Letter www.acsami.org
Exciplex-Forming Co-Host-Based Red Phosphorescent Organic LightEmitting Diodes with Long Operational Stability and High Efficiency Jeong-Hwan Lee,†,‡ Hyun Shin,†,‡ Jae-Min Kim,‡ Kwon-Hyeon Kim,‡ and Jang-Joo Kim*,‡,§ ‡
Department of Materials Science and Engineering and §Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, South Korea S Supporting Information *
ABSTRACT: The use of exciplex forming cohosts and phosphors incredibly boosts the efficiency of organic light-emitting diodes (OLEDs) by providing a barrier-free charge injection into an emitting layer and a broad recombination zone. However, most of the efficient OLEDs based on the exciplex forming cohosts has suffered from the short operational lifetime. Here, we demonstrated phosphorescent OLEDs (PhOLEDs) having both high efficiency and long lifetime by using a new exciplex forming cohost composed of N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T). The red-emitting PhOLEDs using the exciplex forming cohost achieved a maximum external quantum efficiency (EQE) of 34.1% and power efficiency of 62.2 lm W1− with low operating voltages and low efficiency roll-offs. More importantly, the device demonstrated a long lifetime around 2249 h from 1000 cd m−2 to 900 cd m−2 (LT90) under a continuous flow of constant current. The efficiencies of the devices are the highest for red OLEDs with an LT90 > 1000 h. KEYWORDS: exciplex-forming cohost, high efficiency, lifetime, phosphine oxide derivatives, red phosphorescent OLEDs
1. INTRODUCTION Organic light-emitting diodes (OLEDs) are now predominant in small-sized displays and are increasingly being used in large displays, and their application is being expanded to lighting. Still many scientific and technological innovations are occurring in this field. For example, the external quantum efficiency (EQE) of both fluorescent and phosphorescent OLEDs has been improved incredibly in the past few years and now exceeds 30% with no external outcoupling structures, through the appropriate management of the excitons’ singlet and triplet states as well as control of the horizontal ratio of emitting dipoles (Θ).1−24 Improving efficiency itself is important, but it would be much better if high efficiency were realized with excellent device stability. There have been many reports on the long operational lifetime of red and green OLEDs. Unfortunately, most stable OLEDs are not very efficient because the reported stable red OLEDs have had an EQE lower than 15%.25−36 Few studies have focused on the lifetime of highly efficient OLEDs. In fact, many of the recently reported highly efficient red OLEDs are not very stable, as will be apparent from this paper, indicating that operational stability is still a challenging issue in high-efficiency OLEDs. This work presents highly efficient red phosphorescent OLEDs (PhOLEDs) with a long operational lifetime based on a new exciplex-forming cohost consisting of a hole-transporting material [N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4″-diamine (NPB)] and an electron-transporting material containing phosphine oxide and triazine [(1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) or PO-T2T] © XXXX American Chemical Society
doped with two different red phosphorescent dyes: bis(4methyl-2-(3,5-dimethylphenyl)quinoline))Ir(III) (tetramethylheptadionate) [Ir(mphmq)2(tmd)] and bis(2-methyldibenzo[f,h]-quinoxaline)Ir(III) (acetylacetonate) (Ir(MDQ)2(acac)). The red PhOLED with the NPB:PO-T2T exciplex host has a maximum EQE of 34.1% and a power efficiency (PE) of 62.2 lm W1− with low operating voltages of 2.42, 3.32, and 5.19 V at 100, 1000, and 10,000 cd m−2, respectively. More importantly, the device using the NPB:PO-T2T exciplex host has an LT90 of 2249 h at 1000 cd m−2. The efficiencies of the devices are the highest for red OLEDs with an LT90 > 1000 h.
2. EXPERIMENTAL SECTION 2.1. Device and Sample Fabrication. To fabricate red PhOLEDs, the organic layers and a metal cathode were stacked on top of 100 nm-thick indium tin oxide (ITO) prepatterned glass substrate by thermal vacuum-evaporation under a base pressure of
