Electrical and Optical Studies of the Organic Thin Film Devices Produced by Cluster Beam Deposition Methods
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Electrical and Optical Studies of the Organic Thin Film Devices Produced by Cluster Beam Deposition Methods J.Y. Kim, E.S. Kim and J.-H. Choi* Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University, 1, Anam-dong, Seoul 136-701, Korea ABSTRACT The neutral and ionized cluster beam deposition (NCBD and ICBD) methods have been applied to fabricate the new double-layer organic light emitting devices (OLEDs) with the structure of indium-tin-oxide (ITO)-coated glass/spin-coated poly[2-(N-carbazolyl)-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] (CzEH-PPV)/8hydroxyquinoline aluminum (Alq3)/Li:Al. The surface morphology profiles measured by atomic force microscopy (AFM) showed that the cluster beam deposition methods are efficient in producing uniform and smooth film surfaces. Photo- and electroluminescence (PL, EL) spectroscopies demonstrated that while the new hole transporting medium CzEH-PPV is susceptible to the bombardment of energetic ionized beam, the introduction of the neutral buffer layer significantly improves the device characteristics, i.e., lower threshold and turn-on voltages and higher external quantum efficiency (EQE). In addition, the effect of doping of highly fluorescent dye (DCM) into Alq3 layer showed a complete energy transfer, color-tuning capability and higher EQE compared to the undoped devices. INTRODUCTION The recent progress made in the fabrication of efficient light emitting devices utilizing organic and polymeric materials is of extreme importance in developing future flat-panel display applications [1,2]. In preparing such high-quality functional devices, the acquisition of smooth and uniform thin films to form well-defined heterojunctions is one of the important prerequisites. For low molecular weight molecules the most common technique is the simple physical vapour deposition(PVD) method. Another less popular but promising scheme is applying cluster beam deposition methods [3,4]. Neutral and ionized cluster beams have been widely used in gas-phase reactions to understand the effect of solvation and intermolecular interactions at the microscopic level. The advantages of employing such cluster beam are the high directionality and translational kinetic energy obtained when the gas molecules undergo adiabatic expansion in high vacuum. Since the clusters are composed of weakly bound molecules, the collision of cluster beam with the substrate induces facile fragmentation into individual molecules followed by active surface migration resulting in more uniform film surfaces. In particular, the electric charge of the ionized cluster beam can provide more adjustable deposition parameters such as ionizing current, ionization and acceleration voltages, ion current density, etc. In this paper, we describe the fabrication and characterization of the two-layer OLEDs utilizing CzEH-PPV, a newly synthesized derivative of a prototypal conjugated polymer PPV, as a hole transport layer and Alq3 as an electron transport
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layer which is deposited by either NCBD o
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