Jaroslaw Jung

Influence of SiO2 surface energy on the performance of organic field effect transistors based on highly oriented, zone-cast layers of a tetrathiafulvalene derivative

In this paper we present that the surface energy of silicon dioxide employed as the dielectric in bottom gate organic field effect transistors has large impact on the device performance. By the use of the zone-casting simple solution processing technique, we ensured reproducibility of active layer preparation confirmed by the atomic force microscopy and x-ray diffraction that showed high crystalline quality. Electrical measurements revealed that charge carrier mobility based on highly ordered zone-cast tetrakis-(octadecylthio)-tetrathiafulvalene layer was increased 30 times to 0.2 cm2/V s, when dielectric surface energy decreased from 51.8 to 40.1 mN/m.

Photogeneration and transport in thin films of p- and n-type discotic liquid crystals

Thin layers of discotic liquid crystals, p-type: HBC-C8,2, HBC-C12 and n-type: HATNA 4D, RDISC1 and perylene derivative were prepared by drop- and zone-casting. It is shown, that zone-casting yields aligned, anisotropic films with edge-on orientation suitable for FET construction. In the isotropic, obtained by drop-casting films the charge-carrier photogeneration, recombination and transport were investigated by means of xerographic discharge. For the first time it is demonstrated experimentally that in the nominally n-type discotics the majority charge carriers are electrons.

Analysis of electric-field assisted photogeneration in polyparacyclophane doped with 2,4,7-trinitrofluorenone

A new photoconducting polymer with extended cooperative electronics effects, poly((E,E)-[6.2]paracyclophane-1,5-diene) (PDE), doped with a low molecular weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), was investigated by means of steady-state photoconductivity and xerographic discharge methods. The photogeneration quantum yield in the PDE+TNF charge transfer (CT) complex for blue light is comparable with the photogeneration yield in poly(N-vinylcarbazole) (PVK) doped with TNF. It is demonstrated that the electric-field dependence of the photogeneration quantum yield in PDE+TNF systems can be described by the Braun kinetic model of dissociation of the CT complex, and not by the Onsager model of geminate recombination, which is appropriate for PVK+TNF systems.

Anisotropy in structural and physical properties in tetrathiafulvalene derivatives-based zone-cast layers as seen by Raman spectroscopy, UV-visible spectroscopy, and field effect measurements

We have studied anisotropy of thin layers of amphiphilic tetrathiafulvalene derivatives (TTF-4SCn, with n=12, 18, and 22) obtained by zone-casting technique. All the films show optical anisotropy, as seen by polarized optical microscopy and polarized UV-visible spectroscopy. By using polarized Raman spectroscopy an angular dependence of intensity of different vibrational modes in respect to the zone-casting direction was determined. It was found that intensities of the modes related to central and ring C=C vibrations in the TTF core depend very strongly on the angle between the zone-cast direction and polarization plane of incident laser light. Comparison of the deduced orientation of the molecules in one of the films (TTF-4SC18) with its crystal structure shows that the polarized Raman spectroscopy can be useful for controlling orientation of molecules in thin films (e.g., for online monitoring). Organic field effect transistors (OFETs), with channels oriented in parallel and perpendicularly to the zone-...

Photogeneration in N-carbazolyl-substituted polysilanes and their charge-transfer complexes with 2,4,7-trinitrofluorenone

Abstract Photoconductivity studies of N -carbazolyl-substituted silane homo- and copolymers and their charge-transfer complexes with 2,4,7-trinitrofluorenone (TNF) are reported. The quantum yield of photogeneration was investigated by means of xerographic discharge method. The photogeneration yields and their dependencies on electric field were analysed in terms of the Onsager model of geminate recombination and they were compared with previously obtained results for poly( N -vinylcarbazole), pure and doped with TNF.

Balanced Ambipolar Organic Field-Effect Transistors by Polymer Preaggregation

Ambipolar organic field-effect transistors (OFETs) based on heterojunction active films still suffer from an imbalance in the transport of electrons and holes. This problem is related to an uncontrolled phase separation between the donor and acceptor organic semiconductors in the thin films. In this work, we have developed a concept to improve the phase separation in heterojunction transistors to enhance their ambipolar performance. This concept is based on preaggregation of the donor polymer, in this case poly(3-hexylthiophene) (P3HT), before solution mixing with the small-molecular-weight acceptor, phenyl-C61-butyric acid methyl ester (PCBM). The resulting heterojunction transistor morphology consists of self-assembled P3HT fibers embedded in a PCBM matrix, ensuring balanced mobilities reaching 0.01 cm2/V s for both holes and electrons. These are the highest mobility values reported so far for ambipolar OFETs based on P3HT/PCBM blends. Preaggregation of the conjugated polymer before fabricating binary blends can be regarded as a general concept for a wider range of semiconducting systems applicable in organic electronic devices.

Influence of bimolecular recombination on xerographic discharge process

The photoinduced decay of surface potential, known as xerographic discharge, is analyzed as a method of determination of photogeneration quantum yield taking into account the recombination processes. The effects of slowing down the surface potential decay with time of illumination and nonzero residual potential are usually explained by trapping of the charge carriers in the bulk of the photoconductor. However, these effects can be explained also by bimolecular recombination in the illuminated thin surface layer occurring simultaneously with the photogeneration. In this work it is assumed that as a result of the illumination the mobile positive charge carriers and immobile negative recombination centers are generated. The critical value of the recombination cross section is determined for which the nonzero residual surface potential appears. This analysis allows also to determine an influence of the accumulated recombination centers on the photocurrent decay and build up of the residual potential in a seri...

Od algorytmu dynamicznej cieczy sieciowej do dedykowanego komputera równoległego II – maszyna mDLL

The article describes the assumption of design, construction and implementation of a machine intended to simulate the phenomena occurring in complex molecular systems. Presented electronic system was built with the Field Programmable Gate Array (FPGA). In the machine was implemented a model of Dynamic Lattice Liquid (DLL) and the tests of the device operation was performed. (From dynamic lattice liquid algorithm to dedicated parallel computer II – mDLL machine). Słowa kluczowe: programowalne układy FPGA, topologia sieci połączeń, równoległe przetwarzanie danych, symulacje molekularne.