Semitransparency is an attractive and important property in solar cells since it opens new possibilities in a variety of applications such as tandem cell configuration and building-integrated photovoltaics. Metal halide perovskite has the optimal properties to function as the light harvester in solar cells and can be made as a thin film, while its chemical composition can change its band gap. However, achieving high transparency usually compromises the solar cell's efficiency. Here we report on a unique approach to fabricating semitransparent perovskite solar cells that does not rely on their composition or their thickness. The approach is based on a scalable process, inkjet printing of arrays of transparent pillars, which are composed of inert photopolymerizable liquid compositions and are partly covered by the perovskite. This material can be printed at specific locations and array densities, thus providing a digital control of both the transparency and efficiency of the solar cells. The new semitransparent device structure shows 11.2% efficiency with 24% average transparency without a top metal contact. Further development including deposition of a transparent contact enabled the fabrication of fully semitransparent devices with an efficiency of 10.6% and average transparency of 19%.

Vanadium dioxide (VO2) is a unique active plasmonic material due to its intrinsic metal-insulator transition, remaining less explored. Herein, we pioneer a method to tailor the VO2 surface plasmon by manipulating its atomic defects and establish a universal quantitative understanding based on seven representative defective VO2 systems. Record high tunability is achieved for the localized surface plasmon resonance (LSPR) energy (0.66-1.16 eV) and transition temperature range (40-100 °C). The Drude model and density functional theory reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. We further demonstrate their superior performances in extensive unconventional plasmonic applications including energy-saving smart windows, wearable camouflage devices, and encryption inks. This journal is

Significant advancements in the perovskite solar cells/modules (PSCs/PSMs) toward better operational stability and large area scalability have recently been reported. However, semitransparent (ST), high efficiency, and large area PSMs are still not well explored and require attention to realize their application in building-integrated photovoltaics (BIPV). This work employs multiple synergistic strategies to improve the quality and stability of the ST perovskite film while ensuring high transparency. Europium ions, doped in the perovskite, are found to suppress the generation of detrimental species like elemental Pb and I, resulting in higher atmospheric stability. The effect of the top transparent contact is designed to obtain an average visible transparency (AVT) of >20% for full device and a green colored hue. Lastly, the lower current density due to the thinner ST absorber is enhanced by the application of a down-converting phosphor material which harvests low energy photons and inhibits UV-induced degradation. This multimodal approach renders a power conversion efficiency of 12% under dim light conditions and 9.5% under 1 sun illumination, respectively, on 21 cm2 ST-PSM.

The fabrication of Nd-Nb co-doped SnO2/α-WO3 electrochromic (EC) materials for smart window applications is presented in the present paper. Nb is a good dopant candidate for ECs owing to its ability to introduce active sites on the surface of α-WO3 without causing much lattice strain due to the similar ionic radius of Nb5+ and W6+. These active sites introduce more channels for charge insertion or removal during redox reactions, improving the overall EC performance. However, Nb suffers from prolonged utilization due to the Li+ ions trapped within the ECs. By coupling Nd with Nb, the co-dopants would transfer their excess electrons to SnO2, improving the electronic conductivity and easing the insertion and extraction of Li+ cations from the ECs. The enhanced Nd-Nb co-doped SnO2/α-WO3 exhibited excellent visible light transmission (90% transmittance), high near-infrared (NIR) contrast (60% NIR modulation), rapid switching time (∼1 s), and excellent stability (>65% of NIR modulation was retained after repeated electrochemical cycles). The mechanism of enhanced EC performance was also investigated. The novel combination of Nd-Nb co-doped SnO2/α-WO3 presented in this work demonstrates an excellent candidate material for smart window applications to be used in green buildings.

