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Design of red, green, blue transparent electrodes for flexible optical devices

Sungjun Kim, Hyung Won Cho, Kihyon Hong, Jun Ho Son, Kisoo Kim, Bonhyeong Koo, Sungjoo Kim, and Jong-Lam Lee

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Abstract

Controlling the wavelength of electrodes within a desirable region is important in most optoelectronic devices for enhancing their efficiencies. Here, we investigated a full-color flexible transparent electrode using a wavelength matching layer (WML). The WMLs were able to adjust the optical-phase thickness of the entire electrode by controlling refractive indices and were capable of producing desirable colors in the visible band from 470 to 610 nm. Electrodes with tungsten oxide (WO3) having a refractive index of 1.9 showed high transmittance (T = 90.5%) at 460 nm and low sheet resistance (Rs = 11.08 Ω/sq), comparable with those of indium tin oxide (ITO, T = 86.4%, Rs = 12 Ω/sq). The optimum structure of electrodes determined by optical simulation based on the characteristic matrix method agrees well with that based on the experimental method. Replacing the ITO electrode with the WO3 electrode, the luminance of blue organic light-emitting diodes (λ = 460 nm) at 222 mA/cm2 increased from 7020 to 7200 cd/m2.

© 2014 Optical Society of America

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Figures (10)
Fig. 1
Fig. 1 Schematic illustrations of OLEDs with (a) tunable dielectric-metal multilayer with control of thickness, (b) the admittance diagram of the system, (c) tunable dielectric-metal multilayer with control of refractive index, and (d) the admittance diagram of the system.

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Fig. 2
Fig. 2 (a) Sheet resistance of Ag films as a function of Ag thickness. (b) Secondary cut-off spectra of ITO, Ag and Ag/WO3. (c) Secondary cut-off spectra. (d) Power efficiency of OLEDs as a function of WO3 thickness.

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Fig. 3
Fig. 3 Calculated value (line) of transmittance of dielectric/Ag/WO3 as function of (a) refractive indices and (b) thickness of dielectric. Peak transmittance of dielectric/Ag/WO3 as a function of (c) refractive indices and (d) thickness of dielectric and the optical-phase thickness of dielectric layer at the peak transmittance.

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Fig. 4
Fig. 4 Simulated contour plots of transmittance and reflectance for Dielectric/Ag/WO3 multilayers upon variation of the (a) (c) refractive index of the dielectric and (b) (d) thickness of dielectric (n = 1.9). The calculated admittance diagram of the system of (e) variation of refractive index and (f) thickness.

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Fig. 5
Fig. 5 Measured transmittance of (a)WO3/Ag/WO3 with various thickness of WO3 and (b) WO3/Ag/WO3, ZnS/Ag/WO3, and TiO2/Ag/WO3.

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Fig. 6
Fig. 6 Luminance-current density-voltage characteristics of (b) blue OLEDs, (c) green OLEDs, and (d) red OLEDs with ITO and DMD electrodes. (Three emissive layers: TCTA:FIr6 (460 nm), Alq3:C545T (525 nm), CBP:Ir(piq)3 (620 nm) (d) EL spectra of blue, green, and red OLEDs with ITO, and DMD electrodes.. (e) Enhancement factor of devices using DMD compared to ITO with three emissive layers. (f) Sheet resistance after repeated bending as function of the number of cycles for ITO and WO3/Ag/WO3.

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Fig. 7
Fig. 7 Simulated electric field distribution of OLEDs with (a) WO3/Ag/WO3, (b) ZnS/Ag/WO3, (c) TiO2/Ag/WO3 electrodes, and (d) Simulated angular emission pattern of OLEDs with DMD electrodes.

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Fig. 8
Fig. 8 (a) Transmittance at 460 nm and (b) transmittance spectra of Glass/WO3 (30 nm)/Ag (12 nm)/WO3 (20 nm) as a function of incident angle.

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Fig. 9
Fig. 9 Complex refractive indices of (a) Ag and (b) WO3, ZnS, TiO2 (23°C), and TiO2 (400°C) as a function of wavelength.

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Fig. 10
Fig. 10 Simulated contour plots of transmittance for (a) WO3/Ag/WO3, (b) ZnS/Ag/WO3, and (c) TiO2/Ag/WO3 multilayers upon variation of the thickness of the WO3 and Ag layers. Calculated value (line) of transmittance for (a) WO3/Ag/WO3 (λ = 460 nm) (b) ZnS/Ag/WO3 (λ = 520 nm), and (c) TiO2/Ag/WO3 (λ = 620 nm) electrode as a function of outer dielectric layer and Ag thicknesses. Corresponding results obtained from experiments (symbols) are also shown for comparison.

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Equations (4)

Equations on this page are rendered with MathJax. Learn more.

Y=H/E=N y 0 ,
Y= cosδ Y sub +i n f sinδ cosδ+i sinδ n f Y sub .
δ= 2π λ Ndcosθ,
R = | Y a i r − Y Y a i r + Y | 2 ,
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