High performance short channel organic electrochemical transistors for bioelectronics

Abstract

Organic electrochemical transistors (OECTs) are characterized by ion infusion into channel volume during gate modulation. The electric and ionic charges couple within the channel of OECTs, leading to much higher transconductance than that of a field-effect transistor. OECTs are well studied in channel materials and the application of biosensing. However, short-channel devices have been rarely reported. In this thesis, I will focus on the fabrication and application of short-channel OECTs as follows: Firstly, vertical short-channel OECTs are prepared with aluminum oxide as a spacer between source and drain electrodes. Short-channel OECTs with physical channel lengths down to 0.86 nm have been realized, which show high on-off current ratio and low gate leakage. For both p-type and n-type short-channel OECTs, an on-off ratio of 106 in channel currents and an ideal subthreshold slope of 60 mV/dec have been achieved when the channel length is longer than 1.8 nm. Secondly, the transient response of vertical short-channel OECTs is investigated with the variation of channel area, channel thickness, channel length and gate channel distance. Channel area and thickness are two main factors that influence the OECT response time. With a small channel area and thinner channel thickness, the transient response of PEDOT:PSS -based OECT below 1 μs is achieved for the first time. The n-type OECT can also realize a record fast response of 0.32 ms. Using fast response and high transconductance OECTs, electrocardiogram (ECG) and electrooculogram (EOG) recording on human subjects are demonstrated. A complementary inverter based on p-type and n-type OECTs is established with a record high gain (456 V/V), which is attributed to the small subthreshold slope of both devices. Thirdly, biosensors based on OECTs are fabricated for the detection of COVID-19 IgG. OECT enables the detection of antibodies by converting charged biological signals into electrical signals. By adjusting the concentration and pH value of the test electrolyte, the COVID-19 IgG biosensor can realize a detection limit of 1 fM in PBS samples, and 10 fM in saliva and serum samples. The IgG detectable region is from 10 fM to 100 nM in serum and saliva, which can cover those SARS-CoV-2 IgG levels of COVID-19 patients. By adding a voltage pulse during the antibody incubation process, the reaction time can be reduced to only 5 minutes. A portable meter is designed and constructed to conduct wireless detections, which can be operated by mobile phone through Bluetooth. In summary, the simple solution-gated device structure, miniaturized device size and low working voltages of OECT devices will lead to broad potential applications in bioelectronics. The study of new device structures can not only obtain high-performance organic transistors, but also promote the transistor scaling down to the next generation.