Georges Hadziioannou

Supramolecular self-assembly and opto-electronic properties of semiconducting block copolymers

Abstract With continuous and nanometre-scale interpenetrating phases of electron donor and acceptor components, a novel diblock copolymer, in which one block is poly(p-phenylene vinylene) (PPV) and the other is a C60-functionalized polystyrene, is designed to be an efficient photovoltaic material. The synthesis involves the polymerization of a styrene derivative from a PPV-based macroinitiator via living free radical polymerization, and its subsequent functionalization with C60 via atom transfer radical addition. In selective solvents for the polystyrene block, aggregation is detected by means of optical spectroscopy and small-angle neutron scattering. Solid films exhibit honeycomb-structuring at the micrometre level when cast from CS2. As active layer in a device, the donor–acceptor diblock copolymer shows enhanced photovoltaic response relative to a blend of its constituent polymers.

Synthesis of a conjugated macromolecular initiator for nitroxide-mediated free radical polymerization

Controlled synthesis of rod-coil block copolymers is possible with a new macroinitiator for living nitroxide-mediated radical polymerization (NMRP). The synthesis of the new macroinitiator which contains a rodlike, conjugated poly(p-phenylene vinylene) block is described. Electron microscopy imaging indicates microphase separation of the resulting block copolymer when cast from chloroform.

Synthesis and self-organization of PPV-based block copolymers for photonic applications

Rod-coil block copolymers have been synthesized with the objective of enhancing the photovoltaic efficiency by the incorporation of both donor and acceptor components in a molecular architecture that is self-structuring. Block copolymers were obtained by using an end-functionalized rigid-rod block of poly(2,5-dioctyloxy-1,4-phenylenevinylene) (PPV) as a macroinitiator for the nitroxide-mediated controlled radical polymerization of a flexible, styrene-based block. Under suitable conditions the formation of honeycomb structures by PPV-block-polystyrene (PPV-b-PS) is observed, The honeycomb structure consists of a 2-dimensional array of spherical air holes with an inner diameter of 3 - 5 μm in a polymeric film, which results in 6.5 million hexagonally packed holes per square centimeter. The application of these structures as templates for highly ordered functional patterns is demonstrated by evaporating aluminum on this regularly structured surface and subsequently washing away the polymer.

Field-effect mobilities in spin-cast and vacuum-deposited PPV-type pentamers

Abstract Field-effect transistors (FET) have been fabricated to determine the hole mobility of spin-cast films of the five-ring PPV-type oligomer 2,5-di- n -octyloxy-1,4-bis((4′,4″-bisstyryl)styrylbenzene) (Ooct-OPV5). The influence of annealing of the organic layer on the device characteristics and the derived mobility is investigated. In all cases a reasonable saturation of the drain-source hole current is obtained under negative gate bias. Mobilities in the order of 10 −5 xa0cm 2 /Vxa0s were calculated from square-law theory. The reported increase of mobility upon heat treatment is explained by the increase of the size of the crystalline domains in the organic layer, observed with polarized light microscopy. The oligomer/gold electrode interfaces are studied with AFM. Vacuum-deposited films of the five-ring PPV-type oligomer 2-methoxy-5-(2′-ethylhexyloxy)-1,4-bis((4′,4″-bisstyryl)styrylbenzene) (MEH-OPV5) have also been used as semiconducting layers in FETs. Field-effect mobility values up to at least one order of magnitude larger than those found for the spin-cast films are demonstrated.

Formation of Transparent conducting films based on core-shell latices : Influence of the polypyrrole shell thickness

Transparent conducting latex films have been prepared from core-shell latices. The latex particles have a poly(butyl methacrylate) (PBMA) core of about 700 nm and a very thin polypyrrole (PPy) shell. We have studied the film formation of latices with 1, 2, and 4 wt % PPy and compared this with the film formation of the pure PBMA latex. The film formation process was studied by transparency measurements, atomic force microscopy surface flattening, and transmission electron microscopy on ultrathin sections of films after various annealing times at 120°C. It is demonstrated that highly transparent (>90%) and antistatic films can be produced using these latices. The presence of a polypyrrole shell around the PBMA latex particle seriously hinders the deformation of the particles. The amount of polypyrrole, and thus the shell thickness, is the determining factor for the speed of film formation.

Resistance of transparent latex films based on acrylic latexes encapsulated with a polypyrrole shell

Abstract We have prepared films with a transparency of more than 90% and a resistance in the antistatic range from 1xa0wt.% polypyrrole-encapsulated latexes (PPy-encapsulated latexes) by a heat treatment at 120°C for several hours. The initial film resistance is below 1xa0MΩ/□, but after a temporary decrease in resistance upon increasing the film temperature, the film resistance increases strongly. A clear relationship between the temperature and the stability of the film resistance is demonstrated. It is shown that the increase in film resistance upon heating can be limited by the incorporation of a more stable counter-anion, like p -toluenesulfonic acid or by performing the heat treatment in an inert atmosphere. The p -toluenesulfonic acid has been incorporated by two different methods: directly in the in situ synthesis of the PPy shell or later via ion exchange. The direct incorporation of the stable counter-anion has a more positive influence on the resistance stability, but a drawback of this method is that the risk of pure PPy particle formation apart from the core–shell latex particles is found to increase.

Substituent effects on the excited states of phenyl-capped phenylene vinylene tetramers

The singlet and triplet excited states of phenyl-capped tetramers of phenylene vinylene with different alkyl, alkoxy or cyano substituents, were investigated in benzene solution. The lowest singlet states were studied by laser flash-photolysis with time-resolved microwave conductivity and fluorescence. The triplets were generated by pulse radiolysis, via energy transfer from the solvent excited singlet followed by inter-system crossing or by energy transfer from the naphthalene triplet. Their absorption spectra and rates of quenching by oxygen were determined.

Polymers for opto-electronic applications: structure and morphology of thin films and their interfaces

Abstract Organic–organic and metal–organic interfaces are explored. The influence of the morphology of thin films of MEH-phenylene-vinylene oligomer (OPV5):C 60 blends on their photovoltaic characteristics is demonstrated. An interdigitating structure is considered to be favorable for efficient operation. The design of a diblock copolymer for this specific application is discussed. By means of UPS, the electronic energy levels at the interface between OPV5 and gold are assessed. Due to the formation of a dipole layer, the vacuum levels are not aligned and the barrier for charge injection is altered.