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.
Measurements have been performed on thermally equilibrated conjugated-polymer/insulating-polymer bilayers, using specular and off-specular neutron reflectivity. While specular reflectivity is only sensitive to the structure normal to the sample, off-specular measurements can probe the structure of the buried polymer/polymer interface in the plane of the sample. Systematic analysis of the scattering from a set of samples with varying insulating-polymer-thickness, using the distorted-wave Born approximation (DWBA), has allowed a robust determination of the intrinsic width at the buried polymer/polymer interface. The quantification of this width (12 Å ± 4 Å) allows us to examine aspects of the conjugated polymer conformation at the interface, by appealing to self-consistent field theory (SCFT) predictions for equilibrium polymer/polymer interfaces in the cases of flexible and semi-flexible chains. This analysis enables us to infer that mixing at this particular interface cannot be described in terms of polymer chain segments that adopt conformations similar to a random walk. Instead, a more plausible explanation is that the conjugated polymer chain segments become significantly oriented in the plane of the interface. It is important to point out that we are only able to reach this conclusion following the extensive analysis of reflectivity data, followed by comparison with SCFT predictions. It is not simply the case that conjugated polymers would be expected to adopt this kind of oriented conformation at the interface, because of their relatively high chain stiffness. It is the combination of a high stiffness and a relatively narrow intrinsic interfacial width that results in a deviation from flexible chain behaviour.
High aspect-ratio poly(9,9 ′ -dioctylfl uorene) (F8) nanowires are fabricated on top of silicon/amorphous Tefl on substrates, by dewetting of F8 thin-fi lms. The conjugated nature of the polymers enables the self-assembly of continuous molecularly-aligned nanowires of several micrometers in length and tens of nanometers width, without using a template. The sizescale of the dewetted morphology can be controlled by varying the thickness of the F8 and amorphous Tefl on layers. As predicted by van der Waals theory the characteristic sizescale of the dewetted morphology increases with F8 fi lm thickness. However, the dependence of the characteristic sizescale on amorphous Tefl on thickness is not accounted for, even qualitatively, by standard spinodal theory with van der Waals forces as the de-stabilizing force across the F8 fi lm. The Rayleigh instability is strongly suppressed in the F8 nanowires in the late stages of dewetting, compared to isotropic, amorphous polymers. Polarized Raman measurements show a systematic increase in molecular alignment along the axis of the nanowires as their width is reduced below the typical liquid-crystalline domain size in polyfl uorene fi lms. Thus the dewetting process aligns the polymers, and the aligned polymers suppress the Rayleigh instability and enable the formation of high aspect-ratio continuous nanowires.
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