Guest post by Jim Brodrick, U.S. Department of Energy
OLEDs don’t get as much publicity as LEDs, because they’re a good five to seven years behind their inorganic cousins in terms of commercial deployment. But they have considerable potential to save energy, as well as a number of other advantages. So where do we stand with OLED technology today?
Over the past two decades, the widespread use of LEDs in calculators, cell phones, televisions, indicator lamps, and related applications has driven the combination of basic research and manufacturing development needed to produce LEDs that are commercially viable for general illumination. OLEDs, on the other hand, are relative newcomers and find themselves at a critical stage of their development when it comes to general illumination. While they’ve had success as displays in handheld devices and show great promise for use in flat-panel TVs, the commercial demand for them has not been anywhere near as broad as it’s been for LEDs and has been much more recent, so that OLEDs haven’t benefited from the same kind of investment. As a result, they still face a host of challenges in becoming adopted for lighting applications.
One of the biggest challenges is price. The current price of an OLED panel is somewhere in the neighborhood of $1,500/klm – compared with around $30/klm today and falling rapidly for an LED A-type lamp and $4/klm for a fluorescent T8 lamp and ballast. For OLED luminaires to be cost-competitive, the panel price needs to come down by at least a factor of 75 – to ~$20/klm or less – by 2020.
The root cause of this problem can be attributed to the nature of OLED devices. Because OLEDs are large-area, diffuse light sources, they require larger amounts of costly materials and precise manufacturing control and handling over large areas. Increasing the brightness of the OLED panels reduces the device area needed to achieve a given light output, which in turn leads to lower materials and manufacturing costs. Thus, developing an effective low-cost, scalable light-extraction approach would have the biggest near-term impact on efficacy and cost.
And while increasing the lumen density of the OLED panels can have a large impact on the cost of both panels and luminaires, this can’t be done at the expense of panel lifetime. Developing stable novel materials and device architectures that have even higher efficiency and are suited to low-cost manufacturing are the other main challenges. In addition to increasing the efficacy of devices, materials costs need to be simultaneously reduced. In terms of materials, this includes substrates with light-extraction enhancement layers, the organic emission and charge transport layers, and the encapsulation materials and approaches. In terms of device architecture, a low-cost approach needs to be developed that yields consistent performance over very large areas. But merely improving the design of materials and device structures won’t be enough to reach DOE and industry cost targets. Reaching those targets will require increased production volume and manufacturing experience.
What about OLED performance? Considerable advancement has been made in the past year or so, with efficacy up to 60 lm/W, CRI ~80, and brightness increasing to levels considered appropriate for general illumination. But OLEDs have the potential to become much more efficient, so significant headroom remains – particularly in light-extraction efficiency (which is now 70 percent) and reduced operating voltage. There’s also room for improvement in the internal quantum efficiency and spectral efficiency of the panels, as well as in the driver and optical efficiency of the luminaire. Making all of these improvements will allow OLED panel performance to reach 190 lm/W, and OLED luminaire performance to reach DOE’s 2020 target of 140 lm/W. However, these gains need to be developed while keeping costs competitive with other lighting technologies.
As for the future of OLEDs for general illumination, many feel that it hinges on differentiating them from other lighting technologies and demonstrating how they can be complementary rather than an either-or alternative. That shouldn’t be a hard sell, given the inherent advantages OLEDs have. OLEDs can be configured as large-area, diffuse sources whose soft light can be viewed directly, with less need for shading, lenses, diffusers, louvers, or parabolic cells. This makes them ideal for general ambient lighting. On top of that, they can be molded to almost any shape when made on substrates that are thin and flexible, which allows them to be integrated more tightly into architectural designs. This not only shatters conventional conceptions of what a luminaire should look like, but even blurs the distinction between luminaire and light source and opens up such intriguing possibilities as windows that give off their own light at night.
A number of OLED products that are available on the market today show that the future has already begun. For example, Lumiotec came out last year with a portable flat panel equipped with a hook, which can be hung from any projection. And Acuity Brands now offers four separate families of OLED luminaires: Revel™, Kindred™, Trilia™, and Canvis™ – the latter featuring a luminous surface that changes shape by interacting with building occupants through gestural control.
OLEDs are not likely to replace LEDs, but they clearly have the potential to become a viable part of SSL that makes us more energy-efficient while helping to transform the lighting landscape. And although it may take time, they’re moving closer to that point with each passing year.
