W.J. Feast

Indium–tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

BUILT-IN FIELD ELECTROABSORPTION SPECTROSCOPY OF POLYMER LIGHT-EMITTING DIODES INCORPORATING A DOPED POLY(3,4-ETHYLENE DIOXYTHIOPHENE) HOLE INJECTION LAYER

We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca–Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS−. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes’ work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime.

Synthesis, processing and material properties of conjugated polymers

Recent progress in the synthesis and characterization of well-defined conjugated polymers and oligomers is presented. The different synthetic schemes for most of the important conjugated polymers will be discussed using a subdivision into three main sections, covering hydrocarbon polymers, heterocyclic polymers and the oligomers, respectively. Although the materials properties will be referred to, the main emphasis of the review is directed to the different strategies that are now available to produce conjugated polymers and well-defined oligomers in a reproducible way and with full control over most of the important macromolecular properties.

Investigation of the ageing of plasma oxidized PEEK

Abstract Oxygen plasma treatment can be used for increasing the hydrophilicity of polymer surfaces, however, it is widely known that this effect decays significantly with time. This ageing phenomenon is thought to be caused by both migration of low molecular weight fragments and reorientation of modified polymer chains. It has recently been shown that the aged surface becomes transiently hydrophilic before attaining a final surface energy significantly lower than the initially treated surface. X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used to monitor the changes in surface chemistry of plasma oxidized poly(ether ether ketone) (PEEK) during post treatment storage. The decay and transient increase in hydrophilicity were found to be dependent upon crystallinity and storage temperature.

Durham poly acetylene: preparation and properties of the unoriented material

Abstract Polyacetylene prepared by the Shirakawa route is a mat of crystalline fibrills, which are capable of being oxidized or reduced to a highly conducting state. Polyacetylene can also be produced from a soluble precursor polymer by a thermally driven transformation reaction (the ‘Durham’ route) as a solid, coherent film. This transformation consists of the elimination of a substituted benzene unit followed by the isomerization of the largely cis polyacetylene produced to a disordered trans isomer. We have studied the effect of different transformation conditions and from these studies it is possible to gain an insight into these processes. Techniques such as caloremetry, infrared, resonance Raman and optical spectroscopy, e.s.r., X-ray diffraction and electrical conductivity have all been used to probe the transformation and the product. The resultant information gives a consistent picture of the overall transformation processes detailed above but also shows an ordering associated with higher transformation temperatures. A thorough knowledge of these processes enables the orientation of the precursor to produce highly crystalline material.

Solution-state carbon-13 nuclear magnetic resonance studies of polyaniline

Abstract Solution-state 13 C nuclear magnetic resonance spectra are presented for the emeraldine base and leucoemeraldine base forms of polyaniline. The bulk of the leucoemeraldine base sample gives a simple two-line spectrum, which is readily assignable. The emeraldine base samples give spectra of much greater complexity, showing many more lines than there are carbons in the postulated chain repeat unit. This is rationalized in terms of slow interchange of the many conformers available to the emeraldine base form. A quantitative ‘region assignment’ is made. Gel permeation chromatography measurements show that interconversion between the two forms is achieved without either significant crosslinking or chain scission. The minor peaks in the spectrum of the leucoemeraldine base form due to chain defects (including chain ends) are compared with chemical shift values calculated for postulated structures, and some structures can be rejected on the basis of this.

Characterisation of the properties of surface-treated indium-tin oxide thin films

Abstract We studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen and argon plasmas) treatments. The influence of surface treatment on ITO was investigated by surface energy, surface roughness, sheet resistance, and workfunction measurements. We report that oxygen plasma treatment induces: (1) the highest surface energy with the highest polarity, (2) the smoothest surface, (3) the lowest sheet resistance, and (4) the highest workfunction. We also observe that the PL efficiencies of PPV, PDPV and PEDOT/PPV on ITO are affected by the surface treatment of ITO.

Tailored copolymers via coupled anionic and ring opening metathesis polymerization. Synthesis and polymerization of bicyclo[2.2.1]hept-5-ene-2,3-trans-bis(polystyrylcarboxylate)s

Abstract The preparation of polystyrene macromonomers containing a norbornene unit and their living ring opening metathesis polymerization (ROMP) to produce comb graft copolymers is described. The ROMP was initiated by the well defined Schrock initiators, Mo(CHR)(NAr)(OR′)2, where R is (CH3)3C or C6H5C(CH3)2, Ar is 2,6-diisopropylphenyl and R′ is (CH3)3C. Well characterized macromonomers and comb graft copolymers with polystyryl grafts with average degrees of polymerization (DPs) of 4, 7, and 9 were successfully produced. The graft copolymers exhibit single mode molecular weight distributions and narrow polydispersities. Attempts to prepare copolymers with longer polystyryl grafts gave products which exhibited bimodal molecular weight distributions in which one component of the distribution had the same retention time as that of the macromonomer, this observation is rationalized in terms of steric inhibition of the ROMP of such macromonomers.

Efficient green light‐emitting diodes from a phenylated derivative of poly(p‐phenylene–vinylene)

We have used polyvinylcarbazole, PVK, in combination with poly(p‐phenylene vinylene), PPV and a green‐emitting, soluble derivative, poly(4,4′‐diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple‐layer light‐emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn‐on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single‐layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single‐layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents.

Side‐Chain Degradation of Ultrapure π‐Conjugated Oligomers: Implications for Organic Electronics

The degrdn. of two defect-free pi-conjugated oligomers and the participation of their solubilizing side chains in the process are studied in unprecedented detail. The detected intermediate products reveal a mechanism of successive shortening of alkyl and oligo(ethylene glycol) substituents. Eventually, these processes cause chem. modifications in the conjugated backbone, which strongly influence the electronic properties of the materials.