The first substituent‐induced “flip” from p‐ to n‐type conductivity as well as enhanced thermal stability and volatility are found for fluorocarbon‐functionalized sexithiophene 1 (relative to the fluorine‐free analogues 2 and 3). Evaporated films of 1 behave as n‐type semiconductors, and can be used to fabricate thin‐film transistors with field‐effect mobilities as high as 0.02 cm2 V−1 s−1—some of the highest reported to date for n‐type organic semiconductors.

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Using conjugated polymers as the active materials in electronic and optoelectronic devices opens up the possibility of fabricating all-polymer devices using solution processing technologies. The fabrication of good quality field-effect transistors (FETs) is crucial to a number of polymer-based devices, such as active matrix displays and integrated circuits. Central to FET operation is the dielectric/semiconductor interface. Here we look at the interface between a polymer gate dielectric and a conjugated polymer, using neutron reflectivity. By using a mixed solvent (toluene/cyclohexane) to deposit the conjugated polymer directly on top of the polymer dielectric we are able to fabricate bilayer FET architectures with systematically controlled interfacial roughness, and study the impact on transistor performance.The use of conjugated organic materials as the semiconducting component, within electronic devices is an increasingly important technology. Conjugated polymers combine the promise of products with desirable properties such as flexibility, with the processing advantages of solution-based production methods (large areas at low cost). The nature of the interfaces between semiconducting polymers and other materials is of vital importance in the performance of several organic devices. This study focuses on FETs, where the interface between the polymer dielectric and the semiconducting polymer is of crucial importance, as it is adjacent to this interface that current flows within the device.In this work we focus on a bottom gate, top contact architecture, with poly(9,9-dioctylfluorene-altbithiophene) (F8T2) as the semiconducting layer and poly(methylmethacrylate) (PMMA) as the gate dielectric (see figure 1). By dissolving the F8T2 in a miscible mixture of a good solvent (toluene) and a poor solvent (cyclohexane) for PMMA in various volume ratios, and directly spin-coating onto the PMMA layer, it is possible to control the structure of the buried F8T2/PMMA interface. By varying the ratio of the two solvents in this mixture we are able to systematically control interfacial roughness over a broad range and correlate this directly with charge mobility. FETs were also fabricated following the same procedures, but with the addition of a 300nm SiO 2 on top of the silicon substrate. Our goal in this study was to make polymerpolymer bilayers with controlled interfacial roughness. However, only specific combinations of PMMA molecular weight and solvent ratio can produce well-controlled bilayer structures. If the toluene content is too high, or the PMMA molecular weight too low, dissolution of the PMMA film by the solvent used for the deposition of the F8T2 is too rapid, and a complex laterally structured morphology is obtained. For PMMA with molecular weight Mw=313,500 g/mol we are able to produce bilayers for toluene:cyclohexane (T:C) solvent ratios ranging from 4:1 to 1:3, and it is these bilayers that we shall discuss here.

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Ein Wechsel von p‐ zu n‐Halbleitereigenschaften, durch Substituenten hervorgerufen, wurde beim Perfluorhexyl‐funktionalisierten Sexithiophen 1 festgestellt. Dieses ist thermisch stabiler und flüchtiger und hat eine höhere Elektronenaffinität als die nicht fluorierten Verbindungen 2 und 3. Aufgedampfte Filme von 1 verhalten sich wie n‐Halbleiter und sind geeignet zur Herstellung von Dünnfilmtransistoren mit einer elektrochemischen Beweglichkeit von etwa 0.02 cm2 V−1 s−1 – dies ist einer der höchsten für organische n‐Halbleiter beschriebenen Werte.