Showing papers by "Yang Yang published in 1997"

Efficient blue-green and white light-emitting electrochemical cells based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl]

Abstract: Efficient blue-green polymer light-emitting electrochemical cells, based on poly[9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl] (BDOH-PF), have been demonstrated. BDOH-PF has a high photoluminescent quantum efficiency of 73% photon/photon in solid films. The polyether-type side groups of this polymer are built in to promote the ionic conductivity necessary for the operation of light-emitting electrochemical cells (LECs). Blue-green LECs using a blend of BDOH-PF and lithium triflate as the emissive layer, ITO and Al as the electrodes, achieve an external quantum efficiency of 4% photons/electron and a luminous efficiency of 12 lm/W at 3.1 V. The brightness reaches ∼200 cd/m2 at 3.1 V, and exceeds 1000 cd/m2 at 3.5 V. When additional poly(ethylene oxide) (PEO) is blended into the emissive layer, phase separation between PEO and BDOH-PF occurs, and the emission becomes white. These white emitting LECs reach an external quantum efficiency of 2.4% photons/electron and 400 cd/m2 brightness at a bias of 4 V.

Polyfluorenes as materials for photoluminescence and electroluminescence

Abstract: Fluorenes and polyfluorenes having one or two polar-group-containing alkyl substituents on their "9" carbon methylenes are disclosed together with the polymers' use as luminescent materials in organic light-emitting diodes and light-emitting electrochemical cells.

Polymer Electroluminescent Devices

Abstract: Electroluminescence (EL) is the emission of light generated from the radiative recombination of electrons and holes electrically injected into a luminescent semiconductor. Conventional EL devices are made of inorganic direct-bandgap semiconductors, such as GaAs and InGaAs. Recently EL devices based on conjugated organic small molecules and polymers have attracted increasing attention due to easy fabrication of large areas, unlimited choice of colors, and mechanical flexibility. Potential applications of these organic/polymeric EL devices include backlights for displays, alphanumeric displays, and high-density information displays.Electroluminescence from an organic material was first demonstrated in the 1960s on anthracene crystals by Pope et al. at New York University. Subsequently several other groups also observed this phenomenon in organic crystals and thin films. These organic EL devices had high operating voltages and low quantum efficiency. Consequently they did not attract much attention. In 1987 a breakthrough was made by Tang and VanSlyke at Eastman Kodak who found that by using multilayers of sublimated organic molecules, the operating voltage of the organic EL devices was dramatically reduced and the quantum efficiency was significantly enhanced. This discovery touched off a flurry of research activity, especially in Japan. The Japanese researchers, as welt as the group at Kodak, have since improved the device efficiency and lifetime to meet commercial requirements. This progress is reviewed by Tsutsui in this issue.

Light-emitting electrochemical cells from a blend of p- and n-type luminescent conjugated polymers

Abstract: We demonstrate polymer light-emitting electrochemical cells (LECs) made of a blend of p- and n-type luminescent conjugated polymers. These two polymers, poly[9-(3,6,9-trioxadecyl)- carbazole-3,6-diyl] (TOD–PC, a p-type polymer) and poly[2,3-di(p-tolyl)–quinoxaline-5,8-diyl] (DT–PQX, a n-type polymer), are blue and blue–green emission polymers, respectively, both with high photoluminescent quantum efficiency. However, the photoluminescence of the polymer blend is completely quenched, due to the charge transfer between the two polymers. A new and faint orange–yellow photoluminescence emission, which has photonic energy consistent with the energy difference of the π band of TOD–PC and the π* and of DT–PQX, has been observed. LECs fabricated from this polymer blend show strong current injection and bright electroluminescence at this new emission color, which is believed to be due to the interpolymer radiative recombination of the electrons from the n-type polymer and holes from the p-type polymer. Such an independent p doping of TOD–PC and n doping of DT–PQX in the blend and interpolymer radiative recombination provide an interesting way of generating new emission colors in the LEC system.

Mode transition and nonlinear dynamics in the beam-injected plasma

Abstract: Summary form only given. The motions of electron beams in a discharge plasma are simulated using the 1-D particle-in-cell code (XPDP1). With appropriate pressures, beam current, beam velocity, and voltages, there is mode transition from the anode glow mode (AGM) to the temperature limited mode (TLM). The transition happens when the ionized ions which make the potential positive fill the entire space. During transition from AGM to TLM, the dynamics show chaotic motions and the current increases to the injected beam current. After the TLM is established, the dynamics of electron current and potential show self oscillation, periodic doubling, or chaos for various electron to ion number ratios, sheath sizes, and bulk potential levels. The higher bulk potential level is, the larger fluctuation amplitude of current is observed. For higher pressures of neutral gas, the dynamics show more chaotic but small amplitude of fluctuation motions.