Morteza Eslamian

Modeling of Solution Droplet Evaporation and Particle Evolution in Droplet-to-Particle Spray Methods

A mathematical model for the entire process of micro- and nanoparticle formation via one-step, direct conversion of solution droplets to particles with application to spray-drying and pyrolysis techniques is developed. Here we identify three physical stages for drying of solution droplets, including shrinkage, induction, and constant-diameter periods. The characteristics of these periods are studied by considering the drying of a population of solution droplets introduced into a hot wall reactor. The particle evolution and intraparticle phenomena in each period are elucidated. Numerical results show that low reactor wall temperatures, larger droplet number densities, and smaller initial droplet sizes favor the formation of fully filled particles, and the carrier gas flow rate has no effect on particle morphology.

Substrate vibration-assisted spray coating (SVASC): significant improvement in nano-structure, uniformity, and conductivity of PEDOT:PSS thin films for organic solar cells

In an attempt to improve surface wetting and coating characteristics, a novel technique, i.e., imposing ultrasonic vibration on the substrate, is introduced. This technique is combined with conventional ultrasonic spray coating, thus substrate vibration-assisted spray coating (SVASC), and is employed to fabricate PEDOT:PSS thin films. PEDOT:PSS is a co-polymer, commonly used as solar cell buffer layers and thin-film electrodes. Advanced surface characterization techniques, such as atomic force microscopy and confocal laser scanning microscopy are utilized. The results show that the application of the imposed vibration on the substrate results in a significant decrease in surface roughness, film thickness, and the number of defects and pin-holes. In terms of the film functionality, the electrical conductivity of the PEDOT:PSS films made using the SVASC technique shows a four-time increase, compared to those made by conventional ultrasonic spray coating. In conventional ultrasonic spray coating, increasing the number of spray passes or deposition layers usually improves the coating uniformity. For aqueous PEDOT:PSS solution and within the range of the values of the parameters investigated in this work, with imposed substrate vibration, the number of spray passes is immaterial, as far as the film uniformity is concerned. However, the application of multiple spray passes enhances the film’s electrical conductivity. Our unprecedented results on the combined substrate vibration with spray coating provide a platform for low-cost fabrication of solution-processed thin-film solar cell devices, and a forward step toward commercialization of emerging solar cells, such as polymer and perovskite solar cells. The positive effect of using imposed substrate vibration on spray-on solar cell thin films may be deployed in other coating (e.g., spin coating) and spray coating applications as well.

A Critical Review of Thermodiffusion Models: Role and Significance of the Heat of Transport and the Activation Energy of Viscous Flow

Abstract In this paper thermodiffusion models developed to estimate the thermal diffusion factor in nonideal liquid mixtures are reviewed; the merits and shortcomings of each model are discussed in detail. Most of these models are multicomponent in principle; however our focus here is on binary mixtures. Two rather different groups of models are identified: models needing a matching parameter to be obtained usually from the outside of thermodynamics, and the self-contained or independent models. Derivation of the matching parameter models using linear non-equilibrium thermodynamics and the details of how to find the matching parameters are investigated. The physical meaning of parameters such as the net heat of transport and the activation energy of viscous flow is elucidated, as the literature is overwhelmed with confusing and misleading information. The so-called dynamic and static models and their relations to the matching and non-matching parameter models are also discussed. We conclude that modeling the net heat of transport by the activation energy of self-diffusion may provide better results than approximating it by the activation energy of viscous flow. Nonetheless, the matching parameter models, which use the activation energy of viscous flow, are more dynamic and predict the thermal diffusion factor better than the non-matching parameter or static models, such as those of Kempers and Haase.

Improving uniformity and nanostructure of solution-processed thin films using ultrasonic substrate vibration post treatment (SVPT).

