Low voltage, low cost, flexible and balanced ambipolar OFETs based on Br2PTCDI-C18/CuPc fabricated on an Al foil gate substrate with good ambient stability

Abstract

Low cost, flexible, ambient stable, low operating voltage and balanced ambipolar organic field-effect transistors were fabricated with plastic over-head projector (OHP) sheets as transparent substrates using laminated Al foil as a gate electrode without any mechanical or electrochemical polishing, using Br₂PTCDI-C18 (n-channel) and copper phthalocyanine (CuPc) (p-channel) as the active materials and PMMA/PVA or C-PVA (crosslinked PVA) as the dielectric material. The optimized Br₂PTCDI-C18/CuPc heterostructure OFET devices exhibited an ambipolar nature with operating voltages of ±10 V and ±3 V for the PMMA/PVA and C-PVA dielectric materials respectively. The electron and hole mobilities achieved are as high as 1.2 × 10⁻² cm² V⁻¹ s⁻¹ and 5.5 × 10⁻³ cm² V⁻¹ s⁻¹ respectively. These ambipolar devices were also studied under different conditions (vacuum and ambient) and the results are reported here. The hole currents exhibited an anomalous bias stress effect whereas the electron currents decayed with time under vacuum. The anomalous bias stress was explained with the slow polarization of dipoles in the polar dielectric materials. However, under humidity, both currents decayed due to charge trapping at the interface by absorbed water molecules. The devices exhibited reproducible results in the ambient range even after several days of exposure. These results demonstrate the very easy fabrication of ambipolar devices on a low cost substrate and gate with good ambient and electrical stabilities, which are highly desirable for practical applications.