Fabrication of organic field effect transistors using photosensitive active materials
thesisposted on 29.03.2022, 01:16 by Christopher McMahon
The research described in this thesis was conducted with the aim of finding a versatile group of organic semiconducting and conducting materials that could be used in a practical manner for the production of organic field effect transistors (OFETs) via a direct photopatterning process. It was established that a suitable class of materials – photosensitive Diels-Alder bridged pentacene precursors – already existed, but had not been fully exploited for this purpose. Calculations of synthetic UV-visible, infrared and partial density of states spectra, based on models of single molecules with geometries optimised using density functional theory, proved to be useful in understanding the photochemical properties of existing semiconductor materials and precursors. The viability of these calculations was tested by comparing them with the UV-visible spectra and synchrotron based x-ray photoelectron, x-ray emission and near-edge x-ray absorption fine structure spectra of a series of robust n-type organic semiconductors with shared perylene core structures. Narrowband UV exposures of 6,13-dihydro-6,13-methanopentacene-15-one (6,13-MPn) and 5,14-dihydro-5,14-methanopentacene-15-one (5,14-MPn) were carried out. It was found that they were sensitive to light with wavelengths < 325 nm and < 315 nm, respectively. The difference in the photosensitivity of the two pentacene precursors was utilised in a study of the means to independently pattern multiple layers of electronically active materials. It was also found that patterns could be transferred to films of the organic conductor PEDOT:PSS by photobleaching and that the resistance of the films changed in a predictable manner. Following this, the performance of OFETs produced using 6,13-MPn and 5,14-MPn as precursors for the active semiconductor layers was studied. A measureable increase in conductivity through the source-drain channel of 6,13-MPn and 5,14-MPn based transistors resulted when they were illuminated with broadband light and simultaneously annealed at 95 ˚C and 90 ˚C, respectively. Exposing the 6,13-MPn precursor to filtered light in the range from 270-340 nm once again resulted in conversion to a semiconductor with gate voltage enhanced mobility. In the case of the 5,14-MPn transistor, the output curves appeared to be affected by carrier trapping. Following the studies on the precursors of pentacene, a new range of photoprecursors for semiconductors containing thiophene rings was proposed. The photolysis wavelengths of these materials were predicted by investigating the synthetic UV-visible absorption spectra of the materials.