Effect of relative humidity on crystal growth, device performance and hysteresis in planar heterojunction perovskite solar cells

Unveiling the Dual Role of Humidity: The Interplay with Defects Manipulating the Charge Carrier Lifetime in Metal Halide Perovskites

Humidity has exhibited experimentally either beneficial or detrimental effects on the charge carrier lifetime of CH3NH3PbI3 perovskites, leaving the mechanism unresolved. By using ab initio nonadiabatic molecular dynamics simulations, we unveil the dual role of humidity stemming from the complex interplay between water and defects. Beneficially, water passivates iodine vacancies (VI) or grain boundaries (GBs), mitigating electron trapping by reducing nonadiabatic coupling and delaying charge recombination. However, when VI and GBs coexist, water molecules make the two unsaturated lead atoms approach closer and exacerbate electron trapping by deepening the Pb-dimer electron trap that was created by the VI defect, shortening the carrier lifetime to half of pristine CH3NH3PbI3. The study uncovers the origin of the positive and negative effects of humidity on the charge carrier lifetime of perovskites and offers strategies for improving perovskite devices, particularly by avoiding simultaneous point defects and GBs.

Trace Water in Lead Iodide Affecting Perovskite Crystal Nucleation Limits the Performance of Perovskite Solar Cells

The experimental replicability of highly efficient perovskite solar cells (PSCs) is a persistent challenge faced by laboratories worldwide. Although trace impurities in raw materials can impact the experimental reproducibility of high-performance PSCs, the in situ study of how trace impurities affect perovskite film growth is never investigated. Here, light is shed on the impact of inevitable water contamination in lead iodide (PbI2 ) on the replicability of device performance, mainly depending on the synthesis methods of PbI2 . Through synchrotron-based structure characterization, it is uncovered that even slight additions of water to PbI2 accelerate the crystallization process in the perovskite layer during annealing. However, this accelerated crystallization also results in an imbalance of charge-carrier mobilities, leading to a degradation in device performance and reduced longevity of the solar cells. It is also found that anhydrous PbI2 promotes a homogenous nucleation process and improves perovskite film growth. Finally, the PSCs achieve a remarkable certified power conversion efficiency of 24.3%. This breakthrough demonstrates the significance of understanding and precisely managing the water content in PbI2 to ensure the experimental replicability of high-efficiency PSCs.

Fabrication of Hyperbranched Photomechanical Crystals Composed of a Photochromic Diarylethene

We report the fabrication of hyperbranched hollow crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene on a concave surface of the spherical glass substrate by sublimation and their practical photomechanical behaviors. The number of units of the branched structure of the hollow crystals composed of this compound is proportional to the substrate curvature of the substrate. Compared with the sublimation process of the same compound on the flat glass substrate, two kinds of the thin film domains are generated separately in the center and around the edge of the spherical glass substrate. Especially under the high relative humidity condition, the boundaries between these thin film domains move gradually around the edge through the center during as long as 6 h of sublimation time so that the hyperbranched hollow crystals are densely produced on the entire surface of the substrate. These hyperbranched hollow crystals can be prepared with the highly ordered molecular packing due to the very slow formation process of the crystalline walls of the hollow structures. Furthermore, the photo-induced bending behaviors in the few- and highly-branched hollow crystals have the practical roles in moving and bending the minute objects according to their characteristics of these branched shapes.

Thermal-Radiation-Driven Ultrafast Crystallization of Perovskite Films Under Heavy Humidity for Efficient Inverted Solar Cells

Fabricating perovskite solar cells (PSCs) in air is conducive to low-cost commercial production; nevertheless, it is rather difficult to achieve comparable device performance as that in an inert atmosphere because of the poor moisture toleration of perovskite materials. Here, the perovskite crystallization process is systematically studied using two-step sequential solution deposition in an inert atmosphere (glovebox) and air. It is found that moisture can stabilize solvation intermediates and prevent their conversion into perovskite crystals. To address this issue, thermal radiation is used to accelerate perovskite crystallization for integrated perovskite films within 10 s in air. The as-formed perovskite films are compact, highly oriented with giant grain size, superior photoelectric properties, and low trap density. When the films are applied to PSC devices, a champion power conversion efficiency (PCE) of 20.8% is obtained, one of the best results for air-processed inverted PSCs under high relative humidity (60 ± 10%). This work substantially assists understanding and modulation to perovskite crystallization kinetics under heavy humidity. Also, the ultrafast conversion strategy by thermal radiation provides unprecedented opportunities to manufacture high-quality perovskite films for low-temperature, eco-friendly, and air-processed efficient inverted PSCs.

