Physics of organic field-effect transistors and related optical applications

Abstract

Two important merits that involved in the basic operations of organic field-effect transistors (OFETs) are contact resistance and carrier mobility. One is taking the lead on the charge injection, the other is responsible for charge transport. These two parameters are mutually complementary and both are indispensable if we want to bring the OFET into real life products. This thesis aims to study the basic device physics regarding to the contact resistance and mobility extraction, and tackle the bottleneck of the OFET. Next, we demonstrate OFET-integrated optical applications, including transparent transistors and optical memory transistor array, which validate optoelectronic applications with integrated memory function. We propose the surface doping of F4-TCNQ on C8-BTBT based OFETs to improve the contact resistance from 25.7 to 5.2 kΩ cm and bias stress stability (current remaining) from 36 % to 86 %. The systematical study on the doping mechanism implies the density of trap states in the C8-BTBT were reduced through the trap filling process, which was triggered by the spontaneous charge transfer happened at the F4-TCNQ and C8-BTBT molecular interface. This suggests surface doping as a simply yet effective strategy to improve the electrical performance of the large band gap OFETs, as well as other organic devices. For reliable mobility extraction, other than the “double-slope” induced by gated Schottky contacts, the fringe effect resulting from the improper electrodes design is an another source for mobility overestimation. We examined the fringe effect in the OFET with both vacuum-deposited and solution-processed semiconductor layers. An important correction-factor (C-factor) of the W/L was introduced to describe the severity of the mobility overestimation. The results show the C-factor can reach 1.16 and 0.74 in devices under W/L ratio of one for vacuum-deposited and solution-processed transistors respectively. We also provide recommendations on the design of device geometry and channel dimension so that common pitfalls of fringe effect in the device fabrication can be avoided. In terms of optical applications, we demonstrated transparent transistor and optical memory transistor (OMT) array. First, a fully transparent OFET was fabricated by integrating junction-free metal network electrodes as the three terminal electrodes. The devices exhibit a 70 % optical transparency, an average mobility of 0.13 cm2V-1s-1 with an on/off current ratio larger than 107, which provides an alternate route towards the ITO-free transparent electronics. Second, we exploited the intrinsic charge trapping in the organic semiconductors for the OMT without employing floating gate or electret polymer. The average mobility of the device is 7.7 cm2V-1s-1. The photoresponsivity is as high as 433 A W-1 and the memory retention can retain on-off state current ratio ˃E6 after 2E4 seconds. After confirming the trap states originate from the nano-sprouts induced by structural inhomogeneity in the active layer, we further scaled up the OMT to a 16×16 active-matrix array on flexible substrates. The array can map out and encrypt 2D optical images.