To summarize the operational lifetimes of both single element and stacked OLEDs, a comparison is shown in Figure 3.14. The advantage to using a SOLED that distributes the EML between elements, while reducing operating current to achieve a desired brightness, is readily apparent.
Finally, the committee notes that elevated temperature can significantly reduce the OLED operational lifetime. At surface luminances of 8,000 cd/m2, it has been found that a 10°C increase in temperature reduces the lifetime by as much as 30 percent (see panel a of Figure 3.15). Fortunately, because OLEDs are highly distributed lighting sources, their temperature rise during operation is minimal (Levermore et al., 2012). Indeed, with proper packaging, the rise in temperature even at high surface luminances of 3,000 cd/m2 can be less than 1°C using only natural convection present in the ambient surrounding the fixture, as shown in panel b of Figure 3.15.
FINDING: OLEDs are area light sources, and their rise in temperature, even at the highest drive currents (and hence brightness), is minimal. This is a major distinction from LEDs, which are intense point light sources and, hence, operate at high temperatures that require extensive heat sinking and care in their installation. Nevertheless, OLED operational lifetime is very sensitive to temperature increases. As the room temperature rises, the OLED lifetime can be expected to be noticeably decreased.
RECOMMENDATION 3-9: The Department of Energy should support the pursuit of material sets and device architectures that would increase the useful operational lifetimes of high-intensity white organic light-emitting diodes.
FIGURE 3.14 Comparison of lifetimes of three different white PHOLED emitters at two different surface luminance intensities. SOURCE: Courtesy of Universal Display Corporation.
There is as yet no large-scale manufacture of OLED lighting; however, major growth in OLED display technologies may provide both infrastructure and cost reduction and, thus, important incentives for the further development of manufacturing for OLED-based SSL. As of this writing, one company alone, Samsung Mobile Displays (SMD), is producing 30 million such displays per month, with plans to scale these devices to larger, three-dimensional displays. SMD’s major competitor in this space is LG Display, along with a handful of other display companies in Asia of varying
FIGURE 3.15 Time to LT80 for a white PHOLED with an initial surface luminance of Lo = 8,000 cd/m2. (b) Infrared image showing the surface temperature of a 7 cm × 15 cm panel as in (a) after 60 min operation in a 24°C ambient temperature. SOURCE: Levermore et al. (2012).