PEDOT:PSS 박막을 이용한 비휘발성 고분자 메모리 소자에 관한 연구
- PEDOT:PSS 박막을 이용한 비휘발성 고분자 메모리 소자에 관한 연구
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- Electrically bistable devices using organic thin film were first reported in the 1970s but didn’t attract much attention. In the last decade nonvolatile memory devices based on complementary metal oxide semiconductor (CMOS) processes have rapidly scaled down transistor size to lower process costs and increase chip density. However, nonvolatile memory devices are now facing the limitations of scaling down of silicon devices. Accordingly, advanced memory technologies are required.
Nonvolatile memory technology using polymer materials has again attracted attention to overcome the limitations of scaling down of silicon devices. This technology has the advantages of good scalability, mechanical flexibility and ease of processing. Therefore, polymer materials are being actively researched as nonvolatile memory, and polymer memory devices have been reported using various materials.
Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT:PSS) thin films have been widely used in research on write-once-read-many-times (WORM) memory, organic transistors, organic light emitting diodes (OLED), and solar cells. We have fabricated polymer memory devices using PEDOT:PSS thin film with various electrodes and have measured current-voltage (I-V) characteristics of those devices.
The devices had bipolar and unipolar switching behavior, depending on what electrode was used or what operation method was used. The devices using Al, Ti and Au BE basically didn’t have any switching behavior. If a compliance current (CC) was used in the write operation method, the Al/PEDOT:PSS/TEs devices had unipolar switching behavior and the Au/PEDOT:PSS/Al devices had bipolar switching behavior. The ITO/PEDOT:PSS/TE devices had only bipolar switching behavior. Clearly, the electrode material and the write operation method have important roles in the switching behavior and characteristics.
Each switching mechanism can basically be explained as the formation of current paths due to redox behavior of PEDOT:PSS film. When a voltage is applied to the electrode, the PEDOT chains will be oxidized to PEDOT+ chains by the injected carriers. Then current paths will be formed by PEDOT+ chains and the devices will switch from the off state to the on state. However, the two types of devices have different on-off switching processes. In the bipolar switching devices, the PEDOT+ chains are reduced to PEDOT0 chains by injection carriers when a voltage is scanned in the opposite direction. Then the current paths are destroyed and the devices switch from the on state to the off state. In the unipolar switching devices, the current flowing through the current path in the devices is increased beyond the CC when a voltage is applied with a maximum CC. Then the current paths are ruptured by the very large turn-off current and the devices switch from the on state to the off state.
Finally, we have fabricated diode-memory devices for 3D multi-layer and crossbar array applications. The devices were fabricated by incorporating ITO/PEDOT:PSS/ P3HT:PCBM/Al polymer diode devices in series with unipolar Al/PEDOT:PSS/Al memory devices. The devices had either WORM or unipolar switching behavior, depending on how compliance current was used.
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