Materials Engineering

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC 71 BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating process, i.e., the majority of PEG accumulates at the bottom substrate, while the minority resides in the film. During electrode evaporation, PEG molecules kinetically " trapped " near the film surface migrate to the organic/Al interface to reduce the interfacial energy. This diffusion-limited process is enhanced with the initial concentration of PEG in the solution and with thermal annealing after metal deposition. In contrast, annealing the film before metal deposition stalls PEG migration. This mechanism is supported by corresponding OSC devices showing that V oc increases with PEG content at the interface, up to a saturation value associated with the formation of a continuous PEG interlayer. Presence of a continuous interlayer excludes the driving force for further migration of PEG to the interface. Revealing this mechanism provides practical insight for judicious selection of additives and processing conditions for interfacial engineering of spontaneously generated interlayers.

The fundamental structure of all organic electronic devices is a stack of thin layers sandwiched between electrodes, with precise intralayer morphology and interlayer interactions. Solution processing multilayers with little to no intermixing is, however, technically challenging and often incompatible with continuous roll-to-roll, high-speed manufacturing. Here, an overview of a recently developed methodology for self-generation of interlayers positioned between the active layer and metal contact is presented. The interlayer material is blended as an additive in the active layer and migrates to the organic/metal interface during metal deposition. The driving force for this migration is additive-metal interactions. The generated interlayer positions an interfacial dipole that reduces barriers for charge transfer across the organic/metal interface. This methodology is generic and, as reported here, the self-generated interlayers significantly improve the performance of many devices. I...

Migration of additives to organic/metal interfaces can be used to self-generate interlayers in organic electronic devices. To generalize this approach for various additives, metals, and organic electronic devices it is first necessary to study the dynamics of additive migration from the bulk to the top organic/metal interface. In this study, we focus on a known cathode interlayer material, polyethylene glycol (PEG), as additive in P3HT:PC71BM blends and study its migration to the blend/Al interface during metal deposition and its effect on organic solar cell (OSC) performance. Using dynamic secondary ion mass spectroscopy (DSIMS) depth profiles and X-ray photoelectron spectroscopy surface analysis (XPS), we quantitatively correlate the initial concentration of PEG in the blend and sequence of thermal annealing/metal deposition processes with the organic/Al interfacial composition. We find that PEG is initially distributed within the film according to the kinetics of the spin coating...

The structure of bulk heterojunction organic photovoltaic devices generally includes interlayers, thin films positioned between the active layer and one or both of the electrodes, intended to enhance the performance and/or stability. Interlayers can consist of organic or inorganic materials. They play different roles, such as enhancing adhesion and reducing energetic barriers, and are deposited using techniques such as spin coating and thermal deposition. Here we focus on interlayers that do not require a distinct deposition process but rather spontaneously form at the film surface or the active layer/electrode interface. This review paper identifies the underlying mechanisms that induce the interlayer self-generation, and classifies the relevant studies based on (reported or realized) driving forces for interlayer formation. By doing so, this perspective not only offers directive tools for the design of interlayers at surfaces and buried interfaces, but also provides insights on thermodynamic and/or kinetic aspects of processes that occur spontaneously during device fabrication and/or testing but have been overlooked until now.

We report n-type performance enhancement of an ambipolar donor−acceptor (D−A) polymer by suppressing its p-type behavior using a simple and versatile methodology of interlayer processing in top-contact organic field-effect transistors. In this approach, a judiciously selected interlayer material, polyethylene glycol (PEG), is codeposited as an additive with the semiconducting polymer DPP-T-TT active layer. During top-contact aluminum deposition, the PEG molecules migrate from within the bulk film to the organic/Al interface because of additive−metal interactions. The formation of the PEG interlayer was confirmed by X-ray photoelectron spectroscopy and its effect on the polymericinterfacial microstructure was studied using angle-dependent grazing-incidence wide-angle X-ray scattering. We find that the PEG interlayer has two significant contributions to the n-type characteristics of the device: it suppresses hole injection while improving electron injection by tuning the effective work-function, and it enhances the order and crystallinity of the polymer at the interface effectively enhancing charge mobility. 50 example, Sun et al. used poly ethylenimine (PEI) as a hole-51 trapping dopant in an ambipolar D−A semiconducting 52 polymer effectively converting it into a high-performance 53 unipolar n-type film for OFETs. 11

It has been recently shown that doping-like properties can be introduced into functional ceramics by inducing dislocations. Especially for TiO2, donor and acceptor-like behavior were observed depending on the type of introduced mesoscopic dislocation network. However, these early reports could not fully elucidate the mechanism behind it. In this work, we rationalize the electrical properties of dislocations by targeted microelectrode measurements on deformed single crystals, comparing dislocation-rich and deficient regions. With the help of finite element method (FEM) calculations, a semi-quantitative model for the effect of dislocations on the macroscopic electrical properties is developed. The model describes the dislocation bundles as highly conductive regions in which respective space charges overlap and induce temperatureindependent, highly stable electronic conductivity. We illustrate the mechanism behind unique electrical properties tailored by introducing dislocations and believe that these results are the cornerstone in developing dislocation-tuned functionality in ceramics. Teaser (as per journal format requirement/NOT part of the main text) The field of dislocation-tuned functionality came onto the scene about eight years ago. It took few years to develop as it involves a fundamental understanding of both chemistry and mechanics and dedicated characterization. However, this year may be the breakthrough for this field with devices ultimately based on atomic-scale functionality. It will be a completely new materials paradigm, i.e., new devices not made by atomistic doping but by mechanical or 1-dimensional doping. We believe that our work provides a critical understanding of the newly coined dislocation-tuned electrical conductivity in ceramics.

