While organic electroluminescent devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic electroluminescent device is comprised of an anode for hole injection, a cathode for electron injection, and an organic layer sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier OLEDs are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322-334 (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent OLEDs include an organic medium consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic medium” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer OLED structure, described first by Tang et al. U.S. Pat. No. 4,356,429, one organic layer of the organic medium adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer (HTL), and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer (ETL). Recombination of the injected holes and electrons within the organic medium results in efficient electroluminescence.
There have also been proposed three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Tang et al. “Electroluminescence of Doped Organic Thin Films”, J. Applied Physics, 65, 3610-3616 (1989). The LEL commonly consists of a host material doped with a guest material. Still further, there has been proposed by Tang et al. in U.S. Pat. No. 4,769,292 a four-layer OLED adding a hole-injecting layer (HIL) between anode and the HTL. These structures have resulted in improved device performance.
Moreover, in order to further improve the performance of the OLEDs, a new kind of OLED structure called stacked OLED, which is fabricated by stacking several individual OLED vertically, has also been proposed. Forrest et al. in U.S. Pat. No. 5,703,436 and Burrows et al. in U.S. Pat. No. 6,274,980 disclosed their stacked OLEDs. In their inventions, the stacked OLEDs are fabricated by vertically stacking several OLEDs, each independently emitting light of a different color or of the same color. Using their stacked OLED structure can make fall color emission devices with higher integrated density in the display, but each OLED needs a separate power source. In alternative designs, Tanaka et al. in U.S. Pat. No. 6,107,734 and Jones et al. in U.S. Pat. No. 6,337,492 proposed a stacked OLED structure by vertically stacking several OLED without individually addressing each OLED in the stack.
The aforementioned stacked OLEDs use individual OLEDs (anode/organic medium/cathode) as building blocks to fabricate the stacked OLEDs. The complex architecture in these designs presents serious fabrication problems. It is difficult to achieve high optical transparency in the visible light range due to the presence of electrodes internal to the stack (intra-electrodes). This reduces the overall device efficiency.