Household and commercial lamps are designed to emit a broad spectrum of light composed of many wavelengths. But LEDs typically emit light of just one wavelength, which is determined by the energy gap between the excited state and the ground state of the atoms or molecules that make up the LED.

Earlier white-light LEDs were made from a combination of atoms or molecules with different energy gaps, so that the LED emits light at many wavelengths, simulating white light. In practice, however, the different materials used in these devices degrade at different rates, so the spectrum of such white-light LEDs changes over time. This makes them unsuitable for use as lights, which must have a stable spectrum over their entire lifetime.

Duggal’s team has solved this problem by using a technique known as ‘down-conversion’ to generate light of many wavelengths. To make their device, the researchers placed an organic LED that emits blue light ‘face down’ on a glass substrate. On the other side of the glass, they deposited layers of dyed polymers that contained particles of phosphor.

When light from the LED enters the polymer layers, some of the blue photons are split – or ‘down converted’ – into two longer-wavelength photons by the phosphor particles. As the original photons and the new photons encounter subsequent polymer layers, this process is repeated again and again. When the light emerges from the device, it consists of a wide range of wavelengths – that is, it is white light.

Different white light spectra are distinguished by their ‘colour temperature’, which is the temperature at which a perfect black body would emit the same spectrum. A tungsten filament light bulb emits reddish light and has a colour temperature of around 2700 kelvin, while a ‘bluer’ metal halide lamp has a colour temperature of around 6000 kelvin.

The colour temperature of the new device can be tuned between 3000 and 6000 kelvin by changing the thickness and the number of polymer layers. Since the LED is based on light produced by just one type of molecule, Duggal and colleagues say it should be very stable and cheap to produce.