Efficient Charge Balance of Green Organic Light Emitting Diodes Using Mixed Charge Transporting Materials as the Host

Abstract

The mixed layer for an emitting layer (EML) consists of two materials 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) without host material. A series of EML structures were fabricated with three types of single mixed layers (mixing ratio 2:1, 1:2) and double mixed layers (mixing ratio 2:1/1:2), and the EML of using double mixed layer was optimized. The maximum luminous efficiency (LE) and quantum efficiency (QE) were 62.07 cd/A, 18.92%, respectively. Moreover, the roll-off ratio of LE was 20.42% at 20,000 cd/m². Interestingly, this result shows the roll-off ratio was reduced roughly 50% compared to the reference device.

Keywords:

• Organic light emitting diode
• mixed host
• energy transfer
• roll-off ratio
• charge balance

Acknowledgment

This research was supported by the MSIP (Ministry of Science, ICT & Future Planning), Korea, under the ITRC (Information Technology Research Center) support program NIPA-2013-(H0301-13-1004) supervised by the NIPA (National IT Industry Promotion Agency) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP)(CAFDC/Young Kwan Kim/No. 2007-0056090).