Water-induced Degradation Research Articles

Towards water-resistant perovskite solar cells: Electron-beam deposited SiO2 layers for highly stable perovskite solar cells

Metal halide perovskite solar cells (PSCs) incorporating organic–inorganic compounds exhibit promising potential for future photovoltaic devices. One of the main obstacles to commercializing perovskite photovoltaic technology is oxygen and water-induced degradation in ambient conditions. Herein, we demonstrate a general approach to protect PSCs from water-induced degradation using electron-beam evaporated silicon dioxide (SiO2) barrier coating. A number of small (0.25 cm2) and large-area (2.0 cm2) PSCs have been fabricated to obtain statistically significant data. An optimal 1-μm thick SiO2 layer coated over RbCsMAFAPb(IBr)3-PSCs provided remarkable protection even after fully immersing the devices in a water-filled container. After immersing the PSC in water for 1 min, the device efficiency showed only a small reduction in efficiency from 14.3 % to 12.0 %, highlighting the excellent water protection offered by the barrier layer. We show that SiO2 barrier layers can improve the overall stability of the cells. Perovskite devices coated with 1 µm-SiO2 protective layers maintained more than 85 % of their initial power conversion efficiency (PCE) values after 2000 h in ambient storage. The devices coated with the optimum SiO2 layer thickness of 1 µm also showed excellent stability during outdoor sunlight testing for 2000 h, maintaining ∼ 80 % of their initial efficiency values.

Facet engineering of minerals for effective oil-water separation

Minerals have been growing as an emerging ‘hot’ topic in separating water-in-oil emulsions. Whether the intrinsic structure of minerals could precisely regulate water–mineral interactions for enhancing emulsions separation efficiency is still ambiguous. Here, taking half-hydrated gypsum as a case, this study reveals the crystal facet-dependent potential mechanism of minerals in water-in-oil emulsions separation. We show that the (204) facet of half-hydrated gypsum is more susceptible to water-induced degradation and hydration reaction. With combined experimental and theoretical studies, the facet-dependent mechanism for oil–water separation are revealed; the (204) facet that exhibited weak internal hydrogen bond and strong binding energy with water accelerated the dissolution and recrystallization of half-hydrated gypsum, thus enhancing oil–water separation efficiency. Through engineering, half-hydrated gypsum with a higher surface fraction of the (204) facet achieved much higher separation efficiency (99.9%), surpassing those of minerals in disclosed literature. This investigation not only contributes significantly to the comprehension of interactions at the minerals-water interface but also illuminates novel perspectives and avenues for the utilization of facet engineering in advancing research on mineral surface properties.

Enhanced photocatalytic activity and stability of 2D Cs3Bi2Br9 perovskite nanosheets synthesized via modified antisolvent method

Cs3Bi2Br9 perovskite has attracted tremendous research attention in the field of photocatalysis due to its promising light-harvesting properties. However, its practical applications are hindered by water-induced degradation, limiting stability and photocatalytic activity. In this study, we address this challenge by synthesizing stable 2D Cs3Bi2Br9 nanosheets through a modified anti-solvent reprecipitation method. Optimizing the isopropanol amount enabled unprecedented synthesis of 2D Cs3Bi2Br9 nanosheets. SEM and HRTEM images show 2D stacked nanosheets of the sample prepared using 250 mL of isopropanol, while bulks and agglomerations were noticed in the samples prepared using different amounts of isopropanol. The Cs3Bi2Br9 nanosheets exhibits the lowest charge recombination rate, hence achieving the highest degradation ratio of methylene blue, removing ∼80 % of the dye within 90 min under visible light attributed to their stability, facilitating efficient charge separation. Our study sheds light on the pivotal role of 2D morphology in enhancing the stability and photocatalytic performance of Cs3Bi2Br9.