Moisture-triggered fast crystallization enables efficient and stable perovskite solar cells

Understanding the function of moisture on perovskite is challenging since the random environmental moisture strongly disturbs the perovskite structure. Here, we develop various N₂-protected characterization techniques to comprehensively study the effect of moisture on the efficient cesium, methylammonium, and formamidinium triple-cation perovskite (Cs_0.05FA_0.75MA_0.20)Pb(I_0.96Br_0.04)₃. In contrast to the secondary measurements, the established air-exposure-free techniques allow us directly monitor the influence of moisture during perovskite crystallization. We find a controllable moisture treatment for the intermediate perovskite can promote the mass transportation of organic salts, and help them enter the buried bottom of the films. This process accelerates the quasi-solid-solid reaction between organic salts and PbI₂, enables a spatially homogeneous intermediate phase, and translates to high-quality perovskites with much-suppressed defects. Consequently, we obtain a champion device efficiency of approaching 24% with negligible hysteresis. The devices exhibit an average T₈₀-lifetime of 852 h (maximum 1210 h) working at the maximum power point.

Perovskite structure is disturbed by environmental moisture, limiting the device performance. Here, Wei et al. monitor the effect of moisture during the growth by N₂-protected characterization techniques, and obtain an operationally stable perovskite solar cell with efficiency approaching 24%.

Advanced Strategies to Tailor the Nucleation and Crystal Growth in Hybrid Halide Perovskite Thin Films

Remarkable improvement in the perovskite solar cell efficiency from 3.8% in 2009 to 25.5% today has not been a cakewalk. The credit goes to various device fabrication and designing techniques employed by the researchers worldwide. Even after tremendous research in the field, phenomena such as ion migration, phase segregation, and spectral instability are not clearly understood to date. One of the widely used techniques for the mitigation of ion migration is to reduce the defect density by fabricating the high-quality perovskite thin films. Therefore, understanding and controlling the perovskite crystallization and growth have become inevitably crucial. Some of the latest methods attracting attention are controlling perovskite film morphology by modulating the coating substrate temperature, antisolvent treatment, and solvent engineering. Here, the latest techniques of morphology optimization are discussed, focusing on the process of nucleation and growth. It can be noted that during the process of nucleation, the supersaturation stage can be induced faster by modifying the chemical potential of the system. The tailoring of Gibbs free energy and, hence, the chemical potential using the highly utilized techniques is summarized in this minireview. The thermodynamics of the crystal growth, design, and orientation by changing several parameters is highlighted.

Drying and Coating of Perovskite Thin Films: How to Control the Thin Film Morphology in Scalable Dynamic Coating Systems

Hybrid perovskite photovoltaics combine high performance with the ease of solution processing. However, to date, a poor understanding of morphology formation in coated perovskite precursor thin films casts doubt on the feasibility of scaling-up laboratory-scale solution processes. Oblique slot jet drying is a widely used scalable method to induce fast crystallization in perovskite thin films, but deep knowledge and explicit guidance on how to control this dynamic method are missing. In response, we present a quantitative model of the drying dynamics under oblique slot jets. Using this model, we identify a simple criterion for successful scaling of perovskite solution printing and predict coating windows in terms of air velocity and web speed for reproducible fabrication of perovskite solar cells of ∼15% in power conversion efficiency─in direct correlation with the morphology of fabricated thin films. These findings are a corner stone toward scaling perovskite fabrication from simple principles instead of trial and error optimization.

Humidity-Induced Defect-Healing of Formamidinium-Based Perovskite Films

Formamidinium (FA)-based perovskite material holds great potential to deliver highly efficient commercial solar cells. However, the FA-based perovskite films are commonly processed under a strictly controlled environment, which would eventually hinder their way to commercialization. Herein, a systematic study is conducted to investigate the sequential deposition of FA-based perovskite films that are annealed under ambient conditions. Unexpectedly, the films prepared in low humidity condition possess less pinholes and defects and exhibit better device performances than those prepared in the moisture-free condition. A series of in situ and ex situ investigations are conducted which reveal defects in perovskite films are continuously healed during the film annealing process under the humid condition. This extraordinary effect is attributed to the interaction between water molecules and perovskite. The current study should shed light on the ambient fabrication of FA-based perovskite solar cells and foster their real-world applications.

