Trends and Opportunities in Photonics Technologies: Solid-State Lighting and Healthcare (NISTIR 7305)

II. SOLID-STATE LIGHTING (SSL)

Solid-state lighting is the name given to lighting generated through exciton recombination rather than through black body radiation (standard incandescent or halogen) or gas discharge (fluorescent or High Intensity Discharge). The basis of solid-state light emitting devices consists of an electron-rich (or easily injected) region and a hole-rich (or easily injected) region separated by a carrier depleted zone. When a voltage is applied electrons and holes are injected into the depleted region where they recombine, emitting light. This rather broad brush description is applicable to both light emitting diode (LED) based lamps, which are composed of traditional semiconductor materials, and organic light emitting diodes (OLED's), which are made of conduction and emission organic layers.

A. Current Status

The primary driving force for the development of solid-state lighting is the reduction of energy use, with concomitant reductions in chemical pollution, light pollution, and cost to the consumer. According to a 2003 report¹ by the Next Generation Lighting Industry Alliance (NGLIA), in 2001, 22% of the electricity generated in the United States was used for lighting. This is the equivalent output of 100 power plants, each generating 1000 MW, for a total cost of 55 billion dollars. The lighting requirements for 2005 in the United States are shown in Table 1², broken down into residential, commercial, industrial, and outdoor categories and into low, medium, high, and very high color rendering index (CRI)^b bins.

Table 1 shows that commercial lighting is expected to account for 74% of the generated light. For very high CRI, residential applications account for fully 55% of the usage.

According to the NGLIA report¹, when LEDs and OLEDs reach 150 lum/W for white light generation, the United States will be able to reduce its annual consumption of electricity by 6% - 7%, equivalent to about $17 billion.   This reduction is projected to result in reductions of pollutants on the order of 150 x 106 tons of CO2, 0.3 x 106 tons of NOx, and 0.67 x 106 tons of SO2. However, the current status of SSL sources falls well short of the 150 lum/W goal (in 2004, white LEDs lamps achieved = 30 lum/W). Table 2 shows efficacy values³ for three LEDs and one OLED that span the visible spectral range in 1999.

For comparison, an unfiltered incandescent lamp² has an efficacy of 10 - 25 lum/W and fluorescent lamps³ reach 50 - 95 lum/W, depending upon the specific lamp geometry and application. By 2005, commercial LEDs improved to 100 lum/W at 50% wall plug efficiency.⁴ In addition, commercial solid-state white lamps that provide 60 lum are now available, although they only provide about 15 lum/W. At this time, the DOE "World Record" for white lamps⁵ is 74 lum/W for a laboratory LED and 20 lum/W for a laboratory OLED. Clearly, substantial progress needs to occur before the 150 lum/W goal is achieved.

Along with the primary benefit of energy conservation, a number of secondary benefits also accrue with SSL. Possibly the most important secondary benefit is reliability. Today, blue LEDs provide over 5,000 h of light and red LEDs can last well over 100,000 h. When failure occurs, it is seldom catastrophic, as for incandescent and fluorescent lamps; rather, LEDs typically fail via a gradual diminution of light output over time. The long lifetimes and the non-catastrophic failure processes explain why LEDs have already made substantial inroads replacing incandescent traffic signal lamps and have almost completely replaced all other lamps for use in emergency exit signs throughout the United States.⁶ In another aspect of reliability, the absence of breakable filaments contributes to make the idea of SSL attractive as light sources in environments inherently subject to vibration; e.g., instrument control lamps in all forms of transportation, overhead reading lamps in aircraft, brake lamps in automobiles and trucks, and vanity and door panel lamps in automobiles.

In addition to these prosaic, albeit important, benefits, SSL opens the possibility that architects, designers, and casual users of lighting will easily be able to achieve or to modify lighting-induced ambience without having to resort to energy robbing filters or having to maintain a collection of expensive specialty lamps. These possibilities include, but are not limited to, varying lamp color by mixing relative intensities of different colored LEDs or OLEDs, of dimming lamps without altering their color output, and, for OLEDs, of designing entire walls, table tops, or any other continuous surface to act as a controllable, distributed light source.⁷ Finally, there are many niche markets that, while forming only a small part of the overall lighting energy budget, would still provide substantial energy savings if incandescent lamps were replaced by SSL; the most important of these niche applications are holiday lighting and commercial signage.⁶ In another aspect of reliability, the absence of breakable filaments contributes to make the idea of SSL attractive as light sources in environments inherently subject to vibration: e.g., instrument control lamps in all forms of transportation, overhead reading lamps in aircraft, brake lamps in automobiles and trucks, and vanity and door panel lamps in automobiles.

