Chih-Chia An

Wet-process feasible novel carbazole-type molecular host for high efficiency phosphorescent organic light emitting diodes

Wet-process organic light-emitting diodes (OLEDs) are crucial to realize cost-effective and large-area roll-to-roll fabrication of high quality displays and lightings. In this study, a wet-process feasible carbazole based host material, 2-[4-(carbazol-9-yl)butyloxy]-9-[4-(carbazol-9-yl)butyl]carbazole (6), is synthesized, and two other carbazole hosts, 2-[5-(carbazol-9-yl)pentyloxy]-9-[5-(carbazol-9-yl)pentyl]carbazole (7) and 2-[6-(carbazol-9-yl)hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole (8) are also synthesized for comparison. All the three host materials exhibit high triplet energy, and possess high solubility in common organic solvents at room temperature. On doping a green phosphorescent emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)3) into host 6, the device shows an efficacy of 51 lm W−1 and a current efficiency of 52 cd A−1 at 100 cd m−2 or 30 lm W−1 and 40.7 cd A−1 at 1000 cd m−2. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, the ability for excitons to generate on both host and guest, and excellent film integrity.

Organic Light-Emitting Diode with Color Tunable between Bluish-White Daylight and Orange-White Dusk Hue

The varying color of sunlight diurnally exhibits an important effect on circadian rhythm of living organisms. The bluish-white daylight that is suitable for work shows a color temperature as high as 9,000 K, while the homey orange-white dusk hue is as low as 2,000 K. We demonstrate in this report the feasibility of using organic light-emitting diode (OLED) technology to fabricate sunlight-style illumination with a very wide color temperature range. The color temperature can be tuned from 2,300 K to 9,300 K, for example, by changing the applied voltage from 3 to 11 V for the device composing red and yellow emitters in the first emissive layer and blue emitter in the second. Unlike the prior arts, the color-temperature span can be made much wider without any additional carrier modulation layer, which should enable a more cost effective fabrication. For example, the color-temperature span is 7,000 K for the above case, while it is 1,700 K upon the incorporation of a nanoscale hole modulation layer in between the two emissive layers. The reason why the present device can effectively regulate the shifting of recombination zone is because the first emissive layer itself possesses an effective hole modulation barrier of 0.2 eV. This also explains why the incorporation of an extra hole modulation layer with a 0.7 eV barrier did not help extend the desirable color-temperature span since excessive holes may be blocked.

High efficiency yellow organic light-emitting diodes with a solution-process feasible iridium based emitter

Yellow emission plays an important role in many display and lighting applications, such as RGBY display or blue hazard free lighting sources, while a wet-process enables soft devices to be manufactured cost-effectively. We demonstrate here that high efficiency yellow organic light-emitting diodes can be made with an innovative solution-process viable iridium based material, bis[5-methyl-7-fluoro-8-trifluoromethyl-5H-benzo(c)(1,5)naphthyridin-6-one]iridium(picolinate) (FCF3BNO). The device exhibits a power efficacy of 49.2 lm W−1 at 1000 cd m−2 with an external quantum efficiency of 22.7% with a spin-coated emissive layer. A National Television Standards Committee (NTSC) color saturation of 110% is achievable by employing the yellow emission along with the standard RGB. The high efficiency may be attributed to the electron-withdrawing F and CF3 substitutions in the emitter to prevent self-quenching from dense packing. We also found that high efficiency can be achieved for wet-processed devices simply by balancing the injected holes and electrons with the help of energy-level matching hosts. The resultant RGBY composed of pure white light is 8% more friendly than the typical RGB counterpart from a retina protection perspective and also 8% better with respect to melatonin secretion.

Solution-processable naphthalene and phenyl substituted carbazole core based hole transporting materials for efficient organic light-emitting diodes

Solution-processable molecular hole transporting materials (HTMs) are extremely crucial in order to realize low cost, high throughput, and roll-to-roll fabrication of large area organic light emitting diodes for display and lighting applications. In this report, a series of naphthalene and phenyl substituted carbazole core based HTMs, 3-(1-naphthyl)-9-(2-phenylvinyl)carbazole (NPVCz), 3,6-di-(1-naphthyl)-9-phenylvinylcarbazole (DNPVCz), and 3,6-diphenyl-9-(2-phenylvinyl)carbazole (DPPVCz) are successfully synthesized and characterized. The synthesized HTMs possess excellent solubility in common organic solvents. By using a fluorescent tris(8-hydroxyquinolinato)aluminium emitter, we demonstrate an enhancement of 135%, from 1.7 to 4.5 cd A−1, in the current efficiency of an organic light emitting diode (OLED) by replacing the conventional HTM, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), with the NPVCz counterpart. Moreover, the current efficiency of a conventional tris[2-phenylpyridinato-C2,N]iridium(III) based phosphorescent green OLED device increases from 46.4 to 66.2 cd A−1 by substituting the NPB with NPVCz. These findings suggest that this type of solution-processable molecular HTM will be a promising contender for high efficiency OLED devices.