Water's Role in Polymorphic Platinum(II) Complexes

Solvent plays a vital role in the recrystallization process and resulting crystallinity of materials. This role is of such importance that it can control the stability and utility of materials. In this work, the inclusion of a solvent in the crystalline lattice, specifically water, drastically affects the overall stability of two platinum polymorphs. [Pt(tpy)Cl]BF₄ (tpy = 2,2';6'2″-terpyridine) crystallizes in three forms, red (1R) and blue (1B) polymorphs and a yellow nonsolvated form (2). 1R is the more stable of the two polymorphs, whereas 1B loses crystallinity upon dehydration at ambient conditions resulting in the formation of 2. Close examination of the solid-state extended structures of the two polymorphs reveals that 1R has a lattice arrangement that is more conducive to stronger intermolecular interactions compared to 1B, thereby promoting greater stability. In addition, these two polymorphs exhibit unique vapochromic responses when exposed to various solvents.

Inkjet-Printed Full-Color Matrix Quasi-Two-Dimensional Perovskite Light-Emitting Diodes

Full-color matrix devices based on perovskite light-emitting diodes (PeLEDs) formed via inkjet printing are increasingly attractive due to their tunable emission, high color purity, and low cost. A key challenge for realizing PeLED matrix devices is achieving high-quality perovskite films with a favorable emission structure via inkjet printing techniques. In this work, a narrow phase distribution, high-quality quasi-two-dimensional (quasi-2D) perovskite film without a "coffee ring" was obtained via the introduction of a phenylbutylammonium cation into the perovskite and the use of a vacuum-assisted quick-drying process. Relatively efficient emissions of red, green, and blue (RGB) uniform quasi-2D perovskite films with high photoluminescence quantum yields were cast by the inkjet printing technique. The RGB monochrome perovskite matrix devices with 120 pixel-per-inch resolution exhibited electroluminescence, with maximum external quantum efficiencies of 3.5, 3.4, and 1.0% (for red, green, and blue light emissions, respectively). Furthermore, a full-color perovskite matrix device with a color gamut of 102% (NTSC 1931) was realized. To the best of our knowledge, this is the first report of a full-color perovskite matrix device formed by inkjet printing.

Printing High-Efficiency Perovskite Solar Cells in High-Humidity Ambient Environment-An In Situ Guided Investigation

Extensive studies are conducted on perovskite solar cells (PSCs) with significant performance advances (mainly spin coating techniques), which have encouraged recent efforts on scalable coating techniques for the manufacture of PSCs. However, devices fabricated by blade coating techniques are inferior to state-of-the-art spin-coated devices because the power conversion efficiency (PCE) is highly dependent on the morphology and crystallization kinetics in the controlled environment and the delicate solvent system engineering. In this study, based on the widely studied perovskite solution system dimethylformamide-dimethyl sulfoxide, air-knife-assisted ambient fabrication of PSCs at a high relative humidity of 55 ± 5% is reported. In-depth time-resolved UV-vis spectrometry is carried out to investigate the impact of solvent removal and crystallization rate, which are critical factors influencing the crystallization kinetics and morphology because of adventitious moisture. UV-vis spectrometry enables accurate determination of the thickness of the wet precursor film. Anti-solvent-free, high-humidity ambient coatings of hysteresis-free PSCs with PCEs of 21.1% and 18.0% are demonstrated for 0.06 and 1 cm2 devices, respectively. These PSCs exhibit comparable stability to those fabricated in a glovebox, thus demonstrating their high potential.

Combined Computational and Experimental Investigation on the Nature of Hydrated Iodoplumbate Complexes: Insights into the Dual Role of Water in Perovskite Precursor Solutions

Water is generally considered an enemy of metal halide perovskites, being responsible for their rapid degradation and, consequently, undermining the long-term stability of perovskite-based solar cells. However, beneficial effects of liquid water have been surprisingly observed, and synthetic routes including water treatments have shown to improve the quality of perovskite films. This suggests that the interactions of water with perovskites and their precursors are far from being completely understood, as water appears to play a puzzling dual role in perovskite precursor solutions. In this context, studying the basic interactions between perovskite precursors in the aqueous environment can provide a deeper comprehension of this conundrum. In this context, it is fundamental to understand how water impacts the chemistry of iodoplumbate perovskite precursor species, PbI_x^2–x. Here, we investigate the chemistry of these complexes using a combined experimental and theoretical strategy to unveil their peculiar structural and optical properties and eventually to assign the species present in the solution. Our study indicates that iodide-rich iodoplumbates, which are generally key to the formation of lead halide perovskites, are not easily formed in aqueous solutions because of the competition between iodide and solvent molecules in coordinating Pb²⁺ ions, explaining the difficulty of depositing lead iodide perovskites from aqueous solutions. We postulate that the beneficial effect of water when used as an additive is then motivated by its behavior being similar to high coordinative polar aprotic solvents usually employed as additives in one-step perovskite depositions.

Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells

Negative capacitance in the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, which origin is still under discussion. Here we use impedance spectroscopy to analyze these responses in methylammonium lead bromide cells treated with lithium cations at the electron-selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the surface polarization model, we obtain a time constant associated with the kinetics of the interaction of ions/vacancies with the surface, τ_kin, in the range of 10⁰-10² s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding a flattening of the cyclic voltammetry. As a consequence of these results we find that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

An Efficient Approach to Fabricate Air-Stable Perovskite Solar Cells via Addition of a Self-Polymerizing Ionic Liquid

Despite the excellent photovoltaic properties achieved by perovskite solar cells at the laboratory scale, hybrid perovskites decompose in the presence of air, especially at high temperatures and in humid environments. Consequently, high-efficiency perovskites are usually prepared in dry/inert environments, which are expensive and less convenient for scale-up purposes. Here, a new approach based on the inclusion of an in situ polymerizable ionic liquid, 1,3-bis(4-vinylbenzyl)imidazolium chloride ([bvbim]Cl), is presented, which allows perovskite films to be manufactured under humid environments, additionally leading to a material with improved quality and long-term stability. The approach, which is transferrable to several perovskite formulations, allows efficiencies as high as 17% for MAPbI₃ processed in air % relative humidity (RH) ≥30 (from an initial 15%), and 19.92% for FAMAPbI₃ fabricated in %RH ≥50 (from an initial 17%), providing one of the best performances to date under similar conditions.

Good or evil: what is the role of water in crystallization of organometal halide perovskites?

Perovskite solar cells (PSCs) have the potential to become one of the most cost-efficient photovoltaic devices. However, current fabrication methods of PSCs still require strict environment control and ultrahigh purity chemicals, which could prevent their large-scale commercialization. To tackle this challenge, the role of water is the first to be thoroughly understood in a perovskite formation process. Until now, there is still controversy about whether water is harmful or beneficial for perovskite formation, not to mention exactly what role water plays therein. In this Focus article, we review recent studies on water involved chemical reactions, solvent interaction, intermediates, and crystal growth in the perovskite film formation process, in order to bring out a full picture about what water does in the perovskite formation process. As our current understanding stands, a suitable amount of water could be of help for growing high quality perovskite films due to the resultant formation of intermediates, such as MAPbI3·H2O, which facilitates the conversion from precursors to perovskites. However, too much water would induce the formation of relatively stable components, such as (MA)4PbI6·2H2O, which are left in the product-films as impurities resulting in degraded device performance. Continual efforts should be made to further understand and develop water-involved strategies for consistent PSC fabrication under ambient conditions.

Understanding of perovskite crystal growth and film formation in scalable deposition processes

Hybrid organic-inorganic perovskite photovoltaics (PSCs) have attracted significant attention during the past decade. Despite the stellar rise of laboratory-scale PSC devices, which have reached a certified efficiency over 25% to date, there is still a large efficiency gap when transiting from small-area devices to large-area solar modules. Efficiency losses would inevitably arise from the great challenges of homogeneous coating of large-area high quality perovskite films. To address this problem, we provide an in-depth understanding of the perovskite nucleation and crystal growth kinetics, including the LaMer and Ostwald ripening models, which advises us that fast nucleation and slow crystallization are essential factors in forming high-quality perovskite films. Based on these cognitions, a variety of thin film engineering approaches will be introduced, including the anti-solvent, gas-assisted and solvent annealing treatments, Lewis acid-base adduct incorporation, etc., which are able to regulate the nucleation and crystallization steps. Upscaling the photovoltaic devices is the following step. We summarize the currently developed scalable deposition technologies, including spray coating, slot-die coating, doctor blading, inkjet printing and vapour-assisted deposition. These are more appealing approaches for scalable fabrication of perovskite films than the spin coating method, in terms of lower material/solution waste, more homogeneous thin film coating over a large area, and better morphological control of the film. The working principles of these techniques will be provided, which direct us that the physical properties of the precursor solutions and surface characteristics/temperature of the substrate are both dominating factors influencing the film morphology. Optimization of the perovskite crystallization and film formation process will be subsequently summarized from these aspects. Additionally, we also highlight the significance of perovskite stability, as it is the last puzzle to realize the practical applications of PSCs. Recent efforts towards improving the stability of PSC devices to environmental factors are discussed in this part. In general, this review, comprising the mechanistic analysis of perovskite film formation, thin film engineering, scalable deposition technologies and device stability, provides a comprehensive overview of the current challenges and opportunities in the field of PSCs, aiming to promote the future development of cost-effective up-scale fabrication of highly efficient and ultra-stable PSCs for practical applications.