In addition to these prosaic, albeit important, benefits, SSL opens the possibility that architects, designers, and casual users of lighting will easily be able to achieve or to modify lighting-induced ambience without having to resort to energy robbing filters or having to maintain a collection of expensive specialty lamps. These possibilities include, but are not limited to, varying lamp color by mixing relative intensities of different colored LEDs or OLEDs, of dimming lamps without altering their color output, and, or OLEDs, of designing entire walls, table tops, or any other continuous surface to act as a controllable, distributed light source.⁷ Finally, there are many niche markets that, while forming only a small part of the overall lighting energy budget, would still provide substantial energy savings if incandescent lamps were replaced by SSL; the most important of these niche applications are holiday lighting and commercial sinage.⁶

However, for SSL to displace most of these applications and, certainly, for it to disruptively take over any significant portion of the white light market, three advances must occur: 1) the price must come down (for white LEDs, a reduction of x100 is needed), 2) the efficiency must increase, and 3) the reliability must increase (this is especially important for OLEDs although it is also true for blue, green, and yellow LEDs). Because white-light lamps and area lighting form the largest component of the lighting budget and because efficient, reliable, and cheap generation of such white light from LEDs and OLEDs, which are inherently narrow in spectral output, forms the greatest barrier to the displacement of incandescent and fluorescent lamps by SSL, the remaining discussion of SSL will focus primarily on white lamps.

B. White Light Generation

There currently are three mechanisms by which solid-state lamps can produce white light:³

1. use of a blue source surrounded by a matrix containing one or two kinds of fluorescing particles, i.e., particles that will fluoresce in one or two
   wavelengths,
2. use of an UV LED surrounded by a matrix containing several different types of fluorescing particles, and
3. use of multiple active sources, either LEDs or OLEDs, to generate white light (Figure 1). This third approach avoids the need for fluorescent particles.

While it is commonly felt that the third approach will be the most efficient and is the most versatile, the first two approaches are more likely to enter the marketplace first.³ Advantages and disadvantages with each approach are enumerated below.

1. Blue light source with phosphor particles

This approach uses a blue source that is surrounded by a matrix containing a single, yellow-fluorescing particle or a matrix containing two different fluorescing particles, e.g., a red and a green. The phosphor material can be a traditional fluorescent, similar in behavior although quite different in material requirements to coatings in fluorescent lamps, or the material could be made up of quantum dots (QDs). In either case, the lamp concept is the same. Because the phosphor particles only partially occlude the blue light source and because the capture and conversion efficiency of blue light by the fluorescing particles is less than 100%, the light out of the lamp is a combination of the fluorescing wavelength(s) and the original bluesource wavelength. By choosing phosphor particle wavelengths appropriately, the combination of fluorescence and blue source light results in a white output. Two of the main difficulties with this approach are the "halo" effect and weak blue light absorption. The halo effect arises because the light from the blue diode is directional while the light from the phosphors is uniformly distributed. Consequently, while the center of the light spot appears white, the edges are dominated by the phosphor emission. Weak blue absorption requires thick phosphor layers. Currently, research is underway to make the diodes less directional in their output and to develop phosphor materials that are more efficient in both photon capture and photon emission.

2. UV LED with fluorescing particles

This approach is similar to the blue light source approach except that the LED output does not contribute to the visible light output. Therefore, all of the wavelengths required for white light generation must be supplied by the phosphors (i.e., phosphors for blue output must be included into the layer). The UV excitation results in high CRI values but it also means that the phosphor particle, in addition to being thermally stable, must be stable under UV radiation. Additionally, because the LED is not contributing to the visible output, the phosphor particles must be highly efficient in converting the UV light. Finally, because UV radiation typically degrades polymers, this approach is more likely to be used for LEDs than OLEDs.

3. Three Color LEDs/OLEDs

Use of three independent colored LEDs to generate a white light has two important advantages. First, by avoiding the need for phosphor particles, the conversion efficiency of the phosphors no longer limits the efficiency of the lamps. Second, because the light is being generated directly from the LEDs/OLEDs, the output color can be controlled by individual control of the separate LEDs or OLEDs, i.e., intelligent pixel management. However, this control can be difficult, as pointed out in a report from the Optoelectronic Industry Development Association³: while "[i]n the long term, this option may be the preferred method for producing high quality white light for general illumination," these benefits come at a cost. "... [B]ecause the ... different color components have different voltage requirements, different degradation characteristics and different temperature dependencies, a sophisticated control system might be required."³ In addition, the lifetime of the lamp will be determined by the minimum lifetime of the various LEDs/OLEDs that contribute to the white light. Currently the lifetime of red LEDs is 5 times that of blue LEDs. To achieve the desired lamp lifetimes, degradation mechanisms for the blue, green, and yellow sources must be identified and solutions to the degradation must be developed. Finally, there currently are no efficient green LED candidates. Therefore, for solid-state white lamps, without phosphor particles, there is an urgent need for the development of green LEDs to achieve the correct color control.

