Carrier Injection and Transport of Conductive Polymer-Based Devices
Kazuaki Furukawa, Wenping Hu, Hiroshi Nakashima, Yoshiaki Kashimura, Katsuhiro Ajito, and Keiichi Torimitsu
Materials Science Laboratory
Organic molecules attract much interest recently as electronic materials. Research and development for flexible display and thin-film-transistor applications are vital today. In addition, molecular scale device, the device comprising single or small number of molecules, is considered for future organic electronic devices, not only because it has advantage in size but also it is expected to show novel phenomenon originated from single molecule. In this study, we prepared and observed carrier transport properties of organic electronic devices with different electrode sizes and configurations .
Figure 1 shows device structures. Nanogap electrodes (gap 〜100 nm) and conventional sandwich type thin film device (with 2mm × 2mm area) were fabricated. The material is poly(p-phenyleneethynylene) derivative (TA-PPE, Fig.1) , which is know as a rigid π-conjugated polymer.
The remarkable difference between two types of devices was observed for the temperature dependence of current density-voltage (J-V) curves. Although the well known Arrhenius type temperature dependence was observed for the sandwich device, temperature dependence was not clearly observed for the nanogap device. Moreover, after the treatment of the nanogap device exposed in THF vapor, which is a good solvent for TA-PPE, the temperature dependence almost disappeared (Fig. 2). This result suggests that the carrier injection and transport is dominated by the tunneling process. During the exposure of the nanogap device in the vapor of its good solvent, TA-PPE became somewhat fluid, and thus the nanostructure of TA-PPE changed. The increase of the number of bonds between TA-PPE ends and Au electrodes at the interface made the tunneling process through Au-S bond superior.
The present results indicated that the molecular conformations as well as configurations contribute to the characteristics of molecular scale devices. The device design involving the control of molecular conformations is currently under consideration.
 W. P. Hu, et al., Phy. Rev. B (accepted).
 H. Nakashima, et al., Polym. Prepr. 44(1) (2003) 482.
Fig. 1. Molecular structure of TA-PPE and sandwich and nanogap device structures.
Fig. 2. ln(J) plotted vs. 1/T at different applied voltage for TA-PPE nanogap device after exposed in THF vapor.
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