Two-Carrier Multi-Layer Devices

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 Im/W, 3 Im/W, and 6 Im/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

Despite the tremendous progress that has been achieved there remain a number of critical questions that need to be resolved. In single-layer structures, a de- 

State-of-the-art polymer LEDs now have operating lifetimes and luminous efficiencies suitable for a wide variety of commercial applications. Furthermore, it is clear that the fundamental limits of polymer LED performance have not yet been reached. With improvements in material synthesis, fabrication techniques, and device design, significant increases in LED performance are to be expected. These improvements should lead to the extensive use of polymer LEDs in future display applications. 

Greenham, R.H. Friend, in Solid State Physics (Eds. H. Ehrenreich, F. Spaepen), Academic Press, New York 1995, p. 1. 

Matsumoto, in Electronic Display Devices (Ed. S. Matsumoto), John Wiley Sons, New York 1990, Chapter 1. 

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