UC Berkeley

The goal of this research was to develop methods to control the material properties of organic semiconductors, like solubility, stability, charge mobility, and self-assembly, through structural design. Investigations of structure-property relationships were conducted to optimize the properties of organic semiconductors for applications in organic field effect transistors (OFETs) and organic photovoltaics (OPVs).

Chapter 1 gives an introduction to charge transport in organic semiconductors, and describes how the structure of conjugated molecules can affect their electrical performance and facilitate facile solution deposition. Furthermore, factors that can affect the stability of organic semiconductors to ambient conditions are discussed. Also, the device characteristics of OFETs and OPVs are summarized as a reference for subsequent chapters.

Chapter 2 discusses the investigation of the air stability of distyryl oligothiophenes in OFETs. This work made use of thermally labile solubilizing groups to facilitate solution deposition of the oligothiophenes. In addition to device characterization, an extensive analysis of the thin film morphology using AFM, NEXAFS and GIXD is presented. This work revealed the general stability of distyryl oligothiophenes to oxidative degradation, and the high degree of crystallinity in our thin films.

In Chapter 3, the charge transporting properties of pentathiophene monolayer islands is analyzed using current sensing AFM. The pentathiophenes were prepared with carboxylic acid moieties for self assembly, and the sub-monolayer films were transferred onto conductive substrates using the Langmuir-Blodgett technique. The morphology of the monolayers was observed to be sensitive to the alkyl substitution pattern of the pentathiophenes, which in turn affected charge transport.

Hierarchical supramolecular assemblies of oligothiophenes and block copolymers are studied in Chapter 4. The structure of the assemblies is studied by TEM and small angle X-ray scattering, confirming successful formation of supramolecular assemblies. Thin films are studied further by grazing incidence X-ray scattering, and their charge transporting properties are evaluated in OFETs. This work demonstrates that non-covalent self-assembly can be used to access nanostructured thin films of functional organic semiconductors.

Finally, Chapter 5 discusses the photophysical, morphological, and electronic properties of boron(subphthalocyanine)s with conjugated axial ligands. The tendency of these materials to form polycrystalline films instead of amorphous films was shown to be very sensitive to axial ligand structure and coordination geometry. The morphology of thin films was also shown to affect OPV device characteristics, with crystalline films supplying higher photocurrents.