Stable Thin Films Research Articles

In Situ Growth of [hk1]‐Oriented Sb2S3 for Solution‐Processed Planar Heterojunction Solar Cell with 6.4% Efficiency

AbstractBinary compound antimony sulfide (Sb2S3) with its nontoxic and earth‐abundant constituents, is a promising light‐harvesting material for stable and high efficiency thin film photovoltaics. The intrinsic quasi‐1D (Q1D) crystal structure of Sb2S3 is known to transfer photogenerated carriers rapidly along the [hk1] orientation. However, producing Sb2S3 devices with precise control of [hk1] orientation is challenging and unfavorable crystal orientations of Sb2S3 result in severe interface and bulk recombination losses. Herein, in situ vertical growth of Sb2S3 on top of ultrathin TiO2/CdS as the electron transport layer (ETL) by a solution method is demonstrated. The planar heterojunction solar cell using [hk1]‐oriented Sb2S3 achieves a power conversion efficiency of 6.4%, performing at almost 20% higher than devices based on a [hk0]‐oriented absorber. This work opens up new prospects for pursuing high‐performance Sb2S3 thin film solar cells by tailoring the crystal orientation.

Highly stable thin film organic solar cells using poly crystallized silver doped LaPO4

Silver doped lanthanum phosphate (LaPO4:Ag) nano-composite was successfully synthesized at various concentrations of silver using wet chemistry. The newly synthesized particles were used in the solar absorber medium of thin-film organic solar cells to assist in photons harvesting. The photoactive layer of the solar cell was composed of poly-3-hexylthiophene(P3HT) and (6–6) phenyl-C61-butyric acid methyl ester (PCBM) molecules blend with a small concentration of LaPO4:Ag by weight. The solar cells were fabricated in an inverted device architecture consisting of layers of materials as ITO/ZnO/P3HT:PC61BM-LaPO4:Ag/MoO3/AL. As a result, significant power conversion efficiency was recorded as high as 5.23% using the newly synthesized nano-composite at a silver concentration of 0.05 mol. This is an excellent achievement for ambient device preparations conditions which is over 124% growth in PCE compared to undoped devices. The incorporation of LaPO4:Ag into the polymers blend absorber layer is expected to broaden optical absorption and enhance photons harvesting. The inclusion of silver in the composite could also be expected to trigger surface plasmon resonance effect. Moreover, LaPO4:Ag doped solar cells appeared to be highly stable under ambient environment. The optical and morphological properties of the nano-composite are well presented and discussed.

Scanning process of micro-spark assisted chemical engraving (SACE) based on thin laminar flow electrolytes and given narrow machining gaps

Spark assisted chemical engraving (SACE) has the advantages of less contact stress and little/no tool electrode wear, and it is expected to achieve damage-free processing on dielectric workpieces. The key factors of electrolyte depths and machining gaps play an important role in the SACE process, especially for a micro-SACE process. Currently, there is still no convenient method for forming a thin and stable electrolyte depth in a scanning process of micro-SACE, and the existing methods are difficult to specify a micron-level machining gap between a tool electrode and a workpiece. In order to solve these problems, a scanning process of micro-SACE based on the thin laminar flow electrolyte and the narrow machining gap was proposed. This research focuses on the experiments to obtain the controllable rules of a stable thin film of electrolyte for concentrating the processing energy and restricting the processing locality. A novel method of indirect electric contact was presented for measuring workpiece surface height and then precisely leveling the workpiece surface before the scanning process. A micro-feed contact method with CCD (charge coupled device) vision was used to give a narrow machining gap. In experiments, the leveling accuracy of workpiece surface can reach ± 0.1 μm/mm, and the adjusting resolution of micro-machining gaps can be down to a precise scale of ± 0.5 μm. As a result, a micro-structure of 1.2 × 1.2 mm with a depth removal resolution down to ~ 2 μm was achieved on the surface of ZrO2 workpiece with the flow layer electrolyte as thin as 165 ± 5 μm.

