Photoinduced Memory Effects based on Poly(3-hexylthiophene) and Al Interface
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Photoinduced Memory Effects based on Poly(3-hexylthiophene) and Al Interface Keiichi Kaneto, Mitsuru Ujimoto and Wataru Takashima Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Wakamatsu-ku, Kitakyushu, Fukuoka, 808-0196 JAPAN ABSTRACT Organic memory cells have been constructed utilizing conducting polymer, poly(3-hexylthiophene), PHT. The PHT film with Au electrodes at the bottom was coated with ultra thin Al having the thickness of 1 nm on the top. The cell showed the large photoconductivity, which was measured between Au bottom electrodes. The photoconductivity lasted for several hours after turn off the illumination, indicating photo induced memory effect. The phenomenon is discussed in terms of the persistent photoconduction, which is resulted from the electron trapping at Al donor sites in the depletion layer formed near the top of film.
INTRODUCTION Organic electronics devices like electro luminescence (EL) displays, field effect transistors (FET) and solar cells have been intensively interested and studied, because of the lightweight, thin film and low cost manufacturing process. For the fabrication of all organic electronic circuits, organic memory devices are required. Though a large number of studies on the improvement of EL and FET have being carried out, organic memory devices have not been studied. A cell consisting of polyethlenedioxythiophene (PEDOT) and polystyrene sulfonic acid has revealed a write-once, read-many-times (WORM) memory[1]. Bistable memory devices using organic materials of 2-amino-4,5-imidazoledicabonitrile (AIDCN)[2] have shown that the conductivity of the films switched on and off by the application of voltages at certain threshold. The reports have stimulated further interests to realize the rewritable organic memories. In this paper, a preliminary study on the memory effect utilizing a persistent photoconduction in conducting polymers, poly(3-hexylthiophene), (PHT) film with thin Al layer will be mentioned.
EXPERIMENTALS Typical structures of memory devices are shown in Fig.1(A). Au electrodes with the thickness of 30 nm were firstly deposited in vacuum on glass substrate. The distance between Au electrodes and the width of electrodes were 25 µm and 2 mm, respectively. Then PHT was spin-coated with 2000 rpm for several minutes. The thickness of the PHT films was tuned by using various concentrations of PHT in chloroform ranging 0.125−1.0 wt%. The thickness of PHT film was ranging 50−200 nm. Al with thickness of approximately 1 nm was deposited on it for memory cells. Another type cell with thin Al layer deposited on Au electrode underneath PHT was also prepared as shown in Fig.1 (B). The electrical characteristics of the cell were measured in vacuum using the Keithley 6517 electrometer. Light source was a monochromated light of 550nm with the intensity of 50 µW/cm2 from the Xe lamp. The intensity of light was tuned by using neutral optical density filters.
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Ａｌ(1nm) Depletion layer PHT Au
Au Glass
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(A) Al top
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