Ion Migration Process Research Articles

Zr-metal–organic framework based cross-layer-connection additive for stable halide perovskite solar cells

The poor stability of metal halide perovskite seriously affects the large-scale commercial application of perovskite solar cells (PSCs). Here, a large-particle Zr-based metal–organic framework (Zr-MOF), e.g. UiO-66 (NH2), is added to the precursor solution of SnO2 electron transport layer (ETL), realizing a cross-layer modification of (1) ETL, (2) perovskite active layer and (3) theirs interface simultaneously. We find that the SnO2 particles are dispersed into the ordered porous scaffold of Zr-MOF, to form an ETL with improved crystallinity, reduced defect states and increased contact area with active perovskite film. For the subsequently deposited perovskite layer, the crystallization along the vertical direction is significantly optimized, and the defects are passivated by the bottom-up infiltrated Zr-MOF, so as to suppress the ion migration process. The power conversion efficiency (PCE) of one-step fabricated Zr-MOF-modified MAPbI3 based PSCs is optimized from 17.48% to 18.25%, and remaining 99% of its initial value after storage for 800 h under ambient conditions with the relative humidity over 60%. This work provides a simple cross-layer passivation strategy to improve the performance and stability of PSCs using highly designable MOF materials.

Mapping the pathways of photo-induced ion migration in organic-inorganic hybrid halide perovskites

Organic-inorganic hybrid perovskites exhibiting exceptional photovoltaic and optoelectronic properties are of fundamental and practical interest, owing to their tunability and low manufacturing cost. For practical applications, however, challenges such as material instability and the photocurrent hysteresis occurring in perovskite solar cells under light exposure need to be understood and addressed. While extensive investigations have suggested that ion migration is a plausible origin of these detrimental effects, detailed understanding of the ion migration pathways remains elusive. Here, we report the characterization of photo-induced ion migration in perovskites using in situ laser illumination inside a scanning electron microscope, coupled with secondary electron imaging, energy-dispersive X-ray spectroscopy and cathodoluminescence with varying primary electron energies. Using methylammonium lead iodide and formamidinium lead iodide as model systems, we observed photo-induced long-range migration of halide ions over hundreds of micrometers and elucidated the transport pathways of various ions both near the surface and inside the bulk of the samples, including a surprising finding of the vertical migration of lead ions. Our study provides insights into ion migration processes in perovskites that can aid perovskite material design and processing in future applications.

Electric-Controlled Resistive Switching and Different Synaptic Behaviors in p⁺-Si/n-ZnO Heterojunction Memristor

In this study, the resistive switching (RS) and different synaptic behaviors have been observed by controlling the different applied bias voltages in the p <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{+}$</tex-math> </inline-formula> -Si/n-ZnO heterojunction memristor. After a big forming voltage, the formation and rupture of the oxygen vacancy filaments model has been proposed, and different migration amount of oxygen ions realizes the multiple RS behaviors. Prior to the electroforming, different synaptic behaviors have been observed dependent on the applied bias voltage based on the oxygen ion migration processes. Under small bias ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt;$</tex-math> </inline-formula> 2.5 V), the oxygen ion of ZnO film first migrates and inters into p/n-junction barrier region, leading to the increasing barrier height/width. With increasing voltage, the electric field will further induce oxygen ion migration and entrance to Si film, leading to the decreasing barrier height/width. Several synaptic functions are demonstrated, such as nonlinear transmission characteristics, long-term memory (LTM)/short-term memory (STM) and “learning-experience” behavior. Multifunction memristive behaviors and these mechanisms research are important to design and realize the multifunction devices for decreasing the manufacturing complexity.

