Highly Efficient Fluorescent Organic Light-Emitting Diodes using Delayed Fluorescence

Abstract

유기발광소자는 현존하는 디스플레이 및 조명을 구현하는 방법중 독보적인 우위를 점하고 있다. 기존 업계에서 내세우던, 단순한 색순도 및 시야각의 장점을 벗어나, 기판의 선택에 따라 유연하게 휘어지고 투명하게 제작될 수 있어, 유기발광소자의 적용범위는 계속 확장되고 있는 실정이다. 이에 따라, 그 어느때 보다도 유기발광소자의 효율 및 수명을 증진시키는 연구가 물질 및 소자 측면에서 활발하게 진행되고 있다. 이러한 관점에서, 3원색중 청색 유기발광소자 연구가 가장 활발한데, 그 이유로 인광물질과 형광물질에 기반한 청색 유기발광소자의 장점 및 단점에 기인한 경쟁이 있다. 인광물질에 기반한 청색 유기발광소자는 고효율의 외부양자효율을 구현할 수 있지만 단수명으로 인해 상용화의 한계에 도달해 있어, 낮은 외부양자효율을 보이는 형광물질 기반의 청색 유기발광소자가 상용화 되어 사용되고 있다. 하지만, 이론적으로 형광유기발광소자는 25%의 여기자만이 발광에 사용되어, 인광유기발광소자의 외부양자 효율에 접근할 수 없다. 이러한 형광유기발광소자의 낮은 외부양자효율을 높이기 위한 방법으로, 최근 전하이동 복합체의 역항간 교차를 이용하여 삼중항 여기자를 수확한 연구가 활발히 보고 되었다. 분자내 전하이동 복합체인 열활성화지연형광 (TADF) 발광체는 삼중항-일중항 에너지 차이를 줄여 삼중항 여기자를 발광 가능한 단일항 여기자로 수확할 수 있다. 들뜬상태 분자간 전하이동 복합체인 엑시플렉스도 삼중항-일중항 에너지 차이가 작아서 역항간 교차를 이용하여 효율적인 삼중항 수확을 할 수 있다. 본 논문에서는 이러한 역항간교차 방법을 활용해 형광유기발광소자의 외부양자효율을 증진시키는 방법을 유기발광 소자적 측면에서 연구하였다. 제 2장에서는, 형광유기발광소자의 외부양자효율이 인광유기발광소자에 근접할 수 있는지를 확인하였다. 엑시플렉스는 전술한 바와 같이 삼중항-일중항 차이가 적어 역항간 교차를 이용할 수 있는 장점이외에도, 호스트 물질로 사용시 발광층으로의 주입장벽이 없어져 전하-정공 균형을 맞출 수 있으며, 발광층내 여기자가 넓게 분포하여, 여기자 농도에 의한 급격한 효율감소를 방지할 수 있는 등의 장점이 있다. 엑시플렉스에서 도판트로의 에너지 전이를 고려하여, 발광체로는 기존 높은 절대발광효율에도 낮은 외부양자효율이 보고된 바 있는 녹색 열활성화지연형광 물질을 사용하였다. 제작된 녹색형광유기발광소자는 30%의 외부양자효율을 나타내어, 인광유기발광소자의 외부양자효율과 동일한 외부양자효율이 가능함을 증명하였다. 하지만, 엑시플렉스는 이종의 물질 각각의 최고 점유 분자궤도 (HOMO)와 최저 비점유 분자궤도 (LUMO)에 의해 형성되어, 높은 삼중항에너지가 요구되는 청색 도판트에 적합한 엑시플렉스 호스트의 발견이 용이하지 않다. 이에 따라, 소자에 적용시 엑시플렉스가 갖고 있는 양극성에서 나오는 전하균형의 장점을 사용할 수 있도록, 제 3장에서는 혼합 공동호스트를 사용한 고효율 청색형광유기발광소자를 연구하였다. 고효율의 높은 삼중항에너지의 호스트물질의 부재로, 현재까지 청색형광유기발광소자 연구는 단일호스트인 특정물질에 의존해 왔다. 본 연구에서는 높은 절대발광효율에도 불구하고 낮은 외부양자효율을 보이는 청색 열활성화지연형광 물질을 사용하여, 기존 보고된 ~13%의 외부양자효율 대비 향상된 21.8%의 외부양자효율을 발표하였다. 소자내에서 전기적 손실이 없을때를 가정한 이론적 한계수치와 동일한 외부양자효율이다. 소자적인 측면에서, 기존보고된 청색형광유기발광소자 대비 구동전압은 낮고, 20%가 넘는 외부양자효율을 보여 청색형광유기발광소자용으로 적용된 혼합 공동호스트가 효율적임을 증명하였다. 하지만, 기존 보고된 바에 의하면, 색순도와 소자성능 두가지를 모두 구현한 청색형광유기발광소자는 전무한 실정이었다. 제 4장과 5장에서는, 혼합 공동호스트에 아자질린과 트리아진 물질을 기반으로하는 청색열활성화지연형광 발광체를 적용하여, 발광체의 원활한 역항간 교차와 혼합 공동호스트의 장점을 결합하여, 22.3%의 외부양자효율과 y-색좌표기준 0.2이하의 색 순도 높은 청색형광유기발광소자를 구현할 수 있었다. 이러한 결과를 통해 아자질린 물질이 갖는 열활성화지연형광물질에서의 공여체로서의 장점을 발견할 수 있었다. 아자질린 공여체를 활용한 다른 세가지 열활성화지연형광물질을 공동혼합호스트에 적용하여, 각각의 물질들이 갖는 구조와 수용체의 종류에 따른 현상을 소자 및 광물리적 측면에서 분석하였다. 하지만, 청색 열활성화지연형광 물질을 발광체로 사용시, 물질의 전하이동 특성에 따라 소자의 발광 스펙트럼이 넓어져 색수도가 떨어질 수 있으며, 장수명의 청색 열활성화지연형광 물질이 적어 실질적인 산업체 적용에 한계가 있을 수 있다. 이를 극복하기 위해, 일반 청색형광물질을 발광체로 사용하고, 이를 센시타이징 또는 중원자 효과(heavy atom effect)를 활용하여 외부양자효율을 향상시킬 수 있다. 제 6장에서는, 센시타이징과 중원자효과를 복합적으로 활용하여, 일반 청색형광물질 기반의 청색형광유기발광소자의 외부양자효율을 2.5배 이상 향상시킨 연구 결과이다. 본 연구에서 제안된 방법을 통해, 발광에 사용된 여기자가 기존 ~25%에서, ~94%로 증가하여, 일반청색형광유기발광소자에서 비발광 삼중항 여기자를 거의 모두 수확할 수 있었다.

When organic light emitting diodes (OLEDs) appeared commercially aming to replace conventional display devices in early 2000s, it was not easy to guess OLEDs would become a dominant lighting source in display and lighting products industry. With ability of displaying colors in higher quality and wider viewing angle than those of liquid crystal display (LCD), and its adaptability to other technology from being able to be flexible and transparent, now OLEDs have become the ultimate display means. However, there are unsolved issues that need to draw our attention. Competition rose between phosphorescent and fluorescent OLEDs especialy to take better position for the commercial blue OLED. Blue phosphorescent OLEDs exhibit highly efficient blue light, however the stability issue still need to be cleared and further, phosphorescent OLEDs are considered to be toxic for containing heavy metal complex. Therefore, blue fluorescent OLEDs are being used commercially in spite of their low EL efficiency from limitation of exciton production portion of singlet to ~25%. Theoretically, if singlet-triplet splitting () is small enough, then reverse intersystem crossing (RISC) can take place harvesting additional triplets in fluorescent OLEDs. In order to have small, it is necessary to have spatial separation of the highly occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). Recently, by taking advantage of small, efficient intra- and intermolecular charge transfer (CT) materials have been reported, which are referred to thermally activated delayed fluorescence (TADF) material and excited charge transfer complex (exciplex) system, respectively. Especially, OLEDs adopting exciplex system exhibit high EL efficiency from efficient energy transfer (ET) of host to dopant material and charge balance in emission layer (EML). Further investigating into the method to boost EL efficiency in fluorescent OLEDs, there are sensitizing and heavy atom effect (HAE). Implementing phosphorescent or TADF material in EML working as assisted dopants in organized structure of cascading enegy levels of singlet and triplet among the constituents, improved EL efficiencies in fluorescent OLEDs were reported. Also, in CT type host material, HAE can be induced to improve EL efficiency of fluorescent OLED by enhancing mixing of singlet and triplet state taking advantage of both the sensitizing and HAE. In this dissertation, the work is focused on the investigation of full potential of fluorescent OLEDs, especially blue fluorescent OLEDs, attempting to answer the series of questions: 1) Is it possible to turn all the triplets into light in fluorescent OLED? 2) Can there be efficient host system for blue fluorescent OLED? 3) Is it possible to achieve high EL efficiency and color purity in a same blue fluorescent OLED, simultaneously? 4) Is it possible to achieve high EL efficiency in conventional blue fluorescent OLEDs? Starting from the first question of the possibility of achieving high EL efficiency in fluorescent OLED equivalent to that of phosphorescent OLED. In Chapter 2, 100% internal quantum efficiency (IQE) is achieved in a green fluorescent OLED exhibiting 30% external quantum efficiency (EQE) comparable to that of phosphorescent OLED. The OLED comprises an exciplex-forming cohost system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC)

the exciplex-forming cohosts contributed efficient ET and charge balance in the system. Large distribution of exciton in exciplex cohost lowered exciton density in EML resulting in the smallest efficiency roll-off among the other reported OLEDs using the same fluorescent emitter. The orientation of the transition dipole moment of the fluorescent dye is shown to have an influence on the EQE of the device. Motivated by the result of the highly efficient green fluorescent OLED using exciplex cohost system in Chapter 2, the efficient mixed cohost system for blue fluorescent OLED was investigated in Chapter 3. Selecting host material for blue emitter is particulary difficult for the requirement of high triplet energy, therefore the majority of reported efficient blue fluorescent OLEDs have been dependent on the same single host material of high triplet energy, which arouse the second question for efficient host system for blue fluorescent OLEDs. The efficient mixed cohost suggested in this work boosted electorluminescence (EL) efficiency of the blue fluorescent OLED by fully utilizing the ability of blue TADF emitter. The EQE was increased from previously reported ~13% to 21.8% from perfect charge balance through bipolar character of the mixed cohost. The achieved EQE was the highest among the blue fluorescent OLEDs of the same emitting material in the single host. The achieved EQE was one of the highest EQEs in blue fluorescent OLEDs and identical to the theoretically achievable maximum EL efficiency using the emitter under consideration of none electrical loss. Through the works in chapter 2 and 3, the fluorescent OLED was proven to be able to achieve the equivalent EL efficiency to that of phosphorescent OLED achieving 100% IQE, and highly efficient blue fluorescent OLED based on the mixed cohost was reported, therefore the motivation of further investigating into the potential of blue fluorescent OLEDs has risen. Despite of numerous reports on efficient blue fluorescent OLEDs, not many have achieved