Orange and Red Organic Light-Emitting Devices Employing Neutral Ru(II) Emitters: Rational Design and Prospects for Color Tuning

• Published in: Advanced functional materials Vol. 16; no. 12; pp. 1615 - 1626
• Main Authors: Tung, Y.-L., Chen, L.-S., Chi, Y., Chou, P.-T., Cheng, Y.-M., Li, E. Y., Lee, G.-H., Shu, C.-F., Wu, F.-I., Carty, A. J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 04.08.2006; WILEY‐VCH Verlag
• Subjects: Displays; Electroluminescence; Light-emitting diodes; Light‐emitting diodes, organic; organic; Ruthenium complexes
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200500901

A new series of charge‐neutral Ru(II) pyridyl and isoquinoline pyrazolate complexes, [Ru(bppz)2(PPh2Me)2] (bbpz: 3‐tert‐butyl‐5‐pyridyl pyrazolate) (1), [Ru(fppz)2(PPh2Me)2] (fppz: 3‐trifluoromethyl‐5‐pyridyl pyrazolate) (2), [Ru(ibpz)2(PPhMe2)2] (ibpz: 3‐tert‐butyl‐5‐(1‐isoquinolyl) pyrazolate) (3), [Ru(ibpz)2(PPh2Me)2] (4), [Ru(ifpz)2(PPh2Me)2] (ifpz: 3‐trifluoromethyl‐5‐(1‐isoquinolyl) pyrazolate) (5), [Ru(ibpz)2(dpp)] (dpp represents cis‐1,2‐bis‐(diphenylphosphino)ethene) (6), and [Ru(ifpz)2(dpp)] (7), have been synthesized, and their structural, electrochemical, and photophysical properties have been characterized. A comprehensive time‐dependant density functional theory (TDDFT) approach has been used to assign the observed electronic transitions to specific frontier orbital configurations. A multilayer organic light‐emitting device (OLED) using 24 wt % of 5 as the dopant emitter in a 4,4′‐N,N′‐dicarbazolyl‐1,1′‐biphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPB) as the hole‐transport layer exhibits saturated red emission with an external quantum efficiency (EQE) of 5.10 %, luminous efficiency of 5.74 cd A–1, and power efficiency of 2.62 lm W–1. The incorporation of a thin layer of poly(styrene sulfonate)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT) between indium tin oxide (ITO) and NPB gave anoptimized device with an EQE of 7.03 %, luminous efficiency of 8.02 cd A–1, and power efficiency of 2.74 lm W–1 at 20 mA cm–2. These values represent a breakthrough in the field of OLEDs using less expensive Ru(II) metal complexes. The nonionic nature of the complexes as well as their high emission quantum efficiencies and short radiative lifetimes are believed to be the key factors enabling this unprecedented achievement. The prospects for color tuning based on Ru(II) complexes are also discussed in light of some theoretical calculations. The rational design of a new series of charge‐neutral Ru(II) metal complexes is reported here. Ruthenium complexes with isoquinolyl ligands have been utilized as dopant emitters for the fabrication of red and red‐orange OLEDs (see figure). The high efficiencies and low turn‐on voltage obtained using inexpensive Ru marks a significant advance in the field of organic displays.