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One of the basic assumptions in organic field-effect transistors, the mostfundamental device unit in organic electronics, is that charge transport occurstwo-dimensionally in the first few molecular layers near the dielectricinterface. Although the mobility of bulk organic semiconductors has increaseddramatically, direct probing of intrinsic charge transport in thetwo-dimensional limit has not been possible due to excessive disorders andtraps in ultrathin organic thin films. Here, highly ordered mono- totetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy onhexagonal BN. We find that the charge transport is dominated by hopping in thefirst conductive layer, but transforms to band-like in subsequent layers. Suchabrupt phase transition is attributed to strong modulation of the molecularpacking by interfacial vdW interactions, as corroborated by quantitativestructural characterization and density functional theory calculations. Thestructural modulation becomes negligible beyond the second conductive layer,leading to a mobility saturation thickness of only ~3nm. Highly ordered organicultrathin films provide a platform for new physics and device structures (suchas heterostructures and quantum wells) that are not possible in conventionalbulk crystals.

Probing Carrier Transport and Structure-property Relationship of Highly Ordered Organic Semiconductors at Two-dimensional Limit