Solution-based Process Research Articles

Improvement Strategies for Stability and Efficiency of Perovskite Solar Cells.

Recently, perovskites have garnered great attention owing to their outstanding characteristics, such as tunable bandgap, rapid absorption reaction, low cost and solution-based processing, leading to the development of high-quality and low-cost photovoltaic devices. However, the key challenges, such as stability, large-area processing, and toxicity, hinder the commercialization of perovskite solar cells (PSCs). In recent years, several studies have been carried out to overcome these issues and realize the commercialization of PSCs. Herein, the stability and photovoltaic efficiency improvement strategies of perovskite solar cells are briefly summarized from several directions, such as precursor doping, selection of hole/electron transport layer, tandem solar cell structure, and graphene-based PSCs. According to reference and analysis, we present our perspective on the future research directions and challenges of PSCs.

The Growth Dynamics of Organic-Inorganic Metal Halide Perovskite Films.

Organic-inorganic metal halide perovskite films have emerged as potential candidate materials for photoelectric devices because of their superior optoelectronic properties. The performance of these devices depends on the quality of perovskite films defined by their grain size, crystallinity, and absence of pinholes. While solution-based processing is the most cost-effective and scalable approach to producing these films, the impact of the process parameters on the film quality and nanoscale details of these processes are unknown. Specifically, it is unclear how perovskites grow from a liquid precursor to form solid-phase nanocrystals and how these nanocrystals arrange to form a uniform film. Here, using liquid-phase transmission electron microscopy (TEM), we show how perovskite nanocrystals nucleate from a precursor solution and then grow and coalesce to form a polycrystalline film. Furthermore, we show how additives, such as urea, can improve the film crystallinity by increasing perovskite solubility, which induces the dissolution and subsequent redeposition of smaller crystals onto larger grains. Our approach to studying the growth of perovskite films provides an important insight into improving the synthesis of perovskites and other technologically relevant crystalline films.

Quantum dot lasing from a waterproof and stretchable polymer film

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

Recent Developments and Implementations of Conductive Polymer-Based Flexible Devices in Sensing Applications.

Flexible sensing devices have attracted significant attention for various applications, such as medical devices, environmental monitoring, and healthcare. Numerous materials have been used to fabricate flexible sensing devices and improve their sensing performance in terms of their electrical and mechanical properties. Among the studied materials, conductive polymers are promising candidates for next-generation flexible, stretchable, and wearable electronic devices because of their outstanding characteristics, such as flexibility, light weight, and non-toxicity. Understanding the interesting properties of conductive polymers and the solution-based deposition processes and patterning technologies used for conductive polymer device fabrication is necessary to develop appropriate and highly effective flexible sensors. The present review provides scientific evidence for promising strategies for fabricating conductive polymer-based flexible sensors. Specifically, the outstanding nature of the structures, conductivity, and synthesis methods of some of the main conductive polymers are discussed. Furthermore, conventional and innovative technologies for preparing conductive polymer thin films in flexible sensors are identified and evaluated, as are the potential applications of these sensors in environmental and human health monitoring.

Robust superhydrophobic and icephobic surface based on Teflon AF coated multiscale hierarchical ZnO/Cu2O nanostructures

We demonstrate hetero-nanostructured rough surface with solution based hydrophobic coating for robust superhydrophobic (SH) properties. The copper foil was anodically etched to generate Cu2O micro-needles and then ZnO nanorods were grown on Cu2O to create ZnO/Cu2O micro-nano hierarchical rough surface. A thin layer of Teflon AF was applied as a low surface energy coating on the top of the hierarchical surface. These surfaces were characterized for their structural phases using the X-ray diffraction technique and multiscale hierarchical surface morphology was confirmed using FESEM and AFM techniques. Wetting properties like static and dynamic water contact angle (CA) were analyzed using the advancing receding contact angle (ARCA) technique. This surface shows superhydrophobic properties with static CA of 157° and a roll-off angle of 3°. The durability of the SH surfaces was examined by water droplet impact for a range of velocities. The droplet rebounding is consistently seen up to Weber number, We ∼ 220. When the SH surface was cooled to −10 °C, dropwise condensation takes place. In this case, self-propelled jumping of droplets via multiple coalescences of microdroplets was observed. In the freezing rain condition, this surface exhibits poor adhesion to ice formation demonstrating an icephobic property. The surface retains SH properties even to the UVC exposure of about 12 J/cm2. The fabrication of the SH surface involves solution-based processes which are scalable to mass production for applications ranging from enhanced heat conduction to water-repellent coatings.

