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

TL;DR

This study investigates charge transport in ultra-thin, highly ordered organic semiconductors, revealing a transition from hopping to band-like conduction within a few molecular layers, enabled by van der Waals epitaxy.

Contribution

It demonstrates the fabrication of ultra-thin, highly ordered organic semiconductors with controlled structure and reveals the phase transition in charge transport mechanisms at the two-dimensional limit.

Findings

Charge transport is hopping in the first layer and band-like in subsequent layers.

Mobility saturation occurs at approximately 3 nm thickness.

Interfacial vdW interactions modulate molecular packing and transport properties.

Abstract

One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono- to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to band-like in subsequent layers. Such abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ~3nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals.