Coulomb Blockade in a Two-Dimensional Conductive Polymer Monolayer (original) (raw)

Electronic transport was investigated in poly(3-hexylthiophene-2,5-diyl) monolayers. At low temperatures , nonlinear behavior was observed in the current-voltage characteristics, and a nonzero threshold voltage appeared that increased with decreasing temperature. The current-voltage characteristics could be best fitted using a power law. These results suggest that the nonlinear conductivity can be explained using a Coulomb blockade (CB) mechanism. A model is proposed in which an isotropic extended charge state exists, as predicted by quantum calculations, and percolative charge transport occurs within an array of small conductive islands. Using quantitatively evaluated capacitance values for the islands, this model was found to be capable of explaining the observed experimental data. It is, therefore, suggested that percolative charge transport based on the CB effect is a significant factor giving rise to nonlinear conductivity in organic materials. The interface between an organic semiconductor and a dielectric layer plays a critical role in carrier transport in organic field-effect transistors (OFETs), because the intrinsic transport characteristics are governed by only a few molecular layers at the interface. Good organic conductors often have a low-dimensional configuration, e.g., quasi-one-dimensional (1D) structures or two-dimensional (2D) layers. Nevertheless, a large number of fundamental questions remain to be answered regarding the charge-transport mechanism, particularly in low-dimensional structures. One such question concerns the nonlinear behavior that is often observed in the current-voltage (I-V) characteristics of organic conductors. Even for materials that exhibit good linear I-V characteristics near room temperature (RT), nonlinearity can occur as the temperature is reduced, reflecting a decrease in conductivity. This effect has been interpreted using a variety of mechanisms-such as charge hopping, trapping, tunneling, and emission-either within the organic material or at the interfaces. Explanations have been proposed involving classical physical models previously developed for inorganic materials; what these explanations have in common is that the expression for the current contains an exponential term involving the electric field (E) and the temperature (T). However, the observed nonlinearity cannot be fully explained using these conventional models or combinations of them. Recently, it has been reported that the I-V characteristics obey a power-law relationship in low-dimensional organic materials such as polymer nanofibers [1], nanotubes [2], and polymer films [3,4], as is the case for carbon nanotubes and inorganic quantum wires. The observed power-law relationship for polymer materials has been put forward as evidence for tunneling into a 1D Luttinger liquid because of the quasi-1D structure of these materials [2,3]; however, power-law behavior was also observed for a three-dimensional (3D) polymer film [4]. The origin of such power-law behavior in organic materials is still under debate [5]. On the other hand, in inorganic granular materials, the power-law dependence of the I-V characteristics has commonly been attributed to dissipative tunneling processes, such as that associated with a Coulomb blockade (CB) [6-8]. The CB effect has been confirmed during charge transport through a single molecule spanning adjacent electrodes [9,10], although it has rarely been suggested as the origin of nonlinear conduction in larger condensed organic conductor systems [4,11,12]. CB transport occurs in systems consisting of an array of small conductive islands connected by narrow junctions, provided the tunneling resistance between neighboring sites is significantly larger than the quantum resistance (≫h=e 2), the capacitance associated with each island is sufficiently small, and the energy corresponding to an additional electron charge at each site is large compared to k B T. Here, h is Planck's constant, e is the charge of an electron, and k B is Boltzmann's constant. Since the nature of the individual sites (i.e., whether they are metallic, superconducting, or semiconducting) is irrelevant [13], there is no reason why the CB effect should not emerge in organic materials that consist of small conducting segments. In the present Letter, an investigation into charge transport is carried out through a 2D conjugated polymer PRL 115, 196801 (2015) P H Y S I C A L