Inkjet-Printed Organohalide 2D Layered Perovskites for High-Speed Photodetectors on Flexible Polyimide Substrates

The synthesis of solution-processed two-dimensional (2D) layered organohalide (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n = 2, 3, and 4) perovskites is presented, where inkjet printing was used to fabricate heterostructure flexible photodetector (PD) devices on polyimide (PI) substrates. Inks for the n = 4 formulation were developed to inkjet-print PD devices that were photoresponsive to broadband incoming radiation in the visible regime, where the peak photoresponsivity R was calculated to be ∼0.17 A/W, which is higher compared to prior reports, while the detectivity D was measured to be ∼3.7 × 10¹² Jones at a low light intensity F ≈ 0.6 mW/cm². The ON/OFF ratio was also high (∼2.3 × 10³), while the response time τ on the rising and falling edges was measured to be τ_rise ≈ 24 ms and τ_fall ≈ 65 ms, respectively. Our strain-dependent measurements, conducted here for the first time for inkjet-printed perovskite PDs, revealed that the I_p decreased by only ∼27% with bending (radius of curvature of ∼0.262 cm^-1). This work demonstrates the tremendous potential of the inkjet-printed, composition-tunable, organohalide 2D perovskite heterostructures for high-performance PDs, where the techniques are readily translatable toward flexible solar cell platforms as well.

Controlling Homogenous Spherulitic Crystallization for High-Efficiency Planar Perovskite Solar Cells Fabricated under Ambient High-Humidity Conditions

The influence of precursor solution properties, fabrication environment, and antisolvent properties on the microstructural evolution of perovskite films is reported. First, the impact of fabrication environment on the morphology of methyl ammonium lead iodide (MAPbI₃ ) perovskite films with various Lewis-base additives is reported. Second, the influence of antisolvent properties on perovskite film microstructure is investigated using antisolvents ranging from nonpolar heptane to highly polar water. This study shows an ambient environment that accelerates crystal growth at the expense of nucleation and introduces anisotropies in crystal morphology. The use of antisolvents enhances nucleation but also influences ambient moisture interaction with the precursor solution, resulting in different crystal morphology (shape, size, dispersity) in different antisolvents. Crystal morphology, in turn, dictates film quality. A homogenous spherulitic crystallization results in pinhole-free films with similar microstructure irrespective of processing environment. This study further demonstrates propyl acetate, an environmentally benign antisolvent, which can induce spherulitic crystallization under ambient environment (52% relative humidity, 25 °C). With this, planar perovskite solar cells with ≈17.78% stabilized power conversion efficiency are achieved. Finally, a simple precipitation test and in situ crystallization imaging under an optical microscope that can enable a facile a priori screening of antisolvents is shown.

Crystalline Clear or Not: Beneficial and Harmful Effects of Water in Perovskite Solar Cells

Clarification of how water affects the photovoltaic performance of perovskite solar cells is one of the major challenges to successfully develop a large-scale low-cost fabrication process. Many authors have reported beneficial effects of moisture during the fabrication of perovskite solar cells (PSCs), such as enhanced crystallinity, photoluminescence and photovoltage. However, the highest power conversion efficiency reported until this date was obtained under completely dry atmosphere conditions, avoiding the presence of water during perovskite formulation and preserving the damage caused by moisture exposure with encapsulation techniques. This apparent contradiction makes patent the necessity of an extensive clarification to establish the conditions in which water represents a beneficial or harmful factor in the development of high efficiency and stable perovskite devices. In this review, we summarized the effects of water, both as an additive into the perovskite formulation as an additive and as moisture exposure during fabrication. We discuss in depth the structural and chemical effects, analysing also the photovoltaic consequences during operation conditions. As a final input, we remark a useful method to perform high efficiency PSCs under different lab ambient conditions and highlight the latest advances in hydrophobic devices and encapsulation techniques.

