The fabrication of small molecule organic light-emitting diode pixels by laser-induced forward transfer

Laser-induced forward transfer (LIFT) is a versatile organic light-emitting diode (OLED) pixel deposition process, but has hitherto been applied exclusively to polymeric materials. Here, a modified LIFT process has been used to fabricate small molecule Alq(3) organic light-emitting diodes (SMOLEDs). Small molecule thin films are considerably more mechanically brittle than polymeric thin films, which posed significant challenges for LIFT of these materials. The LIFT process presented here uses a polymeric dynamic release layer, a reduced environmental pressure, and a well-defined receiver-donor gap. The Alq(3) pixels demonstrate good morphology and functionality, even when compared to conventionally fabricated OLEDs. The Alq(3) SMOLED pixel performances show a significant amount of fluence dependence, not observed with polymerical OLED pixels made in previous studies. A layer of tetrabutyl ammonium hydroxide has been deposited on top of the aluminium cathode, as part of the donor substrate, to improve electron injection to the Alq(3), by over 600%. These results demonstrate that this variant of LIFT is applicable for the deposition of functional small molecule OLEDs as well as polymeric OLEDs. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4788710

Synthesis of solvent-free processable and on-demand cross-linkable dielectric elastomers for actuators

Dielectric elastomer actuators (DEAs) have gained growing interest during the last decade from both the scientific community and industry. However, the high operating voltages needed to drive these actuators reduce their application potential. To date, several reports have been published on high permittivity elastomers that can be operated at low electric fields; however, little attention has been given to their processability into thin films and reliability as dielectrics in actuators. This work presents the synthesis of low molecular weight polar polysiloxanes, which have low viscosity and can, therefore, be processed into thin films by the solvent-free doctor blade deposition technique. Additionally, the prepared polysiloxanes have vinyl end-groups which are subsequently used for cross-linking into thin films, where multifunctional thiol cross-linkers and a UV initiator are used. The prepared films are elastic and show a permittivity of about two times higher as compared to commercial polydimethylsiloxane elastomers. In addition, their elastic modulus is easily tuned between 1.2 and 0.4 MPa using a dithiol as a chain prolongation reagent, while their viscoelastic behavior is comparable to commercial silicone elastomers. When used as dielectrics in DEAs the developed elastomers allow reliable operation over 180 000 actuation cycles at reduced voltages

Functional Ink Formulation for Printing and Coating of Graphene and Other 2D Materials: Challenges and Solutions

The properties of 2D materials are unparalleled when compared to their 3D counterparts; many of these properties are a consequence of their size reduction to only a couple of atomic layers. Metallic, semiconducting, and insulating types can be found and form a platform for a new generation of devices. Among the possible methods to utilize 2D materials, functional printing has emerged as a strong contender because inks can be directly formulated from dispersions obtained by liquid-phase exfoliation. Printed graphene-based devices are shifting from laboratory applications toward real-world and mass-producible systems going hand in hand with a good understanding of suitable exfoliation methods for the targeted type of ink. Such a clear picture does not yet exist for hexagonal boron nitride (h-BN), the transition metal dichalcogenides (TMDs), and black phosphorous (BP). Rather, reports of applications of these 2D materials in printed devices are scattered throughout the literature, not yet adding to a comprehensive and full understanding of the relevant parameters. This perspective starts with a summary of the most important features of inks from exfoliated graphene. For h-BN, the TMDs, and BP, the characteristic properties when exfoliated from solution and strategies to formulate inks are summarized

Charge generation by ultra-stretchable elastomeric electrets

Novel piezoelectric elastomers are synthesized and their long term stability and piezoelectric properties are investigated. They are thin film composites of specially designed polymer nanoparticles with high glass transition temperatures (T-g) and side groups with large permanent dipoles, which are embedded in chemically crosslinked polydimethylsiloxane matrices. To obtain a piezoelectric material, the initially randomly oriented polar groups in the nanoparticles are poled in a strong electric field while the film is heated above the T-g of the nanoparticles. Under these conditions the polar groups orient in the direction of the electric field and the achieved orientation is subsequently frozen-in by cooling the material back to room temperature. A permanent polarization responsible for the piezoelectricity is induced in the elastomers. All composites are elastic and can be strained up to 600%. The lateral piezoelectric coefficient d(31) of the composites are found to first decrease over a period of several days before ultimately stabilizing. The largest d(31) value obtained, 12.1 pC N-1, is comparable to commercially available materials which are not elastic. Two composites exhibit promising thermal stability at 50 degrees C and generate a maxim of 2.5 V when strained. The novel elastic electret materials described in this paper are likely to find application as stretchable sensors, soft electronics, transducers and energy harvesters. Another important aspect of this work is the abundantly available combinations of elastic matrices and high T-g polar polymers, which will allow the creation of elastic electrets with tailor-made properties in the future

