Electrochromic (EC) smart windows are able to vary their throughput of visible light and solar energy by the application of an electrical voltage and are able to provide energy efficiency and indoor ...comfort in buildings. Section 1 explains why this technology is important and timely by first outlining today's precarious situation concerning increasing energy use and associated effects on the world's climate, and this section also underscores the great importance of enhancing the energy efficiency of buildings by letting them function more in harmony with the environment—particularly its varying temperature—than is possible with current mainstream technologies. This same chapter also surveys recent work on the energy savings and other benefits that are possible with EC-based technologies. Section 2 then provides some notes on the history of the EC effect and its applications. Section 3 presents a generic design for the oxide-based EC devices that are most in focus for present-day applications and research. This design includes five superimposed layers with a centrally-positioned electrolyte connecting two oxide films—at least one of which having EC properties—and with transparent electrical conductors surrounding the three-layer structure in the middle. It is emphasized that this construction can be viewed as a thin-film electrical battery whose charging state is manifested as optical absorption. Also discussed are six well known hurdles for the implementation of these EC devices, as well as a number of practical constructions of EC-based smart windows. Section 4 is an in-depth discussion of various aspects of EC oxides. It begins with a literature survey for 2007–2013, which updates earlier reviews, and is followed by a general discussion of optical and electronic effects and, specifically, on charge transfer absorption in tungsten oxide. Ionic effects are then treated with foci on the inherent nanoporosity of the important EC oxides and on the possibilities to accomplish further porosity by having suitable thin-film deposition parameters. A number of examples on the importance of the detailed deposition conditions are presented, and Section 4 ends with a presentation of the EC properties of films with compositions across the full tungsten–nickel oxide system. Section 5 is devoted to transparent electrical conductors and electrolytes, both of which are necessary in EC devices. Detailed surveys are given of transparent conductors comprising doped-oxide semiconductors, coinage metals, nanowire meshes and other alternatives, and also of electrolytes based on thin films and on polymers. Particular attention is devoted to electrolyte functionalization by nanoparticles. Section 6 considers one particular device construction: A foil that is suitable for glass lamination and which, in the author's view, holds particular promise for low-cost large-area implementation of EC smart windows. Device data are presented, and a discussion is given of quality assessment by use of 1/f noise. The “battery-type” EC device covered in the major part of this critical review is not the only alternative, and Section 7 consists of brief discussions of a number of more or less advanced alternatives such as metal hydrides, suspended particle devices, polymer-dispersed liquid crystals, reversible electroplating, and plasmonic electrochromism based on transparent conducting oxide nanoparticles. Finally, Section 8 provides a brief summary and outlook. The aim of this critical review is not only to paint a picture of the state-of-the-art for electrochromics and its applications in smart windows, but also to provide ample references to current literature of particular relevance and thereby, hopefully, an easy entrance to the research field.
•Critical review of electrochromic oxide thin films and devices.•Variable transmittance of visible light and solar energy.•Fenestration in energy efficient buildings.
CdS thin films were deposited onto glass substrates by the chemical bath deposition (CBD). The influence of the deposition temperature varied from 55
°C to 85
°C in a step of 5
°C on the ...crystallographic structure, morphology as well as optical and electrical properties was investigated in detail. Increasing deposition temperature can promote phase transformation from cubic to hexagonal and improvement of crystallinity in CdS films. CdS film deposited at 75
°C shows compact and smooth surface, and excellent transmission in visible light range. The band gaps are found to decrease from 2.56
eV to 2.38
eV with the increase of deposition temperature, and the sub-band gap of about 2.32
eV is noticed at low deposition temperature of 55–70
°C. All CdS films are of n-type conductivity and the carrier concentration is in the order of 10
12–10
13
cm
−3. The lowest resistivity and highest mobility obtained are in the case of 85
°C.
In order to determine the influence of different types of magnetron sputtering (MS) depositions on the characteristics of Al-doped ZnO (AZO) thin films appropriate for applications as transparent ...electrodes in thin-film solar cells, transparent conducting AZO thin films were prepared on glass substrates at 200
°C by direct current (dc) magnetron sputtering (dc-MS), radio frequency (rf)-MS and rf power superimposed dc-MS (rf
+
dc-MS) depositions using an MS apparatus with the same AZO target. AZO thin films prepared by an rf
+
dc-MS deposition exhibited both a higher deposition rate than that found with rf-MS depositions and a lower resistivity or higher Hall mobility than those found with dc-MS. The lower dc sputter voltage featured in rf-MS and rf
±
dc-MS depositions, producing smoother surface morphology and better crystallinity than obtained with dc-MS depositions. The light scattering characteristics of surface-textured AZO thin films prepared by various types of MS depositions were evaluated by observing the surface texture and measuring the optical transmittance and the diffusive component; wet-chemical etching of the thin film surface was performed in a 0.1% HCl solution. The obtainable haze property in the range from visible to near infrared in AZO films prepared by an rf
+
dc-MS deposition was markedly better than that obtained with dc-MS depositions.
