Achmad Zen

Swivel-cruciform oligothiophene dimers

Synthesis and electronic properties of three swivel-cruciform oligothiophene dimers—bis(terthiophene) (BT3), bis(pentathiophene) (BT5) and bis(heptathiophene) (BT7)—with increased solubility in organic solvents are reported. We obtained a field-effect mobility of 3.7 × 10−5 cm2 V−1 s−1 and a current on/off ratio of >103 for a solution-processed OFET device with dimer BT5 as p-type semiconductor.

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.

Improving the Performance of Organic Field Effect Transistor by Optimizing the Gate Insulator Surface

The effect of oxygen plasma treatment and/or silanization with hexamethyldisilazane (HMDS) on the surface chemistry and the morphology of the SiO2-gate insulator were studied with respect to the performance of organic field effect transistors. Using X-ray photoelectron spectroscopy (XPS), it is shown that silanization leads to the growth of a polysiloxane interfacial layer and that longer silanization times increase the thickness of this layer. Most important, silanization reduces the signal from surface contaminations such as oxidized hydrocarbon molecules. In fact, the lowest concentration of these contaminations was found after a combined oxygen plasma/silanization treatment. The results of these investigations were correlated with the characteristic device parameters of polymer field effect transistors with poly(3-hexylthiophene)s as the semiconducting layer. We found that the field effect mobility correlates with the concentration of contaminations as measured by XPS. We, finally, demonstrate that silanization significantly improves the operational stability of the device in air compared to the untreated devices.

Polarization-sensitive photoconductivity in aligned polyfluorene layers

Polarization-sensitive photodiodes have been constructed utilizing an aligned polyfluorene layer oriented on a photoaligned liquid crystalline photoaddressable polymer. The diodes exhibit a pronounced dependence of the photocurrent on the polarization of the incident light, yielding a polarization sensitivity of more than a factor of 10 at the onset of the absorption. The action spectrum is symbatic for light polarized perpendicular to the polymer alignment direction whereas it does not simply correlate with the absorption characteristics for parallel polarized light. The data can be explained utilizing Ghosh’s model taking into account interference effects within the polyfluorene layer.