Organic semiconductors have the advantage of being cheap to process and can be built on flexible substrates. However, the disadvantage is that these materials generally have much lower charge carrier mobilities than their inorganic counterparts and thus do not conduct electricity as well. This is because it is difficult to grow them on conventional insulating substrates, such as silicon dioxide, glass or plastics, which are highly disordered. The result is an extremely rough interface between the semiconductor and substrate that contains charge traps.

Researchers have managed to overcome this problem to some extent by suspending organic single crystals over an air gap above the substrate. Although such structures are perfect for fundamental charge transport studies, they are completely unsuitable for making real-world electronics devices.

No charge traps

Unlike amorphous substrates, 2D dielectric materials, such as h-BN layers and other so-called van der Waals (vdW) materials, like metal oxides and metal dichalcogenides, might just be the answer here, say team members Chul-Ho Lee and Elton Santos of Stanford University. “These new types of substrate bind to semiconducting organic films via weak vdW interactions and the interface between the substrate is free from dangling bonds, charge traps and electron scattering centres, all of which conspire to deteriorate charge mobilities. What is more, the vdW interactions help the molecules in the semiconductor to assemble in a well ordered way, something that minimizes heterogeneous nucleation processes and produces single-domain crystals that have high charge mobilities.”

The researchers, led by Philip Kim and Colin Nuckolls of Columbia University, grew rubrene films on h-BN using a physical vapour transport method with graphene as the vdW electric contacts. Transmission electron microscopy (TEM) image and selected area electron diffraction (SAED) patterns confirmed that the rubrene films contained large single-crystal domains and that the interface between the substrate and semiconductor was clean. “Most importantly (and despite the imperfect contact between the h-BN/graphene and rubrene), we also measured the highest field-effect mobility (of 11.5 cm2/V/s) for an organic semiconductor grown on any substrate to date,” Santos told nanotechweb.org. “Indeed, such a mobility value is comparable to that of free-standing crystals.”

According to the researchers, organic devices based on vdW heterojunctions might find use in applications such as flexible displays, flat panels and solid-state lighting. “The epitaxial approach we have developed could also be easily applied to a host of other organic and layered material systems, not just h-BN/graphene and rubrene,” said Lee.

The work is detailed in Advanced Materials.