Investigation on bias stress effects in n-type PDI8-CN2 thin-film transistors

In this contribution, we investigate the bias stress phenomenon in n-type PDI8-CN2 thin-film transistors fabricated by evaporation on both bare and hexamethyldisyloxane (HMDS)-treated SiO2 gate dielectrics. Since the morphological properties of PDI8-CN2 films are poorly influenced by the SiO2 treatment, all the differences observed in the DC electrical response and the bias stress performances of these devices can be mainly ascribed to the interface chemistry between the dielectric and the semiconductor. In long-term bias stress experiments, performed in vacuum keeping the devices under fixed voltage polarization, the I-DS(t) decaying behavior shows to saturate when transistors on HMDS-treated substrates were considered. According to our findings, the BS physical origin is related to the occurrence of electrochemical reactions where PDI8-CN2 molecules interact with H2O, producing O-2 and protons (H+) which can initially diffuse in the SiO2 layer barrier. Hence, the possibility that the bias stress effect in these n-type devices can be ruled by the H+ back-diffusion process, occurring from the SiO2 bulk towards the dielectric-semiconductor interface during the prolonged application of positive V-GS voltages, is discussed. (C) 2012 Elsevier B.V. All rights reserved.