α,ω -dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors

While organic semiconductors are being widely investigated for chemical and biochemical sensing applications, major drawbacks such as the poor device stability and low charge carrier mobility in aqueous electrolytes have not yet been solved to complete satisfaction. In this work, solution-gated organic field-effect transistors (SGOFETs) based on the molecule α,ω-dihexyl-sexithiophene (DH6T) are presented as promising platforms for in-electrolyte sensing. Thin films of DH6T were investigated with regard to the influence of the substrate temperature during deposition on the grain size and structural order. The performance of SGOFETs can be improved by choosing suitable growth parameters that lead to a two-dimensional film morphology and a high degree of structural order. Furthermore, the capability of the SGOFETs to detect changes in the pH or ionic strength of the gate electrolyte is demonstrated and simulated. Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications

α,ω -dihexyl-sexithiophene thin films for solution-gated organic field-effect transistors

While organic semiconductors are being widely investigated for chemical and biochemical sensing applications, major drawbacks such as the poor device stability and low charge carrier mobility in aqueous electrolytes have not yet been solved to complete satisfaction. In this work, solution-gated organic field-effect transistors (SGOFETs) based on the molecule α,ω-dihexyl-sexithiophene (DH6T) are presented as promising platforms for in-electrolyte sensing. Thin films of DH6T were investigated with regard to the influence of the substrate temperature during deposition on the grain size and structural order. The performance of SGOFETs can be improved by choosing suitable growth parameters that lead to a two-dimensional film morphology and a high degree of structural order. Furthermore, the capability of the SGOFETs to detect changes in the pH or ionic strength of the gate electrolyte is demonstrated and simulated. Finally, excellent transistor stability is confirmed by continuously operating the device over a period of several days, which is a consequence of the low threshold voltage of DH6T-based SGOFETs. Altogether, our results demonstrate the feasibility of high performance and highly stable organic semiconductor devices for chemical or biochemical applications.This work has been partially supported by the Nanosystems Initiative Munich (NIM) and the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1032.Peer Reviewe

Microdiffraction imaging—a suitable tool to characterize organic electronic devices

Tailoring device architecture and active film morphology is crucial for improving organic electronic devices. Therefore, knowledge about the local degree of crystallinity is indispensable to gain full control over device behavior and performance. In this article, we report on microdiffraction imaging as a new tool to characterize organic thin films on the sub-micron length scale. With this technique, which was developed at the ID01 beamline at the ESRF in Grenoble, a focused X-ray beam (300 nm diameter, 12.5 keV energy) is scanned over a sample. The beam size guarantees high resolution, while material and structure specificity is gained by the choice of Bragg condition.Here, we explore the possibilities of microdiffraction imaging on two different types of samples. First, we measure the crystallinity of a pentacene thin film, which is partially buried beneath thermally deposited gold electrodes and a second organic film of fullerene C$_{60}$. The data shows that the pentacene film structure is not impaired by the subsequent deposition and illustrates the potential of the technique to characterize artificial structures within fully functional electronic devices. Second, we investigate the local distribution of intrinsic polymorphism of pentacene thin films, which is very likely to have a substantial influence on electronic properties of organic electronic devices. An area of 40 μm by 40 μm is scanned under the Bragg conditions of the thin-film phase and the bulk phase of pentacene, respectively. To find a good compromise between beam footprint and signal intensity, third order Bragg condition is chosen. The scans show complementary signal distribution and hence demonstrate details of the crystalline structure with a lateral resolution defined by the beam footprint (300 nm by 3 μm). The findings highlight the range of applications of microdiffraction imaging in organic electronics, especially for organic field effect transistors and for organic solar cells

Surface-directed molecular assembly of pentacene on aromatic organophosphonate self-assembled monolayers explored by polarized Raman spectroscopy

Organophosphonate self-assembled monolayers (SAMPs) fabricated on SiO 2 surfaces can influence crystallization of vapor-deposited pentacene and thus can affect device performance of pentacene-based organic thin film transistors. Polarized Raman spectroscopy is demonstrated to be an effective technique to determine the degree of anisotropy in pentacene thin films deposited on three structurally different, aromatic SAMPs grown on silicon oxide dielectrics. Vibrational characterization of pentacene molecules in these films reveals that the molecular orientation of adjacent crystalline grains is strongly correlated on the SAMP-modified dielectric surface, which results in enhanced interconnectivity between the crystallite domains, well beyond the size of a single grain. It is found that vibrational coupling interactions, relaxation energies, and grain size boundaries in pentacene thin films vary with the choice of SAMP. This information clearly shows that molecular assembly of pentacene thin films can be modulated by controlling the SAMP-modified dielectric surface, with potentially beneficial effects on the optimization of electron transfer rates