Light-Emitting Field-Effect Transistors with π-Conjugated Liquid Crystalline Polymer

Abstract

Organic light-emitting field-effect transistors (OLEFETs) were fabricated with a π-conjugated liquid crystalline polymer of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). Thermal annealing effects on film morphology, crystalline structure and OLEFET characteristics were investigated for spin-coated films of F8T2. The film annealed at the isotropic phase of 350 °C grew in homogeneous polycrystalline while the film grown from the nematic phase at 290 °C was composed of microcrystalline grains and the films annealed at temperatures lower than 250 °C were amorphous. Bottom-contact/top-gate OLEFET devices were fabricated with indium-tin-oxide (ITO) source/drain electrodes, a poly(methyl methacrylate) (PMMA) insulating layer and a gold gate electrode. Under usual DC gate operations, the homogeneous polycrystalline film exhibited well-balanced ambipolar carrier injection. On the other hand, the nematic microcrystalline film was less electron-transporting due to electron traps at the grain boundaries. Light emission from the devices was observed under AC-gate operations. By applying square-wave voltages to the gate, light emission was detected at the both edges of the source and drain electrodes by alternating injection of opposite carriers even when the source and drain were grounded. The highest light emission was obtained with the homogeneous polycrystalline film grown from the isotropic phase. The light intensity was enhanced in the channel region by biasing the source negative and the drain positive so that the holes injected from the drain were transported to recombine with the electrons injected at the source edge. The light emission intensity from the film with nematic grains annealed at 290 °C was lower than that from the amorphous film annealed at 250 °C due to slightly higher electron mobility in the latter film.