CHARACTERIZATION AND STABILITY OF LIGHT-EMITTING-DIODES BASED ON POLY(BITHIAZOLE)S

We report, for the first time, electroluminescence (EL) from a new cla ss of conjugated polymers: poly(bithiazole)'s, based light-emitting di odes. The emission spectra of these polymers range throughout the visi ble, from blue-green to red. These polymers also show high thermal and oxidative stabilities. LED's based on poly(N-methyl-4,4'-dinonyl-2,2' -bithiazole triflate) (MPNBT), one member of this polymer class, have been electrically characterized under ambient conditions, a dc voltage of about 5.0 V is needed to induce the light emission, and a stable e mission is observed when current density is greater than or equal to 9 5 mA/cm(2), However, after the light emission has occurred, a rapid de gradation of diode current level at a fixed voltage has been observed. At the same time, the current-voltage characteristic of the diode str ucture changes from a typical diode-like behavior to a nearly linear o hmic-like behavior, indicating a transformation at/or near the Al/poly mer contact. Further, some bubbles appear at the surface of the top el ectrode after light emission has occurred, showing the loss of adhesio n between the top Al contact layer and the polymer layer. We believe t hat the LED characteristic change and bubble formation originate front the same mechanism-the adhesion loss at the top contact/polymer inter face. The roughness of back contact and uniformity of polymer active l ayer are found to be critical far LED electrical stability.