FIGURE 3.12 Polymer sheet of 5 ìm diameter microlenses attached to the glass surface of an OLED. Outcoupling enhancements of a factor of 2 are possible using this approach. SOURCE: Sun (2008). Reprinted by permission from Macmillan Publishers Ltd.
OLED structure itself without degrading device performance in other, unintended ways.
Generally, to couple waveguide modes into the glass or air, there must be a surface texture inserted at the transparent anode/organic interface. The length scale of the texture cannot be on the order of the emission wavelength; otherwise, an undesirable angular dependence of emission wavelength (i.e., color) and/or intensity may result. Low-index grids consisting of a dielectric such as silicon dioxide residing at this interface have been shown to outcouple almost all of the waveguide modes without significant losses (Sun and Forrest, 2008). The openings in the grids are typically 5 μm, with grid lines of only 1 μm. Combining the grid with the microlenses shown in Figure 3.12 has resulted in the demonstration of 34 percent external efficiency.
FINDING: A number of promising approaches have been developed to increase outcoupling efficiency.
RECOMMENDATION 3-6: The Department of Energy should focus on efforts that result in significant light outcoupling enhancements for OLED that are low-cost to implement and are independent of both wavelength and viewing angle.
As in the case of LEDs, OLEDs also suffer a loss of efficiency as the current (and corresponding brightness) is increased. This is readily apparent in Figure 3.13 where the external quantum efficiencies of archetype fluorescent and phosphorescent devices are shown as functions of drive current. This droop is fundamentally related to the molecular excited state (exciton) that, when de-excited, emits light. At very high intensity, a substantial fraction of the emitting molecules in the EML are excited. When the excitation migrates from molecule to molecule, it has a possibility of colliding with another excitation on the same molecule or on an electron or hole that is transiting the EML. This collision results in the loss (or de-excitation) of one of the two excited states, ultimately resulting in the loss of efficiency. This process is known as “exciton annihilation.” Importantly, this same process leads to the degradation of the molecules and hence a decrease in OLED operational lifetime, as discussed below. Hence, it is essential to find device architectures that minimize exciton annihilation processes. One method to effect this is, for example, extending the thickness or grading the dopant concentration within the EML. However, little work has been done to date to reduce or even eliminate droop.
FIGURE 3.13 Efficiency droop in phosphorescent OLEDs (black rectangles) and fluorescent OLEDs (red circles).
One important difference between OLEDs and LEDs, however, is that in the former case, there is no thermally driven droop effect. That is, as the temperature is varied, the efficiency characteristics in Figure 3.13 are largely unaffected.
FINDING: OLEDs show a decrease in efficiency as the current is increased. This results in a reduction in efficiency at high brightness.
RECOMMENDATION 3-7: The Department of Energy should support research to understand the fundamental nature of efficiency droop at high currents in organic lightemitting diodes and to seek means to mitigate this effect through materials and device architectural designs.