Potential Profile Surveyed Directly by Nanomanipulator with Probing Tip in Al/Poly(3-alkylthiophene)/Au Diodes
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Potential Profile Surveyed Directly by Nanomanipulator with Probing Tip in Al/Poly(3-alkylthiophene)/Au Diodes K. Kaneto, M. Nakagawa and W. Takashima Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Fukuoka 808-0196, Japan ABSTRACT The potential profiles along the channel of Al/Poly(3-hexylthiophene), PHT film/Au diode from Al to Au electrodes have been measured directly using nanomanipulator with potential probing tips. The steep potential cliff is observed at the interface of Al/PHT, indicating the existence of depletion layer with higher resistance than that of bulk region. For the forward bias, the resistance at the depletion layer decreased significantly and the potential gradient in the PHT bulk region is observed above the onset bias of 1V. It is found that the contact resistance of PHT/Au is unexpectedly large in spite of the ohmic behavior and also for Al/PHT at forward bias. The effect of light on the depletion layer will be also mentioned. It is stressed that the contact resistances of PHT and metals are significantly important to improve the performance of organic electronics devices. KEYWORDS: conducting polymers, poly(3-hexylthiophene), potential profile, Schottky diode, nanometric interface, ohmic contact INTRODUCTION Recently, organic materials are subject to utilize opto-electronics devices such as light emitting diodes (LED) [1], solar cells[2] and thin film transistors[3], which are featured by flexibility, low cost and/or large area. Already organic light emitting diodes have been developed into market, and prospectively the other devices are also. The mechanisms of such functions in organic devices have been studied based on the theory of conventional inorganic semiconductor. The behavior and phenomena in organic device are similar to conventional devices. However, it is found that the details of the mechanisms of organic device are quite different from those of conventional models. The organic materials are assembled with the molecules with weak molecular bond and electrons are localized on a molecule. On the other hand, inorganic semiconductors are condensed with strong covalent bond between atoms and the electrons spread over the crystal. Conducting polymers[4] are studied intensively as one of CnH2n+1 the organic electronic active materials, since the conjugated π-electrons are optically and electronically active as a semiconductor. Particularly, poly(alkylthiophene), PAT films as shown in Fig.1 are S studied, because the variety of films can be prepared by controlling S x the content of head-to-tail (HT) coupling and with various alkyl chain lengths. It has been found[5] that the highly regioregular PAT films CnH2n+1 with shorter alkyl chain length show larger carrier mobility and higher performance in the rectification and photovoltaic effect of Au/PAT/Al Fig.1 Chemical structure of diodes. These functions of diodes are commonly generated at the poly(3-alkyllthiophene). interface of semiconductors and metals (the
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