형광체를 이용한 고효율 백색 유기LED소자 연구

Abstract

Organic Light Emitting Diode (OLED) field has been spreading out in the applications such as an information display portable device, cellular phone and MP3 player since the first commercialization in 1993, and recently, it is drawing broad attentions as the most dominant candidate as the new types of light-lamp source and large area flat display. Since OLED as light-lamp source can be produced in flexible plate type with comparatively low electric power, not containing any environment pollution materials like heavy metal, there are extensive on-going researches on use of it as next generation light source technology. In the early stage of WOLED development, attentions were focused on production of full-color devices that do not require the fine patterned mask by applying color filter, and recently, there are active researches on application of WOLED as lighting. In this thesis, new types of White-Light Emitting Diodes (WOLEDs) that emit three primary colors of red, green and blue has been demonstrated. By controlling the number of electrons transferred to the emitting layer, highly efficient three-wavelength WOLEDs were fabricated (Chapter 2). Such WOLEDs are different from those made using simple stacking of RGB emitting layers in that the movement distribution of electrons transferred to emitting layer could be adjusted using the difference in LUMO energy level and that lights of all 3 wavelengths could be emitted through appropriate arrangement of RGB emitting layers. WOLED device with the structure of m-MTDTA (40 nm)/NPB (10 nm)/ Coumarin6 doped Alq3(3%) (8 nm)/Rubrene doped NPB(5%) (15 nm)/NPB (2 nm)/ DPVBi (20 nm)/Alq3 (20 nm)/LiF(1 nm)/Al (200 nm) showed high luminance efficiency of 8.9 cd/A and color purity of (0.31, 0.40). In addition, WOLED device with the thickness of non-doped NPB layer increased from 2nm to 3nm to increase blue light emission showed a luminance efficiency of 7.6 cd/A and color purity of (0.28, 0.36). Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated in Chapter 3. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq3/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm) / NPB (20 nm) / C-545T(3%) doped Alq3 (5 nm) / Alq3 (59 nm) / LiF (1 nm) / Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 hours for initial luminance of 3,000 cd/m2, which is more stable in comparison to the device without balanced drift current. Well-organized highly efficient white OLEDs using fluorescent emitting materials was demonstrated in chapter 4. WOLED is properly laid out with emitting layers so that all three wavelengths of light can be emitted by using fit energy level, and the organic functional layer named white balanced layer (WBL) is introduced. In the case of WOLED device that possesses the device structure of ITO / m-MTDATA 40 nm / NPB 5 nm / NPB+P-1(3%) 3 nm / NPB 0.1 nm / Alq3+C-545T(5%) 0.5 nm / IDE-120+IDE-105(7%) 35 nm / LiF 1 nm / Al 150 nm in this study, color coordinate was (0.341, 0.424) and light emitting efficiency was 16.5 cd/A at current density 10 mA/cm2, so the WOLED demonstrated highly efficient characteristics of over commercial level. In chapter 5, a novel moisture getter was fabricated from a new desiccant triethylaluminum (TEA) and a porous material poly(1-trimethylsilylpropyne) (PTMSP) as a binder and then was applied to a film-casting method, its property was measured. As a result, PTMSP(Mn:50K) created a film with a relatively high porosity. PTMSP (60%) and TEA(40%) were mixed to fabricate a getter system with a good transmittance of over 80 %. The fabricated getter, adopted in a OLED device, showed excellent features, as a level of commercialization : 490-hour shelf lifetime under the conditions of 60°C and 90% RH. Novel highly phenyl-substituted aromatic compounds(spirobifluorene and carbazole derivatives) were synthesized by using the Diels-Alder reaction in Chapter 6. The Diels-Alder reaction for synthesis of OLEDs material could be expected to have several merits such as highly pure material and high yield because there is no catalyst reaction. BDFEC showed sky blue PL spectrum at 481 nm and BTPSF showed ultra-violet PL spectrum at 372 nm in chloroform solution. Also BTPEC and BDFSF exhibited PL spectrum at around the UV region, 390 and 467 nm. Finally, cabazole-based emitting molecules, such as 3,3’-(1,4-phenylene di-2,1-ethenediyl)bis[9-ethyl-(E,E)-9H-carbazole](PEEC) and 3,3’-([1,1’-biphenyl]-4,4’diyldi-2,1-ethenediyl)bis[9-ethyl-9H-carbazole](BPEEC), were synthesized and OLED device using these molecules were fabricated in chapter 7. PEEC showed bluish-green EL(λmax = 496 nm) and turn-on voltage at 6V and 2.4 cd/A (1.3 lm/W) efficiency in ITO/m-MTDATA/NPB/PEEC/Alq3/LiF/Al device. ITO/m-MTDATA/NPB/DPVBi + PEEC(3%)/Alq3/LiF/Al device showed EL spectrum at 462 nm and EL efficiency was increased to 4.5 cd/A (1.7 lm/W) from 3.5 cd/A (the device without doping agent).