Microelectronics Journal 37 (2006) 1325–1328 Enhancement of red organic light-emitting diodes via cascade energy transfer Fuxiang Wei à , Xiaobo Zhang, Jin Cao, M.A. Khan, Wenqing Zhu, Xueyin Jiang, Zhilin Zhang Key Laboratory of Advanced Display and System Application of Ministry of Education, Institute of Materials Science and Engineering, Shanghai University, Jiading campus, Shanghai 201800, P.R. China Received 9 May 2006; accepted 10 July 2006 Available online 22 August 2006 Abstract We have demonstrated that efficient red electroluminescence is obtained via cascade energy transfer from Alq to fluorescent dye Coumarin(C545) and then from C545 to 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). The cell structure was indium tin oxide (ITO)/ N,N 0 -bis-(1-naphenyl)-N,N 0 -biphenyl-1,1 0 -bipheny1-4-4 0 -diamine (NPB)/ tris (8-hydroxyquino- line) aluminum (Alq): C545: DCJTB/Alq/LiF/Al. An additional dopant, C545, was used to assist the energy transfer from Alq to the red dopant. Compared with the devices where the emitting layer is only composed of Alq and DCJTB, the emission efficiency and color purity were improved. We attribute these improvements to the assistant dopant C545 which leads to the more efficient energy transfer from Alq to DCJTB. The co-doping system is a promising method for red organic light-emitting diodes. r 2006 Elsevier Ltd. All rights reserved. Keywords: Red organic light-emitting diodes; Assistant dopant; Energy transfer 1. Introduction Organic light-emitting diodes (OLEDs) have attracted wide attention in view of practical applications for displays and lighting [1,2], since the demonstration of efficient electroluminescence (EL) from a bilayer device by Tang et al. [3]. For realizing practical full-color displays, red emitters with sufficiently high luminous efficiencies and color purity are required. Conventional red OLEDs have an emitting layer, which consists of a host material and a red emitting dopant [4,5]. But it is difficult for the doping system to produce red emission with high luminance efficiency, because the excitation energy of host differs greatly from that of the red dopant and energy transfer from host to guest is not very complete, in addition, the concentration quenching effect by the aggregation of the dopants at high doping concentration. Co-doping of two dyes into one host has been proposed to trigger a complete cascade energy transfer process in OLEDs [6]. The emitting layer of the OLEDs consists of three different fluorescent materials A–C, and the energy transfer process is from A to B and then from B to C, which is a promising method for obtaining red OLEDs. Hamada et al. [6] employed the rubrene as an assistant dopant to facilitate the energy transfer and improved the color purity of red emission. Ohmori et al. [7] enhanced red emission by co-doping DCM and TPP into the Alq layer, utilizing efficient energy transfer and energy confinement in molecules. Lee et al. [8] reported that using cascade energy transfer to tune the emitting color and improve the external quantum efficiency in dye-doped ternary polymer blend light-emitting diodes. In this letter, we propose a new co-doping system for red emission, in which two fluorescent dyes of coumarin (C545) and 4-dicyanomethylene-2-t-butyl-6-1,1,7,7-tetramethylju- lolidyl-9-enyl-4H-pyran (DCJTB) were codoped into the tris (8-hydroxyquinoline) aluminum (Alq) emitting layer. An additional dopant C545, named an emitting assistant (EA) dopant is introduced. The EA dopant works as an intermediary to transfer the excitation energy from Alq to ARTICLE IN PRESS www.elsevier.com/locate/mejo 0026-2692/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2006.07.012 à Corresponding author. Tel.:+86 21 69982587; fax: +86 21 39988216. E-mail address: weifuxiang2001@163.com (F. Wei).