PMMA/High-K Self-Assembled TiO2/PMMA Multi-Layer Gate Dielectric for P3HT Organic Field Effect Transistors

In this work, a multi-layer structure of poly(methyl methacrylate)/ titanium dioxide/poly(methyl methacrylate) (PMMA/TiO₂ /PMMA; PTP) was proposed as a top-gate insulator for P3HT-based organic field-effect transistors (OFETs). Adding a TiO₂ interlayer as a high dielectric constant (high-k) material into PMMA film enables the modification of the dielectric constant of the multi-layers PTP film. The content of TiO₂ in the PTP film, which can be varied by changing the number of soaking cycles in TiO₂ solution, plays a crucial rule in modifying the dielectric constant of the PTP film. The higher the TiO₂ content used in the PTP film, the higher the dielectric constant of PTP film can be obtained. However, using high TiO₂ content led to a reduction in the dielectric constant of the PTP film due to leakage current induced by the agglomeration of TiO₂. The utilization of the top-gate insulator containing TiO₂ significantly enhanced several P3HT-OFETs characteristics, e.g., an increase in the I_on/I_off ratio, and a decrease in the threshold voltage. However, the use of the PTP top-gate insulator with a high content of TiO₂ resulted in regressions in the OFETs characteristics, such as a decrease in carrier mobility and reduction in the I_on/I_off ratio. OFETs operating at the optimum conditions of the PTP gate-insulator, with PTP thickness of 225 nm and RMS roughness of 20.0 nm, provided a dielectric constant of 7.13, a threshold voltage of -8.49 V, a saturation mobility of 2.2× 10^-4 cm²V^-1s^-1, I_on/I_off ratio of 37.9, and a subthreshold slope of 0.39 V/decade.

Keywords: organics field-effect transistors; dielectric constant; TiO2; PMMA; P3HT

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