A stretchable pressure sensor with interlinked interfaces prepared by a template-free process

A stretchable pressure sensor with interlinked interfaces was fabricated by a template-free process. The sensor was composed of stretchable silver nanowire (AgNW)/carbon nanotube (CNT) electrodes and piezoresistive polydimethylsiloxane (PDMS)/CNT foam. The electrode was fabricated by embedding conductive nanoparticles in PDMS, showing a low sheet resistance of 0.25 Ω sq−1, strain insensitivity, excellent stability, and durability over 2000 stretching/releasing cycles. The PDMS/CNT foam was fabricated through a solution-based process without a template exhibiting considerable piezoresistivity. By drop-casting and curing PDMS/CNT emulsion on the electrode, the electrode and foam can be bonded to form a stretchable pressure sensor seamlessly and directly. The sensor exhibited outstanding stretchability to 50%, high fidelity to dynamic pressure and great durability over 3000 loading/unloading cycles. In addition, a sensor array for pressure distribution detection was developed and confirmed to be able to withstand large deformations, such as stretching and poking.

Fabrication of Color Glass by Pearlescent Pigments and Dissolved EVA Film.

In this paper, we propose a single-layer thin-film color glass manufacturing process for building-integrated photovoltaics (BIPV) with excellent aesthetics and high transmittance, through a solution process using pearlescent pigments. As a matrix for the color solution, ethylene vinyl acetate (EVA), which serves as an encapsulant and adhesive for the photovoltaic module (PV), was dissolved and used as a matrix for the color solution. The color glass produced is excellent in securing the aesthetics of buildings, has a high transmittance of 90% or more, outputs a maximum solar power generation efficiency of 91% from a solar cell, and can minimize the deterioration of power generation efficiency. In addition, the characteristics do not change over time, so it is suitable as color glass for BIPV. Through this study, the solution-based color glass manufacturing process for BIPV using dissolved EVA as a matrix forms a single-layer thin film with good color extensions. The choice of EVA as a matrix makes it possible for color glass to be easily attached to a solar panel using a heat press method. This proposed technique makes it easier and simpler to manufacture color glass as compared to the physical vapor deposition process. The adoption of this solution process technique to fabricate pearlescent pigment-based color glass can effectively reduce the time and cost of the process, so it is expected to be applied to the low-cost BIPV market with excellent aesthetics and high transmittance.

Pivotal role of nucleation layers in the hydrothermally-assisted growth of ZnO and its H2 gas sensing performance

Although hydrogen is a non-poisonous gas that can dissipate quickly once is released, it causes suffocation at high concentrations and is very risky on some occasions if not suitably handled. In this study, an effective approach was suggested for developing and tuning hydrogen gas sensors, where the crucial effect of nucleation layers and chemical stabilizing agents on the morphology of hydrothermally-assisted growth and its gas sensing performance was demonstrated. For this purpose, ZnO seed layers were separately synthesized by different solution-based chemical processes based on the successive ionic layer adsorption and reaction, spin, and dip coating techniques followed by a hydrothermal treatment under the same conditions. X-ray diffraction measurements revealed that all the synthesized samples have a similar pure hexagonal wurtzite structure with a big difference in the degree of textural orientation along the c-axis of ZnO. Optical transmittances, forbidden bandgap energies, and Urbach energies of the produced samples were influenced to a great extent by the coating techniques and the chemical solutions used. Aside from that, the morphology of the synthesized samples and their overall thicknesses together with the available oxygen vacancies detected by X-ray photoelectron spectroscopy changed considerably. The change in structure and morphology have influenced the wettability of the synthesized samples. They have also greatly influenced the gas sensing performance of the sensors in terms of optimal working temperature, sensitivity, and response/recovery periods. The hydrogen gas detection mechanism of the fabricated sensors and the logic behind the change that occurred in gas sensing performance based on morphology are discussed.