Carbazole Electroactive Amorphous Molecular Material: Molecular Design, Synthesis, Characterization and Application in Perovskite Solar Cells

In perovskite photovoltaics (PSCs), the role of the hole transporting material (HTM) is highly important as it significantly influents to the global device’s performance and stability. Hole transporter ensures the extraction of hole at the perovskite/HTM interface and transport it towards the cathode. Thus, accurate molecular design affording to efficient and cost-effective HTM is of major interest. Small molecules having glass forming property is an attractive class as it can form morphologically stable thin film. Herein, a carbazole molecular glass bearing a polymerizable function was designed and synthetized. Its characteristics are suitable for application as HTM in PSCs. The preliminary photovoltaic application lead to device efficiency of 14–15% depending on the chemical composition of the perovskite employed. These promising results open the way to design new alternative molecular and polymeric HTMs suitable for solution processed hybrid solar cells.

Extraordinary Mass Transport and Self‐Assembly: A Pathway to Fabricate Luminescent CsPbBr3 and Light‐Emitting Diodes by Vapor‐Phase Deposition

AbstractHalide perovskites have been shown to be promising materials in making light‐emitting diodes. At present, almost all of perovskite materials are made by solution‐based synthesis. There are very limited reports on fabricating perovskite LEDs by vapor‐phase deposition (VPD), a method that can be easily scaled up for commercial production. In this paper, dual‐source VPD is used to fabricate stable CsPbBr3 perovskite thin films with excellent luminescent properties. Scanning electron microscope and atomic force microscope studies show that CsPbBr3 films, when coated with a thin LiBr overlayer, demonstrate an extraordinary mass transport at room temperature to re‐assemble into well‐defined islands. LiBr is also shown to passivate nonradiative defects and boost photoluminescence performance of the CsPbBr3, improving the intensity by a factor of 11 for a nominal 18 nm perovskite film and leading to extremely narrow photoluminescence peaks (16 nm FWHM). This self‐assembled perovskite LED shows major improvement in the electroluminescence performance, almost tripling the brightness of reference devices. X‐ray photoelectron spectroscopy measurement shows that surface LiBr improves Cs/Pb chemical stoichiometry, reduces Br vacancies, and shift the Fermi energy level toward conduction band minimum.

Thermally stable organic thin film transistors based on 2-(anthracen-2-yl)tetracene

Abstract Organic semiconductors with high mobility and high thermal stability are of great importance for practical application of organic electronics. To explore new semiconductors by taking advantage of the intrinsic properties of tetracene molecule, herein, we report the design and synthesis of a novel p-type tetracene derivatives, 2-(anthracen-2-yl)tetracene (TetAnt). Top contact organic thin-film transistors (OTFTs) based on TetAnt show a hole mobility of up to 0.79 cm2 V−1 s−1. In addition, a high mobility of ~0.4 cm2 V−1 s−1 is maintained even after thermal stressed to a high temperature of 290 °C, indicating the excellent thermal stability of TetAnt.

All‐Solution Processed Multicolor Patterning Technique of Perovskite Nanocrystal for Color Pixel Array and Flexible Optoelectronic Devices

AbstractIn the present study, a new patterning method is introduced through the surface modification and stabilization of perovskite nanocrystals, which is compatible with conventional photolithography process based on all‐solution processes. Chemically designed gel‐type silica‐coated CsPbX3 (X = Br, I, etc.) perovskite nanocrystals combined with dip coating method are introduced to form stable and uniform films. Analyses of the physical and chemical states of nanocrystals and investigation of the kinetics in silica formation are conducted. In an optimized condition, physically uniform and chemically stable perovskite thin films are deposited on various substrates such as flexible, stretchable substrates, or even nonflat objects. By adopting these advantages and developing stable photolithographic chemicals, the high resolution patterns are successfully patterned with green and red emitting CsPbBr3 and CsPbBr3I3−x perovskites with the size down to 5 µm of radius and even a multicolor pixel array which can be used for the color filter, light converting or detecting applications. Flexible white light emitting diode is also fabricated with a large color gamut coverage. This work provides a fundamental understanding of perovskite nanocrystals, and also offers a technological breakthrough enabling various optoelectronic applications.

