Migration Of Iodide Ions Research Articles

Suppressing Ion Migration in MAPbI3 Polycrystalline Wafer with BMIMBF4 Ionic Liquid for X‐ray Detection and Imaging

Abstract3D lead‐halide perovskite wafers, recognized for their superior photoelectronic properties and robust fabrication, are promising candidates for advanced X‐ray detectors. However, severe ion migration in 3D perovskite wafers leads to dark current baseline drift and long‐term operational instability, hindering their further development. Herein, 3D MAPbI3 polycrystalline wafers with suppressed ion migration are prepared using the ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIMBF4) and systematically investigated for X‐ray detection. The BMIMBF4‐passivated MAPbI3 wafers exhibit a low defect trap density (ntrap) of 9.45 × 109 cm−3, a high ion activation energy (Ea) of 0.51 eV, a notable iodide ion migration energy barrier of 0.65 eV, and a decreased dark current drift (Idrift) of 3.56 × 10−4 nA cm−1 s−1 V−1. The optimized MAPbI3 wafer‐based detectors exhibit a high sensitivity of 12088.8 µC Gyair−1 cm−2, a low detection limit (LoD) of 107.8 nGyair s−1, and strong operational stability in X‐ray detection. Moreover, these detectors demonstrate outstanding X‐ray imaging capabilities, achieving a high spatial resolution of 5.17 lp mm−1. Consequently, the utilization of ionic liquids with pseudo‐halide anions and polarized electron density distribution provides an innovative strategy for passivating 3D perovskite, advancing the field of powder‐pressed perovskite wafer X‐ray detection.

Robust chelated lead octahedron surface for efficient and stable perovskite solar cells

PbI6 octahedron as a fundamental framework endows the perovskite with excellent photoelectric properties, but also the defective and flimsy surface. Here, we report that the treatment of perovskite surface by bidentate ligands molecules N, N’-Dimethyl-1,2-ethanediamine can in-situ form a lead iodide chelates layer with excellently robust chelated lead octahedron, leading to effectively stabilize and passivate the underlying perovskite. The strong chelation with the lead enables the surface to largely inhibit the defects generation, iodide ion migration and skeleton collapse under external stimuli. It also prolongs the carrier lifetime and adjusts the surface energy-level of perovskite. The resultant perovskite solar cells deliver a power conversion efficiency of 25.7% (certified 25.04%) and retain >90% of their initial value after almost 1000 hours aging at maximum power point under simulated AM1.5 illumination.

Columnar Macrocyclic Molecule Tailored Grain Cage to Stabilize Inorganic Perovskite Solar Cells with Suppressed Halide Segregation

AbstractSolidifying the soft lattice of all‐inorganic mixed‐halide perovskites is of great importance to restrain the notorious halide segregation under persistent light illumination. Herein, a multifunctional columnar macrocyclic molecule additive, namely cucurbituril into perovskite precursor to enhance the crystallization and reduce the defect density in the final perovskite film is introduced. Based on the theoretical calculation and simulation, the cucurbituril molecule with a strong double‐ended negatively‐charged cavity surrounded by terminated oxygen atoms not only coordinates with dangling Pb2+ ions to form host‐guest complexation but also induces an electric dipole field at perovskite grain boundary to effectively repel the iodide ion migration from the inside grain to the defective boundary, significantly suppressing the halide segregation and improving the device performance. As a result, the carbon‐based, all‐inorganic CsPbI2Br solar cell achieves an enhanced efficiency of 15.59% with great tolerance to environmental stresses. These findings provide new insights into the development of a novel passivation strategy with macrocyclic molecules for making high‐efficiency and stable perovskite solar cells.

Unraveling Differences in the Effects of Ammonium/Amine-Based Additives on the Performance and Stability of Inverted Perovskite Solar Cells.

Additive engineering, with its excellent ability to passivate bulk or surface perovskite defects, has become a common strategy to improve the performance and stability of perovskite solar cells (PVSCs). Among the various additives reported so far, ammonium salts are considered an important branch. It is worth noting that although both ammonium-based additives (R-NH3 +) and amine-based additives (R-NH2) are derivatives of ammonia (NH3), the functions of the two can be easily confused due to their structural similarities. Moreover, there is no comprehensive comparative analysis of them in the literature. Here, the differences between phenethylammonium iodide (PEA+) and phenethylamine (PEA) additives are revealed experimentally and theoretically. The results clearly show that PEA outperforms PEA+ in terms of device performance and stability based on the following three factors: i) PEA's defect passivation capability is superior to that of PEA+; ii) PEA has better hydrophobicity to hinder water ingress; and iii) PEA completely improves the stability of PVSCs by enhancing thermal stability and inhibiting iodide migration in perovskite more effectively than PEA+. As a result, the power conversion efficiency (PCE) of the inverted methylammonium triiodide (MAPbI3) device using PEA increases by ≈15% to over 21%. More importantly, this device exhibits greater ability to prevent water invasion, thermal-induce degradation, and inhibit iodide ion migration, resulting in better long-term stability.

