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3 Assessment of LED and OLED Technologies Both inorganic and organic light-emitting diodes (LEDs) are integrated into the entirety of this chapter, some of the offer dramatically new sources of illumination with the committee’s major findings and recommendations are stated potential of greater efficiency, longer lifetimes, exceptional at the outset of this chapter to provide some perspective for control over the colors generated, and differing form factors. the reader and to set the tone for the rest of the chapter. In aggregate, these sources promise to redefine how light- ing that is both economically and energy efficient can be FINDING: LEDs and OLEDs are complementary lighting integrated into our daily lives. Inorganic LEDs, fabricated sources that can together offer a wide range of lighting solu- from light-emitting semiconductors, leverage a great deal tions. OLEDs can provide large-area diffuse lighting, while, of fabrication and manufacturing equipment developed for in the same venue, LEDs form intense point sources, useful electronic semiconductor devices. 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. There has been continual applications, and tremendous energy savings. progress in the wall plug efficiency of these lighting sources, resulting in current values of ~150 lumens per watt FINDING: LED and OLED efficiency and performance (lm/W) for commercial samples and the best in-lab ­ aluesv are still limited by fundamental materials issues. Improve- of 254 lm/W (Cree, 2012). Organic LEDs (OLEDs) for ments in efficiency at the device and materials level, as tar- lighting can build on manufacturing experience gained from geted by the Department of Energy (DOE) solid-state lighting the recent and large-scale production of OLED ­ isplays— d (SSL) roadmap, will have a “lever effect”—­nfluencing the i which in 2012 represent a $3 billion market. Today white design, performance, and cost of the luminaires. Therefore, OLEDs with color rendering indices greater than 80 have improvements in efficiency and performance of the entire been reported (i.e., they can be made to emit with almost SSL system are linked to further fundamental investigations any color; therefore generation of high-quality white light in core technology on emitter materials. is easily achieved) with greater than 100 lm/W efficacy (D’Andrade et al., 2008; Reineke et al., 2009). Figure 3.1 While inorganic LEDs have been manufactured and illustrates the progress in lighting efficiency and the role widely available commercially for some time, there is as yet that LEDs and OLEDs have played in driving that progress. no commensurate large-scale manufacture of OLEDs. Never­ The succeeding sections of this chapter will provide a basic theless, LED yield, cost, and performance would still ben- introduction to both LED and OLED technologies and will efit enormously from further fundamental exploration and discuss the major challenges for each technology in achiev- improvements in the basic technology of materials growth. ing widespread, low-cost, higher-efficiency lighting sources. The technical details that underlie performance of LEDs FINDING: Current LED dies used in SSL lighting suffer and OLEDs have important impacts on the efficacy and reli- from inhomogeneities in the light output, color, and operat- ability of the performance of the total lighting system and ing voltage that necessitate “binning” (hence testing) of dies also determine the subtleties of color quality of the lighting. from a single wafer. This variability severely constrains the Ultimately, understanding these details will allow better yield of the manufacturing process and raises the cost of strategies for developing lower-cost manufacturing tech- the technology. These inhomogeneities are in turn related to nologies. Although specific findings and recommendations fundamental materials and materials growth issues. 34

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 35 300 Lab DemonstraƟons 250 Luminous Efficacy (Lumens per WaƩ) 200 White LED 150 Lamp HID Linear Fluorescent 100 Low High WaƩage WaƩage White 50 OLED Compact Fluorescent Panel Halogen Incandescent 0 1940 1960 1980 2000 2020 FIGURE 3.1  Progress in lighting efficacy. SOURCE: DOE (2012b, p. 38). RECOMMENDATION 3-1: The Department of Energy semiconductor material and the basis of the integrated cir- should continue to make investments in LED core technol- cuits that underlie the fast and compact electronic devices, ogy, aimed at increasing yields, and in fundamental emitter such as computers and cell phones, that are so critical to our research to increase efficacy, including improvements in the daily lives. LEDs are based on a semiconductor material controlled growth and performance of the emitter material. comprised of several different elements. This material is DOE should carefully consider the range and depth of fund- known as a compound semiconductor. The tremendous power ing in its portfolio of investments in these areas, given the of semiconductors lies in their ability to take on a wide range existing technological challenges, in order to determine how of conductivities, from metallic to insulator. This is brought the targeted goals of device performance can indeed be met. about by “doping” the semiconductor with other elements that will donate either positively or negatively charged car- The remainder of this chapter will provide an introduction riers to achieve a desired conductivity. 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. This will semiconductor (see Box 3.1). The general process for light be followed by discussions on the control of the color output e ­ mitted in this manner is referred to as electro­luminescence. of these devices and the important influence of mate­ ials on r The first high-efficiency light-emitting devices were devel- device performance. Because OLEDs for SSL have not yet oped in the 1960s utilizing gallium arsenide (GaAs), alu- been scaled up for large-scale manufacture, the discussion minum gallium arsenide (AlxGa1xAs), gallium phosphide for OLEDs will also encompass issues of reliability and (GaP), and gallium arsenide phosphide (GaAsxP1-x) (Hall manufacturabililty. The chapter will conclude with a com- et al., 1962; Nathan et al., 1962; Pankove and Massoulie, parison and summary of promises and challenges for both 1962; Woodall et al., 1972; Herzog et al., 1969). GaAs and technologies. 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 AN LED PRIMER the early 1990s, efficient blue LEDs based on III-nitride mate­ials began to appear based on the work of Akasaki r Introduction et al. (1992) and ­ akamura et al. (1994). (The III refers N Semiconductor LEDs are a special kind of electronic to elements in the third column of the periodic table, device that emits light upon the application of a voltage indicating that these LEDs can be comprised of alloys of across the device. Silicon (Si) is probably the best-known aluminum nitride (AlN), gallium nitride (GaN), and indium

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36 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING BOX 3.1 Light Emission Mechanism Figure 3.1.1 gives a simple description of the basic light-emission process. Electrons fill up energy states in a valence band, which is separated in energy from a conduction band by an energy gap, with energy Eg (where there are generally no allowed states in which electrons can reside). Providing energy to an electron in the valence band can promote that electron to the higher-energy conduction band, also creating a hole (lack of electron) in the valence band. The electron can subsequently return to its lower-energy state: in radiative recombination, the electron returns to the valence band and releases a photon with the energy of the photon approximately equal to the energy Eg. In an LED, radiative recombination is the desirable outcome for an “energized electron,” but there are also numerous non-radiative recombination processes where the electron or hole may be trapped at defects or imperfections in the material. Such imperfections limit the efficiency of the light generation and, therefore, of the LED. FIGURE 3.1.1  Light emission process. nitride (InN)). The bandgaps of these III-nitrides produce of materials issues that are still of importance today: the light emission across a range of wavelengths spanning the lack of a well-matched material (substrate) upon which to infrared to ultraviolet (UV) parts of the spectrum. The III- form the LED structures and some difficulties in control- nitride LEDs have had an unusually rapid development and ling the electrical properties of the material. Nonetheless, huge impact on appearance of SSL. Although the first GaN the III-nitride materials have been pivotal in the success of LED was reported by Pankove et al. (1971), almost two inorganic SSL, and thus the committee will focus on LEDs decades transpired before substantial further progress was formed from those materials. There are several good reviews made by Akasaki and ­Nakamura. Akasaki demonstrated that of LED device technology (see, for example, Schubert high crystal quality GaN could be grown by metal organic [2006]) as well as III-nitride materials technology (Pankove chemical vapor deposition (MOCVD) using a novel low- and Moustakas, 1998)). temperature buffer (Amano et al., 1986). 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. This provided the understanding of the mechanisms The basis of the LED device is a p-n junction diode, that had limited the conductivity of P-type material and shown schematically in Figure 3.2. As the name implies, allowed for the first time the fabrication of low-voltage p-n there is a junction between the N-type material (rich in junction LEDs and eventually led to the commercialization electrons) and P-type material (rich in holes). Under forward of high-brightness blue and white LEDs for SSL. The wider bias (positive voltage applied to the P-region and negative bandgaps of the III-nitrides enabled the development of voltage applied to the N region) large numbers of electrons efficient LEDs that emit light at blue wavelengths, which are injected into the N region and large numbers of holes are together with green and red LEDs provided the basis for injected into the P region. white light as well as full-color displays. The nitride blue Current flows in the device and the large number of emitters can also be coupled with phosphors to generate injected electrons and holes can combine radiatively, produc- white light, which is currently the dominant approach to ing significant light emission. The basic structure is modified an SSL technology. The later introduction of blue LEDs, in actual LEDs to (1) improve the efficiency of injection of compared to their green and red counterparts, is the result electrons and holes and to (2) “localize” the electrons and

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 37 device performance and on the uniformity of the dies grown from a single wafer. This is further discussed in the section “­ aterials Issues.” In order to connect the device to the M outside world, metal contacts must be deposited by evapora- tion on the N and P regions. Figure 3.3 shows these metal contacts, as well as the transparent and conductive indium tin oxide (ITO) layer that extends the top-side electrical contact over the device surface. Both the sapphire substrate and the ITO spreader contact are transparent to the emitted light, as is necessary for the light to leave the device. High- quality electrical contacts are important to reduce loss due to resistance (R) to current flow (I) in the contact region. This is even more important when the device is operated at high currents or current densities, because loss of power due to resistive heating scales as I2R. In the III-nitride materials, it is a challenge to dope the materials to a sufficient level so that resistances are low, particularly for P-type materials. The formation of the device structure shown in Figure 3.3 is FIGURE 3.2  Schematic of p-n junction diode. just a starting point for the fabrication of the final solid-state “light-bulb.” An individual device must be further “pack- aged” to better control its chemical, thermal, and electrical holes and improve the likelihood of radiative recombination. environment and to better integrate it into the final luminaire. This localization is accomplished by introducing quantum wells in the region of the junction. These are thin slivers of The LED Module lower bandgap-materials that, as their name implies, serve as wells that confine pools of electrons and holes to increase The LED package is the structure in which the LED chip the probability that they will recombine radiatively. is mounted and through which access to the LED terminals is The external view of the typical LED structure is given provided. It is an important part of the finished device. The in Figure 3.3, showing the N-type GaN, the InGaN quan- package serves the following functions: (1) 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); (2) it inte- process. Typically, one 4-inch-diameter sapphire wafer can grates an optical lens structure, which determines the optical produce 5,000 individual devices or “dies.” The 16 percent emission pattern of the structure; (3) it removes heat from the mismatch in natural lattice size between the sapphire sub- device, protecting against degradation due to overheating; strate and the GaN overlayers has important consequences on and (4) it protects the device from electrostatic discharge FIGURE 3.3  A typical GaN LED chip.

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38 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING failure. The packaging processes include placement of the in Figure 3.2, leading to the generation of photons within the device in the chip carriers, attachment of the optical lens, as device, and culminating with the emission (or extraction) of well as electrical and optical device testing and “binning.” the photons from the device. A simple summary of the total Because of the variability in the color accuracy, color quality, external quantum efficiency (EQE or ηEQE) of an LED can and color stability (see section “Controlling the Color Output be expressed as: of the LED”), each device must be individually tested and placed in performance bins. In addition, if phosphor coatings ηEQE = ηIQE • ηout are used in connection with the LED to control the output color, the phosphor must be added to the device or package. where ηIQE is the internal quantum efficiency, and ηout is the A schematic of a typical LED package is shown in outcoupling (or light extraction) efficiency, which will be Figure 3.4. The LED semiconductor chip or “die” is bonded further discussed below. to a ceramic substrate, which provides mechanical support and thermally connects the LED to a thermal pad on the Internal Quantum Efficiency bottom of the substrate. An electrical interconnect layer connects the LED chip to the voltage leads on the bottom Not all electrons and holes that are injected into the LED of the substrate (one of the voltage leads, the cathode, is (e.g., from a battery) will produce photons; for example, shown). A silicone lens above the LED extracts the light defects in the LED material can trap an electron or a hole, that is generated within the chip. Also shown is a transient and prevent the formation of a photon. The percentage of voltage suppressor (TVS) chip which protects the LED chip photons generated, relative to current (of electrons or holes) against electrostatic discharge events. 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. ηIQE also sensitively depends on the quality of formance, and some insights into efficient operation can the LED material. Because the quantum well composition be gleaned by tracing the life cycle of the LED operation and strain varies with the desired emission wavelength, ηIQE beginning with the injection of electrons and holes, shown varies with wavelength. Although ηIQE of today’s best LEDs FIGURE 3.4  Schematic of an LED module. NOTE: TVS = transient voltage supression. SOURCE: Figure provided courtesy of Sudhir Subramanya, Philips Lumileds Lighting Company.

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 39 has reached as high as 80 percent for blue LEDs and 38 per- The ratio of photons leaving the device to the number gen- cent for green LEDs (DOE, 2011a, p. 71), equal efficiency erated within the device is called the outcoupling (or light of LEDs at all colors is important, and further improvements extraction) efficiency. Because the LED material has a higher toward 100 percent ηIQE will require far better control of the index of refraction (n ~ 2.5) than air (n = 1), most photons material defects. incident on the GaN-air interface will be internally reflected and trapped within the LED structure or absorbed (lost) by other materials comprising the device (see Figure 3.5). A thin Current and Thermal Droop metal film can serve as a mirror to direct the light out through Two of the most important issues holding back efficiency the “front surface” of the LED. The internal reflection and at high illumination levels is the droop in efficiency as the trapping of the light can be mitigated by forming a rough, LED is driven at higher currents (e.g., operation at 100 A/cm2 rather than smooth top LED surface; one way of achiev- compared to operation at 35 A/cm2 (DOE, 2011a, Table A1.2, ing this is through the immersion of the device structure p. 71), and the effect of temperature. These issues are known in a simple wet chemical etchant (Fujii et al., 2004). Such in the industry as “current droop” and “thermal droop.” The techniques can improve the extraction efficiency from a few causes and solutions to current droop are still not widely percent to values of 80 percent (Krames et al., 2007). Finally, known. Thermal droop is influenced by the choice of III- the external power efficiency (ηP) is defined as the ratio of the nitride alloy bandgaps and the active layer design, which total optical power output of the LED to the electrical power is limited to thin quantum wells. 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. to maximize ηout produce high power efficiency in LEDs. Similarly, the “droop” at higher current operation is more Maximizing the power efficiency not only increases the effi- pronounced for green LEDs. All major LED companies have cacy of the LED but also reduces the heat removal problem. active research in these areas. FINDING: Efficient operation of LEDs depends on a number of critical factors related to materials defects, struc- Outcoupling (or Light Extraction) Efficiency ture, and strain. Such factors not only limit device efficien- Once the photons have been formed in the LED structure, cies, but also lead to thermal and current droop; all have a care must be taken to ensure that they will exit the device. major impact on the cost and performance of LED lighting. FIGURE 3.5  Improving light extraction efficiency. (a) Much of the light emitted from the quantum well is internally reflected (not extracted). (b) Flipping the LED and placing it above a reflective surface helps to direct the light outwards. (c) Removing the sapphire substrate and then roughening the top of the LED surface. NOTE: p-GaN is P-type (i.e., positive) gallium nitride material (rich in holes); n-GaN is N-type (i.e., negative) gallium nitride material (rich in electrons).

