Flexible accumulation and depletion mode organic field effect transistors with tunable onset-voltage

The emerging appealing possibility of exploiting organic heterojunctions for obtaining new electronics properties derived from the interface between different semiconductors is going to add a further degree of freedom for improving OFET performances. In this paper we report on the possibility to use organic bulk heterojunctions of two derivatives of a conjugated molecule for tuning and controlling charge carrier population and transport within the active layer. In particular, we report on the fabrication and operation of a series of OFETs, which exhibit (i) accumulation and depletion mode operation, (ii) wide tunability of threshold voltage, (iii) mechanical flexibility. We fabricated OFETs on plastic substrates (Mylar®), acting at the same time as gate dielectric, by co-depositing two organic semiconductor materials, sexithiophene (6T) and α,ω-dihexylsexithiophene (DH6T), at various ratios. OFETs comprising pure 6T channels exhibited a slightly negative threshold voltage (VT), thus working in p-type accumulation mode. When only DH6T formed the channel, large positive VT was observed, giving evidence that an accumulation of p-type charge carriers was already present without applying any gate bias and that these OFETs work in the depletion regime. Photocurrent spectroscopy measurements confirmed that a larger concentration of charge carriers is present in DH6T OFETs. For OFETs with mixed 6T:DH6T channels we found a linear dependence of VT on the 6T: DH6T ratio. Moreover, the hole mobility was essentially constant for all mixing ratios (≥ 5x10-3 cm2/Vs), and even higher than for pure 6T flexible OFETs (3x10-3 cm2/Vs). X-ray diffraction and atomic force microscopy studies showed that 6T and DH6T co-deposited films (on Mylar® and Si-oxide) have very similar structural and morphological properties as the pure materials films, and that the two molecules form intercalation compounds. This is particularly interesting because it demonstrates the possibility to adjust the device working point and tune its operational mode without negatively affecting charge carriers transport across the channel. Our work shows that extremely wide OFET function tunability can be achieved by combining molecules with appropriately adjusted properties in molecularly mixed films. A rational approach to precisely control OFET performance has thus been established, providing a considerable extension of the application-potential of organic electronics.