Lattice Strain Regulation and Halogen Vacancies Passivation Enable High-Performance Formamidine-Based Perovskite Solar Cells.

Formamidinium lead iodide (FAPbI3) perovskite has lately surfaced as the preferred contender for highly proficient and robust perovskite solar cells (PSCs), owing to its favorable bandgap and superior thermal stability. Nevertheless, volatilization and migration of iodide ions (I-) result in non-radiating recombination centers, and the presence of large formamidine (FA) cations tends to cause lattice strain, thereby reducing the power conversion efficiency (PCE) and stability of PSCs. To solve these problems, the lead formate (PbFa) is added into the perovskite solution, which effectively mitigates the halogen vacancy and provides tensile strain outside the perovskite lattice, thereby enhancing its properties. The strong coordination between the C═O of HCOO- and Pb-I backbones effectively immobilizes anions, significantly increases the energy barrier for anion vacancy formation and migration, and reduces the risk of lead ion (Pb2+) leakage, thereby improving the operation and environmental safety of the device. Consequently, the champion PCE of devices with Ag electrodes can be increased from 22.15% to 24.32%. The unencapsulated PSCs can still maintain 90% of the original PCE even be stored in an N2 atmosphere for 1440h. Moreover, the target devices have significantly improved performance in terms of light exposure, heat, or humidity.

Unraveling Conductive Filament Formation in High Performance Halide Perovskite Memristor

AbstractHalide perovskites (HPs) are promising materials for memristor devices because of their unique characteristics. In this study, nonvolatile resistive switching memory devices based on thick MAPbI3 perovskite (800 nm) films with structure FTO/MAPbI3/polymethyl methacrylate (PMMA)/Ag are presented. Reproducible and reliable bipolar switching characteristics are demonstrated with an ultra‐low operating voltage (−0.1 V), high ON/OFF ratio (106), endurance (>2 × 103 times) and a record retention time (>105 s). The I–V curve of the first cycle exhibits self‐formed conductive filaments. These are attributed to the presence of metallic Pb resulting from an excess of PbI2 in the perovskite film. The subsequent activation process involves the formation of conductive filaments, consisting of either iodide vacancies or migrated charged metals. Numerical simulations are then carried out to understand the nature of these conductive filaments and the role of the internal electric field in the migration of iodide ions, iodide vacancies, and Ag cations. Finally, an exhaustive model is proposed that explains the set and reset processes governing the first voltage cycle and the steady state, at different voltage ranges. In summary, this work offers a novel and thorough perspective of the complete resistive switching (RS) behavior in a MAPbI3/buffer/Ag memristor, supported by numerical simulations.

Light-induced transformation of all-inorganic mixed-halide perovskite nanoplatelets: ion migration and coalescence

When exposed to light, the colloidal perovskite nanoplatelets (NPLs) in the film can fuse into larger grains, and this phenomenon was thought to be closely related to ion migration. However, the available CsPbBr3 NPLs are not conducive to directly distinguishing this hypothesis. Herein, we prepare mixed-halide perovskite CsPbBr2.7I0.3 NPLs by a ligand-assisted reprecipitation method and investigate the photoluminescence evolution of NPLs under laser irradiation. At a low-irradiation intensity, 4.5-monolayer NPLs exhibit blue-shifted photoluminescence peaks due to the migration of iodide ions. Under higher laser fluence, a new photoluminescence component appears in the long wavelength region after the spectral blue shift, which is attributed to the coalescence of NPLs according to transmission electron microscopy analysis. A similar spectral evolution is also observed in 8-monolayer NPLs, while only the spectral blue shift caused by ion migration is detected in cuboidal CsPbBr2.7I0.3 nanocrystals. The use of strong bonding ligands can inhibit the fusion process of the NPLs, but not to impede ion migration, suggesting that fusion requires ligand detachment rather than ion migration. Similar suppression effects can be achieved in a vacuum atmosphere. Moreover, we demonstrate that mixed-halide NPLs can be used to realize anti-counterfeiting applications with superior photosensitivity.

