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Perovskite solar cells (PSCs) have attracted much attention due to their low-cost fabrication and high power conversion efficiency (PCE). However, the long-term stability issues of PSCs remain a significant bottleneck impeding their commercialization. Inverted PSCs with a p-i-n architecture are being actively researched due to their concurrent good stability and decent efficiency. In particular, the PCE of inverted PSCs has improved significantly in recent years and is now almost approaching that of n-i-p PSCs. This review summarizes recent progress in the development of high-efficiency inverted PSCs, including the development of perovskite compositions, fabrication methods, and counter electrode materials (CEMs). Notably, we highlight the development of charge transport materials (CTMs) and the effects of defect passivation strategies on the performance of inverted PSCs. Finally, we discuss the remaining issues and perspectives of high-efficiency inverted PSCs.Inverted perovskite solar cells (PSCs) with a p-i-n architecture are being actively researched due to their concurrent good stability and decent efficiency. In particular, the power conversion efficiency (PCE) of inverted PSCs has seen clear improvement in recent years and is now almost approaching that of n-i-p PSCs. Here, we systematically review recent progress in the development of high-efficiency inverted PSCs, and highlight the development of charge transport materials and the effects of defect passivation strategies on the performance of inverted PSCs, with the aim of providing constructive suggestions for the future development of inverted PSCs.

The International Telecommunication Union announced a new color gamut standard of broadcast service television (BT 2020) for ultra-high-definition TV in 2012. To satisfy the wide-color gamut standard of BT 2020, monochromatic red (R), green (G), and blue (B) emissions require a small full width at half-maximum, which is an important property for improving color purity. Although organic light-emitting diode (OLED) displays are currently one of the main types of display technologies, their broad emission via strong vibronic coupling between ground and excited states is a major hurdle to overcome in the development of next-generation wide-color gamut displays. Thus, the development of OLED emitters with narrowband R–G–B emissions is of great significance. In this review, the recent progress in the development of OLED materials with narrowband emission is summarized by grouping them into fluorescent, phosphorescent, and thermally activated delayed fluorescent emitters to reveal the correlation between molecular structures, optical properties, and device characteristics. We discuss rational molecular design strategies to achieve narrow photoluminescence and electroluminescence and the underlying mechanisms for controlling the emission bandwidth. Finally, the challenges in the realization of wide-color gamut OLED displays and the future prospects of such devices are discussed.Optoelectronics: An organic route to higher optical purityOrganic light-emitting diodes (OLEDs) with high color purity could be used in the next generation of high-definition televisions. The most widely used semiconductor, silicon, is an inorganic material but a wide range of organic alternatives are now emerging. These alternatives are especially in demand for light-emitting applications, where the performance of silicon is poor. Ji-Eun Jeong, Han Young Woo and colleagues from Korea University in Seoul, South Korea, reviewed recent progress in the development of OLEDs. An OLED tends to emit light over a relatively broad spectrum. This lack of color purity limits the device’s use in future ultra-high-definition TVs. The team presented an overview of the various molecular design strategies that have been used to reduce emission bandwidth and the physical mechanisms forming the basis of these strategies.With a growing demand for new emitters to realize ultra-high-definition displays, various types of organic emitters with narrow emission and high luminescent efficiency have been extensively studied. In this review, we summarized the recent developments of organic emitters (fluorescent, phosphorescent, and thermally activated delayed fluorescent) which show narrowband emission spectra with full-width half-maximum smaller than 50 nm.