UNLABELLED The main goal of this paper is to introduce a novel mechanical method herein terms as substrate vibration post treatment (SVPT) technique, powered by ultrasonic vibration imposed on the substrate to enhance the characteristics and functionality of spun-on thin films or thin films made by similar casting techniques, such as drop and dip coating. In this technique, the as-casted wet films are placed on a substrate vibrated by an ultrasonic transducer with controlled power and duration to improve the film characteristics, such as uniformity and nanostructure. The performance of this technique is examined on spun-on PEDOT PSS thin films used in polymer and perovskite solar cells and unprecedented results are presented. We first explore the influence of the vibration duration time on the characteristics of the films made by pristine PEDOT PSS solution, where it is found that the optimized vibration duration for the pristine PEDOT PSS film is about 10s, resulting in significant increase in the film electrical conductivity and lowered thickness and roughness. In order to further test the generality and merit of the method, thin films made using PEDOT PSS solution modified with various types of surfactants and cured by the SVPT are studied. The results show that the application of the SVPT method combined with surfactant modification leads to an impressive twelve-fold increase in the conductivity of the PEDOT PSS thin films compared with that of the pristine non-vibrated PEDOT PSS thin films. The sole effect of the SVPT is a four-fold increase in the conductivity of pristine PEDOT PSS film compared with that of the non-vibrated film. This remarkable enhancement in conductivity is further explained by the AFM phase images of PEDOT PSS films, showing that the ultrasonic energy could loosen the Coulomb forces between PEDOT and PSS chains, resulting in phase separation and localized reordering of the conducting PEDOT chains leading to an increase in the electrical conductivity of the film. Highly conductive PEDOT PSS thin film is a viable candidate as electrodes in emerging solution-processed solar cells.

Controlled wetting/dewetting through substrate vibration- assisted spray coating (SVASC)

We have recently developed a novel spray-coating method, called the “substrate vibration-assisted spray coating” (SVASC) (Zabihi and Eslamian in J Coat Technol Res 12:711–719, 2015), in which ultrasonic vibration is imposed on the substrate to improve the spray-on film and coating characteristics. In that work, the SVASC method was introduced, and its effectiveness on the uniformity and electrical conductivity of PEDOT:PSS films, used in emerging solar cells, was demonstrated. The present work reports unprecedented results on the effect of the ultrasonic vibration power on wetting/dewetting of PEDOT:PSS films. It is observed that, while the application of a low-power ultrasonic vibration (LPUV) improves the surface wetting and film coverage through improving droplet spreading and coalescence and repairing of the dewetted areas, a high-power ultrasonic vibration (HPUV) promotes dewetting, resulting in less coverage and the formation of a nonuniform film. The improved wetting due to the application of the LPUV has opened a window of opportunity for the fabrication of intact thin films and related thin film devices. On the other hand, the controlled dewetting process due to the application of the HPUV may have novel applications in template fabrication and self-assembly in nanotechnology. Here, we also study the effects of the application of multipass spraying compared with single-pass spraying strategy, and the application of using two co-solvents on PEDOT:PSS film characteristics. The results confirm that the utilization of isopropyl alcohol (IPA) as a co-solvent added to the PEDOT:PSS precursor solution improves the surface wettability and film coverage, compared to the films made using demethylformamide (DMF), as the co-solvent.

Recent advances in nanoparticle preparation by spray and micro-emulsion methods.

Micro- and nano-sized metal, semiconductor, pharmaceutical, and simple or complex ceramic particles have numerous applications in the development of sensors, thermal barrier coatings, catalysts, pigments, drugs, etc. The challenges include controlling the particle size, size distribution, particle crystallinity, morphology and shape, being able to use the nanoparticles for a given purpose, and to produce them from a variety of precursors. There are several methods to produce nanoparticles, each suitable for a range of applications. In this article, two methods that are receiving increasing attention are considered: spray and microemulsion methods. Spray techniques are single-step methods of producing a broad spectrum of simple to multicomponent functional micro and nanoparticles and quantum dots. Microemulsion is a wet chemistry method. A micro-emulsion system consists of aqueous domains, called reverse micelles, dispersed in a continuous oil phase. In this article, the above mentioned methods of nanoparticle production are introduced and recent advances, research directions and challenges, and the pertinent patents are reviewed and discussed.

