Characteristics of polythiophene surface light emitting diodes

Surface light emitting diodes (SLEDs), in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings (250 nm and 10 or 20 μm) and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison (electrode separation 50 nm). In this study, the emitting layer was poly[3-(2′,5′-bis(1″,4″,7″trioxaoctyl)phenyl)-2,2′-bithiophene] (EO–PT), a conjugated polymer based on polythiophene with oligo(ethyleneoxide) side chains. The current–voltage (I(V)) and light–voltage (L(V)) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide (ITO)–aluminum, gold silicide–aluminum, and gold silicide–gold SLEDs, but the turn-on voltages were lowest with the ITO–aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO–PT increased the current and brightness, decreased the turn-on voltages, and made the I(V) characteristics symmetric; thus, a device with an electrode separation of 10 μm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.