In an urbanized city, about a third of total electrical consumption is allocated for indoor lighting and air conditioning system in residential and commercial buildings. The majority of the worldwide energy generation comes from burning of non-renewable fossil fuel which is not sustainable in the long run. The use of smart windows technology may catalyze the effort to reduce energy consumption of building and houses. More than 50% of heat entering a building through windows originate from the solar radiation in the near infrared (NIR) region. This candidate smart window material must exhibit dual-band (visible and NIR) modulation that allows selective modulation of NIR heat without affecting visible light transmission. A good electrochromic material in this respect should possess high visible light transmission, high NIR modulation, fast switching between colored and bleached state, and good stability over prolonged usage. In this work, we propose a novel Nd–Mo co-doped SnO2/α-WO3 electrochromic materials (ECs). As compared to the traditional SnO2/α-WO3 ECs, our Nd–Mo co-doped SnO2/α-WO3 ECs exhibits up to 90% visible light transparency (at λ = 600 nm), 62% NIR modulation (at wavelength 1200 nm), high coloration efficiency ($\sim$200 cm2 C−1), fast switching time with only 31% electrochromic performance drop (vs 59% of undoped sample) after up to 1000 reversible cyclic test. The enhanced electrochromic performance comes from the presence of Nd–Mo co-dopants that limit the trapping of Li + ion within α-WO3 framework, reduce the extent of crystallization of α-WO3 layer and enhancement of the electronic conductivity by transferring their excess electron to the conduction band of the SnO2. To the best of the authors' knowledge, the present composition of ECs offers one of the better candidate materials for electrochromic to be used as thermal management layers on smart windows application.

UV-curable particle-free ceramic compositions for stereolithography-based 3D printing technologies present a promising alternative to the commonly used particle-based compositions. So far, such compositions were mainly based on solutions of pre-ceramic polymers which limit their applications to silicon-containing materials. However, the application of particle-free inks for the fabrication of other ceramic materials, in particular dense polycrystalline ones, is very little explored. We present a new and general fabrication approach based on all-solution compositions, by combining sol–gel chemistry and photopolymerization, for obtaining dense 3D ceramic structures by DLP printing. The process is demonstrated here for the fabrication of barium titanate (BaTiO3). By using chelating solvent and monomer, a stable UV-curable solution is obtained. An aging period of 8–14 days was crucial for obtaining dense ceramic objects without any secondary phases. The heat treatment was found to affect the microstructure, density and hardness of the resulting ceramics. The presented process enables obtaining objects free of carbon materials, having a density as high as 98% of the theoretical value, and a hardness of 4.3 GPa.

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Young's modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors.

Semitransparent solar cells are able to capitalize on land scarcity in urban environments by co-opting windows and glass structures as power generators, thereby expanding the capacity of photovoltaics to meet energy needs. To be successful, devices must be efficient, possess good visual transparency, long-term stability, and low cost. Copper zinc tin sulfide is a promising thin-film material that consists of earth-abundant elements. For optical transparency, the usual molybdenum back contact is replaced with a transparent conducting oxide (TCO). However, due to subsequent high-temperature annealing, the TCO degrades, losing conductivity, or forms a poor interface with CZTS. Lower temperatures mitigate this issue but hinder grain growth in CZTS films. Herein, cadmium substitution and silver and sodium doping are used to aid grain growth and improve film quality at lower annealing temperatures. Thin molybdenum is sputtered on TCO to help improve the interface transition postannealing by conversion to MoS2. Rapid thermal processing is used to minimize high-temperature exposure time to preserve the TCO. With these methods, a semitransparent device with a front illumination efficiency of 2.96% is demonstrated.

Vanadium dioxide (VO2) based thermochromic smart window is considered as the most promising approach for economizing building energy consumption. However, the high phase transition temperature (τc), low luminous transmission (Tlum), and solar modulation (ΔTsol) impose an invertible challenge for commercialization. Currently, smart window research surprisingly assumes that the sunlight radiates in one direction which is obviously not valid as most regions receive solar radiation at various angles in different seasons. For the first time, solar elevation angle is considered and 3D printing technology is employed to fabricate tilted microstructures for modulating solar transmission dynamically. To maximize energy-saving performance, the architecture of the structures (tilt, thickness, spacing, and width) and tungsten (W) doped VO2 can be custom-designed according to the solar elevation angle variation at the midday between seasons and tackle the issue of compromised Tlum and ΔTsol with W-doping. The energy consumption simulations in different cities prove the efficiency of such dynamic modulation. This first attempt to adaptively regulate the solar modulation by considering the solar elevation angle together with one of the best reported thermochromic properties (τc = 40 °C, Tlum(average) = 40.8%, ΔTsol = 23.3%) may open a new era of real-world-scenario smart window research.

Ordered mesoporous silica materials gain high interest because of their potential applications in catalysis, selective adsorption, separation, and controlled drug release. Due to their morphological characteristics, mainly the tunable, ordered nanometric pores, they can be utilized as supporting hosts for confined chemical reactions. Applications of these materials, however, are limited by structural design. Here, we present a new approach for the 3D printing of complex geometry silica objects with an ordered mesoporous structure by stereolithography. The process uses photocurable liquid compositions that contain a structure-directing agent, silica precursors, and elastomer-forming monomers that, after printing and calcination, form porous silica monoliths. The objects have extremely high surface area, 1900 m2/g, and very low density and are thermally and chemically stable. This work enables the formation of ordered porous objects having complex geometries that can be utilized in applications in both the industry and academia, overcoming the structural limitations associated with traditional processing methods.

