Experimental Study of DKPP-βT Polymeric Thin Film Transistor

Abstract

<p>In the last 30 years, the possibility of using polymeric thin film transistors (PTFTs) in flexible display, sensors, radio-frequency identification tag and the potential of using printing or low-cost reel-to-reel fabrication techniques has stimulated much research and technology development in these devices. However, the utilization of PTFTs needs better understanding of the organic semiconductor material’s properties and their physical and chemical mechanisms. In addition, the PTFTs show poor stability compared to the crystalline transistors. The PTFTs can have significant variations of threshold voltage, mobility, on/off ratio even when deposited using the same conditions on the same substrate. Therefore, better understanding of the PTFTs’ physical and chemical properties and the improvement of the characterization techniques are needed.</p> <p>The design and fabrication of the novel polymeric semiconductor, diketopyrrolopyrrole β-unsubstituted quaterthiophene (DKPP-βT) based bottom-gated top-contact PTFT and microfluidics PTFT are introduced in this thesis. The microfluidic PTFT consists of polydimethylsiloxane (PDMS) microchannel which guides liquids flowing over the top of the semiconductor channel.</p> <p>From consecutive electrical measurements, it was found that the threshold voltage (V_T) follows a logarithmic law function of the time. Illuminating the PTFTs results in shifts of the initial value of the threshold voltage linearly towards more positive value. The mobility is unaffected by time or by illumination. However, the off current increased proportionally with light. Also, the contact resistance extracted by the parameter compensated transmission line model (TLM) method is ohmic and gate bias independent for high gate biases.</p> <p>The novel microfluidic PTFT enables the study of the sensing property of the DKPP-βT PTFT of liquid analytes. The threshold voltage evolution in the deionized (DI) water measurements also follows logarithmic function of the time with a slightly steeper slope than in air. The mobility only slightly decreases initially on exposure to DI water. The off current in DI water measurements decrease compared with air measurements. In acid solution measurements, the threshold voltage remains stable and the mobility slightly increased, compared with measurements in water. Additionally, the subthreshold slope and off current in both acid solution and salt water measurements show similar results to the DI water measurements. While the base solution damages the device immediately. The stable performance of DKPP-βT PTFTs with DI water and low-concentration salt water in the microchannel makes it a promising biosensor.</p>