Enhanced Charge Transport in Polymer Thin-film Transistors Through Structural and Morphological Optimization

Book excerpt: Organic semiconductors offer the potential for low cost, large area, and flexible electronic devices. However, the lower performance of organic materials relative to silicon, germanium, and other inorganic components has prevented widespread implementation. Correspondingly, the overarching goals of the work outlined in this dissertation focus on exploring the fundamental properties and intermolecular interactions of conjugated polymers and utilizing the findings to develop routes for improving thin-film transistor performance. Charge transport in organic active layers depends largely on the intrinsic carrier mobility of the semiconductor, the morphology achieved during processing and fabrication, as well as the effectiveness of the post-processing techniques. In this work, we investigate the influence of each and will show how appropriately tuning polymorphism, copolymerization between strong and weakly crystallizing moieties, and fluorine substitution on delocalized cores significantly enhance transistor characteristics. Crystalline organic molecules often exhibit the ability to assemble into multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules in the unit lattice is a viable method for improving charge transport within the material. We have employed grazing-incident X-ray diffraction to demonstrate the formation of tighter stacking poly(3-hexylthiophene-2,5-diyl) polymorphs in films spin coated from ferrocene-containing solutions. As a result, the addition of ferrocene to casting solutions yields thin-film transistors which exhibit approximately three times higher source-drain currents and charge mobilities than neat polymer devices. Nevertheless, thorough analysis of the active layer reveals that all ferrocene is removed during the spin coating process, which may be an essential factor to achieve good mobilities. Such insights gleaned from ferrocene/poly(3-hexylthiophene) mixtures can serve as a template for selection and optimization of other small molecule/polymer systems with greater baseline charge mobilities. Block copolymerization provides yet another avenue for altering the crystal packing behavior and morphology of polymer semiconductors. Our work reveals that covalently coupling a weakly crystalline acceptor polymer with excellent electron mobility to a strongly crystallizing donor unit can induce ordering in the less crystalline block. Grazing-incidence X-ray scattering results confirm that shorter interchain spacing distances are obtained in poly(3-hexylthiophene)-b-poly(fluorene-dithiophene-benzothiadiazole) copolymers compared with neat poly(fluorene-dithiophene-benzothiadiazole) films. An enhancement in the ordering of the acceptor moiety was also observed both in neat homopolymer and copolymer samples after thermal annealing at 195 C. Consequently, the electron mobility of the block copolymer, measured in thin-film transistors with aluminum contacts, surpassed that of either homopolymer and peaked at annealing temperatures between 195 210 C. Several recent reports have surfaced in the literature in which fluorinated analogues of various donor/acceptor copolymers consistently surpass their non-fluorinated counterparts in terms of performance. Prior studies have speculated as to the origin of this fluorine effect, but concrete evidence has not been forthcoming. Using a benzodithiophene and benzotriazole copolymer series consisting of fluorinated, partially-fluorinated, and non-fluorinated analogues, we confirm that the addition of fluorine substituents beneficially impacts charge transport in polymer semiconductors. Transistor measurements demonstrated a 5x rise in carrier mobilities with the degree of fluorination of the backbone. Furthermore, X-ray diffraction data indicates progressively closer packing between the conjugated cores and an overall greater amount of crystallinity in the fluorinated materials. It is likely that attractive interactions between the electron-rich donor and fluorinated electron-deficient acceptor units induce very tightly stacking crystallites, which reduce the energetic barrier for charge hopping. In addition, a change in crystallite orientation was observed from primarily edge-on without fluorine substituents to mostly face-on with dual fluorine groups. We also introduce a promising post-processing technique adapted from existing zone purification and recrystallization methods. Zone annealing and zone refining are proposed for imparting directionality to the crystallization process, thereby increasing the size of crystallites and uninterrupted conjugation lengths within polymer films. A custom nichrome wire-based zone heating apparatus developed for zone refining thin films is described, and preliminary results with poly(3-hexylthiophene) are presented. A comparison with the UV-Vis absorbance of films annealed statically on a hot plate suggests that similar conjugation lengths can be achieved in approximately a sixth of the time with zone refining. Further optimization and investigatory studies are required before the procedure can be successfully extended to transistor samples, but zone crystallization appears to be a highly compatible post-processing approach for large scale manufacturing. The final portion of this work was dedicated to the development of potential integration venues for organic devices. Applications which take full advantage of the unique properties of polymer semiconductors will be needed as organic electronics begin the arduous transition into the commercial sphere. As such, neutron and X-ray detection systems represent two categories where very large area and flexibility would be invaluable. We therefore explore the feasibility of sensitizing conjugated materials towards either neutrons or X-rays through the incorporation of elements possessing excellent neutron capture or X-ray absorption properties. The projected mechanisms and challenges associated with direct radiation detection are discussed, and the results obtained from numerous screening experiments, conducted to determine which compounds maintain acceptable performance in transistors, are included. Based on these trials, boron nitride, 10B-enriched boric acid, and ruthenocene blended with poly(3-hexylthiophene) at extraordinarily high loadings were identified for further scrutiny and eventual response testing with an X-ray or neutron source. In summary, the objectives set forth for this work have been successfully realized. We examined the impact of several parameters governing charge transport in organic semiconductors, and based on our conclusions, we have identified three approaches for substantially augmenting the performance of polymer field-effect transistors. We have also considered a useful post-process treatment for large scale device fabrication and illustrated the benefits and potential for adapting conjugated materials for novel detection applications. The contributions of the research efforts expounded within this dissertation have far reaching implications yet represent only a small part of the general advance of the organic semiconductor field. Significant progress is being made on many critical fronts, and provided the allure of light weight, completely conformable electronics remains strong, we expect to continue witnessing the steady emergence of ever more numerous devices and gadgets based on organic transistors and diodes.