Reduced contact resistance in top-contact organic field-effect transistors by interface contact doping

Emerging organic integrated electronics require capability of high speed and the compatibility with high-resolution structuring processes such as photolithography. When downscaling the channel length, the contact resistance is known to limit the performance of the short channel devices. In this report, orthogonal photolithography is used for the patterning of the source/drain electrodes of the organic field-effect transistors (OFETs) as well as the interface dopant insertion layers for further modifications of the contact resistance. Bottom-gate top-contact pentacene OFETs with different thicknesses of the p-dopant 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile under the Au electrodes show a significant decrease in threshold voltage from −2.2 V to −0.8 V and in contact resistance from 55 k$\Omega$ cm to 10 k$\Omega$ cm by adding a 1 nm thin dopant interlayer. The influence of doping on charge carrier injection is directly visible in the temperature-dependent output characteristics and a charge-transfer activation energy of ∼20 meV is obtained. Our results provide a systematic study of interface contact doping and also show the connection between interface contact doping and improved charge carrier injection by the activation of charge transfer process.