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40 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING CONTROLLING THE COLOR OUTPUT OF THE LED reproducible quantum wells is to look at alternative sub- strates for the growth of the GaN LED structures. An important metric of LED device operation is the control over the accuracy, quality, and stability of its FINDING: The color output of LEDs is extremely sensi- color or peak emission wavelength. Three well-established tive to the control of materials composition and thicknesses approaches to generating white light using LEDs are shown of the LED structure, which in turn are influenced by the in Figure 3.6. These include a blue LED with yellow phos- control of the MOCVD growth process. phors; an ultraviolet (UV) LED with blue and yellow p ­ hosphors (or red, green, and blue phosphors); and a device that combines red, green, blue LEDs. Use of Phosphors The color of emitted light from an LED depends on the Another means of controlling the LED color output is structure and composition of the LED. Achieving a desired through the use of phosphors. The phosphors absorb the photon frequency (hence color) from an LED requires sensi- (typically) blue light from the GaN LED and re-emit light tive control over the thicknesses and material composition at longer wavelengths. The phosphors are chosen so that the of the LED. combination of the direct light from the LED and the light emitted from the phosphor will produce the desired white Quantum Well Thickness and Composition light. A selected few phosphors have garnered consider- able attention, including for example rare-earth (RE) doped The active layers of the current blue LEDs used in SSL yttrium aluminum garnets (YAG:RE, Y3Al5O12(RE)). The are extremely small, 3 nm thick, which classifies them as cerium-doped YAG can absorb blue and UV light and emit it quantum wells. In other words, these nanostructures fall in as yellow light with high efficiency. A critical aspect of this the class of devices in which the light generation mecha- process is that the higher-energy light (e.g., UV or blue) is nism is controlled at the atomic level. Small changes in the being converted into lower energy (e.g., yellow or red). As a indium composition and well thickness affect the emission consequence, LEDs emitting red light—the color having the wavelength and width of the emission. 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). the hue of white light obtained. The MOCVD deposition Phosphors are typically directly added on top of the LED machines used in the manufacture of the LEDs have a huge in the encapsulation material, which is either silicone or influence on the uniformity of the wavelength and yield of epoxy-based. The uniformity of the phosphor coating and white LEDs (see the section “Materials Growth”). One way mixture selection can drastically affect the efficacy and qual- to improve the color consistency and make wider, more FIGURE 3.6  Three types of white LEDs for lighting: (a) blue LED plus yellow phosphors, (b) ultraviolet LED plus three phosphors, (c) three LEDs: red, green, blue connected in parallel. SOURCE: Pimputkar et al. (2009). Reprinted by permission from Macmillan Publishers Ltd.

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 41 ity of light. For example, the use of just a yellow-emitting ticular phosphor involved, but can be disappointing, particu- YAG phosphor with a single blue GaN LED results in high larly compared to incandescent-based light sources. Over the luminous efficacies but relative poor quality (color rendering past few years, manufacturers have found that adding some index, CRI < 75) light. This has led to the perception that all additional energy in the long (red) wavelengths, either with LED lighting is blueish-white and cold. UV LEDs with phos- a red LED or remote phosphor, vastly improves the color phor mixtures provide a better CRI value, but at the expense quality of typical white phosphor LEDs (Hum, 2011). This of poorer efficacy. A combination of three (or more) LEDs solution is now widely used. having different wavelengths (red, green, and blue) may be used if one wishes to dynamically control white light. This Quantum Dot “Phosphors” approach may lead to higher efficacies than the UV-phosphor LED. In addition, moving the phosphor layer away from the There has recently emerged another interesting approach chip and tailoring the optics between the LED chip and the for the color control of both LEDs and OLEDs involving remote phosphor layer has resulted in significant improve- semiconductor nanocrystals or “quantum dots.” These mate- ment in light output and luminous efficacy (­ arendran et N rials are governed by the light emission mechanism described al., 2005). earlier, but these semiconductors, like the phosphors, can also absorb higher-energy photons and emit photons of lower energy or longer wavelength. What is distinctive about General Considerations: Mixed LEDs or Phosphors? these chemically synthesized materials, with diameters of Using narrowband (colored) components to create white a few nanometers, is that the size of the nanocrystals will light, like the examples in Figure 1.9 in Chapter 1, allow influence the wavelength of emission. Initial work on CdSe manufacturers to manipulate the luminous efficacy of radia- nanocrystals began in the late 1980s (Brus, 1991): the color tion (LER) and color qualities of a light source depending on tunability of these structures, their small size, the relative their goals and priorities. At these relatively early stages of ease of production, and their optical robustness encouraged the technology, there are a number of technical difficulties researchers to utilize these quantum dots in a variety of with the use of multi-color LEDs (red, green, blue; red, green, applications, such as selective tagging and in vivo imaging blue, yellow) to produce a white source in SSL products. of features in cells (Michalet et al., 2005). Although much If one of the colored components ages differently from the further assessment will be required to understand the full o ­ thers or responds to heat differently, the color properties of potential of this technology, initial results look promising. the light source will change. Furthermore, inadequate mix- ing of the light will result in colored shadows. Nonetheless, FINDING: A number of approaches have successfully multi-color LED lighting has some advantages. The spectrum been used to achieve and modulate color rendition for LED can be tuned to optimize LER. A wide range of ­chromaticities lighting. Phosphor-converted and color-mixed LEDs show can be achieved by adjusting the relative intensities of the promise but face different challenges. The ultimate choice component colors. In fact, some SSL products currently on of approach will depend on a multiplicity of issues regarding the market allow users to adjust the chromaticity—a ­ eavily h sensitivity of color control, efficiency, reliability, manufac- marketed feature (Philips Solid-State Lighting Solutions, turability, and cost. 2012)—and some in the industry believe that eventually all consumers will expect their lighting products to offer such RECOMMENDATION 3-2: The Department of Energy functionality (Thompson et al., 2011). has provided excellent guidance in its roadmap targets for Even though entire portions of the visible spectrum are both phosphor-converted and color-mixed light-emitting missing from such light sources, narrowband multi-color diodes. Core investment in these technologies should be con- lights can achieve good (three components) or excellent tinued, with consideration for promising new technologies (four or more components) color rendering. These types of (e.g., quantum dot layers replacing phosphors). lights can even exhibit some desirable color-rendering prop- erties that broadband light sources cannot, such as inducing MATERIALS ISSUES FOR WHITE LEDS increases in the colorfulness/vividness of object colors beyond what the CRI deems to be “perfect.” Research on Materials issues in white LED technology affect the cost, this effect suggests that people find these slightly enhanced yield, and reliability of the resulting luminaire at the most object colors to appear more attractive (Jost-Boissard, 2009). fundamental level. 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. This in turn places an ibility in chromaticity as those from multi-color white LEDs, additional burden on the evaluation of individual devices: the phosphors themselves are available in a wide range of “Variability in lumen output, correlated color temperature chromaticities. Color rendering varies depending on the par- (CCT), and forward voltage, is currently handled by test-

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42 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING a) b) Main carrier gas (N2,H2) Group III gas Group V gas (TMGa,TMIn) (NH3) Flow Flange Input gas flow LED Wafers Exhaust gas Susceptor/Heater Rotation Spindle FIGURE 3.7  Schematic of an metal organic chemical vapor deposition system. Panel (b) is a three-dimensional diagram of the gas flow around and under the wafer stage. Panel (b) image courtesy of Veeco Instruments Inc. NOTE: TMGa = trimethyl gallium; TMIn = trimethyl indium. 3.07.eps ing each package and placing it into a specific performance between ammonia and trimethylgallium to occur, forming bin” (DOE, 2009, p. 15). The technology of growth of LED the GaN material, the sapphire substrate must be heated to devices and the choices of substrates for that growth form temperatures of about 1,000°C. This is done using a heated the early components of LED manufacturing and can have metal plate (called a susceptor). There are several possible a profound “lever effect” with long-term implications for configurations for this growth system; however, MOCVD device yield and reliability (DOE, 2011b, p. 14). growth based on a vertical rotating disk design has had broad There are two principal approaches to mitigation or acceptance. The generic rotating disk design is shown in elimination of the materials-related cost and performance Figure 3.7. The sample sits on the rotating disk, which is also issues. The first approach is to improve the uniformity of a susceptor. Gases are injected vertically into and through a the ­ pitaxial growth process. In the second approach, a e showerhead, and the high-speed rotating disk produces stable fundamental breakthrough in native GaN substrate technol- gas flow, aiding the uniformity of the material composition ogy would allow the elimination of a vast number of crystal produced. The MOCVD technology can be used for all of defects and would revolutionize the materials growth pro- the III-nitride materials (Ga, In, Al) utilizing a specific metal cess. The two approaches are discussed below and build on organic gas for each element (for example, trimethyindium an understanding of the MOCVD growth process, which is for indium compounds and trimethyaluminum for aluminum one of the steps in forming the LED devices. compounds). Therefore, all of the elements of the LED struc- ture can be grown in a single run. MOCVD technology has the following several advantages: (1) the ability to grow all of Materials Growth: Mechanisms, Reactors, and Monitoring the III-nitride materials and alloys, (2) the ability to produce As was made evident in the preceding section, the forma- abrupt junctions between dissimilar regions of materials, tion of the materials that comprise the LED plays a critical and (3) the ability to produce thin (almost single atom layer) role in determining the color output and the efficiency of the quantum well regions. device. 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. In these processes, a single-­crystal material flow and gas chemistry in the reactor. Therefore, scale-up in (the overlayer) is grown on a crystalline substrate, and there is reactor size to accommodate larger substrates (and poten- a registry, or relationship, between the structure of the over- tially lower-cost manufacturing) should be straightforward. layer and the substrate. The most commonly used process is The major challenge in MOCVD technology is control of this MOCVD. These complex MOCVD machines are basically complicated growth process over the entire area of the sub- very sophisticated “ovens” used to produce the wafers that strate. Complicating the issues of MOCVD control and mon- are later fabricated into individual LED chips. The typical itoring for the III-nitride materials is the substantial material MOCVD machine costs more than $2 million and can carry differences between the overlayers (GaN LED structure) and out growth on 60 2-inch wafers at a time. the substrate (sapphire). The low thermal conductivity of Technology leadership in this field is still based in the the sapphire means that substrate and overlayer might not United States (VEECO, Applied Materials) and Europe be at the same temperature during the growth process. It is (AIXTRON). In MOCVD technology, ammonia gas and therefore important to accurately measure the temperature of trimethylgallium (called a metal organic gas) are combined the surface of the growing material. The substrate and the in a stainless steel growth chamber. In order for the reaction overlayer have different thermal coefficients of expansion,

