Solution-based metal electrode modification for improved charge injection in polymer field-effect transistors

We report that solution-based treatment of Cu electrodes with strong electron acceptor molecules significantly decreases the contact resistance towards organic semiconductors, which is advantageous for applications such as organic field-effect transistors (OFETs). Spin-coating solutions of tetracyanoquinodimethane (TCNQ) or tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) onto Cu electrodes results in strongly chemisorbed acceptor (sub-)monolayers, which increase the electrode work function from 4.5 eV (bare Cu) up to 5.2 eV (F4-TCNQ-treated) even in air, as evidenced by X-ray photoelectron spectroscopy and photoelectron yield spectroscopy. The use of such modified electrodes in flexible OFETs with poly(3-hexylthiophene)-dithienyltetrafluorobenzene (P3HT-TFT) as semiconductor lead to a twofold increase of the on-current in the saturation regime and a decrease of the threshold voltage from −20 V (bare Cu) to −7 V (F4-TCNQ-treated). These results confirm that this simple solution-based process is viable for lowering organic/metal contact resistances in organic electronic devices.