Fabrication of organic light-emitting diode pixels by laser-assisted forward transfer

Romain Fardel, Matthias Nagel, Frank Nüesch, Thomas Lippert, Alexander Wokaun
2007 Applied Physics Letters  
Fabrication of a polymer light-emitting device was achieved by a laser forward transfer technique using the decomposition of a thin triazene polymer film by a XeCl excimer laser. The dry deposition process allows transfer of a bilayer consisting of the electroluminescent polymer poly͓2-methoxy-5-͑2-ethylhexyloxy͒-1,4-phenylenevinylene͔ covered with an aluminum electrode onto a receiver substrate. The soft transfer results in laterally well resolved pixels ͑Ϸ500 m͒, whose fluorescence as well as
more » ... electroluminescence spectra remain unaltered. The rectifying and smooth current-voltage characteristics add to the merits of this laser-based transfer method that opens up the possibility of direct-writing heat-and UV-sensitive materials. Laser-induced forward transfer ͑LIFT͒ has been developed as a direct-write method for the defined microdeposition of metals, ceramic powders, or organic materials. 1-4 In conventional LIFT procedures, a layer of the transfer material is coated on a transparent donor substrate ͑e.g., fused silica͒. Irradiation by a laser pulse coming in through the substrate leads to an evaporative forward ablation of the transfer material. A receiver substrate that is placed in close vicinity of the donor substrate receives the material to be transfered. Mainly robust and heat-resisting materials can be transferred by this method because of the high thermal load induced by direct light absorption. Sensitive materials, such as organic dyes or semiconductive polymers, are damaged by laser irradiation. To avoid direct laser damaging, an additional dynamic release layer ͑DRL͒ was introduced for lightto-heat conversion needed to transfer sensitive materials. 2,5-8 The common issue of these intermediate layers, predominantly used with infrared lasers, is the intrinsically high thermal load on the sensitive materials to be transferred. 9 To prevent heat-and light-induced damages, we developed an advanced LIFT process using an intermediate sacrificial layer of a UV-absorbing dialkyltriazene polymer. Thin films of this photopolymer are excellent DRLs because they decompose integrally into volatile fragments at very low ablation thresholds ͓Ϸ25 mJ/ cm 2 ͑Ref. 10͔͒. Recently, living mammalian cells were deposited onto a bioreceiver substrate with a pulsed ArF excimer laser using such a triazene polymer DRL. 11 In a similar manner, pixel arrays of sensitive multispectral nanocrystal quantum dots were smoothly transferred with good resolution. 12 Therefore, this LIFT method should be ideally suited to transfer sensitive optoelectronic polymers for the fabrication of organic light-emitting diodes ͑OLEDs͒ as well. Unlike solution-based screen printing, 13 ink-jet methods, 14 and photolithographic techniques, 15 the laser deposition process is a dry transfer method that bears two principal advantages. Firstly, it is able to produce highresolution patterning required for pixelated displays. Secondly, it bears the potential to fabricate well-defined multilayer structures. This is difficult if not impossible to achieve using solution based methods if more than two layers are to be deposited. In contrast with a resonant infrared pulsed laser ablation method, recently applied to fabricate polymer light-emitting diodes ͑PLEDs͒, 16 the advanced LIFT process does not rely on vacuum deposition and shadowing mask techniques. In this letter, we report the LIFT method for the fabrication of polymer light-emitting devices using the archetype poly͓2-methoxy-5-͑2-ethylhexyloxy͒-1,4-phenylenevinylene͔ ͑MEH-PPV͒. 17 We demonstrate microdeposition of a polymer/metal cathode bilayer from a fused silica donor to a transparent conducting oxide receiver using the high-quality triazene photopolymer as sacrificial DRL ͑Fig. 1͒. Photoluminescence, electroluminescence, current-voltage characteristics, as well as structural properties of the transferred pixel devices are discussed. With the goal to produce OLEDs according to the simple device architecture tin-doped indium oxide ͑ITO͒/MEH-PPV/Al, the multilayer donor films were prepared by depositing successively triazene polymer ͑100 nm͒, aluminum ͑70 nm͒, and MEH-PPV ͑90 nm͒ on fused silica substrates ͑see Fig. 1͒ . The triazene polymer ͑poly͓oxy-1,4-phenylene͑3-methyl -1-triazene-1,3-diyl͒-1,6-hexanediyl͑1-a͒
doi:10.1063/1.2759475 fatcat:qp4ra2ayvzeptbx72prpcaggf4