Journal
PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE
Volume 210, Issue 8, Pages 1467-1475Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/pssa.201329107
Keywords
hybrid white organic light-emitting diodes; triplet diffusion; triplet harvesting; quantum efficiency
Funding
- European Community [FP7-224122 (OLED100.eu)]
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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 27.1lmW(-1) (at 1000cdm(-2)) which can be further increased to 46lmW(-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.
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