Semiconductive Single Molecular Bilayers Realized Using Geometrical Frustration

Abstract

A unique solution-based technology to manufacture self-assembled ultrathin organic-semiconductor layers with ultrauniform single-molecular-bilayer thickness over an area as large as wafer scale is developed. A novel concept is adopted in this technique, based upon the idea of geometrical frustration, which can effectively suppress the interlayer stacking (or multilayer crystallization) while maintaining the assembly of the intralayer, which originates from the strong intermolecular interactions between π-conjugated molecules. For this purpose, a mixed solution of extended π-conjugated frameworks substituted asymmetrically by alkyl chains of variable lengths (i.e., (πCore)-C 's) is utilized for the solution process. A simple blade-coating with a solution containing two (πCore)-C 's with different alkyl chain lengths is effective to provide single molecular bilayers (SMBs) composed of a pair of polar monomolecular layers, which is analogical to the cell membranes of living organisms. It is demonstrated that the chain-length disorder does not perturb the in-plane crystalline order, but acts effectively as a geometrical frustration to inhibit multilayer crystallization. The uniformity, stability, and size scale are unprecedented, as produced by other conventional self-assembly processes. The obtained SMBs also exhibit efficient 2D carrier transport as organic thin-film transistors. This finding should open a new route to SMB-based ultrathin superflexible electronics.

Understanding the Meniscus‐Guided Coating Parameters in Organic Field‐Effect‐Transistor Fabrications

“…To date, the uniformity of the organic active layer in the OFETs is still a major bottleneck in scaling up the OFET fabrication. Recently, some MGC grown high quality organic crystals up to wafer scale have been demonstrated, which is believed to be able to advance the pace of the OFET development . A specific investigation of how each processing parameter synchronizes with the others can provide valuable information in designing the fabrication setup.…”

“…Crystallization of the organic molecules occurs at the substrate‐solvent‐air contact line while the substrate moves and the solvent evaporates. Currently, different MGC methods for organic semiconductor thin films deposition, such as dip coating, zone casting and blade coating and others, have been demonstrated. Recent advances on the MGC crystallization mechanisms mainly depend on the surface energy (γ) analysis of the system.…”

Wafer‐Scale Fabrication of High‐Performance n‐Type Polymer Monolayer Transistors Using a Multi‐Level Self‐Assembly Strategy

“…Recently, 2D monolayers of conjugated molecules have been studied using solution processing or vacuum evaporation method . Compared to their thick films counterpart, ultrathin films exhibited some unique properties, such as uniform microstructure, constant carrier density, and high sensitivity as sensors . Like most 2D inorganic materials, fabrication of 2D organic materials is also of great challenges due to their weak van der Waals forces and variable lattice constants …”

“…Many efforts have been devoted to the large‐scale fabrication of the organic thin films, but only a few works realized 2D organic monolayers. Organic monolayers can avoid the film thickness variations and usually show less contact resistances .…”

“…Compared to the p‐type polymers, current state of the art n‐type polymer monolayer FETs showed about 1–2 order lower mobilities (0.002–0.14 cm 2 V −1 s −1 ) . Moreover, wafer‐scale fabrication of either p‐type or n‐type polymer monolayers is also challenging, because, in previous works, necessary film transfer process, unavoidable crystalline boundaries, or different layer thicknesses of 2D organic materials limited their uniformity for large‐scale electronic devices …”