DNA-Programmable AgAuS-Primed Conductive Nanowelding Wires-Up Wet Colloids.

Self-assembly of nanomaterials, directed by molecular or supramolecular interactions, is a powerful strategy to build nanoscale devices. Despite many advantages of such solution-based processes, a big challenge is to realize interparticle ohmic contacts toward facilitated charge transport over a long distance. We report a new concept of primed nanowelding to thread solution-borne nanoparticles in prescribed assemblies. The process starts with a gap-specific deposition of Ag2 E (E=S, Se) materials in pre-assembled gold structures, which spontaneously transform into AgAuE semiconductors via directional gold diffusion. Treatment with tributylphosphine generates alloyed Au/Ag welding spots that conductively wire-up nanoparticles into discrete "molecules" and micron-long "polymers". This method is compatible with DNA programming and delivers a possible way to solve the problem of the carrier-transport dilemma in solution-processed nanostructures for better-functioning nanodevices.

Flexible random laser from a porous polymer film

Flexible random lasers have attracted a great deal of interest due to their potential in light-weight, flexible optical sensors and devices. Among many alternatives, flexible thin film random lasers are very interesting because they can be easily fabricated via a simple solution-based process. Herein, we demonstrate a dye-doped porous polyvinyl alcohol film that can work as a flexible random laser source under optical pumping. The film is fabricated by a drop cast technique and has a nearly circular shape with a diameter of ∼5 mm. Owing to the inverted photonic glass structure that is air voids in the polymer matrix, light emission from dye molecules is multiple scattering and lasing emission is achieved. In the central area, the film laser works reliably with the lasing threshold of ∼166μJ/mm2 and lasing wavelength of around 595 nm. In the edge area, the lasing properties are more fluctuated indicating that the structure may not be uniform. It is found that the lasing wavelength in the edge area is 4.5 nm red-shifted compared to that in the central area of the film. Interestingly, the thin film laser can operate well under bending conditions with a minor change in the lasing wavelength.

Reliable fabrication of transparent conducting films by cascade centrifugation and Langmuir-Blodgett deposition of electrochemically exfoliated graphene.

Electrochemical exfoliation is an efficient and scalable method to obtain liquid-phase graphene. Graphene in solution, obtained through electrochemical exfoliation or other methods, is typically polydisperse, containing particles of various sizes, which is not optimal for applications. We employed cascade centrifugation to select specific particle sizes in solution and prepared thin films from those graphene particles using the Langmuir–Blodgett assembly. Employing centrifugation speeds of 3, 4, and 5 krpm, further diluting the solutions in different volumes of solvent, we reliably and consistently obtained films of tunable thickness. We show that there is a limit to how thin these films can be, which is imposed by the percolation threshold. The percolation threshold is quantitatively compared to results found in literature that are obtained using other, more complex graphene film fabrication methods, and is found to occur with a percolation exponent and percolative figure of merit that are of the same order as results in literature. A maximum optical transparency of 82.4% at a wavelength of 660 nm is obtained for these films, which is in agreement with earlier works on Langmuir–Blodgett assembled ultrasonic-assisted liquid-phase exfoliated graphene. Our work demonstrates that films that are in all respects on par with films of graphene obtained through other solution-based processes can be produced from inexpensive and widely available centrifugal post-processing of existing commercially available solutions of electrochemically exfoliated graphene. The demonstrated methodology will lower the entry barriers for new research and industrial uses, since it allows researchers with no exfoliation experience to make use of widely available graphene materials.

Surface-capping engineering for electrically neutral surface of perovskite films and stable solar cells

Metal halide perovskite solar cells (PSCs) have developed rapidly in recent years, due to their high performance and low-cost solution-based fabrication process. These excellent properties are mainly attributed to the high defect tolerance of polycrystalline perovskite films. Meanwhile, these defects can also facilitate ion migration and carrier recombination, which cause the device performance and the long-term stability of PSCs to deteriorate heavily. Therefore, it is critical to passivate the defects, especially at the surfaces of perovskite grains where the defects are most concentrated due to the dangling bonds. Here we propose a surface-capping engineering (SCE) method to construct ‘dangling-bond-free’ surfaces for perovskite grains. Diamine iodide (methylenediammonium diiodide, MDAI2) was used to construct an electroneutral PbX6-MDA-PbX6 (X = Cl, Br or I) layer at the perovskite surfaces. Compared to the monovalent FA+ which can only coordinate one [PbX6]4− slab, the bivalent MDA2+ can coordinate two [PbX6]4− slabs on both sides, thus realizing a dangling-bond-free surface. Solar cells based on SCE-perovskite films exhibited a higher power conversion efficiency (PCE) of 21.6%, compared with 19.9% of the control group; and maintained over 96% of its initial PCE after 13 h during the maximum power point tracking test under continuous AM1.5G illumination, whereas the control group only lasted 1.5 h. Constructing a dangling-bond-free capping layer on the grain boundary opens new avenues for the fabrication of ultralow-defect polycrystalline semiconductors, paving the way to further improve the PCE and lifetime of PSCs.

Morphology control in nickel cobaltite synthesised via solution routes for electrochemical applications

Research on the synthesis of electrode materials has seen a transition from a “mix and try” approach to a systematic design of nanostructures resulting in exciting morphologies and architectures. Recent research efforts delve rigorously into utilizing the exquisite shape dependant and defect-induced property of electrode materials for phenomenal enhancements in the device performance. Among various strategies for the morphology control at the nano/micro level, the solution-based chemical processes offer pathways for the growth of various morphologies enabling the utility of materials chemistry for enhanced properties. Nickel cobaltite has been explored as an active electrode material for various energy storage and conversion devices, including batteries, supercapacitors, and electrocatalysts. Synthesis of an impressive range of composite structures of nickel cobaltite has already been reviewed and reported. In this article, we collate and review the approaches for morphology control and defect engineering strategies adopted for nickel cobaltite nanostructures. The effect of synthetic parameters such as precursor effects, type of precipitant and additives, calcination temperature etc. On the morphology tuning and defect engineering are analysed. This review aims to establish a structure-property correlation based on the synthesis conditions and the electrochemical properties of nickel cobaltite for meeting the desired needs in various application areas.

(Invited, Digital Presentation) Composites of 2D Materials and Organic Molecules

Atomically thin two-dimensional (2D) materials have aroused an explosive growth of scientific interest for their outstanding physicochemical properties. In addition, decorations of 2D materials with organic semiconductors have also begun to receive scientific attention owing to the tunable optoelectronic properties and the excellent solution-based processability of the organic molecules. In this study, composite films that consisted of C60 with well-exfoliated nanosheets of transition metal dichalcogenides (TMDs) [1], antimony [2], or bismuth [3] were successfully fabricated onto a semiconducting SnO2 electrode via a two-step methodology: self-assembly into their composite aggregates by injection of a poor solvent into a good solvent with the dispersion, and subsequent electrophoretic deposition. In addition, photophysical and photoelectrochemical properties of the composite films were investigated in detail. For example, in a flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurement of a composite material consisting of C60 and few-layered antimonene (FLSb) nanosheets (denoted as (FLSb+C60) m ), a rapid rise in transient conductivity was observed, whereas no conductivity signal was observed in the single components, FLSb and C60. This demonstrated the occurrence of photoinduced charge separation between FLSb and C60 in (FLSb+C60) m . Furthermore, a photoelectrochemical device with an electrophoretically deposited (FLSb+C60) m film exhibited an enhanced efficiency of photocurrent generation, compared to those of the single-components, FLSb and C60, due to the photoinduced charge separation between FLSb and C60. This work provides a promising approach for fabrication of 2D material–organic molecule composites and paves the way for their application in optoelectronics.[1] J. Baek, T. Umeyama, W. Choi, Y. Tsutsui, H. Yamada, S. Seki, and H. Imahori, Chem. Eur. J., 24, 1561-1572 (2018).[2] T. Umeyama, T. Ohara, Y. Tsutsui, S. Nakano, S. Seki, and H. Imahori, Chem. Eur. J., 26, 6726-6735 (2020).[3] T. Umeyama, H. Xu, T. Ohara, Y. Tsutsui, S. Seki, and H. Imahori, J. Phys. Chem. C, 125, 13954-13962 (2021).

