Blue-green-emitting cationic iridium complexes with oxadiazole-type counter-anions and their use for highly efficient solution-processed organic light-emitting diodes

Abstract

Cationic iridium complexes are promising phosphorescent dopants for solution-processed organic light-emitting didoes (OLEDs) and counter-anion control has emerged as a facile approach to tailor their properties for high-performance devices. A series of oxadiazole-type anions, 3-(5-phenyl-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-SO₃⁻), 3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (tBuOXD-SO₃⁻) and (3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)trifluoroborate (tBuOXD-BF₃⁻), have been prepared as counter-anions for blue-green-emitting cationic iridium complexes. The photophysical and electrochemical properties of the anions and the complexes have been comprehensively characterized. The anions do not affect the emission properties of the phosphorescent cation and efficiently transfer their energy to the cations in films. Solution-processed, double-layer OLEDs using the complexes as dopants have shown much higher (×1.4) efficiencies than the device using the reference complex with a PF₆⁻ counter-anion, owing to the improvement of carrier transport/recombination balance by the electron-trapping effect of oxadiazole-type anions. In particular, the blue-green device using the complex with the OXD-SO₃⁻ counter-anion affords a peak current efficiency of 37.6 cd A⁻¹ and a peak external quantum efficiency (EQE) of 15.2%, which is the highest for solution-processed OLEDs based on cationic iridium complexes reported so far.