Fatemeh Zabihi

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.

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.

Fabrication of highly reproducible polymer solar cells using ultrasonic substrate vibration posttreatment

Abstract. Organic solar cells are usually nonreproducible due to the presence of defects in the structure of their constituting thin films. To minimize the density of pinholes and defects in PEDOT:PSS, which is the hole transporting layer of a standard polymer solar cell, i.e., glass/ITO/PEDOT:PSS/P3HT:PCBM/Al, and to reduce scattering in device performance, wet spun-on PEDOT:PSS films are subjected to imposed ultrasonic substrate vibration posttreatment (SVPT). The imposed vibration improves the mixing and homogeneity of the wet spun-on films, and consequently the nanostructure of the ensuing thin solid films. For instance, our results show that by using the SVPT, which is a mechanical, single-step and low-cost process, the average power conversion efficiency of 14 identical cells increases by 25% and the standard deviation decreases by 22% indicating that the device photovoltaic performance becomes more consistent and significantly improved. This eliminates several tedious and expensive chemical and thermal treatments currently performed to improve the cell reproducibility.

Low-cost transparent graphene electrodes made by ultrasonic substrate vibration-assisted spray coating (SVASC) for thin film devices

In an attempt to replace expensive and rigid transparent conductive oxides, used as electrodes in thin film devices, in this study, transparent graphene electrodes (TGEs) are fabricated by conventional spray coating and ultrasonic substrate vibration-assisted spray coating. Systematic characterizations of the TGEs and indium tin oxide (ITO) demonstrate comparable and even better surface and morphological characteristics, film coverage, surface potential distribution and suitable work function of the spray-on TGEs compared to those of the reference ITO electrode. A lower transmittance, electrical conductivity and charge quenching potential are observed in the TGEs compared to those of the reference ITO, which may be further improved by process optimization. As a proof of concept, the TGE was incorporated into a perovskite solar cell, where power conversion efficiency of 3.54% was achieved, which is promising given that the developed graphene electrode was fabricated using scalable and low-cost spray coating.

Preparation of Nano-curcumin with Enhanced Dissolution Using Ultrasonic-Assisted Supercritical Anti-solvent Technique

Abstract Curcumin is the main gradient of “Turmeric” a famous Indian spice and food additive. The marvelous nutritional and medicinal effects of curcumin made it a good alternative to some conventional drugs and food flavoring or coloring materials. However, the low solubility of curcumin is a challenging hindrance which should be seriously addressed. In this work, we prepared nano-curcumin with enhanced aqueous dispersion and dissolution rate. Ultrasonic-assisted supercritical anti-solvent (UA-SAS) technique was used to convert the commercial curcumin to uniform distributed nano-particles with the average size of 20 nm and yielding of 65%. The effect of process parameters including pressure, temperature, solution flow rate, and nature of organic solvent on the average particle size and yielding of products was investigated. The morphology, size, and crystalline pattern of processed curcumin particles were characterized by scanning electron microscopy, mean particle size analyzer, and X-ray diffraction. The champion specimen was achieved when the supercritical fluid was employed at 16 MPa and 35°C. Aqueous suspension of processed nano-curcumin can be stable for more than 2 months. In vitro dissolution experiments showed a remarkable enhancement in dissolution rate of UA-SAS-treated curcumin respecting to the commercial curcumin powder.

A bottom-up approach to design wearable and stretchable smart fibers with organic vapor sensing behaviors and energy storage properties

Realizing the best way to integrate electronics and textiles to develop smart wearable, functional apparel with multiple functionalities such as fibers with a unified capability to store and utilize energy is a significant topic of concern recently. Therefore, presenting a facile approach to obtain fibers with such unique properties in a continuous process is a forward contributing step towards the development of this field. Herein, a bottom-up approach to fabricate stretchable poly(styrene-butadiene-styrene)/few-layer graphene composite (SBS-G) fibers with unique organic vapor sensing behaviors and modified SBS-G fibers coated with electroactive carbon black (CB) nanofibers via modified electrospinning with excellent energy storage properties is presented. Unlike conventional conductive polymer composites (CPCs) that respond only to polar or non/low-polar organic vapors, the fabricated SBS-G composite fibers exhibited high sensitivity, excellent reversibility, and reproducibility as well as fast response to both polar and non/low-polar organic vapors. Moreover, the modified nanofiber-based SBS-G fibers demonstrated a high capacitive performance (78 F cm−3), energy and power density (6.6 mW h cm3 and 692 mW cm3) and excellent flexibility. This study provides guidelines for the fabrication of ideal organic vapor sensors based on polymer composite fibers and an approach to modify any “off-the-shelf fiber” for fiber-based power storage.

Two-step sequential substrate vibration-assisted spray coating (SVASC) as a path to high performance perovskite solar cells

Efficiency of planar heterojunction CH3NH3PbI3-xClx solar cell is strongly influenced by the morphology and crystal size of the perovskite thin film. A novel approach using sequential spraying of the solution onto an ultrasonically vibrating substrate, i.e., two-step substrate vibration-assisted spray coating, (SVASC) is introduced and utilized for the fabrication of perovskite thin films. The perovskite crystal size, morphology and the percentage of coverage are significantly improved by imposing a 5 W ultrasonic vibration on the substrate. The perovskite thin films, prepared by the 2-step SVASC shows a 10% average improvement in light harvesting potential, compared to the thin films deposited on stationary substrates.