Grafting Fluorinated Polymer Nanolayer for Advancing the Electrical Stability of Organic Field-Effect Transistors

Abstract

With the goal of achieving high-performance electrically stable organic field-effect transistors (OFETs), we chemically graft a fluorinated polymer nanolayer onto the polar oxide dielectric surfaces via a simple and easy fabrication process in ambient air. The para-fluorine-thiol click reaction between poly(pentafluorostyrene) (PFS) and mercaptopropyltrimethoxysilane is used to synthesize a graftable fluorinated polymer (gPFS). The surface characteristics of the gPFStreated SiO 2 dielectrics and the crystal structure and grain growth of the overlying organic semiconductors are investigated. Various semiconductor materials are employed for the OFETs prepared with gPFS-treated SiO 2 dielectrics, including vacuum-processed pentacene, N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide, solution-processed 5,11-bis-(triethylsilylethynyl)anthradithiophene, and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di-(2,2′-bithiophen-5-yl)ethane. Three OFETs are prepared with different dielectrics: (i) bare SiO 2 , (ii) gPFS-treated SiO 2 , and (iii) perfluorooctyltriethoxysilane-treated SiO 2 . The OFETs prepared with the gPFS-treated SiO 2 dielectrics display the highest mobilities and smallest hysteresis. Furthermore, the gPFS-treated SiO 2 provides the best device stability under a sustained gate bias, suggesting that the gPFS surface minimize the number of traps present in the OFET.

Balancing Surface Hydrophobicity and Polarizability of Fluorinated Dielectrics for Organic Field‐Effect Transistors with Excellent Gate‐Bias Stability and Mobility

“…In contrast, rich fluorinated surfaces (with γ values of less than 35.0 mJ m −2 ) yielded smaller grains of pentacene, indicating nominal sizes of 0.2–0.7 μm based on the AFM topographies (Figure d–f). Particularly, grains on the PPFS‐treated surface (with γ of 24 mJ m −2 ) were 0.2–0.3 μm in size; this matches well with the size values of pentacene crystallites previously reported for fluorinated surfaces . Note that lateral percolation between small grains on gate dielectrics increased both the number of trap sites and the energy barrier of charge transport in OFETs considerably (will be discussed later) …”

“…Utilization of hydrophobic (or neutral) polymer‐coated polymer or oxide dielectrics has enabled maintenance of low‐voltage operation in OFETs with good stability against a sustained gate‐bias stress, because the polymer layers minimize the diffusion of active species around the dielectric surfaces. Recently, fluoropolymers have been reported to be dielectric materials with excellent electrical stability against a sustained gate‐bias stress . The element fluorine (F) has a small atomic radius that is comparable to that of hydrogen, and the lowest polarizability among all atoms, which confers good hydrophobicity and chemical inertness to fluorinated materials .…”

“…When the dielectric's γ is close to that of the lowest γ crystal plane of the seeding molecules (e.g., 38 mJ m −2 for pentacene), pentacene grows into terrace‐like grains containing the lowest‐ γ ab plane crystal, referred to as (001), and directly contacts the surface. However, the preferred crystal form, known as the “thin film phase”, competes for surface coverage with the other polymorphs, resulting in an increase in the mismatch of γ , film thickness, and/or surface roughness . On many fluoropolymer surfaces, small‐sized and irregular grains of organic semiconductors severely degrade field effect mobility ( μ FET ), which has hindered the introduction of fluoropolymers into gate dielectrics .…”

“…However, the preferred crystal form, known as the “thin film phase”, competes for surface coverage with the other polymorphs, resulting in an increase in the mismatch of γ , film thickness, and/or surface roughness . On many fluoropolymer surfaces, small‐sized and irregular grains of organic semiconductors severely degrade field effect mobility ( μ FET ), which has hindered the introduction of fluoropolymers into gate dielectrics . Due to intrinsic defects in the π‐overlap structure of organic semiconductors on fluorinated surfaces, however, it is still unclear which component of OFETs is the major contributor to device instability under a sustained gate‐bias …”

Solid-state deep blue and UV fluorescent dyes based on para-bis(2-thienyl)phenylene

“…As already mentioned, fluorine groups are not commonly found in fluorescent dyes; nevertheless, generally, fluorinated dyes are considered to be potentially useful materials, as the introduction of electron-withdrawing fluorine lowers both LUMO and HOMO levels [13][14][15]. Therefore, fluorinated compounds may play primary roles as active materials in liquid crystals [16,17], self-assembly applications [18][19][20], OLEDs [21,22], and OFETs [23], exhibiting superior characteristics for the abovementioned applications. Moreover, due to the strength of the C-F bond, fluorine-containing compounds exhibit high thermal and oxidative stability, low polarity, and a low level of intermolecular interactions [24], all of which also increase their potential with respect to use in practical applications.…”