Stress-controlled transfer of high-mobility organic semiconductor nanosheets for thin-film transistors

Abstract

Organic thin-film transistors (TFTs) are key components for large-area and mechanically flexible electronic devices, and reliable fabrication methods beyond direct physical vapor deposition (PVD) are required, particularly for technical and non-planar surfaces. We fabricated organic TFTs by transferring organic small-molecule nanosheets. During the release and transfer process, the nanosheets experience significant mechanical stress, leading to plastic deformation or phase changes that can be quantified by X-ray diffraction (XRD). In accordance with classical bending theory, the mechanical stress can be significantly reduced by limiting the nanosheet thickness to 30 nm or less. Using this optimized thickness, we directly compare transferred nanosheets with thin films prepared by PVD on self-assembled monolayer (SAM)-functionalized bottom contacts. TFTs fabricated with transferred dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) nanosheets exhibit a mobility of 0.32 cm2/V s and a subthreshold swing of 114 mV/dec, approaching state-of-the-art TFTs fabricated by direct PVD, which exhibit here a mobility of 0.42 cm2/V s. XRD indicates that transferred DNTT nanosheets exhibit superior crystalline order on the contacts compared to films grown by direct PVD. However, the PVD growth on the gate dielectric, i.e., on SAM-passivated Al2O3, is slightly better than on the sacrificial polymer layer. These results indicate that the transport properties of the transferred nanosheets are not primarily limited by the transfer process itself, but rather by the thin-film growth on the sacrificial water-soluble polymer layer. Transferable nanosheets have applications in hybrid devices and devices with unconventional geometries, e.g., non-planar transistors or devices containing free-standing thin films, where direct evaporation is challenging or not feasible.