Graphene: Almost The Ideal Replacement for Indium Tin Oxide (ITO)

ITO replacement is also a large and growing market, predicted to reach $8.1 billion by 2021. Image Credits: Bevan Goldswain/shutterstock.com

Indium tin oxide is a transparent conducting material used in most of the world’s touchscreen applications. While it is fantastic material, it does have some limitations.

Firstly, it is expensive, as indium is a scarce commodity. Secondly, it has limited flexibility before it begins to crack, and thirdly it requires deposition in vacuum by physical vapour deposition (PVD), which takes time.

As a result there has been major interest over the last 15 years in using various forms of nanomaterials as a replacement. The ideal material would be cheap, transparent and highly conductive - a combination which has so far eluded most developers.

ITO replacement is also a large and growing market. Touch Display Research forecasts that the non-ITO transparent conductor market will reach $8.1 billion by 2021.

Is Graphene Good Enough?

Of course, the transparent conductive electrode doesn’t have to be 100% transparent to work. ITO lets through almost 90% of the light and if a display has backlighting the transmission can be even lower.

To further widen the field of opportunities, some applications require UV or infra red light to be transmitted.

There are a number of technologies being developed to address this market, many of which are based on some form of nanomaterial and involve printing or coating processes which can be performed continuously, unlike PVD.

The main approaches, as well as graphene, include carbon nanotubes, metal meshes, silver nanowires, and conductive polymers.

ITO Replacements Are A Competitive Market

Cambrios, Blue Nano and Carestream Advanced Materials are all taking the silver nanowire approach. The idea here is to have a highly conductive material such as silver, in a form where it imparts sufficient conductivity to a polymer without affecting its optical properties.

A similar approach has been tried with carbon nanotubes by companies such as Eikos and C3Nano, but the sticking point has often been that the loading of nanotubes required to make the material conductive has degraded its optical properties.

Other companies such as Hereaus are looking at conducting polymers such as their Clevios material.

Getting Graphene Into the ITO Replacement market

Graphene, a single layer of atoms thick, combines transparency with conductivity, making it an obvious choice for research into ITO replacement - it could even be considered the perfect material, at least in theory. There are three broad ways to get graphene into a form that would be useable in displays.

The ideal method would be to deposit a single layer of graphene onto a large substrate, lift this off, and place it onto the front of the screen material where it could then be patterned if necessary.

The problem lies in handling these atom thick layers of graphene. The sheet would have to be removed from the substrate on which it was grown and transferred elsewhere, all without introducing defects which would severely degrade its performance.

While the problem is not insoluble, and this is the source of much research in programs such as the EU Graphene Flagship, at present there is no reliable and cost effective way of doing this.

While there is a window of opportunity for graphene, it is one that is beginning to narrow. Image Credits: shutterstock.com/Ambelrip

A second approach is to use graphene nanoplatelets (GNPs). GNPs are available from an increasing number of suppliers with prices for some types forecast to fall to around $100/kg in the near future, thus addressing the cost issue.

The idea is to formulate the GNPs into ink, which can then be applied to a polymer substrate and patterned, ready for integration into a display. While that may move graphene based solutions to the left in the diagram above, there are some major issues remaining.

The conductivity of GNP based inks is currently rather lower than that required for ITO replacement, which is approaching ten ohms per square. GNPs also exhibit a wide variation in conductivity, depending on their production method, their surface chemistry, edge roughness and size which can range from sub micron to tens of microns in diameter.

To achieve the right conductivity, a relatively high loading of GNPs is require, as with carbon nanotubes, which can give the films a grey cast and reduce light transmission. There have been attempts to dope, or indeed mix GNPs with other materials such as silver to improve conductivity, but this negates any cost savings.

A third approach has been reported by Samsung, who claim to have found a way to synthesise single crystal graphene onto semiconductor wafers. While this may pave the way for graphene electronics, it is currently still very much at the research stage.

The Opportunity Won’t Be Around For Long

While there is a window of opportunity for graphene, it is one that is beginning to narrow. There are plenty of other ITO replacement materials available, from silver nanowires to conductive polymers, and all are much closer to market than graphene.

The danger here, as with many emerging technologies, is that when the business starts you are addressing an unmet industry need, but by the time your product is ready the need no longer exists.

In this case, the unmet need is to move away from the price volatility and scarcity of indium, not to create a touch screen display.

If any of the competing approaches meet that need in the next few years then graphene based solutions may be irrelevant - unless they can be so much better or cheaper than the other approaches, which by then will be the industry standard.

Companies looking at graphene need to get their skates on if they want to be a contender to replace ITO. While at the moment the market is wide open, the accessible market is shrinking as other technologies gain market share.

While graphene will almost certainly be part of the solution to ITO replacement, whether it is a mainstream contender, a niche player or a dead duck depends on how fast someone can get a compelling product to market.

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