However, a fundamental understanding of the role of organic cations in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Here we demonstrate the first quantitative investigation of the anionic interdiffusion across atomically flat 2D vertical heterojunctions. Interestingly, the halide diffusion does not follow the classical diffusion process. Instead, a 'quantized' layer-by-layer diffusion model is proposed to describe the behaviour of the anionic migration in 2D halide perovskites. Our results provide important insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform with an enhanced stability for heterostructure integration.

Halide Diffusion in MAPbX3: Limits to Topotaxy for Halide Exchange in Perovskites

We study halide exchange in the prototypical halide perovskite, methylammonium lead trihalide, MAPbX3 (X = halide), to test and possibly experimentally use halide diffusion in these materials. We u...

Suppressed Interdiffusion and Degradation in Flexible and Transparent Metal Electrode-Based Perovskite Solar Cells with a Graphene Interlayer.

Metal-based transparent conductive electrodes (TCEs) are attractive candidates for application in indium tin oxide (ITO)-free solar cells due to their excellent electrical conductivity and cost effectiveness. In perovskite solar cells (PSCs), metal-induced degradation with the perovskite layer leads to various detrimental effects, deteriorating the device performance and stability. Here, we introduce a novel flexible hybrid TCE consisting of a Cu grid-embedded polyimide film and a graphene capping layer, named GCEP, which exhibits excellent mechanical and chemical stability as well as desirable optoelectrical properties. We demonstrated the critical role of graphene as a protection layer to prevent metal-induced degradation and halide diffusion between the electrode and perovskite layer; the performance of the flexible PSCs fabricated with GCEP was comparable to that of their rigid ITO-based counterparts and also exhibited outstanding mechanical and chemical stability. This work provides an effective strategy to design mechanically and chemically robust ITO-free metal-assisted TCE platforms in PSCs.

Barrier Design to Prevent Metal-Induced Degradation and Improve Thermal Stability in Perovskite Solar Cells

Metal-contact-induced degradation and escape of volatile species from perovskite solar cells necessitate excellent diffusion barrier layers. We show that metal-induced degradation limits thermal stability in several perovskite chemistries with Au, Cu, and Ag gridlines even when the metal is separated from the perovskite by a layer of indium tin oxide (ITO). Channels in a sputtered ITO layer that align with perovskite grain boundaries are pathways for metal and halide diffusion into or out of the perovskite. Planarizing the perovskite morphology with a spin-cast organic charge-transport layer results in a subsequently deposited ITO layer that is uniform and impermeable. We show that it is critical to seal the edges of the active layers to prevent escape of volatile species. We demonstrate 1000 h thermal stability at 85 °C in CH3NH3PbI3 solar cells with complete-coverage silver contacts. Our barrier layer design enables long-term thermal stability of perovskite solar cells, a critical step to commercialization.