Dopant Diffusion Inhibition in Organic Field-Effect Transistors Using Organic Semiconductor/High-Molecular-Weight Polymer Blends

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.