A study on electrical and optical properties of the organic light emitting devices based on a carbazole/thioxanthene-S,S-dioxide host material

Abstract

Organic light-emitting devices (OLEDs) have been currently emerged as future display candidates for promising applications in full-color displays, mobile displays, and white-light sources with excellent advantages of ultrathin thickness, fast response speed, wide viewing angle, low power consumption, and high contrast ratio. The prospect of OLEDs in next-generation lighting sources has led to substantial research and development efforts to fabricate OLEDs with various host materials. However, studies on the OLEDs utilizing host materials with bipolar characteristic, high triplet energy (ET), high thermal endurance has been not conducted yet. To fabricate efficient OLEDs, host materials should have bipolar charge transport capability to achieve balanced hole and electron fluxes through the emitting layer to improve their efficiency and lifetime, and should typically have a minimum ET of 2.0, 2.5, or 2.7 eV to accommodate a red, green, or blue emitter, respectively. However, obtaining a high ET, above 2.7 eV, by combining hole and electron transport moieties in the molecular structure was rather difficult because of the extended degree of p-conjugation. The high cost of materials and the relatively complicated multi-step synthesis routes leading to a relatively low Tg for unipolar and/or bipolar hosts greatly undermine their overall effectiveness for OLED applications. First of all, a simple synthesis procedure introducing a thioxanthene-S,S-dioxide bearing a sulfone group as an electron transport moiety using two non-conjugated carbazole moieties in the 9-position of the thioxanthene-S,S-dioxide ring was described. The carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) molecule shows good solubility, high ET (2.94 eV), high highest occupied molecular orbital energy level (5.95 eV), and high Tg (227oC), which should make it a suitable candidate for use in a high-performance OLEDs. The electrical and optical properties of OLEDs based on an EBCz-ThX host material have been presented to determine the novelty of EBCz-ThX host material. Finally, phosphorescent organic light-emitting devices (PHOLEDs) were fabricated utilizing EBCz-ThX as a host material with bis(2-benzo[b]thiophen-2-ylpyridine)(acetylacetonate) iridium(III) and bis((4,6-difluorophenyl)-pyridinate-N,C20)picolinate as red and blue dopants. Red and blue PHOLEDs showed excellent performances with quantum efficiencies of about and 10 and 12%. Moreover, white OLEDs based on EBCz-ThX were fabricated with current efficiency of 15.3 cd/A which verifying the excellent property as a host material of EBCz-ThX.