Investigation of Sulfide-Based Solid Electrolytes (SSE) Sensitivity Towards Humidity

All-solid-state lithium-ion batteries (ASSBs) represent a new generation of energy storage tools containing only solid components. They are of great interest to researchers and manufacturers thanks to their high energy density, which makes them promising candidates for electric transport applications. Among different types of solid-state electrolytes for lithium-ion batteries (LIBs), sulfide-based electrolytes (SSEs) have several advantages, such as high ionic conductivity and malleability [1, 2]. However, one of their main drawbacks is a high sensitivity towards air humidity [3]. Fast reaction of the SSEs with water results in the degradation of their structure and release of toxic gas (H2S) when exposed to even low concentrations of humidity. This factor leads to significant process cost and safety issues associated with mass production of ASSBs with sulfide-based SEs.The purpose of the present work is to study in details reactivity of SSEs towards humidity by following two main directions: The development of a flow-through setup that allows a better identification and highly precise quantification of gases (H2S or others) generated during reaction between water (humidity) and commercial SSEs: this flow-through setup intends to solve problems usually encountered when using closed-type setups, which are the most used setups in the literature[3, 4]. These problems include over-concentration of detected gases and low reproducibility of measurements;Physico-chemical and electrochemical characterizations of pristine and humidity exposed-SSEs in order to reveal the main factors affecting sensitivity towards humidity and to understand the degradation mechanisms. First, the efficiency of the flow-through gas analysis setup developed had been validated: H2S detection curves obtained for commercial SSE powders (Li6PS5Cl, Li6PS5Br, Li7P3S11, Li3PS4, Li10GeP2S12) in contact with humid Argon (30 to 200 ppm H2O in Ar) are highly reproducible (see example in Fig. 1 and Table 1). Measurements conducted at different humidity levels allowed accessing kinetics of water-induced degradation reactions involved. SSE powders characterizations using XRD, Raman, XPS, SEM, EIS, granulometry before and after exposure to humidity provided an insight into degradation-related phenomena taking place: morphology deterioration, loss of structure and conductive structural groups, drastic increase of ionic resistance. A relationship between the structure of SSE and its reactivity towards water was found and described.Refs[1] Energy Storage Mater. 2018, 14, 58–74 ; [2] Nano Energy 2021, 83, 105858 ; [3] Solid State Ion. 2011, 182 (1), 116–119 ; [4] Chem. Mater. 2020, 32 (6), 2664–2672. Figure 1

Moisture-Induced Degradation of Quantum-Sized Semiconductor Nanocrystals through Amorphous Intermediates.

Elucidating the water-induced degradation mechanism of quantum-sized semiconductor nanocrystals is an important prerequisite for their practical application because they are vulnerable to moisture compared to their bulk counterparts. In-situ liquid-phase transmission electron microscopy is a desired method for studying nanocrystal degradation, and it has recently gained technical advancement. Herein, the moisture-induced degradation of semiconductor nanocrystals is investigated using graphene double-liquid-layer cells that can control the initiation of reactions. Crystalline and noncrystalline domains of quantum-sized CdS nanorods are clearly distinguished during their decomposition with atomic-scale imaging capability of the developed liquid cells. The results reveal that the decomposition process is mediated by the involvement of the amorphous-phase formation, which is different from conventional nanocrystal etching. The reaction can proceed without the electron beam, suggesting that the amorphous-phase-mediated decomposition is induced by water. Our study discloses unexplored aspects of moisture-induced deformation pathways of semiconductor nanocrystals, involving amorphous intermediates.

Mitigating water-induced surface degradation in water-based Ni-rich Li-ion battery electrodes

Replacement of the toxic solvent used during manufacturing of cathode electrodes for Li-ion batteries with water is an essential step on the road towards eco-friendly battery production. This study aims to improve the performance of pilot-scale water-based LiNi0.83Co0.12Mn0.05O2 cathodes. The importance of proper electrode treatment before cell assembly to achieve maximum cell performance is demonstrated. Temperatures ranging from room temperature to 170 °C were applied to the electrodes to investigate the impact of residual moisture in pouch cells. Electrochemical cycling showed that despite a much higher amount of residual moisture a low drying temperature was beneficial. X-ray photoelectron spectroscopy indicated that with increasing temperature a phase reconstruction in the near-surface region of the cathode material particles is the cause for the difference in performance. This was further supported by impedance spectroscopy showing growing charge-transfer resistance arcs with higher temperature, which split into two contributions with ongoing cycling. Already temperatures above 100 °C resulted in more pronounced surface reconstruction and stronger impedance increase lowering the long-term cycling stability, while the residual moisture seemed to have only a minor impact. By optimizing the drying temperature, the long-term cycling stability could be improved from 1000 to 1700 cycles before reaching 80% capacity retention.

