Orange and White Organic Light-Emitting Diodes using Exciplex-forming Co-hosts

Abstract

White organic light emitting diodes (WOLEDs) have great potential for applications in displays, lighting, automobiles, imaging, and medicine. In this thesis, various approaches to develop highly efficient WOLEDs using exciplex forming co-hosts are studied in detail. This thesis consists of four parts

(1) orange OLEDs (chapter 2~4), (2) multi-EML WOLEDs (chapter 5), (3) a mechanism of charge generation units (CGUs) and an efficient CGU, and (4) Tandem WOLEDs. All OLEDs use exciplex forming co-hosts to achieve high efficiency, low driving voltage, and low efficiency roll-off, simultaneously. Chapter 1 summarizes the operating principle of OLEDs and what is the exciplex and how to incorporate the exciplex into OLEDs. Various layouts for WOLEDs are introduced and the quality of WOLEDs for lightings is briefly discussed in terms of the efficiency and color quality. Chapter 2~4 introduce orange OLEDs which are the key building block for development of efficient WOLEDs with high color rendering index (CRI). In chapter 2, an exciplex forming co-host is introduced in order to fabricate orange OLEDs with high efficiency, low driving voltage and an extremely low efficiency roll-off, by the co-doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieved a low turn-on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent-green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs showed low efficiency roll-off with an EQE of over 21% at 10,000 cdm-2. The device displayed a very good orange color (CIE of (0.501, 0.478) at 1,000 cdm-2) with very little color shift with increasing luminance. The color shift is supposed to be originated from a light doping concentration of red dopants. The transient electroluminescence (EL) of the OLEDs indicated that both energy transfer (ET) and direct charge trapping took place in the devices. In chapter 3, the recombination mechanism of red emission in orange OLEDs with co-doping of green and red phosphorescent dopants is studied using transient EL analysis in decay region. The dominant recombination type is the ET from green to red dopants and the efficiencies of ET were calculated about 50% in various orange OLEDs using decay rate constants of green emission without red dopant and ET rate constants from green to red dopants. In addition, we showed the energy of transition dipole with the same orientation is more efficiently transferred to that of the same orientation, because the ET is a function of transition dipole orientation. Furthermore, we demonstrated an unprecedentedly efficient orange OLED exhibiting the maximum EQE of 32.2% using green and red dopants having horizontally oriented transition dipole moments. In chapter 4, a high performance orange OLED where red and green phosphorescent dyes are doped in an exciplex forming co-host as separate red and green emitting layers (EMLs). The OLED shows a maximum EQE of 22.8%, a low roll-off of efficiency with an EQE of 19.6% at 10,000 cd/m2, and good orange color with a CIE coordinate of (0.442, 0.529) and no color change from 1,000 to 10,000 cd/m2. In chapter 5, a novel structure for highly efficient WOLEDs approaching the theoretical limit is studied by combining exciplex forming co-hosts, horizontally oriented phosphorescence dyes, and two emission layers for three colour white emission. The WOLED showed a maximum luminous efficacy (LE) of 68 lm W–1 and maximum EQE of 28.8% without an extra light extraction layer. Furthermore, the device exhibited high LE at high luminance with a LE of 58 lm W–1 at 1,000 cd m–2 originating from a high EQE (27.9%), low operating voltage (3.52 V) and low efficiency roll-off. The device emitted high quality warm white light with a CRI of 82. In addition, we achieved a LE of 106 lm W–1 at 1,000 cd m–2 by attaching an index-matched glass half sphere to the glass substrate. In chapter 6, the rate limiting step of charge generation is studed using the CGUs composed of a p-doped hole transporting layer (p-HTL), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) and n-doped electron transporting layer (n-ETL) where 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC) was used as the HTL. Energy level alignment determined by the capacitance-voltage (C-V) measurements and the current density-voltage characteristics of the structure clearly showed that the electron injection at the HATCN/n-ETL junction limits the charge generation in the CGUs rather than charge generation itself at the p-HTL/HATCN junction. Consequently, the CGUs with 30 mol% Rb2CO3 doped BPhen formed with the HATCN layer generates charges very efficiently and the excess voltage required to generate the current density of 10 mA/cm2 was around 0.17 V, which is extremely small compared with the literature values reported up to now. In chapter 7, tandem WOLEDs with EQE approaching theoretical limit are reported by interconnecting high efficiency orange OLEDs with co-doping of green and red phosphorescent dopants having high  andPL in exciplex forming co-host (concept shown in chapter 2, 3) and blue OLED using exciplex forming co-host with an efficient CGU (shown in chapter 6). A tandem WOLED with a high maximum EQE of 54.3% (PE of 63 lm W–1), EQE of 52.6% (PE of 52 lm W–1) at 1,000 cd m–2, low efficiency roll-off, and high color stability was demonstrated. In addition, an EQE of 90.6% at 1,000 cd m–2 by attaching an index-matched glass half sphere on conventional glass substrate was achieved