Metallic amorphous alloy for long-term stable electrodes in organic sensors and photovoltaics

Amorphous metallic alloy, i.e. metallic glass (MG), is a metal without long-range ordered atomic arrangements. The unique structural property of MGs contributes to the superior mechanical and chemical properties to that of conventional metals. In this research, the unique characteristics of MG is considered for fabrication of long-term stable thin film strain sensors and the electrode for organic solar cells. MG thin film with zirconium (Zr) based composition exhibited encouraging results on strain sensing, showing high mechanical stability with reversible resistive response under the applied strain by bending. Furthermore, MG thin films with aluminum (Al) based composition exhibited high chemical stability, which contributed to the organic solar cell's stable operation without significant degradation in performance. Thus, the results suggest that MG is a promising material to realize long-term sustainable organic photovoltaic device. • Newly designed metallic glasses (MGs) of zirconium (Zr) and aluminum (Al) were investigated for organic thin film devices. • Zr-based MG exhibited improved stable resistive response with respect to exerted strains. • Al-based MG was sputtered onto MoO x layer for long-term stable photovoltaic operations. • MG electrodes’ superior mechanical and chemical stability contribute to enhanced reliability of the flexible devices.

Single step deposition of Cs2SnIxCl6-x thin films with uniform morphology, composition and high air stability

The present work reports single step spray deposition of cesium tin mixed halide perovskite thin films using a precursor solution containing cesium chloride (CsCl) tin chloride (SnCl2) and iodine powder. At the optimized spray conditions, the role of solvents, substrate temperature and iodine concentration on the structure and properties of the perovskite films were investigated. X-ray diffraction patterns and Raman spectra showed that the thin films were Cs2SnIxCl6-x, a mixed halide double perovskite having cubic crystal structure. The elemental composition and their chemical states were determined using X-ray photoelectron spectroscopy. The Cs2SnIxCl6-x perovskite thin films showed bandgap values ranging from 3.1 to 1.5 eV as x varied from 0 to 0.25. Further, the films showed uniform and compact morphology with good ambient air stability. Our results demonstrated that spray deposition is a reproducible technique to synthesize stable inorganic lead-free perovskite thin films.

IR spectroscopic and photoluminescence studies of plasma polymerized organic thin films based on tea tree oil

Plasma-assisted synthesis of transparent, environment friendly, lightweight, flexible and stable organic thin films from naturally occurring precursors were emerging as potential candidates for organic semiconductor industry. In the present study, tea tree (Malaleuca alternifolia) oil based polymer thin films were deposited on glass and silicon substrate by using radio frequency plasma polymerization technique. The polymer thin films were characterized with atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy techniques. AFM images indicate the formation of homogenous film on the substrate surface. FTIR spectra gives bands related to methyl and methylene groups which confirms the formation of chain branching in the polymer films. Relatively intense infrared (IR) bands obtained from films deposited on glass substrate reveals that glass substrate is more favourable for the growth of polymers than silicon substrate. Optical band gap of the polymerized thin film on glass substrate was estimated using Tauc plot which gives a value of 3.19 eV, indicating the semiconducting nature of the material. Photoluminescence (PL) emission in the yellow region were observed from both the samples and its CIE colour coordinates also matches with yellow emission. Broad visible emission observed in the wavelength range 465–695 nm indicates the presence of multichromophores in the polymer film. Samples also gives IR emission in the wavelength range of 850–1090 nm, upon excitation with a wavelength of 785 nm contributed to polaronic transitions.