Improving Thermal Stability of Perovskite Solar Cells by Suppressing Ion Migration

Ion migration presents a formidable obstacle to the stability and performance of perovskite solar cells (PSCs), hindering their progress toward commercial feasibility. Herein, the degradation mechanism of PSCs caused by iodide ion migration, which leads to abnormal changes in photoluminescence transients at the buried interface of perovskite films, is investigated. In light of this problem, a novel strategy is proposed to mitigate ion migration by introducing poly(2‐vinylnaphthalene) into poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] as the hole transport layer with improved ion‐blocking capability. Consequently, this layer effectively reduces defect concentration, suppresses ion migration, and modulates energy level alignment, leading to an impressive efficiency exceeding 23% for doctor‐bladed FAPbI3 PSCs. Moreover, the corresponding unencapsulated devices demonstrate remarkable durability, maintaining over 80% of their initial value after undergoing rigorous stress tests in accordance with the International Electrotechnical Commission 61215 standard for temperature, humidity, and illumination. These tests include 1000 h of thermal cycling and a long‐term operational test lasting 600 h under maximum power point tracking.

Efficient and Highly Stable Lateral Perovskite Single‐Crystal Solar Cells through Crystal Engineering and Weak Ion Migration

AbstractThe lateral device structure for perovskite solar cells (PSCs) has garnered significant attention, primarily due to its elimination of the need for expensive transparent electrodes. However, the performance of lateral devices, which are more sensitive to crystal quality and charge carrier transport bottlenecks, has lagged far behind the predominant vertical PSCs. Herein, by modulating the crystal nucleation and growth processes of thin FA0.75MA0.25PbI3 (FA = formamidinium; and MA = methylammonium) single crystals, crystal quality and carrier transport are improved, resulting in a power conversion efficiency (PCE) of 12.64%, a record for lateral PSCs. Investigation of the device's stability reveals that iodide ion migration is suppressed due to a reduction in the iodide vacancy concentration combined with weak interface iodide ion migration. It is shown that the latter effect is a result of the perpendicular direction of the ion migration and the electric field in the lateral PSCs. Consequently, these lateral single‐crystal PSCs display remarkable operational stability, retaining 100% of their initial PCE after 1200 h of steady‐state output at the maximum power point voltage (Vmpp) under 1 sun illumination. This work highlights the advantages of lateral single‐crystal devices and their potential to address key ion migration issues of PSCs.

Suppressing Ion Migration by Synergistic Engineering of Anion and Cation toward High-Performance Inverted Perovskite Solar Cells and Modules.

Ion migration-induced intrinsic instability and large-area fabrication pose a tough challenge for the commercial deployment of perovskite photovoltaics. Herein, an interface heterojunction and metal electrode stabilization strategy is developed by suppressing ion migration via managing lead-based imperfections. After screening a series of cations and nonhalide anions, the ideal organic salt molecule dimethylammonium trifluoroacetate (DMATFA) consisting of dimethylammonium (DMA+) cation and trifluoroacetate (TFA-) anion is selected to manipulate the surface of perovskite films. DMA+ enables the conversion of active excess and/or unreacted PbI2 into stable new phase DMAPbI3, inhibiting photodecomposition of PbI2 and ion migration. Meanwhile, TFA- can suppress iodide ion migration through passivating undercoordinated Pb2+ and/or iodide vacancies. DMA+ and TFA- synergistically stabilize the heterojunction interface and silver electrode. The DMATFA-treated inverted perovskite solar cells and modules achieve a maximum efficiency of 25.03% (certified 24.65%, 0.1 cm2) and 20.58% (63.74 cm2), respectively, which is the record efficiency ever reported for the devices based on vacuum flash evaporation technology. The DMATFA modification results in outstanding operational stability, as evidenced by maintaining 91% of its original efficiency after 1520 h of maximum power point continuous tracking.

Substitution of lead with tin suppresses ionic transport in halide perovskite optoelectronics.

Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles' heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development.