Environmental degradation due to the carbon emissions from burning fossil fuels has triggered the need for sustainable and renewable energy. Hydrogen has the potential to meet the global energy requirement due to its high energy density; moreover, it is also clean burning. Photoelectrochemical (PEC) water splitting is a method that generates hydrogen from water by using solar radiation. Despite the advantages of PEC water splitting, its applications are limited by poor efficiency due to the recombination of charge carriers, high overpotential, and sluggish reaction kinetics. The synergistic effect of using different strategies with cocatalyst decoration is promising to enhance efficiency and stability. Transition metal-based cocatalysts are known to improve PEC efficiency by reducing the barrier to charge transfer. Recent developments in novel cocatalyst design have led to significant advances in the fundamental understanding of improved reaction kinetics and the mechanism of hydrogen evolution. To highlight key important advances in the understanding of surface reactions, this review provides a detailed outline of very recent reports on novel PEC system design engineering with cocatalysts. More importantly, the role of cocatalysts in surface passivation and photovoltage, and photocurrent enhancement are highlighted. Finally, some challenges and potential opportunities for designing efficient cocatalysts are discussed. The potential of transition metal-based cocatalysts for PEC water splitting has been outlined with a focus on the reported literature, ongoing research, and future perspectives.

Soft sensing technologies have the potential to revolutionize wearable devices, haptic interfaces and robotic systems. However, there are numerous challenges in the deployment of these devices due to their poor resilience, high energy consumption, and omnidirectional strain responsivity. This work reports the development of a versatile ionic gelatin-glycerol hydrogel for soft sensing applications. The resulting sensing device is inexpensive and easy to manufacture, is self-healable at room temperature, can undergo strains of up to 454%, presents stability over long periods of time, and is biocompatible and biodegradable. This material is ideal for strain sensing applications, with a linear correlation coefficient R 2  = 0.9971 and a pressure-insensitive conduction mechanism. The experimental results show the applicability of ionic hydrogels for wearable devices and soft robotic technologies for strain, humidity, and temperature sensing while being able to partially self-heal at room temperature. Versatile ionic gelatin-glycerol hydrogel for soft sensing applications: The sensing device is inexpensive and easy to manufacture, is self-healable at room temperature, can undergo strains of up to 454%, presents stability over long periods of time, and is biocompatible and biodegradable. This material is ideal for strain sensing applications, with a linear correlation coefficient R 2  = 0.9971 and a pressure-insensitive conduction mechanism. The experimental results show the applicability of ionic hydrogels for wearable devices and soft robotic technologies for strain, humidity and temperature sensing while being able to partially self-heal at room temperature.

The triboelectric nanogenerator (TENG) is a new type of energy generator first demonstrated in 2012. TENGs have shown potential as power sources for electronic devices and as sensors for detecting mechanical and chemical stimuli. To date, studies on TENGs have focused primarily on optimizing the systems and circuit designs or exploring possible applications. Even though triboelectricity is highly related to the material properties, studies on materials and material designs have been relatively less investigated. This review article introduces recent progress in TENGs, by focusing on materials and material designs to improve the electrical output and sensing performance. This article discusses the current technological issues and the future challenges in materials for TENG.Nanotechnology: Materials for harvesting energy from motionThe development of materials for a technology that uses the movement of the human body to provide power has been reviewed by scientists in South Korea. A triboelectric nanogenerator converts mechanical energy into electricity by harnessing the fact that two surfaces rubbing against one another can become electrically charged. This is known as the triboelectric effect. One exciting use for these nanogenerators is in wearable electronics, where the motion of the body provides the power. Unyong Jeong and colleagues from Pohang University of Science and Technology have reviewed recent progress in material advances in the four main elements of a triboelectric nanogenerator: the charge-generating layer, the charge-trapping layer, the charge-collecting layer, and the charge-storage layer. These improvements all aim to increase the electrical output of such devices.