Cooperative Covalent Polymerization of N-carboxyanhydrides: from Kinetic Studies to Efficient Synthesis of Polypeptide Materials

ConspectusPolypeptides, as the synthetic analogues of natural proteins, are an important class of biopolymers that are widely studied and used in various biomedical applications. However, the preparation of polypeptide materials from the polymerization of N-carboxyanhydride (NCA) is limited by various side reactions and stringent polymerization conditions. Recently, we report the cooperative covalent polymerization (CCP) of NCA in solvents with low polarity and weak hydrogen-bonding ability (e.g., dichloromethane or chloroform). The polymerization exhibits characteristic two-stage kinetics, which is significantly accelerated compared with conventional polymerization under identical conditions. In this Account, we review our recent studies on the CCP, with the focus on the acceleration mechanism, the kinetic modeling, and the use of fast kinetics for the efficient preparation of polypeptide materials.By studying CCP with several initiating systems, we found that the polymerization rate was dependent on the secondary structure as well as the macromolecular architecture of the propagating polypeptides. The molecular interactions between the α-helical, propagating polypeptide and the monomer played an important role in the acceleration, which catalyzed the ring-opening reaction of NCA in an enzyme-mimetic, Michaelis–Menten manner. Additionally, the proximity between initiating sites further accelerated the polymerization, presumably due to the cooperative interactions of macrodipoles between neighboring helices and/or enhanced binding of monomers. A two-stage kinetic model with a reversible monomer adsorption process in the second stage was developed to describe the CCP kinetics, which highlighted the importance of cooperativity, critical chain length, binding constant, [M]0, and [M]0/[I]0. The kinetic model successfully predicted the polymerization behavior of the CCP and the molecular-weight distribution of resulting polypeptides.The remarkable rate acceleration of the CCP offers a promising strategy for the efficient synthesis of polypeptide materials, since the fast kinetics outpaces various side reactions during the polymerization process. Chain termination and chain transfer were thus minimized, which facilitated the synthesis of high-molecular-weight polypeptide materials and multiblock copolypeptides. In addition, the accelerated polymerization enabled the synthesis of polypeptides in the presence of an aqueous phase, which was otherwise challenging due to the water-induced degradation of monomers. Taking advantage of the incorporation of the aqueous phase, we reported the preparation of well-defined polypeptides from nonpurified NCAs. We believe the studies of CCP not only improve our understanding of biological catalysis, but also benefit the downstream studies in the polypeptide field by providing versatile polypeptide materials.

Experimental study on the reasonable proportions of rock-like materials for water-induced strength degradation in rock slope model test

Water-induced strength deterioration of rock mass is a crucial factor for rock slope instability. To better show the degradation process of rock slope water–rock interaction, we used bentonite as a water-sensitive regulator to build a new rock-like material that matches the features of water-induced strength degradation based on the cement-gypsum bonded materials. Twenty-five schemes of the material mixture proportion were designed using the orthogonal design method considering four factors with five variable levels, and a variety of experiments were conducted to obtain physico-mechanical parameters. In addition, one group of rock-like material proportion was selected and applied to the large-scale physical model test. The experiment results reveal that: (1) The failure mode of this rock-like material is highly similar to that of natural rock masses, and the physico-mechanical parameters vary over a wide range; (2) The bentonite content has a significant influence on the density, elastic modulus, and tensile strength of rock-like materials; (3) It is feasible to obtain the regression equation based on the linear regression analysis to determine the proportion of rock-like material; (4) Through application, the new rock-like material can effectively simulate or reveal the startup mechanism and instability characteristics of rock slopes under water-induced degradation. These studies can serve as a guide for the fabrication of rock-like material in the other model tests.

Study on the mechanism of water-induced degradation of slip zone soils and FDEM coupled simulation of slopes based on multi-scale characteristic

