Abstract

Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/N, N’-diphenyl-N, N’-(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD) (90 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turn-on voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.

© 2005 Optical Society of America

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References

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Appl. Phys. Lett. (10)

M. H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack, and J. J. Brown, �??High-efficiency top-emitting organic light-emitting devices,�?? Appl. Phys. Lett. 81, 3921-3923 (2002).
[CrossRef]

G. Pathasarathy, P. E. Burrows, V. Khalfin, V. G. Kozolov, and S. R. Forrest, �??A metal-free cathode for organic semiconductor devices,�?? Appl. Phys. Lett. 72, 2138-2140 (1998).
[CrossRef]

L. S. Hung and C. W. Tang, �??Interface engineering in preparation of organic surface-emitting diodes,�?? Appl. Phys. Lett. 74, 3209-3211 (1999).
[CrossRef]

S. Han, X. Feng, Z. H. Lu, D. Johnson, and R. Wood, �??Transparent-cathode for top-emission organic light-emitting diodes,�?? Appl. Phys. Lett. 82, 2715-2717 (2003).
[CrossRef]

L. S. Hung, C. W. Tang, M. G. Mason, P. Raychaudhuri, and J. Madathil, �??Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes,�?? Appl. Phys. Lett. 78, 544-546 (2001).
[CrossRef]

C. W. Chen, P. Y. Hsieh, H. H. Chiang, C. L. Lin, H. M. Wu, and C. C. Wu, �??Top-emitting organic light-emitting devices using surface-modified Ag anode,�?? Appl. Phys. Lett. 83, 5127-5129 (2003).
[CrossRef]

R. B. Pode, C. J. Lee, D. G. Moon, and J. I. Han, �?? Transparent conducting metal electrode for top emission organic light-emitting devices: Ca�??Ag double layer,�?? Appl. Phys. Lett. 84, 4614-4616 (2004).
[CrossRef]

G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest, and M. E. Thompson, �??Transparent organic light emitting devices,�?? Appl. Phys. Lett. 68, 2606-2608 (1996).
[CrossRef]

H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieβ, �??Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling,�?? Appl. Phys. Lett. 82, 466-468 (2003).
[CrossRef]

C. C. Wu, C. L. Lin, P. Y. Hsieh, and H. H. Chiang, �?? Methodology for optimizing viewing characteristics of top-emitting organic light-emitting devices,�?? Appl. Phys. Lett. 84, 3966-3968 (2004).
[CrossRef]

Chem. Phys. Lett. (1)

Z. Xie, L. S. Hung, and F. Zhu, �??A flexible top-emitting organic light-emitting diode on steel foil,�?? Chem. Phys. Lett. 381, 691-696 (2003).
[CrossRef]

J. Appl. Phys. (3)

P. E. Burrows, G. Gu, S. R. Forrest, E. P. Vicenzi, and T. X. Zhou, �??Semitransparent cathodes for organic light emitting devices,�?? J. Appl. Phys. 87, 3080-3085 (2000).
[CrossRef]

H. Riel, S. Karg, T. Beierlein, W. Rieβ, and K. Neyts, �??Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study,�?? J. Appl. Phys. 94, 5290-5296 (2003).
[CrossRef]

S. Han, D. Grozea, C. Huang, Z. H. Lu, R. Wood, and W. Y. Kim, �??Al:SiO thin films for organic light-emitting diodes,�?? J. Appl. Phys. 96, 709-714 (2004).
[CrossRef]

Opt. Express (1)

Supplementary Material (1)

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Figures (5)

Fig. 1.
Fig. 1.

Schematic cross-sectional diagram of the TOLED structure.

Fig. 2.
Fig. 2.

(1.6 MB Movie) Photo image of the TOLED, (a) Green light is from ITO glass substrate side; (b) Green light is from top cathode side.

Fig. 3.
Fig. 3.

Current density (a) luminance (b) versus voltage characteristics of the TOLED.

Fig. 4.
Fig. 4.

Current efficiency versus voltage characteristics of the TOLED.

Fig. 5.
Fig. 5.

Transmittance spectra of the TOLED and ITO glass substrate used. For TOLED, the transmittance refers to that of the organic layers and metal cathode, but excluding the ITO glass.

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