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Title: Organic field effect transistors fabricated using solution based aggregation and soft lithography
Author: Mukherjee, Abhimanyu
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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In this thesis, we present a study on thin films and organic field effect transistor (OFET) devices using solution ageing induced aggregation and soft lithography techniques. We first investigated the effect of solution ageing and film thickness of F8T2 (poly(9,9-dioctylfluorene-co-bithiophene)) spin coated from different boiling point solvents. Using the weakly interacting H-J aggregate model, we found that with solution ageing, there was an increase in J-aggregate ordering, characteristic of the beta-phase regime commonly associated with polyfluorenes. In organic transistor devices, this corresponded with a positive correlation with threshold voltage, V_th, and the Vissenberg and Matters density-of-states temperature for disorder, T_0. Azeotropic binary solvent thin films of F8T2 were also investigated in regimes of extreme H and J aggregation, with the latter indicating more presence of the beta-phase was detrimental to OFET performance. Using the soft lithographic approach, patterning of the organic semiconductor layer was achieved by applying the micromolding in capillaries (MIMIC) and solvent assisted micro molding in capillaries (SAMIM) methods. Different architectures of stamp designs and domains using Fluorolink MD 700 perfluoropolyether (PFPE) and polydimethylsiloxane (PDMS) materials were investigated and their effects on OFET device performance are presented. We found that patterned organic semiconductor layers of both small molecules such as TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene), and the polymers F8T2 and P3HT (poly(3-hexylthiophene)) from a solution precursor, were capable of forming sub-micron features using optimised stamp geometries and materials at room temperature. Furthermore, we also investigated the effect of confined crystallisation of F8T2 from mono and azeotropic binary solvents in staggered top gate geometries. Using the highly hydrophobic fluoropolymer CYTOP as the dielectric, saturation mobilities of >0.001 cm^2/Vs, threshold voltages of V_th > -20 V, and gate leakage currents of (|I_D/I_G| > 10^3) were obtained. These results for confined crystallised F8T2 OFETs are of better performance, with the mobility in particular being an order of magnitude higher than that of conventional solution processed counterparts using spin coating and drop casting deposition methods.
Supervisor: Campbell, Alasdair Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral