Ultrahigh Vacuum Seebeck Effect and Conductivity Measurements on N-Doping of C 60 Films
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0965-S13-03
Ultrahigh Vacuum Seebeck Effect and Conductivity Measurements on N-Doping of C60 Films Naoki Hayashi1, Kaname Kanai1, Yukio Ouchi1, and Kazuhiko Seki1,2 1 Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusaku, Nagoya, 464-8602, Japan 2 Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
ABSTRACT We constructed an ultrahigh vacuum (UHV) system for in-situ measurements of conductivity and Seebeck coefficient for organic semiconductors. Using this apparatus, the conductive characteristics of non-doped and acridine-orange-base (AOB) doped fullerene (C60) films were examined under UHV and low pressure oxygen atmosphere. As the oxygen pressure increases, the conductivity of non-doped C60 film drastically decreased. On the other hand, the conductivity of C60 film doped with AOB by co-deposition under light irradiation decreased only moderately. Such decrease in conductivity and the difference caused by AOB doping were interpreted that the oxygen act as carrier traps and AOB doping under light irradiation partly cancels the effect of oxygen. Seebeck coefficient S was also measured for both non-doped and doped samples. Its sign was negative for both samples, indicating electron conduction. The absolute value of S for AOB-doped C60 film increased by increase of oxygen partial pressure. This indicates the lowering of the Fermi level towards the midpoint of the bandgap, in consistency with the proposed mechanism of the effect of AOB.
INTRODUCTION Recently organic devices like electroluminescent devices, field effect transistors, and solar cells attract much attention in relation to their practical applications. In general, these devices consist of organic semiconductors and metal electrodes, and elucidation and control of conduction in the organic layer is important for achieving better device performance. As an important factor in real devices, it is well known that conductive properties are often strongly affected by atmosphere like oxygen during device preparation and operation. Especially, the conductivity of fullerene (C60) film is drastically decreased by the exposure to oxygen.[1-3] Deep understanding of such effect of atmosphere can be obtained by the combination of (1) the preparation and in-situ measurements in ultrahigh vacuum (UHV), and (2) the examination of the effect of the exposure to the gas. For clarifying such atmospheric effect, it is also desirable to use some method besides
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Figure 1. Chemical structure of acridine orange base (AOB)
simple conductivity measurements for obtaining detailed information about the type of carriers and electronic structure. Hall effect measurement can give the sign of the major carriers, but it can not be applied to most of all organic semiconductors because of their low mobility. As another technique, the sign of the carriers can also be determined by the thermoelectric power measurements from the sign of the Seebeck coefficient S. Seebeck coefficient is the thermoelect
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