19 octobre 2021

[Organic Thin Film] – Mobility enhancement of DNTT and BTBT derivative organic thin-film transistors by triptycene molecule modification.

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Organic thin-film transistors (OTFTs) have gained considerable research interest for potential use in next-generation electronic devices, such as imperceptible biosensors  and large-area flexible displays, because of their low-cost processability to ultrathin and flexible plastic substrates. In such devices, high performance (for example, field-effect mobility) is required for OTFTs to enable wide bandwidth and fast responsivity of the analog and digital circuits. To date, various approaches have been used to realize high field-effect mobility in OTFTs, such as material designs for organic semiconductor (OSC) with a large molecular orbital overlap and high environmental stability, fabrication process enabling OSC films with fewer disorders and grain boundaries, and modification on a variety of solid interfaces (for example, metal, metal oxide, and polymer). Among these approaches, interface modification and functionalization between a gate dielectric layer and an OSC layer is a simple and promising technique for enhancing the crystallinity of OSC molecules and reducing trap sites at the interface, resulting in their high carrier mobility. Recent studies  have reported that the interface modification by triptycene, with a few molecular layers, is beneficial for improving the performance of OTFTs based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT). Triptycene, a three-bladed propeller-shaped molecule, can self-assemble to form a 2D nested hexagonalpacking and 1D stacked structure , facilitating large grain size and enhanced crystallinity of DNTT that improves the mobility of OTFTs. However, the versatility and utility of triptycene layers for the different small-molecule thienoacene-based OSCs remain unstudied. It is well-established that thienoacene-based OSCs, such as DNTT [1]benzothieno[3,2-b][1]benzothiophene (BTBT), and their diphenyl- and alkyl-derivatives, such as DPh-DNTT , DPh-BTBT , C10-DNTT , and C8-BTBT, have a high air stability and high hole mobility in OTFTs. In previous studies, vacuum-evaporated DNTT derivatives, with phenyl or alkyl groups in the side chain, were reported to exhibit significantly different thin-film morphologies in the crystal growth processes, compared with those without any side chains. For example, the evaporated thin film of C10-DNTT shows edge-on orientations of crystals with large-terraced grains and lamellae structures on the scale of 100 nm, owing to its long alkyl chain. In contrast, the DNTT thin films are characterized by a relatively flat surface morphology with dendritic crystal grains, and the lamellae-like structure is not very prominent. Because of the inadequate research, the effect of triptycene molecule layers on the thienoacene-based OSCs in OTFTs remains to be intensively studied.

Here, we report a comprehensive effect of triptycene-modified layers on five different small-molecule thienoacene-based OSCs: DNTT, C10-DNTT, DPh-DNTT, C8-BTBT, and DPh-BTBT. Solution-processed triptycene layers formed on the polymer gate dielectrics were found to improve the effective carrier mobility in OTFTs for all the OSC derivatives. The most significant effect of the triptycene layers was observed for C10-DNTT, showing a 20-fold improvement in the average effective mobility. In other OSCs, the triptycene-modified OTFTs showed an improved average effective mobility (the least improvement of 1.5 times was obtained for C8-BTBT). Utilizing the transmission line method (TLM), the intrinsic channel mobilities for the triptycene-modified OTFTs were also found to be higher than those for the OTFTs without triptycene layers, regardless of the OSC type even at a different amount of contact resistance. Furthermore, the morphological observation of OSC films via scanning electron microscopy (SEM) demonstrates that the crystal grain size of the OSCs grown on the triptycene-modified dielectrics, is larger than that of the OSCs grown on pristine dielectrics, facilitating enhancement of the intrinsic channel mobility.

OSCs (DNTT, C10-DNTT, DPh-DNTT, C8-BTBT, and DPh-BTBT) and dopantmolecules (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane: F4-TCNQ) were procured from Nippon Kayaku Co., Ltd. and Tokyo Chemical Industry Co., Ltd., respectively. The insulating materials of parylene (diX-SR) and fluoropolymerCYTOP™ were purchased from Daisan Kasei Co., Ltd. and AGC Inc., respectively. Among a series of paraffinic triptycenes, a triptycene derivative with a methoxy group was chosen (hereinafter, methoxy-derivative triptycene is referred to as “triptycene” in this manuscript). The triptycene molecule film was formed by the blade coating method. The blade coating method is a simple and low-cost thin-film deposition method for organic materials. The fixed blade sweeps the solution in which an organic molecule is dissolved, and the solvent gradually evaporates from the meniscus zone, forming a thin and uniform film with aligned organic molecules. In previous research , blade-coated triptycene films were found to feature highly ordered crystal packing, compared with the other thin-film deposition methods (for example, thermal vacuum evaporation, drop casting, and spin coating). Therefore, we used the blade coating technique to form triptycene molecule films. The blade was made of EAGLE XG® (purchased from Corning Inc.) glass and fixed, ensuring a 20° angle between the perpendicular axis and the substrate surface. The gap between the blade and substrate surface was approximately set to 100 μm. During the coating, the substrate was heated to 50–60 °C, and the sweep speed of the substrate was fixed at 40 μm/s. The triptycene solution was prepared by dissolving a powder of triptycene in the mesitylene solvent (purchased from Wako Pure Chemical Corporation) at a concentration of 0.5 mM. Notably, the blade coating unit used in this study could easily encompass the coating area because the size of the blade could be increased, as shown in Fig. 1(c), facilitating a high scalability for large-area deposition.

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