All-printed liquid crystal display devices made from sol-gel Indium tin oxide(ITO) ink

N. Xia, R.A. Gerhardt
Georgia Institute of Technology,
United States

Keywords: ITO ink, inkjet printing, transparent, conductive

Summary:

Indium tin oxide (ITO) is the most widely used transparent conducting oxide due to its high conductivity and transparency. It is currently used in a wide range of devices that require transparent conducting electrodes. Currently, the commercial ITO coating methods (e.g. sputtering) not only require strict vacuum conditions and the use of high cost equipment, but such a method also causes a large amount of ITO to be wasted during the process. Since solution-based fabrication technology can greatly lower the cost and simplify the process, we have fabricated ITO films and patterns onto glass substrates using an ink-jet printing method utilizing a custom-made sol-gel ITO ink based on a recently published new formulation. When commercial ITO films need patterning, they require photolithography steps as well as etching steps which causes more material to be wasted. In contrast, our ITO inkjet printed patterns only need simple printing and controlled annealing steps. We have found that the thickness of the ITO films and patterns can be well controlled by the printing resolution and the number of printing layers. All of the annealed ITO films and patterns that were inkjet printed were highly transparent with more than 90% transmittance at 550nm, as measured by UV-vis spectrophotometer. The sheet resistance of the ITO films was tested by the four point probe DC method and was around 500 ohms/sq, The ITO films had relatively uniform surface morphology and less than 400nm total thickness when five layers were deposited. The surface structure and cross sectional thickness of the films was characterized by FE-SEM. Other techniques like atomic force microscope (AFM) will be used to investigate more details about the conducting network on the sample surfaces in the future. In order to demonstrate the quality of the optical and electrical properties of these ITO films and patterns, simple LCD devices were fabricated and tested. These devices show clear images that shift in contrast when connected to a voltage source Thus, we have demonstrated that LCD devices can be fabricated by an all-printing method. Other devices will be tested in the future.