No abstract

Poly (3,4-ethylenedioxythiophene) : poly(styrenesulphonic acid) (PEDOT : PSS) is commonly used as an anode in polymer light-emitting diodes (PLED). We have studied the effect of the pH and Na + ion concentration of the aqueous PEDOT : PSS dispersion on the bulk and surface properties of spincoated films by various techniques, including UV-vis-NIR optical absorbance spectrometry, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS). A pH increase by addition of NaOH modifies the PEDOT : PSS properties in a similar way as electrochemical dedoping: the IR absorbance decreases, the Raman peaks shift, sharpen and increase in intensity, and the work function decreases. Consequently, a barrier for hole injection is introduced for several classes of light-emitting polymers. We argue that the mechanism of the pH-effect is different from electrochemical dedoping, and originates from a change in the relative stability of polarons and bipolarons on the doped thiophene. The changes in the electronic properties of PEDOT : PSS point to the determining role of the counter-ion in the stabilisation of oxidised thiophene units. Polymer LEDs comprising Na + -rich, proton poor PEDOT : PSS can show lower lifetime and efficiency than the corresponding Na + -free, proton-rich devices. For light emitting polymers which suffer from the addition of sodium to the hole injecting PEDOT : PSS, the decreased lifetime hints at hole injection as limiting factor in the degradation of these PLEDs.

No abstract

No abstract

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.

No abstract

No abstract

Abstract:The commercial breakthrough of phosphorescent organic white light sources is presently hampered due to the unavailability of a stable blue phosphorescent emitter material. Moreover, only few analytical investigations have been made regarding the chemical degradation of the phosphorescent emitter materials during the processing or the operation of the devices. Organic light emitting devices (OLEDs) containing phosphorescent metal complexes with iridium as central ion were investigated. Special attention was paid to the chemical degradation of the material. The devices were analyzed by means of high performance liquid chromatography coupled with mass spectrometry (HPLC/MS). Electron spray ionization (ESI) was employed as ionization source. Isomerization phenomena of the blue-green emitting heteroleptic iridium complex FIrpic could be observed after the device manufacture and after operation. These findings could give hints on the mechanisms that influence the lifetime of PhOLEDs based on FIrpic or similar blue emitters.© Versita Warsaw and Springer-Verlag Berlin Heidelberg.