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Impedance Characterization of Dielectric and Semiconducting Materials with Organic Capacitors for Organic Transistors

C.M. Li, L.K. Pan, C.Q. Sun, J. Zhang and D. Gamota
Nanyang Technological University, SG

organic transistor, impedance modeling, material characterization

Characterization of the properties of OFET materials and devices have been conducted by I-V or C-V curves that used in MOS devices for provide information of basic shape, threshold voltage, mobility effects, velocity saturation, scaling, channel charge, junction capacitance, etc. However, interfacial resistances between source (drain)/organic conducting layer, dielectric/gate, and dielectric/organic conducting layer play important roles in OTFT performance. Conventional I-V and C-V experiments are not convenient to differentiate different interfacial resistances. In addition, conventional I-V and C-V measurements are based on fabricated devices, which are costive and time-consuming for screening great amount of OFET materials. In this report, a simple model capacitor device was made from dielectric and organic semiconducting materials to characterization OFET materials. A new impedance analysis method was used to measure the interfacial resistances. An equivalent circuit was built to simulate and differentiate interfacial resistances for different material combinations. This method can simultaneously characterize dielectric and conductive behaviours of materials, and also distinguish individual contributions to electrical conduction or to polarization from different sources such as dielectric layer, semiconductor layer, and interface in OFETs. Different dielectric materials show different conductivity dependence on temperatures. It has been observed that dielectric materials with lower conductivity, non-metallic conduction behaviour at high temperature, and lower interface resistance are more suitable for OFET. A good dielectric material appears an enhancement in conductivity by heating following an Arrhenius law with an activation energy transition from 0.002 to 0.24 eV at ~307 K, which originates from band tail hopping that occurs around the Fermi edge. At ~314 K, a dielectric transition also occurs, which is interpreted as a combination of electron polarization associated to the band tail hopping. This method is very useful for investigation of OFET materials and characterization of device performance. The new method can be used in efficiently screening OFET materials such as dielectric, semiconducting and source/drain materials. The screening materials were used to fabricate OFETs, and further proved the power and accuracy of the novel method.

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