Host Selection and Configuration Design of Electrophosphorescent Devices

• Published in: Advanced functional materials 2007-07, Vol.17 (11), p.1757-1764
• Main Authors: Xia, H., Li, M., Lu, D., Zhang, C. B., Xie, W. J., Liu, X. D., Yang, B., Ma, Y. G.
• Format: Article
• Language: English
• Subjects: Electroluminescence; Light-emitting diodes; Light‐emitting diodes, organic; organic; Phosphorescence; Polymers; Structure-property relationships
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200600882

Recent studies on electrophosphorescent polymeric devices have demonstrated that charge‐trapping‐induced direct recombination on the phosphorescent dopant is of crucial importance. In this paper, we show that the electrochemical properties of phosphorescent molecules, which reflect their carrier‐trapping ability, may be a basic design criterion for the selection of host and device configuration. The systems, consisting of a red phosphorescent [Ru(4,7‐Ph2‐phen)3]2+ dopant and two blue hosts 2‐biphenyl‐4‐yl‐5‐(4‐tert‐butyl‐phenyl)‐[1,3,4]oxadiazole (PBD) and poly(vinylcarbazole) (PVK), are intensively studied. The triplet energy level of PVK and PBD is higher than that of the [Ru(4,7‐Ph2‐phen)3]2+, and both hosts show the ability of efficient energy transfer to the dopant, however, efficient electroluminescence (EL) is only obtained in the PVK‐host system. The combined studies of photoluminescence (PL), EL, and electrochemistry for doped films demonstrate that [Ru(4,7‐Ph2‐phen)3]2+, which undergoes a multielectron trapping process as it is used as a dopant in electron‐rich (n‐type) hosts, for instance, PBD, may induce an inefficient recombination for the resulting emission. Whereas using a hole‐rich (p‐type) polymer, such as PVK, as a host and inserting both hole‐blocking and electron‐transfer layers can effectively increase the efficiency of the corresponding devices up to 8.63 Cd A–1, because of the reduced probability of multielectron trapping at the [Ru(4,7‐Ph2‐phen)3]2+ sites. The charge‐trapping ability of phosphorescent molecules is reflected by their electrochemical behavior. The electrochemical properties of these molecules can, thus, be used to design efficient electroluminescent (EL) devices by choosing the right host materials and device structure. For instance, using [Ru(4,7‐Ph2‐phen)3]2+ as a dopant in a p‐type host can effectively increase the efficiency of EL devices, while the light emission is inefficient when using an n‐type host (see figure).