MORPHOLOGICAL AND ENERGETIC EFFECTS ON CHARGE TRANSPORT IN CONJUGATED POLYMERS AND POLYMER-NANOWIRE COMPOSITES

Organic semiconductors have wide applications in organic-based light-emitting diodes, field-effect transistors, and thermoelectrics due to the easily modified electrical and optical properties, excellent mechanical flexibility, and solution processability. To fabricate high performance devices, it is important to understand charge transport mechanisms, which are mainly affected by material energetics and material morphology. Currently it is difficult to control the charge transport properties of new organic semiconductors and organic-inorganic nanocomposites due to our incomplete understanding of the large number of influential variables. Molecular doping of π-conjugated polymers and surface modification of nanowires are two means through which charge transport can be manipulated. In molecular doping, both the energetics and microstructures of polymer films can be changed by controlling the degree of oxidation of the conjugated polymer backbone. For surface modification of inorganic nanowires, the energetics and morphology can be influenced by the properties of the surface modifiers. Meanwhile, the energy band alignment, which can be controlled by surface modification and molecular doping, may also alter the charge transport due to the variation in energetic barriers between the transport states in the organic and inorganic components. To reveal the effects of morphology and energetics on charge transport in conjugated polymers and organic-inorganic nanocomposites, the influence of surface modifier on the electrical and morphological properties of nanocomposites was first probed. Silver nanowires modified with different thiols were blended with poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS) to fabricate thin films. The modified nanowires provided a means of controllably altering the nanowire dispersability and compatibility with solvents and polymers. The results also demonstrated that charge transport between the nanowires was facilitated due to low wire-to-wire junction resistance. To further figure out the charge transport mechanism in organic-inorganic nanocomposites and the potential applications, tellurium nanowires and ferric chloride doped poly (3-hexylthiophene-2,5-diyl)(P3HT) were used to characterize energy band alignment effects on charge transport, electrical conductivity, and thermoelectric properties. The results showed that charge transfer between nanowires can be mediated by the polymer and may potentially increase the electrical conductivity as compared to the pure polymer or pure nanowires; while the observed enhancement of power factor (equal to electrical conductivity times the square of Seebeck coefficient) may not be affected by the energy band alignment. It is important to investigate the change of polymer morphology caused by molecular doping and processing method to determine how the morphology will influence the electrical and thermoelectric properties. Various p-type dopants, including ferric chloride and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Motfd3), were examined for us in P3HT and other polymers. The results showed that: i) At light doping levels, the electrical conductivity and power factor of polymers doped with the large electron affinity (EA) dopants were larger than small EA dopants; ii) At heavy doping levels, the large size dopants cannot effectively dope polymers even for the dopants with large EAs; iii) For the same dopant, as the IE of the polymer increased, the doping efficiency gradually decreased