Andrew J. Lovinger

Soluble and processable regioregular poly(3‐hexylthiophene) for thin film field‐effect transistor applications with high mobility

The electrical characteristics of field‐effect transistors using solution cast regioregular poly(3‐hexylthiophene) are discussed. We demonstrate that both high field‐effect mobilities (ca. 0.045 cm2/V s in the accumulation mode and 0.01 cm2/V s in the depletion mode), and relatively high on/off current ratios (greater than 103) can be achieved. We find that the film quality and field‐effect mobility are strongly dependent on the choice of solvents. In addition, treating a film with ammonia or heating to 100 °C under N2 can increase the on/off ratio without decreasing the mobility.

A soluble and air-stable organic semiconductor with high electron mobility

Electronic devices based on organic semiconductors offer an attractive alternative to conventional inorganic devices due to potentially lower costs, simpler packaging and compatibility with flexible substrates. As is the case for silicon-based microelectronics, the use of complementary logic elements—requiring n- and p-type semiconductors whose majority charge carriers are electrons and holes, respectively—is expected to be crucial to achieving low-power, high-speed performance. Similarly, the electron-segregating domains of photovoltaic assemblies require both n- and p-type semiconductors. Stable organic p-type semiconductors are known, but practically useful n-type semiconductor materials have proved difficult to develop, reflecting the unfavourable electrochemical properties of known, electron-demanding polymers. Although high electron mobilities have been obtained for organic materials, these values are usually obtained for single crystals at low temperatures, whereas practically useful field-effect transistors (FETs) will have to be made of polycrystalline films that remain functional at room temperature. A few organic n-type semiconductors that can be used in FETs are known, but these suffer from low electron mobility, poor stability in air and/or demanding processing conditions. Here we report a crystallographically engineered naphthalenetetracarboxylic diimide derivative that allows us to fabricate solution-cast n-channel FETs with promising performance at ambient conditions. By integrating our n-channel FETs with solution-deposited p-channel FETs, we are able to produce a complementary inverter circuit whose active layers are deposited entirely from the liquid phase. We expect that other complementary circuit designs can be realized by this approach as well.

Organic field‐effect transistors with high mobility based on copper phthalocyanine

Organic field‐effect transistors that employ copper phthalocyanine (Cu–Pc) as the semiconducting layer can function as p‐channel accumulation‐mode devices. The charge carrier mobility of such devices is strongly dependent on the morphology of the semiconducting thin film. When the substrate temperature for deposition of Cu–Pc is 125 °C, a mobility of 0.02 cm2/V s and on/off ratio of 4×105 can be obtained. These features along with the highly stable chemical nature of Cu–Pc make it an attractive candidate for device applications.

Structure and morphology of poly(propylenes): a molecular analysis

Abstract The progress made in the understanding of the crystal polymorphism and morphology of isotactic and syndiotactic poly(propylene) (iPP and sPP) is reviewed. In 1960, the crystal structure of the stable form of iPP, the α phase, was already solved and the general principles of polyolefin chain conformation and crystallography established. The molecular analysis of the lamellar branching of αiPP—a case of homoepitaxy specific to this polymer and this crystal form—was given in 1986. The γ phase of iPP, solved in 1989, is the first and so far unique example of a polymer structure with non-parallel chain stems. The β phase is the first example of a frustrated structure in polymers, and rests on packing of isochiral helices in the unit-cell. The stable, high temperature form of sPP described in 1988 is based on full antichiral packing of helices, whereas the original structure corresponds to another limiting (but not experimentally observed in pure form) case based on packing of isochiral helices. Details of the molecular structure of iPP and sPP, such as the hand of individual helical stems and setting of chains, are accessible by crystallographic considerations which make use of molecular markers provided by the α-α and α-γ epitaxies in iPP or by direct visualization by atomic force microscopy.

Electronic sensing of vapors with organic transistors

We show that organic thin-film transistors have suitable properties for use in gas sensors. Such sensors possess sensitivity and reproducibility in recognizing a range of gaseous analytes. A wealth of opportunities for chemical recognition arise from the variety of mechanisms associated with different semiconductor–analyte interactions, the ability to vary the chemical constitution of the semiconductor end/side groups, and also the nature of the thin-film morphology.