Synthesis of Highly-Oriented Black CsPbI3 Microstructures for High-Performance Solar Cells

Synthesis of a thin layer of perovskite with less pinholes and defects is vital for the construction of a high-performance perovskite solar cell (PSC). We report on an oriented attachment strategy to fabricate a black phase CsPbI3 highly oriented quasi-single-crystalline microstructure film for constructing a mesoporous PSC device with enhanced power conversion efficiency, from 13.4% to 16.0%. The theoretical calculations and detailed in/ex situ characterizations demonstrate that the oriented attachment is driven by the predominant adsorption of dimethyl sulfoxide (DMSO) on the {212} facets of the yellow phase δ-CsPbI3 crystals which initially form a relatively oriented structure followed by phase transition to α-CsPbI3 crystals and merging into a large highly oriented quasi-single-crystalline microstructure film with {200} planes exposed under annealing. Our work demonstrates that the oriented attachment strategy enables the formation of highly oriented quasi-single-crystalline perovskite microstructure films which are essential for realizing high-efficiency and stable thin film PSCs.

Self-Sustaining 3D Thin Liquid Films in Ambient Environments.

Thin liquid films (TLF) have fundamental and technological importance ranging from the thermodynamics of cell membranes to the safety of light-water cooled nuclear reactors. The creation of stable water TLFs, however, is very difficult. In this paper, the realization of thin liquid films of water with custom 3D geometries that persist indefinitely in ambient environments is reported. The wetting films are generated using microscale "mounts" fed by microfluidic channels with small feature sizes and large aspect ratios. These devices are fabricated with a custom 3D printer and resin, which were developed to print high resolution microfluidic geometries as detailed in Reference 26. By modifying the 3D-printed polymer to be hydrophilic and taking advantage of well-known wetting principles and capillary effects, self-sustaining microscale "water fountains" are constructed that continuously replenish water lost to evaporation while relying on surface tension to stabilize their shape. To the authors' knowledge, this is the first demonstration of stable sub-micron thin liquid films (TLFs) of pure water on curved 3D geometries.

Morphological and Structural Properties of Sol-Gel Derived ZnO Thin Films Spin-Coated on Different Substrates

In this work, ZnO thin films were prepared by the low-cost sol-gel deposition method onto six different substrates (glass, ITO coated glass, sapphire (Al2O3), p-Si, p-GaN and polyethylene terephthalate (PET)) to study the effects of these substrates on the morphological and structural properties of the produced films. Precursor solution is Zinc acetate dihydrate based dissolved in ethanol with monoethanolamine (C2H7NO) added to act as a stabilizing agent to the sol. The corresponding ZnO thin films were characterized using field emission scanning electron microscopy (FESEM), high resolution X-ray diffraction (XRD) and atomic force microscopy (AFM). Results revealed distinct morphological and structural properties of ZnO thin films deposited on each substrate. The most uniform morphology was identified on glass, owing to the acquisition of the averagely stable grain sizes (58 nm – 61 nm) and thin film thicknesses (280 nm – 325 nm). High resolution XRD analysis showed that the films deposited on glass, ITO, p-Si, and p-GaN were attributed to hexagonal crystallite structures while the films deposited on sapphire and PET substrates exhibited amorphous phases. Amongst the samples, the ZnO thin film spin coated on p-Si demonstrated preferred orientation in (002) direction.

High-performing self-driven ultraviolet photodetector by TiO2/Co3O4 photovoltaics

Ultraviolet (UV) may provide the essential to maintain the human body and metabolism, however, excessive exposure to UV light is absolutely harmful. We may take advantage of the strong UV energy for energy conversion devices. A stable, simple and scalable heterojunction (TiO2/Co3O4) thin film was developed for UV-absorbing and visible-blind photovoltaic. Effective hole transport layer (HTL, NiO) insertion established photovoltaics, having a power conversion efficiency of 15.8%. The heterojunction exhibits the improved carrier lifetime from 1.94 ms to 9 ms, resulting in the enhancement of both photovoltage and photocurrent from 0.17 V to 0.78 V and from 0.54 mA cm−2 to 13.5 mA cm−2, respectively. The TiO2/Co3O4 device exhibits a photoresponse to a UV light intensity of 60 μW cm−2 in the presence of sunlight and works stable for over a year. The results indicate metal oxide heterojunction (TiO2/Co3O4) photovoltaic device could be useful for monitoring UV radiation exposure and cost-effective self-powered UV optoelectronics. The functional utilization of metal oxide layers would provide strong benefits for photoelectric devices, such as photodetectors and transparent solar cells.

