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.

Layered crystallinity of organic semiconductors is crucial to obtain high-performance organic thin-film transistors (OTFTs), as it allows both smooth channel/gate-insulator interface formation and efficient two-dimensional carrier transport along the interface. However, the role of vertical transport across the crystalline molecular layers in device operations has not been a crucial subject so far. Here we show that the interlayer carrier transport causes unusual nonlinear current-voltage characteristics and enormous access resistance in extremely high-quality single-crystalline OTFTs based on Ph-BTBT-C 10 that involve inherent multiple semiconducting π-conjugated layers interposed respectively by electrically-inert alkyl-chain layers. The output characteristics present layer-number (n)-dependent nonlinearity that becomes more evident at larger n (1≤ n ≤ 15), demonstrating tunnelling across multiple alkyl-chain layers. The n-dependent device mobility and four-probe measurements reveal that the alkyl-chain layers generate a large access resistance that suppresses the device mobility from the intrinsic value of about 20 cm 2 /Vs. Our findings clarify the reason why device characteristics are distributed in single-crystalline OTFTs.

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We investigated a system based solely on a single molecular species, Cu(tmdt) 2 , accommodating d and π orbitals within the molecule.

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