C. Scientific and Technical Barriers

The process of generating white light from solid-state sources requires several steps, each of which involves its own loss mechanisms and concomitant cost in efficiency. Because the overall device efficiency is the product of these individual efficiencies, each step in the process must be optimized to maximize the overall efficiency of the final device. A simplistic description of the key processes involved in developing a white lamp will help clarify the research and development prioritization schemes that are discussed below. In SSL, photons are generated via recombination of electrons and holes. Mechanisms that reduce carrier concentration, carrier mobility, or non-radiative exciton recombination limit the net output of the device. Therefore, such things as electric fields that inhibit charge injection, Fermi surfaces that reduce mobility, and point/distributed defects that act as alternative trapping sites, need to be identified and reduced or eliminated in order to increase light generation. Once generated, photons must escape the material without being reabsorbed or scattered into non-productive pathways. Consequently, the active region needs to be thin, the index of refraction for the surrounding material needs to be large and the geometry of the active region needs to be designed to minimize internal reflection. If the white lamp depends on down-conversion of blue or UV light, the efficiencies of the phosphor particles in capturing the excitation photons and in converting the energy of the captured photons into light act as additional limiting factors on the overall efficiency of the lamp. Additionally, for down-conversion lamps, the phosphor particles and their surrounding matrix must withstand high intensities of blue or UV light with minimal property degradation in order for the lamp to achieve the desired lifetime of multiple tens of thousands of hours. Finally, thermal management of the lamp is essential at all scales. While SSL generates light by direct electron/hole recombination rather than by joule heating, as an incandescent lamp does, the fact remains that, if a lamp has a 50% wall plug efficiency, as the best commercial LEDs currently do, then half of the energy used by the lamp will be converted to heat. If SSL is improved to achieve the goal of 80% wall plug efficiency, there will still be 20% of the energy being used to generate heat within the lamp. While much less energy is being used to generate heat in SSL than in incandescent lamps, the heat is being generated in very small volumes and can result in locally large thermal excursions that lower the efficiency, alter the output wavelengths, and degrade the lifetimes of the lamps.   Therefore, thermal management is a critical aspect of SSL design from the semiconductor size scale all the way through the lamp and housing scale. These various issues form the basic conceptual barriers that must be addressed for SSL to function. However, for SSL to succeed as an industry, the solutions to these barriers must be economical and practical as well as scientifically possible.

At the spring Department of Energy (DOE) SSL Workshop and Program Review⁸ held in San Diego, CA, February 3-4, 2005, industry, national lab, and university representatives discussed the scientific and technical barriers that must be overcome for SSL to displace incandescent and fluorescent lighting. The identified barriers were prioritized in terms of urgency and, based upon that prioritization, were placed into 2 - 3 year or 5 - 10 year time-frame bins. Because the barriers are essentially the same as those previously identified by the Optoelectronic Industry Development Association (OIDA),³ the NGLIA,¹ National Electrical Manufacturers Association (NEMA),⁹ and a presentation made by a SSL company, LUMILEDS, at a DOE conference on SSL,¹⁰ the discussion of technical barriers and priorities below will focus predominately on the consensus-based conclusions reached at the February 2005 DOE Program Review.

Figure 1 lays out the primary barriers that must be overcome for SSL to break into the lighting industry at a substantive level. The figure is divided into issues associated with inorganic SSL, or LEDs (Fig. 1a), and those associated with organic SSL, or OLEDs (Fig. 1b). Issues are divided into needed progress for both LEDs and OLEDs:

• further research and understanding are necessary (Core Technology)
• basic understanding is (or will be) in place and the remaining barriers are engineering and economic (Product Development).

Items that are italicized were, by consensus of the participants, determined to be the most urgent items; i.e., issues that need to be addressed in the 1 - 3 year time frame. The remaining items need to be addressed in the 5 - 10 year time frame or, in a few cases, are expected to be issues that would be addressed by individual companies on a proprietary level rather than on a consortium level. The following discussion addresses topics in both the 1 - 3 year time frame and the 5 - 10 year time frame. The ordering presented in the 1 - 3 year time frame discussion has no significance; the five items at the workshop that received the largest number of votes in each category were designated as the most urgent. The number of attendees at the workshop was not large enough to allow a meaningful prioritization of the five "most urgent" items in each category.