The evolution of the mechanical properties of slip zone soils had a greater impact on the structure of soil slopes with large upper porosity and excellent infiltration conditions. Especially under the rainfall effect, it was easy to be affected by the infiltration effect of internal injury, which will lay a ‘hidden danger’ for the slope body to trigger sliding in the secondary transformation process. In this paper, based on Haijiao ping landslide in Guizhou, SEM image recognition techniques were used to reveal the water-induced degradation mechanism of the sliding zone soil from multiple scales and the damage parameters were obtained. Meanwhile, the FDEM numerical model was established to simulate the stability of the slope coupled with indoor TCT test results. The results proved that the angle of internal friction decreased linearly with the increase of water content on the macroscopic scale, but the cohesive force showed an increase and then a significant decrease. The effect of matrix suction in the microscopic scale was significant at lower water content, the internal cohesion formed a large number of agglomerate structures to resist external deformation, but the microstructure was loose and porous after sufficient water immersion. The pore space spreads directionally and the area increases by 2.66 times. The cross-scale discrete-finite element coupled simulation method based on image recognition can visually respond to the macroscopic mechanical properties and stability change response of the slope body caused by microscopic damage. The water-induced degradation effect of rainfall on slip zone soils was the inherent factors for the initiation deformation of landslide. The artificial excavation was the external factor that triggered the slope to slide. This type of landslide was more concealed in its natural state and prone to deformation when excavated after long-term rainfall.

Analysis of Rainfall Infiltration and Improvement of the Analytical Solution of Safety Factors on Unsaturated Inner Dump Slopes: A Case Study

Rainfall infiltration is one of the main triggers of inner dump slope failure in the process of mining coal. Changes in water content throughout the process of rainfall infiltration have rarely been studied. The reductions in soil strength due to water migration have seldomly been considered in the existing analytical solution of the safety factor (F.S) for unsaturated inner dump slopes in an open-pit mine. In this work, a new mechanical model was developed by improving the conventional analytical solutions of F.S for unsaturated inner dump slopes to accommodate water-induced degradation in the mechanical strength of waste material. Parameter analysis was carried out via a case study of the Shengli #1 open-pit coalmine. The results showed that the wetting front depth increased with increasing rainfall time, and the increasing rate was constant during the non-compressive infiltration stage, while it decreased gradually in the compressive infiltration stage. The F.S of the transition layer decreased at first and then increased with increasing infiltration depth. By considering the water migration in the inner dump slope, the calculation result of F.S by the analytical solution in the paper can more precisely represent the in situ conditions. It was larger than that of the saturated strength, but smaller than that of the natural strength. The position of the minimum F.S did not alter in the wetting front, but was close to the position of the wetting front. The depth of the potential slip surface can be calculated by the converse solution of the analytical equation when F.S = 1 for rainfall infiltration, and the most dangerous slope surface can be determined. The depth (hmin) of the potential slip surface increases with increasing wetting front (hf) by a linear function, and increases with increasing depth ratios of the saturation layer (λ). The depth ratio (i) of the minimum F.S increases with increasing λ by an exponential function. The improved analytical solution can be used to evaluate the potential sliding surface under rainfall conditions, which is helpful for evaluating slope stability and analyzing dangerous surfaces under rainfall conditions and providing guidance for reinforcement schemes.

Synthesis of Cu–ZnO–Pt@HZSM-5 catalytic membrane reactor for CO2 hydrogenation to dimethyl ether

CO2 hydrogenation to dimethyl ether (DME) has drawn increasing interest in science and industry. However, the conversion of CO2 to DME is challenging due to the limitation of thermodynamic equilibrium and the water-induced degradation of the catalysts in a catalytic fixed bed reactor (CFBR). In this study, a novel reaction-separation coupling Cu–ZnO–Pt@HZSM-5 catalytic membrane reactor (CMR) was fabricated for CO2 hydrogenation to DME. Owing to continuous separation of the by-product steam by using an HZSM-5 membrane, the limitation of thermodynamic equilibrium can be broken effectively, thus leading to a substantially enhanced CO2 conversion (from 24.9% in the CFBR to 41.1% in the CMR) and DME selectivity (from 53.7% in the CFBR to 100% in the CMR). Further, water-induced degradation of the catalyst can be restrained because of water removal, thus keeping a high catalytic activity for a long time.

Atomistic insights into the debonding of Epoxy–Concrete interface with water presence

In this study, molecular models are developed to investigate the water-induced bond degradation of the epoxy–concrete interface. Concrete is simulated using the CSH binder. The results indicate that the interfacial chemical bonds, including Ca–O, Ca–N, and H-bond, are reduced due to the existence of water at the interface. Two different roles of water molecules are characterized in the interfacial structure, including the filling and enlarging roles. The water presence degrades the interfacial bond strength and accelerates the interface debonding process, attributed to the weakened interaction between the epoxy and the CSH and the weakened load transfer of water molecules. The fracture position is transferred from the internal epoxy to the interface between the epoxy and the CSH. These atomic-level findings facilitate a better understanding of the interfacial deterioration of epoxy-bonded systems, e.g., fiber-reinforced polymer (FRP)-strengthened concrete structures with water presence at the interface.

