Organic light-emitting diodes (OLEDs) have recently attracted much interest among researchers as well as engineers as promising high quality self-emissive displays for all kinds of portable devices such as cellular phones, personal organisers, etc. While monochrome operation is sufficient for some applications, ultimately multi-color devices such as signs or even RGB (red, green, blue) matrix displays will be requested by the customer in the future. So far, this goal has been achieved with small-molecule devices fabricated by vacuum deposition. In contrast, electroluminescent (EL) polymers, which are commonly deposited by solution processing, seemed to be only poorly suited for this purpose owing to the lack of high-resolution patterning processes. Recent attempts, therefore, focus on the adaptation of common printing techniques such as screen printing and ink jetting, both having severe technical difficulties and drawbacks, such as limited resolution in the former and wetting issues in the latter case requiring extensive pre-treatment of the substrates. We demonstrate the use of a new class of EL polymers, which can be applied similar to a standard photoresist. Soluble polymers with oxetane sidegroups were crosslinked photochemically to yield insoluble polymer networks in the desired areas. The resolution of the process is sufficient to fabricate common pixelated matrix displays. Consecutive deposition of the three colors yielded a RGB device with efficiencies comparable to state-of-the-art EL polymers, even slightly reduced onset voltages, and improved efficiencies at high luminance levels. The improved thermal and morphological stability promises better performance in passive-matrix displays requiring high drive currents. The new method potentially allows efficient manufacturing of high-resolution multi-color polymer-based displays on large area using common lithography techniques.
PEDOT [Poly(3,4-ethylenedioxythiophene)] layers were prepared by electrochemical polymerization of the respective monomer. Thereafter, these layers were electrochemically adjusted to different equilibrium potentials and they were investigated with Kelvin Probe measurements. A change in work function could be observed yielding a linear correlation with the pre-adjusted electrochemical potential. These PEDOT layers have been utilized in OLEDs in their accessible range of work functions. Internal energy conditions of the OLEDs were characterized in a photovoltaic setup yielding a linear correlation of the open circuit voltage on the pre-adjusted potential. In a second step the efficiency was determined for devices with Ca cathode (space-charge-limited electron current) as well as for devices with Al cathode (injection-limited electron current). These devices could be optimized in efficiency by adjusting the hole current to the electron current, which was determined by the work function of the cathodic metal. The optimum could be explained in zero-order approximation in terms of a balanced bimolecular reaction between holes and electrons.