Study On Performance Improvement In Blue Top-emitting Organic Light-emitting Devices

This paper is focus on how to improve performances of top-emitting Organic light-emitting devices (TEOLEDs) from the following aspects: the optimization of the suitable anode buffer layer, the out-coupling layer, and the suitable electron buffer layer and cathodes. In addition, the design of the total thickness of organic layers and the out-coupling layer with microcavity theory and transfer matrix theory, respectively.1. In order to improve the hole-injection ability of Ag, MoO_x, which is a strong electron acceptor, was deposited on the surface of the Ag anode, and a series of devices with the sturctre Ag(70 nm)/MoO_x(X nm) or O₂ plasma/ N,N'-Bis(naphthalen-1-yl)-N,N'-bi(phenyl)-benzidine (NPB, 50 nm)/ tris-(8-hydroxyquinoline) aluminum(Alq3, 50 nm)/Sm(5 nm)/Ag(25nm) were fabricated to testify the effect of MoO_x. By comparison, we found that the device employing a 3 nm MoO_x buffer layer showed a best brightness (6760 cd/m²), and a maximum current efficiency (1.77 cd/A), which is 5.6 times higher than those with O₂ plasma.2. The blue emission TEOLED was fabricated with the conventional used thickness of 105 nm for the organic layer. The brightness is improved with the utilization of wide-angle interference accompany with the restraint of multiple-beam interference using a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP) layer, the optimization of the organic layers and the metal layers and the metal cathodes by using a transfer matrix theory, and the consideration of the banlance of the injection carriers. The device structure is glass/Ag (70 nm)/MoO_x (3 nm)/4,4',4"-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA, 25 nm)/NPB (X nm)/4,4'-Bis(carbazol-9-yl)biphenyl (CBP): iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) (7 wt. %, 30 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen, Y nm)/Sm (4.5 nm)/Ag (25 nm)/BCP (Z nm). The devices achieve maximum luminance of 8029 cd/m² at 14 V, and maximum luminous efficiency are 4.02 (at 25 mA/cm²) cd/A, when X is 10 nm, Y is 40 nm, and Z is 35 nm. The device demonstrates a better electroluminescent (EL) performance indicating the wide-angle interference and the effective transport of holes and electrons by adjusting the NPB and BPhen thickness are beneficial to improve the EL performance. The BCP layer leads to the blue shift of the CIE coordinates changing from (0.24, 0.47) to (0.17, 0.35). At last, as an important parameter in full-color display applications, pixel contrast ratio (PCR) was also explored. In order to enhance the PCR of the devices, a polarizer was added onto the top cathode of the blue TEOLEDs. With as high as the on-state brightness and ambient illumination of 1000 cd/m2 and 140 lx, the PCR achieved 113:1. Such high-contrast TEOLEDs incorporated into active matrix OLED displays would possess a good readability under a strong ambient illumination.3. A blue emission TEOLED, whose EL spetral didn't alter with the viewing angle, was fabricated with the thickness optimization of Sm and Ag layer. With the suitable thickness of 15 and 15 nm for Sm and Ag, the chromaticity of the blue emission was as good as (0.16, 0.38), even better than (0.20, 0.41) in the bottom-emitting blue OLEDs. The blue TEOLEDs with a good chromaticity and a wide viewing angle were expected to have fine prospection in full color display applications in the future.