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 43 producing a strain at the interface between the substrate FINDING: Production-scale MOCVD growth of LEDs is and overlayer that can result in a curvature or “bowing” of a complex process. The uniformity and yield of the structures the wafer. The curvature is accentuated by the mismatch in grown (and hence of the optical performance of the LEDs) lattice constant between the sapphire and the LED over­ is strongly and negatively affected by small variations in the layers (see section below). This curvature in turn aggravates MOCVD growth process. The thermal and lattice mismatch non-uniformity in temperature and the flux of materials seen between substrate and overlayer exacerbates the sensitivity by each wafer. The most important issue for the MOCVD of the growth process. Further difficulties of growth con- growth of nitride LEDs is control of the growth temperature. trol are anticipated with use of substrates with increased Wafer temperature is important for determining the growth diameter. rate as well as the composition of the indium-containing ­layers in the structure. Even small changes in temperature are RECOMMENDATION 3-3: The Department of Energy enough to change the optical emission (color) of the LED. should fund research to develop instrumentation for in situ The technology that has been developed to accomplish monitoring and dynamic control of the metal organic chemi- monitoring of MOCVD is based largely on optical reflec­ cal vapor deposition growth process. tivity. Early technology development was supported by DOE through Sandia National Laboratories (Sandia National The Search for an Improved Substrate Laboratories, 2004) and has resulted in a U.S.-based start-up company, k-space Associates, that manufactures MOCVD Epitaxial growth processes work best when the substrate monitoring products. There is a similar company in Europe, (e.g., sapphire) that serves as the template for the material LayTec, which manufactures similar technology. Both growth has a structure (lattice constant) that matches that of k-Space1 and LayTec2 have developed specific systems, the finally formed material. Without the one-to-one registry which can in “real time” monitor either the wafer tempera- of the overgrown material to the template, there will be a ture or the growth rate or the curvature (hence the strain) of strain in the overlayer that may eventually give rise to dis­ the growing layer. However, a complete picture of the state locations and defects in the material (107 to 108 cm–2 disloca- of the reactor, and therefore of the material being grown, tions in the best case for GaN on sapphire). Such defects will requires information about all of these parameters. It is pos- compromise the performance and reliability of the devices sible to incorporate several of these monitors into a single formed from the material, and this in turn leads to “uncer- reactor, but this requires the careful design of the reactor tainty in the long-term performance of the luminaire system” to best accommodate and integrate the monitors. In addi- and “makes it difficult to estimate and warrant the lifetime of tion, careful cross-calibration of the monitor outputs with LED-based luminaires” (DOE, 2012a, p. 30). the actual grown materials is required in order to extract Thus, a key issue in the growth of III-nitrides is the lack meaningful data about the monitored growth conditions and of a native or lattice-matched substrate. Currently, the sub- the actual material characteristics. There are currently no strates used for the III-nitrides are sapphire, SiC, or Si; at systems, which can directly monitor the composition of the the moment, there are no GaN substrates of suitable size and growing film. Initial efforts have been made to implement quality available. Were they available, they might provide a many of systems with feedback reactor control (Haberland, better match to the overlayer III-nitride material, within the 2008). However, for commercial reactors only wafer tem- limitations imposed by different lattice constants for InGaN, perature has been implemented as part of the control loop. GaN, and so forth. Besides improvement in device efficiency Current sapphire substrates are 4 inches in diameter. As and reliability, a better thermal match between the substrate the production reactors scale up to 8-inch-diameter sapphire and overlayer would reduce wafer bowing, increasing the substrates, the interrelated problems of substrate tempera- yield of fabricated devices. ture, wafer bow, and change in materials composition can From the discussion above it is easy to see that there is a only get worse. In order to achieve the desired wafer yield, great impetus for the development of GaN native substrates— ­ optical monitoring needs to continue to develop. Goals for among which is the growth of GaN bulk substrates. DOE is future optical monitoring system include the following: considering several competing technologies for substrates. (1) integration of growth rate and wafer bow measurements There are a variety of approaches to the growth of GaN bulk with reactor control in commercial systems, (2) development substrates, which have been recently reviewed in a special of tools to make real time composition measurements on issue of the Proceedings of the IEEE and in other journals the growing layers, and (3) development of tools that will (Avrutin et al., 2010; Ehrentraut and Fukuda, 2010; Paskova provide full wafer maps of the important parameters (tem- et al., 2010). Further progress in the formation of GaN perature, composition, strain, and growth rate). bulk substrates has considerable technological challenges; furthermore, GaN substrates will have to compete with the 1 See, for example, Data Sheets KSA 400, KSA MOS, KSA Rate Rat lower-cost availability of (lattice-mismatched) substrates Pro, and KSA BandiT (k-Space Associates, 2012). such as 8-inch sapphire. 2 See, for example, Application Notes 49, 53, 45, 50, and 34 at http://www.laytec.de/compounds-applicationnotes.html.

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44 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING However, the committee believes that a breakthrough in TABLE 3.1  Internal Quantum Efficiency Values of Light- native GaN substrate technology would allow the elimination Emitting Diodes of a vast number of crystal defects, revolutionize the mate- Metric(s) 2010 Status 2020 Target(s) rials growth process, and have profound benefits for LED efficiency, reliability, and yield. DOE workshop participants IQE at 35 A/cm2 80% (blue) 90% (blue, green, red) 38% (green) speculate that “in principle, the use of a GaN substrate, if it 75% (red) were available at reasonable cost, might simplify the buffer EQE at 35 A/cm2 64% (blue) 81% (blue, green, red) layer technology (thinner buffer layers with shorter growth 30% (green) times) and allow flat, uniform epiwafers to be manufactured” 60% (red) (DOE, 2012a, p. 35.). Power Conversion Efficiency 44% (blue) 73% (blue, green, red) @ 35 A/cm2 21% (green) FINDING: Significant improvements in LED efficiency, 33% (red) yield, and reliability are possible by using GaN substrates Relative EQE at 100 A/cm2 77% 100% and latticed-matched epitaxial growth processes. Currently, versus 35 A/cm2 (droop) there are no viable techniques for producing high-quality, SOURCE: DOE (2011a, Table 2.1, p. 71). low-cost GaN substrates. While realization of low-cost GaN substrates is not assured, the potential payoff of this research is immense. solutions (e.g., heat sinking) at the packaging level, which RECOMMENDATION 3-4: The Department of Energy may increase the overall cost. For example, Krames et al. should make a long-term investment in the development and (2007) have calculated that an improvement in IQE from deployment of gallium nitride substrates. 2010 values (Table 3.1) to 2020 values could result in a four- fold reduction in the amount of wasteful heat generated in a 70 lm/W device. The ancillary issues of increased device CHALLENGES AND PROMISES FOR LEDs lifetime and reliability will also have an impact on cost. The development of III-nitride LED technology has Thus, investments in improving the control and unifor- brought many surprises to the semiconductor community. mity of the epitaxial growth process can have a profound Never before have production devices been formed in a effect on long-term device performance, reliability, and cost. materials system in which the light-emitting layers were Improvement in the cumulative manufacturing yield of the produced on non-native substrates with thermal and lattice LED ­ odule, currently in the range of 50 to 70 percent to m mismatch. Although GaN-based devices have worked well more than 95 percent, will further lower the cost and improve enough to initiate a lighting revolution, materials issues the quality of SSL. But although not directly shown in the have re-emerged as defining elements in the technology. projection of LED package costs (Figure 3.8), improve- As 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 pro- internal quantum efficiency achievable. cess, such as improved uniformity in the epitaxial process. The DOE (2011a) roadmap goals relating to device effi- These improvements will exercise a “lever” effect on the ciency, shown in Table 3.1, can only be achieved by substan- cumulative yield and have a large impact on the final device tial improvements in the control and quality of the materials cost and selling price through improved binning yield (DOE, growth and in the reduction of defects that arise through 2011b). the growth and fabrication processes, which are aggravated by the strain between substrate and overlayers. Moreover, FINDING: LED efficiency and performance is still lim- improvements in the basic technology that forms the starting ited by materials issues. Improvements in efficiency at the materials of the LEDs will have a profound feed-forward device level, as targeted by the DOE SSL roadmap, will have effect that will influence yield, and thus cost, at every stage of a “lever effect,” influencing design, performance, and cost of the LED package formation and performance. For example, the luminaires. Improvements in efficiency and performance strain between the substrate and the overlayer results in are linked to further fundamental investigations in core tech- the non-uniformity of LED characteristics across wafers, nology on emitter materials. leading to the wasteful practice of “binning.” Fluctuations in the composition of the LED layers, and particularly in RECOMMENDATION 3-5: The Department of Energy the quantum well region, compromise control over the LED should continue to make investments in light-emitting diode emission wavelength. Defects have an impact on the electri- core technology and fundamental emitter research. Its cal resistance of the LEDs, increasing power dissipation and port­olio of investments in these areas should be extensive f limiting higher-temperature performance, as well as lifetime. enough to ensure that the targeted goals of device perfor- Limitations at the device level necessitate compensating mance can indeed be met.

