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Title: Charge modulation spectroscopy of all-polymer transistors
Author: Deng, Yvonne Yalda
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2004
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The semiconducting polymer poly(9,9’-dioctyl-fluorene-co-bithiophene) (F8T2) is incorporated into a thin-film transistor structure with a polymer dielectric layer deposited by spincoating and inkjet printed polymer electrodes. Charge modulation spectroscopy (CMS) is used to study the optical absorptions in the presence of charges in-situ in the transistor structure. This phase-sensitive technique allows absorptions to be measured at a signal level 4-5 orders of magnitude below the neutral absorption. A prominent charge-induced absorption at 1.65eV is observed as well as a shoulder at 1.3eV and a tail extending towards the absorption edge. The bias-dependence of the CMS signature confirms that intermixing of the polymer layers is minimal, as expected from the excellent transistor characteristics.  Various efforts aimed at changing the morphology of the semiconductor-dielectric interface and their effects on CMS spectra are presented. The liquid-crystalline properties of F8T2 are exploited by manufacturing uniaxially aligned transistors and measuring the dependence of the charge-induced absorptions on the polarisation direction of the incident light. It is found that the main feature at 1.65eV is strongly polarised whereas the shoulder is unpolarised. This observation as well as further experimental evidence lead to the conclusion that while the main absorption is attributable to the intrinsic, polaronic absorption in F8T2, the shoulder is likely to originate from a defect state. Finally, the CMS technique is applied to study bias stress induced charge trapping in F8T2 transistors. The effect is responsible for a decay of the transistor current under operation. The trapped charge responds slowly to an applied modulation and various studies, including decreasing the modulation frequency and illuminating the device, are presented to investigate the trapping mechanism.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available