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학술지 Highly Efficient Green, Blue, and White Phosphorescent Inverted Organic Light-Emitting Diodes by Improving Charge Injection and Balance
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이현구, 맹민재, 홍종암, Rokeya Najnin, 문제현, 조현수, 이종희, 유병곤, 박용섭, 조남성
Journal of Materials Chemistry C : Materials for Optical and Electronic Devices, v.5 no.38, pp.9911-9919
Royal Society of Chemistry (RSC)
17HB1700, 고해상도 광대역 실감 입체 영상용 소재 및 소자 기술, 김태엽
To improve the performance of inverted organic light-emitting diodes (OLEDs), we investigated the electrical, optical, and interfacial properties of three different lithium (Li)-doped electron transport materials (ETMs): tris(3-(3pyridyl)mesityl)borane (3TPYMB), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB). The electron injection barriers (EIBs) between indium-tin-oxide and the ETMs were deduced for both pristine and Li-doped cases from ultraviolet photoelectron spectroscopy measurements and optical band gap values. The Li-doped ETMs showed EIB values of approximately 0.03 eV, 0.77 eV, and 0.81 eV for 3TPYMB, TmPyPB, and BmPyPB, respectively, which are much lower than those of their pristine counterparts of 0.94 eV, 1.14 eV, and 1.48 eV, respectively. The Li-doped ETMs were employed as electron injection layers (EILs) of inverted bottom-emission OLEDs (IBE-OLEDs) with green phosphorescence. IBE-OLEDs with 3TPYMB, TmPyPB, and BmPyPB EILs exhibited driving voltages of 3.6 V, 4.0 V, and 4.5 V at 1000 cd m-2 and maximum external quantum efficiencies (EQEs) of 20.3%, 19.7%, and 16.5%, respectively. From the low EIB of Li-doped 3TPYMB, we also demonstrated highly efficient blue and white phosphorescent IBE-OLEDs. We optimized the device structure to improve the charge balance and out-coupling efficiency by changing the hole injection layer and the thickness of the hole and electron transport layers with optical simulation. The blue device showed a maximum EQE and luminous current efficiency of 22.9% and 43.1 cd A-1, respectively. In addition, the white device exhibited a high EQE and luminous efficacy of 19.3% and 37.8 lm W-1 at 3 mA cm-2 (~1000 cd m-2), respectively. To the best of our knowledge, the efficiencies of these green, blue, and white devices are the highest values obtained to date with a low driving voltage for IBE-OLEDs without any additional light-extraction structure. Since the Li-doped 3TPYMB has an extremely low EIB and shows good device performance, it can be utilized as an effective EIL in inverted-structure devices.
KSP 제안 키워드
Device structure, Electron transport materials, Extraction structure, Hole and electron transport, Hole injection layer(HIL), Interfacial Properties, Inverted organic light-emitting diodes, Li-doped, Luminous current efficiency, Optical band gap, Optical simulation