1. 1 - 3 year

a) Inorganic Core Research

In this area, the five most urgent research areas are: 1) large area substrates, buffer layers, and wafer research; 2) high-efficiency semiconductor materials; 3) device architecture approaches, structures, and systems; 4) strategies for improved light extraction and manipulation; and 5) phosphors and conversion materials. Some of the key aspects of each of these areas are:

1)   Large area substrates, buffer layers, and wafer research

1. Defects in substrates that propagate into the film active areas act as non-radiative recombination sites that degrade the efficiency of the light-emitting devices built on the substrates. Therefore, defect reduction in substrates is a high priority. However, defect reduction becomes increasingly difficult as substrates with larger areas are grown.
2. Ideal buffer layers between the substrates and the actives layers will not only isolate the active layers from defect propagation from the substrate but will also reduce dislocation formation in the active layers that results from thermal expansion mismatch between the substrate and subsequent film layers. The buffer layers need to accomplish this while allowing a high level of crystalline perfection in the active layer.
3. Research into fabrication of high quality (i.e., low defect density) wafers that come close to lattice matching the active layers could result in much lower dislocation densities in the active layers and avoid the need for the buffer layers. The nitrides are prime contenders in this area. In addition, ZnO is being investigated as a material that provides an alternate path.

2)   High-efficiency semiconductor materials

1. As mentioned previously, one of the highest priorities for multicolor LED white lamp fabrication is the development of an efficient, long-lived green LED.
2. In addition to development of an adequate green LED, cost reduction and reliability improvement of all of the nitride-based and ZnO-based LEDs remain high priority research areas. Increasing the active area, i.e., defect free region, of nitride films would provide an economy of scale as well as lead to more efficient LEDs.

3)   Device architecture approaches, structures, and systems

4)   Strategies for improved light extraction and manipulation

5)   Phosphors and conversion materials

1. Optimization of deposition methods, particle packing and distribution, particle layering, particle conversion efficiency, and matrix optical properties are critical to enhancing the efficiency of the down-conversion of the incident light. This task also includes issues such as design requirements to enhance forward scattering, rather than backscattering of the incident light and reduction of secondary absorption in the phosphorescing particles.
2. In addition to control of initial optical properties and conversion efficiencies of the phosphorescent particles, SSL requires long-term stability. Therefore, degradation mechanisms, via exposure to high intensity incident light, high local thermal excursions, chemical interface interactions, etc., must be identified and quantified, and strategies for minimizing degradation must be developed.

b) Inorganic Product Development

The corresponding five most urgent items for inorganic LED "Product Development" are: 1) integration of manufactured materials, 2) integration of LED packages and packaging materials, 3) optical coupling and modeling, 4) integration of electronics design, and 5) integration of thermal design.

1) Integration of manufactured materials

2)   Integration of LED packages and packaging materials

1. Integrated packages must include compatibility with both electrodes, and commercial design requirements as well as a thermal management system ( i.e., thermal pathways to dissipate heat without interfering with optical requirements).
2. Compatible electrodes must be chosen to fit package design requirements, e.g., transparent or opaque, low resistance, high adhesion.

3)   Optical coupling and modeling

4)   Integration of electronics design

5)   Integration of thermal design

It is important to bear in mind that, in the DOE SSL framework, the tasks listed under "Product Development" are expected to require minimal additional research. At this stage, the expectation is that the tasks should be headed toward commercialization. However, with regard to the NIST mission, the Product Development tasks remain important for two reasons. First, as commercialization is begun, issues associated with standards become more evident. Second, the steps between basic understanding, i.e., "Core Technology," and commercialization lie precisely in the mandated mission of the Advanced Technology Program. Therefore, knowledge of these issues could allow DOE and NIST/ATP to leverage off of each other's resources, to the benefit of all.

c)  OLED Core Research

Exactly the same process was used to identify the five most urgent issues on "Core Research" for OLEDS:

1. High efficiency, low voltage materials;
2. Approaches, structures, and systems for improved-performance;
3. Transparent electrodes;
4. Low-cost encapsulation and packaging technology; and
5. Investigation of low-cost fabrication and patterning techniques.

d) OLED Product Developmen

1. Substrates for electro-active organic materials.
2. High efficiency, low voltage, stable materials.
3. Implementing strategies for improved light extraction and manipulation.
4. Develop architectures that improve device robustness, increase lifetime and increase efficiency.
5. OLED packaging for lighting applications.

