Effect of the alkali metal content on the electronic properties of PEDOT:PSS

\u3cp\u3eThe effect of the sodium and cesium ion surface concentration on the electronic properties of spin-coated poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonic acid) films, known as PEDOT:PSS, has been studied by means of ultraviolet and X-ray photoelectron spectroscopy. The sodium and cesium concentration in the film has been varied by the addition of NaOH or CsOH to the PEDOT:PSS dispersion. Hydrogen ions of the acid PSSH are exchanged for sodium or cesium ions, resulting in the salt PSSNa or PSSCs without changing the oxidation state of PEDOT, i.e., without doping/dedoping the material. The work function changes from 5.1 to 4.0 eV with increasing alkali surface concentration. The ionization potential remains constant at 5.0 eV above 1 at% alkali metal content and coincides with the work function below 1 at%. Thus, the material changes from a semiconductor-like to a metal-like state.\u3c/p\u3

Modification of PEDOT:PSS as hole injection layer in polymer LEDs

\u3cp\u3ePoly(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\u3csup\u3e+\u3c/sup\u3e 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\u3csup\u3e+\u3c/sup\u3e-rich, proton poor PEDOT:PSS can show lower lifetime and efficiency than the corresponding Na\u3csup\u3e+\u3c/sup\u3e-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.\u3c/p\u3

Preparation of monodisperse polymer particles and capsules by ink-jet printing

\u3cp\u3eMicron-sized particles with a narrow size distribution are prepared by ink-jet printing technology. Droplets of a polymer solution are printed with the nozzle submerged into an aqueous phase. The particles are formed upon removal of the solvent for the polymer. The particle size can be accurately predicted from the initial drop size and the polymer concentration. The method is also suitable for the preparation of monodisperse capsules with a well-defined core and shell using an additional non-solvent for the polymer. Hollow capsules are prepared by removal of the non-solvent from core of the capsules using freeze-drying. This leads to gas-filled capsules with a well-defined polymeric shell and a diameter in the range of 5 μm, which may be applied as an ultrasound contrast agent.\u3c/p\u3

High-temperature thin-film barriers for foldable AMOLED displays

\u3cp\u3eWe present a thin-film dual-layer bottom barrier on polyimide that is compatible with 350°C backplane processing for organic light-emitting diode displays and that can facilitate foldable active-matrix organic light-emitting diode devices with a bending radius of <2 mm. We demonstrate organic light-emitting diodes that survive bending over 0.5 mm radius for 10.000× based on the high-temperature bottom barrier. Furthermore, we show compatibility of the bottom barrier with the backplane process by fabricating active-matrix organic light-emitting diode displays on GEN1-sized substrates.\u3c/p\u3