Gert H. ten Brink

Elimination of the light soaking effect and performance enhancement in perovskite solar cells using a fullerene derivative

In this work, we investigate how electron extraction layers (EELs) with different dielectric constants affect the device performance and the light-soaking phenomenon in hybrid perovskite solar cells (HPSCs). Fulleropyrrolidine with a triethylene glycol monoethyl ether side chain (PTEG-1) having a dielectric constant of 5.9 is employed as an EEL in HPSCs. The commonly used fullerene derivative [60]PCBM, which has identical energy levels but a lower dielectric constant of 3.9, is used as a reference. The device using PTEG-1 as the EEL shows a negligible light soaking effect, with a power conversion efficiency (PCE) of 15.2% before light soaking and a minor increase to 15.7% after light soaking. In contrast, the device using [60]PCBM as the EEL shows severe light soaking, with the PCE improving from 3.8% to 11.7%. Photoluminescence spectroscopy and impedance spectroscopy measurements indicate that trap-assisted recombination at the interface between the hybrid perovskite and the EEL is the cause of the light soaking effect in HPSCs. The trap-assisted recombination is effectively suppressed at the perovskite/PTEG-1 interface, while severe trap assisted recombination takes place at the perovskite/[60]PCBM interface. We attributed these experimental findings to the fact that the higher dielectric constant of PTEG-1 helps to screen the recombination between the traps and free electrons. In addition, the electron donating side chains of PTEG-1 may also contribute to the passivation of the electron traps. As a consequence, the devices using PTEG-1 as the EEL display a considerable increase in the efficiency and a negligible light soaking effect.

Counterion-Mediated Ligand Exchange for PbS Colloidal Quantum Dot Superlattices

In the past years, halide capping became one of the most promising strategies to passivate the surface of colloidal quantum dots (CQDs) in thin films to be used for electronic and optoelectronic device fabrication. This is due to the convenient processing, strong n-type characteristics, and ambient stability of the devices. Here, we investigate the effect of three counterions (ammonium, methylammonium, and tetrabutylammonium) in iodide salts used for treating CQD thin films and shed light on the mechanism of the ligand exchange. We obtain two- and three-dimensional square-packed PbS CQD superlattices with epitaxial merging of nearest neighbor CQDs as a direct outcome of the ligand-exchange reaction and show that the order in the layer can be controlled by the nature of the counterion. Furthermore, we demonstrate that the acidity of the environment plays an important role in the substitution of the carboxylates by iodide ions at the surface of lead chalcogenide quantum dots. Tetrabutylammonium iodide shows lower reactivity compared to methylammonium and ammonium iodide due to the nonacidity of the cation, which eventually leads to higher order but also poorer carrier transport due to incomplete removal of the pristine ligands in the QD thin film. Finally, we show that single-step blade-coating and immersion in a ligand exchange solution such as the one containing methylammonium iodide can be used to fabricate well performing bottom-gate/bottom-contact PbS CQD field effect transistors with record subthreshold swing.

Tuning structural motifs and alloying of bulk immiscible Mo-Cu bimetallic nanoparticles by gas-phase synthesis

Nowadays bimetallic nanoparticles (NPs) have emerged as key materials for important modern applications in nanoplasmonics, catalysis, biodiagnostics, and nanomagnetics. Consequently the control of bimetallic structural motifs with specific shapes provides increasing functionality and selectivity for related applications. However, producing bimetallic NPs with well controlled structural motifs still remains a formidable challenge. Hence, we present here a general methodology for gas phase synthesis of bimetallic NPs with distinctively different structural motifs ranging at a single particle level from a fully mixed alloy to core-shell, to onion (multi-shell), and finally to a Janus/dumbbell, with the same overall particle composition. These concepts are illustrated for Mo-Cu NPs, where the precise control of the bimetallic NPs with various degrees of chemical ordering, including different shapes from spherical to cube, is achieved by tailoring the energy and thermal environment that the NPs experience during their production. The initial state of NP growth, either in the liquid or in the solid state phase, has important implications for the different structural motifs and shapes of synthesized NPs. Finally we demonstrate that we are able to tune the alloying regime, for the otherwise bulk immiscible Mo-Cu, by achieving an increase of the critical size, below which alloying occurs, closely up to an order of magnitude. It is discovered that the critical size of the NP alloy is not only affected by controlled tuning of the alloying temperature but also by the particle shape.

N-Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host-Dopant Miscibility

In this contribution, for the first time, the polarity of fullerene derivatives is tailored to enhance the miscibility between the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain (PTEG-1) is used as the host and (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine n-DMBI) as the dopant. Thereby, the doping efficiency can be greatly improved to around 18% (<1% for a nonpolar reference sample) with optimized electrical conductivity of 2.05 S cm-1 , which represents the best result for solution-processed fullerene derivatives. An in-depth microstructural study indicates that the PTEG-1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n-DMBI dopants are mainly incorporated in the side chain plane without disturbing the π-π packing of PTEG-1. This new microstructure, which is rarely observed for codeposited thin films from solution, formed by PTEG-1 and n-DMBI molecules explains the increased miscibility of the host/dopant system at a nanoscale level and the high electrical conductivity. Finally, a power factor of 16.7 µW m-1 K-2 is achieved at 40% dopant concentration. This work introduces a new strategy for improving the conductivity of solution-processed n-type organic thermoelectrics.

