Structural and Electrostatic Complexity at a Pentacene/Insulator Interface

The advancement of organic electronics for applications in solar-energy conversion, printed circuitry, displays, and solidstate lighting depends upon the optimization of a variety of organic-semiconductor interfaces. Organic-semiconductor/ insulator (O/I) interfaces, in particular, are critical to the operation of organic thin-film transistors (OTFTs) currently being developed for printed flexible electronics. In an OTFT, the presence or absence of gate-induced charge at the O/I interface determines whether the OTFT is ON (current flows) or OFF (no current flows). Structural and electrostatic disorder in the first few layers of the organic semiconductor next to the insulator (i.e., in the O/I interfacial region) lowers the OTFT ON current, reduces the switching speed, and can increase the threshold voltage, all of which are undesirable. Minimizing disorder at O/I interfaces is therefore essential for improving device performance. Unfortunately, rational approaches are hampered by limited understanding of both interfacial structure and the correlation of structure with electrical properties. Here we report scanning probe microscopy (SPM) observations of polycrystalline microstructure and defects in 2–5 nm thick interfacial layers of the benchmark organic semiconductor pentacene (C22H14) grown on the insulator silicon dioxide. We demonstrate that grain boundaries (GBs) and high line-dislocation densities produce striking and unexpected variations in the surface electrostatic potential of the interfacial pentacene layers on SiO2. The surface-potential distributions of O/I interfaces are critically important to OTFT operation, as discussed below. Correlations between structure and surface potential therefore have significant implications for electricaltransport models and for understanding structure–property relationships in OTFTs. Visualization of rich structural and electrostatic complexity at crystalline O/I interfaces also opens future opportunities for rational efforts to tailor the properties of these interfaces by physical or chemical means.