Bryan McCulloch

Effect of Interfacial Properties on Polymer–Nanocrystal Thermoelectric Transport

The electrical behavior of a conducting-polymer/inorganic-nanowire composite is explained with a model in which carrier transport occurs predominantly through a highly conductive volume of polymer that exists at the polymer-nanowire interface. This result highlights the importance of controlling nanoscale interfaces for thermoelectric materials, and provides a general route for improving carrier transport in organic/inorganic composites.

The relationship between morphology and performance of donor–acceptor rod–coil block copolymer solar cells

Self-assembled functional rod–coil block copolymers (poly(3-hexylthiophene)-b-poly(n-butyl acrylate-stat-acrylate perylene)) containing electron donor (poly(3-hexylthiophene)) and acceptor (perylene) moieties were synthesized, characterized, and studied in photovoltaic devices. The block copolymers were synthesized by a combination of the McCullough route yielding monodisperse polythiophene, living radical polymerization and finally “click chemistry”. The self-assembled nanostructure was tuned via time to control the degree of order. As a result, devices with active layers which were completely disordered (molecularly mixed), contain short range order in which the nanodomains were molecularly pure, but were poorly organized, or consisted of cylindrical fibrils with their long axes running parallel to the electrodes were compared. Active layers with well formed but poorly organized nanodomains had the highest photovoltaic efficiencies indicating that molecular scale segregation has a significant effect on device performance. The poor performance of the well defined cylindrical nanostructures is probably a reflection of the poor charge transport properties associated with the misorientation of the long axes parallel to the electrodes.

Poly(3-alkylthiophene) Diblock Copolymers with Ordered Microstructures and Continuous Semiconducting Pathways

Conjugated rod-coil diblock copolymers self-assemble due to a balance of liquid crystalline (rod-rod) and enthalpic (rod-coil) interactions. Previous work has shown that while classical block copolymers self-assemble into a wide variety of nanostructures, when rod-rod interactions dominate self-assembly in rod-coil block copolymers, lamellar structures are preferred. Here, it is demonstrated that other, potentially more useful, nanostructures can be formed when these two interactions are more closely balanced. In particular, hexagonally packed polylactide (PLA) cylinders embedded in a semiconducting poly(3-alkylthiophene) (P3AT) matrix can be formed. This microstructure has been long-sought as it provides an opportunity to incorporate additional functionalities into a majority phase nanostructured conjugated polymer, for example in organic photovoltaic applications. Previous efforts to generate this phase in polythiophene-based block copolymers have failed due to the high driving force for P3AT crystallization. Here, we demonstrate that careful design of the P3AT moiety allows for a balance between crystallization and microphase separation due to chemical dissimilarity between copolymer blocks. In addition to hexagonally packed cylinders, P3AT-PLA block copolymers form nanostructures with long-range order at all block copolymer compositions. Importantly, the conjugated moiety of the P3AT-PLA block copolymers retains the crystalline packing structure and characteristic high time-of-flight charge transport of the homopolymer polythiophene (μ(h) ~10(-4) cm(2) V(-1) s(-1)) in the confined geometry of the block copolymer domains.

Tuning the optoelectronic properties of P3EHT block copolymers by surface modification

P3AT-containing block copolymers (BCPs) have potential for use as electroactive polymer brushes for stimuli-responsive surfaces, with reversible tethering of the P3AT block. This kind of tethering is highly dependent on the P3AT blocks affinity for the surface. We have investigated the effect of surface modification on P3EHT-b-PS and P3EHT-b-PtBA films deposited on ITO-coated glass substrates modified by electrodeposition of poly(terthiophene). Optical and electrochemical properties of the BCPs deposited both on PTTh films and on bare ITO were investigated by UV-Vis spectroscopy and cyclic voltammetry, respectively. These characterisations reveal interplay between deposition conditions and the optoelectronic behaviour of BCPs investigated, and also provide insight as to their morphology under various conditions as it impacts on such behaviour. Significantly, electrochemically-driven switches in the morphology of BCPs were observed, however polymer-solvent interactions dominated. Absorbance spectra revealed a strong interaction between BCPs and the poly(terthiophene) substrates, manifesting as disruptions in crystallinity in the BCPs.