Understanding mercury methylation in the changing environment: Recent advances in assessing microbial methylators and mercury bioavailability

Methylmercury (MeHg) is a neurotoxin, mainly derived from microbial mercury methylation in natural aquatic environments, and poses threats to human health. Polar regions and paddy soils are potential hotspots of mercury methylation and represent environmental settings that are susceptible to natural and anthropogenic perturbations. The effects of changing environmental conditions on the methylating microorganisms and mercury speciation due to global climate change and farming practices aimed for sustainable agriculture were discussed for polar regions and paddy soils, respectively. To better understand and predict microbial mercury methylation in the changing environment, we synthesized current understanding of how to effectively identify active mercury methylators and assess the bioavailability of different mercury species for methylation. The application of biomarkers based on the hgcAB genes have demonstrated the occurrence of potential mercury methylators, such as sulfate-reducing bacteria, iron-reducing bacteria, methanogen and syntrophs, in a diverse variety of microbial habitats. Advanced techniques, such as enriched stable isotope tracers, whole-cell biosensor and diffusive gradient thin film (DGT) have shown great promises in quantitatively assessing mercury availability to microbial methylators. Improved understanding of the complex structure of microbial communities consisting mercury methylators and non-methylators, chemical speciation of inorganic mercury under geochemically relevant conditions, and the pathway of cellular mercury uptake will undoubtedly facilitate accurate assessment and prediction of in situ microbial mercury methylation.

Highly Stable and Rapid Switching Electrochromic Thin Films Based on Metal-Organic Frameworks with Redox-Active Triphenylamine Ligands.

Metal-organic frameworks (MOFs), known for their tailorable porous structures and large specific surface areas, are appealing for electrochromic applications as their abundant pores may greatly benefit the charge transport required for electrochromic switching. Herein, for the first time, a simple, scalable, and cost-effective electrochemical deposition method for fabrication of high-performance and durable MOFs-based electrochromic films with redox-active ligands was developed. The fabricated film can achieve rapid switching speed (both coloration and bleaching time <5 s) because the inherent cavities of the MOFs greatly facilitate ion insertion and extraction. In addition, the film constructed with optimized parameters shows a high optical contrast of 65%@700 nm and can be stably switched for 1000 cycles with <5% contrast attenuation, which is by far the best cycling performance for MOFs-based electrochromic materials ever reported. Furthermore, our method enables the scalable preparation of large-area MOFs-based electrochromic thin films without using large high-pressure reaction vessels, and the as-prepared film in this work could be switched well between colored and bleached states. This new method, therefore, opens up a new avenue to broaden the use of MOFs-based thin films for electrochromic applications.

Fluorene based amorphous hole transporting materials for solution processed organic light-emitting diodes

Abstract For an organic light-emitting diode (OLED), enhancing hole injection into the emissive layer for charge balance is a priority to achieve efficient device performance, while solution processing enables organic devices to be fabricated cost-effectively with a large area-size via continuous roll-to-roll manufacturing. However, a limited number of small molecules-based hole-transporting materials (HTMs) were reported with the solution process feasibility and high performance. Here, we demonstrate a series of low cost, efficient and solution processable small molecule HTMs, namely, 2,7-Di(4-fluorophenyl)-9,9-diethylfluorene (Fl-PyEyF), 2,7-Di(3,5-difluorophenyl)-9,9diethylfluorene (di-Fl-PyEyF) and 2,7-Di(2,4,6-trifluorophenyl)-9,9-diethylfluorene (tri-FlPyEyF), designed by using two fluorophenyl, difluorophenyl or trifluorophenyl fragments as common end capping groups with 9,9-Diethylfluorenes cores, respectively, for highly efficient OLEDs. The glass transition temperatures of the molecules were estimated to be higher than 80 °C, which can provide morphologically stable amorphous thin films. All the molecules possess good solubility in common organic solvents. Moreover, all the synthesized molecules have not only appropriate highest occupied molecular orbital energy levels for good hole injection ability, but also sufficient lowest unoccupied molecular orbital for electron blocking capability, adequate ionization potentials (6.15 eV) and suitable triplet energies, which make them suitable hole transporting materials. The current efficiency of a conventional iridium(III) bis(4phenylthieno[3,2-c]pyridinato-N,C-2՜) acetylacetonate based phosphorescent yellow OLED device increases from 40.2 to 61.0 cd/A, an increment of 51.7% by substituting the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the tri-Fl-PyEyF counterpart. These findings suggest that the reported materials can serve as potential molecular HTMs in solution-processed OLED.