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46 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING that is located on the same molecule as the electron). This decay process often emits light whose energy is equal to that of the difference in energies between the electron and hole. By changing the composition or structure of the molecule, the wavelength (color) of light emission can be varied. In fact, small chemical modifications can change the color emission from the ultraviolet, through the blue and green, to the red. In all cases, the light emission can be extremely efficient, with 100 percent conversion of electrons to pho- tons having been reported across the visible light spectrum (Willner et al., 2012). FIGURE 3.9 Archetype organic light-emitting diode structure. Metrics of Device Performance SOURCE: Willner et al. (2012). ©IEEE (2012). Reprinted, with In a manner similar to the calculation of the EQE of an permission from Proceedings of the IEEE. inorganic LED, the EQE of an OLED depends on both an intrinsic efficiency, for the material and device, and an outcoupling or extraction efficiency, where transport layer that moves electrons from the cathode metal ηEQE = φ • γ • hout • c (3.1) contact to the light emissive layer, or “EML.” This layer is typically composed of two different molecules, a charge where φ is the absolute efficiency of a molecule to emit light conductive “host” molecule into which is doped a molecule once excited, γ is the probability that every injected electron at very small concentration (~1 to 8 percent by weight) that and a hole can simultaneously exist on a light-emissive gives off light of the desired color (or wavelength) under molecule, hout is the outcoupling efficiency to be discussed excitation from electrons and holes in the device. This below, and χ is the ratio of emissive molecular excited states d ­ opant is called the light emissive “guest.” The “HTL” is that an electron and hole can reside on in a single molecule the hole transport layer whose purpose is to transport posi- to the total number of possible excited states. χ is also known tively charged “holes” from the anode contact to the EML. as the excited state ratio. For the best emissive molecules, The transparent conducting anode through which the light φ = 1, which is often the case with state-of-the-art materials. is viewed is invariably composed of ITO, and the cathode Furthermore, γ = 1 in properly engineered device structures. is a metal (such as aluminum doped with lithium) capable The power efficiency (hP) of the light source is its most of forming an ohmic contact with the ETL for the efficient important operational parameter. 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. How- Vλ ever, in all cases, the total thickness of organic layers rarely hP = θηEQE (3.2) V exceeds 100 nanometers (1 nanometer = 10–7 centimeter) (Willner et al., 2012). The committee also notes that in con- Here, θ is the overlap of the light source with the spectral trast to LEDs, OLEDs can be made integral to the luminaire sensitivity of the eye, and Vλ is related to the energy of the rather than being added to it, in contrast to all alternative emitted photon. The operating voltage of the OLED is V— lighting solutions. This structural adaptability provides new clearly the power efficiency decreases as V increases. For a design possibilities for SSL. given device geometry, the operating voltage is related to the The mechanism for light emission in organic, thin- device drive current and thus also has an important influence film OLEDs (Box 3.2) is fundamentally different than in on the device lifetime. i ­norganic semiconductor LEDs described earlier in this In conventional OLEDs fabricated on glass substrates, chapter. When an electron and its oppositely charged through mechanisms similar to those in inorganic LEDs, counter­ art, the hole, are conducted to the same molecule p much of the emitted light is trapped within the glass sub- within the EML, they put the molecule into an excited state. strate or absorbed in the layers that comprise the device This excitation is maintained for a brief period of time (from (see Figure 3.10), resulting in an extraction or outcoupling nanoseconds to microseconds). While it exists, the excitation efficiency of only ~20 percent. However, low-cost schemes can hop from molecule to molecule, which are very densely have been reported that can increase this efficiency to 40 to packed within the EML. This mobile excitation (called an 60 percent (see below). Nevertheless, one of the grand chal- “exciton”) eventually decays by the recombination of the lenges facing OLEDs is how to extract more of the emitted electron and the hole (i.e., the electron “falls into” the hole light in a cost-effective and highly efficient manner. This will

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 47 BOX 3.2 How Light Is Emitted in OLEDs Shown in Figure 3.2.1 is a pictorial view of the light-emitting layer in an organic light-emitting diode (OLED). This layer is typically sand- wiched between electron and hole transporting layers. The blue background represents the thin film that is comprised of a molecular species that transports the charges injected from contacts at the boundaries of the OLED itself. The red dots are the dopant molecules that are interspersed at low density within the charge transporting matrix. These dopants can either be fluorescent molecules or phosphorescent molecules. Phosphorescent molecules can produce devices with the highest internal quantum efficiency. The inset on the lower left shows a typical phosphor molecule. It can be very inexpensive and is only used in trace amounts. Ultimately, it consists of carbon, nitrogen, and hydrogen atoms (open circles) that are bonded together (lines) along with a heavy metal atom (typically iridium) in its center (red dot). Light emission occurs when an electron injected from the cathode travels to the same molecule as the hole (positive charge) injected from the anode, forming a mobile excitation or “exciton.” Light is then generated when the electron and hole (or exciton) recombine on the edges of the dopant molecule. This emission process is depicted by the yellow burst around the dopant molecule in the emitting region. By varying the structure of the molecule, the entire visible and near-infrared spectra can be accessed. FIGURE 3.2.1  Pictorial view of the light-emitting layer of an OLED. be discussed further in the section on necessary technology developments. Finally, the excited state ratio is χ = 0.25 for fluorescent emitting molecules, and χ = 1 for phosphors, as will be dis- cussed in the following section (Baldo et al., 1999b). Putting all of the efficiencies together, it is demonstrated that ηEQE is 20 to 60 percent in the very best cases. 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) photon following emission from a luminescent molecule (shown as yellow star). of the light emitted. White light is generated by mixing red,