Roughness controlled superhydrophobicity on single nanometer length scale with metal nanoparticles

Here we demonstrate high water pinning nanostructures and trapping of water droplets onto surfaces via control of roughness on a single nanometer length-scale generated by deposition of preformed gas phase distinct copper nanoparticles on hydrophilic and hydrophobic surfaces. It was found that the contact angles of the water droplets were increased to the superhydrophobic limit ∼150° at high nanoparticle coverages (≥80%) independent of the initial type of surface. The water droplets were trapped onto the surfaces by high adhesion forces similar like the rose petal effect. The droplets are in a Wenzel state at their outer part. Local nanocapillarity can force liquid into crevices between nanoparticles and push trapped air within the center of the droplet forming a Cassie–Baxter metastable state. Hence our approach to alter the wetting state is extremely straightforward without involving special micro/nano structuring facilities, but instead using direct single nanoparticles deposition on any type of surfaces creating a rough surface on a single nanometer length-scale, allowing due to its peculiar high water pinning and nanoporous structure liquid trapping phenomena.

Copper nanoparticle formation in a reducing gas environment

Although copper nanoparticles are used as model nanomaterial because of their small nucleation barrier, their oxidization sensitivity hampers production of fully metallic nanoparticles with controlled size and shape. Nevertheless, we demonstrate here synthesis of copper nanoparticles, via high pressure magnetron sputtering, having highly tunable sizes and shapes over a size range spanning two orders of magnitude. This is achieved by exploiting a reducing gas environment to mediate proper nucleation conditions, allowing size control of nanoparticles with robust motifs for particle sizes ∼5–300 nm. However, due to rapid coalescence oxidation-free nanoparticles cannot be produced monodisperse for sizes larger than ∼30 nm.

Synthesis and morphology of iron-iron oxide core-shell nanoparticles produced by high pressure gas condensation

Core-shell structured Fe nanoparticles (NPs) produced by high pressure magnetron sputtering gas condensation were studied using transmission electron microscopy (TEM) techniques, electron diffraction, electron energy-loss spectroscopy (EELS), tomographic reconstruction, and Wulff shape construction analysis. The core-shell structure, which is composed of an Fe core surrounded by a maghemite (γ-Fe2O3) and/or magnetite (Fe3O4) shell, was confirmed by fast Fourier transform (FFT) analysis combined with EELS. It was found that the particle size and shape strongly depend on the gas environment. Moreover, extensive analysis showed that NPs with a size between 10-20 nm possess a truncated cubic morphology, which is confined by the 6 {100} planes that are truncated by the 12 {110} planes at different degrees. For NPs larger than 20 nm, the rhombic dodecahedron defined by the 12 {110} planes is the predominant crystal shape, while truncated rhombic dodecahedrons, as well as non-truncated and truncated cubic NPs, were also observed. The NPs without truncation showed a characteristic inward relaxation indicating that besides thermodynamics kinetics also plays a crucial role during particle growth.

Improved efficiency of NiOx-based p-i-n perovskite solar cells by using PTEG-1 as electron transport layer

We present efficient p-i-n type perovskite solar cells using NiOx as the hole transport layer and a fulleropyrrolidine with a triethylene glycol monoethyl ether side chain (PTEG-1) as electron transport layer. This electron transport layer leads to higher power conversion efficiencies compared to perovskite solar cells with PCBM (phenyl-C61-butyric acid methyl ester). The improved performance of PTEG-1 devices is attributed to the reduced trap-assisted recombination and improved charge extraction in these solar cells, as determined by light intensity dependence and photoluminescence measurements. Through optimization of the hole and electron transport layers, the power conversion efficiency of the NiOx/perovskite/PTEG-1 solar cells was increased up to 16.1%.

Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Dynamics of GeSbTe phase-change nanoparticles deposited on graphene

Phase-change Ge2Sb2Te5 nanoparticles (NPs), that are promising for next-generation phase-change memory and other emerging optoelectronic applications, have been deposited on graphene support layers and analyzed using advanced transmission electron microscopy techniques allowing high quality atomic resolution imaging at accelerating voltages as low as 40 kV. The deposition results in about three times higher NP coverage on suspended graphene than on graphene containing an amorphous background support. We attribute this to the variation in surface energy of suspended and supported graphene, indicating that the former harvests NPs more effectively. Hydrocarbon contamination on the graphene profoundly enhances the mobility of the NP atoms and after prolonged (weeks) exposure to air resulted in more severe oxidation and spreading of NPs on the suspended graphene than on supported graphene because the network of hydrocarbons develops more extensively on the suspended rather than on the supported graphene. Due to this oxidation, GeO x shells are formed out of NPs having a uniform composition initially. The present work provides new insights into the structure and stability of phase-change NPs, graphene and their combinations.