A dual-mode assay kit using a portable potentiostat connected to a smartphone via Bluetooth communication and a potential-power angle-based paper device susceptible for low-cost point-of-care testing of iodide and dopamine

BackgroundConsidering that the brain controls most of the body's activities, it is very important to measure the factors affecting its function, such as dopamine and iodide. Due to the growing population in the world, it is necessary to provide fast, cheap and accurate methods with the capability of on-site analysis and without the need for invasive sampling and operator skill. As a result, there is a strong desire to replace laboratory instruments with small sensors for point-of-care testing. Paper-based analytical devices (PADs) are one of the popular zero-cost approaches to achieve this goal. ResultsWe developed a simple and disposable diagnostic paper system based on electroanalytical and potential-power angle-based methods. First, we prepared an angle-based analytical system capable of performing semi-quantitative iodide analysis simply by reading the colored angle traveled. This system design is based on a channel containing complex reagents and two pencil-drawn electrodes to apply a constant voltage accelerating the anions migration. Meanwhile, a three-electrode system based on conductive pencil graphite is developed to measure dopamine concentration based on linear sweep voltammetry. For the quantitative analysis, the voltammetric data was wirelessly transmitted to a mobile device via Bluetooth communication. In this context, a power supply providing the required voltage for the migration of iodide ions, a portable potentiostat system, and a mobile application for measuring dopamine were developed. The calibration curves for I− and dopamine range from 3.5 × 10−4-47.0 × 10−4 and 10.0 × 10−6-1000.0 × 10−6 mol L−1 with LODs of 2.3 × 10−4 and 5.0 × 10−6 mol L−1, respectively. Significance and noveltyA new portable dual-mode voltage-assisted integrated PAD platform was designed for iodide and dopamine analysis. The characteristics of this device allow non-experts to carry out in-field analysis using sub-100 μL saliva sample with a time-to-result of <10 min along with reducing the overall cost and operational complexity.

Mitigating lattice strain and phase segregation of mixed-halide perovskite films via dual chloride additive strategy toward highly efficient and stable perovskite solar cells

Mixed halide perovskites could be used to fabricate the high-efficient single junction perovskite solar cells (PSCs) and also be assembled into the tandem solar cells. However, those perovskites always suffered from microstrain and phase segregation issues, which tend to bring in extra non-radiative recombinations and aggravate the energy losses and degradation of PSCs. Here, we developed a dual chloride additive strategy to overcome these issues. Compared with the pristine and single additive films, the perovskite films with dual chloride additive possessed the lowest microstrain and the least defects, and thus the activation energy related to phase segregation of those films was improved from 40.21 kJ mol−1 to 59.08 kJ mol−1. Density functional theory revealed that the iodide ion migration also had been inhibited by the dual chloride additive as the energy barrier increased from 0.41 eV to 0.57 eV. PSCs with dual chloride additive showed a efficiency of 22.94%, higher than that of the pristine PSCs (20.62%) and the single additive PSCs. Moreover, the unencapsulated PSCs with dual chloride additive exhibited better working stability maintained 93% of their original efficiency after 1000 s of MPP tracking operation in ambient air, while their counterpart PSCs maintains 46%.

In Situ Observation of Photoinduced Halide Segregation in Mixed Halide Perovskite

Mixed–halide perovskites have emerged as a promising candidate for optoelectronics, due to their tunable optical properties. However, photoinduced phase segregation remains an obstacle for stable performance in solar cells. Here, we have conducted both bulk and nanoscale measurements to elucidate the mechanism of halide ion migration that leads to phase segregation. By utilizing Kelvin probe force microscopy (KPFM) under illumination, we have observed the time-evolution of ion migration to and from grain boundaries that agreed with bulk photoluminescence spectra of perovskites with a wide range of band gaps (1.67–1.88 eV), Cs0.15FA0.65MA0.20Pb(IxBr1–x)3 where x = 0.47–0.80. By visualizing the changes of band bending at grain boundaries, we deduce that halide segregation is dominantly caused by iodide ions, given faster ion migration in perovskite materials with a higher iodine content. Further, we verify that the changing rate of band bending at grain boundaries is consistent with the emerging rate of I-rich phase at grain boundaries, suggesting the influence of iodide ion migration toward grain boundaries on I-rich phase transition. This work will help provide insight for interpreting the mechanism of light-halide ion interactions.

Two Schiff base iodide compounds as iodide ion conductors showing high conductivity.

Here, we reported the crystal structures, dielectric and conducting properties of two Schiff base iodide compounds [m-BrBz-1-APy]I3 (1) and [o-FBz-1-APy]I3 (2). The Schiff base cations build irregular channel frameworks, and the polyiodide anions are located in the channel. The impedance spectra demonstrated that the two compounds show intrinsic iodide ion conductance with higher conductivity of 1.03(4) × 10-4 S cm-1 at 343 K for 1 and 4.94(3) × 10-3 S cm-1 at 353 K for 2. The dielectric modulus analysis further confirmed that the conductance contributed to the migration of iodide ions.

