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Investigation of triplet harvesting and outcoupling efficiency in highly efficient two‐color hybrid white organic light‐emitting diodes
Author(s) -
Hofmann Simone,
Furno Mauro,
Lüssem Björn,
Leo Karl,
Gather Malte C.
Publication year - 2013
Publication title -
physica status solidi (a)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.532
H-Index - 104
eISSN - 1862-6319
pISSN - 1862-6300
DOI - 10.1002/pssa.201329107
Subject(s) - oled , phosphorescence , common emitter , quantum efficiency , singlet state , materials science , electroluminescence , optoelectronics , iridium , luminous efficacy , fluorescence , exciton , optics , chemistry , physics , atomic physics , excited state , nanotechnology , layer (electronics) , biochemistry , quantum mechanics , catalysis
We investigate singlet and triplet transfer processes in white triplet harvesting (TH) organic light‐emitting diodes (OLEDs), comprising fluorescent and phosphorescent emitter molecules. By analyzing electroluminescence spectra and performing time‐resolved measurements, we prove direct TH from the blue fluorescent emitter N , N ′‐di‐1‐naphthalenyl‐ N , N ′‐diphenyl‐[1,1′:4′,1″:4″,1′′′‐quaterphenyl]‐4,4′′′‐diamine (4P‐NPD) to the yellow phosphorescent emitter bis(2‐(9,9‐dihexyluorenyl)‐1‐pyridine)(acetylacetonate)iridium(III) (Ir(dhfpy) 2 (acac)). Singlet transfer is identified as a second exciton transfer mechanism in the TH OLEDs under investigation. The CIE coordinates of these devices can be adjusted over a wide range by varying the distance between exciton generation and TH zone. For an OLED with CIE color coordinates of (0.42/0.40), close to the warm white color point A, we obtain nearly perfect Lambertian emission characteristics. This device achieves an external quantum efficiency (EQE) of 9.4% and a luminous efficacy (LE) of 27.1 lm W −1 (at 1000 cd m −2 ) which can be further increased to 46 lm W −1 using outcoupling enhancement techniques. Optical modeling of the electromagnetic field inside the TH OLED and a comparison to the position of the emitters within the device confirms that the devices operate close to the optical optimum and yields an estimated internal quantum efficiency (IQE) of 48.7% and a singlet/(harvested) triplet ratio of 0.28/0.72 = 0.39.