US scientists have combined fluorescence and phosphorescence to create the most efficient white OLED yet

US chemists and electrical engineers have combined fluorescence and phosphorescence to create the most efficient white organic light emitting diode (OLED) yet developed. 

OLEDs are carbon-based semiconductor devices that generate light from electricity through the production of excitons - electron-hole pairs that recombine to emit photons. The electric current generates two types of excitons - high energy singlets and lower-energy triplets - with one singlet produced for every three triplets. Normally, only the singlet excitons generate light, so OLEDs were not very efficient.

In 1998, researchers from Princeton University and the University of Southern California, led by Stephen Forrest, found that adding compounds based on platinum or iridium to OLEDs meant that triplet excitons could also generate light via phosphorescence (a relatively slow acting form of fluorescence). Using phosphorescent compounds that shone red, green and blue, Forrest and colleagues produced OLEDs that generated white light. Unfortunately, blue phosphorescent compounds tend to be fairly unstable and inefficient, and this has held back the development of white OLEDs.

Forrest’s team has now developed a relatively efficient white OLED, which contains a blue fluorescent compound in conjunction with red and green phosphorescent compounds. The OLED was designed so that singlet excitons can only travel to the fluorescent compound, where they generate blue light. The triplet excitons reach the phosphorescent compounds, where they generate red and green light.

The OLED is much more efficient at generating white light than any previous OLED, and is almost twice as efficient as conventional light bulbs. ’The beauty of this approach is that it harvests the energy in a near-optimal manner, whilst also avoiding the stability problems associated with the use of blue phosphors,’ said John de Mello, a lecturer in nanomaterials at Imperial College, London.

John Evans