Characterization of ohmic contacts in polymer organic field-effect transistors

• Previous TLM analysis of Rc(Vds and Vgs) surface requires unrealistic assumptions.
• An improved TLM analysis of Rc(Isd and σ) surface is demonstrated here.
• Rc shows little dependence on Isd and σ for modified source−drain Au contacts that impose strong hole-doping of the DPPT2-T interface.
• The resultant Rc(Isd, σ) surface is modelled to yield ρc of the metal/OSC interface, a key parameter of the contact that has so far been neglected in OFETs.
• The ultimate ρc attained is ca. 1 W cm2, independent of Isd and σ. This is a hallmark of a true metal/OSC ohmic contact.
• The lowest ρc reached here shortens LT down to ca. 5 µm, enabling short electrode lengths to be advantageously employed in technology.

It is well known that contact resistance Rc limits the performance of organic field-effect transistors (OFETs) that have high field-effect mobilities (μFET ≳ 0.3 cm2 V−1 s−1) and short channel lengths (Lch ≲ 30 μm). The usual transfer-line method (TLM) to analyze Rc calls for extrapolation of total resistance to zero Lch at constant drain and gate voltages. This requires an unrealistic assumption that Rc does not vary with source−drain current Isd (nor with channel carrier density σ). Here we describe a self-consistent TLM analysis that instead imposes the condition of constant Isd and σ. The results explicitly reveal the dependence of Rc on Isd and σ. We further describe how this Rc(Isd, σ) surface can be modelled to yield the specific contact resistivity ρc of the metal/organic semiconductor (OSC) interface, a key parameter that has so far been neglected in OFETs. We illustrate the application of these analyses to high-performance staggered top-gate bottom-contact poly(2,5-bis(alkyl)-1,4-dioxopyrrolo [3,4-c]pyrrole-3,6-diyl-terthiophene-2,5″-diyl) (DPPT2-T) OFETs fabricated on bottom Au source–drain electrode arrays, with high contact-corrected μFET of 0.5 cm2 V−1 s−1. We show that when these electrodes are modified to impose weak, and then strong hole-doping of the DPPT2-T interface, Rc diminishes and its dispersion, i.e. dependence on Isd and σ, weakens. The ultimate ρc attained for the strongly hole-doped contact is ca. 1 Ω cm2, broadly independent of Isd and σ, which we propose is a hallmark of a true metal/OSC ohmic contact. For comparison, the bare Au/DPPT2-T contact gives ρc of the order of 10 Ω cm2 with a marked σ dependence. The lowest ρc reached here shortens the current transfer length down to ca. 5 μm, enabling short electrode lengths to be advantageously employed in technology.

Here we describe an improved TLM analysis that imposes the condition of constant Isd and σ which can decouple and reveal the Rc dependences on Isd and σ separately. We further show that the resultant Rc(Isd, σ) surface can be modelled to yield the specific contact resistivity ρc of the metal/organic semiconductor (OSC) interface, a key parameter of the contact that has so far been neglected in OFETs. We obtained an ultimate ρc attained is ca. 1 Ω cm2, independent of Isd and σ from a source−drain contacts modified to impose weak and then strong hole-doping of the DPPT2-T interface in a top-gate bottom-contact DPPT2-T OFETs.Figure optionsDownload as PowerPoint slide