Improvement of Electrical Characteristic of P3HT Organic Electrochemical Transistors with Ionic Liquid by Slow Cooling in Annealing Process

Organic field-effect transistors (OFETs) are widely studied because they have a potential to realize a light-weight, flexible, and large-area devices. Although it is expected that they are applied to an organic electroluminescence display system, this requires further low voltage and high current operation. Recently, organic electrochemical transistors (OECTs) using an ionic liquid have been reported, which are operated by an electrochemical doping into an organic active layer, and they can flow a large driving current [1]. We previously showed that the field effect mobility of OFETs of poly (3-hexylthiophene-2,5-diyl) (P3HT) was improved due to ordered P3HT crystals induced by slow cooling from a temperature above a glass transition point [2]. In this study, we fabricated P3HT OECTs with an ionic liquid, by varying conditions of annealing, and investigated an effect of P3HT crystallinity on characteristic of P3HT OECTs.

We fabricated a bottom-contact and top-gate type of P3HT OECT samples as shown in Fig. 1. The P3HT layer was formed by spin-coating P3HT solution in 1,2-dichlorobenzene solvent (10 mg/mL) on the Au/Ti/glass substrate treated with hexamethyldisilazane (HMDS), and by annealing in a nitrogen atmosphere. These samples were annealed at different temperatures (110-300 °C ) and cooling rates (0.5 °C/min. or immediately stopped). Acetone in which an ionic liquid of 1-Butyl-3-methylimidazolium hezafluorophosphate ([Bmim][PF6]) and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) were dissolved, was spin-coated on the P3HT layer and annealed to form the layer of an ion gel immobilized by PVDF-HFP. Al was finally evaporated on the ion gel layer as a gate electrode.

Figure 2 and 3 show transfer characteristic and transconductance of the fabricated OECTs, respectively. The drain current and transconductance of the sample annealed at 250 °C and slowly cooled (at the rate of 0.5 °C/min.) was drastically improved, compared with the sample annealed 110 °C and rapidly cooled (immediately stopped). Based on the result that both the field effect mobility of P3HT OFETs and P3HT crystallinity were improved by the same annealing condition [2], we can assume that crystallizing the P3HT layer contributes to improving the characteristic of OECTs. As you can see, the threshold voltage in Fig. 2 was also lower in the case of annealing of 250 °C-slow, indicating that the P3HT crystallinity was improved due to the fact that an electrochemical doping is priorly occurred into an area of higher crystallinity polythyophene [3].

In summary, we conclude that increasing a crystallinity of an organic active layer by slow cooling in annealing process plays an important role in improving an electrical characteristic of OECTs as well as OFETs.

Acknowledgements

This work was partially supported by JSPS Core-to-Core Program, A. Advanced Research Networks "International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration". 

References

[1]  J. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. Renn, T. Lodge, C. Frisbie, Nature Mater., 7 (2008) 900-906.

[2]  S. Iino, D. Tadaki, T. Ma, J. Zhang, Y. Kimura, M. Niwano, #2183, 224th ECS Meeting.

[3]  X. Jiang, Y. Harima, K. Yamashita, Y. Tada, J. Ohshita, A. Kunai., Chem. Phys. Lett., 364 (2002) 616-620.