4D printed objects are 3D printed structures whose shape, property, and functionality are able to self-transform when exposed to a predetermined stimulus. The emerging field of 4D printing has attracted wide interest from both academia and industry since first introduced in 2013. Stimuli-responsive hydrogels have become a competitive and versatile group of materials for 4D printed devices due to their good deformability, promising biocompatibility, simple manufacturing, and low cost. This review aims to provide a summary of the current progress of hydrogel-based 4D printed objects and devices based on their fabrication techniques, materials, and applications. Herein, presented are: the characteristics of different additive manufacturing methods such as direct ink writing, fused deposition modeling, and stereolithography; the properties of various stimuli-responsive hydrogels such as poly(N-isopropylacrylamide) and poly(N,N-dimethylacrylamide), alginate, etc.; and diverse applications of 4D printed hydrogels such as actuators, cellular scaffolds, and drug release devices. Opportunities and challenges for 4D printed hydrogels are discussed and prospects for future development are elaborated.

With the advancement of wearable electronics, stretchable energy harvesters are attractive to reduce the need of frequent charging of wearable devices. In this work, a stretchable kirigami piezoelectric nanogenerator (PENG) based on barium titanate (BaTiO3) nanoparticles, Poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) matrix, and silver flakes-based electrode is fabricated in an all-3D printable process suited for additive manufacturing. The 3D printable extrusion ink is formulated for facile solvent evaporation during layer formation to enable heterogenous multilayer stacking. A well-designed modified T-joint-cut kirigami structure is realized to attain a non-protruding, high structural stretchability performance, overcoming the out-of-plane displacement of the typical kirigami structure and therefore enabling the pressing-mode of a kirigami-structured PENG. This PENG can be stretched to more than 300% strain, which shows a great potential for application in wearable electronic systems. Furthermore, a self-powered gait sensor is demonstrated using this PENG.

Privacy and energy-saving are key functionalities for next-generation smart windows, while to achieve them independently on a window is challenging. Inspired by the cephalopod skin, we have developed a versatile thermo- and mechano-chromic design to overcome such challenge and reveal the mechanism via both experiments and simulations. The design is facile with good scalability, consisted of well-dispersed vanadium dioxide (VO2) nanoparticles (NPs) with temperature-dependent localized surface plasmon resonance (LSPR) in transparent elastomers with dynamic micro wrinkles. While maintaining a fixed solar energy modulation of (ΔTsol), the design can dynamically control visible transmittance (Tvib) from 60% to 17%, adding a new dimension to VO2-based smart windows. We prove that the optical modulation relies on the microtexture-induced broadband diffraction and the plasmon-enhanced near-infrared absorbance of VO2 NPs. We further present a series of modified designs towards additional functionalities. This work opens an avenue for independent dual-mode windows and it may inspire development from fundamental material, optic, and mechanical science to energy-related applications.

A novel approach for fabricating selective absorbing coatings based on carbon nanotubes (CNTs) for mid-temperature solar–thermal application is presented. The developed formulations are dispersions of CNTs in water or solvents. Being coated on stainless steel (SS) by spraying, these formulations provide good characteristics of solar absorptance. The effect of CNT concentration and the type of the binder and its ratios to the CNT were investigated. Coatings based on water dispersions give higher adsorption, but solvent-based coatings enable achieving lower emittance. Interestingly, the binder was found to be responsible for the high emittance, yet, it is essential for obtaining good adhesion to the SS substrate. The best performance of the coatings requires adjusting the concentration of the CNTs and their ratio to the binder to obtain the highest absorptance with excellent adhesion; high absorptance is obtained at high CNT concentration, while good adhesion requires a minimum ratio between the binder/CNT; however, increasing the binder concentration increases the emissivity. The best coatings have an absorptance of ca. 90% with an emittance of ca. 0.3 and excellent adhesion to stainless steel.

