期刊
ADVANCED FUNCTIONAL MATERIALS
卷 24, 期 29, 页码 4681-4688出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adfm.201303453
关键词
organic light emitting diodes; Langevin recombination; exciplex emission; energy transfer; trap
类别
资金
- Industrial strategic technology development program - MKE/KEIT [10035225]
- Korea Evaluation Institute of Industrial Technology (KEIT) [10035225] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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.
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