Iodide Migration Research Articles

Towards highly efficient and stable perovskite solar cells: suppressing ion migration by inorganic boric acid stabilizer

The poor stability of organic-inorganic perovskite solar cells is commonly ascribed to elevated ion migration, stemming from the low electronegativity of iodine. To address this issue, boric acid was chosen as a stabilizer for perovskite thin films. As a Lewis acid, the boric acid has a sp2 hybridized boron atom, which can readily accept a pair of electrons from iodine ion into their vacant unhybridized p orbital, and the formation of the Pb-O bond further increases the iodide migration barrier. The significantly increased barrier of the iodine ion migration was demonstrated by the improved phase stability of the perovskite film under an electric field and the obviously enhanced stability of the perovskite films under strong ultraviolet light. PSCs that included the BA stabilizer were able to achieve an enhanced PCE of 25.52%. The initial efficiency of BA-modified devices remained at 80% even after being aged for 1000hours at 85 ℃ under around 30% relative humidity (RH). When subjected to maximum power point tracking and 20-25% RH, the PCE of BA-modified devices maintained an initial efficiency of 80% after 1500hours.

Evaluation of imidazole blocking layers for perovskite stability

The prevention of AgI formation is critical for ensuring long term stability of perovskite solar cells. While sputtered metal oxide films are known for uniform dense film formation, solution phase nanoparticle coatings of metal oxides are prone to pinholes through which free iodide can migrate leading to AgI formation at the back contact. Iodide migration can be prevented through the addition of passivator or blocking layers at the perovskite/charge transport or charge transport/Ag interfaces. In this paper we evaluated a series of imidazoles as interfacial layers to prevent iodide migration from the perovskite to the Ag back contact through solution phase deposited Y:SnO2. We identified structural features of the imidazoles that contribute to an effective blocking layer. Specifically, it was found that imidazoles with conjugated planer systems and hydrogen bonding heteroatoms were able to prevent ion migration while retaining device performance. Of the materials evaluated 4-(Imidazol-1-yl)phenol had the best performance with a 60 % increase in stability verse the control under illumination at short circuit conditions.

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.

Danger in the Dark: Stability of Perovskite Solar Cells with Varied Stoichiometries and Morphologies Stressed at Various Conditions.

The long-term stability of perovskite solar cells (PSCs) remains a bottleneck for commercialization. While studies on the stoichiometry and morphology of PSCs with regard to performance are prevalent, understanding the influence of these factors on their long-term stability is lacking. In this work, we evaluate the impact of stoichiometry and morphology on the long-term stability of cesium formamidinium-based PSCs. We demonstrate that the lead iodide (PbI2) to formamidinium iodide (FAI) ratio influences stability under various stress factors (elevated temperature and light). A high molar ratio (PbI2/FAI > 1.1) in the perovskite precursor displays drastic degradation under ISOS-L1 (100 mW/cm2, 25 °C, maximum power point tracking) conditions. However, postdegradation analysis contradicts these results. Devices with PbI2/FAI ≤ 1.1 are stable under light, but intermittent current density-voltage characterizations indicate that device performance decreases during storage in the dark. Migration of iodide (I-) ions to the electron-transport layer (ETL) and iodine vacancies (VI-+) to the hole-transport layer (HTL) forms localized shunts in the absorber layer. Pinhole formation, surrounded by FA+-rich regions, explains the extent of damage in comparably aged films. In summary, this work emphasizes the importance of reporting stability under different stress conditions, coupled with postdegradation and dark recovery analyses of PSCs to better understand the complexities of perovskite instability under real-life conditions such as expected during outdoor operation.

Anion Binding Interaction Enhances the Robustness of Iodide for High-Performance Perovskite Solar Cells.

Owing to the ionic bond nature of the Pb-I bond, the iodide at the interface of perovskite polycrystalline films was easily lost during the preparation process, resulting in the formation of a large number of iodine vacancy defects. The presence of iodine vacancy defects can cause nonradiative recombination, provide a pathway for iodide migration, and be harmful to the power conversion efficiency (PCE) and stability of organic-inorganic hybrid perovskite solar cells (HPSCs). Here, in order to increase the robustness of iodides at the interface, a strategy to introduce anion binding effects was developed to stabilize the perovskite films. It was demonstrated that the N,N'-diphenylurea (DPU), characterized by high anionic binding constants and a Y-shaped structure, provides a relatively strong hydrogen bond donor site to effectively reduce the iodine loss during film preparation and inhibits iodide migration in the device working condition. As expected, the reduced iodine loss considerably improves the quality of the perovskite films and suppresses nonradiative recombination. The performance of the device after DPU modification was significantly increased, with the PCE rising from 23.65 to 25.01% with huge stability enhancement as well.

