Taiwanese researchers have made an OLED structure that can emit all the hues of sunlight, simply by changing its terminal voltage.
The emitter is a series of layers including red/orange (590nm), green (495nm) and blue (460nm) OLED materials.
Holes move faster in the selected materials so, according Professor Jwo-Huei Jou of National Tsing-Hua University, without the hole modulation layer they would mostly make it to the all the way to the red OLED before they combined with sluggish electrons leaving the electron transport layer - making it a red emitter.
However, Jou's hole modulation layer restricts the number of holes that can pass into the red layer - fixing the maximum intensity of red emissions.
The result is that the excess holes concentrate in the green and blue layers where they combine with electrons that easily pass through the hole modulator.
Increasing the terminal voltage therefore increases green and blue emission whilst red remains constant.
By careful choice of layer thickness, Jou gets the output to follow the daylight locus of the CIE colour chart.
At 3V emission is 2,500k (sunset), at 5.5V it is 5,500K (noon), all the way up to 7V where the output is at 8,000K (noon at high latitudes).
Brightness increases with voltage across the same range, from 100 to 1000cd/m2, and power efficiency drops from 7.0 to 2.2lm/W.
Jou predicts efficiency can be boosted.
"Currently, the employed materials are all fluorescent type," he told Electronics Weekly. "Phosphorescent materials can generate much higher lumens per Watt."
The colour temperature range over which the device operates can be set at manufacture by altering the amount of dopant in the red section. Ranges from 1,875-6,030K to 3,100-18,800K have been demonstrated.
OLEDS are notoriously hard to keep alive. Does Jou see anything fundamental stopping this becoming long life material?
"OLED devices with pretty good life-span are easily obtainable with proper encapsulation," he said.
Full details can be found in Applied Physics Letters (Vol.95, Issue 1)
See also: Electronics Weekly's roundup of content related to LEDs, with a special focus on both white LEDs and coloured LEDs:
LED technology - White LEDs
LED technology - Coloured LEDs
LED technology - LEDs general
LED technology - OLEDs
LED technology - LEDs Lighting