Cavity Light-Emitting Diode for Durable, High-Brightness and High-Efficiency Lighting Applications: First Budget Period Technical Report

Abstract

A COLED device consists of a top electrode (anode) and a bottom electrode (cathode) separated by a thin dielectric layer. In this metal/dielectric stack, numerous small wells, or cavities, are etched through the top electrode and the dielectric layer. These cavities are subsequently filled with LEP molecules. When a voltage is applied between the top and bottom electrodes, holes (from the top electrode) and electrons (from the bottom electrode) are injected into the polymer. Light emission is generated upon recombination of holes and electrons within the polymer along the perimeters of cavities. Figure 1 compares the structures of the COLED and the traditional OLED. The existing COLED fabrication process flow is illustrated in Figure 2. A COLED can potentially be 5 times more efficient and can operate at as much as 100 times higher current density with much longer lifetime than an OLED. To fully realize these potential advantages, the COLED technology must overcome the following technical barriers, which were the technical focused points for Years 1 and 2 (Phase I) of this project: (1) Construct optimum thickness dielectric layer: In the traditional OLED structure, the optimal thickness of the LEP film is approximately 80-100 nm. In a COLED device, the effective LEP thickness roughly equals the thickness of the dielectric layer. Therefore, the optimal dielectric thickness for a COLED should also be roughly equal to 80-100 nm. Generally speaking, it is technically challenging to produce a defect-free dielectric layer at this thickness with high uniformity, especially over a large area. (2) Develop low-work-function cathode: A desired cathode should have a low work function that matches the lowest unoccupied molecular orbital (LUMO) level of the LEP molecules. This is usually achieved by using a low-work-function metal such as calcium, barium, lithium, or magnesium as the cathode. However, these metals are very vulnerable to oxygen and water. Since the cathode of the COLED will be exposed to air and processing chemicals during the COLED fabrication process, these low-work-function metals cannot be used directly in the COLED structure. Thus, new materials with low work function and better chemical stability are needed for the COLED cathode. (3) Increase active device area: Since photons are only generated from perimeters of the cavities, the actual active area in a COLED device is smaller than the device surface area. The cavity diameter and cavity spacing of the COLED devices previously produced at SRI by conventional photolithography processing are typically in the range of 3 to 7 {mu}m with an estimated active area of 2-3%. To achieve the same brightness of a traditional OLED at the same applied voltage, the active device area of a COLED should be at least 20% (1/5) of the device surface area, provided the COLED has 5 times higher EQE. This requires reducing the cavity diameter and cavity spacing to the sub-micrometer region, which can be achieved by electron-beam lithography or nanoimprint lithography. (4) Improve metal/polymer interfaces: The polymer/metal interfaces are critical issues to improve and optimize since they directly affect the effectiveness and balance of hole and electron injection, and consequently the device performance. Conventional approaches for improving a metal/polymer interface include deposition of a special interfacial material on the selected electrode surface or applying a proper surface treatment prior to deposition of the LEP. Since these approaches are generally nonselective to the cathode and anode, they cannot be directly adopted for COLED devices. Generally, the interface integration in current OLED technology still needs a better chemical approach. Hence, advanced methodology developed for the COLED technology as promoted in this project may be also suitable for other OLED devices.