Contact resistance limits the performance of organic field-effect transistors, especially those based on high-mobility semiconductors. Despite intensive research, the nature of this phenomenon is not well understood and mitigation strategies are largely limited to complex schemes often involving co-evaporated doped interlayers. Here, this study shows that solution self-assembly of a polyelectrolyte monolayer on a metal electrode can induce carrier doping at the contact of an organic semiconductor overlayer, which can be augmented by dopant ion-exchange in the monolayer, to provide ohmic contacts for both p- and n-type organic field-effect transistors. The resultant 2D-doped profile at the semiconductor interface is furthermore self-aligned to the contact and stabilized against counterion migration. This study shows that Coulomb potential disordering by the polyelectrolyte shifts the semiconductor density-of-states into the gap to promote extrinsic doping and cascade carrier injection. Contact resistivities of the order of 0.1–1 Ω cm2 or less have been attained. This will likely also provide a platform for ohmic injection into other advanced semiconductors, including 2D and other nanomaterials.

Contacts with suitable work functions are important not only for ohmic injection of carriers but also to set up the built-in potential required for various semiconductor processes including current rectification, light emission and photovoltaic generation. For two-dimensional (2D) materials, one way to shift the work function is by intercalation doping with suitable donors or acceptors. Here, using an atomic sheet transfer methodology, we report layer-by-layer assembly of centimeter-square sizes of graphene–fluorofullerene multilayers through directed stacking of chemical-vapor-deposited (CVD) graphene sheets and self-assembly of fluorofullerene acceptors, for example C60F48, to give a 1 eV increase in work function to 5.7 eV, which is unprecedented for a well-defined compound. These assemblies exhibit an unusual motif of fully-ionized large dopants in open packing with the graphene sheets. As a consequence, they show a sizeable electrostatic dipole to give ultrahigh work function at acceptor-terminated surfaces even for a moderate hole doping level of 1.6 × 1013 cm−2 per sheet. They exhibit little additional carrier scattering and a remarkable chemical stability. Hall measurements reveal unity doping efficiency with temperature-independent hole density, mobility and electrical conductivity down to 2.5 K, which are atypical of conventional graphite intercalation compounds. These materials provide the first examples of a novel domain of doped 2D assemblies where large ions are incorporated through room-temperature solution processing, which opens new opportunities beyond van der Waals semiconductor heterostructures.

•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.

Although all graphites share the same idealized chemical structure, marked differences in fact exist between their reactivities, such as the propensity for oxidation, that need to be taken into consideration for the development of applications. Here we show that five different commercially sourced natural and synthetic graphites differ significantly in their response to a modified Staudenmaier oxidation that produces substoichiometric graphene oxides (sub-GOx). The dominant oxidation product is hydroxyl groups, which can be dehydrate to epoxy groups under mild heating even below 120 °C. The extent of oxidation correlates broadly with the defect band intensity in the starting graphites as measured by Raman spectroscopy. FTIR shows there is a significant concentration of H defects at the % atom level. The results suggest that defects in the graphite plane are more prevalent than previously thought. Finally, the properties of the thermally reduced sub-GOx are also different. The product from the least defective starting graphite ultimately exhibits the lowest activation energies for both electron and hole transport, of the order of 10 μeV below 25 K, that is characteristic of band-like transport. These results are important because they show that the quality of the starting graphite significantly affects the properties of the derived products.Keywords: chemical oxidative reactivity; graphene; graphene oxide; graphite; graphite defects; nanographene domain; oxidation; suboxidized graphene oxide; substoichiometric graphene oxide;

We report an efficient solvothermal process to achieve deep deoxidation of octadecylamine functionalized sub-stoichiometric graphene oxides (sub-GOx) in organic solvents. An initial average carbon oxidation state of ca. 0.6 (i.e., 0.6 OH per basal-plane C) could be reduced to ca. 0.15, while retaining a critical density of the alkyl chains for solvent processability, ca. 0.02 chains per basal-plane C. The products can thus be characterized as single-sheet dispersions of alkyl-functionalized disordered “graphenes”. The oxidation state was characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy. We found a strong solvent effect in the relative rates of alkyl chain degrafting and/or chain scission versus deoxidation and/or regraphenization. The desired deoxidation and/or regraphenization are promoted at higher temperatures and by the use of aprotic amide solvents. Furthermore we found that carbon monoxide (CO) was a remarkably efficient fugitive deoxidizer. A key advantage of this solvothermal deoxidation process is that the resultant dispersion of conductive graphenes can be directly used for printing, coating or formulating into nanocomposites, without further purification or heat treatment. Thin films of the deoxidized sub-GOx give dc conductivities of up to 40 S cm−1, which is the highest reported to date for solvent-processable graphene derivatives. These films show temperature independent conductivity, which suggests that its conductivity is largely limited by tunneling between the alkyl chain spacers.

