Transparent Conductive Films and Smartglass

Transparent conductive films are the ‘secret superpower’ of the smartglass sector. They enable the electrical connection of auto-tinting, privacy, or photovoltaic layers, while maximizing daylighting for sustainable green architecture.

In a Nutshell

When we look through a smart window and discover internal layers that conduct electricity, we might ask ourselves:

Where are the electrical conductors?

The answer lies in internal films of electrically-conductive materials which are transparent enough to transmit daylight.

The following diagram shows the typical structure of an auto-tinting smartglass panel with a switchable interlayer sandwiched between transparent conductive films. 

Two further layers of glass complete the laminated structure.

The structure below is typical of dye-doped liquid crystal, PDLC and SPD smartglass, but we can also find transparent conductors in electrochromic and transparent photovoltaic smartglass.

indium tin oxide ITO smartglass structure

Basic structure of smart glass (plastic protection layers not shown for clarity)

Transparent conductive thin films

Transparent conductive thin films find uses not only as electrodes in smartglass and in photovoltaic solar panels, but also in the following applications:-

  • transparent antennae in automotive windshields
  • infrared blocking layers for smart homes and automotive applications
  • flat panel televisions
  • antistatic coatings
  • transparent heating elements for auto glass defrosting 
  • transparent electromagnetic interference screens for displays, monitors, screens
  • transparent electrodes for capacitive touch screens in mobile devices

In this article, we will learn why transparency and electrical conductivity are actually opposing trade-offs.

First, let’s look at the most prevalent transparent conductive material in use today, Indium Tin Oxide.

Indium Tin Oxide (ITO)

The most common transparent conductive material at the moment is Indium Tin Oxide (called ITO in the industry), a compound of indium oxide (which is transparent in thin films) and tin oxide, which increases the overall electrical conductivity of the material.

Currently, ITO offers the best combination of:

  • high optical transmittance (typ. 80% or more)
  • low electrical resistivity (<0.05 ohms per square)
  • low haze (<2%)
  • good chemical and mechanical stability
  • strong resistance to corrosion
  • infrared (solar heat) reflection of up to 80%

Typically, we deposit ITO on glass by either magnetron sputtering or pulsed-laser deposition (PLD).

ITO Supply Chain

Unfortunately, ITO suffers from volatile market price fluctuations due to its sparse distribution in the earth’s crust and is mechanically fragile, which makes it unsuitable for very large area displays.

Indium is not officially one of the 17 rare earth elements but is considered ‘rare’ by some due to its relative difficulty and cost of mining. Indium is in fact more abundant than silver, gold and platinum (but less abundant than copper or aluminium).

The current (April 2024) price of Indium is USD 573 per kg which represents a 50% increase since Jan 2018 but a substantial drop since its peak price in 2014 (see the graph below). 

Indium is mined across Japan, Canada, Peru and Mexico, with approximately 70% coming from China, and about 50% of all indium being refined globally for manufacture of ITO.

market price indium

Indium market price, showing minimum purity (source: Strategic Metals Invest)

Alternatives to ITO

For many years now, the industry has been looking for alternatives to ITO, including both inorganic and organic materials. 

Below we can find some of the most promising technologies. 

Future articles on Smartglass World will centre on some of these in more detail.

Inorganic:

  • fluorine doped tin oxide (FTO)
  • aluminium zinc oxide (AZO)
  • metallic nanowires
  • nanowire meshes
  • metal grids

Organic:

  • carbon nanotubes
  • conducting polymers
  • graphene

Bandgap

Fundamentally important to the functioning of any transparent conductive film (TCF) is the bandgap, which is the ‘energy gap’ between:

  • the valence band (the upper band occupied by electrons where they are still ‘bound’ to an atom or molecule) and 
  • the conduction band (the band in which electrons can move freely and conduct electricity).

Solid state electronic bandgap

The separation between the conduction and valence bands is the bandgap and represents the energy that electrons would need to absorb from incoming light photons in order to ‘jump up’ to the conduction band and become ‘free electrons’.

Why does ITO transmit light?

For a material to be transparent, photons of light cannot be absorbed or reflected but rather must be transmitted unimpeded through a material.

For this light transmission to happen, the energy of each photon must be less than the bandgap of that material, measured in electron-Volts (eV). 

This means that the photon will not have the energy needed to excite electrons from the valence band up to the conduction band. The photon therefore does not interact with free electrons and is not reflected or absorbed.

It is transmitted.

And since visible light has wavelengths of 380 nm to 760 nm, this corresponds to photon energies of 1.6 eV to 3.2 eV.

Therefore, the bandgap (in the visible region) must be greater than 3.2 eV in order for the material to transmit photons of visible light.

ITO has a bandgap of 4.0 eV which explains why it is transparent to visible light.

Why does ITO reflect infrared?

This does not explain why ITO reflects infrared, when infrared photons have less energy than visible light. Surely the same applies to infrared photons?

The explanation is simple:

ITO actually reflects infrared due to the (small amount of) ‘free electrons’ already present in the material (thanks to the tin oxide), which oscillate in response to the incident infrared waves, leading to absorption and reflection of infrared only (but not visible light).

Net result:

ITO transmits visible daylight into a building beautifully, but rejects the solar heat that can cause overheating and excessive air conditioning costs.

How cool is that?

Furthermore, researchers can engineer a variety of optical properties (e.g. transmittance, heat rejection, haze), based on the requirements of each application. 

This results in products that companies can ‘fine-tune’ according to individual market needs by altering the proportions of the constituent elements

Trade Off

So, we see that transparency and conductivity are actually at odds, since transparency to visible light requires a large bandgap but conductors imply a small bandgap.

ITO does both, by having a large bandgap in the visible range but (thanks to the tin oxide) having a higher electrical conductivity than the indium oxide alone, turning the material from an insulator to a semiconductor.

Just as an aside, metals have no bandgap (their valence and conduction bands actually overlap), allowing for their high conductivity.

ITO is not a metal, but a metal oxide (classified as a semiconductor), having an electrical conductivity between that of metals and insulators.

In fact transparent conductive films in general occupy a strange niche, seemingly giving them ‘superpowers’, without which smartglass would simply not exist.

Outlook

The smartglass industry is heavily invested in Indium Tin Oxide at the moment but research is advancing in alternatives that exhibit better light transmittance, electrical conductivity and mechanical stability.

We should remember that any transparent conductive layer represents only a small section of the overall smartglass stack. Even drastic improvements may be thwarted by the limitations of the other material layers present in the stack.

Having said that, it is unlikely that indium tin oxide will be completely replaced any time soon. Despite the volatile market prices, indium may still be a pretty good commodity to invest in for the time being.

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