Blue-Greenish Electroluminescent Poly(<i>p</i>‑phenylenevinylene) Developed for Organic Light-Emitting Diode Applications

A novel electroluminescent poly­(<i>p</i>-phenylenevinylene) (PPV) derivative was synthesized via the Gilch route, which emits in the blue-greenish region. The required monomer synthesis is a multistep process starting from catechol and does not involve any critical step. The polymer synthesis itself proceeds via standard Gilch conditions and results in constitutionally homogeneous and extraordinary high-molecular-weight PPVs. The characterization of these materials was carried out using nuclear magnetic resonance spectroscopy and size exclusion chromatography measurements. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels were estimated by combining information provided by cyclic voltammetry and UV–vis measurements. Finally, the electroluminescent behavior of the polymer was confirmed in an organic light-emitting diode

Inverse <i>I</i>–<i>V</i> Injection Characteristics of ZnO Nanoparticle-Based Diodes

Simple Al/ZnO­(NP)/Au diodes produced by spin coating of ZnO nanoparticle dispersions (ZnO­(NP)) on Al/Al₂O₃ and Au substrates and subsequent Au deposition have been investigated to understand electron injection properties of more complex devices, incorporating ZnO­(NP) as injection layer. Inverse I–V characteristics have been observed compared to conventional Al/ZnO­(SP)/Au diodes produced by reactive ion sputtering of ZnO. SEM micrographs reveal that the void-containing contact of ZnO­(NP) with the bottom Al electrode and the rough morphology of the top Au electrode are likely to be responsible for the observed injection and ejection probabilities of electrons. A simple tunneling model, incorporating the voids, explains the strongly reduced injection currents from Al whereas the top electrode fabricated by vapor deposition of Au onto the nanoparticle topology adopts the inverse ZnO­(NP) morphology leading to enlarged injection areas combined with Au-tip landscapes. These tips in contrast to the smooth sputtered ZnO­(SP) lead to electric field enhancement and strongly increased injection of electrons in reverse direction. The injected charge piles up at the barrier generated by voids between ZnO­(NP) and the bottom electrode forcing a change in the barrier shape and therefore allowing for higher ejection rates. Both effects in combination explain the inverse <i>I</i>–<i>V</i> characteristic of nanoparticle based diodes

Order Induced Charge Carrier Mobility Enhancement in Columnar Liquid Crystal Diodes

Discotic molecules comprising a rigid aromatic core and flexible side chains have been promisingly applied in OLEDs as self-organizing organic semiconductors. Due to their potentially high charge carrier mobility along the columns, device performance can be readily improved by proper alignment of columns throughout the bulk. In the present work, the charge mobility was increased by 5 orders of magnitude due to homeotropic columnar ordering induced by the boundary interfaces during thermal annealing in the mesophase. State-of-the-art diodes were fabricated using spin-coated films whose homeotropic alignment with formation of hexagonal germs was observed by polarizing optical microscopy. The photophysical properties showed drastic changes at the mesophase-isotropic transition, which is supported by the gain of order observed by X-ray diffraction. The electrical properties were investigated by modeling the current–voltage characteristics by a space-charge-limited current transport with a field dependent mobility

Interrelation between Chemical, Electronic, and Charge Transport Properties of Solution-Processed Indium–Zinc Oxide Semiconductor Thin Films

<i>Solution-processed</i> metal oxide semiconductors are of high interest for the preparation of high-mobility transparent metal oxide (TMO) semiconductor thin films and thin film transistors (TFTs). It has been shown that the charge transport properties of indium–zinc oxide (IZO) thin films from molecular precursor solutions depend strongly on the preparation conditions, in particular on the precursor conversion temperature <i>T</i>_pc and, to some surprise, also on the concentration of the precursor solution. Therefore, the chemical and the electronic structure of solution-processed IZO thin films have been studied in detail with X-ray photoelectron spectroscopy (XPS) under systematic variation of <i>T</i>_pc and the concentration of the precursor solution. A distinct spectral feature is observed in the valence band spectra close to the Fermi level at <i>E</i>_B = 0.45 eV binding energy which correlates with the trends in the sheet resistivity, the field effect mobility μ_FE, and the optical gap <i>E</i>_g^opt from four-point-probe (4PP), TFT, and UV–vis measurements, respectively. A comprehensive model of the interrelation between the conditions during solution-processing, the chemical and electronic structure, and the charge transport properties is developed

New Columnar Zn-Phthalocyanine Designed for Electronic Applications

Columnar liquid crystals are composed of disk-shaped aromatic molecules surrounded by flexible side chains, where molecules self-assemble in columns and thereby form large surface-oriented domains. These systems are known for their good charge and exciton transport along the columns, with mobilities approaching those of aromatic single crystals. Such semiconducting materials are promising for devices applications, since the output efficiency can be tuned by properly aligning columns. In the work presented here, the synthesis and characterization of a new Zn-phthalocyanine (ZnPc) is described which exhibits remarkable properties, such as hexagonal columnar order, achieved by cooling down from the isotropic phase to room temperature. Such order was confirmed by optical microscopy and X-ray diffraction experiments. Diodes were constructed using spin-coated films, and the conductive properties were investigated by current versus voltage analysis, where mobilities of 10^–3 and 10^–2 cm²/(V s) were obtained for the nonannealed and annealed films, respectively