Sky-blue-emitting cationic iridium complexes with oxadiazole/triazine-type counter-anions and their use for efficient solution-processed organic light-emitting diodes

Abstract

Counter-anion control has emerged as a facile approach to tune the properties of cationic iridium complexes for optoelectronic applications. Here we report sky-blue-emitting cationic iridium complexes with electron-deficient oxadiazole/triazine-type counter-anions and their use for highly efficient solution-processed organic light-emitting diodes (OLEDs). A sky-blue-emitting complex [Ir(meoppy)2(dmapzpy)]+ (meoppy is 4-methoxy-2-phenylpyridine and dmapzpy is 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine) has been developed as the phosphorescent cation. Hexafluorophosphate (PF6−), 3,5-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-7-SO3−), 4-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-p-SO3−) and 3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzenesulfonate (TRZ-m-SO3−) have been developed as the counter-anions in complexes R, 1, 2 and 3, respectively. In doped films, the complexes afford similar sky-blue emission peaked at around 472 nm with high phosphorescent efficiencies of around 0.80. Solution-processed, double-layer OLEDs using the complexes as dopants afford sky-blue electroluminescence peaked at around 476 nm. The devices using complexes 1–3 show largely enhanced performances compared to the device using the reference complex R, owing to the improved carrier-recombination balance imparted by the electron-trapping effects of the oxadiazole/triazine type counter-anions. The devices based on complexes 2 and 3 exhibit higher performances than that based on complex 1, because of the stronger electron-trapping effects of TRZ-p-SO3− and TRZ-m-SO3− compared to OXD-7-SO3−. The double-layer device using complex 3 shows a peak current efficiency of 29.4 cd A−1 and an external quantum efficiency of 12.4%, which are the highest for blue OLEDs using cationic iridium complexes reported so far.