DOE report updates LED research priorities

15 Apr 2009

Warm-white light, lower costs and a better idea of device lifetimes are near the top of the US wishlist

Targets set by the US Department of Energy (DOE) for solid-state lighting (SSL) product development are being achieved, but not all at the same time within a given luminaire.

That is one of the many conclusions reached by the DOE's latest annual report on SSL research and development, a document that details the wide range of work in the US aimed at fundamentally changing the country's lighting infrastructure to one based on LEDs instead of bulbs.

Milestones that had previously been set for 2008 included the development of an LED-based product with 80 lm/W efficacy, a device price of $25 per kilolumen, a 50,000 hr lifetime and a "warm-white" colour-correlated temperature (CCT) of less than 5000 K.

"These goals have been met individually," states the report. "In fact, some commercial products have achieved device efficacies greater than 100 lm/W." The only problem is that there is, as yet, no single product that can meet all of those targets simultaneously.

Despite that, the DOE report authors are confident that interim goals set for 2010 will be met. These include device efficacies of 140 lm/W and a commercial price of just $10 per kilolumen.

Jim Brodrick, who co-ordinates SSL activity at the DOE, says that the latest report represents something of an overhaul: "We completely revised the research and development task structure and updated priorities in recognition of how far we have come with the technology," he said.

The chief priorities highlighted by Brodrick include:

Achieving warm-white light: so far, the best device efficacies have all been achieved with cool-white LED emitters, more suited to task lighting than for created a relaxed home environment.
Lowering the cost of LEDs: this is very much seen as a key barrier to market entry, and even though the cost of SSL is declining it is still an expensive technology.
Gaining a better understanding of LED lifetimes: "It is still unclear how long they truly last," Brodrick remarked.
Changing the common misperception that LEDs can only offer dim light, seen as a significant barrier to market.
Encouraging more investment in organic LED development: although OLEDs are now widely used in small display applications, devices suitable for lighting remain a much more distant prospect.

One new element in the DOE's March 2009 report focuses on system reliability. Task "A6.3" calls for the development of models and experiments to determine the lumen depreciation of an integrated SSL luminaire, using accelerated lifetime testing.

The target set for 2015 calls for 99% lumen maintenance after 6000 hours of operation, and 90% lumen maintenance after 50,000 hours.

Although the report confirms that much work remains to be done before SSL reaches true commercial viability, Brodrick is clear that good progress has been made.

"Milestones are being met," he said. "And DOE-funded prototypes are in the process of becoming marketable products. It is an exciting time."

• This article first appeared on compoundsemiconductor.net.