color purity and high EL efficiency at the same time. In Chapter 4, the mixed cohost was further utilized to realize deep blue emission and high EL efficiency at the same time using blue TADF emitter based on azasiline unit. For electroluminescence with delayed fluorescence, the azasiline unit has been introduced for the first time as a donor in a TADF material. The TADF material of 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPDDA) shows strong intramolecular CT character with large spatial separation with the acceptor of triazine leading to narrow splitting of singlet and triplet excited states for the efficient RISC. A blue OLED based on DTPDDA not only displayed deep blue emission in the Commission Internationale de LEclairage (CIE) coordinates of (0.149, 0.197) but also exhibited EQE of 22.3% which is the highest value ever reported for a blue fluorescent OLED. Theoretical prediction based on transient photoluminescence (PL) and optical simulation result agrees well with the achieved EQE indicating the successful conversion of triplet excitons to singlet in the blue fluorescent OLED by using DTPDDA. In Chapter 5, further utilizing azasiline unit for blue TADF emitter, donor-connector-acceptor (D-C-A) and donor-acceptor-donor (D-A-D) type blue TADF emitters were implemented into the mixed cohost in order to investigate the full potential of blue fluorescent OLEDs based on azasilin unit. Utilizing the azasiline unit, 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1-biphenyl]-4-yl)-10,10- diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DTPPDDA), the TADF blue emitter of D-C-A type resulted in deep blue emission with CIE coordinate of (0.151, 0.087), close to the blue standard of the National Television System Committee (NTSC) of (0.140, 0.080) with 4.7% EQE. In D–A–D type materials of bis(4-(10,10-diphenyldibenzo[b,e]-[1,4]azasilin-5(10H)-yl)phenyl)methanone (BDAPM) and 5,5-(sulfonylbis(4,1-phenylene))bis(10,10-diphenyl-5,10-dihydro dibenzo[b,e][1,4]azasiline) (SPDDA), carbonyl and sulfone units were used as the acceptors where the azasiline moeity was used as the donor unit, respectively. The sulfonyl unit contributed to a large twist of the molecular structure while the carbonyl unit led to a small twist of the molecular structure. As a result, the blue fluorescent OLEDs containing BDAPM and SPDDA demonstrated 11.4% and 2.3% EQEs with CIE y-values of 0.310 and 0.107, respectively. However, emitting from TADF material often shows broad EL spectrum due to strong CT state of the material, and the stability of the TADF based OLEDs still need to be improved. As an alternative, conventional fluorophore can be used as an emitter, where assisted dopants either phosphorescent or TADF material is strategically implemented to enhance spin mixing of singlet and triplet excited states. Recently, the high EL efficiencies were reported from blue fluorescent OLEDs based on conventional blue fluorescent dye, taking advantage of TADF material as a sensitizer. With smallof TADF material, RISC can occur effectively harvesting additional triplet excitons. Also, adopting heavy metal compound of platinum or iridium enhances spin mixing of singlet and triplet from HAE, consequently EL efficiency of OLED can be improved. Even more, increased spin reversal of triplet to singlet was observed by taking advantage of CT state of host on the induction of HAE from iridium complex, eventually enhancing the singlet to triplet ratio (ηs/T). In Chapter 6, almost all the triplets were harvested in the conventional blue fluorescence OLED by promoting both sensitizing and HAE through co-doping TADF material and phosphorescent material into EML. Comparison of the theoretical and experimental data indicates that ηs/T was increased from 0.23 to 0.94 with increased EQE from 5.6% to 12.3% in blue fluorescent OLED when the both TADF material and phosphorescent material were doped together as assisted dopants, indicating the nearly all the triplets harvested in the conventional blue fluorescent OLED.