It is immediately clear that many of the issues associated with LEDs and OLEDs are the same, albeit, because of the differences between material properties of semiconductors and polymers, typically different solutions to technical barriers will be required. Nevertheless, efficiency, improved device structure, improved light extraction, low cost packaging technology, increased lifetimes, and thermal management are critical areas that need to be addressed in both materials systems. Indeed, one of the suggestions that came out of the workshop⁸ is that there needs to be "more cross-fertilization between LED and OLED" activities. However, because of the inherent differences in long-term stability between semiconductors and polymers, there is substantially greater emphasis in the OLEDs on low voltage materials, improved performance, and improved lifetimes.

2. 5 - 10 year time frame

Longer-term research and development requirements exist. Some of the items are considered long term because they reflect ongoing improvements and extensions of the current state of understanding in the basic sciences that underpin SSL. Others tasks are considered long term because their implementation is predicated on solutions being found for the more urgent, 1 - 3 year tasks discussed above.

a) Inorganic Core Technology

High priority inorganic "Core Technology" research issues in the 5 - 10 year time frame are: 1) development of reliability predictions and defect physics for improved LED lifetime; 2) development of improved encapsulate and packaging materials; 3) development of improved electrodes and interconnects; 4) development of measurement metrics and human factors for SSL; 5) improved physical, chemical, and optical modeling for epitaxial processes; and 6) design and development of in situ diagnostic tools for epitaxial processes.

1)   Development of reliability predictions and defect physics for improved LED lifetime.

1. An improved theoretical understanding and associated experimental tools are desired for predicting material or device properties as well as
   device failure interpretation. Current models are more empirical and/or qualitative than desired for device design or lifetime prediction, particularly as materials used in SSL evolve.
2. Droop, i.e., efficiency degradation at high temperature and current density, is a well recognized but poorly understood phenomenon. Accurate theoretical analysis with experimental confirmation is needed for droop to be overcome in current device designs and avoided in future devices.

2)   Development of improved encapsulate and packaging materials.

1. Similar to the phosphor particles and the active light generation medium that are highlighted in tasks for the 1 - 3 year time frame, packaging materials have to withstand elevated temperature, high intensity of visible and, perhaps, UV radiation, and environmental attack while allowing efficient optical extraction. In addition, the packaging material has to maintain its properties for the lifetime of the lamp.
2. As discussed previously, thermal management is a ubiquitous and potentially limiting issue that must be addressed at every level of lamp design.

3)   Development of improved electrodes and interconnects.

4)   Development of measurement metrics and human factors for SSL.

1. The interface with human usage and perception is critical for the widespread adoption of SSL. Human experience, expectations, and preferences for lighting need to be incorporated into lamp design; regardless of lamp efficiency and output power, if the lamp output is not perceived to be attractive, convenient, or desirable, SSL will not replace current lamps in the general lighting market.
2. Industry definitions of "white" light, and "quality" of light need to be developed and adopted. The variation of both white light and quality of light with specific applications need to be designed into lamps.
3. Standards for photometric measurements need to developed and adopted by the industry.

5)   Improved physical, chemical, and optical modeling for epitaxial processes.

6)   Design and development of in-situ diagnostic tools for epitaxial processes.

b) Inorganic Product Development

The 5 - 10 year time frame tasks for the inorganic SSL "Product Development" are closely tied to success of the 1 - 3 year tasks of the inorganic "Core Research".   In particular, four tasks in Materials Development are clearly dependent upon success in development of large, low defect density substrates, improved buffer layers, new high-efficiency LEDs that span the visible spectrum, and improved high-efficiency phosphors. These tasks are: 1) substrate, buffer layer, and wafer engineering and development; 2) high-efficiency semiconductor materials; 3) implementing strategies for improved light extraction and manipulation; and 4) device architectures with high power conversion efficiencies. Two other long-range goals, modeling of coupling issues for device integration and luminaire design and field tests, build upon the short-range tasks 1 and 2 (i.e., packaging issues) for inorganic "Product Development".   The final two "Product Development" long-range activities, (a) evaluation of human factors and evaluation of lifetime and (b) performance characteristics, are tightly coupled to similar long term tasks laid out under "Core Research".

c)  OLED Core Research

In the 5 - 10 year time frame, the following tasks exist for OLED "Core Research":

1)   Research into electro-active organic materials for substrates.

1. Addresses the difficulties associated with achieving efficient charge injection into the polymer semiconductors from the perspective of material development

2)   Improved contact materials and surface modifications for improved charge injection.

