in hopping events could result in an increased mobility. Further experiments will be required to determine the exact nature of chain packing and charge transport.We conclude that the MW of RR P3HT has a substantial effect on the way that the chains pack on each other, and that this, in turn, causes the mobility to vary by at least four orders of magnitude. Our data does not show that disordered films are intrinsically better than highly crystalline films. It does show that factors other than crystallinity can have a strong effect on mobility, and that these other factors must be considered when optimizing polymer devices. It is possible that some of the variations in mobility observed in previous studies were at least in part due to variations in MW instead of some of the variables that were reported. As attempts are made to make FETs with higher mobilities and photovoltaic cells with higher energy conversion efficiency, it will clearly be important to optimize the MW of the polymers. We have not yet found a MW at which the mobility saturates or declines. We therefore speculate that higher mobilities could be obtained if longer chains could be synthesized without introducing defects.
ExperimentalRegioregular poly(3-hexylthiophenes) of Group A were synthesized by a modified McCullough route [14,15]. The molecular weight of the samples was varied by adjusting the polymerization time (5 min, 11 min, 17 min, and 12 h, respectively). The polymerization was terminated by quenching with methanol. After precipitation from methanol, the polymer was purified by continuous extraction with methanol and then chloroform under nitrogen atmosphere. Polymers of Group B were prepared by performing continuous extraction of a sample from Aldrich with increasingly better solvents (hexane, dichloromethane, and chloroform). Polymers of Group C were synthesized similarly to the modified McCullough route, except that the polymerization system was a mixture of the monomer and a 5-methylthienylzinc chloride in a ratio of n:1. The methylthienylzinc chloride can statistically incorporate into the chain ends of the polymer and terminate the chain growing. The ratio n:1 was adjusted to give variety of molecular weights and degrees of polymerization. After polymerization, the polymer batches of each group were purified by precipitation in methanol and continuous washing of the precipitate with methanol for 24 h under nitrogen atmosphere. The regioregularity of our samples were estimated with
In situ X-ray Diffraction (XRD), X-ray Absorption Fine Structure (XAFS) spectroscopy and Small Angle X-ray Scattering (SAXS) have been used in conjunction with Diffuse Optical Spectroscopy (DOS) to characterise cadmium oxide particles nucleated within cadmium exchanged sodium zeolite Y treated with sodium hydroxide. The zeolite structure is maintained up to 750°C at which point it collapses to an amorphous intermediate. At lower temperatures the synthesis of cadmium oxide can be followed from cadmium XAFS and SAXS. XAFS identifies the nanocrystalline nature of the product and uv-vis spectroscopy the associated effects of quantum confinement. Microstructural changes evident from SAXS are similar to those occurring when cadmium oxide is synthesised from a hydroxyl gel but nanocrystals are smaller with no increase below 200°C, pointing to incarceration in the zeolite network. Subsequent growth of cadmium oxide may be responsible for the eventual collapse of the zeolite structure at 750°C.
The combined XAS/XRD technique has been used to investigate the stability of the microporous structure during calcination process for a series of ZnAPSO-44 materials. Incorporation of large amounts of Zn is found to produce materials that are unstable upon removal of the structure directing organic template molecule. However, simultaneous incorporation of Si is found to increase the thermal stability of the materials, yielding catalysts that are well suited for acid catalysed reactions.
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