Realizing low-voltage operating crystalline monolayer organic field-effect transistors with a low contact resistance

Abstract

Enhancing charge injection from electrodes to the conducting channel and reducing the contact resistance (R_c) are valid strategies not only to improve the performance of organic field-effect transistors (OFETs), but also to enable low-voltage operating devices with a short channel. In order to improve the charge injection efficiency, many efforts have been made to reduce the interfacial injection resistance (R_c,int) in the past few decades, such as by adjusting the metal work function, inserting a buffer layer and so on. However, the method of decreasing the bulk injection resistance (R_c,bulk) in the vertical direction of the semiconductor layer is rarely studied. In this paper, two-dimensional (2D) monolayer and multilayer single-crystal devices of 1,4-bis((5′-hexyl-2,2′-bithiophen-5-yl)ethynyl)benzene (HTEB) were fabricated to study the dependence of the contact resistance of OFETs on crystal thickness. Devices based on monolayer molecular crystals (MMCs), and 4-layer and 15-layer single-crystal devices were found to have R_c of 1.77 kΩ cm, 26.9 kΩ cm and 5.15 MΩ cm, respectively. Thanks to the lower R_c, the MMC devices possess more steady linear mobility at a varied V_DS, and a smaller deviation of the linear and saturation mobility. Finally, a low-voltage operating (−2 V) HTEB MMC device was fabricated on a 30 nm HfO₂ insulating layer with an ultra-low R_c of 540 Ω cm.