Influence of Water-Induced Degradation of Polytetrafluoroethylene (PTFE)-Coated Woven Fabrics Mechanical Properties

The impact of water-induced degradation on the mechanical properties of the chosen two PTFE-coated, glass threads woven fabrics is investigated in this paper. The paper begins with a survey of literature concerning the investigation and determination of coated woven fabric properties. The authors carried out the uniaxial tensile tests with an application of flat and curved grips to establish the proper values of the ultimate tensile strength and the longitudinal stiffness of groups of specimens treated with different moisture conditions. Despite the water resistance of the main materials used for fabrics manufacturing, the change of the mechanical properties caused by the influence of water immersion has been noticed. The reduction in the tensile strength resulting under waterlogged is observed in the range from 5% to 16% depending on the type of investigated coated woven fabric and direction of weft or warp.

Water-Induced Breaking of Interfacial Cohesiveness in a Poly(lactic acid)/Miscanthus Fibers Biocomposite.

The impact of the immersion in water on the morphology and the thermomechanical properties of a biocomposite made of a matrix of poly (lactic acid) (PLA) modified with an ethylene acrylate toughening agent, and reinforced with miscanthus fibers, has been investigated. Whereas no evidence of hydrolytic degradation has been found, the mechanical properties of the biocomposite have been weakened by the immersion. Scanning electron microscopy (SEM) pictures reveal that the water-induced degradation is mainly driven by the cracking of the fiber/matrix interface, suggesting that the cohesiveness is a preponderant factor to consider for the control of the biocomposite decomposition in aqueous environments. Interestingly, it is observed that the loss of mechanical properties is aggravated when the stereoregularity of PLA is the highest, and when increasing the degree of crystallinity. To investigate the influence of the annealing on the matrix behavior, crystallization at various temperatures has been performed on tensile bars of PLA made by additive manufacturing with an incomplete filling to enhance the contact area between water and polymer. While a clear fragilization occurs in the material crystallized at high temperature, PLA crystallized at low temperature better maintains its properties and even shows high elongation at break likely due to the low size of the spherulites in these annealing conditions. These results show that the tailoring of the mesoscale organization in biopolymers and biocomposites can help control their property evolution and possibly their degradation in water.

TEOS-Based Superhydrophobic Coating for the Protection of Stone-Built Cultural Heritage

Tetraethyl orthosilicate (TEOS) is extensively used in the conservation of stone-built cultural heritage, which is often subjected to water-induced degradation processes. The goal of this study was to produce and study a TEOS-based material with the ability to repel liquid water. A sol solution of TEOS and 1H,1H,2H,2H-perfluorooctyl triethoxysilane (FAS) was prepared and deposited on marble. The static contact angles (CAs) of water drops on the coated marble surface were >170° and the sliding angles (SA) were <5°, suggesting that superhydrophobicity and water repellency were achieved on the surface of the synthesized TEOS-based coating. FTIR and SEM-EDS were employed to characterize the produced coating. The latter offered good protection against water penetration by capillarity, reducing the breathability of marble only by a small extent and with practically no effect on its aesthetic appearance. The durability of the coating was evaluated through various tests that provided very promising results. Finally, the versatility of the method was demonstrated as the TEOS-based coating was successfully deposited onto glass, brass, wood, silicon, paper and silk, which obtained extreme wetting properties.

Research uncovers why water degrades certain solar cell materials

Research into the water-induced degradation of hybrid inorganic-organic perovskite could lead to better solar cell technology.

Sodium Dodecylbenzene Sulfonate Interface Modification of Methylammonium Lead Iodide for Surface Passivation of Perovskite Solar Cells.

Perovskite solar cells (PSCs) have been developed as a promising photovoltaic technology because of their excellent photovoltaic performance. However, interfacial recombination and charge carrier transport losses at the surface greatly limit the performance and stability of PSCs. In this work, the fabrication of high-quality PSCs based on methylammonium lead iodide with excellent ambient stability is reported. An anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), is introduced to simultaneously passivate the defect states and stabilize the cubic phase of the perovskite film. The SDBS located at grain boundaries and the surface of the active layer can effectively passivate under-coordinated lead ions and protect the perovskite components from water-induced degradation. As a result, a champion power conversion efficiency (PCE) of 19.42% is achieved with an open-circuit voltage (VOC) of 1.12 V, a short-circuit current (JSC) of 23.23 mA cm-2, and a fill factor (FF) of 74% in combination with superior moisture stability. The SDBS-passivated devices retain 80% of their initial average PCE after 2112 h of storage under ambient conditions.

