ORGANIC LIGHT EMITTING DIODES WITH STRUCTURED ELECTRODES
Applicants: THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
Abstract:
A cathode that contain nanostructures that extend into the organic layer of an OLED has been described. The cathode can have an array of nanotubes or a layer of nanoclusters extending out from its surface. In another arrangement, the cathode is patterned and etched to form protruding nanostructures using a standard lithographic process. Various methods for fabricating these structures are provided, all of which are compatible with large-scale manufacturing. OLEDs made with these novel electrodes have greatly enhanced electron injection, have good environmental stability.
[0008]As shown in the schematic in FIG. 1, an OLED 100 has an emissive layer 110, a transport layer 120, an anode 130 and a cathode 140, all on a substrate 150. The layers 110, 120 are made of organic semiconducting small molecules or polymers. When a voltage is applied across the OLED 100 such that the anode 120 is positive with respect to the cathode 140, the cathode 140 injects electrons 145 into the emissive layer 110 and the anode 130 injects holes 135 into the transport layer 120. The electrons 145 and the holes 135 move toward each other and they recombine. The recombination produces an emission of radiation 160 whose frequency is typically in the visible, may also be in the infrared and ultraviolet regions.
Nanotube and Nanocluster Cathodes
[0022]FIG. 2 is a schematic cross section drawing that shows an embodiment of the invention that uses a nanotube-based cathode. An OLED 200 has a cathode 240, a light-emitting organic layer 210, a transport layer 220, an anode 230, and a substrate 250. The anode 230 may be made of a transparent material, such as indium tin oxide (ITO) and the substrate 250 may be any known substrate such as plastic, glass, and the like. Light may be emitted in the direction of arrows 260 or in the opposite direction.
[0023]The cathode 240 has a plurality of nanostructures 242 extending outwardly into the light-emitting organic layer 210. The nanostructures 242 may be any type of structure such as nanotubes, nonorods, or nanoclusters. The nanostructures 242 can be nanotubes grown out from a cathode substrate 240. Alternatively, the nanostructures 242 can be nanoclusters deposited onto the cathode substrate 240.
[0024]Nanotubes are good field emitters because of their small tip radii, which can range from approximately one nanometer to as much as a micron. The smaller the tip radius the stronger the concentration of the electric field at the tip. A high electric field at the tip causes a high electron ejection rate, which results in very efficient ejection of electrons. In addition to improving electron injection from the cathode 240 to the light-emitting organic layer 210, the small tips and even distribution of the nanotubes 242 provide a balanced charge distribution in the device, reduce exciton quenching near the cathode 240, and allow for the use of lower voltages to achieve electron emission. Furthermore, carbon nanotubes are chemically stable, decreasing the environmental sensitivity of the cathode 240.
USPTO
WIPO
