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Langevin and Trap‐Assisted Recombination in Phosphorescent Organic Light Emitting Diodes
Author(s) -
Lee JeongHwan,
Lee Sunghun,
Yoo SeungJun,
Kim KwonHyeon,
Kim JangJoo
Publication year - 2014
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201303453
Subject(s) - phosphorescence , phosphorescent organic light emitting diode , materials science , optoelectronics , oled , electroluminescence , dopant , excimer , diode , doping , optics , nanotechnology , fluorescence , physics , layer (electronics)
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.