Fabrication and Morphological Characterization of High-Efficiency Blade-Coated Perovskite Solar Modules

Organo-metal halide perovskite demonstrates a large potential for achieving highly efficient photovoltaic devices. The scaling-up process represents one of the major challenges to exploit this technology at the industrial level. Here, the scaling-up of perovskite solar modules from 5 × 5 to 10 × 10 cm² substrate area is reported by blade coating both the CH₃NH₃PbI₃ perovskite and spiro-OMeTAD layers. The sequential deposition approach is used in which both lead iodide (PbI₂) deposition and the conversion step are optimized by using additives. The PbI₂ solution is modified by adding methylammonium iodide (MAI) which improves perovskite crystallinity and pore filling of the mesoporous TiO₂ scaffold. Optimization of the conversion step is achieved by adding a small concentration of water into the MAI-based solution, producing large cubic CH₃NH₃PbI₃ grains. The combination of the two modifications leads to a power conversion efficiency of 14.7% on a perovskite solar module with an active area of 47 cm².

An Air Knife-Assisted Recrystallization Method for Ambient-Process Planar Perovskite Solar Cells and Its Dim-Light Harvesting

The photovoltaic performance of perovskite solar cells is highly dependent on the control of morphology and crystallization of perovskite film, which usually requires a controlled atmosphere. Therefore, fully ambient fabrication is a desired technology for the development of perovskite solar cells toward real production. Here, an air-knife assisted recrystallization method is reported, based on a simple bath-immersion to prepare high-quality perovskite absorbers. The resulted film shows a strong crystallinity with pure domains and low trap-state density, which contribute to the device performance and stability. The proposed method can operate in a wide process window, such as variable relative humidity and bath-immersion conditions, demonstrating a power conversion efficiency over 19% and 27% under 1 sun and 500-2000 lux dim-light illumination respectively, which is among the highest performance of ambient-process perovskite solar cells.

Luminescent Intermediates and Humidity-Dependent Room-Temperature Conversion of the MAPbI3 Perovskite Precursor

Preparation of metal-halide perovskites under room temperature attracts attention because of energy saving by removing thermal annealing. Room-temperature transformation of spin-cast wet films consisting of methylammonium (MA) iodide, PbI₂, and dimethylformamide toward solid MAPbI₃ perovskite proceeds via several intermediate crystalline states and is strongly dependent on ambient humidity. Light transmission and photoluminescence (PL) microscopy and spectroscopy were used to monitor the growth of crystals and transformation of their properties in time under nitrogen atmosphere at room temperature. Under low humidity, a highly luminescent intermediate phase with low absorption in the visible range appears, with the PL spectra composed of several bands in the range from 600 to 760 nm. We assign these bands to low-dimensional (nanocrystals and two-dimensional inclusions) MAPbI₃ intermediates, where the exciton confinement shifts the spectrum to higher energies in comparison with the bulk MAPbI₃. The intermediate levels of ambient humidity (10-50%) appear to catalyze the conversion of the intermediate phase to MAPbI₃. At a high ambient humidity (>80%), the initially formed MAPbI₃ is quickly transformed to the transparent hydrate phase of MAPbI₃. The role of ambient water catalyzing the material transformation by competing for Pb coordination with the solvent molecules is discussed.

Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes

Metal halide perovskites (MHPs) have numerous advantages as light emitters such as high photoluminescence quantum efficiency with a direct bandgap, very narrow emission linewidth, high charge-carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost. Based on these advantages, MHPs have recently shown unprecedented radical progress (maximum current efficiency from 0.3 to 42.9 cd A^-1 ) in the field of light-emitting diodes. However, perovskite light-emitting diodes (PeLEDs) suffer from intrinsic instability of MHP materials and instability arising from the operation of the PeLEDs. Recently, many researchers have devoted efforts to overcome these instabilities. Here, the origins of the instability in PeLEDs are reviewed by categorizing it into two types: instability of (i) the MHP materials and (ii) the constituent layers and interfaces in PeLED devices. Then, the strategies to improve the stability of MHP materials and PeLEDs are critically reviewed, such as A-site cation engineering, Ruddlesden-Popper phase, suppression of ion migration with additives and blocking layers, fabrication of uniform bulk polycrystalline MHP layers, and fabrication of stable MHP nanoparticles. Based on this review of recent advances, future research directions and an outlook of PeLEDs for display applications are suggested.

