Cascade Connected Organic El Diodes for Multi-Color Emission
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(a) PPCP
(b) Alq 3
( CH3)O C(CH 3)3 00
3 (H)
(c) BPPC
& CH(C 3
H'6
(d) TPD
Fig.1. Molecular structure of fluorescent dyes used for fabricating multi-color emission device. (a) 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP), (b) 8-hydroxyquinoline aluminum (Alq 3), (c) N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BPPC), (d) N,N'-diphenylN,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD)
In Fig. 2, device structure which has been realized R-G-B emission is schematicallly shown. The device consists of two parts, i.e., two-color emission part and single-color emission part. It consists of an indium-tin-oxide (ITO) coated glass substrate, fluorescent dye layers, a half-transparent aluminum (Al) electrode, fluorescent dye layers, and an indium-containing magnesium (Mg:lIn) as shown in Fig. 2. The layer structure was fabricated by organic molecular beam deposition (OMBD) on an ITO-coated glass substrate at a background pressure of about 10-5 Pa for EL devices. For optical measurement, the layers were deposited on a quartz substrate. The powders of fluorescent dyes were loaded into separate Knudsen cells (K-cells), then the cells were subsequently heated upto their sublimation temperatures, and deposited onto a substrate. The typical deposition rates were 2-3 nm/min in this experiment. The layer thickness of the deposited materials was determined in-situ using an oscillating quartz thickness monitor. Al and Mg:In electrodes were vapor deposited at a background pressure of about 10-1 Pa. For two-color emission part, 25-nm-thick BPPC as red emissive layer, 40-nm-thick TPD as electron blocking layer, 30-nm-thick PPCP as blue emissive layer, 5-nm-thick Alq 3 as electron transporting layer, and a half-transparent Al electrode were deposited on an ITO electrode. The Alq3 layer, which is inserted between the Al electrode and the PPCP layer, act as to increase electron injection from Al electrode to the PPCP layer. Since the lowest unoccupied molecular 570
orbital (LUMO) of Alq 3 is lower than that of PPCP, insertion of Alq3 layer helps electron injection into PPCP layer. For single-color emission part, 40-nm-thick TPD as hole transporting layer, 40nm-thick Alq3 as green emissive layer, and a Mg:In cathode were deposited on the half-transparent Al electrode. The Al electrode, whose transmittance is about 17% at 515nm of the green emissive wavelength, plays roles as cathode to the two-color emission part and, at the same time, as anode to the single-color emission part, simultaneously. The active electrode area of the device was 4mm2 . Electrical properties such as current-voltage (I-V) ,current-emission intensity (I-L) characteristics, and the emission spectra were measured by conventional methods, under DC, pulsed or AC bias conditions, which was reported by our previous paper [5]. The measurements were carried out at liquid nitrogen temperature (77K) or at room temperature (RT). The devices showed similar emission characteristics to those measured at 77K, but the emission intensity is not so
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