05 May 2006
Organic electroluminescent devices made using low-cost inkjet printing or reel-to-reel fabrication could one day surpass the power efficiency of commercial fluorescent lamps say researchers in the US.
A team from Cornell University, US, has overcome the hurdles of high voltage and ac operation to devise a mains powered OLED that could pave the way for energy efficient lighting. Today, lighting accounts for around 20% of the energy used in the US, with an estimated environmental cost of over 10 million metric tons of carbon emission per year. (J. Appl. Phys 99 074502)
By cascading several low-voltage OLEDs in series, the researchers were able to design a panel that can support a mains level supply of 110V or more. However, the challenge of ac operation proved to be harder to solve. Conventional LEDs require an ac/dc converter or dc voltage offset before they can be used with an ac supply.
One option is to formulate the OLED using ionic transition metal complexes (iTMCs), which emit light for both positive and negative voltages and therefore suit a pure ac supply. Unfortunately, these devices can suffer from slow turn-on times, resulting in lights that flicker or blink.
To tackle this, the Cornell team decided to look at iTMCs with a high intrinsic ionic conductivity based on recent success in the lab. Other approaches such as reducing the thickness of the iTMC layer or increasing the applied bias have been shown to improve turn-on time, but can adversely affect device efficiency and degradation rate.
"We had achieved a significant improvement in the dc turn-on times of iTMC devices based on iridium and ruthenium with ionic liquids, but we didn't know exactly how fast we could get the OLEDs to switch," Jason Slinker of Cornell University's Laboratory for Organic Electronics told Optics.org. "When we connected one of these devices to a function generator at 60 Hz, we were pleasantly surprised to observe the stable emission of light - at that point we had all of the necessary pieces to make a mains powered device."
Lifetime and efficiency still remain as barriers in terms of taking devices to the lighting market. However, Slinker believes that the efficiencies of the group's iTMC-based devices are now comparable to incandescent bulbs, although prototypes have yet to reach the standard of fluorescent lighting.
Ultimately, he thinks that devices will need to move from their current level of around 25 lm/W to more than 50 lm/W before the technology will make a big impression on the lighting sector. "Some improvement can be made by clever engineering, but I suspect that most of the progress will come from careful design and synthesis of new complexes," he added.
As for lifetime, degradation of the device's complex layer currently limits its operation to around 1000 hours. Slinker feels that researchers are now closing in on the key factors, but at least an order of magnitude improvement will be required for most commercial applications.
James Tyrrell is News Editor on Optics.org and Opto & Laser Europe magazine.
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