High Performance n-Channel Organic Field-Effect Transistor of Conjugated Polymers with Fluorine-Substituted Phenylene Unit

Abstract

Numerous organic semiconductors have been developed to replace conventional inorganic ones. Particularly, conjugated organic polymers have attracted much interest because of their unique properties including solution processability, flexibility and low-cost mass production with fast and large area printing. In the past decade, semiconducting conjugated polymers have contributed to significant progress in organic electronics, especially field effect transistors, light-emitting diodes and organic photovoltaics. Recently, semiconducting polymers based on diketopyrrolopyrrole (DPP) units have emerged as promising active materials for the optoelectronic applications such as organic field-effect transistors (OFETs) and organic solar cells due to its high molar absorptivity and high hole mobility. Although various DPP-based conjugated polymers have recently been reported to show high hole mobilities over 1 cm2V?1s?1 in OFETs, conjugated polymers with n-channel charge carrier transport are scarce because most conjugated polymers usually exhibit p-channel dominant transport characteristics. Herein, we present novel conjugated polymers composed of DPP and fluorinated phenylene for n-channel dominant transport in OFETs. For the purpose, we synthesized a series of alternating copolymers (PDPPnFP) based on thiophene end-capped diketopyrrolopyrrole (DPP) and fluorine-substituted phenylenes (nFT where n=1, 2, 4). Here, n represents the number of fluorine substitution on phenylene unit. Substitution of fluorine has frequently been used to modify the electronic properties of conjugated polymers because the fluorine atom has the highest Pauling electronegativity and its small atomic size hardly induces the steric hindrance for configuration and molecular packing of the polymer. All PDPPnFP show similar optical absorption spectra with nearly identical band gaps of about 1.5 eV, and their cyclic voltammograms indicate that molecular orbital energy levels (both LUMO and HOMO) are lowered stepwise as the number of fluorine substitution on phenylene unit increases. All the fluorinated polymers (PDPPnFP) exhibit ambipolar properties for OFETs with much enhanced charge carrier mobilities compared to non-fluorinated polymers (PDPP0FP). In particular, PDPP4FP which has tetra-fluorophenylene unit exhibits an outstanding electron mobility, 1.7 cm2V-1s-1, which is among the highest electron mobility in OFETs using conjugated polymers to the best of knowledge. The optoelectronic, electrochemical, and charge carrier transport characteristics of the PDPPnFP are also discussed in terms of the number of fluorine substitution.