Luminescence Enhancement by the Introduction of Disorder into Poly(p-phenylene vinylene)

A method is described for increasing luminescence in poly(p-phenylene vinylene) (PPV) light-emitting diodes. Cis linkages were engineered into the PPV chain. These linkages interrupt conjugation and interfere with the packing of the polymer chains, which results in the formation of amorphous PPV. Large-area electroluminescent devices were prepared from this polymer. Devices made of an aluminum electrode, PPV as the luminescent layer, and an electron-transporting layer have internal quantum efficiencies of 2 percent, a turn-on voltage of 20 volts, and can carry current densities of 2000 milliamperes per square centimeter. The current density is at least an order of magnitude higher than previously obtained.

Status of and prospects for organic electroluminescence

We review the device and materials science behind organic electroluminescent diodes made both using discrete evaporable molecules and spin-cast organic polymers. A great deal of progress has been made in improving the efficiencies and spectral properties of organic light-emitting diodes, and these are now adequate for many applications. More work is necessary to understand the stability and degradation of emissive and charge-transporting organics, but some systems have been shown to be stable for 10 4 hours at display brightness. Major challenges still face the community in terms of developing satisfactory systems design and processing techniques if organic electroluminescence is to realize either performance or economic advantages over technologies and significantly penetrate the display market. We present an analysis of the suitability of organic light-emitting diodes for various applications, and consider the materials and manufacturing obstacles that must be overcome.

Poly(Vinylidene Fluoride)

This chapter presents an extensive review of the important ferroelectric polymer, poly(vinylidene fluoride). Covered topics include: polymerization, molecular structure, and defects; dilute-solution properties; crystalline structure of the various polymorphic phases, crystallization, and morphology; polymorphic transitions induced through a variety of means (e.g. thermal, mechanical, electrical); relaxational characteristics; melting behaviour and thermal degradation; ferroelectric properties, with emphasis on piezoelectricity and pyroelectricity; copolymers and compatible blends with other polymers and general engineering properties and uses.

Crystallographic changes characterizing the Curie transition in three ferroelectric copolymers of vinylidene fluoride and trifluoroethylene: 1. As-crystallized samples

Abstract Copolymers of vinylidene fluoride/trifluoroethylene of molar composition 65 35 , 73 27 and 78 22% respectively, are ferroelectric and undergo a Curie transition to the paraelectric state at high temperatures. In contrast to the irregular structure found earlier for the 52 48 mol % copolymer, the structures of these three compositions in the low-temperature state are all well ordered and analogous to that of β-poly(vinylidene fluoride): they consist of molecular chains in a polar trans conformation whose order is improved with increasing vinylidene fluoride content, packed pseudo-hexagonally in unit cells whose dimensions decrease with increasing vinylidene fluoride content. In their paraelectric phase, the chains assume a partly disordered conformation consisting of irregular TG, TḠ and TT sequences and are packed on an expanded pseudo-hexagonal lattice. The Curie transitions were found to occur over a broad temperature range, encompassing ∼30°C, and in the case of the 78 22 mol % copolymer to extend into the melting region; they were also found to exhibit hysteresis by occurring at much lower temperatures upon cooling than upon heating.

Kinetic analysis of the crystallization of poly(p-phenylene sulphide)

Growth rates of spherulites were measured in poly(p-phenylene sulphide) crystallized from the melt and the quenched glass over the temperature range 100°C–280°C, possibly the most extensive overall range yet reported for any polymer and, as such, most propitious for study of regime III crystallization. For a medium M.wt. polymer, a regime II → III transition was obtained at 208°C using values of transport parameters common to many polymers (U∗ = 1400 cal mol−1, T∞ − Tg = 30°C) together with experimentally determined values of T0m(315°C) and Tg(92°C). Under these conditions, the regime III/II slope ratio was found to be 2.07 (i.e. only 3.5% higher than predicted by regime theory), and reasonable estimates of surface free energies and of the work of chain folding were obtained. Other choices of the transport terms, including WLF and zero values, did not allow successful kinetic analyses. Although a regime I → II transition is predicted to occur at the high-temperature end of our growth-rate data, we found no experimental evidence for it. For a low M.wt. polymer, our analysis showed that regime III kinetics is obeyed at low temperatures, while at higher ones there is a continuous departure from that behaviour without, however, full attainment of regime II kinetics.