Molecular contact doping in organic field-effect transistors (OFETs) has been proved to be a very efficient strategy to reduce the device contact resistance. It consists of inserting a dopant layer between the organic semiconductor (OSC) and the top gold contacts to reduce the energy barrier required to inject/release charges. However, a main bottle-neck for its implementation is that the dopant diffuses toward the OFET channel with time, doping the OSC, and hampering the on/off switching device capability. In this work, we fabricated OFETs based on the benchmark OSC 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8) by a solution shearing technique. First, we show that the OFET performance of these devices is significantly improved when a layer of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is inserted before the evaporation of the gold source/drain contacts. Remarkably, we demonstrate that the dopant diffusion can be controlled by blending the OSC with polystyrene (PS) of different molecular weights. In-depth electrical characterization combined with studies of surface and in-depth distribution of the dopant by time-of-flight secondary ion mass spectrometry (ToF-SIMS) unambiguously show that in thin films of OSC blends with high-molecular-weight PS, the dopant remained drastically confined into the contact areas, which was reflected by an enhanced long-term device stability.

This work was funded by the project GENESIS PID2019-111682RB-I00 from MICINN/AEI, the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), and the Generalitat de Catalunya (2021 SGR 00443). J.L. is enrolled in the UAB Materials Science PhD program and acknowledges the Scholarship from the Chinese Council. E.C., P.B., and L.T. acknowledge funding from INFN through the CSN5 FIRE project. The authors also thank A. Tamayo for his help in the AFM images analysis.

With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Peer reviewed