Usefulness of Polar and Bulky Phosphonate Chain-End Solubilizing Groups in Polymeric Semiconductors

Abstract

A series of highly soluble copolymers (EH4P-Th, EH4P-Se, EH4P-TT, and EH4P-BT) based on phosphonate chain-end functionalized diketopyrrolopyrrole monomer and four different counterpart comonomers with varied electron-donating strength and conjugation length have been synthesized, characterized, and used in pchannel organic field-effect transistors (OFETs). It was found that introducing different counterpart comonomers into the main backbone alters the copolymers' intrinsic properties, including absorption, frontier energy levels, molecular microstructure, and charge transport in OFETs. In OFETs fabricated on n-octadecyltrimethoxysilane (OTS)-treated silicon (Si)/silicon dioxide (SiO2) surfaces, the copolymers exhibit good hole transport with maximum hole mobility (mu h) of 1.46 x 10-1 cm2 V-1 s-1 in EH4P-TT, which is attributed to edge-on packing, fibrillar intercalating networks, and large crystalline pi stacking. More intriguing is the fact that high solubility and polarity of the resulting copolymers are induced via polar and bulky phosphonate chain-end groups, allowing for proper OFET operation using not only OTS-untreated Si/SiO2 substrates but also an eco-friendly 2-methyltetrahydrofuran solution process. These results demonstrate promising applications of phosphonate chain-end groups in the design of conjugated polymers for various purposes.