Tuning the ambipolar behaviour of organic field effect transistors via band engineering

We report on a method for fabricating balanced hole and electron transport in ambipolar organic field-effect transistors (OFETs) based on the co-evaporation of zinc-phthalocyanine (ZnPc) and its fluorinated derivative (F8ZnPc). The semiconducting behaviour of the OFET can be tuned continuously from unipolar p-type, with a hole mobility in the range of (1.7 ± 0.1) × 10−4 cm2/Vs, to unipolar n-type, with an electron mobility of (1.0 ± 0.1) × 10−4 cm2/Vs. Devices of the pristine ZnPc and F8ZnPc show a current on/off ratio of 105. By co-evaporating the p-type ZnPc with the n-type F8ZnPc, we fabricate ambipolar transistors and complementary-like voltage inverters. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio with hole and electron mobilities of (8.3 ± 0.2) × 10−7 cm2/Vs and (5.5 ± 0.1) × 10−7 cm2/Vs, respectively. Finally we demonstrate application of the ambipolar devices in a complementary-like voltage inverter circuit with the performance comparable to an inverter based on separate ZnPc and F8ZnPc OFETs.