Characterization and stability of light-emitting diodes based on poly(bithiazole)'s

We report, for the first time, electroluminescence (EL) from a new class of conjugated polymers: poly(bithiazole)'s, based light-emitting diodes. The emission spectra of these polymers range throughout the visible, 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 emission is observed when current density is greater than or equal to 95 mA/cm(2), However, after the light emission has occurred, a rapid degradation of diode current level at a fixed voltage has been observed. At the same time, the current-voltage characteristic of the diode structure changes from a typical diode-like behavior to a nearly linear ohmic-like behavior, indicating a transformation at/or near the Al/polymer contact. Further, some bubbles appear at the surface of the top electrode after light emission has occurred, showing the loss of adhesion between the top Al contact layer and the polymer layer. We believe that the LED characteristic change and bubble formation originate front the same mechanism-the adhesion loss at the top contact/polymer interface. The roughness of back contact and uniformity of polymer active layer are found to be critical far LED electrical stability.