Gate Voltage Dependent Resistance of a Single Organic Semiconductor Grain Boundary

Abstract

Conducting probe atomic force microscopy (CP-AFM) was used to examine electrical transport through an individual grain boundary (GB) in the organic semiconductor sexithiophene (6T, E gap ∼ 2.3 eV). The sample consisted of a pair of grains grown by vapor deposition onto an SiO 2 /Si substrate. A variable channel length transistor was constructed using a microfabricated Au electrode contacting one grain, a Au-coated AFM tip as a positionable electrode, and the doped Si substrate as a gate. The GB resistance was found to be gate voltage dependent and large, on the order of 10 9 -10 10 Ω for a 1 µm boundary length. Resistances across single 6T grains were an order of magnitude lower. The results indicate that GBs can be the principal bottleneck to charge transport in polycrystalline organic semiconductor films, particularly at low gate fields, consistent with a recent model that proposes potential barriers exist between grains. We estimate the GB barrier height to be on the order of 100 meV.We report the use of conducting probe atomic force microscopy (CP-AFM) 1 to measure resistances associated with individual grain boundaries in the organic semiconductor sexithiophene (6T, E gap ∼ 2.3 eV). 2 Grain boundaries (GBs) play an important role in the transport properties of field effect transistors (FETs) based on polycrystalline organic semiconductor films. 3 In particular, it is expected that GBs trap and scatter mobile charge carriers, increasing the overall film resistance. Understanding of GB effects and the extent to which they dominate transport in organic semiconductors can be improved by microscopic measurements on indiVidual grains and GBs. In the work described here, we have taken advantage of the submicron resolution of CP-AFM to show that GB resistances can be an order of magnitude higher than single grain resistances in sexithiophene. Furthermore, the GB resistances are gate voltage dependent, as predicted by a recently proposed transport model. 3b Our studies employ a commercial AFM 4 and the experimental configuration shown in Figure 1. We use a Au-coated AFM tip as a positionable electrical contact to 6T grains that are also contacted by a fixed microfabricated Au electrode. The doped Si substrate serves as a gate. Current-voltage (I-V) measurements are made in air as a function of the probe position and gate field by applying negative voltages to the microfabricated electrode while keeping the probe tip at ground. Effectively, the structure in Figure 1 constitutes a FET in which the source (hole injecting) contact is an AFM tip that can be positioned at arbitrary points on the grain. Figure 2A is an AFM topograph showing two 6T grains on SiO 2 /Si sharing a common boundary approximately 1 µm in length. Both grains display typical 2.3 nm thick terraces corresponding to single layers of 6T molecules. Near the GB, the total thickness of each crystal is 18 nm, or 8 6T layers stacked on top of one another. The well-defined facets on the grains allow identification of their crystallographic orientations; the...

Organic Thin-Film Transistors as Plastic Analytical Sensors

“…Traps can also be located at grain boundaries; the increase in mobility with larger grain size is proof of this (49). In addition, the resistance of the locally measured grain boundaries drops as the gate bias increases (22).…”

“…Depending on the material properties, transport either within the grain or across the boundary dominates, with the slower process acting as the rate-determining step. Thermally evaporated thiophene oligomers exhibit a transport that is limited by thermionic emission over the potential barrier at the grain boundaries (22). A similar mechanism applies also to the charge injection at contact barriers; for PTCDI-C5, the contact barriers were ~20% higher than E A (51).…”

“…In transistors with undoped active semiconductor materials, V t is extracted from the I-V curves along with µ FET . V t is related to low mobility electronic levels, and it corresponds to the voltage required to fill such trap states in the organic material or at the interface with the gate dielectric (21,22).…”

Organic semiconductors for device applications: current trends and future prospects

“…The concept of using single crystals in polymeric device fabrication was primarily motivated by successfully assessing the intrinsic charge carrier transport properties in the absence of grain boundaries and molecular disorders [21], which are invariably there in polycrystalline thin films [591,592], degrading mobility and impairing device performance. Although improvements made in this direction could produce ruberene single-crystal OFETs exhibiting charge carrier mobilities as high as 20 cm 2 /Vs [303], there were problems such as poor-quality electrical contacts and difficulty in handling the fragile crystals that limited their uses in device fabrications.…”

Inducement of Azimuthal Molecular Orientation of Pentacene by Imprinted Periodic Groove Patterns for Organic Thin‐Film Transistors

“…For polycrystalline pentacene, several models have been considered that fully account for the effects of the grains and grain boundaries of the pentacene layer. [37][38][39] The grain boundaries play a dominant role in the charge-carrier transport because they trap and scatter the mobile charge carriers, thereby increasing the overall film resistance. The effective mobility can be obtained from the grain boundary trapping model using the following equation.…”