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48 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING green, and blue emission from different regions of the OLED. F/P WOLEDs That is, the emission spectrum of a particular molecule The F/P WOLED is based on the recognition that approxi- is insufficiently broad to efficiently generate the desired mately 25 percent of the color content of white light is blue. broadband white radiation to produce high-quality illumina- To achieve lower voltage operation and perhaps longer tion, so the use of several different molecular species within lifetime, which is currently limited by phosphorescent blue the emission region of the OLED is required to achieve the EMLs (see Box 3.3), and because 25 percent of the injected desired white spectrum. charge forms fluorescent states, this device uses a fluores- A given molecule within an EML will emit with a well- cent blue segment and harvests the remaining green and red defined spectral shape. Hence, unlike the case for inorganic excited states using phosphorescent (i.e., heavy-metal-atom LEDs, binning is not needed to select those devices that emit containing) molecular compounds. In principle, this particu- at the appropriate wavelength. However, the white balance or lar device has the lowest drive voltage and hence highest chromaticity is ultimately determined not only by the well- efficacy of all alternative architectures. The F/P design can defined spectra of the constituent molecules in the EML, but also be incorporated into stacked and striped architectures. also by the details of the device structure, which may vary Hence, the device still achieves 100 percent internal quantum from run to run. Hence, several schemes have been developed efficiency because all excitons are harvested by a combina- for lighting applications that are both efficient and have a tion of blue fluorescent dopants and red and green phosphors. stable, predictable, and highly controllable white chromatic- ity. The highest performance is achieved using a variant of one of the three designs shown in Figure 3.11—the striped, Stacked WOLEDs white OLED (WOLED), the fluorescent/phosphorescent The compact SOLED design stacks two or three white- (F/P) WOLED, and the stacked WOLED (SOLED). The emitting segments, with each segment separated by a very latter design is most effective in achieving long lifetime and thin and transparent “charge generating layer.” In this case, high brightness and can be combined with the F/P design as a single injected electron can recombine with a positively well as others for illumination purposes. charged hole in each segment, generating a photon. Thus, a 2 to 3 times higher quantum efficiency is achieved with this Striped WOLEDs device compared to the other designs, but at 2 to 3 times higher voltage (where the multiplier is equal to the number The simple design of the striped WOLED places stripes of of elements in the final stack). Hence, the efficacy of this red (R), green (G), and blue (B) PHOLEDs (phosphorescent device is no higher than that of the other designs shown, but OLEDs) side by side. The R-G-B pattern is repeated on a there are significant benefits of increased device lifetime. very small scale so that the separate colors cannot be resolved For example, the SOLED of Figure 3.11 comprises a by an observer. By injecting current into each stripe, the B PHOLED as one stacked element and an R-G PHOLED viewers will perceive the mixture of the three primary colors, as the second element. Other examples of SOLEDs include which will appear white. An advantage of this design is that a complete white-emitting phosphorescent R-G-B EML in each of the three color elements can be separately optimized each element. This three-element SOLED is known as a to emit with 100 percent internal efficiency, and variation of W-W-W SOLED, as shown in Figure 3.11. 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. A disadvantage is the complexity of driving the are often pursued are to use very broadly emitting white WOLED with three different current sources. FIGURE 3.11  Three examples of white organic light-emitting diode designs.

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 49 organic vapor phase deposition (OVPD); Shtein et al., 2002), although, like polymers, they can also be deposited from BOX 3.3 liquid solution. OLED Drive Voltage Note that the demands placed on the deposition process (and tools) are quite high. As discussed above, a typical According to Equation 3.2, the white organic light-emitting high-performance OLED display consists of at least five diode (WOLED) efficacy (i.e., power efficiency) is inversely l ­ ayers, with the thickness of the entire stack seldom exceed- proportional to the total voltage dropped across the device. ing 100 nm. White-emitting OLEDs used in lighting appli- Low-voltage operation is, therefore, desirable for high efficacy cations have at least double this number of layers. Hence, (and also results in lower electrical power dissipation); because deposition must occur over very large substrate surfaces the conductivity of organic materials is low, this requires the (exceeding 1 m2 in production environments), with minimum use of thin device layers. Unfortunately, such thin layers can individual layer thicknesses of ~5-10 nm. To maintain device also decrease device yields, because any flaws in the film (e.g., performance uniformity, layer thickness variations of only a pinholes, clusters, etc.) can lead to shorts between the closely few percent are tolerable across the entire substrate surface. spaced electrodes (typically, the organic layers are ~100-200 nm Fortunately, in-line VTE and OVPD have been proven to thick in total). A particular constraint for low voltage operation achieve these demanding specifications in display produc- resides with OLEDs emitting in the blue, requiring at least ~3.5 V tion facilities, suggesting that such targets are realizable for to excite the emitting (dopant) molecule. To transfer the excitation lighting as well. from the host material to the dopant molecule requires ~3.5 V for To produce high-quality, long-lived devices, it is impera- fluorescent emission in the blue, but >4 V to achieve phospho- tive that the organic materials, most of which are easily syn- rescent emission in the blue. As discussed earlier, the excited thesized using well-known methods, be highly purified prior state ratio, χ is only 0.25 for fluorescent-emitting molecules, but to use. 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. allows a lower-voltage operation. High purity is achieved by evaporation of the volatile (and Another effective means for decreasing voltage is to lightweight) impurity molecules in a vacuum system that increase the conductivity of the charge transporting layers exhausts the volatile evaporants. u ­ sing ­ onductivity-enhancing dopants (Blochwitz et al., 1998; c The layering of numerous materials with different func- D’Andrade et al., 2008; Pfeiffer et al., 2002). This strategy has been tions needed to confine both charges and photons (see successfully pursued by Novaled and the COMEDD Fraunhofer Figure 3.9) is a relatively simple task when deposition occurs Institute in Dresden, Germany. from the vapor phase, and given that the process generally occurs in vacuum, this too aids in the prolonged operational lifetime of the OLEDs. As will be discussed below, extended lifetimes, particularly of the blue emitting molecules, remains a challenge. OLEDs (e.g., emission from a single molecular species that In addition to polymer and small-molecular-weight spans an unusually broad spectrum or uses a blue OLED to OLEDs, as mentioned in the section “How Light Is ­Emitted,” “pump” red and green organic phosphors located external there are two systems of emissive dopants available: those to the OLED pump). Unfortunately, in both cases the effi- that emit from fluorescent molecules and those from phospho- ciency is considerably less than direct PHOLED emission rescent molecules. Note that while fluorescent devices (Tang and, hence, are generally not viewed as adequate for meeting and VanSlyke, 1987) can have a theoretical maximum emis- the stringent demands of advanced SSL sources. sion efficiency of only 25 percent (because χ = 25 percent in Equation 3.1, in this case), phosphorescent devices can have 100 percent internal quantum efficiency. This implies that MATERIALS FOR OLEDS PHOLEDs are ideal for both displays and lighting. Follow­ All current significant manufacturing of OLEDs employs ing the demonstration of “electro­ hosphorescence” using p small-molecular-weight organic materials. These materials metalorganic compounds (Baldo et al., 1998, 1999a) in 1998, consist of a single molecular unit with a well-defined number the materials have found widespread adoption in all OLED of constituent atoms. This is in contrast to polymers that are devices currently introduced into the market. chains of units of indeterminate length and, hence, number As shown in Table 3.2, extremely high-efficiency emitting of atoms. Polymers are typically used in plastics, whereas molecules (with 100 percent IQE) are available for emission small molecules are used as pigments in common dyes for across this entire spectral region. clothing, ink-jet printing, and so on. Small-molecular-weight Several other materials are used in high-efficiency organic compounds are commonly deposited from the vapor PHOLED structures beyond the simplified design shown in phase (i.e., either vacuum thermal evaporation (VTE) or Figure 3.9, including the conductive host, the electron and

