Van der Waals assembled organic field-effect transistors and their applications

Abstract

Organic field-effect transistors (OFETs), due to their cost-effectiveness and suitability for large-area processing, have emerged as pivotal components in the next generation of flexible electronics. With their thin-film device architecture and lightweight characteristics, OFETs hold considerable promise for applications such as e-skin and electric walls. The objective of this thesis is to devise a practical fabrication technique for crafting high-performance flexible OFETs and to delve into both the potentials of OFETs and their circuits for functional applications. Firstly, an efficient fabrication strategy for thermally evaporated flexible OFETs is proposed. The assembly process involves two key components: a 40 nm DNTT layer paired with a CYTOP dielectric, and gate electrodes, along with 50 nm gold electrodes encased within a PMMA encapsulation layer. A 2 μm flexible OFET is then showcased as a conformal sensor, successfully demonstrated on a simulated pig liver for signal sensing. Moreover, the thesis covers the fabrication of high-pass and low-pass filters in thin-film structures based on DNTT and high-κ dielectrics. Conclusively, the fabrication of a DNTT-based rectifier circuit, exhibiting a rectifying frequency in the MHz range, is demonstrated, hinting at its potential for RFID applications. Secondly, the thesis explores the fabrication of large-area monolayer organic single crystals for high-performance OFETs, achieved through the solution-shearing process. A scalable semiconductor transfer method is introduced to surmount substrate limitations during the solution-shearing process. PTS-modified Si/SiO2 is identified as a reusable substrate for C10-DNTT crystals. The transfer process is facilitated by a PVA/PMMA/PDMS stamp with water assistance. The cleanliness of transferred crystals is confirmed through AFM imaging. To avoid thermal damage during evaporation, gold source/drain electrodes are transferred onto the crystals. The transferred C10-DNTT OFET demonstrates impressive fidelity, showcasing a high mobility of 10.4 cm^2V^-1s^-1 and an exceptional on-off ratio of 10^8. Moreover, the thesis proposes methods for the matched large-area transfer of electrodes and semiconductor patterning. The polymer transfer method attains a 100% success rate in transferring centimeter-scale gold electrodes. Photolithography-driven parylene patterning removes unwanted semiconductor regions via static electric forces, minimizing leakage currents and crosstalk in OFETs. Contact resistance analysis is performed on both bottom and top contact transferred C10-DNTT OFETs. The top contact OFET, featuring an HfO2 dielectric, demonstrates favorable contact resistance of 109 Ω·cm at VG-Vth = -3.5 V. The poorer contact resistance in bottom contact OFETs can be attributed to an air gap at the contact, as verified through STEM examination. Finally, the thesis scrutinizes the device performance of C10-DNTT OFETs on hydrophobic SAM-modified dielectrics through the semiconductor transfer method. The bias stability improves by approximately fivefold, transitioning from hydrophobic PTS-treated Si/SiO2 to FC10PA-modified AlOx. Facilitated by the pure dielectric and the semiconductor interface, the OFET exhibits a low subthreshold swing of 63 mV/Decade. The inverter circuit manifests an ultrahigh gain of 650, transforming into a low-power ECG sensor through the amalgamation of gel electrodes and 3D printing molds. The innovative ECG sensor achieves signal amplification hundreds of times and a signal-to-noise ratio of 34 dB, rendering it highly suitable for diagnostic purposes.