The counter anion effect of ion-type phosphorescent dye tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) complexes as dopant for light-emitting diodes

We present here the effect of counter anion size in tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) complexes [Ru(dpp)3X2] (X=Cl−, BF4−, ClO4−, PF6− and AsF6−) on their doped electrophosphorescent device properties. The performance of organic light-emitting diodes (OLEDs) based on these complexes is influenced by the counter anions, that is the efficiency enhances by increasing the counter anion size from small Cl− to large AsF6−. Among the five complexes, [Ru(dpp)3(AsF6)2] shows the best single layer diode performance with the luminous efficiency of 4.25 cd A−1. The possible reasons are carefully studied on the complex properties and their luminescence processes in light-emitting layers. It is found that the size of counter anion affects properties of complex in many aspects such as solubility and photophysical properties. The result shows that the differences in luminescence quantum yields of complexes, the energy-transfer abilities, as well as the charge injection, transfer and trapping abilities of devices related to the counter anions are the major factors in the different performance of doped OLEDs.