Multilevel resistive switching in stable all-inorganic n-i-p double perovskite memristor

Memristors are promising information storage devices for commercial applications because of their long endurance and low power consumption. Particularly, perovskite memristors have revealed excellent resistive switching (RS) properties owing to the fast ion migration and solution fabrication process. Here, an n-i-p type double perovskite memristor with "ITO/SnO2/Cs2AgBiBr6/NiOx/Ag" architecture was developed and demonstrated to reveal three resistance states because of the p-n junction electric field coupled with ion migration. The devices exhibited reliable filamentary with an on/off ratio exceeding 50. The RS characteristics remained unchanged after 1000s read and 300 switching cycles. The synaptic functions were examined through long-term depression and potentiation measurements. Significantly, the device still worked after one year to reveal long-term stability because of the all-inorganic layers. This work indicates a novel idea for designing a multistate memristor by utilizing the p-n junction unidirectional conductivity during the forward and reverse scanning.

Understanding and Hindering Ion Migration in Er,Yb:LiLuF4 Core–Shell Nanoparticles for Nanothermometers with Enhanced Photoluminescence

Developing core–shell nanoparticles is a common approach to improve the luminescence performance of nanomaterials, as an inert shell shields the core, which is doped with luminescent ions, from the quenching influence of the environment. Core-only nanoparticles are susceptible to solvent overtones, which can couple with the emissive dopants on the surface and quench their intensity. This is undeniably highly undesired for any luminescence application. It was recently shown by some of us that previously unreported ion migration takes place in 2%Er,18%Yb:LiLuF4@LiLuF4 core–shell nanoparticles, which was proven both with energy-dispersive X-ray (EDX) maps and high-temperature luminescence measurements. These findings lead us to the investigation presented here, which explores in detail how to hinder the migration of Er3+ and Yb3+ ions in LiLuF4 and LiYF4 hosts by implementing an interface region. First, different synthesis routes were explored to see if the chosen approach had any effect on the ion migration process from the doped core into the inert shell in a homogeneous core–shell system. Next, a LiYF4 inert shell was grown around a 2%Er,18%Yb:LiLuF4 core forming a heterogeneous core–shell system. We observed that the heterogeneous 2%Er,18%Yb:LiLuF4@LiYF4 and LiLuF4@2%Er,18%Yb:LiYF4 core–shell combinations showed significantly less ion migration based on EDX maps and high-temperature luminescence measurements exhibiting a behavior similar to the well-studied 2%Er,18%Yb:NaYF4@NaYF4 core–shell structures that, according to previous studies, show no or very little ion migration.

Understanding the Role of Small Cations on the Electroluminescence Performance of Perovskite Light‐Emitting Diodes

AbstractThe delicate engineering of monovalent cations in perovskite material has led to continuous performance breakthroughs and stability improvement for the perovskite light‐emitting diodes (PeLEDs). However, the exact role of A‐site cations on the electroluminescence (EL) performance and degradation mechanism of PeLEDs has not been systematically answered yet. Herein, it is demonstrated that the most commonly used methylammonium cation (MA+) has an adverse effect on the electrochemical reaction at the interface between perovskite and metal‐oxide layer, leading to deteriorated EL performance as compared to that of the formamidinium cation (FA+)‐based perovskite. It reveals that the accelerated deprotonation process of MA+ under an electric field will aggravate the reaction between iodide and metal ion in oxide layer. The further substitution of a small portion of FA+ with inorganic cesium cation (Cs+) results in much enhanced crystallinity and enlarged crystal size, leading to an optimized peak external quantum efficiency of 21.3%. The ion migration process in the PeLEDs can be significantly suppressed with Cs+ incorporation, leading to a smaller roll‐off under large current density and an elongated half‐lifetime of 190.1 h under a current density of 20 mA cm‐2, representing one of the most stable PeLEDs based on 3D perovskite layer.

