Colorless Dopants Help Optimize OLEDs

For several decades scientists have been dreaming of making inexpensive sheets of white organic light-emitting diodes (OLEDs) to serve as lighting tiles and wallpaper. Although some products have reached the market, they don’t yet have all the desired properties. One goal has been to develop a fully transparent hole-transport layer. Tungsten or molybdenum oxides and fluorinated tetracyanoquinodimethanes are dopant molecules typically used for this layer. But these colored compounds lead to undesired hues and reduce optical efficiency. A team led by Günter Schmid of the electronics company Siemens AG in Erlangen, Germany, and Marina A. Petrukhina of SUNY Albany have now devised fluorinated copper(I) carboxylates as colorless dopants for the hole-transport layer (Adv. Mater. 2013, DOI: 10.1002/adma.201303252). The dopant normally interacts with a polyaromatic amine electron hole conductor, generating delocalized holes that travel to phosphorescent or fluorescent dyes in an adjacent layer where the light is emitted. The team showed that copper(I) pentafluorobenzoate interacts sufficiently with the amine to stimulate hole transport but not so strongly as to induce charge transfer in the visible region to produce the unwanted color. The German lighting company Osram has used the new dopants to make OLED prototypes that perform as well as previous versions but with transparent hole-transport layers and higher power efficiency.