Stephen Barlow

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Rylene and Related Diimides for Organic Electronics

Organic electron-transporting materials are essential for the fabrication of organic p-n junctions, photovoltaic cells, n-channel field-effect transistors, and complementary logic circuits. Rylene diimides are a robust, versatile class of polycyclic aromatic electron-transport materials with excellent thermal and oxidative stability, high electron affinities, and, in many cases, high electron mobilities; they are, therefore, promising candidates for a variety of organic electronics applications. In this review, recent developments in the area of high-electron-mobility diimides based on rylenes and related aromatic cores, particularly perylene- and naphthalene-diimide-based small molecules and polymers, for application in high-performance organic field-effect transistors and photovoltaic cells are summarized and analyzed.

Perylene-3,4,9,10-tetracarboxylic Acid Diimides: Synthesis, Physical Properties, and Use in Organic Electronics

Perylene-3,4,9,10-tetracarboxylic acid diimides (perylene diimides, PDIs) have been used as industrial pigments for many years. More recently, new applications for PDI derivatives have emerged in areas including organic photovoltaic devices and field-effect transistors. This Perspective discusses the synthesis and physical properties of PDI derivatives and their applications in organic electronics.

Design of Polymethine Dyes with Large Third-Order Optical Nonlinearities and Loss Figures of Merit

Dying by Design To make optical-switching applications a reality, losses from scattering and other absorption processes have to be minimized. Hales et al. (p. 1485, published online 18 February; see the Perspective by Haque and Nelson) present a strategy to explore the refraction and absorption properties of a group of cyanine dyes for designing materials that have properties corresponding to technologically interesting telecommunications windows. The optical properties of the cyanine molecule was controlled by adding heavy chalcogen atoms (selenium) into the end groups of the molecular structure. While producing a series of molecules meeting criteria for feasible application, the work also demonstrates a route to improve the performance of nonlinear optical materials. Nonlinear optical materials are designed and characterized for potential applications in all-optical switching. All-optical switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. We present a design approach that involves enhancing the real part of the third-order polarizability (γ) of cyanine-like molecules through incorporation of polarizable chalcogen atoms into terminal groups, while controlling the molecular length to obtain favorable one- and two-photon absorption resonances that lead to suitably low optical loss and appreciable dispersion enhancement of the real part of γ. We implemented this strategy in a soluble bis(selenopyrylium) heptamethine dye that exhibits a real part of γ that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of γ, a measure of nonlinear loss, that is smaller by two orders of magnitude. This combination is critical in enabling low-power, high-contrast optical switching.

Copolymers of perylene diimide with dithienothiophene and dithienopyrrole as electron-transport materials for all-polymer solar cells and field-effect transistors

Electron-accepting solution-processable conjugated polymers consisting of perylene diimide moieties alternating with dithienothiophene, oligo(dithienothiophene), or N-dodecyl dithienopyrrole units have been synthesized. All these polymers possess excellent thermal stability with decomposition temperatures over 400 °C. The glass-transition temperatures vary from 155 to 263 °C. These polymers show broad absorption extending from 250 to 900 nm with electrochemical and optical bandgaps as low as 1.4 eV; the maximum absorbance increases and the bandgap decreases with increasing the conjugation length of oligo(dithienothiophene), while the bandgap can also be decreased by the replacement of dithienothiophene by dithienopyrrole. The electrochemical onset reduction potentials range from −0.8 to −1.0 V vs. ferrocenium/ferrocene, suggesting that the electron affinities are essentially unaffected by the specific choice of donor moiety, while the onset oxidation potentials (+0.6 to +1.0 V) are a little more sensitive to the choice of donor. The mono dithienothiophene and the dithienopyrrole polymers were found to exhibit electron mobilities as high as 1.3 × 10−2 and 1.2 × 10−3 cm2V−1s−1, respectively, in top-contact organic field-effect transistors. Power conversion efficiencies in the range 0.77–1.1% were obtained under simulated AM 1.5, 100 mW/cm2 irradiation for all-polymer solar cells using the dithienothiophene-based polymers as acceptors in a 1 : 1 ratio with a polythiophene derivative as a donor. The device performance varies with the conjugation length of oligo(dithienothiophene) in the polymer acceptors, and for the best-performing material it can be further optimized to give a power conversion efficiency of 1.5% by increasing the donor/acceptor weight ratio to 3 : 1.

Effects of electronegative substitution on the optical and electronic properties of acenes and diazaacenes

Large acenes, particularly pentacenes, are important in organic electronics applications such as thin-film transistors. Derivatives where CH units are substituted by sp(2) nitrogen atoms are rare but of potential interest as charge-transport materials. In this article, we show that pyrazine units embedded in tetracenes and pentacenes allow for additional electronegative substituents to induce unexpected redshifts in the optical transitions of diazaacenes. The presence of the pyrazine group is critical for this effect. The decrease in transition energy in the halogenated diazaacenes is due to a disproportionate lack of stabilization of the HOMO on halogen substitution. The effect results from the unsymmetrical distribution of the HOMO, which shows decreased orbital coefficients on the ring bearing chlorine substituents. The more strongly electron-accepting cyano group is predicted to shift the transitions of diazaacenes even further to the red. Electronegative substitution impacts the electronic properties of diazaacenes to a much greater degree than expected.

A High-Efficiency Panchromatic Squaraine Sensitizer for Dye-Sensitized Solar Cells

Keywords: dyes/pigments ; light harvesting ; sensitizers ; solar cells ; squaraines ; Films Reference EPFL-ARTICLE-171211doi:10.1002/anie.201101362View record in Web of Science Record created on 2011-12-16, modified on 2017-12-03

Efficient all-polymer solar cells based on blend of tris(thienylenevinylene)-substituted polythiophene and poly[perylene diimide-alt-bis(dithienothiophene)]

A narrow band-gap alternating copolymer of perylene diimide and bis(dithienothiophene) (2) and a polythiophene derivative substituted by a tris(thienylenevinylene) conjugated side chain (4) are used as acceptor and donor, respectively, in all-polymer solar cells (SCs). The optimized device based on the blend of 4 and 2 in the ratio 3:1 (w/w) gives a short circuit current (Jsc) of 5.02 mA cm−2 and a power conversion efficiency of 1.48%, under simulated AM 1.5 illumination at 100 mW cm−2. These values are among the highest values reported for all-polymer SCs.

6,13-Diethynyl-5,7,12,14-tetraazapentacene

A new relative of pentacene: The dialkynylated tetraazapentacene (see figure) was prepared by a two-step synthesis from the corresponding quinone derivative. The heteroacene is an air-stable, dark-blue, crystalline material and is of great interest as a potential organic n-electron-transport material.

Hybrid rylene arrays via combination of Stille coupling and C-H transformation as high-performance electron transport materials.

Hybrid rylene arrays have been prepared via a combination of Stille coupling and C-H transformation. The ability to extend the π system along the equatorial axis of rylenes not only leads to broadened light absorption but also increases the electron affinity, which can facilitate electron injection and transport with ambient stability.