Suppression of halide migration and improved stability in double-coated cesium lead halide perovskite nanocrystals for application in down-conversion white-light-emitting diodes

In the past few years' cesium lead halide (CsPbX3: X = I, Br, Cl) perovskite nanocrystals (NCs) have shown tremendous progress in the field of white and color-tuning lighting technologies for their favorable color tunability, superior emission intensity, narrow emission spectra, and better color purity. However, the perovskite NCs degrade very easily in presence of water and are prone to the ion migration process. It's also very challenging to synthesize stable red-emitting perovskite NCs which is a major component for generating white light-emitting diodes (WLEDs). Here we report the synthesis of highly luminescent green-emitting Zn-doped CsPbX3 NCs via the ligand-assisted reprecipitation (LARP) synthesis method at high humid conditions (RH>60%). The red-emitting Zn-doped CsPbBrxI3−x NCs were synthesized from pre-synthesized Zn-doped CsPbBr3 NCs via the halide exchange process. We also grew polymer shells around the silica-coated NCs to further improve the stability. These double shelling around the NCs facilitates improved structural stability and demonstrates higher resistance to the halide exchange process among different NCs. Finally, we fabricated WLEDs by mixing double-coated green and red NCs and deposited them on a blue LED chip. The optimized WLED device exhibited a luminous efficiency of 28.08 Lm/W, a color gamut of 106.67%, and CIE chromaticity coordinates of (0.334, 0.326). Our research work provides very insightful ideas for synthesizing stable color-tunable NCs and also the fabrication of single-layer WLEDs, which will boost the development of high-performance lighting technologies.

Non-invasive probing of dynamic ion migration in light-emitting electrochemical cells by an advanced nanoscale confocal microscope

In this study, we firstly propose an optical approach to investigate the ion profile of organic films in light-emitting electrochemical cells (LECs) without any invasive sputtering processes. In contrast to previous literatures, this pure optical strategy allows us to record clear and non-destructive ion profile images in the (Ru(dtb-bpy)3(PF6)2) consisted organic layer without interferences of complex collisions from the bombardment of secondary sputter induced ions in a conventional time-of-flight secondary ion mass spectrometry. By using the advanced position sensitive detector (PSD)-based Nanoscale Confocal Microscope, ion distribution profiles were successfully acquired based on the observation of nanoscale optical path length difference by measuring the refractive-index variation while the thickness of the LEC layer was fixed. Dynamic time-dependent ion profile displayed clear ion migration process under a 100 V applied bias at two ends of the LEC. This technique opens up a new avenue towards the future investigations of ion distributions inside organic/inorganic materials, Li-ion batteries, or micro-fluid channels without damaging the materials or disturbing the device operation.

Defects Passivation Strategy for Efficient and Stable Perovskite Solar Cells

AbstractLarge area, high efficiency, and long‐term device stability are integral performance requirements for the commercialization of perovskite solar cells (PSCs). However, the high defect states density formed during the process of device preparation plays a cardinal effect in the charge recombination and ion migration process. The existence of defect states will not only seriously impair the stability but also hinder the efficiency enhancement of PSCs. A large number of studies aiming to solve this problem show that the defects can be reduced to enhance the photovoltaic performance and stability of derived PSCs by tailoring the terminal groups on the perovskite surface and modifying the surface chemical environment. Herein, a systematic and comprehensive review on the development of interfaces passivation (including the anode buffer layer, electron transport layer, the perovskite grains, and hole transport layer) for PSCs is summarized. Based on the defect passivation engineering, various feasible strategies are proposed to break the bottleneck of the commercialization process PSCs.

Light-Controlled Reconfigurable Optical Synapse Based on Carbon Nanotubes/2D Perovskite Heterostructure for Image Recognition.

Two-dimensional (2D) halide perovskite material is characterized by a mixed conducting behavior that possesses both electronic and ionic conductivity. The study on the influence of the light on ion migration in the 2D perovskite is helpful to improve the performance of perovskite-based optoelectronic devices. Here, we constructed an exfoliated 2D perovskite/carbon nanotubes (CNTs) heterostructure optical synapse, in which CNTs can be used as nanoprobes to qualitatively observe the ion aggregation or dissipation process in 2D perovskite, and found that light significantly changes the memory curve of the reconfigurable optical synapses. Through the molecular dynamic simulation, the dynamic process of ion migration in the heterostructure was simulated and the electrostatic interaction effect of nonequilibrium charge distribution of CNTs on iodide ion was demonstrated. Finally, an effective light-controlled process was realized through the synapses, which in situ regulated the performance of the weight-value discretized BP (WD-BP) neural network. This work lays a foundation for the future development of intelligent nano-optoelectronic devices.