Vascular grafts are artificial conduits properly designed to substitute a diseased blood vessel. However prosthetic fail can occur without premonitory symptoms. Continuous monitoring of the system can provide useful information not only to extend the graft's life but also to optimize the patient's therapy. In this respect, various techniques have been used, but all of them affect the mechanical properties of the artificial vessel. To overcome these drawbacks, an ultrathin and flexible smart patch based on piezoelectric Aluminum Nitride (AlN) integrated on the extraluminal surface of the prosthesis is presented. The sensor can be conformally wrapped around the external surface of the prosthesis. Its design, mechanical properties and dimensions are properly characterized and optimized in order to maximize performances and to avoid any interference with the graft structure during its activity. The sensorized graft is tested in vitro using a pulsatile recirculating flow system that mimics the physiological and pathological blood flow conditions. In this way, the ability of the device to measure real-time variations of the hemodynamics parameters has been tested. The obtained high sensitivity of 0.012 V Pa −1 m −2 , joint to the inherent biocompatibility and non-toxicity of the used materials, demonstrates that the device can successfully monitor the prosthesis functioning under different conditions, opening new perspectives for real-time vascular graft surveillance.

Glycine nano-crystals were grown inside alumina nano-pores due to a precipitation process from over-saturated aqueous liquid solutions. The a-glycine polymorph crystals were formed at a higher over-saturation concentration than that of the b-glycine polymorph crystals. The results indicate that the type of the glycine polymorph formed inside the alumina pores is kinetically controlled. A model is suggested to explain the competition between formations of the two polymorphs inside nano-pores. The b-glycine polymorph crystals are distinguished from the a-glycine polymorph crystals not only by XRD measurements but mainly by piezoelectric measurements, where only the non-centro-symmetric b-glycine polymorph crystals show a piezoelectric current response to applied mechanical pressures as low as 1 Pa in the environmental pressure of 1 atm (10-3% pressure change).

A piezoelectric current response to small applied mechanical pressure changes, as low as 1 Pa, in environmental 1 atm pressure conditions, is reported for Lithium Sulfate monohydrate (LSM) nano-crystals grown inside alumina pores. A higher piezoelectric sensitivity is obtained in crystals having a lower residual strain explained by a mechanical spring behavior. The crystals are grown by an isothermal precipitation process from over-saturated aqueous solutions only inside the pores. The crystals are grown with a preferred crystallographic orientation along the longitudinal axis of the pores, indicating a preferred nucleation at the bottom of the pores due to a lower surface energy and negative electric charges embedded in the alumina pore's walls. The nucleation and growth of the LSM crystals inside the pores is described with schematic drawings of the atoms arrival and arrangement in the unit-cell.

The preferred crystallographic orientation of nanocrystals plays a significant role in determining their properties. From the wide variety of nanocrystal growth techniques, we focus in this paper on crystal growth by precipitation from liquid solutions inside porous substrates, and discuss the progress that has been made during the last decade concerning the control of crystal growth direction through this method. In this overview, the motivation and principal mechanisms of achieving highly oriented nanocrystals are presented. Moreover, different experimental challenges within the described growth technique are probed. The paper presents the thermodynamic and kinetic considerations for favoring crystal growth inside pores rather than bulk growth. A special focus is made on the origin of obtaining preferred crystallographic orientations in various types of materials, including varying perspectives of thermodynamic and kinetic driving forces. The paper ends with technological application of crystal growth with preferred crystallographic orientation inside nano-pores.

Development of highly-sensitive detectors of low mechanical pressures in the scale of few Pascal and below in environmental conditions are of a high technological demand for a variety of applications such as a remote control of infants breathing and a leakage alert of hazardous gases. This paper reports on a highly sensitive piezoelectric detector of low applied mechanical pressures down to 0.1 Pa in atmospheric environment (10-4 % pressure change). It is made of SN (sodium nitride) nanocrystals grown inside alumina pores with a preferred crystallographic orientation that can be controlled by their formation conditions based on a thermodynamic driving force. The SN nanocrystals inside the pores show a piezoelectric current response to low applied mechanical pressure which depends on the crystallographic orientation of the crystals and the level of residual or applied static strains. The piezoelectric current response is correlated with the unique molecular structure of the SN crystals.