Perovskites-Based Solar Cells: A Review of Recent Progress, Materials and Processing Methods

With the rapid increase of efficiency up to 22.1% during the past few years, hybrid organic-inorganic metal halide perovskite solar cells (PSCs) have become a research “hot spot” for many solar cell researchers. The perovskite materials show various advantages such as long carrier diffusion lengths, widely-tunable band gap with great light absorption potential. The low-cost fabrication techniques together with the high efficiency makes PSCs comparable with Si-based solar cells. But the drawbacks such as device instability, J-V hysteresis and lead toxicity reduce the further improvement and the future commercialization of PSCs. This review begins with the discussion of crystal and electronic structures of perovskite based on recent research findings. An evolution of PSCs is also analyzed with a greater detail of each component, device structures, major device fabrication methods and the performance of PSCs acquired by each method. The following part of this review is the discussion of major barriers on the pathway for the commercialization of PSCs. The effects of crystal structure, fabrication temperature, moisture, oxygen and UV towards the stability of PSCs are discussed. The stability of other components in the PSCs are also discussed. The lead toxicity and updated research progress on lead replacement are reviewed to understand the sustainability issues of PSCs. The origin of J-V hysteresis is also briefly discussed. Finally, this review provides a roadmap on the current needs and future research directions to address the main issues of PSCs.

Aqueous-Containing Precursor Solutions for Efficient Perovskite Solar Cells

Perovskite semiconductors have emerged as competitive candidates for photovoltaic applications due to their exceptional optoelectronic properties. However, the impact of moisture instability on perovskite films is still a key challenge for perovskite devices. While substantial effort is focused on preventing moisture interaction during the fabrication process, it is demonstrated that low moisture sensitivity, enhanced crystallization, and high performance can actually be achieved by exposure to high water content (up to 25 vol%) during fabrication with an aqueous-containing perovskite precursor. The perovskite solar cells fabricated by this aqueous method show good reproducibility of high efficiency with average power conversion efficiency (PCE) of 18.7% and champion PCE of 20.1% under solar simulation. This study shows that water-perovskite interactions do not necessarily negatively impact the perovskite film preparation process even at the highest efficiencies and that exposure to high contents of water can actually enable humidity tolerance during fabrication in air.

Enhanced Thermochemical Stability of CH3NH3PbI3 Perovskite Films on Zinc Oxides via New Precursors and Surface Engineering

Hydroxyl groups on the surface of ZnO films lead to the chemical decomposition of CH₃NH₃PbI₃ perovskite films during thermal annealing, which limits the application of ZnO as a facile electron-transporting layer (ETL) in perovskite solar cells. In this work, we report a new recipe that leads to substantially reduced hydroxyl groups on the surface of the resulting ZnO films by employing polyethylenimine (PEI) to replace generally used ethanolamine in the precursor solutions. Films derived from the PEI-containing precursors are denoted as P-ZnO and those from the ethanolamine-containing precursors as E-ZnO. Besides the fewer hydroxyl groups that alleviate the thermochemical decomposition of CH₃NH₃PbI₃ perovskite films, P-ZnO also provides a template for the fixation of fullerene ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) owing to its nitrogen-rich surface that can interact with PCBM. The fullerene was used to block the direct contact between P-ZnO and CH₃NH₃PbI₃ films and therefore further enhance the thermochemical stability of perovskite films. As a result, perovskite solar cells based on the P-ZnO/PCBM ETL yield an optimal power conversion efficiency (PCE) of 15.38%. We also adopt P-ZnO as the ETL for organic solar cells that yield a remarkable PCE of 10.5% based on the PBDB-T:ITIC photoactive layer.

