Band-like Electron Transport with Record-High Mobility in the TCNQ family

TL;DR

This study reveals that F2-TCNQ organic crystals exhibit record-high electron mobility with band-like transport, and highlights the importance of crystal and electron-phonon interactions for understanding organic electronic properties.

Contribution

It demonstrates that F2-TCNQ crystals achieve exceptional transport properties and introduces the Fx-TCNQ family as a model system for fundamental studies of organic electron transport.

Findings

F2-TCNQ shows very high electron mobility and increases upon cooling.

Mobility in TCNQ and F4-TCNQ is lower and decreases with cooling.

Band structure alone is insufficient; electron-phonon coupling is crucial.

Abstract

In highest quality organic single-crystal field-effect transistors, electron transport occurs in the band-like regime, with the carrier mobility increasing upon lowering temperature. Neither the microscopic nature of this regime, nor why it occurs only in a small number of materials is currently understood. Here, comparative studies of closely related materials, exhibiting high-quality reproducible transport properties are needed. We performed a study of electron transport in single-crystals of different TCNQ (tetracyanoquinodimethane) molecules, combined with band structure calculations. We show that F2-TCNQ devices exhibit very high electron mobility and an unprecedented increase in mobility upon cooling, whereas in TCNQ and F4-TCNQ the mobility is substantially lower and decreases upon cooling. We analyze the crystal and electronic structures of these materials and find that F2-TCNQ crystals are indeed ideal to achieve outstanding transport properties. Our analysis also shows that to understand the difference between the three materials, studying their band structure is not sufficient, and that the electron-phonon coupling needs to be investigated as well. Besides the outstanding transport properties of F2-TCNQ, a key result of our work is the identification of the Fx-TCNQ family as a paradigm to investigate the most fundamental aspects of electronic transport in organic crystals.