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Enhancement of light-extraction efficiency of organic light-emitting diodes using silica nanoparticles embedded in TiO2 matrices

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Abstract

We investigate two types of internal light-extraction layer structures for organic light-emitting diodes (OLEDs) that consist of silica nanoparticles (NPs) embedded in high-refractive-index TiO2 matrices. The composite of silica NPs and TiO2 matrices was coated on the glass substrate and fabricated with and without a SiO2 planarization layer. An increase in the optical out-coupling efficiency by a factor of 2.0 was obtained at a high luminance of 3,000 cd/m2 from OLEDs containing the silica NPs embedded in TiO2 matrices between glass substrates and Zn-doped In2O3 (IZO) electrodes after additional planarization processes. This is consistent with the analytical result using the finite-difference time-domain (FDTD) method. Randomly distributed silica NPs acting as scattering centers could reduce the optical loss when extracting light. By using additional planarization processes with a PECVD-derived SiO2 layer, one can assure that smoother surfaces provide higher out-coupling efficiency, which attain 100% and 97% enhancements in power (lm/W) and current (cd/A) efficiencies, respectively.

© 2014 Optical Society of America

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Figures (9)

Fig. 1
Fig. 1 The schematic drawings (a,b) and in-plane AFM images (c,d) for devices A and B. The tilted AFM images are added as insets.
Fig. 2
Fig. 2 TEM cross-sectional images of the light out-coupling layers introduced to (a)−(c) device A and (d)−(f) device B. The overview (a,d), clear interfaces between layers (b,e), and the composite layers of SiO2 and TiO2 (c,f) are shown with high resolution, respectively.
Fig. 3
Fig. 3 SEM cross-sectional images of light-scattering NPs layers for (a) device A and (b) device B, confirming the air voids, silica, and TiO2 structures.
Fig. 4
Fig. 4 J–V characteristics of fabricated reference OLEDs (Refs. A and B) and devices A and B.
Fig. 5
Fig. 5 (a,d) Luminance (cd/m2) and (b,e) power (lm/W), and current efficiency (cd/A) as a function of current density (mA/cm2) for the reference OLEDs and devices. In addition, (c) and (f) show the enhancement ratio for both efficiencies as a function of luminance, and (f) includes photographs of OLEDs without (left) and with (right) a light-extraction layer (inset).
Fig. 6
Fig. 6 Angular distribution of emitted light for (a) Ref. A and device A and (b) Ref. B and device B. The solid lines represent Lambertian distributions. The emitted light intensities of each reference are normalized by those of the corresponding device (A and B, respectively) in the normal direction.
Fig. 7
Fig. 7 The simulated light-extraction enhancement ratios for different structures of (a) silica beads embedded in a TiO2 composite layer without SiO2 planarization, (b) a smooth TiO2 matrix with a SiO2 planarization layer, and (c) a rough TiO2 matrix with a SiO2 planarization layer.
Fig. 8
Fig. 8 Light-extraction enhancement contour map resulting from FDTD numerical analysis for a simple structure using a silica and TiO2 composite.
Fig. 9
Fig. 9 The variation of the enhancement ratio with respect to the air void content.

Tables (2)

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Table 1 Optical Characteristics and Surface Roughness Measurements for Each Device

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Table 2 Power Efficiencies of the Fabricated Devices and Their Simulated and Measured Enhancements

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