Benzotriazole-Containing Planar Conjugated Polymers with Noncovalent Conformational Locks for Thermally Stable and Efficient Polymer Field-Effect Transistors Seungjib Yum, †,Δ Tae Kyu An, ‡,Δ Xiaowei Wang, †,Δ Wonho Lee, † Mohammad Afsar Uddin, † Yu Jin Kim, ‡ Thanh Luan Nguyen, † Shuhao Xu, † Sungu Hwang, † Chan Eon Park,* ,‡ and Han Young Woo* ,† † Department of Nanofusion Engineering, Department of Cogno-Mechatronics Engineering, Pusan National University, Miryang, Gyeongsangnam-do 627-706, Korea ‡ POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, North Gyeongsang 790-784, Korea * S Supporting Information ABSTRACT: We report a series of benzotriazole-based semicrystal- line π-conjugated polymers with noncovalent conformational locks for applications in polymer field-effect transistors. The benzotriazole moiety is a versatile electron-deficient building block that offers two chemically functionalizable sites, 2(N) and 5, 6(C) positions, allowing easy modulation of the solution processability and electronic structures of the resulting polymers. Fluorine or alkoxy substituents were introduced to the benzotriazole unit to enhance the molecular ordering through intra- and intermolecular F···S, F··· H-C, C-F···π F , or S···O attractive interactions. The fluorinated polymer (PTBTz-F) showed remarkably enhanced hole mobility (μ h = 1.9 cm 2 /(V·s), on/off ratio = 8 × 10 7 ) upon thermal annealing at 305 °C, compared to the unsubstituted one (PTBTz)(μ h = 7.0 × 10 -3 cm 2 /(V·s), on/off ratio = 3 × 10 6 ). Alkoxy unit substitution (PTBTz-OR) also improved the carrier mobility up to 0.019 cm 2 /(V·s) with an on/off ratio of 4 × 10 5 . Fluorine or alkoxy substitution induced tight interchain ordering with edge-on orientation, as confirmed by X-ray diffraction measurements. In particular, fluorinated PTBTz-F showed high thermal stability (T d 453 °C) and the remarkable device characteristics with deep frontier orbital levels. ■ INTRODUCTION Polymer field-effect transistors (PFETs) have been studied extensively because of their low-cost, large-area and solution processability, mechanical flexibility along with their potential applications in smart cards, radio frequency identification, and displays. 1-3 The molecular design and synthesis of conjugated polymers for effective charge transports are of prime importance to realize highly efficient PFETs. 4-6 Although conjugated polymers are suitable materials for easy device fabrication via a solution process, the carrier mobility of PFETs is limited because of the poor interchain packing of the polymers and macroscopic defects originating from the weak van der Waals forces among the adjacent polymers. 7 Favorable molecular ordering with high crystallinity is a key factor for realizing high performance PFETs. A number of D-A (donor- acceptor) type copolymers have been synthesized, realizing high hole mobility surpassing ∼1 cm 2 /(V·s). 8-10 Incorporating appropriate structural moieties into the D-A copolymers can fine-tune the chain curvature, torsional angle, electronic structure of the polymer chains, and resulting intermolecular interactions, thereby affecting charge transport. 11 An alternating copolymer containing cyclopentadithiophene (CDT) as a donor unit and benzothiadiazole (BT) as an acceptor unit showed a hole mobility of ∼3 cm 2 /(V·s). 4 The combination of diketopyrrolopyrrole (DPP) or isoindigo (IDG) units with a range of thiophene derivatives has attracted considerable attention because of their extremely high hole mobility of up to ∼12 cm 2 /(V·s). 5,12,13 In addition to the high charge carrier mobility, thermal, oxidation, and temporal device stability should be also considered carefully for the design of ideal active materials for PFETs. A benzotriazole moiety has been widely studied as an electron-deficient unit in D-A copolymers for photovoltaic applications, showing power conversion efficiencies (PCE) of up to ∼7%. 14-16 The benzotriazole unit offers two chemically functionalizable sites (Scheme 1). i) The central nitrogen position can be functionalized with an alkyl chain (to endow solution processability) separate from a conjugated backbone that reduces the steric hindrance, thereby enhancing the effective intrachain π-conjugation and interchain packing. ii) The 5- and 6-positions on the benzotriazole unit can be modified chemically by introducing other substituents 17 (i.e., alkoxy and fluorine groups, etc.), modulating the frontier orbital levels and oxidational stability of the resulting molecules. The Received: December 29, 2013 Revised: February 21, 2014 Published: February 22, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 2147 dx.doi.org/10.1021/cm4042346 | Chem. Mater. 2014, 26, 2147-2154