Plasmonic Color Printing via Bottom-Up Laser-Induced Photomodification Process.

Plasmonic color printing has received significant attention owing to its advantages such as nonfading and nontoxic color expression, without necessitating the use of chemical dyes. Recently, color generation from laser-induced plasmonic nanostructures has been extensively explored because of its simplicity, cost-effectiveness, and large-scale processability. However, these methods usually utilize a top-down method that causes unexpected background colors. Here, we proposed a novel method of plasmonic color printing via a bottom-up type laser-induced photomodification process. In the proposed method, selective silver nanoparticles (Ag NPs) structure could be fabricated on a transparent substrate through a unique organometallic solution-based laser patterning process. A set of color palettes was formed on the basis of different processing parameters such as laser fluence, scanning speed, and baking time. This color change was verified by finite-difference time-domain (FDTD) simulations via monitoring the spectral peak shift of the localized surface plasmon resonance (LSPR) at Ag NPs. It was also confirmed that the colors can be fabricated at a relatively high scanning speed (≥10 mm/s) on a large substrate (>300 mm2). Since semitransparent color images can be patterned on various transparent substrates, this process will broaden the application range of laser-induced plasmonic color generation.

Processing of Lead Halide Perovskite Thin Films Studied with In-Situ Real-Time X-ray Scattering.

Lead halide perovskites have been of paramount interest for solution-processable solar cells, reaching power conversion efficiencies larger than 25%. In this spotlight, we will provide a systematic overview of the influence of different solution-based processing routes of lead halide perovskites on their phase transformation and conversion as revealed through in-situ X-ray-scattering experiments. These experiments were performed in conditions closely mimicking thin film processing methods and conditions used for thin film solar cell device fabrication and therefore provide critical information about the mechanism of the phase transformation, its onset, the kinetics, as well as the emergence and disappearance of various (meso)phases along the way. The measurements capture the overall solidification and conversion process of lead halide perovskite inks into solid films via so-called one-step and two-step spin-coating processes as well as blade coating and hot casting. Processing routes are applied to films based on basic components as well as mixtures of different anions and cations, solvents, and antisolvents, all of which deeply affect the thin film microstructure and morphology of the light-absorbing semiconductor and associated solar cell devices.

BaTiO3 nanocubes-Gelatin composites for piezoelectric harvesting: Modeling and experimental study

Flexible composites containing BaTiO3 nanoparticles into Gelatin bio-polymer matrix were designed and investigated. Following the idea that the electric field concentration in corners/edges at the interfaces between dissimilar materials give rise to enhanced effective permittivity in composites, cuboid-like BaTiO3 nanoparticles have been employed as nanofillers into Gelatin matrix by using an inexpensive solution-based processing method. As predicted by finite element method simulations developed for cubic-like inclusions into a homogeneous polymer matrix, the experimental permittivity of xBT-(1-x)Gelatin composites increases when increasing the high-permittivity filler addition. For the composition x = 40 wt% (corresponding to 12 vol% BaTiO3 addition), permittivity reaches εr ∼15.7 with respect to εr ∼9.8 of pure Gelatine (measured at 105 Hz), while the average piezoelectric coefficient d33 as determined by piezoelectric force microscopy shows a remarkable increase up to 21 pm/V in composites with x = 40 wt%, in comparison to ∼7 pm/V in pure Gelatin. By using the experimentally determined material constants, the simulated piezoelectric voltage output vs. time has shown a similar increase (about a doubling of its amplitude) of the harvesting signal in the composite with x = 40 wt% BT, with respect to one of the polymer matrix, thus demonstrating the beneficial role of embedding BT nanoparticles into the biopolymer for increasing the mechanical harvesting response.

