(A) Study on Blue Common Layer Structure for Full Color OLEDs

다국어 초록 (Multilingual Abstract)

OLEDs featuring wide range of working temperature, low power consumption, wide viewing angle, fast response time, high contrast, and vivid color are very attractive for the next generation flat-panel-display applications. There have been a lot of reports related to color patterning method to realize full color OLEDs. Among them, the most common method to form individually patterned EMLs is to utilize a fine metal mask as a key component for patterning. However, such a method shows very bad accuracy which normally requires large process margin (e.g. typical bank gap between aperture areas ~ 20 um). In addition to this, fine metal masking (FMM) method has many problems such as the mismatching of glass, mask sagging extent, mask lifetime, cleaning, particle contamination, thermal expansion of masks, etc. Especially, the different sagging behavior between glass substrate and shadow mask gives rise to a show effect which is the major drawback of the FMM method for fabricating high resolution and / or large-area displays.
To circumvent such shadow effect of the FMM method, several alternative patterning methods have been demonstrated such as ink-jet printing, laser-induced pattern-wise sublimation, laser-induced thermal imaging (LITI), radiation-induced sublimation transfer, etc. Among them, more recent and viable method for patterning full color OLEDs may be LITI process. However, organic materials as donor and /or receptor layers can be deformed because the LITI process can’t avoid the thermal energy for patterning. Meanwhile, large-area TVs fabricated by white OLED with color filter (CF) and side-by-side RGB OLED are recently released, but the manufacturing cost might be very expensive due to essentially utilized CFs to make white OLED and the performances of white OLEDs is generally unfavorable compared to those of side-by-side RGB OLEDs.
For that reason, the FMM method is still regarded as the best patterning method despite its demerits abovementioned. There have been constant exertions to simplify the process so as to cut down the manufacturing cost and increase product yield. Especially, there have been a few previous reports for reduction of a one FMM step by introducing blue emitting layer (BCL) as a common layer. However, that concept is unsuitable and difficult to realize when phosphorescent red and green EMLs are combined with fluorescent blue EML due to triplet exciton quenching which causes blue side-emission in red or green devices.
Thus, we proposed a newly derived full color OLEDs with BCL. Before fabricating the final devices, we performed optical simulation to investigate the best microcavity structure of full color OLEDs with BCL. In the case of the full color OLED with TBCL utilizing CBP host for both the green and red EMLs, at a given constant luminance of 1000 cd/m2, the current and power efficiencies were 13.5 cd/A and 10.4 lm/W for the blue device, 39.7 cd/A and 25.0 lm/W for the green device, and 14.7 cd/A and 14.8 lm/W for the red device, respectively. The driving voltage to reach 1000 cd/m2 is 4.0, 5.0, and 5.3 V for blue, red, and green devices, respectively. Meanwhile, the blue side-emission was observed in both the green and red devices, which means that both the green and red triplet excitons couldn’t be completely confined in their EMLs. We thought that this might be because the excessively biased recombination zone toward the blue EML.
Thus, we thought the degradation by triplet excion quenching which resulted in the side-emission could be reduced when an electron type host material is used as a host for both the green and red EMLs. We fabricated a full color OLEDs with BCL utilizing an electron type hosts such as bepp2 and bebq2. At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 75.4 cd/A and 48.7 lm/W for the green device and 19.9 cd/A and 12.1 lm/W for the red device, respectively. The driving voltage to reach 1000 cd/m2 is 4.9 and 5.1 V for the green and red devices, respectively. However, the side-emission was still seriously observed in the red device. This is probably because the difference of lowest unoccupied molecular orbital (LUMO) levels between the red host and dopant (CBP–Ir(phq)2acac: 0.6 eV and Bebq2–Ir(phq)2acac: 0.3 eV) which causes electron trap at an interface in between red and blue EMLs is large.
So, we thought that the fundamental solution to suppress such kind of side effect is to separate the blue EML from both the green and red EMLs. We called the device structures having such separated EMLs system as a BBCL structure. The BBCL structure could be realized by inserting HATCN layer between the underlying blue EML and HTL in OPCL. At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 83.4 cd/A and 62.7 lm/W for the green device and 21.7 cd/A and 14.7 lm/W for the red devices, respectively. The driving voltage to reach 1000 cd/m2 is 4.2 and 4.6 V for the green and red devices, respectively. We could obtain highly efficient green and red PHOLEDs which are enable to be applied to a full color OLED without any color mixing. We attributed the main reasons of the improved device characteristics to the separated EML system as well as HATCN interlayer preventing electron injection to the blue EML and degradation of hole transporting property by non-radiative coulombic interaction.
Based on the simulated 2nd order microcavity structure, a full color microcavity OLED with BBCL was fabricated to investigate the possibility of applying the current mass-production. At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 4.9 cd/A and 3.0 lm/W for the blue device, 125.2 cd/A, and 85.0 lm/W for the green device, and 39.3 cd/A and 27.7 lm/W for the red device. The driving voltages to reach 1000 cd/m2 were 5.2, 4.6, and 4.5 V for the blue, green, and red devices, respectively. The CIE 1931 coordinates were (0.13, 0.13), (0.20, 0.73), and (0.65, 0.35) for the blue, green, and red devices, respectively.
In this work, we have studied on the fabrication process to realize highly efficient full color OLEDs with BBCL. Our proposed fabrication method based on two FMM processes can bring solutions to reduce process TACT time, mask cost and alignment error as well as the manufacturing cost.

목차 (Table of Contents)

• 1. Introduction to blue common layer structure 1
• 1.1. Introduction 1
• 1.2. Previous study on blue common layer structure 3
• 2. Experimental method 8
• 2.1. Device fabrication 8
• 2.1.1. Substrate cleaning 8
• 2.1.2. Organic / Metal layer evaporation 8
• 2.1.3. Encapsulation 8
• 2.2. Measurement and evaluation 9
• 2.2.1. Measurement of layer thickness 9
• 2.2.2. Evaluation of device characteristics 9
• 3. Results and Discussion 10
• 3.1. Structure of full color OLEDs with blue common layer 10
• 3.1.1. Materials 10
• 3.1.2. Phosphorescent OLEDs 12
• 3.1.3. Fluorescent OLEDs 12
• 3.2. Full color OLEDs with top blue common layer 13
• 3.2.1. Optical design of device structure with top blue common layer 13
• 3.2.2. Performances of full color OLEDs with top blue common layer 18
• 3.3. Full color OLEDs with bottom blue common layer 31
• 3.3.1. Optical design of device structure with bottom blue common layer 31
• 3.3.2. Performances of full color OLEDs with bottom blue common layer 35
• 3.3.3. Performances of full color microcavity OLED with bottom blue common layer 41
• 3.4. Film Patterning for Blue common layer structure by two FMMM processes 47
• 4. Conclusion 50
• References 52
• Abstract 55