New Organic LED Designs Presented
Anne L. Fischer
Organic LEDs (OLEDs) are of interest as low-cost replacements for LCDs because they are easier to fabricate and do not require a backlight to function, potentially making them more energy efficient. Two groups of researchers at National Taiwan University in Taipei have developed techniques for OLED design that may improve the performance of these emitters.
In a new design for an organic LED (OLED), the emitters are moved from the first to the second antinode. The OLED’s internal emission is coupled into different modes (a). The forward brightness of the OLEDs is shown as a function of the distance between emitters and the reflective metal electrode (b).
The traditional OLED features organic layers sandwiched between electrodes. A problem is that much of the internally generated light is lost as a result of total internal reflection associated with the high refractive indices of organic materials and substrates, and because of the optical losses associated with the strongly reflective metal electrodes. The first group of researchers thus moved the emitters from the first to the second antinode of the reflective metal electrode to try to boost output.
The device structure for the microcavity two-unit tandem OLED is shown (a). A conventional and a microcavity two-unit tandem device demonstrated a two- and a fivefold enhancement in efficiency, respectively (b).
Chung-Chih Wu, a professor of electrical engineering at the university, explained that the scientists knew that the emitters had to be placed on the antinodes of the metal electrodes to obtain constructive interference and to optimize the light extracted from the device. In conventional designs, the emitters are placed at the first antinode, but the investigators wondered if there would be a benefit in moving them to a farther antinode.
To do so meant that they had to use a thicker carrier-transport layer. The trick was to keep the device’s operating voltage low, so the carrier-transport layer had to have high carrier mobility and electrical conductivity. With the recent development of high-mobility organic materials and conductive doping techniques, they achieved this without degrading carrier injection or increasing the voltage.
The result was an enhancement of the total emission by a factor of 1.2 and of the forward brightness by a factor of 1.6. Wu sees the 1.6 as particularly significant, especially when considering the simplicity of the design change compared with methods such as the incorporation of microcavities, microlenses and photonic crystals.
Wu worked with the other team on decreasing the operating current of an OLED by using a microcavity tandem structure. This effort was driven by the demand for higher brightness in some applications, requiring a higher operating current and, thus, a trade-off in reliability and lifetime.
In conventional tandem OLEDs, the brightness enhancement is proportional to the number of stacked emitting units. Theoretically, the more stacked emitting units, the more complicated the fabrication. The researchers, however, demonstrated a fivefold enhancement by incorporating a microcavity structure into a tandem OLED with only two stacking units.
Where a more complicated design is not a problem, Wu said, the two techniques could be combined to boost OLED emission even further. Unfortunately, the microcavity design prefers a thinner device structure, while the second-antinode design requires a thicker one. And in tandem devices, the structure already is relatively thick.
The two groups will work on their individual projects to boost the optical output and reduce the operating current of OLEDs, and they are working to overcome the complications and combine the methods. They are promoting the second-antinode design approach for display applications. The microcavity tandem design may be useful for display and lighting applications.
Applied Physics Letters, Feb. 20, 2006, 081114, and March 13, 2006 111106.
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