Tandem organic light-emitting diodes (OLEDs), with multiple electroluminescent (EL) units connected electrically in series, have been fabricated. Using an optically transparent doped organic “p-n” junction as the connecting unit between adjacent EL units, excellent light out-coupling and carrier-injection properties have been realized. The luminous efficiency is found to scale almost linearly with the number of EL units in the stack, giving values as high as 32 or 136 cd/A for a three-unit tandem OLED using a fluorescent or a phosphorescent emitter, respectively.

When a tandem light-emitting diode (OLED) utilizes unoptimized electroluminescent (EL) units, it is fairly easy to improve the power efficiency of such a device. However, when a tandem OLED utilizes optimized EL units, improved power efficiency can only be achieved if each intermediate connector has excellent carrier injection capabilities along with a negligible voltage drop across it. Four organic intermediate connectors were studied in this work, one of which consisting of a Li-doped 4,7-diphenyl-1,10-phenanthroline layer and a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile layer, exhibits the best power efficiency improvement for tandem OLEDs.

ment effect at the higher energy side more easily than quantum wires, we expect the SiNTs to be more useful for future optoelectronic devices. ExperimentalPorous alumina templates were prepared by an anodization process. An aluminum foil (99.99 %, 0.13 mm thick, Aldrich) was electropolished in a perchloric acid/ethanol solution (ethanol (95 vol.-%)/perchloric acid (70 vol.-%) = 5:1) at 9 C at a constant dc (direct current) voltage of 18 V for 1.5 min. This cleaned aluminum sheet was then anodized in a 0.3 M oxalic acid solution at 5 C at a constant applied voltage of 40 V for 6 h. The resultant aluminum oxide layer was removed by dipping into an aqueous mixture of phosphoric acid (6 wt.-%) and chromic acid (1.8 wt.-%) at 60 C. The second anodization was performed for 20 min at the same conditions in order to form a regular pore array of hexagons. After pore widening by dipping into an aqueous solution of 0.1 M phosphoric acid for 10 min, we performed heat treatment to enhance the crystallinity of the alumina templates at 500 C for 30 min under Ar atmosphere. A regular array of hexagonal porous alumina was formed on both surfaces of aluminum film. This template was brought into the MBE chamber to grow SiNTs, where the chamber was evacuated to a pressure of 5 10 ±10 torr. The Si atoms/clusters were supplied for 10 min by electron-beam evaporator with a growth rate of 0.07 s ±1. The substrate temperature was maintained at 400 C in order to prevent the aluminum layer from being melted. After growth, the sample was further heat treated at 600 C or 750 C under ambient conditions for oxidation.The morphology of the SiNTs was observed by the field-emission scanning electron microscope (FESEM, JEOL-JSM6770F). High-resolution transmission electron microscopy (HRTEM, JEOL-JEM3011, 300 keV) was also used to determine the atomic details of the SiNTs. For HRTEM observations, small pieces of SiNTs were peeled off from the alumina surface and suspended, followed by further dispersion in isopropanol simply by stirring and sonication. The solvent containing the SiNTs was dropped on a holey carbon micro-grid. The photoluminescence (PL) spectra were measured with a He±Cd laser (325 nm). The laser beam diameter was about 0.3 mm with a power of 10 mW.