Piezoelectricity and pyroelectricity, traditionally encountered in certain single crystals and ceramics, have now also been documented in a number of polymers. Recently, one such polymer-poly(vinylidene fluoride)-and some of its copolymers have been shown to be ferroelectric as well. The extraordinary molecular and supermolecular structural requirements for ferroelectric behavior in polymers are discussed in detail, with particular emphasis on poly(vinylidene fluoride). Piezoelectric, pyroelectric, and ferroelectric properties are also briefly reviewed, as are some promising applications of such polymers.
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.
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 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.
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