Assessment of Advanced Solid-State Lighting (2013) / Chapter Skim
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3 Assessment of LED and OLED Technologies
3 Assessment of LED and OLED Technologies
Pages 34-56
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From page 34... ...
Although red, green, and for spot illumination and downlighting. The committee finds yellow LEDs have been available since the 1970s, the advent value in supporting rapid developments in both technologies, of high-brightness blue LEDs in 1993 made high-efficiency because they both represent large possible markets, new white lighting sources possible.
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From page 35... ...
to both inorganic and organic LEDs in a parallel approach. Semiconductors can also absorb and emit light, and The LED and OLED primers will first focus on the basic the relevant wavelengths are related to the bandgap of the device structure and metrics of device performance.
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From page 36... ...
. In 1992, Nakamura, working at Nichia, developed an industrially robust process The LED Device Structure for p-doping of GaN that led to the first high-brightness blue LEDs.
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From page 37... ...
it passivates or tum wells, and the P-type GaN. Most GaN LED devices protects the active semiconductor material from degradation are formed on a sapphire substrate through the MOCVD due to the environment (principally moisture)
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From page 38... ...
that is injected into the device is reflected in the IQE. ηIQE can be maximized by using quantum well structures as described above, by utilizing defect-free semiconductor material, and Metrics of Device Performance by ensuring high-quality, very-low-resistance metal contacts Efficiency is an important metric of LED device per- to the device.
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From page 39... ...
As was discussed above, input. Low resistive power loss, high ηIQE, and good design ηIQE of green LEDs is much lower than that of blue LEDs.
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From page 40... ...
Currently, blue LEDs lowest energy in the visible spectrum -- cannot be used with have a peak wavelength of 455 nm and a width of 15 nm. phosphors to generate white light; instead, a short-wavelength Any changes in the peak position or width can visibly affect UV, violet, or blue LED is required (Denbaars et al., 2013)
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From page 41... ...
A particularly critical issue is that the cur Phosphor white LEDs have a clear ease-of-use advantage rent white LED materials substrate and growth technology and dominate the current SSL general illumination market. do not produce LED devices that are uniform with respect Although the resultant products do not offer the same flex- to their color quality or efficiency.
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From page 42... ...
Sensitive control over the composite layers of the crys- Prior research on MOCVD technology has established the talline LED device structure, some of which are only nano- fundamental understanding of reactor design and scale-up. meters in thickness, is achieved through the use of pitaxial e Excellent numerical codes are available to simulate the gas growth processes.
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From page 43... ...
GaN, and so forth. Besides improvement in device efficiency Current sapphire substrates are 4 inches in diameter.
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From page 44... ...
projection of LED package costs (Figure 3.8) , improveAs has been discussed above, current devices suffer from a ments in the cumulative yield will benefit enormously from high concentration of defects and dislocations that limit the improvements in the earlier part of the manufacturing prointernal quantum efficiency achievable.
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From page 45... ...
primarily deployed today in very large numbers for displays on handheld appliances such as smart phones. The excitement surrounding OLED technology stems from several The OLED Device Structure and Operation unique aspects of its manufacture and performance.
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From page 46... ...
Here the optical power injection of electrons. Typical OLED structures used in high- out per the input electrical power is related to the quantum efficiency and high-reliability applications are considerably efficiency following the formula: more complex than the structure shown in Figure 3.9.
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From page 47... ...
Even with these limi tations, the power efficiency of phosphorescent white organic light-emitting devices can exceed 150 lm/W, making them especially attractive for use as efficient lighting sources. CONTROLLING THE COLOR OUTPUT OF THE OLED For OLEDs, changing the composition of the molecular FIGURE 3.10 Illustration of the optical pathways taken by a components of the material influences the wavelength (color)
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From page 48... ...
the current through each of the elements can be used to tune Finally, the committee notes that there are several other the color, from their constituent color to any desired white approaches to generating white light. Two alternatives that chromaticity.
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From page 49... ...
That is, small concentrations of molecular impurities fortuitously, only 25 percent of the color content of white light is can lead to rapid degradation in device performance, and blue. Thus using fluorescent molecules for blue OLED emission hence considerable steps must be taken to ensure their purity.
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From page 50... ...
Other losses due to absorption in emit light. Hence, the use of an EBL has greatly increased the organic or transparent conducting oxide anode layers, as the efficiency of PHOLEDs such that 100 percent IQE is well as excitation of so-called plasmons at the metal cathode routinely obtained using optimized materials sets.
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From page 51... ...
is should support research to understand the fundamental increased. This is readily apparent in Figure 3.13 where the nature of efficiency droop at high currents in organic lightexternal quantum efficiencies of archetype fluorescent and emitting diodes and to seek means to mitigate this effect phosphorescent devices are shown as functions of drive cur- through materials and device architectural designs.
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From page 52... ...
Finally, white light does not require the use of deep low-cost fabrication of large area OLED lighting sources blue (i.e., high-energy) emission.
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From page 53... ...
FIGURE 3.14 Comparison of lifetimes of three different white 3.14.eps PHOLED emitters at two different surface luminance intensities. FINDING: OLEDs are area light sources, and their rise bitmap SOURCE: Courtesy of Universal Display Corporation.
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From page 54... ...
mother glass substrates, is resulting Energy should aggressively fund the development of all in a precipitous decrease in the cost of OLED technology, possible routes leading to significant (100×) cost reduction while increasing performance, as the industry grows, thereby in organic light-emitting diode lighting sources.
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From page 55... ...
and costs of scaling up this device technology into a practi 1999a. Very high efficiency green organic light emitting devices based cal and ubiquitous lighting source.
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From page 56... ...
2009. Solid State Lighting Research pplied Physics Letters 64:1687-1689.
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