Combinatorial Fabrication and Studies of Small Molecular Organic Light-Emitting Devices
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Combinatorial Fabrication and Studies of Small Molecular Organic Light-Emitting Devices G. Li, L. Zou, K. O. Cheon, and J. Shinar* Ames Laboratory - USDOE* and Department of Physics and Astronomy Iowa Sate University, Ames, Iowa 50011 Abstract Various combinatorial matrix arrays of UV-violet, white, and blue-to-red organic light-emitting devices (OLEDs), fabricated using a sliding shutter technique, are described. In the UV-violet devices, which contain a UV-violet emitting layer of 4,4'-bis(9-carbazolyl) biphenyl (CBP), the optimal radiance R and external quantum efficiency ηext were determined with respect to the thicknesses of the hole transporting layers. In the blue-to-red devices, which contained a blue-emitting layer of 4,4'-bis(2,2'-diphenyl-vinyl) -1,1'-biphenyl (DPVBi) and a red-emitting 5 wt.% dye-doped guest-host layer, the color of the devices evolved continuously from blue to red as the thickness of the doped layer increased from 0 to 35 Å. The (nominal) 2 Å-thick doped layer device exhibited the highest brightness L ~ 120 Cd/m2 and external quantum efficiency ηext ~4.4 % at a current density of 1 mA/cm2. In the white OLEDs, which were similar to the blue-to-red devices but with lightly doped emission layer, the highest brightness Lmax was over 74,000 Cd/m2; in all devices Lmax exceeded 50,000 Cd/m2. The maximum efficiencies were 11.0 Cd/A, 5.96 lm/W and 4.6% at 5.8 V, 0.6 mA/cm2, and 68 Cd/m2 in a 0.25 wt.%, 2 nm-thick doped layer device. Introduction The performance of small molecular and polymeric red-to-blue and white organic light-emitting devices (OLEDs) has improved dramatically over the past decade.1 However, determining the optimal parameters (e.g., thicknesses) in multilayer structure is tedious and introduces both systematic errors and random variations from batch to batch. Combinatorial fabrication of OLEDs using a sliding shutter technique removes these errors and variations,2 as it enables fabrication of two-dimensional arrays of OLEDs in which two parameters are varied systematically. Apart from these two parameters the OLEDs are identical, and the comparison between them is more reliable. This paper describes the fabrication and studies of combinatorial UV-violet, blue to red, and white OLEDs. The UV-violet devices were based on 4,4'-bis(9-carbazolyl) biphenyl (CBP) emission, with copper phthalocynine (CuPc) as the hole injection layer (HIL) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (Bu-PBD) as the electron transport layer (ETL).3 The optimal radiance R and external quantum efficiency ηext were determined with respect to the thicknesses of the HIL and ETL. At J = 10 mA/cm2, a peak R = 0.38 mW/cm2, yielding ηext = 1.25%, was obtained for 15 nm CuPc and 18 nm Bu-PBD.3 The blue-to-red devices were fabricated by varying the thickness of a 5 wt.% red-emitting dye-doped (N,N'diphenyl-N,N'-bis(1-naphthylphenyl)-1,1’-biphenyl-4,4’–diamine (NPB)) layer, adjacent to the blue emitting 4,4-bis(2,2-diphenyl-ethen-1-yl)-biphenyl (DPVBi)), from 0 to 35 Å.7 For the white OLE
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