Nanoscale morphology and phase structure of regioregular poly(3 -hexylthiophene) in single- and multicomponent systems

Poly(3-hexylthiophene) exemplifies the potential of semiconducting polymers in solution processable electronic devices. This work was focused on the nanoscale morphology and phase structure of poly(3-hexylthiophne)s with molecular weight below 15 kDa and low polydispersity. Specifically, the nature of packing of polymer chains into elongated structures called nanofibrils was studied. Despite the recognition of the impact of the amorphous component on charge transport along with numerous reports of correlation of morphology with charge carrier mobility, very little was known about the chain packing, electron density profile and the ratio of amorphous to crystalline phases. This work addresses these deficiencies based on one of the central findings of this thesis: the presence of alternating high and low packing density regions is one of the fundamental consequences of the fringed edge structure of the fibrillar aggregates, which is rooted in the width of the molecular weight distribution of the polymer. The semicrystalline character of nanofibrillar poly(3-hexylthiophne) results from the nature of packing and not the presence of unaggregated polymer. The signature of this periodic structure was present in x-ray difractograms, which allowed the use of an autocorrelation analysis to estimate the crystallinity and electron density difference between the phases. The free volume of the inter-fibrillar boundaries pointed to the possibility of accumulation of acceptor material for bulk heterojunction solar cells. This suggestion was subsequently experimentally confirmed. This realization amounted to a new view of phase structure in multicomponent systems involving rr-P3HT and its derivatives, with particularly important consequences for the design of photovoltaic systems. The role of other components of this strongly interacting system, such as alkyl side-chain placement, polymer chain end-groups and pre-aggregation in solution, in pure polymer and blends was also studied. The sparse side-chain polythiophene derivatives were shown to mix intimately with acceptor molecules. This resulted in low performance in solar cells (measured by Plextronics, Inc., Pittsburgh, PA) based on these polymers due to the presence of separate domains, which made it impossible for a significant fraction of charge carriers to reach the electrodes. During the course of the studies it was observed that the equilibrium fibrillar morphology can be altered by addition of polymer chain end functionalizing groups. The recognition of the elements that affected self-assembly allowed better control of the interfaces between different phases, which is vital in advancement of polymer-based innovative technologies in areas such as energy conversion and storage, chemsensors and others.