Extensive environmental pollution caused by worldwide industrialization and population growth has led to a water shortage. This problem lowers the quality of human life and wastes a large amount of money worldwide each year due to the related consequences. One main solution for this challenge is water purification. State-of-the-art water purification necessitates the implementation of novel materials and technologies that are cost and energy efficient. In this regard, graphene nanomaterials, with their unique physicochemical properties, are an optimum choice. These materials offer extraordinarily high surface area, mechanical durability, atomic thickness, nanosized pores and reactivity toward polar and non-polar water pollutants. These characteristics impart high selectivity and water permeability, and thus provide excellent water purification efficiency. This review introduces the potential of graphene membranes for water desalination. Although literature reviews have mostly concerned graphene’s capability for the adsorption and photocatalysis of water pollutants, updated knowledge related to its sieving properties is quite limited. Graphene: making seawater drinkable with custom membranes Water desalination plants stand to markedly reduce their energy consumption once nanoporous graphene-based membranes are optimized. Purifying brackish water with reverse osmosis works most efficiently when membranes are as thin, selective and strong as possible. Mady Elbahri from Aalto University in Finland reviews efforts to achieve these goals using graphene monolayers perforated with nanoscale pores. Recent roll-to-roll fabrications of graphene onto polymer supports show that inexpensive, large-scale production of these ultrathin membranes is feasible. It remains challenging, however, to tailor the nanochannels and selectivity of single-atom-thick material. An alternative may be to switch to multilayered graphene oxide membranes. These substances can be engineered with different molecular cross-linking agents between each carbon layer, opening room to insert negatively charged functional groups that target and electrostatically repel sodium ions. Graphene nanomaterials hold great promise for the development of advanced water purification membranes, especially for water desalination. Their atomic thickness, extraordinary mechanical stability and potential for size-selective transport are ideal features, encouraging the membrane scientist across the world to investigate their applicability for water desalination. Graphene can potentially desalinate water either as monolayer or as multilayer membranes. In this review, we discuss these different classes of graphene membranes and highlight their merits and shortcomings. In addition, the theory behind their performance is presented in detail.

Electrospun nanofibers have received considerable attention in the field of soft electronics owing to their promising advantages and superior properties in flexibility and/or stretchability, conductivity, and transparency; furthermore, their one-dimensional nanostructure, high surface area, and diverse fibrous morphologies are also desirable. Herein, we provide an overview of electrospun nanofiber-based soft electronics. A brief introduction of the unique structure and properties of electrospun nanofiber materials is provided, and assembly strategies for flexible/stretchable electronics are highlighted. We then summarize the latest progress in the design and fabrication of representative flexible/stretchable electronic devices utilizing electrospun nanofibers, such as flexible/stretchable conductors, sensors, energy harvesting and storage devices, and transistors. Finally, a conclusion and several future research directions for electrospun nanofiber-based soft electronics are proposed.Flexible electronics: Spinning fibers for wearable technologyThe development of low-cost, efficient, and large-scale methods for fabricating ‘soft’ electronics, conducting materials with improved flexibility and stretchability, increases the range of possible applications. Flexible electronics are useful for foldable displays, healthcare monitoring, artificial skins and implantable bioelectronics. One approach to fabricating these devices is to construct them from conductive nanofibers. Takao Someya from the University of Tokyo and colleagues review recent advances in constructing nanofiber-based soft electronics using a technique called electrospinning. Electrospinning works by drawing a molten material through a nozzle into an electric field to produce strands much finer than a human hair. The authors review the structure and properties of electrospun nanofiber materials and the various strategies for assembling flexible and stretchable electronic devices such as sensors, transistors, and components for energy harvesting and storage.

An ideal hydrogel for biomedical engineering should mimic the intrinsic properties of natural tissue, especially high toughness and self-healing ability, in order to withstand cyclic loading and repair skin and muscle damage. In addition, excellent cell affinity and tissue adhesiveness enable integration with the surrounding tissue after implantation. Inspired by the natural mussel adhesive mechanism, we designed a polydopamine–polyacrylamide (PDA–PAM) single network hydrogel by preventing the overoxidation of dopamine to maintain enough free catechol groups in the hydrogel. Therefore, the hydrogel possesses super stretchability, high toughness, stimuli-free self-healing ability, cell affinity and tissue adhesiveness. More remarkably, the current hydrogel can repeatedly be adhered on/stripped from a variety of surfaces for many cycles without loss of adhesion strength. Furthermore, the hydrogel can serve as an excellent platform to host various nano-building blocks, in which multiple functionalities are integrated to achieve versatile potential applications, such as magnetic and electrical therapies. Biomedical engineering: tough, self-healing hydrogel mimics mussels A self-healing, super-resilient hydrogel that can accelerate skin regeneration has been made using an adhesive mechanism inspired by mussels. Hydrogels have similar structures to soft biological tissues and have great potential for tissue engineering applications. However, most are too fragile for use in the body and lack the ability to self-heal and adhere to tissue. Now, Xiong Lu from China's Southwest Jiaotong University and co-workers have synthesized a self-healing, super-resilient hydrogel using a process that preserves PDA's catechols – substances that impart mussels with high adhesiveness – when embedded in an elastic polymer matrix. The numerous non-covalent bonds between PDA catechols enable the hydrogel to perfectly re-form after being sliced open and help it stretch over 30 times its initial length without breaking. The material could also carry magnetic or conductive nanoparticles for future integrated healthcare applications. Inspired by mussel chemistry, a novel polydopamine–polyacrymide hydrogel simultaneously possesses super stretchability, stimuli-free self-healing properties, cell affinity and tissue adhesiveness. The current hydrogel lasts its adhesiveness for a long term, and can be repeatedly adhered on/stripped from a variety of substrates. The hydrogel can host various nano-building blocks and be tuned to magnetic and conductive hydrogels with above-mentioned properties.

