Overcoming Crucial Challenges in High-Brightness White LEDs

Editor’s note: In last month’s magazine, ST columnist Bob Klausmeier addressed “Display Lifespan and Service” (see November 2007, page 46). Bob wrote, “Commonly, LED manufacturers caution that luminance declines after the first year of use, but real-world data suggests these projections are considerably understated, even though the life-expectancy question hasn’t been answered.” Bob exampled many long-life, LED-lamped displays, including the daily-use, Las Vegas-based Bellagio display, installed in 1999.

In the same issue, on page 32, columnist Marcus Thielen includes information from the Dept. of Energy’s (DOE) LED lamp study. The DOE has released an independent research report that says commercially available LED lamps, such as compact fluorescent lamp (CFL) replacements reach between 11 and 19.5 lumen/watt luminous efficiency, contrary to higher claims. The DOE concluded available LEDs can’t replace CFL or fluorescent (neon) lamps, regarding efficiency (www.eere.energy.gov). Although some circles regard this announcement as headline news, readers should note the DOE evaluation regarded fully fabricated lamps applied to common lighting uses, such as household lamp replacements. The DOE report says, “The pilot tests examined two ‘downlights’ – the type of spotlight that is typically recessed into a ceiling – as well as a task light and an under-cabinet light.”

Apparently, there are difficult, technological obstacles to overcome before applying high-brightness LEDs or other solid-state lighting (SSL) to general lighting applications, including the backlighting systems in plasma or LCD displays. Here, Kahn explains these obstacles.

After having seen color LEDs used as a light source in numerous electronic products for many decades, I’ve observed white LEDs slowly penetrate general lighting markets (including flat-panel displays illumination) in the last few years. The penetration is slow, because, unlike indicator or mixed-batch (RGB) LED lamp systems, humans prefer general (room or sign) illumination and retail-display lighting to have a certain desirable white glow and brightness that’s difficult to achieve in solid-state lighting.

The LED or solid-state lighting (SSL) market growth has been remarkable in the last five years, but, white, high-brightness LEDs (HB-LEDs) for general-illumination and retail-merchandise applications still face tough challenges.

The most successful white HB-LED applications would be general illumination of homes and offices, but this also appears to be most difficult to realize. Secondary applications include cabinet signage and illuminating retail-merchandise cases, including refrigerator cases in grocery stores. Although the concept of using LEDs for general illumination has spread among leading lighting manufacturers and some users, the public hasn’t seen any significant product-development announcement, or product introductions, of a white HB-LED-based lighting system for general-use applications.

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The challenges in developing general-purpose, LED-based lamping systems are multifold, and sometimes controversial. Obviously, they involve both business and technical issues that lead to high-cost investments, which prevent many companies from pursuing advanced systems.

One primary challenge, for example, is how to overhaul incandescent and fluorescent light fixtures, and other incumbent products, so that SSL systems can be suitably retrofitted, especially when the current lighting units generate substantial profits for large companies. Producing workable solutions for problems requires building new manufacturing systems, accompanied

by the gradual abandonment of existing manufacturing lines – an expensive investment. Applying a successful solution, however, will accelerate product and market development, which will convince LED and lighting manufacturers – and investors and the public – of the environmental and cost benefits.

One drawback appears to be communication among the LED, luminaire and lighting companies. The industry also lacks testing norms and a standard rating scale for LED modules’ overall performance and lifetimes. Expectedly, these issues complicate the business challenge and further delay white HB-LED’s technical and market development.

Crucial challenges
The business challenge is familiar: How do large companies replace (or slow down) their current, bread-and-butter production lines with the hope that new-technology products (that still have unsolved technology issues and require large investments and resources) will soon replace their set, profitmaking products?

Solutions are difficult because semiconductor-based technology, especially the newer, semiconductor optoelectronics technology, is a specialized field. Only a few scientists and engineers are familiar with the core science, and these limitations constrain growth, which also limits both investor and corporate decisions.

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A similar situation occurred during the laser-development period, although the task was easier, because laser-based industries (fiberoptic telecom and medical) are low-volume/high-cost fields. The risk was less, and, therefore, manufacturers easily adopted high-tech solutions.

Oppositely, general-lighting consumers demand low-cost and easy-to-use products. Further, residential users are accustomed to the lower-Kelvin degrees (a warm glow) of incandescent bulbs, a hue that’s difficult to produce in both fluorescent and LED lamps.

Addressing the challenges
An LED luminaire is a multi-component system. It comprises an LED chip, an encapsulation system, driver electronics, cooling (heat-sinks and fins), beam-control optics and more. A general-illumination, LED-based luminaire requires an HB-LED that typically comprises several phosphor-coated LED chips, or an ensemble of multiple-color LED chips. Each manufactured ensemble needs to produce a comparable white light.

