Modulation of carrier mobility of diketopyrrolopyrrole and quaterthiophene containing copolymer with self-assembled monolayers on gate dielectrics of thin film transistors

We fabricated organic thin-film transistors (OTFTs) using diketopyrrolopyrrole (DPP) and quaterthiophene (QT) containing a conjugated polymer, poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt- 2,2′:5′,2″:5″,2â€́-quaterthiophene] (P(DPP-alt-QT)), as an active layer, with chemically modified SiO₂ gate dielectrics. The effect of the surface treatment of SiO₂ with alkyltrichlorosilanes on the electric characteristics of P(DPP-alt-QT) was investigated. The self-assembled monolayers (SAMs) were elaborated with n-octyltrichlorosilane (OTS) and n-octadecyltrichlorosilane (ODTS) on the SiO₂ surface. After the treatment with SAM, the contact angles and surface free energy of the dielectric layers were measured to study the variation in the surface properties. Using a high-performance P(DPP-alt-QT) polymer, we fabricated thin-film transistor devices with the following dielectric layers: bare SiO₂/Si, OTS-SiO₂/Si, and ODTS-SiO₂/Si. In the electrical measurements, typical I-V characteristics of TFTs were observed. The values of best field-effect mobility, μ, were 1.13 and 3.46 cm² V^-1 s^-1 for P(DPP-alt-QT)-based TFTs containing OTS- and ODTS-treated SiO₂ dielectric layers, respectively. The ODTS-treated TFT device with the lowest dielectric surface free energy exhibited the highest mobility among the three devices constructed using three different dielectric layers.