Printed Electronics Materials Outlook: Printed OLED Lighting

Printed lighting has been around in the form of EL lighting for many years, but is limited in the applications it can serve, by the lack of brightness. Every so often there are innovations in EL materials, but no one is really expecting major changes that could revive the fortunes of EL, whose traditional markets are slowly being eaten up by high brightness LEDs. At the other end of the scale an entire new generation of printed lighting may emerge as the result of current R&D and productization work being carried out using the emissive properties of carbon nanotubes.

With all that said, most of the interest and opportunities are to be found in OLED lighting a technology that may address a broad range of lighting applications and which for the last couple of years has received funding from both government and private interests. Potentially, OLEDs bring a number of very attractive features to the lighting marketplace. They are low power consuming, bright, can be fabricated on flexible substrates, and can serve as floodlights. As far as the last of these items is concerned, HB-LEDs are more spotlight-like, so OLEDs are potentially complementary to HB-LEDs. The next year should prove a formative one for the OLED lighting industry, because some of the R&D projects are coming to an end and the performance of the first products (perhaps in 2008,) will set the pace for how fast this segment grows. Today, printability is certainly a secondary issue in the OLED lighting space. However, some research groups are specifically looking at printing OLED lighting panels.

While many of the materials issues that come up in the context of printed OLEDs are the same whether the applications are displays or lighting, but there are certainly some differences when it comes to the requirements for efficiencies, lifetimes, brightness, color quality and environmental resilience, for example. This opens up the market to specialist materials, although it will be several years before volume opportunities in this space.

Implications for Materials:

Some of the biggest names in electronics and lighting are involved in R&D on OLED lighting including Add-Vision, GE, Kodak, Novaled, Osram, Philips, and Siemens. All of these firms are involved in materials issues to some extent. And OLEDs are very much a materials game: a full 40 percent of the Euros going to projects in the European Community's OLLA OLED development program (see below), for example, were for materials development.

Obviously, many of these issues and the ones involved with printing OLED lighting are very similar to those discussed for OLED displays, but there are certainly some differences when it comes to the requirements for efficiencies, lifetimes, brightness, color quality and environmental resilience, for example. Requirements also differ across the gamut of lighting applications, as they do across different FPD application spaces. What is suitable for an OLED used as an LCD backlight, for instance, may not serve for general lighting applications, and vice versa; just as a passive monochrome OLED FPD is fine for the secondary display of a cell phone, but not for its primary display. Today, printability is certainly a secondary issue in the OLED lighting space. However, we note that the U.K.'s Department of Trade and Industry's Knowledge Transfer Network project specifically has an objective of printing lighting.

Generally, speaking the brightness demands on OLED lighting are more extreme than they are on OLED FPDs. By analogy with current LCD-based products, an OLED display for a laptop computer screen or desktop monitor would be required to deliver about 200 nits of brightness in most situations, and as much as 400 nits or so in some niche industrial and military applications. An OLED lamp used to backlight an LCD on a laptop, on the other hand, would need to crank out upwards of 4,000 nits to deliver just 200 nits to the user. The reason is that LCDs are extremely inefficient in the amount of generated light that gets through to the eye. Size is also a major challenge; for an entry-level OLED lamp aiming to replace a conventional fluorescent fixture, the area requirement would be 3 feet on the diagonal or more--quite a challenge for OLEDs, which are today usually just a few square inches.

Achieving uniformity in the large-area deposition of OLED materials may also pose a challenge for lighting applications.

The state of the art for OLED lamps today is roughly in the 1030 lm/W range, but that's likely to be a short-term frontier. The DOE's technology roadmap points to possible efficiencies of 100- to 150-lumens/W for OLED lighting in the long run. Blue has traditionally been the most difficult color to deal with, for both OLEDs and ILEDs, but there's a great deal of work being done in this area to good effect.

Lifetime requirements for OLED lamp applications are, like other parameters, highly application dependent. A few thousand hours of life may be adequate for a cell phone backlight, but other applications such as TV backlights will require one hundred thousand hours of life or more. For early lighting applications, such as effect lighting, the Osram-OS team believes a lifetime of at least 1 Khr for 1,000 nits is required. Add-Vision believes that 1 Khr life is "a key commercialization target for our partners and customers," but this company is focused on some backlighting segments for which 100--200 nits is sufficient.

The lifetime of OLEDs has grown by leaps and bounds over the past few years. However, the gods of organic chemistry have imposed the law of differential aging among the primary red, green, and blue (RGB) colors conventionally used to create white light. As the brightness of the material in, say, a green lamp declines over time, the lamp grows dimmer. But as the green material in an RGB white lamp glows less, the color balance of the RGB system is thrown off. Color stability is a serious issue for the lighting materials market, complicated by this differential aging issue. As usual, blue is the worst offender.

In concluding this section, a couple of comments on EL and CNT lighting materials may be worth making. New developments in EL lighting materials occur from time to time, but none of them seem likely to propel EL lighting into new markets in any significant way. It is worth noting, however, that all EL lighting is printed. Applied Nanotech is a firm that has been behind many of the announcements for CNT lighting. However, commercialization in this segment is a relatively minor activity when compared with the work being done on OLED lighting, although the work on CNT lighting does seem to typically involve CNT inks.