Metal-to-insulator transition (MIT) behaviors accompanied by a rapid reversible phase transition in vanadium dioxide (VO2) have gained substantial attention for investigations into various potential applications and obtaining good materials to study strongly correlated electronic behaviors in transition metal oxides (TMOs). Although its phase-transition mechanism is still controversial, during the past few decades, people have made great efforts in understanding the MIT mechanism, which could also benefit the investigation of MIT modulation. This review summarizes the recent progress in the phase-transition mechanism and modulation of VO2 materials. A representative understanding on the phase-transition mechanism, such as the lattice distortion and electron correlations, are discussed. Based on the research of the phase-transition mechanism, modulation methods, such as element doping, electric field (current and gating), and tensile/compression strain, as well as employing lasers, are summarized for comparison. Finally, discussions on future trends and perspectives are also provided. This review gives a comprehensive understanding of the mechanism of MIT behaviors and the phase-transition modulations.

The separation of oily wastewater, especially emulsified oil/water mixtures, is a worldwide challenge because of the large amount of oily wastewater produced in many industrial processes and daily life. For the treatment of oily wastewater, membrane technology is considered the most efficient method because of its high separation efficiency and relatively simple operational process. In this short review, the recent development of advanced filtration membranes for emulsified oil/water mixture separation is presented. We provide an overview on both traditional filtration membranes, including polymer-dominated and ceramic-based filtration membranes, and recently developed nanomaterial-based functional filtration membranes, especially one-dimensional nanomaterials, for effectively treating emulsified oil/water mixtures. The liquid flux and antifouling property, which are the most important factors for membrane performance evaluation, are described for different types of membranes. Conclusions and perspectives concerning the future development of filtration membranes are also provided. Oil/water separation, especially emulsified oil/water mixture separation, has become a widespread concern because of the severe fouling problem caused by the easy adsorption of oil droplets onto the surfaces of filtration membranes. Many strategies have been employed to eliminate the fouling problem, but it remains a challenge and impedes the development of membrane technology. In this short review, we discuss the recent development of membrane technology for emulsified oil/water separation. As shown in the image, in addition to polymer- and ceramic-dominated traditional membranes, nanomaterial-based membranes have recently demonstrated their superiority and have achieved high performance. Filtration technology: Separating oil–water emulsions using filtration membranes Investigating the mechanics of oil–water separation is important for developing efficient membrane filtration technologies for oily wastewater. Oil-containing effluent is produced in vast quantities — both industrially and domestically — and oil removal, particularly from emulsions, poses a significant challenge for wastewater treatment. Jian Jin and colleagues from the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, China, review the most promising membrane filtration technologies for separating oil–water emulsions. Advancements in polymer- and ceramic-based filtration membrane technologies have improved both anti-fouling and flow rate performance. In the laboratory, ultrathin membranes based on one-dimensional nanomaterials such as carbon nanotubes show particular promise in providing high separation performance, even for surfactant-stabilized water-in-oil emulsions. With further development, nanomaterial-based membrane filter technologies have the potential to offer high-performance emulsion separation at industrial scales.