Several modern cutting edge technologies, including the superconducting technology, green energy generation/storage technology, and the emerging 5G networks technology, have some form of thin-film ...coatings. Hence, this critical review showcased the historical evolution, conventional deposition technologies with its application areas, growth modes, specific thin-film stress-state micro/nano-level measurement and models, and prospects projection of thin-film coatings. Specifically, the authors included simple schematics of the primary thin-film coating methods (chemical vapor deposition and physical vapor deposition methods), growth modes, residual stress evolution behavior from valuable up to date models to enhance in-depth understanding of the underlying principles of thin-film coatings techniques and challenges. Also, the authors pointed out specific deficiencies in the reported thin-film stress measurement/models approaches. It is scientifically shown that no coating technique or model has superior results in all scenarios, selecting a suitable coating technique or model depends on the targeted materials and functions of the thin-film system. According to the evaluated reports, the societal demand and specific challenge in the fabrication/applications of thin-film systems indicated that thin-film coatings and its associated challenges would remain vibrant and active research areas for periods far into the future. Thus, this report would serve as a guide and reference material for potential researchers in these areas for a considerable time.
The growing demand of flexible electronic devices is increasing the requirements of their power sources. The effect of bending in thin-film batteries is still not well understood. Here, we ...successfully developed a high active area flexible all-solid-state battery as a model system that consists of thin-film layers of Li
4
Ti
5
O
12
, LiPON, and Lithium deposited on a novel flexible ceramic substrate. A systematic study on the bending state and performance of the battery is presented. The battery withstands bending radii of at least 14 mm achieving 70% of the theoretical capacity. Here, we reveal that convex bending has a positive effect on battery capacity showing an average increase of 5.5%, whereas concave bending decreases the capacity by 4% in contrast with recent studies. We show that the change in capacity upon bending may well be associated to the Li-ion diffusion kinetic change through the electrode when different external forces are applied. Finally, an encapsulation scheme is presented allowing sufficient bending of the device and operation for at least 500 cycles in air. The results are meant to improve the understanding of the phenomena present in thin-film batteries while undergoing bending rather than showing improvements in battery performance and lifetime.
The effect of deposition time on the structural, electrical and optical properties of SnS thin films deposited by chemical bath deposition onto glass substrates with different deposition times (2, 4, ...6, 8 and 10
h) at 60
°C were investigated. The obtained films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and optical absorption spectra. All deposited films were polycrystalline and had orthorhombic structure with small crystal grains. Their microstructures had changed with deposition time, and their compositions were nearly stoichiometric. Electrical parameters such as resistivity and type of electrical conduction were determined from the Hall Effect measurements. Hall Effect measurements show that obtained films have p-type conductivity and resistivity values of SnS films have changed with deposition time. For allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, band gap values varied in the range 1.30–1.97
eV, 0.83–1.36
eV, 0.93–1.49
eV and 0.62–1.23
eV, respectively.
Low‐temperature solution processing opens a new window for the fabrication of oxide semiconductors due to its simple, low cost, and large‐area uniformity. Herein, by using solution combustion ...synthesis (SCS), p‐type Cu‐doped NiO (Cu:NiO) thin films are fabricated at a temperature lower than 150 °C. The light doping of Cu substitutes the Ni site and disperses the valence band of the NiO matrix, leading to an enhanced p‐type conductivity. Their integration into thin‐film transistors (TFTs) demonstrates typical p‐type semiconducting behavior. The optimized Cu5%NiO TFT exhibits outstanding electrical performance with a hole mobility of 1.5 cm2 V−1 s−1, a large on/off current ratio of ≈104, and clear switching characteristics under dynamic measurements. The employment of a high‐k ZrO2 gate dielectric enables a low operating voltage (≤2 V) of the TFTs, which is critical for portable and battery‐driven devices. The construction of a light‐emitting‐diode driving circuit demonstrates the high current control capability of the resultant TFTs. The achievement of the low‐temperature‐processed Cu:NiO thin films via SCS not only provides a feasible approach for low‐cost flexible p‐type oxide electronics but also represents a significant step toward the development of complementary metal–oxide semiconductor circuits.
A solution combustion synthesis is utilized to fabricate p‐type oxide thin‐film transistors (TFTs) at 150 °C. The doping of Cu into the NiO matrix can replace the Ni sites and enhance the p‐type conductivity. The optimized Cu5%NiO TFTs on both Si and ITO (indium tin oxide)/glass with ZrO2 gate dielectrics exhibit an average hole mobility of >1 cm2 V−1 s−1 and Ion/Ioff of 104.