Enhanced fill factor and stability of perovskite solar cells with a multifunctional additive of starch-iodine complex

The efficiency and stability of perovskite solar cells (PSCs) can be greatly affected by various factors such as passivating the perovskite film, oxidizing the hole-transport material of 2,2′,7,7′-tetras(N,N-p-methoxyaniline)-9,9′-spirodifluorene (Spiro-OMeTAD), and inhibiting the iodide migration. Here we introduce a multifunctional starch-iodine complex in the perovskite film to enhance the fill factor and stability of PSCs. Results demonstrate that the starch-iodine complex from the perovskite film can release iodine molecules to gently oxidize the Spiro-OMeTAD, which can significantly increase the PSC's fill factor. Moreover, the remaining starch can effectively passivate the perovskite film to obviously improve the PSC's long-term stability. Furthermore, the starch-iodine complex can absorb and store the iodide ions to generate a starch-triiodide complex, which can release iodide ions to promote iodide defect self-healing and suppress iodide migration in the perovskite films, resulting in a further enhancement of the PSC's long-term stability. As a result, the PSC based on the starch-iodine complex obtains a superior cell efficiency of 22.60% with a high fill factor of 79.54%, and keeps 91.12% of its original efficiency after 1500 h.

Iodide retention characteristics on oxidized Cu coupon in saline and alkaline environments: Perspectives on high-level radioactive waste disposal

Radioactive iodine, particularly 129I, is one of the most hazardous radioisotopes found in deep geological repositories for high-level radioactive waste (HLW). This study investigated the iodide retention capacity of Cu coupons, simulating Cu canisters, under potential repository conditions. In saline environments (0.1 M NaCl + 0.01 M MgSO4), iodide was immobilized through CuI(s) formation on the oxidized surface of Cu coupons. This process was facilitated by the favorable affinity between Cu+ oxidized from the Cu coupons and I−. The iodide retention capacity of Cu coupons exhibited notable sensitivity to the pH and chemical composition of the solutions. Cu+ remained stably complexed with Cl− under saline conditions, leading to the CuI(s) formation. However, in distilled water, iodide retention decreased over time due to the transformation of CuI to CuO through Cu oxidation. In an alkaline solution (0.1 M KOH), iodide retention was negligible. These results indicate that radioactive iodide ions, released in the event of canister failure, can be effectively immobilized, and their migration to the ecosystem can be impeded through CuI(s) formation on oxidized Cu canisters in saline conditions. Consequently, Cu canisters can serve as an effective barrier against radioactive iodide migration and as a long-term isolation medium for HLW, particularly under saline conditions.

Improved photovoltaic performance and stability of perovskite solar cells by adoption of an n-type zwitterionic cathode interlayer.

Recently, inverted perovskite solar cells (PeSCs) have witnessed significant advancements; however, their long-term stability remains a challenge because of the oxidation of silver cathodes to form AgI by mobile iodides. To overcome this problem, we propose the integration of an electron-deficient naphthalene diimide-based zwitterion (NDI-ZI) as the cathode interlayer. Compared to the physical ion-blocking layer, it effectively captures ions by forming ionic bonds via electrostatic Coulombic interaction to suppress the migration of iodide and Ag ions. The NDI-ZI interlayer also suppresses the shunt paths and modulates the work function of the Ag electrode by forming interface dipoles, thereby enhancing charge extraction. FA0.85Cs0.15PbI3 based PeSCs incorporating NDI-ZI exhibited a noticeably high power conversion efficiency of up to 23.3% and outstanding stability, maintaining ∼80% of their initial performance over 1500 h at 85 °C and over 500 h under continuous 1-sun illumination. This study highlights the potential of a zwitterionic cathode interlayer in diverse perovskite optoelectronic devices, leading to their improved efficiency and stability.