Lithium-Silicon Compounds as Electrode Material for Lithium-Ion Batteries

The element silicon is currently considered as one of the most promising alternative electrode materials for lithium-ion batteries. During lithiation, silicon experiences a large volume increase, which often leads to material failure and significant irreversible loss of capacity. The production of amorphous thin layers of lithium-silicon compounds as pre-lithiated electrode material is a promising approach to reduce significant capacity losses and to “homogenize” volume expansion. We demonstrate that it is possible to produce stable amorphous lithium-silicon thin films with variable stoichiometry using ion beam co-sputtering. The electrochemical experiments show that the different starting compositions of the electrode material (different x for LixSi) compared to pure amorphous silicon cause significant differences in the electrochemical behavior and in the electrode kinetics. With an increasing amount of lithium in the lithium-silicon electrode, the specific charge and discharge capacity decreases, but irreversible capacity losses are reduced in the first cycle and the Coulomb efficiency stabilizes at a value of almost 100%. Thin films with a low Li concentration, such as Li0.06Si, are likely not stable to the lithiation process during long term cycling due to material failure, whereas Li0.4Si shows a much better charge/discharge stability up to 100 cycles.

Persistent thin water films encapsulate oil droplets crossing an oil-water interface

Understanding of oil-water interfacial phenomena is essential for predicting mixing and/or phase-separation in environmental and industrial systems. Time-resolved digital holography and planar laser-induced fluorescence are used for examining processes occurring after ascending buoyant oil droplets of varying viscosity cross a stratified oil-water interface. Previous studies have focused on the crossing process, and the current belief is that once this droplet becomes immersed in the oil, its content can mix with the bulk fluid. In contrast, we show that the droplets remain encapsulated by a stable continuous submicron thin water film, which prevent them from mixing. This film forms even in pure oil and water with minimal surfactant concentration and persists for periods that are three to four orders of magnitude longer than those of the crossing process. Observations following the film evolution reveal that segments located close to the interface appear to be attracted to the bulk water, causing the entire droplet to flatten slowly. The resulting reduction in the peripheral radius of curvature eventually breaks up the film into suspended submicron droplets. The morphology of this flattening process varies with oil viscosity, and its duration increases from seconds to nearly one hour as the oil viscosity increases from one to fifty cSt. In processes involving multiple oil droplets crossing the interface, they form a separate persistent long-lasting layer containing a complex thin-film structure that does not mix with the bulk oil. In contrast, thin oil films do not form around a descending water droplet after it crosses the interface.

UV-Cured Poly(Ethylene Glycol) Diacrylate/Carbon Nanostructure Thin Films. Preparation, Characterization, and Electrical Properties

Carbon nanoallotropes such as carbon nanotubes, graphene, and their derivatives have been combined with a plethora of polymers in the last years to develop new composite materials with interesting properties and applications. However, the area of photopolymer composites with carbon nanostructures has not been analogously explored. In the present article, we study the photopolymerization of poly(ethylene glycol)diacrylate (PEGDA) enriched with different carbon nanoallotropes like graphene, pristine and chemically modified carbon nanotubes (CNTs and fCNTs), and a hybrid of graphene and CNTs. The products were characterized by several microscopic and spectroscopic techniques and the electrical conductivity was studied as a function of the concentrations of carbon nanoallotropes. In general, stable thin films were produced with a concentration of carbon nanostructures up to 8.5%, although the addition of carbon nanostructures in PEGDA decreases the degree of photopolymerization, and PEDGA/carbon nanostructure composites showed electrical conductivity at a relatively low percentage.