| Summary: | 博士 === 義守大學 === 電機工程學系 === 93 === Owing to the urgent marketing requirement in illumination light sources fabricated by white organic light-emitting diodes (OLED), more studies engaged in the luminescent performance, driving voltage, and color purity of white OLED are still needed. Therefore, the main purpose of this doctoral thesis is to analyze the luminescent properties of white OLED between “Double-Wavelength Type” and “Triple-Wavelength Type” device structures, and accomplish the optimal design for fabricating the structured white OLED as well as its photoelectronic characteristics analyses.
Among the “double-wavelength type” white OLEDs, we contribute to the understanding of the origins of various hole injection layer (HIL) thickness influences on device luminescent performance and chromaticity deviation phenomena in white OLEDs. We found that a dense film structure and fine surface morphology of the m-MTDATA provides a uniform conducting path for holes to cross the ITO/HIL interface, causing a superior luminescent performance. Furthermore, for achieving a superior white OLED luminescent performance associated with high color purity, the device was further improved to a structure of ITO/m-MTDATA (650 Å)/NPB (600 Å)/Anthracene:0.4% DCM2 (20 Å)/Anthracene (280 Å)/Alq3 (400 Å)/Mg:Ag. A bright white OLED with the CIE coordinate at (x=0.34, y=0.33) reaches a luminance of 7685 cd/m2 under the current density of 100 mA/cm2 and has a maximum luminous efficiency of 1.72 lm/W at 5.5 V.
On the other hand, we completely discuss the photoelectronic characteristics and emission mechanisms of “triple-wavelength type” white OLEDs. Among them, we present a comprehensive interpretation of the causes of high-current-induced fluorescent quenching and chromaticity deviation in “triple-wavelength type” white OLEDs. The measurement results showed that as long as the driving current density was greater than 250 mA/cm2 (defined as high-current-injection), a considerable luminance degradation accompanied the blue-shift in the electroluminescent (EL) spectrum in white OLED was always existent. The main factor for high-current-induced luminance degradation is suggested to be the damage caused by thermal and electrochemical instability. In resisting this damage, both the blue-emitting host (Anthracene) and blue-dopant (BczVBi) materials evidently perform superior to the red- and green-emitting host (Alq3) and red-dopant (DCM2) materials. These results lead to a considerable luminance degradation accompanied with the EL spectrum shifting to blue in White OLED under high-current injection.
Moreover, we also studied the emission mechanism of the white OLED using DCJTB dye and a phosphorescent sensitizer of Ir(ppy)3 co-doped in a TPBI emitting layer. Evidence showed that the white OLED devices with efficient energy transfer from TPBI to Ir(ppy)3 and from Ir(ppy)3 to DCJTB. By doping suitable concentration of Ir(ppy)3 and DCJTB, we could obtain a balanced white emission with the CIE coordinates of (x=0.33, y=0.35).
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