Showing papers by "Xiong Gong published in 2006"

New Architecture for High-Efficiency Polymer Photovoltaic Cells Using Solution-Based Titanium Oxide as an Optical Spacer

Abstract: reported under AM1.5 (AM: air mass) illumination, this efficiency is not sufficient to meet realistic specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the implementation of new materials and the exploration of new device architectures. Polymer-based photovoltaic cells are thin-film devices fabricated in the metal-insulator-metal configuration sketched in Figure 1a. The absorbing and charge-separating bulk-heterojunction layer with a thickness of approximately 100 nm is sandwiched between two charge-selective electrodes; a transparent bilayer electrode comprising poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS) on indium tin oxide (ITO) glass for collecting the holes and a lower-work-function metal (here, Al) for collecting the electrons. The work-function difference between the two electrodes provides a built-in potential that breaks the symmetry, thereby providing a driving force for the photogenerated electrons and holes toward their respective electrodes. Because of optical interference between the incident (from the ITO side) and back-reflected light, the intensity of the light is zero at the metallic (Al) electrode; Figure 1a shows a schematic representation of the spatial distribution of the squared optical electric-field strength. [9–11] Thus, a relatively large fraction of the active layer is in a dead-zone in which the photogeneration of carriers is significantly reduced. Moreover, this effect causes more electron–hole pairs to be produced near the ITO/PEDOT:PSS electrode, a distribution which is known to reduce the photovoltaic conversion efficiency. [12,13] This “optical interference effect” is especially important for thin-film structures where layer thicknesses are comparable to the absorption depth and the wavelength of the incident light, as is the case for photovoltaic cells fabricated from semiconducting polymers. In order to overcome these problems, one might simply increase the thickness of the active layer to absorb more light. Because of the low mobility of the charge carriers in the polymer:C60 composites, however, the increased internal resistance of thicker films will inevitably lead to a reduced fill factor. An alternative approach is to change the device architecture with the goal of spatially redistributing the light intensity inside the device by introducing an optical spacer between the active layer and the Al electrode as sketched in Figure 1a. [11] Although this revised architecture would appear to solve the problem, the prerequisites for an ideal optical spacer limit the choice of materials: the layer must be a good acceptor and an electron-transport material with a conduction band edge lower in energy than that of the lowest unoccupied molecular orbital (LUMO) of C60; the LUMO must be above (or close to) the Fermi energy of the collecting metal electrode; and it must be transparent to light with wavelengths within the solar spectrum.

Red electrophosphorescence from a soluble binaphthol derivative as host and iridium complex as guest.

Abstract: The investigation of the optical properties, carrier injection, and transport into a soluble small molecule, 6,6‘-dicarbazolyl-2,2‘-dihexyloxy-1,1‘-binaphthol (BA), was reported. The results demonstrated that BA is a blue-emitting molecule, which can be used as a host for the fabrication of electrophosphorescent light-emitting diodes (LEDs). The single-layer electrophosphorescent LEDs fabricated from toluene solution containing BA with tris[2,5-bis-2‘-(9‘,9‘-dihexylfluorene)pyridine-κ2NC3‘]iridium(III) [Ir(HFP)3] emitted red light from Ir(HFP)3 triplet emission. The results from photoluminescence (PL) and electroluminescence (EL) demonstrated that the dominated operational mechanism in EL was charge trapping rather than Forster transfer, which was the dominated mechanism in PL. The single-layer OLEDs with 1wt % of Ir(HFP)3 have a luminance (L) of 1000 cd/m2 at 22 V and a luminous efficiency (LE) of 0.88 cd/A at 11 mA/cm2. Double-layer electrophosphorescent LEDs fabricated by casting the emitting layer fro...

Enhanced electron injection in polymer light-emitting diodes: polyhedral oligomeric silsesquioxanes as dilute additives

Abstract: By blending monochlorocyclohexyl- polyhedral oligomeric silsesquioxanes (MCC-POSS) into poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1.4-phenylenevinylene (MEH-PPV), enhanced luminous efficiency (LE, cd A −1 ) and brightness were observed in polymer light-emitting diodes (PLEDs) using Al as the cathode. PLEDs made from MEH-PPV with 0.5 wt.% of MCC-POSS exhibit LE of 1 cd A −1 , four times higher than that observed in simple MEH-PPV devices. x-ray diffraction studies, measurements of photovoltaic response and measurements of photoconductivity from the films of pure MEH-PPV and MEH-PPV with MCC-POSS demonstrated that the improved device performance results from mixing of MCC-POSS with MEH-PPV. The enhanced electron-injection into the emissive layer results from the effect of the ionic conductivity introduced by the addition of MCC-POSS.

