UC San Diego

Mechanical flexibility and deformability are at the core of the advantages offered by organic semiconductors. Therefore, an in-depth understanding of the mechanical properties of these materials is crucial to the design of robust organic electronic devices such as solar cells, sensors, and displays. Since these devices are typically fabricated as thin films, on the order of 200 nm, it can be difficult to measure the mechanical properties using traditional techniques, such as tensile testing, that require bulk samples. This thesis examines and compares various methods of testing the mechanical properties of thin films and correlates the molecular structure of organic semiconductors to such properties. Chapter 1 and Appendix A use mechanical buckling and crack-onset strain techniques to measure the elastic modulus and the strain at fracture in fullerene-based semiconductors. These methods were used to examine the effect of incompletely separated grades of electron acceptors on the mechanical deformability of organic solar cells in an effort to simultaneously improve the mechanical robustness of the organic solar cells and reduce the energy of production. Chapter 2 and Appendix B use mechanical buckling, crack-onset strain, and the onset of wrinkles (collectively known as film-on-elastomer techniques) to show a decrease in the stiffness and an increase in the ductility of small-molecule semiconductors that bear side-chains in the backbone structure and compare the results to fullerene-based semiconductors. Chapter 3 and Appendix C compare the results from two different methods of measuring thin-film mechanical properties; film-on-elastomer and film-on-water methods. The film-on-water method uses water to support thin films while conducting a tensile test. These methods were used to measure the mechanical properties of poly(3-hexylthiophene) in a range of molecular weight and the results were directly compared. In Chapter 4 and Appendix D a technique known as scratch testing was used, for the first time, to measure the cohesion and adhesion of semiconducting polymers. The cohesive and adhesive strength were measured as a function of the length of the side chain in poly(3-alkylthiophenes) and molecular weight in poly(3-hexylthiophene).