Buried interface management via bifunctional NH4BF4 towards efficient CsPbI2Br solar cells with a Voc over 1.4 V

CsPbI2Br perovskite solar cells (PSCs) have drawn tremendous attention due to their suitable bandgap, excellent photothermal stability, and great potential as an ideal candidate for top cells in tandem solar cells. However, the abundant defects at the buried interface and perovskite layer induce severe charge recombination, resulting in the open-circuit voltage (Voc) output and stability much lower than anticipated. Herein, a novel buried interface management strategy is developed to regulate interfacial carrier dynamics and CsPbI2Br defects by introducing ammonium tetrafluoroborate (NH4BF4), thereby resulting in both high CsPbI2Br crystallization and minimized interfacial energy losses. Specifically, NH4+ ions could preferentially heal hydroxyl groups on the SnO2 surface and balance energy level alignment between SnO2 and CsPbI2Br, enhancing charge transport efficiency, while BF4− anions as a quasi-halogen regulate crystal growth of CsPbI2Br, thus reducing perovskite defects. Additionally, it is proved that eliminating hydroxyl groups at the buried interface enhances the iodide migration activation energy of CsPbI2Br for strengthening the phase stability. As a result, the optimized CsPbI2Br PSCs realize a remarkable efficiency of 17.09% and an ultrahigh Voc output of 1.43 V, which is one of the highest values for CsPbI2Br PSCs.

Unveiling of a puzzling dual ionic migration in lead‐ and iodide‐deficient halide perovskites (d‐HPs) and its impact on solar cell J–V curve hysteresis

AbstractHalide perovskite solar cells are characterized by a hysteresis between current–voltage (J–V) curves recorded on the reverse and on the forward scan directions, and the suppression of this phenomenon has focused great attention. In the present work, it is shown that a special family of 3D perovskites, that are rendered lead ‐and iodide‐ deficient (d‐HPs) by incorporating large organic cations, are characterized by a large hysteresis. The strategy of passivating defects by K+, which has been successful in reducing the hysteresis of 3D perovskite perovskite solar cells, is inefficient with the d‐HPs. By glow discharge optical emission spectroscopy (GD‐OES), the existence of the classic iodide migration in these layers is unveiled, which is efficiently blocked by potassium cation insertion. However, it is also shown that it co‐exists with the migration of the large organic cations characteristics of d‐HPs which are not blocked by the alkali metal ion. The migration of those large cations is intrinsically linked to the special structure of the d‐HP. It is suggested that it takes place through channels, present throughout the whole perovskite layer after the substitution of PbI+ units by the large cations, making this phenomenon intrinsic to the original structure of d‐HPs.

Determining the influence of LC2 and FA on the iodide diffusion coefficient of in seawater and sea sand concrete: A novel modified RIM test method

A novel modified Rapid Iodide Migration (RIM) test method was used to determine the influence of LC2 and FA on iodide ion diffusion coefficient of seawater and sea sand concrete (SSC). A 0.1 mol/L AgNO3 solution was selected as the indicator to develop the colour of iodide in SSC, and a lowered mass fraction of NaI solution was selected to improve the RIM test. An X-Ray Fluorescence (XRF) test was employed to detect the feasibility of the modified RIM test. Additionally, the effects of the w/c ratio, the type and substitution rate of supplementary cementitious materials (SCMs), and the compressive strength on the iodide ion diffusion coefficient of SSC were studied using the modified RIM test. The results show that it is completely feasible to use AgNO3 solution as an indicator. A lower mass fraction of NaI solution not only does not change the iodide ion diffusion coefficient of concrete but also reduces the test cost by 77.46% when using a 5 wt% NaI solution. Moreover, using SCMs to replace cement, liking fly ash (FA) and calcined clay and limestone (LC2), significantly reduces the iodide ion permeability of SSC, especially the use of LC2. Furthermore, there is a nonlinear relationship between the compressive strength and the iodide ion diffusion coefficient of SSC. This work is beneficial for the promotion of the application of modified RIM tests in studies of concrete contaminated with chloride and can provide a theoretical basis for the application of seawater and sea sand in engineering practices.

Correlation between Interfacial Structures and Device Performance: The Double-Edged Sword Effect of Lead Iodide in Perovskite Solar Cells.

The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI2 ) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH3 NH3 PbI3 and C60 with and without the modification of PbI2 using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH3 NH3 PbI3 /C60 interface with the modification of PbI2 as compared to that without PbI2 . Therefore, PSCs with PbI2 modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI2 undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH3 NH3 PbI3 /C60 interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI2 as an additive.

