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Influences of Out-Of-Plane Lattice Alignment on the OFET Performance of TIPS-PEN Crystal Arrays

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posted on 17.10.2016, 00:00 by Kuan-Yi Wu, Chou-Ting Hsieh, Liang-Hsun Wang, Chih-Hao Hsu, Shu-Ting Chang, Shih-Ting Lan, Yi-Fan Huang, Yu-Ming Chen, Chien-Lung Wang
In organic field-effect transistors (OFETs), the quality of charge-transport pathway, controlled by crystal structures of organic semiconductors (OSCs), strongly affects the performance of the device. To achieve higher charge mobility, solution-processed single-crystal (SPSC) techniques have been used to decrease crystal defects by aligning the crystals of OSCs in the in-plane direction. Nonetheless, through SPSC techniques, whether the crystalline lattices are well-aligned in the out-of-plane direction and how the out-of-plane lattice misorientaion affects OFET performances remain unclear. Here, a characterization protocol based on polarized optical microscope, X-ray diffraction, and electron diffraction is established to identify the lattice structure, the in-plane and out-of-plane lattice alignment in the crystal array of 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-PEN). Regardless of the solvents used in the PDMS-assisted crystallization, the characterization protocol confirms that all the crystal arrays share the same lattice structure (form I phase), and have similar in-plane lattice alignment. However, TIPS-PEN molecules have sufficient time to unify their out-of-plane orientation and prevent the formation of low angle grain boundary (LAGB) during crystal growth if high boiling temperature solvents are used. The improved out-of-plane lattice alignment increases the hole mobility and decreases the performance fluctuations of devices. The results confirm that the out-of-plane lattice alignment significantly impacts the performance of the devices and the reproducibility of the solution-processed TIPS-PEN OFETs.