Electroluminescent Properties of Organic Light-Emitting-Diode Doped with Squarylium Dye

In order to obtain highly pure red emission, the authors prepared organic light-emitting-diodes (LEDs) with a squarylium (Sq)-doped aluminum quinoline (Alq3) emission layer. We discussed their electroluminescence (EL) and electrical properties. The EL from Sq had a peak wavelength of _??_680nm and a half-width of _??_30nm. Only the EL from Sq (purely red) could be observed at the doping concentration of >14mol%, but the luminance was low (a few cd/m2) and unpractical. Although Sq molecules are thought to act as trap site in Alq3 molecules, they are concluded to act as hopping site at doping concentration of >7mol%. When Al-Li alloy with lower work function than Al is used as a cathode, the EL efficiency of Sq-doped Alq3 LED was not improved in spite of that of undoped Alq3 LED enhanced seven times. Therefore, the EL mechanism of organic Sq-doped Alq3 LED cannot be explained by simple carrier trap and energy transfer models.