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Chloroform is a general solvent for poly(3-hexylthiophene) (P3HT) active layers in field-effect transistors. However, its low boiling point and rapid evaporation limit the time for crystallization during the spin-coating process, and field-effect mobilities achieved for P3HT films spin-coated from chloroform are typically on the order of 0.01 cm2/(V s). Here we investigate a range of solvents with higher boiling points. We find that 1,2,4-trichlorobenzene with good solubility and a high boiling point significantly improves the field-effect mobilities up to 0.12 cm2/(V s) with on:off ratios of 106. By controlling the microstructure through the choice of solvent while keeping the molecular weight fixed, we observe a clear correlation between the field-effect mobility and the degree of microcrystalline order as measured by X-ray diffraction, as well as the strength of polaronic relaxation of charge carriers in the accumulation layer as measured by optical spectroscopy of field-induced charge.

Interchain interactions have a profound effect on the optical as well as charge transport properties of conjugated polymer thin films. In contrast to oligomeric model systems in solution-deposited polymer thin films the study of such effects is complicated by the complex microstructure. We present here a detailed study of interchain interaction effects on both charged polarons as well as neutral excitons in highly crystalline, high-mobility poly-3-hexylthiophene ͑P3HT͒ as a function of molecular weight. We find experimental evidence for reduced exciton bandwidth and increased polaron delocalization with increasing conjugation length and crystalline quality. From comparative studies of field-effect transistor characteristics, film morphology, and optical properties our study provides a microscopic understanding of the factors which limit the charge transport in P3HT to field-effect mobilities around 0.1 cm 2 / V s, and which will need to be addressed to improve mobility further.

Given that self-efficacy has emerged as a key construct in health psychology, this study set out to explore its utility in the context of blood donation as defined within the Theory of Planned Behaviour (TPB). An Ajzen and Fishbein-type questionnaire was administered to 100 undergraduate students at the University of Ulster, Coleraine. A hierarchical multiple regression analysis provided strong support for the role of self-efficacy as a major determinant of intention. It not only helped to explain some 73% of the variance, but also made a greater contribution to the prediction of intention than the other main independent variables of the model-past behaviour and self-identity. Demonstrating the utility of self-efficacy in the context of blood donor behaviour not only has several important practical implications, but serves to further highlight its importance within the TPB.

In spite of the success achieved by the theory of reasoned action, researchers continue to suggest that new components should be added to the original model. Indeed, Ajzen has himself acknowledged that some behaviours may be subject to control problems and thus proposed the theory of planned behaviour to handle behaviours of this kind. This study set out to explore the predictive superiority of the revised theory of planned behaviour by focusing on blood donation, a behavioural domain in which volitional control was thought to be incomplete. It was also the intention to explore the extent to which the belief-based measure of attitude exists as a multidimensional construct. An Ajzen & Fishbein type questionnaire was administered to 141 undergraduate students. Multiple regression analyses provided strong support for the theory of planned behaviour, suggesting that perception of control has an important impact on a person's behavioural motivation. Also, some support for a multidimensional representation of the belief-based measure of attitude was provided.

The carrier transport of carefully purified regioregular poly(3-hexylthiophene) films has been studied using time-of-flight photocurrent measurements. We find balanced ambipolar transport with a room-temperature mobility for holes of 3 Ã 10 −4 cm 2 V −1 s −1 and for electrons of 1.5Ã 10 −4 cm 2 V −1 s −1 at electric fields ജ10 5 V / cm. The transport is relatively field independent and weakly temperature dependent, pointing to a high degree of chemical regioregularity and purity. Conjugated polymers are of increasing interest as new materials for electronic applications, offering the potential for low fabrication cost, easy processing, and flexibility. Organic light-emitting diodes (OLEDs), field-effect transistors (FETs), photodiodes, and solar cells are all applications under intense study and first products are already emerging. One of the limitations for the utilization of such organic semiconductors is that they have relatively poor carrier transport properties in terms both of absolute mobility and with respect to the balance between hole and electron mobility. Low mobilities can limit practical applications, for instance, of OLEDs in passive matrix addressed displays due to low current densities in the space-charge-limited current regime, and unbalanced transport can reduce OLED-based display emission efficiency. Low mobility also impacts on switching speed in FETs and on solar cell efficiency, and again unbalanced transport has additional drawbacks in both of these applications.In general, electron transport in conjugated polymers is much worse than hole transport, and tends to be highly dispersive, with no clear transit time observed in time-offlight (ToF) photocurrent measurements. 1 Better electron transport polymers are, therefore, much needed in order to achieve the desired combination of enhanced and balanced carrier transport. We report here a study that reveals unexpected promise for poly(3-hexylthiophene) (P3HT) as a material with both good electron and hole transport. P3HT has been widely studied in the past and is well known as being an effective hole transport polymer, with higher hole mobility than many other conjugated polymers, including the poly(phenylenevinylene)s. 2 Initially, however, rather low hole mobilities were reported for regiorandom P3HT. 3 However, when head-to-tail regioregularly substituted P3HT became available, the hole mobilities were found to be dramatically improved. [4][5][6] The increase in mobility was related to side-chain-induced self-organization into a well-ordered twodimensional lamellar structure. 2 In contrast, electron transport has not been reported up to now in P3HT, most probably because electrons are heavily trapped. Molecular oxygen impurities have been proposed as one possible trap, 7 but little has been done to systematically address the issue. The extensive literature for P3HT highlights the importance of both structural and chemical influences on transport properties.In this letter, we present results concerning carrier transport in regioregular (RR)...

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The charge transport properties of conjugated polymer semiconductors are governed by strong electron phonon coupling, leading to polaron formation as well as the presence of structural and electronic disorder. However, the relative contribution which polaronic relaxation and disorder broadening make to the temperature activation of the mobility of these materials is not well understood. Here we present a combined study of the temperature and concentration dependences of the field-effect mobility and the optically induced electrontransfer transitions of a series of poly͑3-hexylthiohene͒ field-effect transistors of different molecular weight. We apply a vibronic coupling model to extract the reorganization energy and the strength of electronic coupling from the optical spectra. We observe a transition from a localized to a delocalized transport regime as a function of molecular weight and crystalline quality. Polaron activation is comparable to disorder-induced activation in the low-mobility regime ͓ϳ10 −3 cm 2 / ͑V s͔͒ and needs to be taken into account when interpreting the field-effect mobility, while disorder becomes the dominant mechanism to limit charge transport in the high-mobility regime with mobilities Ͼ10 −2 -10 −1 cm 2 / ͑V s͒.