1. Addresses the difficulties associated with achieving efficient charge injection into the polymer semiconductors from the perspective of interface effects
2. Replaces the insulating substrate and the conducting film that lies between the substrate and the semiconducting polymer with an electro-
   active polymer substrate that has a work function comparable to the HOMO level of the semiconductor.
3. One consequence of low charge injection and mobility is the need for higher driving voltages (e.g., 10 - 20 volts instead of the desired 4 volts).

3)   Improved understanding of the fundamental physics associated with OLEDs.

1. The details of charge injection, mobility, and recombination need to be better understood for OLEDs.

4)   Strategies for improved light extraction.

1. This item is similar to the light extraction issues with LEDs discussed previously. Non-radiative recombination processes, scattering, and re-absorption need to be reduced to improve the efficacy of OLEDs.

5)   Down-conversion materials.

1. Again, the issues in down conversion are the same as the down conversion requirements in LEDs: quantum efficiency, long-term reliability.

6) Integration of electrodes and interconnects.

7) Measurement metrics and human factors.

1. The issues associated with measurement metrics and human factors (e.g., color quality, color acceptability) are identical to those mentioned above for LEDs. The longer time frame for this item in OLEDs reflects the feeling that efficacy, reliability, voltage, etc. are a more urgent priority for OLEDs and that human factors will follow after more immediate problems are solved.

8)   Improved physical, chemical, and optical modeling.

1. It is generally felt that modeling tools are not well established for OLEDs. This reflects the general feeling that understanding of the basic physics in OLEDs is not yet adequate.

d)  OLED Product Development

In the same time frame, the OLEDs "Product Development" tasks are:

1. Improved contact materials for improved charge injection;
2. Demonstration of various device architectures;
3. Simulation tools for modeling OLED devices;
4. Voltage conversion, current density, power distribution and driver electronics;
5. Luminaire design and field tests;
6. Module and process optimization and manufacturing;
7. Manufacturing scale-up of OLED material; and
8. Tools for manufacturing lighting modules.

Issues associated with manufacturability are built closely upon the perceived Core Research needs discussed previously. In addition, a number of these tasks mirror similar tasks associated with the LED lamp fabrication. These include strategies for improved light extraction, improved down-conversion materials, integration of electrodes and interconnects, measurement metrics and human factors, improved physical, chemical, and optical modeling, demonstration of various device architectures, driver electronics, luminaire design and field tests, and tools for manufacturing lighting modules. However, other tasks are more specifically related to the polymer aspects of OLEDs. In particular, the issues associated with charge injection, electrically active substrates, and manufacturing scale-up in OLEDs are quite different from the same issues associated with LED lamps. Finally, it should be noted that, while modeling and theory development are highlighted as needs for both LEDs and OLEDs, improved understanding of the fundamental physics is most pressing for OLED development.

3. Additional considerations

In addition to obtaining information from DOE, OIDA (Optoelectronic Industry Development Association), and industry sponsored workshop summaries and reports, information was also obtained from numerous individuals within various private companies involved in SSL development. Conversations were explicitly restricted to nonproprietary topics, focusing on generic, industry wide issues that NIST might address, i.e., standard reference materials, standard measurement procedures, basic research advances. While most of the topics that arose have been covered above, there was a repeated emphasis on the thin film aspects of SSL that was not explicitly addressed in any of the workshops or reports that we have seen. In particular, concerns exist over appropriate measurement techniques and data interpretation for very thin film systems, e.g., whether material properties remain the same as films are reduced to tens of nm in thickness, and what might be the impacts of reducing size to the point that systems are dominated by surface and interface properties rather than by bulk volume properties. In particular, questions exist regarding the proper procedures for measuring and interpreting thermal conductivity in systems composed of layers of very thin films. How does thermal conductivity of a film behave as film thickness decreases? These are particularly important problems as device size continues to shrink; designs for thermal management in these complicated thin film systems are made using models that assume bulk material thermal properties.  Similar concerns accrue to other material properties as film thicknesses continue to shrink; e.g., how do optical properties like index of refraction behave?   Will delamination become a problem as thicknesses continue to decrease? What impact do the large surface/interface areas have on long term reliability? These questions become increasingly difficult to answer as size scales in SSL devices move into the nanometer range.

D. NIST Interaction Opportunities in SSL

Several organizations exist in the United States that either are currently active in SSL or have been heavily involved in the recent past. These organizations include government agencies, industry consortia, individual companies, and universities. A list of key organizations whose output and conclusions have been used in this report is presented below. We also provide a short discussion of potential interaction benefits to both the NIST laboratories and the NIST ATP for each organization.

