Unidirectionally Crystallized Stable n‐Type Organic Thin‐Film Transistors Based on Solution‐Processable Donor–Acceptor Compounds

Abstract

solution processes such as solution shearing, [5] double-shot inkjet printing, [7] edge casting, [8,9] hot-spin coasting, [3] offcenter spin casting, [4] and bar coating [10] allow the formation of large-area singlecrystalline or unidirectionally crystallized polycrystalline thin films by controlling the crystallization kinetics. Molecular materials that show a high-layer crystallinity are best matched with tailored solution techniques as they can take advantage of the self-organized molecular layer formation within the solution, especially at air-solution interfaces. [11][12][13] A wide range of molecular materials has been explored so far to obtain highperformance, solution-processable organic semiconductors for solution-based thin-film processing. Most of these materials are categorized as p-type, whereas the number of stable n-type organic semiconductors is still strictly limited. [18][19][20][21][22][23] This is related to the difficulty in designing single-component organic semiconductors that simultaneously show high-layer crystallinity, stable n-type operation against oxidation, and high solvent solubility. A unique exception is N,N′-1H,1Hperfluorobutyldicyanoperylene-carboxydi-imide (PDIF-CN 2 ), which is reported to have high-performance, stable n-type characteristics. [2,9,23] Recently, a distinct class of n-type organic semiconductors was reported with the use of charge-transfer (CT) compounds composed of organic donor and acceptor molecules. [24][25][26] Especially, a high degree of layer crystallinity can be achieved when 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (diC 8 BTBT), a renowned single-component organic semiconductor, [15] is used as the donor component: In CT compounds of (diC 8 BTBT) (fluorinated derivatives of tetracyanoquinodimethane (F n TCNQ)) (n = 0, 2, 4), a 2D semiconducting layer is formed with π-conjugated donor and acceptor skeletons, which are separated by alkyl chain layers derived from the diC 8 BTBT end groups. In the compound, the conduction and valence bands are primarily composed of the lowest unoccupied molecular orbital (LUMO) of F n TCNQ and the highest occupied molecular orbital (HOMO) of diC 8 BTBT, respectively (Figure 1). [27] The unique nature is that the semiconducting ground state is fully stabilized by the intermolecular CT interactions between diC 8 BTBT and F n TCNQ. These features afford OTFTs that exhibit n-type field-effect characteristics with both high air stability and high Thin-film formation by solution-shearing technique is examined for layered-crystalline donor-acceptor charge-transfer compounds composed of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene as a donor and optionally fluorinated derivatives of tetracyanoquinodimethane (n = 0, 2, 4) as acceptors. Polycrystalline thin films of the compounds whose crystalline size is over 2 mm along the blade-scan axis are successfully fabricated, and the formed films demonstrate anisotropic alignment of the crystalline grains where the crystal a-axis with the largest transfer integral is parall...

Mixed Stack Organic Semiconductors: The Anomalous Case of the BTBT-TCNQFx Series

“…2−5 Benzothieno-benzothiophene (BTBT) alkylated derivatives that shown outstanding p-channel semiconducting properties 6,7 have recently been combined with TCNQF x acceptors to explore the possibilities of the two-component approach. 8,9 Surprisingly enough, the obtained transistors are of n-type, 9,10 and the degree of charge transfer do not correlate with the TCNQF x acceptor strength, as happens for instance with other mixed stack CT crystals, e.g., the perylene-TCNQF x series. 3,4 In order to disentangle the possible effects of the alkyl sidechains on this behavior, we decided to investigate the physical properties of the charge transfer crystals of the TCNQF x series of acceptors (x = 0, 2, 4) with unsubstituted BTBT.…”

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…The number of potential donor–acceptor combinations is extremely large, especially when considering 1:2, 1:3, or 2:1, 3:1 instead of simply 1:1. Several examples have been reported, often involving TCNQ, F2‐TCNQ 19 , or F4‐TCNQ as acceptors . The field of organic donor–acceptor complexes as novel organic semiconductors has recently been reviewed .…”

Role of Polymorphism and Thin-Film Morphology in Organic Semiconductors Processed by Solution Shearing

“…They provide information related to the crystal packing and also the orientation of the crystals with respect to the surface. Polarized absorption and Raman spectroscopies can also be used to find out about the in-plane orientation of the molecules in thin films. ,, Lattice-phonon confocal Raman spectroscopy has also been proved to be a highly prevailing tool to identify OSC polymorphs in thin films because it is very sensitive to intermolecular interactions. ,− …”

“…Polarized absorption and Raman spectroscopies can also be used to find out about the in-plane orientation of the molecules in thin films. 27,34,69 Lattice-phonon confocal Raman spectroscopy has also been proved to be a highly prevailing tool to identify OSC polymorphs in thin films because it is very sensitive intermolecular interactions. 36,70−72 For the characterization of OSCs, thin films are of great interest to gain information about the molecular ordering of the molecular layer close to the substrate.…”