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Joule's law opens a straightforward way to explore the operating mechanism of organic field-effect transistors, from the angle of inner transported heating. The microscopic dissipated power is calculated from the local conductivity and electric field, which solves the widespread difficulties in building a macroscopic model to determine the contact resistance. The result quantitatively discloses the correlation between the contact resistance and the charge transport properties covering energetic disorder, band-like and hopping transport as well as carrier mobility anisotropy. It turns out that in a staggered configuration with ideal Ohmic contact, the contact resistance is highly affected by the charge transport. [ABSTRACT FROM AUTHOR]

Highlights: [•] Optimal contacts should have smallest size but possess minimum contact effects. [•] Six times of semiconductor film thickness is found to be optimal for contact length. [•] It increases in nanoscale channels with more hopping and anisotropic transport. [•] This optimal contact length is independent of semiconductors and variable mobility. [ABSTRACT FROM AUTHOR]