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Title: Organic field-effect transistors with printed dielectrics and semiconductors
Author: Vaklev, Nikolay Lyubomirov
ISNI:       0000 0004 6348 1059
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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This thesis presents the development of organic field-effect transistors with printed dielectric and semiconductors. The device architecture was bottom-gate bottom-contact. The electrodes were fabricated via standard photolithography. The first milestones were to gravure-coat the dielectric and structure it with photolithography. Dielectric formulations were screened for their ability to photopattern with radical photoinitiators. Variable processing conditions were also investigated such as annealing time and temperature. The preferred formulation and processing conditions gave ca. 130 nm thick dielectric films and 1-2 nm root-mean-square surface roughness. The dielectric films were tested in parallel capacitors and field-effect transistors. For comparison, the dielectric was also spin-cast and processed analogously to the gravure-coated films. Under the same conditions gravure-coating gave films with equal uniformity and insulating properties as spin-casting. Transistors were prepared with thermally-evaporated pentancene and TIPS-pentacene (6,13-bis(triisopropylsilylethynyl) pentacene) via spin- or zone-casting. The measured mobilities were amongst the highest reported in the literature for this material set and device architecture. The highest transistor mobility with TIPS-pentacene was achieved by blending the semiconductor with poly(a-methylstyrene) (PaMS). Device characteristics such as mobility, threshold voltage and sub-threshold swing voltage were calculated and their evolution with blending ratio followed. The semiconductor was either spin-cast onto pre-deposited PaMS layers with different thickness or TIPS-pentacene was blended with PaMS already in the ink. The work concluded with structured gravure-printing of the dielectric and semiconductor. Dielectric inks with different viscosity were printed and print quality investigated. The film thickness ranged between 60 and 500 nm. Exemplary films were used in the fabrication of transistors and complementary inverters. TIPS-pentacene was directly blended with polymer binders and printed.
Supervisor: Campbell, Alasdair Sponsor: European Community
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral