Abstract
The design of novel exciplex-forming co-host materials provides new opportunities to achieve high device performance of organic light emitting diodes (OLEDs), including high efficiency, low driving voltage and low efficiency roll-off. Here, we report a comprehensive study of exciplex-forming co-host system in OLEDs including the change of co-host materials, mixing composition of exciplex in the device to improve the performance. We investigate various exciplex systems using 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl-3,9-diphenyl-9H-carbazole, 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9-phenyl-9H-3,9′-bicarbazole, and 2-(3-(6,9-diphenyl-9H-carbazol-4-yl)phenyl)-4-phenylbenzo[4,5]thieno[3,2-d]pyrimidine, as electron transporting (ET: electron acceptor) hosts and 9,9′-dipenyl-9H, 9′H-3,3′-bicarbazole and 9-([1,1′-biphenyl]-4-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole as hole transporting (HT: electron donor) hosts. As a result, a very high current efficiency of 105.1 cd/A at 103 cd/m2 and an extremely long device lifetime of 739 hrs (t95: time after 5% decrease of luminance) are achieved which is one of the best performance in OLEDs. Systematic approach, controlling mixing ratio of HT to ET host materials is suggested to select the component of two host system using energy band matching and charge balance optimization method. Furthermore, our analysis on exciton stability also reveal that lifetime of OLEDs have close relationship with two parameters; singlet energy level difference of HT and ET host and difference of singlet and triplet energy level in exciplex.
Highlights
Where γ, ηr, Φp, ηp are charge balance, exciton formation efficiency, quantum yield, device out-coupling efficiency, respectively
In addition to the introduction of new high performance electron transporting (ET: electron acceptor) and hole transporting (HT: electron donor) host materials, we suggest that mixing ratio of ET to HT host materials in emission layer can be governing factors to allow for high device efficiency and long lifetime
Our results clearly indicate that the mixing ratio of ET to HT host materials should be controlled as considering the transport properties of each layer to achieve charge balance
Summary
Sangyeob Lee 1, Hyun Koo[1], Ohyun Kwon1,Young Jae Park[1], Hyeonho Choi[1], Kwan Lee[2], Byungmin Ahn 2 &Young Min Park[1,3]. The design of novel exciplex-forming co-host materials provides new opportunities to achieve high device performance of organic light emitting diodes (OLEDs), including high efficiency, low driving voltage and low efficiency roll-off. A very high current efficiency of 105.1 cd/A at 103 cd/m2 and an extremely long device lifetime of 739 hrs (t95: time after 5% decrease of luminance) are achieved which is one of the best performance in OLEDs. Systematic approach, controlling mixing ratio of HT to ET host materials is suggested to select the component of two host system using energy band matching and charge balance optimization method. High-efficiency and low-driving voltage phosphorescent OLEDs using exciplex-forming co-hosts were demonstrated for green[21], blue[22,23,24,25], orange[26] and yellow[27,28] emitting devices. We found out optimal condition to enhance the device lifetime of exciplex-forming co-host system by considering excited state energy level difference between HT, ET molecules and exciplex molecules
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