Multilayer polymer light-emitting diodes for solid state lighting applications

Abstract: Multilayer polymer light-emitting diodes (PLEDs) are demonstrated using semiconducting polymers blended with organometallic emitters as the emissive layer and one or both of an electron transport layer and a hole transparent layer on the appropriate electron injection and hole injection sides of the emissive layer. The transport layers reduce energy potential gaps between the hole injection electrode and the emissive polymer and between the electron injection electrode and the emissive polymer. A solvent-processing based procedure for preparing these devices is also disclosed It uses nonpolar solvent-based solutions of emissive polymers to form the emissive layer and polar solvent-based solutions to form the transport layers to minimize etching and other undesirable interactions as the multiple layers are being laid down. Illumination quality white light can be obtained with stable Commission Internationale de l'Eclairage coordinates, stable color temperatures, and stable color rendering indices, all close to those of “pure” white light. These multilayer white light-emitting PLEDs are useful as backlights for liquid crystal displays and for solid state lighting applications.

Mehrschichtige Licht emittierende Polymer-Diode für Festkörper Beleuchtungs-Anwendungen

Abstract: Licht emittierende Vorrichtung auf der Basis von organischen Polymeren, umfassend eine Elektronen-Injektion-Elektrode und eine Loch-Injektion-Elektrode auf gegenuberliegenden Seiten einer emittierenden Schicht, wobei die emittierende Schicht mindestens ein lumineszierendes erstes halbleitendes Polymer umfasst, das zur Fluoreszenz-Emission in der Lage ist und als Wirt fur mindestens einen zugemischten Phosphoreszenz Emitter dient, der zur Phosphoreszenz-Emission in der Lage ist, wobei die Verbesserung mindestens eines der folgenden umfasst eine organische Elektronen-Transport-Schicht, umfassend ein zweites halbleitendes Polymer, das zwischen der Elektronen-Injektions-Elektrode und der emittierenden Schicht angeordnet ist und ein niedrigstes nicht besetztes Molekular-Orbital aufweist, mit einer Energie nahe dem unteren Ende der π*-Bande des lumineszierenden Polymers; und eine organische Loch-Transport-Schicht, umfassend ein drittes halbleitendes Polymer, das zwischen der Loch-Injektions-Elektrode und der emittierenden Schicht angeordnet ist und ein hochstes besetztes Molekular-Orbital aufweist, mit einer Energie nahe der oberen π-Bande des lumineszierenden Polymers.

Multilayer white lighting polymer light-emitting diodes

Abstract: Organic and polymer light-emitting diodes (OLEDs/PLEDs) that emit white light are of interest and potential importance for use in active matrix displays (with color filters) and because they might eventually be used for solid-state lighting. In such applications, large-area devices and low-cost of manufacturing will be major issues. We demonstrated that high performance multilayer white emitting PLEDs can be fabricated by using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer, and water-soluble (or ethanol-soluble) polymers/small molecules (for example, PVK-SO 3 Li) as the hole injection/transport layer (HIL/HTL) and water-soluble (or ethanol-soluble) polymers/small molecules (for example, t-Bu-PBD-SO 3 Na) as the electron injection/transport layer (EIL/HTL). Each layer is spin-cast sequentially from solutions. Illumination quality light is obtained with stable Commission Internationale d'Eclairage coordinates, stable color temperatures, and stable high color rendering indices, all close to those of pure white. The multilayer white-emitting PLEDs exhibit luminous efficiency of 21 cd/A, power efficiency of 6 Im/W at a current density of 23 mA/cm 2 with luminance of 5.5 x10 4 cd/m 2 at 16 V. By using water-soluble (ethanol-soluble) polymers/small molecules as HIL/HTL and polymers/small molecules as EIL/ETL, the interfacial mixing problem is solved (the emissive polymer layer is soluble in organic solvents, but not in water/ ethanol). As a result, this device architecture and process technology can potentially be used for printing large-area multiplayer light sources and for other applications in plastic electronics. More important, the promise of producing large areas of high quality white light with low-cost manufacturing technology makes the white multilayer white-emitting PLEDs attractive for the development of solid state light sources.