Origins and mechanisms of hysteresis in organometal halide perovskites

Inorganic-organic halide organometal perovskites, such as CH₃NH₃PbI₃ and CsPbI₃, etc, have been an unprecedented rising star in the field of photovoltaics since 2009, owing to their exceptionally high power conversion efficiency and simple fabrication processability. Despite its relatively short history of development, intensive investigations have been concentrating on this material; these have ranged from crystal structure analysis and photophysical characterization to performance optimization and device integration, etc. Yet, when applied in photovoltaic devices, this material suffers from hysteresis, that is, the difference of the current-voltage (I-V) curve during sweeping in two directions (from short-circuit towards open-circuit and vice versa). This behavior may significantly impede its large-scale commercial application. This Review will focus on the recent theoretical and experimental efforts to reveal the origin and mechanism of hysteresis. The proposed origins include (1) ferroelectric polarization, (2) charge trapping/detrapping, and (3) ion migration. Among them, recent evidence consistently supports the idea that ion migration plays a key role for the hysteretic behavior in perovskite solar cells (PSCs). Hence, this Review will summarize the recent results on ion migration such as the migrating ion species, activation energy measurement, capacitive characterization, and internal electrical field modulation, etc. In addition, this Review will also present the devices with alleviation/elimination of hysteresis by incorporating either large-size grains or phenyl-C61-butyric acid methyl ester molecules. In a different application, the hysteretic property has been utilized in photovoltaic and memristive switching devices. In sum, by examining these three possible mechanisms, it is concluded that the origin of hysteresis in PSCs is associated with a combination of effects, but mainly limited by ion/defect migration. This strong interaction between ion motion and free charge carrier transport can be modulated by the prevalent crystalline structure, chemical passivation, and an external photo/electrical field.

Fully Ambient-Processed Perovskite Film for Perovskite Solar Cells: Effect of Solvent Polarity on Lead Iodide

Fully ambient-processed and highly efficient methylammonium lead iodide (MAPbI₃) perovskite films are very desirable for industrial manufacturing of perovskite solar cells (PSCs). To date, most reported highly efficient MAPbI₃ PSCs rely on the fabrication of lead iodide (PbI₂) films inside the glovebox. Here we report a simple fabrication method using extra dry isopropanol (IPA100) for obtaining uniform and loosely packed PbI₂ film, which leads to a uniform and highly crystalline MAPbI₃ film under ambient conditions. Compared with recently reported results (10%-15%) using IPA treatment in the glovebox, we achieved over 16% efficiency of PSCs while fabricating perovskite films in fully ambient conditions. We have found the removal of even trace amounts of water from IPA to be a key factor for the successful ambient fabrication of PbI₂ films, as the high polarity of water negatively influences the crystallinity and morphology of the PbI₂ film.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Interface modification with PCBM intermediate layers for planar formamidinium perovskite solar cells

Efficient planar formamidinium perovskite solar cells without the hysteresis effect were fabricated by a two-step method with PCBM as the intermediate layers.

Ionic and Optical Properties of Methylammonium Lead Iodide Perovskite across the Tetragonal-Cubic Structural Phase Transition

Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI₃ ). However, the ionic and optical properties of MAPbI₃ in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 °C in the ionic/electrical properties of MAPbI₃ owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. These observations can be explained by the "continuous" nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal-cubic structural phase transition occurs.

MAPbI(3-x)Br(x) mixed halide perovskites for fully printable mesoscopic solar cells with enhanced efficiency and less hysteresis

Hybrid lead-halide perovskite solar cells (PSCs) are promising alternatives to silicon-based cells due to their high photovoltaic performance and low cost. We report herein fully printable perovskite solar cells with a mesoporous TiO2/Al2O3/C architecture in combination with mixed-halide MAPbI(3-x)Br(x) perovskites. A maximum conversion efficiency of 13.49% can be achieved with an increased open circuit voltage of 1.01 V, which is higher than the MAPbI3-based devices. A reduced anomalous hysteresis in the J-V curve measurement has been demonstrated in perovskite solar cells based on MAPbI2.4Br0.6 perovskite, which is directly linked to the characteristically slow kinetics measured through electrochemical impedance spectroscopy.

Systematic study on the impact of water on the performance and stability of perovskite solar cells

Sequentially spin-coated inverted perovskite solar cells were constructed with systematically varied concentrations of water in the PbI₂precursor solution. Surprisingly, small concentrations of water improved long-term stability, although with lower initial efficiency.

Humidity versus photo-stability of metal halide perovskite films in a polymer matrix

A polymer–perovskite matrix demonstrates enhanced humidity and photo-stability owing to the improved TiO₂/perovskite interface.

The interaction between hybrid organic–inorganic halide perovskite and selective contacts in perovskite solar cells: an infrared spectroscopy study

The interaction of hybrid organic–inorganic halide perovskite and selective contacts is crucial to get efficient, stable and hysteresis-free perovskite-based solar cells.