In any configuration, high-density current passes through the LED chip, which causes the amount and the quality of white light to degrade over time. Slowing this degradation depends on the chip design, its phosphor technology and thermal-design system. It requires sophisticated, systems-engineering optimization methods.

In truth, most manufacturers cannot produce the required, high-performance lamp systems.

Such multi-disciplinary, core-competency engineering is rare in the industry. Moreover, the industry is facing a measurement and calibration discrepancy for light output and efficacy (lumen/watt) values, as recently acknowledged by the August 2007 DOE SSL Commercial Product Testing Program reports (www.netl.doe.gov/ssl/comm_testing .htm).

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Combining correlated-color temperature (CCT), color-rendering index (CRI) and lifetime-performance ratings magnifies this divergence. Various standards organizations are working to adopt some urgently needed standards. Energy Star®, a U.S. Environmental Protection Agency (EPA) joint program, and the U.S. Dept. of Energy (DOE), have set new standards on luminaires (search for “luminaires” at www.energystar.org).

While some organizations are working to develop SSL standards, certain luminaire companies are carefully measuring key HB-LED characteristics that occur under accelerated-aging and high-temperature conditions, to ensure that the LEDs used in their systems will meet customer requirements. Obviously, the results are more objective when luminaire manufacturers test manufacturer-supplied LED chips and modules.

Nualight (www.nualight.com) in Ireland is such a luminaire manufacturer; it supplies LED lighting solutions for the retail-display market.

The critical junction
The p-n junction (TJ) temperature helps determine LED lifetime, and, although related articles have appeared in various publications, their general focus is how TJ influences LEDs, the brightness, wavelength and forward voltage, but not the credible lifespan.

Another prime focus is measurement techniques. Unfortunately, no one in the SSL industry has yet provided an analytical expression (an accurate, quantitative definition) of TJ, nor has anyone listed its dependent parameters. Without such formulation, it’s difficult to accurately compare the lifetimes and time-varying performance of different suppliers’ LEDs. The absence of standard formulation often misleads lighting companies.

Still, certain key factors of TJ can help establish a quantitative definition:

• Chip-current density, which depends on semiconductor bandgap (the energy difference between the conduction and valence band), material morphology and chip-layout design; and
• Module design, the thermal, electrical, mechanical and optics construction that collectively determines how the heat is removed from the junction area.

Such material, design and packaging technologies establish how ambient temperature translates into TJ and what TJ level becomes destructive to an LED. Clearly, a more rigorous study is required to define TJ and to correlate it more accurately with LED lifetime for various chip and module configurations. Although higher TJ may be tolerated in some LEDs, typically a TJ temperature exceeding 85° C isn’t recommended for durable HB-LEDs.

Some groups have extrapolated the LED lifetime from TJ by applying indirect measurement techniques – infrared imaging and forward voltage – and combining this with an extrapolation of statistical data. This technique isn’t entirely successful with LEDs; it lacks measurement standards and, further, different vendors engineer different chip and module designs.

Sign illumination
Small-signage illumination, LED display and LCD backlights can utilize low-current LEDs where an increase in TJ has minimum effect on degradation. If needed, degradations in these applications can also be handled through electronic tuning (which happens while the lamp operates). However, large signs, plus general and retail cabinet-display lighting, require power ratings of 0.5W and above, thus adding tuning features that may not be realistic.

Industry spokespeople often tout the latest lumen/W values for white HB-LEDs, but the real improvements lie in consistently achieving an efficacy equal or beyond CFL, while maintaining the requirements for CCT, CRI, luminous intensity, long lifetime, minimal degradation, and, of course, low cost.

For wider acceptance within the supply chain, the industry must acquire an accurate understanding of LED-lamp characteristics and the necessary system-level performance requirements. This will occur upon further development of white HB-LEDs, as well as their standards, and the adoption of accurate and reliable measurement methods.

Such progress will substantially diminish the confusion that currently lingers between the LED and luminaire suppliers, and provide consistent and credible feedback to lighting companies.

As the clouds clear for lighting companies and regulators, they can expedite products for the mass media, including the retail-display industry, by widely offering general-illumination, HB-LED products and education.

In relating this information, it helps to remember previous disruptive lighting technologies, such as CFL, which, despite being seen as technologically superior to incandescent lighting, has been slowly adopted. Winning the standards challenges and establishing a better understanding of semiconductor-optoelectronic device parameters (such as the junction temperature) across the supply chain, will prepare the public for the SSL industry’s most potent dream: general illumination within a few years.