Inkjet printed surface cell light-emitting devices from a water-based polymer dispersion

We report on the fabrication of polymer light-emitting diodes (PLEDs) and light-emitting electrochemical cells (LECs) in planar surface cell geometry (anode as well as the cathode are made of gold; interelectrode spacing: 1 mu m) by means of inkjet printing. The active material for PLEDs is an aqueous poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) dispersion, and for LECs blends thereof with poly(ethylene oxide) (two different molecular weights: 100,000 g/mol (PEO-100,000) and 30,000 g/mol (PEO-30,000)) and lithium-triflate, building the solid state electrolyte. The surface PLEDs reveal very poor device performance with extremely high current and light emission onset voltages. However, adding the solid state electrolyte to the luminescent material, leading to the device type of an LEC, distinctly improves the performance obtaining onset voltages slightly above 3 V and remarkable enhanced light output. Due to the exchange of the high molecular weighted PEO-100,000 by the PEO-30,000, which leads to an elimination of the undesired bead-on-a-string effect during the inkjet printing process, the reproducibility of the device fabrication can be conspicuously improved. Additionally, the location of the light emission zone of a surface LEC can be easily determined, since one has a direct view between the electrodes. For such a device the light generation occurs near the cathode. (c) 2007 Elsevier B.V. All rights reserved.