Electrochromic smart windows, with the ability to dynamically modulate thermal radiation transmission, are the key technologies to preserve energy expenditure for indoor lighting and air-conditioning. Despite receiving numerous exertions on design and fabrication technique, smart windows have rarely been commercially employed in the building industry due to unreliable lifetime, poor heat switching performance as well as high fabrication costs. Herein, we introduce a novel strategy in designing smart glass device, which focuses on the development of functionalized MxSnO2 nano-frameworks for electrochromic coating. The hybrid structures based on such nano-frameworks do not change the amorphous nature of electrodeposited tungsten trioxide (ɑ-WO3) layer and therefore are able to preserve its excellent electrochromic properties. Novel hybrid nano-structures of MxSnO2/ɑ-WO3 are able to encompass all desired features of a smart window, including the ability to block more than 95% NIR radiation in colored state while still allow about 80% of visible light transmittance in bleached state, rapid electro-optical response time of about 10 s and improved coloration efficiencies. More importantly, the advanced MxSnO2/ɑ-WO3 nanostructures can also retain their structure and functionality for at least 1000 switching cycles due to the enhanced binding strength. In addition, the synthetic recipe of such functionalized nano-framework is facile and cost-effective, enabling the fabrication on any template type and size.

Graphene and its derivatives have been reported as materials with excellent electrical and thermal conductivity, allowing for various promising applications. In particular, the large-scale surface coating of graphene-based materials can be employed to minimize cross-sectional heat transfer through the glass window. This study introduces a facile and cost-effective method to fabricate graphene quantum dots (GQDs) thin film on Fluorine-doped Tin Oxide (FTO) glass via casting of the GQDs dispersion and stabilizing with poly-vinyl-pyrrolidone (PVP). The thin film possesses excellent optical properties of GQDs and allows more than 80 % of visible transmittance. The presence of the GQDs thin film shows effective reduction in the cross-sectional thermal diffusivity of FTO glass, from 0.55 mm2/s to zero when measured with laser flash over a 4-second period. This low cost and eco-friendly GQDs thin film will be a promising material for heat management in smart window applications.

Second skin is a topically applied, skin-conforming material that mimics human skin properties and bears potential cosmetic and e-skin applications. To successfully integrate with natural skin, characteristics such as color and skin features must be matched. In this work, we prepared bio-based skin-like films from cross-linked keratin/melanin films (KMFs), using a simple fabrication method and non-toxic materials. The films retained their stability in aqueous solutions, showed skin-like mechanical properties, and were homogenous and handleable, with non-granular surfaces and a notable cross-linked structure as determined by attenuated total reflection (ATR). In addition, the combination of keratin and melanin allowed for adjustable tones similar to those of natural human skin. Furthermore, KMFs showed light transmittance and UV-blocking (up to 99%) as a function of melanin content. Finally, keratin/melanin ink (KMI) was used to inkjet-print high-resolution images with natural skin pigmented features. The KMFs and KMI may offer advanced solutions as e-skin or cosmetics platforms.

A near-perfect black solar absorber made of carbon nanotubes (CNTs) prepared by a low-cost wet-deposition method on a reflective metal surface for mid-temperature non-evacuated concentrated solar power (CSP) applications is demonstrated. The dispersed CNTs in an alumina–silica matrix exhibit an absorptance of 0.985 in the entire solar spectrum and emittance of 0.90 in the infrared (IR) region. The coating shows high durability and is super-hydrophilic (0° contact angle) after plasma treatment, without affecting the solar absorptance and excellent coating adhesion. The efficiency of the coating is evaluated by analytical models, which implies that it has higher efficiency at low temperature and at a high solar concentration ratio than that of previously reported selective coatings.

The 3D printing process enables to print objects having various functionalities at pre-designed locations in the object. Hereby, we report on the printing of superhydrophobic objects composed of patterns of micropillars. Superhydrophobicity is an important property of surfaces that has applications in various fields such as self-cleaning, drag reduction, increased buoyancy, and air conditioning. Most existing methods for the fabrication of superhydrophobic surfaces are complicated and time-consuming. Here, we performed a simple and cost-effective process for the fabrication of superhydrophobic (SH) objects by Digital Light Processing (DLP) 3D printing. To the best of our knowledge, this is the first study that has used DLP 3D printing to fabricate SH 3D objects without further coating process. We designed a novel ink, which contained non-fluorinated acrylates and Hydrophobic Fumed Silica (HFS). We studied the effects of HFS concentration and pillar-array design for imparting the SH property, which was measured in terms of the contact and rolling angle of water droplets on the surface. As proof of the concept of increased buoyancy by superhydrophobicity, we demonstrated the floatation of the printed SH objects in comparison to their non-SH counterparts even after forcefully submerging them into water.