Solid-State Organic Light-Emitting Diodes Based on Tris(2,2‘-bipyridine)ruthenium(II) Complexes

In this contribution we report organic light-emitting diodes (OLEDs) fabricated on ITO (indium tin oxide, 100 Ω/square)coated glass substrates with solid films of Ru(bpy)3(ClO4)2 and its derivative (bpy)2Ru(II)[bpy(COOC12H25)2](ClO4)2 [represented as C12-Ru(bpy)3(ClO4)2] as light emitters (Figure 1). The ITO was the positive contact and a Ga:In eutectic (75.5:24.5 wt %, mp 15.7 °C) or a vacuum-evaporated aluminum contact served as the electron injecting (negative) electrode. The currentand luminance-voltage plots show a diode-like behavior with a turnon voltage of 2.3 V. A brightness of 500 cd/m2 at 3 V bias and 2000 cd/m2 at 4.5 V bias, quantum and power efficiencies of 1.4 and 0.8%, respectively, and a delay time (time required to reach maximum luminance) of 1 to 2 s under a constant 3 V bias were realized. An emission maximum at 660 nm was clearly visible as a bright red emission in a lighted room (Figure 2). Electrogenerated chemiluminescence (ECL) of tris-chelated ruthenium(II) complexes in solution has been studied extensively.1-5 In dry acetonitrile, a quantum efficiency of 24% for ECL emission has been reported.1 ECL has also been observed in polymer films of poly-Ru(vbpy)3 and molten Ru(bpy) salts with the attachment of two ester-linked poly(ethylene glycol) tails,8 although their ECL efficiencies were low (0.035-0.1%). More recently, with the growing interest in solid-state OLEDs, efforts have been made to investigate OLEDs based on thin films of polymers or small molecules.9 These have included blends of tris-chelated ruthenium(II) complex and a conducting polymer10 as well as films of tris-chelated ruthenium(II) complex polymers.11,12 The external quantum efficiencies of these devices ranged from 0.06 to 1%, with turn-on voltages of 2.5 to 3 V, and rather low luminance levels (typically 25 to 200 cd/m2 under a 6-10 V bias) and long turn-on times (taking about 0.5-2.5 min to reach maximum luminance under a 6 V constant bias). A recent report13 demonstrated an increase in emission brightness by employing salts of Ru(bpy)3 and the hydroxymethylated and esterified derivatives as emitters. These exhibited luminance levels of 200 cd/m2 at 3 V and 1000 cd/m2 at 5 V (∼1% quantum efficiency). However, the delay time to maximum emission with these was significant (10-15 min at 3 V and 1 min at 5 V).