Study of High-Efficiency and High-Color-Stability White Organic Light-Emitting Devices
Date Issued
2010
Date
2010
Author(s)
Hsiao, Chih-Hung
Abstract
The core of this doctoral dissertation is based on how to design a suitable and simplified emitting layer (EML) in white organic light emitting devices (WOLED) for implementing phosphorescence-sensitization (chapter 3) and improving device efficiency and color stability (chapter 4). In addition to the steady state measurement such as B-I-V and efficiency performance, transient measurements will be utilized for further specifying the carrier dynamic and emission mechanism in OLEDs (chapter 5).
In chapter 3, efficient phosphorescence-sensitization (PS) consisting of tris(phenylpyridine)iridium (Ir(ppy)3) sensitizers and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) (DCJTB) exciton acceptors in a host material, N,N’-dicarbazolyl-3, 5-benzene (mCP), was implemented by a simplified emitting layer structure with selectively doped DCJTB in the Ir(ppy)3-doped region. This codoped region away from the recombination zone peak in the Ir(ppy)3-doped region significantly improved the PS efficiency without affecting the carrier transport characteristics. Coupled with an efficiently phosphorescent blue emitter, a WOLED with this PS-EML was demonstrated to have 5.51% external quantum efficiency, 17.4 cd/A and 12.2 lm/W. With a 3-nm spacer for manipulating the exciton diffusion in the emitting layer, a slight CIE coordinates variation of (-0.008, -0.006) was obtained at practical luminance levels of 1000 to 4000 cd/m2.
In the first-half of chapter 4, we investigated the strong influence of the thickness of blue EML (B-EML) on color stability. The large voltage drop across the B-EML resulted in a higher sensitivity of the carrier transport and injection properties to the applied external voltage. According to carrier mobility measurements by the time-of-flight method, the electron mobility of the mCP exhibited a strong dependence on the electric field. Therefore, at a higher driving voltage, the more rapidly increasing electron mobility of the mCP and the decreasing energy barrier height on the electron transport path would extend the recombination zone from the B-EML to the Ir(ppy)3-doped mCP green emitting layer (G-EML) in devices with thinner B-EMLs. Coupled with the fluctuations of the recombination zone, stronger triplet-triplet exciton annihilation occurring in the thinner B-EMLs led to an even more evident deterioration of the color stability. After circumventing these two negative factors, a green-blue OLED with ultra-high color stability was demonstrated, with the CIE coordinates slightly shifted from (0.256, 0.465) to (0.259, 0.467) with increased luminance from 48.7 to 12700 cd/m2. Further adding a red phosphorescent dopant into this green-blue EML backbone, we successfully fabricated a WOLED with high color stability, which exhibited a nearly invariant CIE coordinate throughout the practical luminance range from 1050 ((0.310, 0.441)) to 9120 cd/m2 ((0.318, 0.446)) and maximum efficiencies of 26.4 cd/A and 19.8 lm/W.
In the second-half of chapter 4, an ultra-high color stability of WOLED based on the same backbone was achieved by deliberately engineering B-EML with a selectively doping profile. The WOLED showed that CIE coordinate shifted from (0.399, 0.483) to (0.395, 0.479) as luminance increased from 145 to 12100 cd/m2 and from (0.401, 0.481) to (0.400, 0.479) as luminance from 1240 and 4850 cd/m2, the practical luminance range for display and lighting applications. In addition to the small CIE coordinate variation of (-0.004, -0.004) over wide luminance variation of about two order of magnitude, the device efficiency achieved a high value of 34.1 cd/A and kept larger than 30 cd/A below 2000 cd/m2.
In chapter 5, by investigating OLED with different Ir(ppy)3-doped regions with transient EL measurement, carrier injection and transport in Ir(ppy)3 was found to show hole-transporting and electron-injection in mCP host. For PS-based OLEDs, an unusual increasing intensity of DCJTB after switching-off the applied voltage pulse indicated the occurring of PS process.
Subjects
organic-light emitting devices
phosphorescence-sensitization
color stability
high efficiency
selectively doping method
transient EL
Type
thesis
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