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50 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING TABLE 3.2  Representative Commercial Phosphorescent Molecules and Their Corresponding PHOLED Performances Operating Lifetime (hours) CIE 1931 PHOLED Performance at 1000 cd/m2 Chromaticity Coordinates Luminous Efficiency (cd/A) L95 L50 Deep red (0.69, 0.31) 17 14,000 250,000 Red (0.69, 0.34) 24 25,000 600,000 Red (0.64, 0.36) 30 50,000 900,000 Green-yellow (0.46, 0.53) 72 70,000 1,400,000 Green (0.34, 0.62) 78 18,000 400,000 Light blue (0.18, 0.42) 47 600 20,000 NOTE: Results are for bottom-emitting structures (with no cavities). Lifetime data are based on accelerated current drive conditions at room temperature without any initial burn-in. L95 and L50 are the time to which the luminance has decreased to 95 percent and 50 percent, respectively, of initial values. SOURCE: Universal Display Corporation (undated). hole transporting molecules, and the exciton blocking layer modes because of total internal reflection when conventional, (EBL). This EBL layer, positioned at the cathode-side of the low-cost glass substrates are employed. A large remaining EML, is required in PHOLEDs because of their long excited- fraction (again about 20 percent) of the light is trapped in state lifetimes and corresponding diffusion length. The the glass substrate (glass modes). And finally, 33 percent of EBL prevents diffusion of the excited states to the cathode the light is emitted along the plane of the organic thin films, contact where they may quench before they can radiatively forming waveguide modes. 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. surface, can also lead to reductions in efficiency. Because of the relative sizes of the effects and ease in modal access, most efforts have been directed at eliminating KEY ISSUES FOR IMPROVED DEVICE PERFORMANCE glass and waveguide modes. The effective elimination of these losses can result in a tripling of the external efficiency Light Outcoupling from 20 to 60 percent. Perhaps the largest efficiency gain that has yet to be Any practical solution of the outcoupling problem must achieved is through increased light outcoupling from the be extremely low cost to implement, and it must not affect substrate. As noted in Equation 3.1 and the ensuing discus- the wavelength or angular intensity distribution of the emit- sion, only 20 percent of the light emitted by the WOLED is ted light. With these considerations in mind, glass modes are coupled into the air and is, therefore, viewable if the device most effectively eliminated by the attachment of microlens is deposited on a conventional glass substrate. The remainder arrays onto the substrate/air surface (Möller and Forrest, is trapped in the glass, or is absorbed by the materials that 2001; Sun and Forrest, 2008). Microlenses are typically comprise the device, as shown in Figure 3.10. Outcoupling of 5-10 mm diameter transparent hemispheres that can be made light in OLEDs is particularly difficult when compared with by molding into large plastic sheets (see Figure 3.12, for LEDs because of the large areal dimension and integrated example). This omnidirectional, wavelength-independent form factor of the OLEDs. That is, they are “area” rather than solution has been shown to increase the efficiency of the “point” lighting sources, whereby a large surface area must OLEDs by nearly a factor of 2 (with external efficiencies be coated with emitting materials to generate the desired as high as 40 percent measured). Alternative solutions for level of illumination. As noted above, this is a generally extracting glass modes include roughening of the glass desirable feature because the lighting source (the OLED) surface, placing OLEDs on the surfaces of plastic blocks and the luminaire form a single integrated unit. Yet, it also with tapered edges, or using high index of refraction plastic poses challenges because there is little access to the light substrates and a large hemispherical lens. trapped within the substrate and emissive layers. Numerous Extracting waveguide modes (i.e., that light emitted out­ oupling efforts have therefore explored ways of cost- c within the organic layers themselves and propagating in effectively harvesting a greater proportion of the trapped the plane of the substrate) that consume 33 percent of the light (Wang et al., 2011). emission is more difficult. It needs to be done very near There are three principal optical pathways an emitted to the point of emission (i.e., at the location of the excited photon can take. The first is the air mode: i.e., the light that molecular dopant) to avoid losses due to waveguiding within escapes from the substrate and can be viewed as useful light. the absorptive organic or transparent conducting oxide anode As noted above, only 20 percent of the light is emitted into air layers. Such solutions, therefore, must be integrated into the

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 51 FIGURE 3.12  Polymer sheet of 5 μm diameter microlenses a ­ ttached to the glass surface of an OLED. Outcoupling enhance- ments of a factor of 2 are possible using this approach. SOURCE: Sun (2008). Reprinted by permission from Macmillan Publishers FIGURE 3.13  Efficiency droop in phosphorescent OLEDs (black Ltd. rectangles) and fluorescent OLEDs (red circles). OLED structure itself without degrading device performance rent. This droop is fundamentally related to the molecular in other, unintended ways. excited state (exciton) that, when de-excited, emits light. At Generally, to couple waveguide modes into the glass or very high intensity, a substantial fraction of the emitting mol- air, there must be a surface texture inserted at the transparent ecules in the EML are excited. When the excitation migrates anode/organic interface. The length scale of the texture can- from molecule to molecule, it has a possibility of colliding not be on the order of the emission wavelength; otherwise, with another excitation on the same molecule or on an elec- an undesirable angular dependence of emission wavelength tron or hole that is transiting the EML. This collision results (i.e., color) and/or intensity may result. Low-index grids in the loss (or de-excitation) of one of the two excited states, consisting of a dielectric such as silicon dioxide residing at ultimately resulting in the loss of efficiency. This process this interface have been shown to outcouple almost all of is known as “exciton annihilation.” Importantly, this same the waveguide modes without significant losses (Sun and process leads to the degradation of the molecules and hence Forrest, 2008). The openings in the grids are typically 5 mm, a decrease in OLED operational lifetime, as discussed below. with grid lines of only 1 mm. Combining the grid with the Hence, it is essential to find device architectures that mini- microlenses shown in Figure 3.12 has resulted in the dem- mize exciton annihilation processes. One method to effect onstration of 34 percent external efficiency. this is, for example, extending the thickness or grading the dopant concentration within the EML. However, little work FINDING: A number of promising approaches have been has been done to date to reduce or even eliminate droop. developed to increase outcoupling efficiency. One important difference between OLEDs and LEDs, however, is that in the former case, there is no thermally RECOMMENDATION 3-6: The Department of Energy driven droop effect. That is, as the temperature is varied, should focus on efforts that result in significant light outcou- the efficiency characteristics in Figure 3.13 are largely pling enhancements for OLED that are low-cost to imple- unaffected. ment and are independent of both wavelength and viewing angle. FINDING: OLEDs show a decrease in efficiency as the current is increased. This results in a reduction in efficiency at high brightness. OLED Efficiency Droop As in the case of LEDs, OLEDs also suffer a loss of RECOMMENDATION 3-7: The Department of Energy efficiency as the current (and corresponding brightness) 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 light- external 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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52 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING ISSUES FOR OLED DEVICE RELIABILITY AND RECOMMENDATION 3-8: To create a highly envi- MANUFACTURING ronmentally robust organic light-emitting diode (OLED) lighting technology, the Department of Energy should invest OLED Reliability in materials and packaging technologies that make OLEDs resistant to degradation over their long operational lifetimes. As in all electronic devices, there are numerous sources In particular, important areas for investment include finding of OLED degradation that limit the operating lifetime of the low-cost means to eliminate glass as a primary package lighting source. These mechanisms fall into two categories: constituent, devising molecules and device architectures extrinsic and intrinsic, or fundamental. Typically, the usable that are resistant to degradation on exposure to atmo- lifetime of a device is indicated by the time to which the light sphere, and developing sealing technologies that are fast, output has dropped to a given percentage of initial luminance precise, and robust to bending. (Lo) (see Table 3.2). Typically, lighting requires qualification to at least the so-called L70 limit set forth in the industry As can be observed in Table 3.2, both red- and green- standard, LM-80, issued by the Illumination Engineering emitting PHOLEDs used in analogous display applications Society (2008). For displays, a differential luminance loss have lifetimes of several hundreds of thousands to millions of only 10 percent on a highly used area of the display field of hours. Devices emitting with these colors, therefore, is easily perceptible, rendering the appliance useless. Once significantly exceed the lifetimes required for practical light- again, experience being gained in the large-scale deploy- ing sources. However, blue PHOLEDs have a significantly ment of reliable displays has promoted the advances in shorter lifetime. Early blue failure is not due to environ­mental lighting applications as well. However, in the case of white factors, but rather to properties intrinsic to organic molecules light sources, their exceptionally high surface brightness (Giebink et al., 2008). The principal intrinsic failure mode (typically 3,000 cd/m2 compared to 100 cd/m2 required for is excited state-charge and excited state-­ nnihilation reac- a displays) places additional stress on the devices that results tions that occur when, at the molecular scale, a charge ends in a shortened lifetime. Furthermore, the differential aging up on an excited molecule (Giebink and Forrest, 2008)—a of one color component versus another leads to perceptible process similar to that responsible for a decrease in quan- and unacceptable shifts in the CRI or luminaire color tem- tum efficiency as current is increased (see Figure 3.13). perature over time. Today, the useful life of blue phosphorescent devices is The primary extrinsic source of aging in OLEDs is degra- only approximately 10,000 to 20,000 hours (Table 3.2), also dation of materials and cathode interfaces due to exposure to setting the limit for the lifetime of white lighting sources. moisture and oxygen (Burrows et al., 1994). For this reason, Fluorescent blue-emitting materials have somewhat longer packaging of OLEDs is important in controlling the local lifetimes because of the very short existence of the blue environment. OLEDs are sealed against ambient ingress fluorescent excited state (~nanoseconds) compared to that by packaging in an ultrahigh purity nitrogen environment. of blue phosphorescence (~tens of microseconds). Hence, A conventional package consists of a glass substrate and a the less efficient fluorophores support annihilating collisions metal back cap that has a slight recess filled with a dessicant with charges and other excited states for durations that are such as BaO to scavenge any residual oxygen or moisture far shorter than for phosphors, increasing the operational in the package. The seal is typically made using a bead of lifetime of the device. UV-curable epoxy. Although it is by no means a perfect One significant challenge that must be overcome to ensure seal, the long-term degradation of most OLEDs is extended the widespread deployment of WOLEDs for lighting, there- to well within acceptable industrial standards. Other routes fore, is to increase the lifetime of the blue-emitting element to extrinsic failure include incorporation of impurities from to hundreds of thousands of hours. As noted, use of fluores- source materials and chemical reactions in the guest-dopant cent blues is one avenue for improvement, as is designing conductive layer. All such extrinsic mechanisms can be phosphorescent blue molecules that have shorter excited state reduced to acceptable levels through the proper handling and emission times. Further techniques might include extending purification of source materials and by careful selection of the thickness of the blue EML, thereby reducing the density materials sets for a particular OLED structure. of excited molecules at high brightness. Indeed, the SOLED architecture does this effectively by distributing several of FINDING: The lifetime of OLEDs is very sensitive to the blue-emitting regions within the several elements in the extrinsic factors such as exposure to air and moisture. The stack. 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. Rather, light blue (cyan) requires a high degree of fabrication competency that can emission is desirable for this application. Because this is a ensure package hermiticity along the entire large package lower energy of emission, the problem is partially resolved periphery and scavenge excess water and oxygen that might simply by the judicious choice of blue-emitting wavelength. have been enclosed during the package manufacture. Nevertheless, more rapid degradation of blue intensity ver- sus red or green inevitably leads to color shifting during the

