The use of ionic salt dyes as amorphous, thermally stable emitting layers in organic light-emitting diodes

The conversion of two neutral dye molecules (D) to ionic salts $({\mathrm{H}}_{\mathrm{2}}{\mathrm{N–D–NH}}_{\mathrm{2}}\mathrm{\cdot 2HX})$ and their utilization as emitting layers in organic light-emitting diodes (OLEDs) is described. The dye salts, $\mathrm{AEQT\cdot 2HCl}$ and $\mathrm{APT\cdot 2HCl},$ can be deposited as amorphous films using conventional evaporation techniques. X-ray diffraction and scanning electron microscopy analysis, coupled with thermal annealing studies, demonstrate the resistance of the films to crystallization. This stability is attributed to strong ionic forces between the relatively rigid molecules. OLEDs incorporating such salts for emitting layers exhibit better thermal stability compared with devices made from the corresponding neutral dyes $({\mathrm{H}}_{\mathrm{2}}{\mathrm{N–D–NH}}_{\mathrm{2}}\mathrm{).}$ These results suggest that ionic salts may more generally enable the formation of thermally stable, amorphous emitting, and charge transporting layers.