Synergistic effect of amide and fluorine of polymers assist stable inverted perovskite solar cells with fill factor > 83%

Exploring hole transporting materials (HTMs) with defect passivation function is an effective way to suppress the defect-induced non-radiative recombination (NRR) process in perovskite solar cells (PSCs). To date, most of these passivation materials are mainly devoted to passivate the uncoordinated Pb2+ of perovskite layer. However, ion migration processes of MA+ and I- in PSCs also cause a series of negative effects, which severely deteriorate the efficiency and device stability. Thus far, very few HTMs that can simultaneously passivate both the MA+/Pb2+ cations and I- anion defects in ABX3 structured perovskite. Herein, three dopant-free HTMs (BNs, BNs-F, and BNo-F) featuring multisite defect passivation function were developed for inverted PSCs. The carbonyl groups C = O and N–H in the amide linkers of BNs-F contributed to passivate the uncoordinated Pb2+ and the halide related defects, respectively. Simultaneously, the F atoms in bridges could interact with MA+ in perovskite via N-H‧‧‧F bond. Interestingly, an intermolecular hydrogen bond (O = C-N-H‧‧‧F) between amide linkers and F atoms was found in our work, which enabled BNs-F to achieve a high hole mobility of 2.42 × 10-4 cm2 V−1 s−1. Consequently, benefiting from the alleviated NRR behavior and enhanced charge transport ability, the optimized devices based on BNs-F realized a remarkable fill factor of 0.834, accompanying a champion efficiency up to 20.56% with superior long-term stability, exceeding those with BNs (19.03%), BNo-F (19.07%) and PTAA (19.56%). Our findings provide a guidance for design molecules with comprehensive passivation effect on MA+/Pb2+/I- defects to achieve efficient and stable PSCs.

Coupling between Ion Drift and Kinetics of Electronic Current Transients in MAPbBr3 Single Crystals.

The optoelectronic properties of halide perovskite materials have fostered their utilization in many applications. Unravelling their working mechanisms remains challenging because of their mixed ionic–electronic conductive nature. By registering, with high reproducibility, the long-time current transients of a set of single-crystal methylammonium lead tribromide samples, the ion migration process was proved. Sample biasing experiments (ionic drift), with characteristic times exhibiting voltage dependence as ∝ V–3/2, is interpreted with an ionic migration model obeying a ballistic-like voltage-dependent mobility (BVM) regime of space-charge-limited current. Ionic kinetics effectively modify the long-time electronic current, while the steady-state electronic currents’ behavior is nearly ohmic. Using the ionic dynamic doping model (IDD) for the recovering current at zero bias (ion diffusion), the ionic mobility is estimated to be ∼10–6 cm2 V–1 s–1. Our findings suggest that ionic currents are negligible in comparison to the electronic currents; however, they influence them via changes in the charge density profile.

