(A) Study on New Electrode and Emissive Layer Technologies for Organic Light-Emitting Diode Lighting
저자
발행사항
서울 : 경희대학교 대학원, 2019
학위논문사항
학위논문(박사)-- 경희대학교 대학원 : 정보디스플레이학과 2019. 8
발행연도
2019
작성언어
영어
주제어
DDC
530-A 판사항(22)
발행국(도시)
서울
형태사항
x, 104 p. : 삽화 ; 26 cm
일반주기명
경희대학교 논문은 저작권에 의해 보호받습니다.
지도교수: 권장혁
참고문헌: p. 90-104
UCI식별코드
I804:11006-200000222419
소장기관
This thesis is about the study of the device technologies for organic light-emitting diodes (OLEDs) lighting technologies of device architecture and electrode for lighting applications. Although white OLED is potential technique of next generation lighting applications, current white OLEDs confront the several technical challenges as follow; 1) limitation of efficiency enhancement due to the low-efficient blue fluorescent emitter; 2) complicate structure of conventional tandem structure which is currently applied as white OLED architecture; 3) luminance non-uniformity issue caused by the relatively high sheet resistance of indium tin oxide (ITO) electrode which is widely used transparent conducting electrode (TCE); 4) complex fabrication process of ITO electrode formation accompanied photolithography process for patterning. Hence, to surmount those plights, we studied the device structures to boost efficiency and simplify the architecture and also investigated the electrode for a simple manufacturing process with high luminance uniformity.
Above all, several device studies for boosting the efficiency characteristics were performed. Because the limitation of efficiency improvement is owing to the low internal quantum efficiency (IQE) of only 25% of blue fluorescent emitter and absence of blue phosphorescent emitter, the introduction of triplet excitons harvesting techniques is necessary. First of all, deep-blue fluorescent OLED device based on triplet fusion (TF) system which utilizes singlet excitons generated by triplet-triplet annihilation (TTA) process was fabricated. Even if TF system is relatively lower triplet harvesting rate than the other system, it has pros of device lifetime. Since singlet generation rate in TF system is proportional to the concentration of triplet excitons, we applied electron transport layer (ETL) material with high electron mobility characteristics. Additionally, 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine (PCzAc) was adopted as an exciton blocking layer (EBL) to improve hole injection property and confine singlet and triplet excitons in emitssive layer (EML). As a result, high external quantum efficiency (EQE) of 9.18% with deep-blue color coordinates of (0.135, 0.115) at the luminance of 1,000 cd/m2 was attained at low driving voltage of 4.5 V. We attribute the performance improvement to the charge recombination increment by the improved charge injection. To verify the hypothesis, time-transient electroluminescence analysis was performed. Consequently, it was revealed that the intensity of delayed fluorescence by TTA process is linearly proportional to the charge recombination rate. Therefore, from that relation, expected EQE when the charge recombination ideally arises with 100% can be increased up to 10.94%. From the optimum structure, fluorescent blue / phosphorescent yellow-green / phosphorescent red hybrid 3-tandem white OLED was fabricated and evaluated. We achieved high device performance of 50.1 lm/W power efficiency and 48.3% EQE with color rendering index (CRI) of 85 at 1,000 cd/m2 luminance.
However, tandem structure is too complicate to fabricate, therefore, we also studied the single stack white OLED. But, in case of fluorescent blue emitter, triplet exciton cannot be fully harvested in cool-white emissive OLEDs. So we introduced thermally activated delayed fluorescent (TADF) blue emitter which utilize the reverse intersystem crossing (RISC) from triplet to singlet by virtue of its low difference between singlet and triplet energy level. Thus, we design the phosphorescent yellow and TADF blue hybrid white OLED by considering the optical conditions of each emitting color. Based on the optical simulation, we adopt the 2nd order antinode position of blue and yellow EML with 150 nm thickness of ITO. By considering the optical and electrical characteristics of blue host and dopant, the kind of blue host and the position of the yellow EML is carefully determined. As a result, high EQE of 23.1% and cool white emission of (0.324, 0.337) color coordinates was achieved.
Meanwhile, one of obstacles of the OLED lighting for commercialization is the non-uniform luminance distribution caused by the sheet resistance of ITO. Hence, we investigated the luminance non-uniformity according to the device structure of OLED of which are single EML, 2-tandem, and 3-tandem to ensure uniform luminance distribution. The luminance distribution was simulated via a circuit analysis program with the performances of unit OLED devices and the simulation results were compared with the measurement results of the fabricated panels. In the luminance distribution calculation results of the yellow-green phosphorescent OLED panel with 30 × 80 mm2 emission area, a 3-tandem structure indicated the lowest non-uniformity (1.34%) while that of single-EML (5.67%) and 2-tandem (2.78%) showed the higher values at 1,000 cd/m2. Through the cross-validation between results of non-uniformity simulation and analysis of the current characteristics of unit devices, we revealed the conductance of OLED affect the luminance non-uniformity of the panel. From the measurement of the luminance distribution of the panel, we confirmed 3-tandem structure had an excellent luminance non-uniformity (4.1%) compared with that of single EML (9.6%) and 2-tandem (6.4%) at 1,000 cd/m2. Additionally, we also identified the size of the panel with 3-tandem structure can be enlarged up to ~5,000 mm2 without any auxiliary electrode to secure the luminance non-uniformity of under 10%.
Finally, we studied new transparent electrode of Organic /Ag / Organic (OAO) architecture based on thermal evaporation process with low deposition rate to replace ITO electrode. Herein, 1,4-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene(p-bPPhenB) was employed as a wetting inducer to increase the wettability of silver and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) which has high hole injection characteristics was applied as a capping material. As a result, high transmittance of 81.34% at 550 nm and very low sheet resistance of about 9.51 Ω sq-1 was obtained in OAO electrode. We confirmed OAO electrode is also available in flexible lighting, which have a durability for bending radius of 5mm with very slight increment (9.90 Ω sq-1) after 2,000 bending cycles. In green phosphorescent OLED device applied in OAO electrode, very high current efficiency of 75.1 cd/A and EQE of 23.1% was achieved at 3,000 cd/m2.
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