A novel solution processable donor–acceptor (D–A) based low band gap polymer semiconductor poly{3,6-difuran-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene} (PDPPF-TVT), was designed and synthesized by a Pd-catalyzed Stille coupling route. An electron deficient furan based diketopyrrolopyrrole (DPP) block and electron rich thienylenevinylene (TVT) donor moiety were attached alternately in the polymer backbone. The polymer exhibited good solubility, film forming ability and thermal stability. The polymer exhibits wide absorption bands from 400 nm to 950 nm (UV-vis-NIR region) with absorption maximum centered at 782 nm in thin film. The optical band gap (Eoptg) calculated from the polymer film absorption onset is around 1.37 eV. The π-energy band level (ionization potential) calculated by photoelectron spectroscopy in air (PESA) for PDPPF-TVT is around 5.22 eV. AFM and TEM analyses of the polymer reveal nodular terrace morphology with optimized crystallinity after 200 °C thermal annealing. This polymer exhibits p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices. The highest hole mobility of 0.13 cm2 V−1 s−1 is achieved in bottom gate and top-contact OTFT devices with on/off ratios in the range of 106–107. This work reveals that the replacement of thiophene by furan in DPP copolymers exhibits such a high mobility, which makes DPP furan a promising block for making a wide range of promising polymer semiconductors for broad applications in organic electronics.

Thin films of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C14–PBTTT) exhibit a monolayer-terraced morphology that indicates a pronounced lamellar order with π-stacks of extended polymer chains. Previously this remarkable state of order was thought to be promoted by the interdigitation of alkyl side chains between the lamellae during cooling from the liquid-crystalline (LC) phase. Here we establish that the key to this ordering in fact is the formation of unentangled π-stacks of extended polymer chains in dilute solutions of chlorobenzene (CB) or 1,2-dichlorobenzene (o-DCB), which though routinely used as the “best” solvents are in fact borderline solvents. Film formation causes these π-stacks to deposit substantially oriented in the film plane, while the subsequent anneal and cool from LC phase accentuates this incipient order to develop the monolayer-terraced morphology. This mechanism is supported by the following lines of evidence. (i) Hydrodynamic and viscometry measurements respectively of the Kuhn segment length and Mark–Houwink–Sakurada exponent of PBTTT reveal that CB is a near-Θ solvent, and PBTTT is significantly stiffer than regioregular polythiophene. (ii) Solution-state UV–vis spectroscopy reveals an early coil → rod transition in highly dilute solutions, which gives rise to unentangled π-stacks. (iii) Solid-state UV–vis spectroscopy, atomic force microscopy and variable-angle spectroscopic ellipsometry together reveal the as-deposited π-stacks are already substantially oriented in the film plane. We further demonstrate that this monolayer-terraced morphology can also be induced in regioregular poly(3-hexylthiophene) films using a borderline solvent mixture of chlorobenzene and mesitylene, and in very dilute CB where the incipient π-stacks do not entangle. Therefore, this dilute π-stacking mechanism is general. Processing with a borderline solvent or solvent additive thus provides a general route to obtain superior supramolecular order in π-stackable conjugated polymers.

We report an efficient solvothermal process to achieve deep deoxidation of octadecylamine functionalized sub-stoichiometric graphene oxides (sub-GOx) in organic solvents. An initial average carbon oxidation state of ca. 0.6 (i.e., 0.6 OH per basal-plane C) could be reduced to ca. 0.15, while retaining a critical density of the alkyl chains for solvent processability, ca. 0.02 chains per basal-plane C. The products can thus be characterized as single-sheet dispersions of alkyl-functionalized disordered “graphenes”. The oxidation state was characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy and Raman spectroscopy. We found a strong solvent effect in the relative rates of alkyl chain degrafting and/or chain scission versus deoxidation and/or regraphenization. The desired deoxidation and/or regraphenization are promoted at higher temperatures and by the use of aprotic amide solvents. Furthermore we found that carbon monoxide (CO) was a remarkably efficient fugitive deoxidizer. A key advantage of this solvothermal deoxidation process is that the resultant dispersion of conductive graphenes can be directly used for printing, coating or formulating into nanocomposites, without further purification or heat treatment. Thin films of the deoxidized sub-GOx give dc conductivities of up to 40 S cm−1, which is the highest reported to date for solvent-processable graphene derivatives. These films show temperature independent conductivity, which suggests that its conductivity is largely limited by tunneling between the alkyl chain spacers.

A novel solution processable donor–acceptor (D–A) based low band gap polymer semiconductor poly{3,6-difuran-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene} (PDPPF-TVT), was designed and synthesized by a Pd-catalyzed Stille coupling route. An electron deficient furan based diketopyrrolopyrrole (DPP) block and electron rich thienylenevinylene (TVT) donor moiety were attached alternately in the polymer backbone. The polymer exhibited good solubility, film forming ability and thermal stability. The polymer exhibits wide absorption bands from 400 nm to 950 nm (UV-vis-NIR region) with absorption maximum centered at 782 nm in thin film. The optical band gap (Eoptg) calculated from the polymer film absorption onset is around 1.37 eV. The π-energy band level (ionization potential) calculated by photoelectron spectroscopy in air (PESA) for PDPPF-TVT is around 5.22 eV. AFM and TEM analyses of the polymer reveal nodular terrace morphology with optimized crystallinity after 200 °C thermal annealing. This polymer exhibits p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices. The highest hole mobility of 0.13 cm2 V−1 s−1 is achieved in bottom gate and top-contact OTFT devices with on/off ratios in the range of 106–107. This work reveals that the replacement of thiophene by furan in DPP copolymers exhibits such a high mobility, which makes DPP furan a promising block for making a wide range of promising polymer semiconductors for broad applications in organic electronics.