Langevin and Trap-Assisted Recombination in Phosphorescent Organic Light Emitting Diodes

Bimolecular and trap‐assisted recombination mechanisms are investigated in small molecule‐based phosphorescent organic light emitting diodes (PhOLEDs) using the current−voltage−luminance characteristics in the diffusion current region, along with transient electroluminescence and capacitance measurements. Two different PhOLEDs, one with a single host, 4,4′‐Bis(carbazol‐9‐yl)biphenyl, and the other with an exciplex‐forming co‐host, are studied. Trap‐assisted recombination with a large number of trapped charges is dominant in the PhOLED with the single host because of the large energy gap between the host and the dopant state. In contrast, bimolecular Langevin recombination is dominant in the PhOLED with the exciplex forming co‐host, where a phosphorescent dye is doped in the co‐host. As a result, the accumulated charge density is lower in the co‐host system than in the single host emission layer, leading to high efficiency that approaches the theoretical limit, with an extremely low efficiency roll‐off. The origin of the ultimate efficiency with extremely low efficiency roll‐off in a PhOLED with exciplex‐forming co‐host (quasi‐single host) is unveiled. Unlike the other PhOLEDs with single host which is dominantly governed by trap assisted recombination, Langevin assisted recombination is dominated in the PhOLED with quasi‐single host system, resulting in low accumulation of charges in the device.