Revisiting Solution-Based Processing of van der Waals Layered Materials for Electronics.

Following the significant discovery of van der Waals (vdW) layered materials with diverse electronic properties over more than a decade ago, the scalable production of high-quality vdW layered materials has become a critical goal to enable the transformation of fundamental studies into practical applications in electronics. To this end, solution-based processing has been proposed as a promising technique to yield vdW layered materials in large quantities. Moreover, the resulting dispersions are compatible with cost-effective device fabrication processes such as inkjet printing and roll-to-roll manufacturing. Despite these advantages, earlier works on solution-based processing methods (i.e., direct liquid-phase exfoliation or alkali-metal intercalation) have several challenges in achieving high-performance electronic devices, such as structural polydispersity in thickness and lateral size or undesired phase transformation. These challenges hinder the utilization of the solution-processed materials in the limited fields of electronics such as electrodes and conductors. In the meantime, the groundbreaking discovery of another solution-based approach, molecular intercalation-based electrochemical exfoliation, has shown significant potential for the use of vdW layered materials in scalable electronics owing to the nearly ideal structure of the exfoliated samples. The resulting materials are highly monodispersed, atomically thin, and reasonably large, enabling the preparation of electronically active thin-film networks via successful vdW interface formation. The formation of vdW interfaces is highly important for efficient plane-to-plane charge transport and mechanical stability under various deformations, which are essential to high-performance, flexible electronics. In this Perspective, we survey the latest developments in solution-based processing of vdW layered materials and their electronic applications while also describing the field's future outlook in the context of its current challenges.

A route towards the fabrication of large-scale and high-quality perovskite films for optoelectronic devices.

Halide perovskite materials have been extensively explored for their unique electrical, optical, magnetic, and catalytic properties. Most notably, solar cells based on perovskite thin films have improved their power conversion efficiency from 3.8% to over 25% during the last 12 years. However, it is still a challenge to develop a perovskite-based ink, suitable for upscaling the fabrication process of high-quality perovskite films with extreme purity, good crystallinity, and complete coverage over the deposition area. This is particularly important if the perovskite films are to be used for the scaled production of optoelectronic devices. Therefore, to make halide perovskites commercially available for various applications, it is vital to develop a reliable and highly robust deposition method, which can then be transferred to industry. Herein, the development of perovskite precursor inks suitable for use at low-temperature and vacuum-free solution-based deposition processes is reported. These inks can be further tailored according to the requirements of the deposition method, i.e., we propose their use with the industrially viable deposition technique called “slot-die coating”. Furthermore, a route for the preparation of low-cost and high-volume manufacturing of perovskite films on both rigid and flexible substrates is suggested in this paper. The presented approach is suitable for the fabrication of any functional layers of perovskites, that can be employed in various scaled applications, and it seeks the potential and the methodology for perovskite film deposition that is scalable to industrial standards.

Instabilities as the Origin of Large-Area Self-Assembled and Aligned Organic Semiconductor Nanocrystals

Aligned nanocrystals of organic semiconductors (OSCs) are highly desirable for electronic devices and biomedical and photonic applications. Solution-based wet processing routes have the potential to produce aligned nanocrystals over large areas in small time frames. Herein, we demonstrate that by optimizing the hydrodynamic evaporative processes, controlled long-range crystalline assemblies of OSCs can be achieved (longest nanocrystal ∼3 mm) purely through physical processes: namely, from fingering instabilities. Self-assembly is achieved here without strong noncovalent interactions such as hydrogen-bonding interactions. Experimentally our approach involves just placing a drop of a solution on an inclined substrate. Nanocrystals with widths of 300–800 nm and lengths of millimeters (length/width aspect ratios >105) are formed in less than 2–8 s. A hydrazine chemiresistive sensor based on the aligned crystalline patterns show unprecedented responsivity (∼10–6), 2 orders greater than those of stick–slip patterns. Finally, experimental parameters that need optimization to achieve nanocrystal patterns are investigated in detail and pointers to fabricate such OSC nanocrystals are provided.