Laser surface remelting can be used to manipulate the microstructure of cast material. Here, we present a detailed analysis of the microstructure of Fe2VAl following laser surface remelting. Within the melt pool, elongated grains grow nearly epitaxially from the heat-affected zone. These grains are separated by low-angle grain boundaries with 1°-5° misorientations. Segregation of vanadium, carbon, and nitrogen at grain boundaries and dislocations is observed using atom probe tomography. The local electrical resistivity was measured by an in-situ four-point-probe technique. A smaller increase in electrical resistivity is observed at these low-angle grain boundaries compared to high-angle grain boundaries in a cast sample. This indicates that grain boundary engineering could potentially be used to manipulate thermoelectric properties.

It is well-known that grain boundaries (GBs) increase the electrical resistivity of metals due to their enhanced electron scattering. The resistivity values of GBs are determined by their atomic structure; therefore, assessing the local resistivity of GBs is highly significant for understanding structure−property relationships. So far, the local electrical characterization of an individual GB has not received much attention, mainly due to the limited accuracy of the applied techniques, which were not sensitive enough to detect the subtle differences in electrical resistivity values of highly symmetric GBs. Here, we introduce a detailed methodology to probe in situ or ex situ the local resistivity of individual GBs in Cu, a metallic model system we choose due to its low resistance. Both bulk Cu samples and thin films are investigated, and different approaches to obtain reliable and accurate resistivity measurements are described, involving the van der Pauw technique for macroscopic measurements as well as two different fourpoint-probe techniques for local in situ measurements performed inside a scanning electron microscope. The in situ contacts are realized with needles accurately positioned by piezodriven micromanipulators. Resistivity results obtained on coincidence site lattice GBs (incoherent Σ3 and asymmetric Σ5) are reported and discussed. In addition, the key experimental details as well as pitfalls in the measurement of individual GB resistivity are addressed.

The scale effect (or "size effect") on the brittleness of solid materials (mainly for brittle solids such as ceramic, glasses, etc.) was first appreciated by Lawn et al., 1-4 who defined the brittleness incorporating the ratio of hardness and fracture toughness. Later, Gerberich et al. 5,6 revealed and discussed the scale-dependent brittleness transition in silicon. However, the exact role played by dislocations, the one-dimensional defects which are one of the main carriers of plastic deformation in crystals, on the scale-dependent "brittle-ductile transition" was insufficiently discussed for brittle oxides. Contrasting the conventional belief that the majority of oxide ceramics are brittle macroscopically at room temperature, 7 there is abundant evidence for dislocation motion at room temperatures in various ceramics. 8-12 Furthermore, the dislocation-based functionality in oxides is witnessing a research upsurge in recent years. Dislocations are introduced into various oxides to harvest the functional properties. For instance, dislocations have been engineered

Titanium thin films were deposited on silicon nitride (SiN x) coated Si, NaCl, and sapphire substrates varying the deposition conditions using e-beam evaporation to investigate thin film growth modes. The microstructure and texture evolution in dependence of substrate, deposition rate, film thickness, and substrate temperature were studied using X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Thin films obtained on SiN x and NaCl substrates were nanocrystalline, while the films deposited on sapphire transformed from nanocrystalline to single crystalline at deposition temperatures above 200 • C. Predominantly, a surface plane orientation of (0002) was observed for the single crystalline films due to the minimization of surface energy. The orientation relationship of epitaxial single crystalline films grown on C-plane sapphire substrate is found to be (0002) T i (0006) S apphire , 1120 T i 0330 S apphire. In this orientation relationship, both the total surface and strain energy of the film are minimized. The results were complemented by resistivity measurements using the four-point probe method reporting an increase from ∼60 µ Ω cm to ∼95 µ Ω cm for single crystalline and nanocrystalline films, respectively. 32 Apart from the deposition parameters, the lattice matching, 33 symmetry of surface planes, substrate miscut, and the planes 34 forming terraces and steps on the substrate surface [15] are vari-35 ables that can alter the orientation and morphology of thin films, 36 while maintaining substrate chemistry. The adaption of an ori-37 entation relationship (OR) between a substrate and a film is a 38 complex phenomenon [16]. Even with a known substrate sur-39 face symmetry and chemical composition, it is in most cases not 40 possible to predict the preferred OR [17]. Although the (0002) 41 plane of Ti has the lowest surface energy, making it the domi-42 nant surface plane, Ti films have been reported to grow with at 43 least six other surface plane orientations [18].

There is an increasing reckoning that the thermoelectric performance of a material is dependent on its microstructure. However, the microstructure-properties relationship often remains elusive, in part due to the complexity of the hierarchy and scales of features that influence transport properties. Here, we focus on the promising Heusler-Fe2VAl compound. We directly correlate microstructure and local properties, using advanced scanning electron microscopy methods including in-situ four-point-probe technique for electron transport measurements. The local thermal conductivity is investigated by scanning thermal microscopy. Finally, atom probe tomography provides near-atomic scale compositional analysis. To locally manipulate the microstructure, we use laser surface remelting. The rapid quenching creates a complex microstructure with a high density of dislocations and small, elongated grains. We hence showcase that laser surface remelting can be employed to manipulate the microstructure to reduce the thermal conductivity and electrical resistivity, leading to a demonstrated enhancement of the thermoelectric performance at room temperature.