Organic semiconductors (OSCs) have been the cornerstone of organic electronics in a variety of applications, including photovoltaics, thin-film transistors (OTFTs) and electrochromics. Although OSCs promise to be lower in cost, more flexible and compatible with roll-to-roll processing compared to inorganic semiconductors, they suffer from a lack of long-range order, especially when deposited from solution over large areas. Defects, such as grain boundaries and dislocations, in as-cast OSC films, have been widely recognized to impede charge transport. While the introduction of an external field during solution processing, such as that during blade coating, has resulted in high quality millimetric single-crystalline domains and ribbons, these techniques still face formidable drawbacks because they employ chlorinated organic solvents that pose significant health and environmental hazards, and are often trapped in the resulting OSC films. Herein, we introduce a scalable, solvent-free technique to prepare single crystalline OSC films with low defect densities over several centimeters. This approach leverages the liquid crystallinity of OSCs; depositing the OSCs at temperatures within their smectic window over a metal frame forms a freely suspended membrane, which is defect-free and uniform across its entire area. Transferring this membrane onto desired substrates and cooling it slowly from its smectic phase into its crystal phase preserves this high quality assembly. In the particular example of a equimolar blend of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT), we have produced BTBT cocrystalline films with single-crystalline domains that span ca. 30 mm laterally. To assess the electrical properties of these single crystalline films, we fabricated OTFTs with a channel width of 200 mm and a channel length of 500 mm. We chose a particularly large channel length of 500 mm to demonstrate the uniformity and quality of our films over large areas. Such devices exhibit an average hole mobility of 8.5 ± 2.3 cm2V-1s-1 and an average on/off ratio that exceeds 107, independent of device location across the entire film. This solvent-free scalable approach of producing single crystalline OSC films at wafer dimensions (ca. 50 mm) should be amenable for broad applications in organic electronics.

Solvent-free coating of organic semiconductor membranes with large-area single-crystalline domains

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.