Learning the making of transparent conductive film for digital devices from mother nature

What is transparent conductive film?

Transparent conductive film is not a material that is generally known to the public, but it is an integral part of the liquid crystal displays used in devices all around us.

It passes visible light and is made of a substance that also passes electricity.

For example, many people imagine metal when they think of a material that conducts electricity. Metal conducts electricity very well, because it has many players that move electrons. However, these electrons’ interaction with visible light prevents visible light from passing through. Therefore, metal is not transparent to the human eye.

On the other hand, let’s think of glass―a transparent material. While glass appears transparent due to it passing visible light, as it normally does not have any player moving the electrons, there is no electricity passing, which classifies it as an insulator.

Then there are some semiconductors―whose electrical resistance value is in the midpoint of those of metal and insulators―that do not interact with a degree of energy as such of visible light. Specifically, it is a substance that passes electricity well―but not as conductive as metal―and transmits visible light.

We artificially create transparent conductive films from such materials and apply them to a variety of industrial products.

For example, a liquid crystal display has a panel structure with liquid crystal sandwiched between transparent conductive films.

The transparent conductive films apply voltage to the liquid crystal to change the sequence of the liquid crystal molecules and control the transmission of light. This means that the material for applying voltage to the liquid crystal must be transparent in order to function as a display.

High-performance thin films, including transparent conductive films, are, in general, made using a method called vapor deposition. It is a highly sophisticated method, but this production process also has a problem.

The problem with the vapor deposition for making transparent conductive films

Today, there are two major vapor deposition methods for producing thin films: physical vapor deposition (PVD) and chemical vapor deposition (CVD).

For example, typical transparent conductive films are made from a metal oxide called indium tin oxide (ITO) using sputter deposition, a physical vapor deposition method. Sputtering is a process where plasma energy (ionized gas molecules) is used on the solid ITO in high vacuum, which causes an impact that displaces the atoms and deposits them onto the substrate, making a thin film.

The advantage of this method is that, because deposition takes place in high vacuum, it is immune from contamination by impurities, which helps create a very clean thin film. As transparent conductive films are commonly used in precision electronic devices, being able to produce a high-quality thin film is a huge advantage.

At the same time, using vapor deposition for the production of ITO film has problems. First of all, the material indium is a rare metal. It is a rare metal that did not see much demand in the past, but starting in the 2000s when the production of liquid crystal panel increased, demand surged. The price of indium skyrocketed, investment in indium started, and indium was entangled in diplomatic strategies.

Research into alternative materials started at that time. For example, thin films made of tin oxide, zinc oxide, and titanium oxide, technologies that makes metal into thin, nano-scale wires that are further fabricated into a mesh, and carbon nanotubes made from carbon were also developed.

However, research into alternative materials eventually dies down in the area of the thin film manufacturing process.

The reason behind this is the development of a material recovery technique for sputtering. In fact, the rate of material usage with the sputtering technique is only a few percent. This is because only the material that was ejected as atoms and was successfully deposited on the substrate is used. That is to say, most of the material ended up as waste, making for an incredibly inefficient method.

However, upon facing crises such as price surge and export controls, improvements have been made to the sputtering technique, giving rise to technology that recovers and reuses the sputtered material. This has significantly reduced the risk of using indium. Nevertheless, the problem of resource limitations, with indium being a rare metal, still persists, and I believe that alternative materials need to be developed.

The other problem, I believe, exists in the essential part of vapor deposition. That problem is that vapor deposition needs high energy to transform the material into a gas, or atomic or molecular state.

Liquid phase method learned from mother nature

The reason why thin film manufacturing techniques PVD and CVD are called vapor deposition is because they turn materials like ITO into a gas. This means that atomic connections need to be forced out of their solid state, turning the material into a high-energy state: atoms and molecules. This process requires large-scale vacuum exhaust facilities and the massive amount of electrical power to operate them.

And if you look at how people are generating power, we are relying on fossil fuel to the most part. This, of course, is not a sustainable energy source.

Transparent conductive films are and will probably be an essential material for the future society. If we go by that, then I believe that we also need the mindset of sustainability in the manufacturing process as well. For that cause, I am studying material making using the liquid phase method.

First of all, thin film fabrication is a technique to deposit some kind of substance on a substrate. As I mentioned earlier, in the case of ITO, ITO as a solid is forcibly broken down into the gas or atomic state for deposition.

Then, if you ask whether this process of decomposing a solid into molecules and atoms and depositing them onto a substrate can only be caused artificially, that is not the case. This is actually a phenomenon that normally occurs primarily through liquid in nature.

For example, human bone is made up as a complex composite of organic matter and hydroxyapatite, a calcium phosphate crystal. While bone formation is achieved everyday inside a human body, this formation process does not require high energy. Among living creatures, there are some that produce inorganic matter such as metal oxides or metal sulfides as part of their body.

Then, how do living things produce inorganic substances? Most natural life forms use chemical reaction in water. In other words, with water acting as the solvent, a living organism effectively accumulates the raw materials―existing in water in their disassembled form―inside its body as material.

Learning from this wisdom of nature, the liquid phase method decomposes the material substance using solvent action and creates the material through re-accumulation and crystallization.

Let’s think about a liquid phase process that uses water as solvent (aqueous solution process) for example. Water is a substance that exists in mass volume on earth, and there is little risk of resource limitation. Additionally, while the boiling point of water is approximately 100 degrees Celsius at atmospheric pressure, since this reaction is mostly used at temperatures below the boiling point in most cases, material can be produced at relatively low temperatures. This means that we can use a resource that is ubiquitously available on earth to produce material without the need for a lot of energy.

Which brings me to the point, by learning from nature and applying its laws, we can create technologies that achieve the sustainability likewise that of nature.

Naturally, many issues exist in the production of materials using such technologies. One is the contamination of produced material by solvent molecules. Specifically, the solvent molecules are taken into the target substance as impurities during the accumulation or crystallization process and become part of it. As another example, while the alignment of atoms and molecules is important, the use of low energy limits atomic and molecular movements, thus making the target alignment difficult to achieve.

We are engaged in research with the aim to solve the many issues, including those above, and apply thin films fabricated via a liquid phase process to functional materials. In particular, we are currently working on creating different electronic materials, including transparent conductive films, using an aqueous solution process as well as base metals.

Today, utilization of computers, smartphones, and other digital devices is promoted for the purpose of increasing our productivity and/or convenience in various aspects of society and our regular lives. One of the fundamental materials for such devices is the transparent conductive film. Unfortunately such transparent conductive films are produced by consuming immense energy and rare metals.

Such issues are not often visible and remain mostly unknown to the public at large. I believe that more people in our positions need to communicate that.

If more people understand that modern product making rests upon mass consumption of energy, I think there would be more interest in the development of technologies such as the liquid phase process deriving from nature. This, I believe, will lead to the consideration of how to achieve a sustainable society.

* The information contained herein is current as of March 2021.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.

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