Stefan Sax

Designed Suppression of Aggregation in Polypyrene: Toward High-Performance Blue-Light-Emitting Diodes

2010 WILEY-VCH Verlag Gm Pyrene is one of the most important and thoroughly investigated organic chromophores. Among the attractive features of pyrene is its exceptionally long fluorescence lifetime, the sensitivity of its excitation spectra to microenvironment changes, and its high propensity for forming excimers. This excimer formation has been utilized over the last 50 years in the investigation of water-soluble polymers, making pyrene, by far, the most frequently applied dye in fluorescence-labeled polymers. Despite its chemical stability and high charge carrier mobility, its strong tendency to form excimers leads to a red-shift of its emission as well as a decrease in its fluorescence efficiency in condensed media, which has prohibited its use as an emissive material in organic light-emitting devices (OLED)s. Since the report of the first double-layer thin-film OLED by the Kodak Company in 1987, OLEDs have attracted enormous attention in the scientific community thanks to their high technological potential for the next generation of fullcolor-flat-panel displays and lighting applications. Whether polymers or small molecules, to date only redand greenemitters have shown sufficient efficiencies and lifetimes to be of commercial value. Here, we present a novel non-aggregating polypyrene, which can be synthesized via a simple three-step chemical route from pyrene in good yields. Our poly-7tert-butyl-1,3-pyrenylene (3) shows a high solid-state fluorescence quantum yield with blue emission, excellent solubility and stability, no aggregation in thin films, and good electro-optical performance in single-layer OLEDs. The use of pyrene, a large conjugated aromatic system, as emitting material in OLED applications has been limited, due to aggregation between planar pyrene molecules. The formation of p-aggregates/excimers causes the quenching of fluorescence, resulting in low solid-state fluorescence quantum yields. In recent years, there has been an increasing interest in the use of pyrene units in OLEDs, including oligothiophenes with pyrenyl side groups, bipyrenylbenzene molecules, as well as pyrene-carbazole and pyrene-fluorene systems, due to their emissive properties combined with high charge carrier mobility. However, the pyrene derivatives that have been reported so far as efficient blue-emitters for OLED applications still present some degree of aggregation in the solid state. A successful effort in the prevention of aggregation in small molecules was achieved with tetrasubstituted, highly sterically hindered pyrenes, which can emit blue light in solution as well in the solid state and with high quantum yield. The well-known 1,3,6,8-tetraphenylpyrene, for example, has been used in OLEDs, organic field-effect transistors (OFET)s, as well in organic light-emitting field-effect transistors (OLEFET)s. Additional tetrasubstituted systems including different phenyl derivatives or pyridyl units at the 1-, 3-, 6-, and 8-positions have been reported as well. Recently, we have reported the suppression of aggregation in a strongly twisted multichromophoric dendrimer made up exclusively from pyrene units substituted at the 1-, 3-, 6-, and 8-positions, which revealed a very high fluorescence quantum yield relative to unsubstituted pyrene. Furthermore, 1,10-bipyrenyl and linear 1,6-disubstituted oligopyrenes have also been investigated. In comparison to small molecules, conjugated organic polymers have the advantage of being applicable in larger display sizes and lighting devices at much lower manufacturing costs via solution-based deposition techniques. Only a small number of investigations concerning the attachment of pyrene to the polymeric chain or the use of pyrene along the polymeric backbone were reported as new materials for molecular electronics. Polypyrenes formed via electrochemical polymerization of pyrenes by a 1 10 coupling were described to give insoluble and unprocessable film or alternatively soluble materials with few repeat units. Thereby, the low degree of polymerization is presumed to be a consequence of the low solubility caused by the strong self-assembly of pyrene segments. In contrast, we present a simple chemical route to a highly soluble and processable polypyrene, 3, with a defect-free structure and the highest degree of polymerization reported up to now for polymers made up exclusively from pyrene units. Compound 3 was synthesized in a simple three-step route as depicted in Scheme 1. Pyrene was first mono-tert-butylated to afford 2-tert-butylpyrene (1), which was then treated with bromine (2 equivalents) in CH2Cl2 at 78 8C to provide the 1,3-dibromo-7-tert-butylpyrene (2) in 89% yield. The use of tert-butyl groups was strategic in order to selectively obtain the 1,3-dibromo pyrene derivative. The polymerization ofmonomer 2 was carried out in a Yamamoto coupling with a Ni(0) catalyst, followed by end-capping with bromobenzene. After precipitation in a mixture of HCl and methanol (1:1) and subsequent removal

Core, shell, and surface-optimized dendrimers for blue light-emitting diodes.

