Thomas Kugler

Active Matrix Displays Based on All‐Organic Electrochemical Smart Pixels Printed on Paper

An organic electronic paper display technology (see Figure and also inside front cover) is presented. The electrochromic display cell together with the addressing electrochemical transistor form si ...

The electronic structure of polymer-metal interfaces studied by ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy has come of age. UPS can take its place beside its older, better-known sister, ESCA (or XPS) as a surface sensitive method which has become more useful in learning certain specific things about interfaces at distances significantly larger than the typical electron elastic mean-free-paths dictated by the photon energies employed. In particular, the emergence of UPS as a real tool for interfacial studies has been applications driven, evolving after needs within polymer-based electronics applications. The situation is clarified through the use of several examples, drawn from the applications-spectroscopy literature.

Electric current rectification by an all-organic electrochemical device

An all-organic printed electrochemical rectifier is reported. The device is based on a patterned layer of poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) that interfaces a patterned electrolyte top layer. Overlap between the electrolyte layer and the conducting polymer pattern results in the formation of two electrochemically active areas within the conducting polymer pattern. When bias voltage is applied across the conducting polymer pattern, the PEDOT in the negatively biased areas is reduced electrochemically, while the PEDOT in the positively biased area is further oxidized. Reducing PEDOT from its p-doped, pristine state to the neutral state results in a marked loss of electrical conductivity. Due to the unsymmetrical device geometry, the current through the device may be shut off for one polarity of applied bias voltage with an electrical current rectification ratio of 100 compared to the opposite polarity. The output characteristics of a corresponding half wave rectifier as well as those from a full wave bridge rectifier show stable performance at frequencies below 15 Hz.

Transparent low-work-function indium tin oxide electrode obtainedby molecular scale interface engineering

A redox reaction between a monolayer of electron–donor molecules, tetrakis(dimethylamino)ethylene, and the indium tin oxide (ITO) surface results in a decrease of the ITO work function down to 3.7eV. The modified ITO surface may be used as electron injecting electrode in polymer light-emitting devices. Photoelectron spectroscopy measurements show that the low-work-function of the modified electrode remains upon exposure to air or gentle annealing; thus, making it a good candidate for inexpensive fabrication of organic/polymeric (opto)electronic devices.

Energy level alignment in organic-based three-layer structures studied by photoelectron spectroscopy

The alignment of energy band edges at interfaces in three-layer structures, important in polymer-based electroluminescent devices, have been studied using photoelectron spectroscopy. Careful choice of the materials allows investigation of the role of the substrate work function. These results are in agreement with the energy level diagram commonly assumed for the metal/polymer/metal structure, with the potential drop over the polymer layer being equal to the difference between the work functions of the metal contacts. Direct indication of the alignment of the electrochemical potential across the three-layer structure has been observed by photoelectron spectroscopy.

Paper electronics and electronic paper

The manufacturing, converting and ennobling processes of paper are truly large area and reel-to-reel processes. Here, we describe a project focusing on using the converting and ennobling processes of paper in order to introduce electronic functions onto the paper surface. As key active electronic materials we are using organic molecules and polymers. We develop sensor, communication and display devices on paper and the main application areas are packaging and paper display applications.

Switchable Optical Polarizer Based on Electrochromism in Stretch-Aligned Polyaniline

We report on the polarizing electrochromic (EC) effect of a conjugated polymer. This has been achieved in a planar flexible electrochemical device cell comprised of a patterned stretch-aligned thin ...

Photoelectron spectroscopy study of the energy level alignment at polymer/electrode interfaces in light emitting devices

Abstract The band alignment at the interface between electroluminescent polymers and the electrodes in polymer-LEDs was studied using photoelectron spectroscopy. Chemical factors like the formation of InCl3 during conversion of precursor-PPV on ITO could be directly monitored with XPS. Films of electroluminescent polymers were studied on a range of ITO and metal electrodes with different work functions, as well as with an intermediate, electrically conducting polymer layer, using UPS. Furthermore, the study of the band alignment at polymer electrode interfaces was extended to three-layer structures: the results confirm the common assumption that the potential drop over the polymer layer in polymer LEDs is equal to the difference between the electrode work functions.

Chemical species at polymer/ITO interfaces : consequences for the band alignment in light-emitting devices

The influence of chemical species present at the interface between the electroluminescent polymer and the ITO electrode in light-emitting devices on the band edge energies of overlayers of semiconducting conjugated polymers has been studied using photoelectron spectroscopy. The formation of InCl3 during the conversion of precursor-PPV on ITO was directly monitored with XPS. Ultrathin films of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) were studied directly on ITO, as well as with an intermediate layer of an electrically conducting polymer using UPS. The initial work function of the ITO was varied chemically from 4.4 eV to 4.8 eV. In addition, the work function of ITO was changed in situ, within a given sample, by exposure to X-rays. For the polymer spin-coated directly on ITO, the vacuum levels are aligned. With the electrically conducting polymer blend, poly(3,4-ethplenedioxythiophene) doped with poly(4-styrene sulfonate) spin-coated on ITO, the Fermi levels are aligned, as expected. Therefore, with a conducting polymer blend intermediate layer between the polymer and the ITO, the polymer bands align to the vacuum level of the conducting polymer blend on ITO, and the barrier to hole injection into the polymer is determined by the work function of the conducting polymer blend instead of the work function of the ITO

Organic Electrochemical Smart Pixels

Major efforts have been spent during recent years in worldwide attempts to achieve an electronic paper technology; the common name for novel flexible displays utilizing substrates such as paper, plastics or thin metal sheets. Various kinds of technology are available that potentially will be used for an electronic paper, which differs from each other mainly with respect to the choice of active materials, substrates and manufacturing techniques. There are many applications for electronic paper technology, ranging from high-resolution displays used in electronic books to updateable large-area billboards. The latter suggests a novel electronic display function that could extend the utilization of cellulose-based paper, which is one of the most common materials ever produced by mankind, by using the paper as a thin and flexible carrier. The requirement for fast update speed in such large area applications would probably be a bit more relaxed compared to traditional display technologies, while low-power consumption and bi-stability are among the factors that should be further emphasized, together with the utilization of well-established printing techniques to enable low-cost manufacturing of the displays. The choice of active materials is therefore crucial in order to reach these objectives in reality and this paves the way for printable conjugated polymers with electrochromic properties. Chemical synthesis of these materials during the last decades has resulted in a vast variety of electrochromic polymers with custom-tailored functionality covering a broad range of optical absorption and electrical conductivities. This thesis review the studies done on the electrochemical switching of poly(3,4-ethylenedioxythiophene) (PEDOT). For this material both the electrical conductivity and the optical absorption is controlled by the oxidation state. Active matrix addressed displays that are printed on flexible substrates have been obtained by arranging electrochemical smart pixels, based on the combination of electrochemical transistors and electrochromic display cells, into cross-point matrices. The resulting polymer-based active-matrix displays are operated at low voltages and the same active material can be used in electrochemical transistors and conducting lines and in electrochromic display cells employing the electronic and the opto-electonic properties of the material, respectively. In addition to this, a switchable optical polarizer is briefly discussed. This is a device utilizing electrochromism of stretch-aligned polyaniline (PANI). The combination of two identical devices in a vertical architecture, orthogonally oriented with respect to each other, results in a filter in which the orientation of the polarized optical absorption is governed by the voltage polarity applied to the device.