Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes

The stability and the degradation processes of two highly efficient blue-emitting phosphorescent materials, iridium(III) bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl) borate (FIr6) and bis(2-(4,6-difluorophenyl) pyridyl-N,C2')iridium(III)picolinate (FIrpic), which are commonly used as emitters in organic light emitting diodes (OLEDs), are investigated. Using single layers devices, the optical response and the half-lifetime behavior of the materials are investigated. Layers of FIr6 exposed to UV-light show the formation of a red emitting degradation product. We analyze the chemical reactions of the materials using laser desorption/ionization time-of-flight mass spectrometry. Several products related to the chemical dissociation of the FIr6 molecule as well as charge complex formation between the emitter and the emitter dissociation products are detected. FIr6 and FIrpic are also compared by lifetime studies on commonly used OLED structures. We show that single layers and OLEDs based on FIrpic exhibit higher stability than those based on FIr6. An explanation for this behavior can be found by considering the chemical structure of the molecules.