1. DOE

The Department of Energy (DOE) has become the national driver for SSL. This reflects three facts: 1) energy conservation is part of thr DOE mission, 2) as the head of a collection of national laboratories, the DOE has substantial research resources that it can direct towards SSL and 3) as the primary source of non-Department of Defense research funding in the United States, the DOE has financial resources to support industry efforts in both basic SSL research as well as in commercialization efforts. Two parts of the DOE are involved in the SSL program: the Office of Energy Efficiency and Renewable Energy (EERE)/Building Technologies and the National Energy Technology Lab (NETL). The overall program is directed by Dr. James Brodrick of the Office of Building Technologies.

a)  NIST Laboratories
There are three benefits for the NIST laboratories in working with the DOE SSL program. First is the possibility for setting up collaborations with industry or DOE laboratory partners who are either currently receiving funding or are attempting to put together a research plan in order to receive future funding. Second is that the SSL program provides a prioritized list of research areas with clearly associated industry impacts that can help to guide NIST laboratories research choices. Third is the potential to leverage funding for LED or OLED material or device issues for both agencies. Priorities are re-evaluated yearly at the DOE program reviews, in light of yearly progress and potentially evolving priorities.

b) ATP
As discussed previously, the DOE SSL program is divided into two components: "Core Research" and "Technology Development". Funding for Core Technology is intended to provide resources for overcoming scientific barriers to SSL. Funding for Technology Development is not intended for research; rather, it is intended for product development and commercialization. One feature of the DOE SSL program is that an industrial organization, the Next Generation Lighting Industry Alliance (NGLIA), provides technical guidance and has access to patented non-commercialized intellectual property (IP) that is developed under SSL Core Technology funding (see discussion below).   An opportunity for the ATP is to provide funding to small companies that have patented IP but have remaining high risk scientific/technological barriers to overcome prior to commercialization. These conditions precisely meet the ATP mandate.

2.  National Electrical Manufacturers Association (NEMA)

NEMA "is the leading trade association in the U.S. representing the interests of electroindustry manufacturers of products used in the generation, transmission and distribution, control, and end-use of electricity."9 As part of its activities, NEMA develops manufacturing roadmaps, develops standards, and lobbies for the interests of manufacturers in the electric and electronic industries. Recently, NEMA has expanded its activities to include SSL. As one of the activities associated with SSL, NEMA manages NGLIA, although NGLIA is not a part of NEMA.

a)  NIST Laboratories
Many NIST staff members participate in NEMA road-mapping efforts and in standards activities involving NEMA. NIST management also has a history of involvement with NEMA, e.g., attending meetings and giving presentations. These activities provide guidance as to industrial directions, allow NIST staff to develop relationships with industrial counterparts, and enhance opportunities for the NIST/industry collaborations.

b) ATP
Outreach activities to NEMA, via road-mapping activities, standards work, or meeting attendance, would provide ATP staff with the same opportunities as the laboratory staff to develop personal relationships with industry members as well as, both formally and informally, educate industry counterparts regarding the existence and purpose of the ATP. In addition, participation of ATP staff in road-mapping and/or standards discussions enhances ATP stature as being technically aware, informed and active.

3.  Next Generation Lighting Industry Alliance (NGLIA)

The NGLIA is an industry consortium focused on SSL under the DOE program. One of the benefits of being a member of the NGLIA is that any patented IP developed under the DOE SSL "Core Research" Program that is not in the process of being commercialized one year after the patent award is available to any consortium member under a non­exclusive license for "reasonable terms".11 The consortium is managed by, though not a part of the National Engineering Manufacturing Association (NEMA). As of February, 2005, the consortium members are: Corning, Cree, Dow/Corning, General Electric, GELcore, Kodak, Lumileds, OSRAM, Philips, and 3M.

a) NIST Laboratories
The primary potential benefit to the NIST laboratories for interaction with NGLIA is that NGLIA research directions and goals are shared by the major companies involved in the SSL industry. This provides guidance to the laboratories regarding the potential economic impact associated with SSL research directions. In addition, because the NGLIA provides technical guidance to the DOE regarding SSL road mapping, NGLIA endorsement of the importance of a proposed NIST laboratory activity enhances the importance that the DOE will place on the proposal.

b) ATP
If the ATP is conducting outreach activities to NEMA and sets up a relationship with DOE that will allow presentations at the DOE SSL program reviews, an explicit outreach activity to NGLIA sponsored meetings may be repetitive. This is particularly true since most of the members of NGLIA are clearly aware of both the existence and purpose of ATP, having submitted proposals in the past.

