Effects of a highly lipophilic substituent on the environmental stability of naphthalene tetracarboxylic diimide-based n-channel thin-film transistors

Abstract

N,N′-Bis(4-trifluoromethylthiobenzyl)naphthalene-1,4,5,8-tetracarboxylic acid diimide (NTCDI-BSCF₃) is synthesized. It shows a similar molecular packing structure and intermolecular transfer integral to N,N′-bis(4-trifluoromethoxybenzyl)naphthalene-1,4,5,8-tetracarboxylic acid diimide (NTCDI-BOCF₃), but demonstrates different behaviors in terms of electron mobility and air stability. NTCDI-BSCF₃ based organic thin-film transistors (OTFTs) exhibit much better environmental stability when compared with NTCDI-BOCF₃ due to their high hydrophobicity which prevents the diffusion of moisture and oxygen into the devices. In addition, the electron mobility of NTCDI-BSCF₃ shows good thermal stability in relation to the deposition temperature, and achieves a value as high as 0.17 cm² (V s)⁻¹ in air, although it is lower than that of NTCDI-BOCF₃. The lower mobility may be attributed to the unexpected crystal growth mode after the deposition of the second monolayer and an insufficient quality of the thin films of NTCDI-BSCF₃, especially their inadequate crystallinity. This contrasts with the Stranski–Krastanov (SK) (layer-plus-island) growth mode with the expected crystal growth direction and good crystallinity of NTCDI-BOCF₃. Nevertheless, it can be concluded that the introduction of the trifluoromethanesulfenyl (SCF₃) group at the N-group of naphthalene tetracarboxylic diimide (NTCDI) is an effective approach for enhancing the environmental stability of NTCDI based n-channel OTFTs.