Analysis of Iodide Transport on Methyl Ammonium Lead Iodide Perovskite Solar Cell Structure Through Operando Hard X-ray Photoelectron Spectroscopy

To analyze the dynamic of ion species through a MAPbI3–xClx (HaP) solar cell, an operando hard X-ray photoelectron spectroscopy (HAXPES) study was proposed. Stacked Au/Ag/AZO/PCBM/HaP/NiO/ITO/glass structures, where AZO, PCBM, NiO, and ITO stand for Al-doped zinc oxide, phenyl-C61-butyric acid methyl ester, nickel oxide, and indium tin oxide, respectively, were evaluated. Experimental core level analysis under an applied bias voltage highlighted that a certain threshold voltage (above the open-circuit voltage (VOC)) is required to trigger the iodide (I–) migration from the HaP to the adjacent electron and hole transport layers. Study also demonstrated that the diffusion of I– in AZO/PCBM over time is unidirectional and follows the gradient descent in concentration. We observed that the irreversibility of the I– transport leads to the absorption of I– by the Ag layer to form a silver iodide (AgI) compound. Furthermore, this work also pointed out the efficiency of an ITO layer when deposited between Ag and AZO to serve as the ion diffusion barrier layer. Finally, the results demonstrated that the presence of ITO restrains the AgI formation and contributes to improving cell performances.

Chloride transport resistance of alkali-activated concrete exposed to combined chloride, sulfate and carbonation environment

The chloride transportation in alkali-activated fly ash (AAFA), alkali-activated slag (AAS) and alkali-activated fly ash/slag (AAFS) concrete exposed to the environments of single chloride and combined of chloride, sulfate and carbonation were investigated. The iodide migration, chloride diffusion and capillary absorption tests were carried out before and after exposure, as well as the analysis of their microstructure and compositions. It was found that the initial chloride transport of AAFA concrete was the highest but decreased gradually with the increase of exposure period. For AAFA and AAFS specimens with a relatively lower strength, chloride and sulfate salt crystallization led to the surface spalling and strength reduction. The coupling of carbonation would further accelerate the deterioration of AAFA, but the inhibitory effect was acted for AAFS specimen. Although the appearance of AAS specimens had little change under various aggressive environments, carbonation led to the decalcification of C-(A)-SH gels and deterioration of pore structure, both the porosity and the harmful 10–50 nm pores content of the specimens were higher compared with that before exposure, and their chloride transport increased with time. However, the deterioration of AAFS specimens resulted from carbonation was less serious, due to the formation of C-(N)-ASH gels with a higher degree of cross-linking and polymerization, thereby it had the best long term performance under the combined environment of chloride, sulfate and carbonation.

Iodide and charge migration at defective surfaces of methylammonium lead triiodide perovskites: The role of hydrogen bonding

Methylammonium lead triiodide perovskite (CH3NH3PbI3) solar cells have exhibited impressive potential in photovoltaic applications, encouraging further work to improve operating stability. Ionic migration has been proposed to be a possible cause of the current–voltage hysteresis which is responsible for the instability of the CH3NH3PbI3 solar cells. Motivated by the variation of the surface geometries of the CH3NH3PbI3 material when going from the pristine to defective surfaces with the iodine vacancies, we adopt the density functional theory method to study the iodide migration along different paths at the defective surfaces followed by employing the time-dependent density functional theory method to calculate the light-induced electronic states along the paths. We find that the iodide movement is largely tuned by the reorientation and rotation of the organic cations with the disruption and formation of the hydrogen bonding during the iodide migration. Besides, the migration of the light-induced charge carriers varies along the paths with different orientations of the organic cations. We attribute the complicated dynamics of the mobile iodide ions and the associated trap states to the hydrogen bonding at the surfaces, providing crucial guidance for improving the stability and charge carrier lifetime in the CH3NH3PbI3 and other metal halide perovskites.

Electrically driven ionic transport in the RCM and RIM: Investigations based on experiments and numerical simulations

In this work, the rapid chloride migration test (RCM) and the rapid iodide migration test (RIM) were utilized to investigate ionic transport in concrete with the aid of an externally applied electric field. Unlike most existing work, this study establishes computational models of ionic transport in chloride-contaminated and chloride-free concrete. The model is validated by the consistency between the numerical and experimental chloride profiles. It is found that the chloride migration depth in the RCM test is obviously affected, and the results for chloride-contaminated concrete are 50% larger compared with those for chloride-free concrete. However, the difference between the two kinds of concretes is only 5% when measured using the RIM test. Thus, the results of the RIM test are more accurate and provide a better choice for applications to characterization of chloride-contaminated concrete.

