ON THE ORDERING, MICROSTRUCTURE AND HOLE TRANSPORT CORRELATIONS IN SEMI-CRYSTALLINE POLY(3-HEXYLTHIOPHENE)

This dissertation focuses on describing the research work done on poly(3-hexylthiophene) (P3HT), which represents one of the most important p-type semi-conducting polymers widely used in the field of organic optoelectronics. P3HT is also identified as a typical semi-crystalline material comprising different phases that would yield distinct impacts on its properties when integrated as an active component in optoelectronic devices. In particular, as the material finds great use as a hole-conductor, the objective of the dissertation is to develop a fundamental and quantitative understanding of the relationship between the semi-crystalline morphology and hole transport properties in P3HT. The first section provides a general introduction of the material P3HT and its role as the hole conducting material in various devices including organic photovoltaic solar cells, organic field effect transistors (OFET) and time-of-flight (TOF) devices. Characteristics of the OFET and TOF measurements are discussed. In parallel, structural characterizations of P3HT involving various methods are also described, followed by the introduction of current research progress in the field, and the motivations of the research presented in this dissertation. Three projects are detailed following the introduction section. In the first project, a correlation between the hole transport and corresponding structural properties of the bulk regioregular poly(3-hexylthiophene) (rr-P3HT) is studied as a function of temperature by the time-of-flight (TOF) and wide angle X-ray diffraction (WAXD) techniques. Combining the measured transport characteristics and structural evolutions, two temperature regions with distinct transport mechanisms are identified. At T\u3c120oC, the transport-related structural changes are negligible, and the hole transport is limited by the amorphous phase and can be thermally activated. At T\u3e120oC, a microscopic thermal expansion along the π-π stacking direction within the nanocrystals and a macroscopic deterioration in the ordering both contribute to the decrease in the hole mobility at high temperatures. As demonstrated in the first project, the semi-crystalline morphology at different length scales plays a crucial role in dictating the hole transport properties in P3HT. The second project is aimed to gain a quantitative understanding of the ordered structures of P3HT at different length scales. Specifically, by utilizing a combination of wide angle X-ray diffraction (WAXD), density and 13C solid-state nuclear magnetic resonance (NMR) measurements, the absolute degrees of crystallinity in different P3HTs are determined and compared. The results suggest that, in addition to the two-phase picture pervading in the literatures, a 10wt% local short-range ordering in the amorphous phase should be included, which may greatly influence the resulting macroscopic hole transport characteristics in P3HT-based optoelectronic devices. As an extension of the first and second projects, the third project presents a detailed investigation of the effect of ordering and microstructures on the hole transport properties involving P3HT with different molecular characteristics. Interestingly, two important features are universally resolved in different materials: (i) a significant increase of the hole mobility measured by TOF at low temperatures in physically aged samples; (ii) an abrupt jump in the hole mobility at high temperatures. Taking advantage of the sensitivity of 13C solid-state NMR to local structures, the low temperature aging effects and high temperature mobility jump are attributed to the growth of the local ordered phase in the non-crystalline region during physical aging and an improvement of the π-π stacking within the crystalline phase, respectively. Based on the research results summarized in the three projects, the last chapter provides insights on the possible routes to further the understanding of structure-property relationships not only in the P3HT but also in other classes of semi-conducting polymers of similar semi-crystalline nature. The new understanding and strategies developed on the model P3HT materials in this dissertation are expected to shed light on improving the future design and processing of new types of high-performance semi-conducting polymers