Research and Development

Tomoyuki Nakayama, Kunimasa Hiyama, Keiichi Furukawa, and Hirofumi Ohtani
Konica Minolta Technology Center, Inc., Display Technology R&D Laboratories,

Major performance is shown in Table 1. As for a warm white color device whose color temperature is 4000K and CIE1931 color coordinates of (0.39, 0.43), 20%　EQE, drive voltage of 3.7V and power efficiency of 37lm/W were obtained at 1000cd/m². With light out-coupling technique in which optical film was set up on surface of a glass substrate, 34% EQE and 64lm/W of power efficiency were achieved at 1000cd/m². In addition, CRI value was 76. We can change color temperature and CRI value comparatively by balancing blue, green and red dopants.

Fig.2 shows dependence of EQE on luminance. It shows that 20% EQE was obtained at 1000cd/m².. The dependence of EQE on luminance is small although there is a tendency that EQE slightly falls in the high luminance side for a peak at 100cd/m². Therefore we can assume that the organic layer structure maintains good carrier balance over a wide luminance range. Fig.3 shows dependence of luminance on drive voltage.

These performances were achieved by the following technology.

Primarily, we use blue phosphorescent dopant and host materials with high emission efficiency and long Lifetime. The performance of these materials was 17% EQE and Lifetime of 16000 hours at initial luminance of 300cd/m². Using these materials developed, realization of the all phosphorescent white OLED device which had high efficiency was enabled.

Second, we designed the emission layers to achieve 20% EQE.　External quantum efficiencies of the blue, yellow-green and red dopants in the layer structure which was similar in the white OLED were 17%, 17%, 12% each. It is possible to calculate EQE of white OLED by using these values. If each dopant emits light with above EQE in the white OLED, the calculated EQE of white light becomes only about 15%. This calculated efficiency does not reach to 20% EQE of our target. One possibility to raise the EQE further is to use carrier blocking technique in which carrier blocking layers are set at both sides of the emitting layer. However, this technique is often accompanied with deterioration of Lifetime because of carrier accumulation in emission layer – blocking layer interface. We achieved EQE of 20% without using such a technique by optimizing layer structure and dopant distribution in the emission layers.

Third, doping technology to the electron transport layer was introduced to achieve low drive voltage. The drive voltage fell 2V - 3V in comparison with a case that Alq₃/LiF was used in electron transport and injection layers. It contributes improvement of power efficiency greatly. Although it is known till now about metal doping technique to electron transport layer, there is concern about the durability. We were able to obtain the electron transport layer with superior durability described in the following chapter by developing electron transport materials.