Emission colour-tunable phosphorescent organic light-emitting diodes based on the self-absorption effect and excimer emission

Abstract

Abstract Two kinds of emission colour-tunable phosphorescent organic light-emitting diodes (PhOLEDs) were realized by changing the Ir(III) bis [2-(4,6-difluorophenyl)-pyridinato-N,C 2′ ] (picolinate) (FIrpic) doping levels in the host polymer poly(9-vinylcarbazole) (PVK) or by adjusting the driving voltage on the devices with neat FIrpic film as the emission layer. The Commission Internationale del'Eclairage (CIE) coordinates of PhOLEDs could be changed from (0.14, 0.37) to (0.29, 0.55) by adjusting the FIrpic doping levels from 0.08 : 1 to 2 : 1 in the host polymer PVK. It is worthwhile to mention that all PhOLEDs with a FIrpic : PVK film as the emission layer show high emission colour stability dependence on the driving voltage. The photoluminescence emission colour tunability of the FIrpic : PVK films was also observed under the same excitation conditions when the FIrpic : PVK doping level was changed, which is very similar to that observed from electroluminescence (EL) emission of PhOLEDs. For devices with a neat FIrpic film as the emission layer, the EL emission colour of the PhOLEDs could be adjusted by the driving voltage. The CIE coordinates of the PhOLEDs could be tuned from (0.31, 0.50) to (0.44, 0.50) along with an increase in driving voltage from 8 to 20 V. The emission colour tunability of the neat FIrpic or FIrpic : PVK films should be attributed to the FIrpic self-absorption effect and the occurrence of a new emission peak at about 530 nm. The emission peak at about 530 nm was proved to be FIrpic excimer emission by investigating this emission intensity dependence on FIrpic doping levels in PVK or photo/electric-excitation density on FIrpic : PVK and neat FIrpic films.