First-principles simulations of Li boracites Li4B7O12Cl and Li5B7O12.5Cl

Lithium boracite crystals have been identified as promising ion conductors for possible use in all-solid-state batteries. With the help of first-principles modeling techniques, we are able to show that these materials have structures with natural interstitial sites which play important roles in Li ion migration processes. The arrangements of these natural interstitial sites follow from group theory analyses of the computed and experimentally analyzed structures. Specifically, the low-temperature $\ensuremath{\alpha}$ phase of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ is computed to have the face-centered rhombohedral $R3c$ structure which is closely related to an ideal face-centered cubic $F\overline{4}3c$ structure with 24 natural interstitial sites per conventional unit cell. Li ion diffusion in this material is found to proceed largely by a concerted motion involving these interstitial sites and two neighboring host lattice sites, consistent with the large measured ionic conductivity of this material. Adding one addition ${\mathrm{Li}}_{2}\mathrm{O}$ cluster per formula unit to ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ forms ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$, which crystallizes in the face-centered cubic $F23$ structure, a subgroup of the $F\overline{4}3c$ structure. While these $F23$ crystals have 16 natural interstitial sites per conventional unit cell, their distribution is such that they do not participate in Li ion diffusion mechanisms, as is consistent with the negligible ionic conductivity measured for this material. Phonon spectral analyses of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ in its $R3c$ structure and ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$ in its $F23$ structure show that both crystals are dynamically stable. Chemical stability of these materials is indicated by convex hull analysis of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ and ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$ with respect to their building blocks of LiCl, ${\mathrm{Li}}_{2}\mathrm{O}$, and ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$.

Ion Migration Process and Cyclic Stability of Voltage-Induced Vanadium Dioxide Phase Transition

Ion Migration Process and Cyclic Stability of Voltage-Induced Vanadium Dioxide Phase Transition

The ion migration process and cyclic stability of voltage-induced vanadium dioxide phase transition

The composite film structure of LiF/VO2 can effectively suppress the ion capture problem in single layer VO2, thereby improving the phase transition capability of voltage-induced ion-gated VO2.

Reduced Barrier for Ion Migration in Mixed-Halide Perovskites.

Halide alloying in metal halide perovskites is a useful tool for optoelectronic applications requiring a specific bandgap. However, mixed-halide perovskites show ion migration in the perovskite layer, leading to phase segregation and reducing the long-term stability of the devices. Here, we study the ion migration process in methylammonium-based mixed-halide perovskites with varying ratios of bromide to iodide. We find that the mixed-halide perovskites show two separate halide migration processes, in contrast to pure-phase perovskites, which show only a unique halide migration component. Compared to pure-halide perovskites, these processes have lower activation energies, facilitating ion migration in mixed versus pure-phase perovskites, and have a higher density of mobile ions. Under illumination, we find that the concentration of mobile halide ions is further increased and notice the emergence of a migration process involving methylammonium cations. Quantifying the ion migration processes in mixed-halide perovskites shines light on the key parameters allowing the design of bandgap-tunable perovskite solar cells with long-term stability.

(Invited) Nanoscale Probing of Field-Driven Ion Migration in TaOx for Neuromorphic Memristor Applications

In semiconducting metal oxides, application of electric fields often causes migration of the constituent ions (oxygen ions, metal cations, and ionic vacancies) via ionic conduction and diffusion. The field-driven ion migration has been considered a key phenomenon in the recent development of oxide-based resistive random access memory (ReRAM) and memristive neuromorphic devices because their operations are generally made through the migration and resultant changes in the valence/resistance states of the oxide layers. Semiconducting amorphous TaOx is known as a practical material of switching layers in ReRAMs, and moreover, has recently been considered of critical importance in the development of neuromorphic hardware because of its variety of neuromorphic resistive switching functions. Many important functions, including multilevel (or analog) resistive switching, stochastic switching, and spike-rate/spike-timing dependent plasticity, have been demonstrated in junctions with a structure of metal/TaOx/metal, and the origin of the neuromorphic functionality has been ascribed to the high ionic conductivities (both for O2− and Ta5+) and degree of structural freedom of amorphous TaOx.Regarding the mechanisms of the neuromorphic switching, however, limited understanding has been achieved in the previous works. The mechanisms have so far been inferred from the switching behaviors in electrical measurements or numerical modeling because of the experimental difficulty in observing nanoscale ion migration during the neuromorphic resistive switching. To directly design of the neuromorphic functions and establish oxide-based neuromorphic computing, therefore, experimental probing of the ion migration processes is now importantly required for neuromorphic resistive switching of TaOx.In this study, we fabricated ultra-flat thin films of amorphous TaOx (3.0–10 nm thick) with a root-mean-square roughness of <0.2 nm by the pulsed laser deposition method, which will offer suitable conditions for ion migration analysis by probe microscopy. In the conductive tip atomic force microscopy investigations, we observed a stable resistive switching phenomenon at arbitrary positions on the TaOx surface by voltage applications from a probe. The positional independence of the switching characteristics ensures the experimental advantages of the ultra-flat films in the investigations. Additionally, this indicates that "easy spots" for resistive switching, of which the formation has been suggested in other metal oxides, are not present in the thin films of amorphous TaOx, and initial clustering of oxygen vacancies is not importantly involved in the switching. When multiple voltage stresses of >3.0 V were applied from a probe, we observed a multilevel switching phenomenon in the TaOx thin films. In the measurements, we observed that the multilevel switching appears with very small amount of structural changes of TaOx (<1.0 nm in the out-of-plane direction), and relatively large-scale structural deformations (2.0 nm in the out-of-plane direction and 150 nm in the in-plane direction) started to be observed only after breakdown-like behaviors in the resistance with formation of a 10 nm-sized conductive spot. This indicates that the intrinsic ion migration distance involved in the multilevel switching of TaOx is in a range of <1.0 nm in out-of-plane direction, which is smaller than previous predictions. From these results, the spatial extents of neuromorphic resistive switching of TaOx were revealed in this study, which will offer important insights into the process development for TaOx-based neuromorphic memristor.

