Laser induced forward transfer (LIFT) and laser sintering of metal nanoparticle inks constitute a two-step digital fabrication technique which has been proven a key enabling technology for the fabrication of flexible microelectronic devices. In this work we will present the investigation of the laser printing and sintering process of Ag nanoparticle inks for the production of a conductive grid comprised of parallel lines as replacement for the bottom Indium Tin Oxide (ITO) electrode in organic photovoltaics (OPVs). We study the effect of a range of laser parameters and their impact on the morphological characteristics and the electrical performance of the laser printed conductive grid. The electrical conductivity of the laser printed lines is calculated by means of electrical measurements in a 4-point probe IV station while their morphological characteristics are assessed with profilometry measurements. As a result, flexible ITO-free OPVs incorporating laser-printed Ag grids as a bottom electrode on PET substrates will be presented. The results confirm that the laser printing and sintering combination is an advantageous technique, which can offer a distinguishing solution for applications in highly efficient ITO-free OPVs.
Most current electronics manufacturing technologies utilise subtractive processing that is expensive, wasteful and energy intensive. Printed electronics is revolutionising the electronics industry by enabling additive processing that significantly reduces expense, waste and energy consumption.
The EU-funded PLASMAS project demonstrates the capability of printed electronics based on novel nanoparticle Cu inks with favourable cost to performance ratios, through development of large area printed circuit boards and printed logic as well as OLED and OPV elements with printed Cu nanoparticle electrodes.
However, a number of challenges need to be overcome when printing these metal nanoparticle inks – the typical feature height of printed structures of several 100 nm tend to exhibit a rough surface, which can lead to shorts in the device after subsequent overcoating of the organic active layer materials. Furthermore, the sintering temperature of the nanoparticle inks needs to be low (< 130 °C) in order to allow deposition and curing on transparent flexible substrates such as PET.
We therefore present the process development of solution-processed electrodes based on inkjet-printed Cu grids, by embedding the inkjet-printed metal grids to yield ITO-free optoelectronic devices. Secondly, we present roll-to-roll inkjet-printed RFID antennas based on Cu inks. Finally, we demonstrate a truly low-temperature sintering route for a Cu nanoparticle ink by using a reducing atmosphere of formic acid, yielding stable highly conducting layers.
The results of the project highlight overall parameters for solution processing and implementation of novel metal nanoparticle materials and architectures in printed electronics.
KEYWORDS: Solar cells, Electrodes, Organic photovoltaics, Field effect transistors, Heterojunctions, Gold, Polymers, Metals, Thin film devices, Electron transport
The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact
bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. We report the
creation of an efficient electron selective bottom contact based on a solution-processed Titania layer on top of Indium
Tin Oxide. The use of o-xylene as the casting solvent creates an efficient carrier collection network with little vertical
phase segregation, providing sufficient performance for both regular as well as inverted solar cells. We demonstrate
inverted layer sequence OPVs with AM 1.5-calibrated power conversion efficiencies of over 3%.
In this study molecular doping in non-conjugated polymeric systems is utilized in order to obtain high efficiency electrophosphorescent light emitting devices (PHOLEDs). The device consists of a light emitting thin film layer composed of hole and electron transporting moieties dispersed in a polymer matrix of polyvinylcarbazole (PVK). Light emission is obtained by harvesting singlet as well as triplet excitons by means of a phosphorescent dye, Iridium (III) tris(2-(4-tolyl)pyridinato-N,C2) (Ir(m-ppy)3), also dispersed in the polymer matrix. By incorporating a low conductivity polyethylene dioxythiophene-polystyrene-sulfonate (PEDOT) hole injection layer between the indium tin oxide transparent anode and the light emitting molecularly doped layer, the efficiency of these devices reaches values as high as 41 cd/A with a peak luminous efficacy of 28 lm/W. At the same time, triplet quenching by the hole transporting moiety as well as the electrodes are expected to be limiting the efficiency of these devices. In this paper we discuss several alternative device architectures studied in order to understand the factors affecting the device performance. In particular the effect of incorporating alternative hole transporting moieties and hole blocking layers are addressed.
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