The Functional Hydrophobic Buffer Layer for Enhanced Performance of Organic Thin Film Transistors.

Abstract

Over the past decade, organic thin film transistors (OTFTs) have attracted much attention for their versatile usages and remarkably progressed in their performances. Especially, to increase the characteristics of OTFTs, various approaches to modify the surface of dielectric layer artificially have been extensively studied because it is a powerful and simple method to improve characteristics of organic semiconducting layer. By controlling the surface characteristic of dielectric, we can obtain a well-organized structure of organic semiconducting layer which is critical to achieve the high performances of OTFTs. Many methods such as self-assembled monolayer and plasma treatment have been investigated so far. However, these conventional techniques are not suitable for practical applications because of inevitable damages of dielectric layer during the process, and complex procedure. In particular, these cause the poor ordering structure and the reduction of insulating property of organic semiconducting layer, which result in the decreased mobility and poor on/off current ratio of OTFTs. In this research, we introduce the functional buffer layer to modify the surface characteristic of dielectric layer without damaging it. We prepare the material named H1 by using an ingredient of water-proof agent. It has a high hydrophobicity which is known good to improve the characteristic of the organic semiconducting layer. Also, it does not damage the insulating property of the dielectric layer unlike the other methods. As the thin film of H1 can be simply formed by spin-coating and be cured by low temperature process, this method is very powerful to enhance the device performance with simple process. To verify the affinity change of the dielectric layer, we measured the change of the contact angles on the conventional organic insulating material (polyvinyl alcohol, PVA) by introducing H1. A PVA was used as the dielectric material due to its good insulating property. The contact angle was drastically increased from 40° to 107° after spin-coating H1. The unit capacitances of H1 coated PVA and bare PVA were measured as 1.99 x 10-8 F/cm2 and 1.94 x 10-8 F/cm2 at 1MHz, respectively. This variation is very small compared to other results. These show that the hydrophobic characteristic of the dielectric’s surface can be successfully obtained by introducing the proposed buffer layer without losing the insulating property of the device. In final, the OTFTs were prepared by using the pentacene as an organic semiconductor. From the electrical transfer characteristics, the mobility of pentacene on PVA and H1 coated PVA were 6.5 x 10-3 cm2/Vs and 0.18 cm2/Vs, respectively. Almost 30 times increased mobility of OTFTs was obtained by using proposed method in our experiment.