Date Published: December 01, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Wook Song, Jun Yeob Lee, Yong Joo Cho, Hyeonghwa Yu, Hany Aziz, Kang Mun Lee.
A new concept of host, electroplex host, is developed for high efficiency and long lifetime phosphorescent organic light‐emitting diodes by mixing two host materials generating an electroplex under an electric field. A carbazole‐type host and a triazine‐type host are selected as the host materials to form the electroplex host. The electroplex host is found to induce light emission through an energy transfer process rather than charge trapping, and universally improves the lifetime of red, yellow, green, and blue phosphorescent organic light‐emitting diodes by more than four times. Furthermore, the electroplex host shows much longer lifetime than a common exciplex host. This is the first demonstration of using the electroplex as the host of high efficiency and long lifetime phosphorescent organic light‐emitting diodes.
Long‐term operational stability is one of the most challenging issues in the field of organic light‐emitting diodes (OLEDs) because of the intrinsic chemical instability of organic or organometallic emitters.1 Although the operational stability of OLEDs has reached lifetimes that are required for commercial applications, there is still strong demand to increase the lifetime of green and blue phosphorescent OLEDs (PhOLEDs) because their lifetime is not long enough for some practical applications like large size TVs.2
Electroplexes have been reported in several publications, but they were not used in OLEDs because of the poor emission efficiency of the electroplex and difficulty managing the electroplex formation in the devices.10 However, the electroplex can be emitted from an intermolecular complex of two materials and transfer the energy to other materials under an electric field like an exciplex.11 An electroplex is a relatively weak intermolecular complex with a weak binding energy compared to an exciplex, which can make it easy to develop a high emission energy blue electroplex. In an exciplex, the high exciton binding energy due to the strong binding of two molecules leads to a large decrease in the exciplex emission energy relative to the emission energy of each host participating in the exciplex formation. In contrast, the exciton binding energy of the electroplex is relatively small compared to that of the exciplex, which facilitates the development of a high emission energy electroplex. Therefore, electroplexes can be used as a host in green and blue PhOLEDs, which require high emission energy in the host material. In addition, the underlying concept of using the electroplex as the host of PhOLEDs is to take advantage of an energy transfer process for improved lifetimes in the emission process of dopant materials like exciplexes. The electroplex can behave like an exciplex when used as a host material. Furthermore, the electroplex can provide high emission energies for green and blue PhOLEDs.
In conclusion, a CBP:DBFTrz electroplex host and an mCBP:DBFTrz electroplex host were developed as high efficiency and lifetime‐extending hosts for red, yellow, green, and blue PhOLEDs, respectively. These hosts improved the lifetime of red, yellow, green, and blue PhOLEDs by a factor of higher than four relative to control devices like single host, mixed host, and exciplex host devices. The light emission mechanism of the electroplex host was attributed to energy transfer rather than charge trapping by ideality factor analysis and delayed transient EL analysis. We found that the devices were stable in the presence of holes and electrons due to the large HOMO/LUMO offset and energy transfer emission mechanism. These were key factors that helped improve the lifetime of the electroplex host. The blue electroplex host was easy to prepare, unlike exciplex hosts, which are rather difficult to develop. Therefore, the electroplex type host can be useful for both high efficiency and long lifetime phosphorescent OLEDs.
Device Fabrication and Materials: The red, yellow, and green devices employed indium tin oxide substrates (ITO) as an anode;N,N′‐diphenyl‐N,N′‐bis‐[4‐(phenyl‐m‐tolyl‐amino)‐phenyl]‐biphenyl‐4,4′‐diamine (DNTPD) as a hole injection layer; N,N,N′N′‐tetra[(1,1′‐biphenyl)‐4‐yl]‐(1,1′‐biphenyl)‐4,4′‐diamine (BPBPA) as a hole transport layer; 9,9‐dimethyl‐10‐(9‐phenyl‐9H‐carbazol‐3‐yl)‐9,10‐dihydroacridine(PCZAC) as an electron blocking layer; Ir(mphmq)2tmd, PO‐01, and Ir(ppy)2acac as red, yellow, and green phosphorescent dopants; CBP, mCBP, and TCTA as hole transport type hosts; DBFTrz and TPBI as electron transport type hosts; LiF as an electron injection layer; and Al as a cathode. The whole device structure was ITO (120 nm)/DNTPD (60 nm)/BPBPA (20 nm)/PCZAC (10 nm)/emitting layer (30 nm)/TPBI (35 nm)/LiF (1.5 nm)/Al (200 nm). The emitting layers of the devices were either single hosts of CBP, and DBFTrz or mixed hosts of CBP:TPBI, CBP:DBFTrz, and TCTA:DBFTrz doped with phosphorescent emitters. The ratio of the two hosts in the mixed hosts was 50:50, and the dopant concentration was 5 wt%.
The authors declare no conflict of interest.