Flexible Accumulation and Depletion Mode Organic Field Effect Transistors With Tunable Threshold Voltage

摘要

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~R), 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. Despite the identical conjugated moiety, the two materials gave rise to a very different electrical behaviour, OFETs comprising pure 6T channels exhibited a slightly negative threshold voltage (V_T), thus working in p-type accumulation mode. When only DH6T formed the channel, large positive V_T 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. DH6T has a 0.15 eV lower ionization energy than 6T, consequently, DH6T is more easily p-doped by atmospheric oxygen than 6T, explaining why pure DH6T OFETs exhibit depletion mode operation (heavily doped) and pure 6T ones are accumulation type (intrinsic). As a result, for OFETs with mixed 6T:DH6T channels we found a linear dependence of V_T on the 6T: DH6T ratio. Moreover, the hole mobility was essentially constant for all mixing ratios. 6T and DH6T co-deposited films have very similar structural and morphological properties as the pure materials films, and the two molecules form intercalation compounds. Our work 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. A rational approach to precisely control OFET performance has thus been established. Moreover, this approach is not influenced by the particular substrate employed as gate dielectric, and it is therefore suitable for tuning the electrical properties in flexible plastic devices, providing a considerable extension of organic electronics application-potential.