We present a novel core-shell-surface multifunctional structure for dendrimers using a blue fluorescent pyrene core with triphenylene dendrons and triphenylamine surface groups. We find efficient excitation energy transfer from the triphenylene shell to the pyrene core, substantially enhancing the quantum yield in solution and the solid state (4-fold) compared to dendrimers without a core emitter, while TPA groups facilitate the hole capturing and injection ability in the device applications. With a luminance of up to 1400 cd/m(2), a saturated blue emission CIE(xy) = (0.15, 0.17) and high operational stability, these dendrimers belong to the best reported fluorescence-based blue-emitting organic molecules.

Efficient Blue-Light-Emitting Polymer Heterostructure Devices: The Fabrication of Multilayer Structures from Orthogonal Solvents

[*] Prof. E. J. W. List, Dr. S. Sax, A. Neuhold NanoTecCenter Weiz Forschungsgesellschaft mbH Franz-Pichler-Straße 32, A-8160 Weiz (Austria) E-mail: list@ntc-weiz.at Prof. K. Müllen, N. Rugen-Penkalla Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz (Germany) E-mail: muellen@mpip-mainz.mpg.de Prof. S. Schuh, Prof. E. Zojer, E. J. W. List Institute of Solid State Physics Graz University of Technology Petersgasse 16, A-8010 Graz (Austria)

Organic Non‐Volatile Resistive Photo‐Switches for Flexible Image Detector Arrays

A unique implementation of an organic image detector using resistive photo-switchable pixels is presented. This resistive photo-switch comprises the vertical integration of an organic photodiode and an organic resistive switching memory element. The photodiodes act as a photosensitive element while the resistive switching elements simultaneously store the detected light information.

Unravelling the Nature of Unipolar Resistance Switching in Organic Devices by Utilizing the Photovoltaic Effect

The origin of resistive switching in organic devices is studied by photovoltaic methods and impedance spectroscopy. The results show that the most commonly proposed charging/discharging mechanisms can be excluded as working mechanism. There is solid evidence that resistive switching is due to the formation/rupture of filaments. Further, it is shown that this is a universal property of metal/organic/metal thin-film devices.

WPLEDs prepared from main-chain fluorene–iridium(III) polymers

A novel heteroleptic iridium(III) complex containing 5,7-dibromoquinolinolate as co-ligand has been used to prepare main-chain iridium(III) polymers which produced white light emission when used as the active layer in polymeric light-emitting devices.

Inkjet printed polymer light-emitting devices fabricated by thermal embedding of semiconducting polymer nanospheres in an inert matrix

An aqueous dispersion of semiconducting polymer nanospheres was used to fabricate polymer light-emitting devices by inkjet printing in an easy-to-apply process with a minimum feature size of 20μm. To form the devices, the electroluminescent material was printed on a nonemitting polystyrene matrix layer and embedded by thermal annealing. The process allows the printing of light-emitting thin-film devices without extensive optimization of film homogeneity and thickness of the active layer. Optical micrographs of printed device arrays, electroluminescence emission spectra, and I∕V characteristics of printed ITO/PEDOT:PSS/PS/SPN/Al devices are presented.

Highly Efficient Color‐Stable Deep‐Blue Multilayer PLEDs: Preventing PEDOT:PSS‐Induced Interface Degradation

Highly efficient and stable blue light emission is observed in novel copolymers that are produced from specially designed building blocks. A PEDOT:PSS-induced chemical degradation of the polymer light-emitting diodes (PLEDs) is identified at the interface, and it is found to be accompanied by a shift in the emission color. A method to prevent this highly undesirable interaction is presented.

A planar waveguide optical sensor employing simple light coupling

The novel optical sensor concept utilizes the sensing layer as light propagating layer and employs a new method to couple light into a planar waveguide.

Direct observation of conductive filament formation in Alq3 based organic resistive memories

This work explores resistive switching mechanisms in non-volatile organic memory devices based on tris(8-hydroxyquinolie)aluminum (Alq3). Advanced characterization tools are applied to investigate metal diffusion in ITO/Alq3/Ag memory device stacks leading to conductive filament formation. The morphology of Alq3/Ag layers as a function of the metal evaporation conditions is studied by X-ray reflectivity, while depth profile analysis with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry is applied to characterize operational memory elements displaying reliable bistable current-voltage characteristics. 3D images of the distribution of silver inside the organic layer clearly point towards the existence of conductive filaments and allow for the identification of the initial filament formation and inactivation mechanisms during switching of the device. Initial filament formation is suggested to be driven by field assisted diffusion of silver from abundant structures formed during the top electrode evaporation, whereas thermochemical effects lead to local filament inactivation.