Building a better OLED

For several years organic LEDs have been used in devices such as phones, cameras, and more recently, televisions. They're thinner and lighter than crystalline LEDs, and organic molecules can be made to emit in every part of the visible spectrum. But not every organic molecule makes for a good OLED. Because electrons and holes are generated with random spin, electrically excited molecules are about three times as likely to end up in a spin triplet state, from which phosphorescence is quantum mechanically forbidden, than in a spin singlet state that readily fluoresces. Quantum efficiency is thus capped at an impractically low 25%. In 1998 researchers at Princeton University and the University of Southern California boosted the efficiency to near 100% by incorporating atoms of platinum or iridium, whose strong spin–orbit coupling facilitates phosphorescence. But Pt and Ir are expensive, and phosphorescent OLEDs have other disadvantages. Now Chihaya Adachi (Kyushu University in Japan) and colleagues have found another path to high OLED efficiency. They designed a class of organic molecules in which the energy difference ΔE between the singlet and triplet states is less than 100 meV (compared with the more typical 0.5–1.0 eV). Thermal energy can thus promote triplet-state molecules into the singlet state, resulting in thermally activated delayed fluorescence, as shown in the figure. The TADF OLEDs can be made to emit in a wide range of colors and are poised to rival phosphorescent devices in efficiency. (H. Uoyama et al., Nature 492, 234, 2012.)—Johanna Miller