Flexible Threshold Switching Based on CsCu2I3 with Low Threshold Voltage and High Air Stability.

Halide perovskites featuring remarkable optoelectronic properties hold great potential for threshold switching devices (TSDs) that are of primary importance to next-generation memristors and neuromorphic computers. However, such devices are still in their infancy due to the unsolved challenges of high threshold voltage, poor stability, and lead-containing features. Herein, a unipolar TSD based on an all-inorganic halide perovskite of CsCu2I3 is demonstrated, exhibiting the fascinating attributes of a low threshold voltage of 0.54 V, a high ON/OFF ratio of 104, robust air stability over 70 days, a steep switching slope of 6.2 mV·decade-1, and lead-free composition. Moreover, the threshold voltage can be further reduced to 0.23 V using UV illumination to reduce the barrier of iodide ion migration. The multilevel threshold switching behavior can be realized through the modulation of either the compliance current or the scan rate. The TSD with mechanical compliance and transparency is also demonstrated. This work enriches TSDs with expanded perovskite materials, boosting the related applications of this emerging class of device families.

Inhibiting Ion Migration by Guanidinium Cation Doping for Efficient Perovskite Solar Cells with Enhanced Operational Stability

Perovskite solar cells (PSCs) develop great potential to make photovoltaic power generation systems more cost‐effective due to the high power conversion efficiency (PCE), low material cost, and easy fabrication. Alloyed A‐site cations surrounded by PbI6 octahedra play decisive roles in the crystal structure, bandgap, and phase stability, as well as ion migration. Herein, based on the temperature‐dependent ion conductivity measurement, the activation energy for iodide ion migration is systematically studied with different proportions of guanidinium cation (GA+) substitution. It is found that partial GA+ doping could effectively suppress iodide ion migration. The triple‐cation perovskite (MA0.8FA0.1GA0.1PbI3) PSCs achieve a PCE of 22.17% with superior operational stability maintaining 90% of their initial efficiency after 1200 h under continuous light soaking. Furthermore, it is extended to mini perovskite solar modules, 14 cm2 active area, and achieves a PCE of 19.18%.

The effect of multiple ion substitutions on halide ion migration in perovskite solar cells

This work shows how substitutions to the perovskite lattice at multiple sites can affect iodide ion migration. The triple cation perovskite, Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3, shows a higher barrier to iodide ion migration than materials with only substitutions at the A-site.

Rear Interface Engineering to Suppress Migration of Iodide Ions for Efficient Perovskite Solar Cells with Minimized Hysteresis

AbstractAccurate interface engineering can effectively inhibit iodide ion migration, thereby improving the stability and photovoltaic performance of perovskite solar cells (PvSCs). The time‐of‐flight secondary‐ion mass spectrometry reveals that in an aged n–i–p‐type PvSC, the iodide ions will move toward the rear side and enter the FTO cathode. In this regard, the authors describe a simple thermal evaporation strategy for introducing an NdCl3 interface layer (NdCl3‐IL) at the rear interface of perovskites to interdict the iodine ion migration pathway, leading to reduced trap densities throughout the whole perovskite region. As a result, a boosted open‐circuit voltage (VOC) is achieved, resulting in power conversion efficiency (PCE) up to 22.16% with negligible hysteresis. The NdCl3‐IL also enhances the device stability, maintaining 83% of initial PCE after the maximum‐power‐point tracking test for 100 h. More encouragingly, a certified PCE of 21.68% is demonstrated on a large‐area (1 cm2) device with combined 2D/3D passivation strategies.

Suppression of ion migration through cross-linked PDMS doping to enhance the operational stability of perovskite solar cells

Although perovskite solar cells have achieved great development in the device performance, the device stability is still a crucial issue hinder the further development. The ion migration is a main reason for the poor stability of the perovskite solar cells. Here, we use a cross-linking material, polydimethylsiloxane (PDMS) to dope the perovskite layer to improve the operational stability. We systematically studied the effect of curing agent content in PDMS on the device performance and stability. The characterization of perovskite film indicates that PDMS primarily doped in the perovskite layer and as well as enriched at the surface of perovskite film. Doping of PDMS promoted the crystallization of perovskite phase, leading to increase of short circuit density (JSC) and fill factor (FF) of the solar cells. In addition, PDMS doping improved the operational stability of device due to the suppressing the migration of iodide ions during aging. Moreover, the higher content of curing agents used in PDMS, the better stability of the devices. This work indicates PDMS doping the perovskite layer with a suitable content of curing agent can produce a better operational stability of the perovskite solar cells.