Visualization and quantitative research of ion migration process in cement-based materials

Visualization and quantitative research of ion migration process in cement-based materials

Coupling effect of the connected pores and pore solution on chloride ion migration in cement-based materials

In this study, a new method for determining the connected pore porosity, tortuosity and constrictivity was developed based on AC impedance tests. The effects of the pore factor χ1, pore solution factor χ2, pore–pore solution factor χ3 and coupling factor χ on the non-steady state chloride ion migration coefficient (Dm) were evaluated. The results indicated that χ1, χ2, χ3 and any single parameter did not have good correlations with Dm, and χ had an excellent linear correlation with Dm. This confirms that the coupling effect of the pores, the pore solution and their interaction control the chloride ion migration process. The contributions of χ1, χ2 and χ3 to Dm accounted for 54.44%, 13.05%, and 31.27%, respectively. Therefore, the migration of chloride ions can be controlled to a large extent by reducing χ1 and χ3, which can be obtained through a reduction in the connected porosity and the accompanying reduction in δ. This means that the connected pores play a decisive role in controlling the migration of chloride ions in cement-based materials, whose conductivity can be used for quickly determining the extreme value of the chlorine-related durability.

Ion migration process and influencing factors in inland river basin of arid area in China: a case study of Shiyang River Basin.

A thorough understanding of the processes and driving factors of ion migration, dilution, and enrichment in arid inland river basins is the basis for implementing water resources management. In this study, we analyzed the water chemistry of streamflow, groundwater, and precipitation and the behavior of main elements in the Shiyang River Basin by means of the hydrochemical diagram and multivariate statistical analysis. The spatial variation of water chemistry was obvious, and the conversion between different water bodies was frequent. The ions migrated from the mountain area to the oasis and desert and accumulated near the terminal lake finally. There were obvious differences in hydrochemistry between surface water and groundwater. From the mountain to the basin, the hydrochemical type of surfer water has varied, and the hydrochemical type of groundwater has changed from Ca-Cl type to Na-Cl type. The hydrochemistry of the basin was controlled by silicate weathering. However, the influence of water-rock interaction on surface water and groundwater was different, and the surface water was more complex. Significantly, agricultural activities and sewage discharge had a negative impact on the water environment. Interbasin water transfer (IBWT) was a form of external ions input from outside the basin, which affected the chemical characteristics of surface water in the lower reaches to a certain extent. In arid areas, human impact on water chemistry needs to be paid attention. These results are helpful to strengthen the understanding of the relationship between different regions and different water bodies in the arid basin.