A Macrospirocyclic Carbazole-Fluorene Oligomer as a Solution-Processable Matrix Host Material for Blue Phosphorescent Organic Light-Emitting Diodes with Low Turn-On Voltage and Efficiency Roll-Off

The use of mixed hosts has been demonstrated to be an effective way to improve the efficiency of phosphorescent organic light-emitting diodes (PhOLEDs). In this study, a macrospirocyclic oligomer (Cz-F)₄ based on carbazole and fluorene is synthesized as a solution-processable matrix host material for blue PhOLEDs. The oligomer exhibits a high triplet level, good film-forming ability, solution processability, and doping compatibility. As a result, a blue solution-processed PhOLED based on a (Cz-F)₄/TAPC [1,1-bis((di-4-tolylamino) phenyl) cyclohexane] mixed host achieves a maximum current efficiency of 18.8 cd/A with a turn-on voltage of 3.3 V. When a ternary mixed-host system composed of (Cz-F)₄, TAPC, and OXD-7 [1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene] is employed, the device exhibits a low turn-on voltage of 2.8 V, and the driving voltages are only 3.8-4.6 V at the practical luminance of 100-1000 cd/m². Even at 5000 and 10 000 cd/m², the driving voltages are only 5.7 and 6.6 V, respectively. The current efficiency roll-off is only 12% from 100 to 5000 cd/m². This study demonstrates that monodisperse oligomers can serve a matrix host for highly efficient blue solution-processed PhOLEDs with low turn-on voltage, low driving voltage, and suppressed efficiency roll-off.