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 53 operational lifetime of the WOLED that must ultimately be minimized. 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. 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. 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, oper- Manufacturing Issues ate at high temperatures that require extensive heat sinking and care in their installation. Nevertheless, OLED opera- There is as yet no large-scale manufacture of OLED tional lifetime is very sensitive to temperature increases. lighting; however, major growth in OLED display tech- As the room temperature rises, the OLED lifetime can be nologies may provide both infrastructure and cost reduction expected to be noticeably decreased. and, thus, important incentives for the further development of manufacturing for OLED-based SSL. As of this writing, RECOMMENDATION 3-9: The Department of Energy one company alone, Samsung Mobile Displays (SMD), is should support the pursuit of material sets and device archi- producing 30 million such displays per month, with plans tectures that would increase the useful operational lifetimes to scale these devices to larger, three-dimensional displays. of high-intensity white organic light-emitting diodes. SMD’s major competitor in this space is LG Display, along with a handful of other display companies in Asia of varying (a) (b) 10000 27.5° C Lifetime to LT80 [hrs] 1000 100 10 290 300 310 320 330 340 350 t = 60 min 28.7 ° C Temperature [K] FIGURE 3.15  Time to LT80 for a white PHOLED with an initial surface luminance of Lo = 8,000 cd/m 2. (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). Figure 3-15 redrawn

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54 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING sizes. All of this manufacturing, today occurring on Gen 5.5 RECOMMENDATION 3-10: The Department of (1,300 × 1,500 × 0.6 mm) 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. positioning these early-entry companies to become com- petitive in producing low-cost, ultrahigh efficiency, easily Extended Operational Lifetime dimmable OLED sources for the consumer lighting market. Indeed, several companies are concerned only with the light- FINDING: Extending the lifetime of blue phosphores- ing applications of OLEDs (i.e., not their uses in displays), cent OLEDs is a primary area where investment will have such as General Electric, Osram, Moser-Baer, and Philips. substantial payoff. It involves a combination of advances Equipment makers, providing key infrastructure that is in the development of new materials, device architectures, required to provide a strong growth in manufacturing, are encapsulation, and contact technologies, as well as a fun- also starting to take notice of the possibilities for large, damental advance in the understanding of degradation developing markets in OLED displays and lighting. Chief processes. Interactions between the phosphor and the con- among the OLED manufacturing equipment suppliers is ductive host will have an influence on mitigating efficiency Aixtron, SE, the largest producer of MOCVD equipment for droop, or the de-excitation of the molecules in the OLED. LED lighting, which also produces (on still a small scale) The mechanisms for thermally induced degradation also organic vapor phase deposition systems for OLEDs. Applied require clarification. Encapsulation compatible with flex- Materials is the world’s largest supplier of equipment for ible, lightweight substrates is also an important area of low-temperature polysilicon deposition on glass substrates development. used as active matrix display drivers (Nathan et al., 2005), and its division Applied Films supplies in-line deposition RECOMMENDATION 3-11: Given the interactions sources for front-plane OLED display materials deposi- between the phosphor and the conductive host molecules, tion. The committee notes, however, that the current lack the Department of Energy should direct studies for determin- of a complete tool set for manufacture of OLEDs remains a ing what chemical structural combinations lead to the most limiting factor in their widespread and low-cost deployment robust materials sets. Fundamental studies of the degradation as lighting sources. mechanisms should be carried out both at room and elevated temperatures. Research on understanding contact and ambi- ent degradation routes and their minimization should also SUMMARY OF OLED CHALLENGES be supported. Based on the foregoing discussion, phosphorescent WOLEDs provide an unusual opportunity to complement Low-Cost Light Outcoupling LEDs as an important solution for areal SSL. Yet there remain significant barriers to their adoption, and as a result FINDING: Increased outcoupling remains the single their development still lags that of inorganic LEDs as the most beneficial route to increasing device efficiency from the preferred white illumination source current 100 lm/W to nearly three times that value. ­Methods to The committee’s findings and recommendations on the achieve this should be inherently very low cost and deploy- major challenges that should be the focus of near-term invest- able over very large areas, even in the context of roll-to-roll ment are given below. manufacture. The outcoupling technology should have the additional attributes of being wavelength and intensity independent, and the light source should exhibit no color Cost Reduction shifts as the viewing angle is varied from normal to highly FINDING: This is potentially the single most important oblique. Clearly, a viable outcoupling technology should not metric to meet in OLED lighting. It requires simplification otherwise impact or degrade OLED performance. of device structure, use of ultralow-cost substrates such as Addressing the critical challenges for OLED lighting metal foils, development of replacements for costly transpar- should enable the realization of increased power efficacy ent anodes (current technology is indium tin oxide), low-cost and the realization of the targets set by DOE, as shown in encapsulation technologies, and so on. Also, investment Table 3.3. in equipment infrastructure is essential for the success of low-cost, manufacturable products. In-line vacuum deposi- SUMMARY AND COMPARISON OF LED AND OLED SSL tion sources, roll-to-roll processes on flexible substrates, ultrahigh-speed organic vapor phase deposition, and in situ The LED and OLED technologies explored in this chapter encapsulation techniques will all require substantial infra- provide new, energy-efficient approaches to lighting with structure development. exceptional control over the chromaticity and the quality of the light produced. As shown in Table 3.3, both technolo-

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ASSESSMENT OF LED AND OLED TECHNOLOGIES 55 TABLE 3.3  Comparison of Lighting Sources by Various Metrics Incandescent Fluorescent LEDs OLEDs Efficacy (lab demo) 231 lm/W – white 100 lm/W 150 lm/W – warm white Efficacy (commercial) 17 lm/W 100 lm/W 100-120 lm/W – white 50 lm/W panel CRI 100 80-85 85 – white Up to 95 95 – warm white Form factor Heat generating Long or compact gas filled Point source high-intensity Large area thin diffuse source glass tube lamp Flexible, transparent Safety concerns Very hot Contains mercury Very hot operation None to date Lifetime (hours) 1,000 20,000 50,000 30,000 Dimmable? Yes, but much lower efficacy Yes, efficiency decreases Yes, efficiency increases Yes, efficiency increases Noise No Yes No No Cost ($/klm) 0.50 1.0 7.0 100-250 gies have enjoyed an unprecedented rapid progress in lumi- technology, both at the fundamental level of materials and nous efficacy. 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