Efficiency Roll-Off in Blue Emitting Phosphorescent Organic Light Emitting Diodes with Carbazole Host Materials

The efficiency roll-off in blue phosphorescent organic light emitting diodes (OLEDs) using different carbazole compounds as the host is systematically studied. While there is no significant difference in device efficiency, OLEDs using ter-carbazole as the host show a reduction in efficiency roll-off at high luminance. Data from transient photoluminescence and electroluminescence measurements show that the lower triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) rates in devices with the ter-carbazole host compared with other carbazole hosts are the reasons for this reduced efficiency roll-off. It is also found that the host materials with low glass transition temperatures are more susceptible to the efficiency roll-off problem.