The terahertz (THz) band has very attractive characteristics for sensing and biosensing applications, due to some interesting features such as being a non‐ionizing radiation, very sensitive to weak ...interactions, thus, complementing typical spectroscopy systems in the infrared. However, a fundamental drawback is its relatively long wavelength (10–1000 µm) which makes it blind to small features, hindering seriously both thin‐film and biological sensing. Recently, new ways to overcome this limitation have become possible thanks to the advent of metasurfaces. These artificial structures are planar screens usually made of periodic metallic resonators and whose electromagnetic response can be controlled at will by design. This design freedom allows metasurfaces to surpass the restrictions of classical THz spectroscopy, by creating fine details comparable to the size of the thin films or microorganisms under test. The strong field concentration near these small metasurface details at resonance makes them highly sensitive to tiny variations in the nearby environment, allowing for an enhanced detection more accurate than classical THz spectroscopy. The main advances in THz metasurface sensors from a historical as well as application‐oriented perspective are summarized. The focus is put mainly on thin‐film and biological sensors, with an aim to cover the most recent advances in the topic.
The exciting topic of terahertz (THz) sensing using metasurfaces, is comprehensively addressed using a historical as well as an application‐oriented perspective. Three main groups are covered: thin‐film sensors, biosensors, and the latest manufacturing techniques, able to achieve extreme sensitivity values. The compendium here presented should serve to frame this rapidly evolving topic with an updated thorough historical view.
•Synthesis of CuO/ZnO (C/Z) bilayer thin film via sol gel spin coating technique.•Improved CO2 response of C/Z bilayer thin film with respect to CuO and ZnO thin films.•CO2 adsorption modeling of C/Z ...bilayer thin film.•Estimation of activation energy, EA, and heat of adsorption, Q for CO2 adsorption on C/Z bilayer thin film.•Description of CO2 sensing mechanism in CuO, ZnO and C/Z bilayer thin films.
A CuO/ZnO (C/Z) bilayer thin film was fabricated with a porous top CuO layer to facilitate a sensitive and selective response towards CO2 gas. Such a sensor architecture allowed optimum oxygen and CO2 gas adsorption in the interfacial region. The C/Z thin-film sensor exhibited a good response (47%) for 2500 ppm CO2 at 375 °C as opposed to CuO (15%) at 300 °C and ZnO (16%) at 350 °C. The sensor was selective to CO2 in respect of CO and CH4 gases at 375 °C with selectivity factor κCO2 ∼ 5 and ∼ 8 for CO and CH4 respectively. By analyzing the conductance-time transients for the gas, the adsorption behavior of CO2 on the heterogenous C/Z bilayer thin-film sensor was established. CO2 obeyed an extended Freundlich model of adsorption. Theoretical analysis of the said adsorption model was performed through which the activation energy (EA) and heat of adsorption (Q) of CO2 gas were estimated. A complementary relationship between EA and Q was established. It was shown that EA decreases with increasing concentration from 123.95 to 108.36 kJ/mol for 1000–2500 ppm CO2 for energetically heterogeneous surfaces. Alternatively, Q values increase with increasing concentration from 59.73 to 71.65 kJ/mol for 500–2500 ppm CO2. The CO2 sensing mechanism was elucidated based on surface defects for CuO and ZnO. CO2 sensing in the C/Z bilayer thin-film sensor was controlled by the adsorption of oxygen forming a space charge layer at the surface and interface of the p-n heterojunction and by band-bending as a result of the change of electron concentration across the junction.
This paper describes the present status and prospects for further development of transparent conducting oxide materials for use as Indium-Tin-Oxide (ITO) substitutes in the thin-film transparent ...electrodes of liquid crystal displays (LCDs), currently the largest use of ITO, and, thus, of indium. The best substitute material for the ITO transparent electrodes used in LCDs is impurity-doped ZnO, e.g., Al- and Ga-doped ZnO (AZO and GZO). From resource and environmental points of view, AZO is the best candidate. The most important problems associated with substituting impurity-doped ZnO for the ITO used in LCDs have already been resolved in laboratory trials. Under the present circumstances, (rf and dc)-magnetron sputtering (rf
+
dc-MS) deposition, both with and without H
2 gas introduction, has been found to be the best deposition method to prepare impurity-doped ZnO thin films for practical use; AZO thin films with a resistivity on the order of 10
−
4 Ω cm were prepared on glass substrates with an approximately uniform resistivity spatial distribution and a thickness above 100 nm. In order to improve the resistivity stability, AZO and GZO thin films co-doped with another impurity have been newly developed. A 50 nm-thick V-co-doped AZO (AZO:V) thin film was stable enough to be acceptable for use in practical transparent electrode applications. However, it seems likely that obtaining a stability comparable to that of ITO using impurity-doped ZnO will be difficult for thin films with a thickness below approximately 30 nm.
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