Iodide and chloride ions diffusivity, pore characterization and microstructures of concrete incorporating ground granulated blast furnace slag

Innovative approaches are under research to study the resistance of chloride ion penetration in concrete containing chloride ions, to minimize the impact of chloride ion penetration test errors in coastal reinforced concrete (RC), which is helpful to the design of coastal RC structures. In this study, the diffusion depth, free ion concentration and diffusion coefficient of chloride, iodide ions with different curing ages and GGBFS content were measured by the Rapid Chloride Migration Test (RCM) and Rapid Iodide Migration tests (RIM). The SEM-EDS and MIP were used to analyze the microstructures, pore size distribution and the hydrated products. The results show that the performance of GGBFS concrete against the diffusion of corrosive ions is affected by the curing age and the content of GGBFS. With the increase of GGBFS content, especially concrete with 60% GGBFS, the influence of chloride, iodide ion penetrating into concrete gradually becomes smaller. The long-age curing system is more conducive to the concrete resistance to the migration and diffusion of chloride, iodine ions. Compared with the ordinary concrete, the total porosity of concrete mixed with GGBFS is lower, the internal microstructures have fewer cracks and defects, the density is better, and the diffusion coefficient of chloride and iodide ions is also lower. In addition, using the concept of corrosive ion adjustment coefficient (conversion coefficient of diffusion between chloride ion and iodide ion) and applying the data regression analysis (DRA), it is found that there is a good quadratic parabolic function relationship between the GGBFS content and the ions adjustment coefficient.

Oxidative Annulation of Diphenylpropanamides via In Situ Hypervalent Iodine-Promoted Intramolecular C–N/C–O Bond Formation

AbstractAn aryl iodide catalyzed intramolecular oxidative transformation of diphenylpropanamide derivatives is described that can readily afford the C–N/C–O coupling products in a single step. The speed of the 1,3-aryl iodide migration process determines the diversity of target compound generation in this reaction. This straightforward approach can be performed with the use of inexpensive and readily available catalyst, transition-metal-free, mild conditions and good functional group tolerance.

Humidity Effect on Resistive Switching Characteristics of the CH3NH3PbI3 Memristor.

Organic-inorganic hybrid halide perovskites (OIHPs) with inherent mixed ionic-electronic conduction ability have been proposed as promising candidates for memristors with unique optoelectronic characteristics. Despite the great achievements toward understanding the working mechanism and exploring their functionality as water-sensitive materials, the humidity effect on the resistive switching (RS) characteristics still remains to be studied. This study investigates the humidity effect on the RS characteristics of Au/CH3NH3PbI3/FTO memristor. The memristor works well at moderate relative humidity (RH, <75%) and degrades rapidly at higher RH of 90%. An obvious decrease in low resistance states on increasing the RH level is observed, which could be attributed to water-induced reduction of the iodide migration barrier. Raman and X-ray diffraction analyses indicate that the migration barrier reduction possibly originated from the weakening of the Pb-I bond caused by the intercalation of water molecules into the crystal lattice. The humidity-sensitive RS characteristics of the memristor could extend the scope of OIHP application for sensing and security applications and also prompt researchers to pay attention to the humidity effect on memristor devices with OIHPs.

Defect compensation in formamidinium–caesium perovskites for highly efficient solar mini-modules with improved photostability

Formamidinium–caesium mixed-cation perovskites have shown better thermal stability than their methylammonium-containing counterparts but they suffer from photoinstability induced by iodide migration and phase segregation. Here we improve their photostability by adding slightly excessive AX (at a molar percentage of 0.25% to Pb2+ ions), where A is formamidinium or caesium and X is iodine. The excessive AX does not improve the initial solar cell efficiency. It compensates iodide vacancies and suppresses ion migration and defects generation during long-term illumination by around tenfold compared with AX-deficient devices. Consequently, generation of hole traps and phase segregation is impeded, with the former limiting solar cell efficiency after degradation. The perovskite mini-modules reached a certified stabilized efficiency of 18.6% with an aperture area of ~30 cm2, corresponding to an active area efficiency of 20.2%. The mini-module maintains 93.6% of the initial efficiency after continuous operation under 1 sun illumination for >1,000 h at 50 ± 5 °C in air. Methylammonium-free perovskite solar cells have achieved promising efficiency and thermal stability yet iodide migration limits their operational stability. Deng and colleagues show that an excess of formamidinium or caesium iodide precursor suppresses iodide vacancies preventing ion migration and eventually the generation of hole traps during device operation.