FIGURE 3.1 Progress in lighting efficacy. SOURCE: DOE (2012b, p. 38).
RECOMMENDATION 3-1: The Department of Energy should continue to make investments in LED core technology, aimed at increasing yields, and in fundamental emitter research to increase efficacy, including improvements in the controlled growth and performance of the emitter material. DOE should carefully consider the range and depth of funding in its portfolio of investments in these areas, given the existing technological challenges, in order to determine how the targeted goals of device performance can indeed be met.
The remainder of this chapter will provide an introduction to both inorganic and organic LEDs in a parallel approach. The LED and OLED primers will first focus on the basic device structure and metrics of device performance. This will be followed by discussions on the control of the color output of these devices and the important influence of materials on device performance. Because OLEDs for SSL have not yet been scaled up for large-scale manufacture, the discussion for OLEDs will also encompass issues of reliability and manufacturabililty. The chapter will conclude with a comparison and summary of promises and challenges for both technologies.
Semiconductor LEDs are a special kind of electronic device that emits light upon the application of a voltage across the device. Silicon (Si) is probably the best-known semiconductor material and the basis of the integrated circuits that underlie the fast and compact electronic devices, such as computers and cell phones, that are so critical to our daily lives. LEDs are based on a semiconductor material comprised of several different elements. This material is known as a compound semiconductor. The tremendous power of semiconductors lies in their ability to take on a wide range of conductivities, from metallic to insulator. This is brought about by “doping” the semiconductor with other elements that will donate either positively or negatively charged carriers to achieve a desired conductivity.
Semiconductors can also absorb and emit light, and the relevant wavelengths are related to the bandgap of the semiconductor (see Box 3.1). The general process for light emitted in this manner is referred to as electroluminescence. The first high-efficiency light-emitting devices were developed in the 1960s utilizing gallium arsenide (GaAs), aluminum gallium arsenide (AlxGa1xAs), gallium phosphide (GaP), and gallium arsenide phosphide (GaAsxP1-x) (Hall et al., 1962; Nathan et al., 1962; Pankove and Massoulie, 1962; Woodall et al., 1972; Herzog et al., 1969). GaAs and AlGaAs LEDs produced light with infrared wavelengths, ~850 nanometers (nm), while the gallium phosphide-based LEDs produced light in the red and green wavelengths. In the early 1990s, efficient blue LEDs based on III-nitride materials began to appear based on the work of Akasaki et al. (1992) and Nakamura et al. (1994). (The III refers to elements in the third column of the periodic table, indicating that these LEDs can be comprised of alloys of aluminum nitride (AlN), gallium nitride (GaN), and indium