Top‐Gate Organic Field‐Effect Transistors with High Environmental and Operational Stability

Abstract

Over the past several years, great progress has been made in the development of organic fi eld-effect transistors (OFETs). Prototypes of electronic devices such as drivers for fl at-panel displays, [ 1 ] complementary circuits, [ 2 , 3 ] radio-frequency identifi cation tags, [ 4 ] and chemical or biological sensors [ 5 , 6 ] have already been demonstrated. While charge-carrier mobility values have improved [ 2 , 3 , 7–9 ] with comparable values for both n and p -channel transistors, long-term environmental and operational stability remain two major issues that need to be resolved before OFETs can realize their full commercial potential. Recently, much effort has been devoted to improve the stability of OFETs. [ 10–18 ] For instance, to improve the environmental stability of OFETs, air-stable organic semiconductors have been synthesized [ 10 , 11 ] or encapsulation layers have been developed. [ 12 , 13 ] On the other hand, achieving operational stability is still a major challenge faced by OFETs as well as other fi eld-effect transistor (FET) technologies, such as those based on a -Si:H, poly-Si, and metal-oxide semiconductors. The operational stability of a FET is in general related to dipolar orientation and charge trapping/de-trapping events at all its critical interfaces and in the bulk of the semiconductor and gate dielectric. [ 14–18 ] The degradation of the performance of a FET during operation is refl ected by changes of its current-voltage characteristics that result from changes of mobility ( μ ), of threshold voltage ( V th ), or variations of the capacitance density ( C in ) of the gate dielectric. The dynamics of the physical and/or chemical mechanisms producing these changes, intrinsic or extrinsic, affect the performance of a FET on different time scales. [ 14 ] The stability of a FET is determined by the total effects produced by several physical and/or chemical processes, but in general, one tends to dominate over the others. This has caused current approaches to improve the stability to focus on mitigating individual processes. [ 15–18 ] Furthermore, the stability of OFETs has been primarily evaluated in devices with a bottom-gate geometry. OFETs with a top-gate geometry are relatively rare because the choice of gate dielectric material is limited since its deposition can potentially damage the organic semiconductor layer underneath. The use of an amorphous fl uoropolymer, CYTOP,