Sub-Band Filling, Mott-like Transitions, and Ion Size Effects in C60 Single Crystal Electric Double Layer Transistors

Tuning Gate Potential Profiles and Current-Voltage Characteristics of Polymer Electrolyte-Gated Transistors by Capacitance Engineering

We demonstrate that the transfer characteristics of electrolyte-gated transistors (EGTs) with polythiophene semiconductor channels are a strong function of gate/electrolyte interfacial contact area, i.e., gate size. Polythiophene EGTs with gate/electrolyte areas much larger than the channel/electrolyte areas show a clear peak in the drain current vs gate voltage (ID-VG) behavior, as well as peak voltage hysteresis between the forward and reverse VG sweeps. Polythiophene EGTs with small gate/electrolyte areas, on the other hand, exhibit current plateaus in the ID-VG behavior and a gate-size-dependent hysteresis loop between turn on and off. The qualitatively different transport behaviors are attributed to the relative sizes of the gate/electrolyte and channel/electrolyte interface capacitances, which are proportional to interfacial area. These interfacial capacitances are in series with each other such that the total capacitance of the full gate/electrolyte/channel stack is dominated by the interface with the smallest capacitance or area. For EGTs with large gates, most of the applied VG is dropped at the channel/electrolyte interface, leading to very high charge accumulations, up to ∼0.3 holes per ring (hpr) in the case of polythiophene semiconductors. The large charge density results in sub-band-filling and a marked decrease in hole mobility, giving rise to the peak in ID-VG. For EGTs with small gates, hole accumulation saturates near 0.15 hpr, band-filling does not occur, and hole mobility is maintained at a fixed value, which leads to the ID plateau. Potential drops at the interfaces are confirmed by in situ potential measurements inside a gate/electrolyte/polymer semiconductor stack. Hole accumulations are measured with gate current-gate voltage (IG-VG) measurements acquired simultaneously with the ID-VG characteristics. Overall, our measurements demonstrate that remarkably different ID behavior can be obtained for polythiophene EGTs by controlling the magnitude of the gate-electrolyte interfacial capacitance.

Individual and synergetic charge transport properties at the solid and electrolyte interfaces of a single ultrathin single crystal of organic semiconductors

The chemical structures and morphologies of organic semiconductors (OSCs) and gate dielectrics have been widely investigated to improve the electrical performances of organic thin-film transistors (OTFTs) because the charge transport therein is a phenomenon at the semiconductor-dielectric interfaces. Here, solid and ionic gel gate dielectrics were adopted on the lower and upper surfaces, respectively, of a single, two molecule-thick single crystals of p-type OSCs to study the charge transport properties at individual interfaces between the morphologically compatible OSC surface and different gate dielectrics. Using the four-probe method, the solid and ionic gel interfaces were found to exhibit hole mobilities of 9.3 and 2.2 cm² V^-1 s^-1, respectively, which revealed the crucial impact of the gate dielectric materials on the interfacial charge transport. Interestingly, when gate biases are applied through both dielectrics, i.e., under the solid/ionic gel dual-gate transistor operation, the hole mobility at the solid gate interface is improved up to 14.7 cm² V^-1 s^-1, which is 1.5 times greater than that assessed without the ionic gel gate. This improvement can be attributed to the electric double layer formed at the ionic gel/uniform crystal surface, which provides a close-to-ideal charge transport interface through dramatic trap-filling. Therefore, the present dual-gate transistor technique will be promising for investigating the intrinsic charge-transport capabilities of OSCs.

Charge polarity-dependent ion-insertion asymmetry during electrochemical doping of an ambipolar π-conjugated polymer

Electrochemical doping is central to a host of important applications such as bio-sensing, neuromorphic computing and charge storage. However, the mechanisms that enable electrochemical dopability and the various parameters that control doping efficiencies are poorly understood. Here, employing complementary electrochemical and spectroelectrochemical measurements, we report a charge-polarity dependent ion insertion asymmetry in a diketopyrrolopyrrole-based ambipolar π-conjugated polymer. We argue that electrostatic interactions are insufficient to fully account for the observed charge-specific ion insertion into the polymer matrix. Using polymer side-chain dependent electrochemical doping studies, we show that electron density donating and accepting tendencies of polymer side-chains sufficiently describe the observed charge-polarity dependent electrochemical doping. Our observations are akin to the solvation of dopant ions by polymer side-chains. We propose that Gutmann donor/acceptor number framework qualifies the 'solvent-like' properties of polymer side-chains and provides a rational basis for designing π-conjugated polymers with favorable mixed ionic electronic transport properties.