|유기발광소자는 초박막 두께, 빠른 응답 속도, 넓은 시야각, 저전력 등의 장점으로 완전 천연색 디스플레이, 모바일 디스플레이 및 백색 광원 분야의 유망한 응용 분야로 차세대 디스플레이 선도하고 있다. 백색 유기발광소자에 대한 연구는 광원 분야에 활용을 목적으로 다양하게 연구되었고 많은 연구 결과들이 발표되었다. 하지만, 최근 백색 유기발광소자를 기반으로 한 높은 해상도 및 색 재현성을 가진 대형 디스플레이 패널이 상용화되면서 관련 분야의 연구가 주목 받고 있다. 특히, 유기발광소자의 호스트 물질에 대한 필요성은 다양한 연구를 통해서 강조되었다. 유기발광소자의 호스트 물질의 높은 Tg는 발광 과정에서 발생하는 열에 강한 내성을 가져 유기발광소자의 수명을 늘릴 수 있다. 높은 효율의 유기발광소자를 제작하기 위해 일반적으로 2.0, 2.5 또는 2.7 eV의 ET값을 가져야 적색, 녹색, 또는 청색의 호스트 물질로 사용할 수 있다. 그러나 p-conjugation의 확장 정도 때문에 2.7 eV이상의 높은 ET를 가진 호스트 물질을 합성은 어렵다. 또한 bipolar한 특성을 가진 호스트 물질은 합성과정이 복잡하고 동시에 Tg값을 떨어뜨리기 때문에 위에서 언급한 세 가지 특성을 가지는 호스트 물질에 대한 연구가 필요하다. 따라서 본 연구에서는 Carbazole / thioxanthene-S, S-dioxide (EBCz-ThX) 호스트 물질을 기반으로 제작 된 유기발광소자의 전기 및 광학 특성에 대한 연구를 진행하였다. Thioxanthene-S, S-dioxide 고리의 9 번째 위치에 두 개의 비공역 carbazole을 접합하였고 전자수송 부분으로 sulfone를 갖는 thioxanthene-S, S-dioxide를 사용하여 물질을 합성하였다. EBCz-ThX 분자는 우수한 용해도, 높은 ET (2.94 eV), 높은 HOMO 에너지 준위 (5.95 eV) 및 높은 Tg (227oC)의 특성을 가지기 때문에 고성능 유기발광소자의 제작에 사용하기 적합하다. 또한 EBCz-ThX는 간단한 제작과정에도 bipolar한 특성을 가지고 있어 호스트 물질로 적합하다. EBCz-ThX 호스트 물질을 사용한 유기발광소자의 전기적 및 광학적 특성을 분석하여 EBCz-ThX 호스트 물질의 우수성을 강조하였다. 적색Ir(bt)2acac와 청색 FIrpic 인광 발광물질을 사용하여 적색 및 청색 인광유기발광소자를 제작하여 그 전기적 및 광학적 특성을 분석하였다. EBCz-ThX의 호스트 물질을 사용한 적색 및 청색 인광유기발광소자는 10 및 12 %의 양자 효율을 갖는 우수한 성능을 보였다. 또한, EBCz-ThX을 사용한 백색 유기발광소자가 제작 되었다. 백색유기발광소자는 15.3 cd / A의 높은 전류효율을 나타내며 호스트 재료EBCz-ThX의 우수한 특성을 검증하였다.; Organic light-emitting devices (OLEDs) have been currently emerged as future display candidates for promising applications in full-color displays, mobile displays, and white-light sources with excellent advantages of ultrathin thickness, fast response speed, wide viewing angle, low power consumption, and high contrast ratio. The prospect of OLEDs in next-generation lighting sources has led to substantial research and development efforts to fabricate OLEDs with various host materials. However, studies on the OLEDs utilizing host materials with bipolar characteristic, high triplet energy (ET), high thermal endurance has been not conducted yet. To fabricate efficient OLEDs, host materials should have bipolar charge transport capability to achieve balanced hole and electron fluxes through the emitting layer to improve their efficiency and lifetime, and should typically have a minimum ET of 2.0, 2.5, or 2.7 eV to accommodate a red, green, or blue emitter, respectively. However, obtaining a high ET, above 2.7 eV, by combining hole and electron transport moieties in the molecular structure was rather difficult because of the extended degree of p-conjugation. The high cost of materials and the relatively complicated multi-step synthesis routes leading to a relatively low Tg for unipolar and/or bipolar hosts greatly undermine their overall effectiveness for OLED applications. First of all, a simple synthesis procedure introducing a thioxanthene-S,S-dioxide bearing a sulfone group as an electron transport moiety using two non-conjugated carbazole moieties in the 9-position of the thioxanthene-S,S-dioxide ring was described. The carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) molecule shows good solubility, high ET (2.94 eV), high highest occupied molecular orbital energy level (5.95 eV), and high Tg (227oC), which should make it a suitable candidate for use in a high-performance OLEDs. The electrical and optical properties of OLEDs based on an EBCz-ThX host material have been presented to determine the novelty of EBCz-ThX host material. Finally, phosphorescent organic light-emitting devices (PHOLEDs) were fabricated utilizing EBCz-ThX as a host material with bis(2-benzo[b]thiophen-2-ylpyridine)(acetylacetonate) iridium(III) and bis((4,6-difluorophenyl)-pyridinate-N,C20)picolinate as red and blue dopants. Red and blue PHOLEDs showed excellent performances with quantum efficiencies of about and 10 and 12%. Moreover, white OLEDs based on EBCz-ThX were fabricated with current efficiency of 15.3 cd/A which verifying the excellent property as a host material of EBCz-ThX.