Elastomers with tunable dielectric and electromechanical properties

Novel electroresponsive silicone elastomers modified with nitrile groups are presented, whose dielectric permittivity (epsilon') is tuned from epsilon' = 4.3 to epsilon' = 17.4. They are prepared in a one-step process starting from a high molecular weight poly(methylvinylsiloxane) to which polar nitrile groups and cross-links are introduced in thin films. Different ratios of butanethiol/3-mercaptoproprionitrile are used to vary the amount of nitrile groups in these elastomers, while 2,20-(ethylenedioxy) diethanethiol is used as a crosslinker. Because of the systematic nature of this investigation, we not only present promising elastic materials with remarkable dielectric, mechanical, and electromechanical properties but also provide a guideline for materials design aimed at dielectric elastomer actuator applications

Gravure printed Ag/conductive polymer electrodes and simulation of their electrical properties

In this paper, some practical issues related to the manufacturing, and design criteria related to the application in devices of a hybrid silver grid/poly (3,4-ethylenedioxythiophene) semi-transparent electrode are discussed. The electrodes are fabricated by gravure printing and screen printing. Experiments showed that defects in the printed grid due to imperfect ink transfer from the gravure roll to the substrate are detrimental to electrode performance. Various parameters like gravure cell design, printing speed, or particle size of the ink were investigated to minimize the fraction of defects and to obtain highly conductive grids. It will be demonstrated that overprinting of the lines is a feasible strategy to minimize the number of defects without noticeably broadening the lines. While a defect-free grid is prerequisite for such a hybrid device, for applications even stronger design criteria hold. That is addressed in the second part of the paper, where the electrical properties of the printed grids are simulated. With two exemplary device architectures, namely an organic light-emitting diode (OLED) and an organic photovoltaic cell, artificial load layers are integrated into the device structure, and potential maps are calculated. The examples show how simulations can be deployed to design and optimize grid electrodes for a specific application

Electroluminescent device

The invention concerns a electroluminescent device with a multilayer structure comprising: i) a first electrode including a layer, consisting of a transparent or translucent conductive material selected among metal oxides and metal nitrides, said layer being deposited on a transparent support, consisting of a glass, silicon, alumina plate, or a polymer sheet; ii) a second electrode; iii) a layer, arranged between the two electrodes, comprising a semiconductor and electroluminescent solid organic substance, said layer being optionally bordered with one or several intermediate layers, consisting of electrocatalysts; and iv) a layer with monomolecular structure, arranged between the layer consisting of the conductive material and the layer consisting of the electroluminescent substance. Said device is furher characterised in that said layer consists of a dipolar organic compound whereof the structure has an electronic system pi, a functional group, vicinal or not of the electronic system pi. Moreover, the dipolar organic compound is chemically bound by the functional group to the conductive material and has chemical affinity for the organic electroluminescent substance

Mild synthesis of mercaptonitriles from vinyl nitriles and their cyclization reactions

Thiol-ene addition of thioacetic acid A is widely used in the synthesis of thiols from vinyl precursors, but so far has not been conducted on non-conjugated vinyl nitriles. The challenge when vinyl nitriles are used is to selectively conduct the thiol-ene addition, while avoiding the nucleophilic addition of A to the nitrile group. We have found that vinyl nitriles give selective UV-induced thiol-ene addition in the presence of photoinitiators as long as a stoichiometric amount of A to the vinyl group and sterically unhindered vinyls are used. In contrast, when a sterically hindered vinyl is used, the nucleophilic addition of the nitrile is favoured. The prepared mercaptonitriles can easily undergo cyclization reactions in basic and acidic conditions as well as in the presence of silica gel. This illustrates the high reactivity of nitriles towards thiol addition. 1,2-Ethanedithiol B is presented as an alternative reagent to A as it allows conversion of vinyl nitriles directly into mercaptonitriles under mild and non-acidic reaction conditions