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Improved light extraction efficiency in organic light emitting diodes with a perforated WO3 hole injection layer fabricated by use of colloidal lithography

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Abstract

We present an organic light emitting diode with a perforated WO3 hole injection layer to improve the light extraction efficiency. The two-dimensionally perforated WO3 layer was fabricated by use of colloidal lithography. The light extraction efficiency was improved due to Bragg scattering of waveguide modes and surface plasmon polaritons, and the operating voltage was also decreased. As a result, the external quantum efficiency and the power efficiency were increased as compared with those of conventional organic light emitting diodes without WO3 layer. The angular dependence of emission characteristics was investigated by measuring radiant intensity profiles for emission angles and azimuthal angles.

©2012 Optical Society of America

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Figures (6)

Fig. 1
Fig. 1 (a) Hexagonal grating period required to extract the waveguide modes for the emission wavelength. The dotted horizontal line represents the grating period of 330 nm, which is selected as the grating period in the experiment. (b) Normalized electric field intensity profile of the waveguide modes.
Fig. 2
Fig. 2 Graphical representation of the fabrication process of OLED with the perforated WO3 layer: (a) self-assembled polystyrene monolayer, (b) non-closely packed self-assembled polystyrene monolayer after the air-plasma etching, (c) WO3 layer deposited on the self-assembled polystyrene monolayer, (d) a perforated WO3 layer, and (e) a structure of the OLED with the perforated WO3 layer.
Fig. 3
Fig. 3 SEM images of (a) the self-assembled polystyrene monolayer and (b) the perforated WO3 layer on the ITO-coated glass substrate. Inset: FIB-SEM image of the cross-section of the OLED with the perforated WO3 layer.
Fig. 4
Fig. 4 (a) Current density (J)-voltage (V) characteristics, (b) current efficiencies in normal emission, (c) external quantum efficiency (EQE), and (d) power efficiency of devices A, B, C and D.
Fig. 5
Fig. 5 Emission spectra from devices A and D in normal emission.
Fig. 6
Fig. 6 Radiant intensity profiles of (a) device A and (b) device D at all emission (θ) and azimuthal (ϕ) angles. (c) Radiant intensity profiles of devices A and D for the emission angles while the azimuthal angle was fixed; dotted lines indicate the radiation profiles of a Lambertian source.

Tables (1)

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Table 1 Structure and Thicknesses of Each Layer of the Fabricated Devices

Equations (3)

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k inplane = k air sinθ
| k guide |( cos ϕ guide sin ϕ guide )+m g1+n g2=| k inplane |( cos ϕ inplane sin ϕ inplane )
g1=( 2π Λ 2π 3 Λ ),g2=( 0 4π 3 Λ )
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