Wouter Marchal

Combustion deposition of MoO3 films: from fundamentals to OPV applications

A systematic study of a combustion precursors thermal decomposition pathway is undertaken, in both powders and thin films, to obtain insights in the various parameters influencing the combustion process. The study focuses on MoO3 as a hole transporting layer (HTL) for applications in organic photovoltaics (OPV). Via evolved gas analysis, it was found that fuel volatility occurs prior to the actual combustion reaction of the acetylacetonate based precursor, affecting the optimal oxidizer to fuel ratio. Moreover, close investigation showed that the high rate combustion reaction disappears with increasing surface to volume ratio. Nonetheless, thermal analysis performed on films suggests that with the right heating rate, an oxidative complete decomposition still occurs in films, exemplified by the film composition and specific morphological differences in the resulting layers and through analysis of the evolved components. Finally, the discussed synthesis route allows organic free, crystalline MoO3 films to be obtained affording organic solar cell devices with promising current density–voltage characteristics.

From liquid to thin film: colloidal suspensions for tungsten oxide as an electrode material for Li-ion batteries

Using a colloidal suspension, tungsten oxide thin films (150 nm) have been prepared via ultrasonic spray deposition using two different current collectors, namely TiN and Pt. First, the precursor chemistry was studied, revealing that the tungsten present is reduced due to the formation of chlorine gas. Due to a dehydrogenation 1,1-diethoxyethane (DEE) and hydrogen chloride (HCl) evolve from the precursor, reducing the chloride content of the precursor. The thin films were annealed at 400 and 500 °C, yielding tetragonal tungsten oxide without the presence of chlorides. Electrochemical analysis indicated that the TiN current collector has a pronounced positive effect on cycling behavior of the WO3 thin film. A higher annealing temperature yields an improved performance, but annealing at temperatures as low as 400 °C also yielded electrochemically active WO3. The current study presents a versatile method to produce electrochemically active tungsten oxide thin films with a high volumetric capacity (640 mA h cm−3) at relatively low temperature to be applied in all-solid-state Li-ion batteries.

Ultrasonically spray coated silver layers from designed precursor inks for flexible electronics

Integration of electronic circuit components onto flexible materials such as plastic foils, paper and textiles is a key challenge for the development of future smart applications. Therefore, conductive metal features need to be deposited on temperature sensitive substrates in a fast and straightforward way. The feasibility of these emerging (nano-) electronic technologies depends on the availability of well-designed deposition techniques and on novel functional metal inks. As ultrasonic spray coating (USSC) is one of the most promising techniques to meet the above requirements, innovative metal organic decomposition (MOD) inks are designed to deposit silver features on plastic foils. Various amine ligands were screened and their influence on the ink stability and the characteristics of the resulting metal depositions were evaluated to determine the optimal formulation. Eventually, silver layers with excellent performance in terms of conductivity (15% bulk silver conductivity), stability, morphology and adhesion could be obtained, while operating in a very low temperature window of 70 °C-120 °C. Moreover, the optimal deposition conditions were determined via an in-depth analysis of the ultrasonically sprayed silver layers. Applying these tailored MOD inks, the USSC technique enabled smooth, semi-transparent silver layers with a tunable thickness on large areas without time-consuming additional sintering steps after deposition. Therefore, this novel combination of nanoparticle-free Ag-inks and the USSC process holds promise for high throughput deposition of highly conductive silver features on heat sensitive substrates and even 3D objects.

Steering the Properties of MoOx Hole Transporting Layers in OPVs and OLEDs: Interface Morphology vs. Electronic Structure

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.

