Chapter 4 Results and discussion
4.5 Comparison of the white devices
In this section, I compared the white devices in this research. The performances of the devices are shown in the following Table 4-8. The performances are retrieved under a current density of 10 mA/cm2 for devices B, C, D, E and 20mA/cm2 for device A. For all the the devices, the one that owns the best characteristic is circled for comparison.
(1) Voltage
Device A and Device E-A while using fluorescent emissive layers have the lowest operation voltage of nearly 5 volts; then followed by devices C, D and E-C, while these devices contain only a part of phosphorescent materials; Device B had the highest operating voltage of over 9 volts due to an extremely thick phosphorescent emissive layer.
(2) Current Efficiency
Devices B and D owns the highest efficiencies of over 16 cd/A, nearly twice of that using fluorescent emission layers. The efficiencies of Set E are not enough since the single blue emissive layer is not efficient enough, and also there is always some loss due to the ECTL.
(3) Power Efficiency
Devices C, D owns the highest power efficiencies, due to the improvement in operation voltage, but still remain highly efficient.
(4)External Quantum Efficiency
For the external quantum efficiency, Devices B, C, D, while all
contained at least one phosphorescent material can reach over 5%.
Among these, the totally phosphorescent device (Device B) truly owns the highest of 7.8%.
(5) CIE Positions
The CIE color are better for fluorescent based devices, while the phosphorescent ones do not have a saturated enough blue emissive color.
This cause the illumination color to be closer to illumination point A.
(6) Color Rendering Index
Device A and Device C both own performable color rendering index for illumination usages. Device B which uses FIrpic as the blue emitter, does not have a saturated blue emission, so the CRI is only fair. For device D, the red emitter DCJTB only has a peak near 580 nm, which is not red enough, so the CRI is even poorer.
(7) Color shifting issue
Devices Set E which contains only one emissive layer had absolutely no color shifting issue of the devices from 200 nits till over 10000 nits.
Table 4-8 Comparsion of the devices
Figure 4-26 The CIE 1931 positions and photograph of the devices Devices Voltage
Chapter 5
Conclusions
In conclusion, we demonstrated WOLEDs with tetra-chromatic fluorescent structrure with a luminance efficiency of 8.7 cd/A, NTSC ratio of 66.9% and CRI=87, and a specific future of this device is that under low drive current density, a CIEx,y near (0.40, 0.44) can be readily obtained which is near illuminant point A for color temperature near 3,000K. Under higher current density, a very white CIE position could be reached. By using such a device as in the day and night the regulation of the human circadian rhythm can be fulfilled. We also fabricated devices based on phosphorescent materials, a fluorescent blue plus phosphorescent green and red emission layer device had an efficiency of 13.5 cd/A and CRI=83, which is acceptable for illumination. Finally, we used a phosphorescent sensitizer to excite a fluorescent dye through resonant energy transfer by adopting the fluorescent red material DCJTB to replace the phosphorescent red material Ir(piq)3. By using this mechanism, and by inefficient transfer from the green to red, a higher efficiency of 16.7 cd/A could be reached due to the higher eye sensitivity of the orange-reddish emission color of DCJTB. Furthermore, we simplified the fabrication process of OLED devices by using a single color emission layer plus an external color tuning layer in order to fabricate white light. In this part, we reached the results that by using a
graded phosphorescent blue device plus the ECTL, the highest efficiency can be reached.
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Biography
James Chang
1984.9. 28 Born in Taipei
1990 Moved to New York, U.S.A.
1994 Moved back to Taiwan
2002 Graduated from Yan Ping High School
2006.6 Received a Bachelor's Degree in Material Science and Engineering from National Chiao Tung
University
2008.8 Received Degree of Master of Science in Display Institute from National Chiao Tung University
會議文獻 會議文獻
會議文獻 會議文獻 (Poster Presentation)
1. Chi-Sheng Chang, Po-Tsun Liu, Meng-Huan Ho, Chin H. Chen
“Tetra-chromatic White OLED for solid state lighting with tuneable ability”, ODF’08, Taipei, 10PS-099, 2008.
2. Chi-Sheng Chang, Po-Tsun Liu and Chin H. Chen,“White Organic Light-emitting Diodes with an External Color Tuning layer”, TDC’08 DIGEST, Taipei, A-020, 2008.
3. Chi-Sheng Chang, Po-Tsun Liu, Meng-Huan Ho, Chin H. Chen,
“Tetra-Chromatic white phosphorescent organic light-emitting diodes with an external color tuning layer”, IMID/IDMC’08, Daejeon, P-85, 2008.