The cold blue light emitted by fluorescent tubes and the orange glow from incandescent bulbs are familiar sights that underscore the difficulties in reproducing natural daylight artificially.

Jwo-Huei Jou and colleagues from the National Tsing Hua University1 now describe the development of organic light-emitting diodes (OLEDs) that not only reproduce natural-looking light, but which can be tuned so that their emission follows the changing color of natural light throughout the day. The device can be tuned simply by adjusting a voltage to produce the desired output — whether a bright high-noon glare or a gentle dusk-like glow.

The researchers used a multilayer OLED configuration consisting of layers of semiconducting conjugated molecules sandwiched between two electrodes. They built the layers up so that each layer in the stack emitted a different color: blue, green or red.

Fig. 1: A sunlight-like color-tunable OLED in operation, emitting warm-white light (top) and cool-white light (bottom).

When the device is switched on, electrons are injected from one electrode and holes are injected from the other. Where the two charge carriers meet, they recombine to produce energetically excited states known as ‘excitons’. These excited states emit their excess energy as light, the color of which depends on the layer of the device in which the exciton is formed.

As in most OLEDs, the holes are the fastest-moving charge carriers in the devices fabricated by Jou and his colleagues. So at low voltages, the holes travel much further than the electrons and meet them in the red-emitting zone. As the voltage is increased, however, the electron mobility increases, so the electrons can travel further into the device and meet the holes at first in the green, and then in the blue emission zones, thus changing the overall emission from a reddish glow — like twilight — to the bluer light of noon. The researchers also used a hole-transport-modulating layer, in which the concentration of red dopants could be changed, to adjust the color of the light emitted.

“These simple, single-OLED devices will find use as indoor illumination in sunlight-lacking areas, such as high-latitude countries,” says Jou. In the future, the researchers will improve the power efficiencies of their devices.