It is widely accepted that atmospheric oxygen can work as an electron-accepting dopant mainly to p-type organic semiconductors. We have examined the effect of oxygen on a pentacene field effect transistor (FET) with and without exposure to light using the displacement current measurement. Under vacuum conditions, the change in the displacement current due to hole injection from the source and drain electrodes to the pentacene layer is clearly observed, suggesting that the origin of the mobile carriers in the pentacene FET is carrier injection. When the FET is exposed to oxygen under dark conditions, a very small change in the threshold gate voltage for hole injection is observed. In contrast, with exposure to both oxygen and light, we observed that the threshold voltage is lowered and shifted across the zero bias and even to the polarity against hole injection. This photoinduced doping effect induces a distinct increase in the drain current of the FET, and it is maintained for at least several hours even after the irradiation is turned off. This finding suggests that the performance of organic semiconductor devices is affected not only by atmospheric oxygen but also by ambient light even during the processing and storage of the devices.

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PACS. 62.20 -Mechanical properties of solids (related to microscopic structure). PACS. 62.65 -Acoustic properties of solids. PACS. 61.40 -Amorphous and polymeric materials.Abstract. -In icosahedral quasi-crystals the second-order elastic constant tensor is known theoretically and experimentally to be isotropic; however, elastic isotropy does not hold if higher-order elasticity tensors are considered. In particular the stress dependence of sound velocities which involves at least a sixth-rank tensor is expected to be anisotropic in quasi-crystals. In this paper, the pressure dependence of sound velocities is calculated for both longitudinal and shear ultrasonic waves propagating in an icosahedral structure and the fist measurements of sound velocities in a quasi-crystal under pressure are reported; the longitudinal and shear waves velocities are also measured from 4 to 300 K. The temperature dependence of sound velocities c o n f i i s the elastic isotropy, while the pressure dependence exibits an unusual behaviour.

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