4. Optoelectronic Industry Development Association (OIDA)

OIDA is an industry consortium made up of predominately U.S. companies involved in photonics. As recently as 2000, manufacturers making up 80% of the dollar value of photonics manufacturing and sales in the United States were members of the OIDA. Until the recent economic downturn, fiber optic communications, networking, and optical computing were the main thrusts of OIDA. Around 2001, the OIDA began promoting the importance of SSL and educating Congress regarding the relevant issues. Since that time, the DOE has become the champion for SSL and the OIDA has turned its focus toward optical sensors. Nevertheless, the OIDA has organized excellent workshops and published reviews of the issues associated with SSL that provide important and relevant information.

a)  NIST Laboratories
The NIST laboratories have had a close working relationship with the OIDA for many years. This is particularly true for the Optoelectronics Division in the EEEL, which has based much of its choice of technical direction upon OIDA-generated roadmaps. However, because the OIDA represents such a large portion of the optoelectronics industry, CSTL, MSEL, and PL also have a track record of interacting with the organization, both in conducting research suggested by OIDA roadmaps and in participation at OIDA meetings. These roadmaps have provided and continue to provide valuable guidance to NIST laboratory research programs for assessing future directions of the photonics industry as a whole.

b) ATP
In addition to the NIST laboratories, the ATP also has a history of close interactions with the OIDA, with several ATP staff routinely both attending OIDA meetings and workshops and giving presentations. Because the OIDA intersects such a large percentage of the U.S. photonics industry, ATP outreach activities at OIDA meetings provide an excellent mechanism for alerting large segments of the photonics industry to the existence of ATP as a potential funding source for cutting edge technology development.

E. NIST Laboratory Research Opportunities

There are several groups at NIST that are working on areas identified as near-term or long-term needs for SSL. In addition, there are other groups that could use their expertise in microelectronics to address similar issues in photonic semiconductor systems and devices. In the outline below, we highlight some of the topical areas important in SSL that scientists at NIST could address for both the near term, i.e., one to five year period, and the long-term, i.e., five to ten year time frame.

1. Inorganic devices

Large area substrates, buffer layers and wafer research

i.   Uniformity

1. Across wafer uniformity
2. Wafer to wafer uniformity
1. Strain at buffer/substrate interface
2. Surface finish and structure

ii.   Defect structure

1. Defect identification
2. Cluster behavior
   

-    High-efficiency semiconductor materials
-    Reliability and defect physics for improved emitter lifetime and efficiency

i.   Defect identification

1. Sources
2. Effect on multilayer structures

1. Failure mechanisms
2. Lifetime prediction

-    High-efficiency phosphors and conversion materials

i.   Quantum efficiency

ii.   Reliability

1. Thermal effects
2. UV bleaching
3. Degradation processes

iii.   Distribution of conversion material

iv.   Interface scattering

-    Encapsulants and packaging materials

i.   Interface scattering

ii.   Reliability

1. Thermal damage
2. UV damage
3. Degradation processes
   

i.   Adhesion

ii.   Local heating

-    Measurement metrics and human factors

i.   Standards development

ii.   Color assessment

1. Device inter-comparison
2. Interface with human expectations

-    Physical, chemical, optical modeling measurement, and experimentation for substrate and epitaxial processes

i.   Efficiency

1. Self absorption
2. Interface scattering
   

1. Lattice mismatch effects
2. Thermal loading
1. Distributed
2. Local
3. Delayed failure
   

iii.   Chemical stability of very thin films

iv.   Index measurement

-    Evaluate systems lifetime and performance characteristics

i.   Lifetime prediction

1. Failure mechanisms
   1. Static and cyclic electrical and thermal loading
   2. Defects

i.   Perturbing effects

ii.   Generation and coalescence

iii.   Quantum efficiency degradation

1. Over time
2. At high current
   
3. Color control

2. Organic specific

Substrates for electro-active organic materials research

i.   Improved work function

ii.   Increased charge injection

-    High-efficiency, low voltage, stable materials

i.   Increased mobility

ii.   Increased light generation

1. Reduced non-radiative recombination
2. Enhanced reliability
1. Thermal stability
2. UV/blue degradation resistance
3. Reduced sensitivity to water degradation

-    Improved understanding of fundamental physics

____________________
The CRI is a parameter that quantifies how closely the appearance of a set of eight pastel colors illuminated by a test lamp compares to the appearance when illuminated by an accepted "standard" lamp of the same light temperature (i.e., warmth). The maximum value of the CRI is 100, which corresponds to an appearance identical to that obtained from the reference lamp.

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Date created: May 22, 2006
Last updated: June 7, 2006

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