Another Way to Build a Better White Light

Hank Hogan

When it comes to replacing today’s white lights, don’t count out solid-state light-emitting electrochemical cells. Researchers from National Taiwan University in Taipei recently demonstrated that cells based on cationic transition metal complexes can produce white light with a peak external quantum efficiency of 4 percent and a peak power efficiency of 7.8 lm/W.

Based on these results, the devices could be a promising alternative to white organic LEDs. Moreover, they would have what chemistry professor Ken-Tsung Wong noted could be a significant advantage.

Because of their single-layer structure, light-emitting electrochemical cells cost less to make.

Researchers designed red and blue light-emitting electrochemical complexes that produced a white light when combined. A CIE color chart shows the coordinates where the emissions occurred. LEC = light-emitting electrochemical cell. Reprinted with permission from the Journal of the American Chemical Society.

Other advantages include the ability to process the light-emitting layer from solutions and air-stable electrodes such as gold or silver. Also, the devices operate at very low voltages yet have high power efficiency.

In a light-emitting electrochemical cell, an electrolyte carries current, releasing light in the process. In their work, the investigators used iridium, a cationic transition metal complex, in part because such complexes are intrinsically ionic and, therefore, conductive. They also are phosphorescent, which enables high electroluminescent efficiencies.

The researchers fabricated devices by spin-coating an iridium-containing solution to form a 100-nm-thick film on glass substrates previously coated with indium tin oxide. They then used thermal evaporation to form a 150-nm-thick silver top contact as a cathode.

A key to the process was the molecular design of the iridium complex. Wong noted that a critical step was a substitution at a particular site in the diazafluorene ligand. Without it, the diazafluorene would be too reactive toward the formation of the iridium complex. After trying various compounds, the researchers found that alkyl substitutions exhibited wavelengths with the shortest emission.

That characteristic was important because the devices consisted of mixtures of blue- and red-emitting complexes. The combination of the two resulted in white light, with the blue a significant factor in making the light whiter. “That drove us to design a better blue emitter,” Wong said.

The researchers then characterized the assembled emitters, driving a voltage through them and measuring the resulting light intensity with a silicon photodiode made by UDT (now part of OSI Optoelectronics in Hawthorne, Calif.). They measured the electroluminescent spectra of the devices with an Ocean Optics spectrometer equipped with a CCD array detector. The blue emitter showed no redshift in solution or when in a thin film. When combined with the red emitter in the proper ratio, the resulting white light had a color rendering index of up to 80, high enough to be an important characteristic for solid-state lighting.

Although these results are promising, Wong noted that issues remain that must be addressed. One is the response time — the time the device takes to reach maximum brightness. Another is device lifetime. The researchers are investigating possible solutions to both.

Journal of the American Chemical Society, March 19, 2008, pp. 3413-3419.