On the Effect of Water-Induced Degradation of Thin-Film Piezoelectric Microelectromechanical Systems

Lifetime and reliability in realistic operating conditions are important parameters for the application of thin-film piezoelectric microelectromechanical systems (piezoMEMS) based on lead zirconate titanate (PZT). Humidity can induce time-dependent dielectric breakdown at a higher rate compared to dry conditions, and significantly alter the dynamic behavior of piezoMEMS-devices. Here we assess the lifetime and reliability of PZT-based micromirrors with and without humidity barriers operated at 23°C in an ambient of 0 and 95 % relative humidity. The correlation of the dynamic response, as well as the ferroelectric, dielectric, and leakage properties, with degradation time was investigated. In humid conditions, the median timeto-failure was increased from 2.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> [1.9×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> -4.0×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ] s to 1.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> [0.9×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> -1.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> ] s at 20 VAC continuous unipolar actuation, by using a 40 nm thick Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> humidity barrier. However, the initial maximum angular deflection, polarization, and dielectric permittivity decreased by about 6, 11, and 12 %, respectively, for Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> capped devices. For both bare and encapsulated devices, the onset of electrothermal breakdown-events was the dominant cause of degradation. Severe distortions in the device's dynamic behavior, together with failure from loss of angular deflection, preceded time-dependent dielectric breakdown in 95% relative humidity. Moreover, due to the film-substrate stress transfer sensitivity of thin-film devices, water-induced degradation affects the reliability of thin-film piezoMEMS differently than bulk piezoMEMS.

Phase degradation of all-inorganic perovskite CsPbI2Br films induced by a p-type CuI granular capping layer

It is necessary to evaluate the interactions between the different functional layers in optoelectronic devices to optimize device performance. Recently, the I-rich all-inorganic perovskite CsPbI2Br has attracted tremendous attention for use in solar cell applications because of its suitable band gap and favorable photo and thermal stabilities. It has been reported that the undesirable phase degradation of the photoactive α phase CsPbI2Br to the non-perovskite δ phase could be triggered by high humidity. To obtain stable devices, it is thus important to protect CsPbI2Br from moisture. In this paper, CuI, a non-hygroscopic p-type hole-transporting material, is found to induce the phase degradation of α-CsPbI2Br to the δ-CsPbI2Br. The rate and extent of phase degradation of CsPbI2Br are closely associated with the heating temperature and coverage of a CuI granular capping layer. This discovery is different from the widely reported water-induced phase degradation of CsPbI2Br. Our work highlights the importance of careful selection of hole-transporting materials during the processing of I-rich all-inorganic CsPbX3 (X=Br, I) perovskites to realize high-performance optoelectronic devices.

Nanoscale Studies at the Early Stage of Water-Induced Degradation of CH3NH3PbI3 Perovskite Films Used for Photovoltaic Applications

Understanding the surface properties of hybrid perovskite materials is a key aspect to improve not only the interface properties in photovoltaic cells but also the stability against moisture degradation. In this work, we study the local electronic properties of two series of CH 3 NH 3 PbI 3 perovskite films by atomic force microscopybased methods. We correlate nanoscale features such as the local surface potential (as measured by Kelvin probe force microscopy) to the current response (as measured by conductive atomic force microscopy). CH 3 NH 3 PbI 3 perovskites made using lead acetate as a precursor result in films with high purity and crystallinity and also result in heterogeneous local electrical properties, attributed to variations in the density of surface states. In contrast, when using lead iodide as a precursor, the perovskite surface exhibits a uniform distribution of surface states. This work also aims to understand the early stages of water-induced degradation at the surface of those films. Through high-precision exposure to small amounts of water vapor, we observe a higher stability for surfaces prepared with lead iodide precursors. More importantly, each precursor-based fabrication route is associated with either nor p-type behavior of the films. These characteristics are determined by the type of surface states, which also and eventually preside over stability. This work should help discriminate between perovskite synthesis routes and improve their stability in photovoltaic cell applications.