Reversible Surface Engineering via Nitrone-Mediated Radical Coupling

Efficient and simple polymer conjugation reactions are critical for introducing functionalities on surfaces. For polymer surface grafting, postpolymerization modifications are often required, which can impose a significant synthetic hurdle. Here, we report two strategies that allow for reversible surface engineering via nitrone-mediated radical coupling (NMRC). Macroradicals stemming from the activation of polymers generated by copper-mediated radical polymerization are grafted via radical trapping with a surface-immobilized nitrone or a solution-borne nitrone. Since the product of NMRC coupling features an alkoxyamine linker, the grafting reactions can be reversed or chain insertions can be performed via nitroxide-mediated polymerization (NMP). Poly( n-butyl acrylate) ( Mn = 1570 g·mol-1, D̵ = 1.12) with a bromine terminus was reversibly grafted to planar silicon substrates or silica nanoparticles as successfully evidenced via X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry, and grazing angle attenuated total reflection Fourier-transform infrared spectroscopy (GAATR-FTIR). NMP chain insertions of styrene are evidenced via GAATR-FTIR. On silica nanoparticles, an NMRC grafting density of close to 0.21 chains per nm2 was determined by dynamic light scattering and thermogravimetric analysis. Concomitantly, a simple way to decorate particles with nitroxide radicals with precise control over the radical concentration is introduced. Silica microparticles and zinc oxide, barium titanate, and silicon nanoparticles were successfully functionalized.

Aqueous solution-based synthesis and deposition of crystalline In-Ga-Zn-oxide films with an enhanced mobility

AbstractIn-Ga-Zn-oxide (IGZO), in its amorphous state, is known to have a high electron mobility and low off-state current inside transistor devices, which may even be further improved by crystallization. Thin films of the IGZO superlattice structure require an optimal layer homogeneity in addition to precise control of the stoichiometry, which can be achieved by using a solution-based process. In this work, an aqueous precursor system is developed, starting from the respective metal (hydr)oxides. A stable multimetal precursor is obtained, which is ideally suited for solution-deposition via spin-coating. Through an optimized multi-step thermal treatment, crystalline thin films of IGZO are obtained that show a preferential c-axis orientation after rapid-thermal annealing at 1000 °C in inert conditions. The resulting film shows a good optical transparency (>70%) and an improved carrier mobility (27.2 cm²/Vs) compared to typical solution-processed amorphous IGZO films, and is therefore promising for further application. Top: Overview of the precursor synthesis, which consists of the metal (hydr)oxides as a starting product, citric acid (ligand) and ammonia (pH adjustment and bridging ion). Middle: Schematic route of the film processing. The precursor is applied on a substrate via spincoating. Intermediate hotplate steps (200–400–600 °C) decompose the precursor into an oxide film. Finally, a thermal treatment (1000 °C) inside an RTP (double-substrate setup) crystallizes the oxide films. Bottom: Thermogravimetric plot of the thermal decomposition of the precursor solution // Plane view SEM micrograph of the film morphology after annealing at 1000 °C for 500 s // XRD diffractogram of the film after annealing at 1000 °C for 500 s.HighlightsSolution-based deposition starting from an aqueous multimetal citrate precursor systemCrystalline IGZO films that show a preferential c-axis orientation after annealing at 1000 °CAn improved carrier mobility compared to typical solution-processed amorphous IGZO filmsA good optical transparency has been observed in the visible-light range

Wet-chemical synthesis of 3D stacked thin film metal-oxides for all-solid-state Li-ion batteries

By ultrasonic spray deposition of precursors, conformal deposition on 3D surfaces of tungsten oxide (WO3) negative electrode and amorphous lithium lanthanum titanium oxide (LLT) solid-electrolyte has been achieved as well as an all-solid-state half-cell. Electrochemical activity was achieved of the WO3 layers, annealed at temperatures of 500 °C. Galvanostatic measurements show a volumetric capacity (415 mAh·cm−3) of the deposited electrode material. In addition, electrochemical activity was shown for half-cells, created by coating WO3 with LLT as the solid-state electrolyte. The electron blocking properties of the LLT solid-electrolyte was shown by ferrocene reduction. 3D depositions were done on various micro-sized Si template structures, showing fully covering coatings of both WO3 and LLT. Finally, the thermal budget required for WO3 layer deposition was minimized, which enabled attaining active WO3 on 3D TiN/Si micro-cylinders. A 2.6-fold capacity increase for the 3D-structured WO3 was shown, with the same current density per coated area.