Displacement Current Measurement of MIS Devices with Ionic Liquids to Explore Carrier Behaviors in Model Interfaces of O
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Displacement Current Measurement of MIS Devices with Ionic Liquids to Explore Carrier Behaviors in Model Interfaces of Organic Devices Taiki Yamada1, Yutaka Noguchi1, 2, Yukio Ouchi3, Hisao Ishii1, 2 1 Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan. 2 Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan. 3 Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8602, Japan. ABSTRACT Charge injection property of organic thin film devices is a key issue to understand the device operation. Displacement current measurement (DCM) is a powerful technique to probe the charge injection behaviors in terms of a change in the apparent capacitance of test devices. However, it requires to suppress actual current flowing through the device for investigating the details of interface phenomena. We propose here the use of ionic liquids (ILs) as a top contact insulator in organic metal-insulator-semiconductor (MIS) structures. Because of the high stability and dielectric constant of the ILs, the external applied voltage was applied mainly to the organic layer with suppressing the actual current. The DCM curves of Pt wire/IL/α-NPD/ITO structure were measured, and they actually show the signals due to the hole injection from the ITO to α-NPD layer and accumulation at the IL/α-NPD. INTRODUCTION Organic electronic devices such as organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) or organic solar cells (OSCs) have been investigated because of their possible applications to large-area, low-cost, and flexible devices [1]. For these devices, interfacial phenomena are one of the crucial factors to determine the performances. For example, in the case of OFETs, carriers which play a role in conduction are injected from an electrode to a semiconductor. In addition, a channel is formed at an interface between a semiconductor and insulator. Luminescence occurs nearby the interface in OLEDs, and charge separation occurs at a hetero junction in OSCs. The carrier behavior in such key electronic processes should be investigated to understand and improve organic devices. Displacement current measurement (DCM) has great advantages to monitor the carrier behavior in organic devices [2, 3]. In this technique, a triangular wave voltage is applied to a device and its current response is measured. Since the displacement current depends on the effective capacitance of the device, useful information on carrier injection and accumulation can be observed. In the previous work of DCM, a metal-insulator-semiconductor (MIS) structure which involves a model interface to be investigated is measured. But, there are some problems: (i) the voltage across an organic semiconductor layer cannot be specified since the voltage drop across an insulating layer is significant; (ii) the process to deposit an insulating layer on an organic layer is cumbersome and sometimes induces damages. In or
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