Effect of Atomization Method on the Morphology of Spray- Generated Particles

Effect of various atomization methods and solute concentration on the morphology of spray dried magnesium sulphate particles is investigated. Four types of atomizers are characterized and tested including (i) a vibrating mesh nebulizer, (ii) a splash plate nozzle, (iii) an air mist atomizer, and (iv) a pressure atomizer. Several types of particle morphologies are identified in this research. Spray characteristics, such as droplet number density, droplet size, and velocity, and accompanying atomizing air have major influence on the drying and morphology of the particles. High initial solute concentrations result in the formation of thick-walled particles, and this prevents the particles to burst. It is found to be difficult to obtain fully filled magnesium sulphate particles, even for saturated solutions at room temperature because the solution equilibrium saturation changes substantially with temperature.

Evaporation and Evolution of Suspended Solution Droplets at Atmospheric and Reduced Pressures

Thermal history and solute precipitation behavior of suspended solution droplets of sodium chloride (NaCl), magnesium sulphate (MgSO4), and zirconium hydroxychloride (ZrO(OH)Cl) evaporating at atmospheric and reduced pressures are studied. Experimental measurements on the variation of droplet diameter, solution concentration, and temperature during the evaporation period are presented and discussed. The results of solute precipitation behavior in solution droplets observed under an optical microscope are displayed and discussed. Results indicate that reducing the pressure (∼ 33 kPa) results in a change in the solution droplet evaporation rate, but the thermal histories of a particular solution droplet are similar at the atmospheric and reduced pressures. At atmospheric and reduced pressures used in this study, the d 2 law for solution droplets is valid at early stages of the evaporation and before the solute precipitation initiates. Drying of MgSO4 and ZrO(OH)Cl solution droplets results in the formation of spherical particles, whereas drying of spherical NaCl solution droplets results in the formation of cubic particles.

Effects of Self-Assembled Monolayer Modification of Nickel Oxide Nanoparticles Layer on the Performance and Application of Inverted Perovskite Solar Cells

Entirely low-temperature solution-processed (≤100 °C) planar p-i-n perovskite solar cells (PSCs) offer great potential for commercialization of roll-to-roll fabricated photovoltaic devices. However, the stable inorganic hole-transporting layer (HTL) in PSCs is usually processed at high temperature (200-500 °C), which is far beyond the tolerant temperature (≤150 °C) of roll-to-roll fabrication. In this context, inorganic NiOx nanoparticles (NPs) are an excellent candidate to serve as the HTL in PSCs, owing to their excellent solution processability at room temperature. However, the low-temperature processing condition is usually accompanied with defect formation, which deteriorates the film quality and device efficiency to a large extent. To suppress this setback, we used a series of benzoic acid selfassembled monolayers (SAMs) to passivate the surface defects of the NiOx NPs and found that 4-bromobenzoic acid could effectively play the role of the surface passivation. This SAM layer reduces the trap-assisted recombination, minimizes the energy offset between the NiOx NPs and perovskite, and changes the HTL surface wettability, thus enhancing the perovskite crystallization, resulting in more stable PSCs with enhanced power conversion efficiency (PCE) of 18.4 %, exceeding the control device PCE (15.5 %). Also, we incorporated the above-mentioned SAMs into flexible PSCs (F-PSCs) and achieved one of the highest PCE of 16.2 % on a polyethylene terephthalate (PET) substrate with a remarkable power-per-weight of 26.9 W g-1 . This facile interfacial engineering method offers great potential for the large-scale manufacturing and commercialization of PSCs.

A Mathematical Model for the Design and Fabrication of Polymer Solar Cells by Spray Coating

Spraying of solution-processable materials such as organic molecules, polymers, nanoparticles, and quantum dots is a viable candidate for the coating process and fabrication of thin film solar cells and other similar semiconductor devices. Spray coating, similar to spray painting in the automotive industry, is a fast process and can be scaled up and used for the roll-to-roll fabrication of solar panels. In this paper, attempts are made to understand various steps of the process and develop a simple model as a design tool. The model equations are solved numerically for the spray coating of a P3HT-PCBM active layer in a polymer solar cell using ultrasonic atomization to investigate the effect of process parameters on the thin film characteristics, such as the film thickness and heat consumption needed to vaporize the solvent. It is concluded that when using spray coating with a small thermal budget, large areas with desired submicron- and nanometer-sized thicknesses can be made in a fast process. Cost of thermal energy and materials decreases with an increase in the substrate speed and nozzle-substrate distance.