Understanding the limitation of organic field-effect transistors

Abstract

Organic field-effect transistors (OFETs) are the keystone of organic electronics and have achieved impressive progress recently. The versatile applications including chemical or biological sensors, integrated circuits, memory device etc. have revealed the great potential of OFETs in the next generation of commercialized functional electronics. In the current thesis, my aim is to understand the limitation in the OFETs, particularly, unveiling the deposition parameters of solution processed organic semiconductor thin films, revealing the origin of contact resistance in monolayer OFETs, and eventually pacing up the advancement of OFETs. On the one hand, the deposition parameters are systematically studied during the meniscus-guided coating (MGC) process, which is widely utilized for single crystalline organic thin film growth and high mobility OFETs. In the study, the tool of blade coating is employed with the organic semiconductor of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT). During the coating process, the organic semiconductor of C8-BTBT reveals three distinct morphology types by varying the processing factors. The best OFET performance is achieved when using the Type II semiconductor thin films as the active channel layer with the highest mobility of 7.68 cm2 V-1 s-1 and average mobility of 5.88 cm2 V-1 s-1. Four elementary factors including shearing speed (v), solution concentration (c), deposition temperature (T) and solvent boiling point (Tb) are unified to analyse the organic semiconductor growth behaviours. It is confirmed that the shearing speed modulates the thickness while the solution concentration is linearly proportional to the crystal growth rate. Most importantly, the crystal growth rate is determined by the solvent boiling point and the deposition temperature in the Arrhenius form. The understanding on the processing variables imparts the valuable information towards large area and uniform thin film deposition. On the other hand, the origin of contact resistance is investigated in the popular monolayer OFET. The metal-semiconductor interface forms by transferring the gold (Au) electrodes on the surface of monolayer 2,9-didecyldinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (C10-DNTT) with the van der Waals force. From the transmission line method (TLM), the gate voltage independent access resistivity of 2.2 × 10-2 cm2 and the intrinsic mobility of 12.2 cm2 V-1 s-1 are extracted under ultra-low drain to source bias (VDS) of -1 mV, while the Schottky diode at the metal-semiconductor is negligible. By further increasing the VDS, the diode effect would escalate and ultimately dominate the performance of monolayer OFET. The technology computer-aided design (TCAD) simulation results show that under saturation, the drain current (IDS) has mild dependence on the channel length (L) when L is lower than 20 m. In the meantime, above 1 m, the source overlap length (LSO) loses the control on IDS. The current restriction and the dependence on L and LSO are actually originated from the carrier depletion under high VDS. With the employment of the high dielectric constant () of HfO2, the source depletion has been diminished by reducing the driving voltages of monolayer OFET. The study on the origin of contact resistance is critical towards OFET downscaling and high density and intricate organic electronics.