Transparent Conductive Thin Films
Developing for the Future
With the maturation of Indium recycling and the stabilization of reactive ZnAl and dc-sputtered AZO (aluminum-doped zinc oxide), a majority of coating applications are able to enjoy solutions bound only by the size, cost and annealing required to maximize spectral properties at the proper resistivity. In this article, we will review the issues relating to coatings on polymers, the status of the oxides, and the challenges of producing conductive coatings that retain high transparency farther into the Infrared region. (For more background, see our previous discussion in Transparent Conductive Coating that focused on ITO (Indium Tin Oxide) and AZO (Aluminum Doped Zinc Oxide).
ITO Is Expensive
In the not so distant past, reliance on newly mined and refined Indium, coupled with rapid expansion of the display industry, often caused the price of Indium and ITO to fluctuate wildly. Even with the recycling of Indium helping to stabilize supply and robust ITO TCOs produced by planar and cylindrical sputtering, the threat of price volatility is still of critical concern to cost-conscious users. Because Indium is a key component which is rare and expensive, finding a replacement and optimizing an innovative route - coupled with refinement - remain on the front line of development.
There are cases of successfully addressing the balance of cost and performance. One contemporary solution can be seen in the novel use of TCO coatings on polymer substrates. Pictured is a specialty approach wherein highly uniform, single-digit resistivity coatings are placed on thermally sensitive polymer substrate. A conventional ITO process can potentially crosslink, melt or distort the substrate – thus limiting performance and erasing the advantage offered by a lightweight component. This process circumvented such issues. It is hoped that future technological developments will further resolve cost challenges for the expanding marketplace.
Specialty High-Speed Web ITO TCO Coater Photo Credit: Buellton Advanced Materials
New Approaches to Indium Tin Oxide (ITO) Materials
Lightweight, high performance optical components are increasingly in demand by a variety of industries where sputtering ITO materials play a critical role in display, lighting and OPV technologies. Moldable, durable and conformal coatings are commonly used for applications in defense, communications and sensing. Materion has responded to diverse market requirements stretching beyond the classic ceramic 90/10 ITO sputtering targets with custom oxide mixtures or doped ITO and Titania targets. In addition, since AZO cannot be consistently Ebeam deposited, a 90/10 ITO evaporation material has been developed for mobile electronics and remote sensing.
After years of development, AZO is perhaps the most successful ITO alternative in mass production, providing lower cost and addressing temperature issues. It offers metal reactive and full oxide sputtering, once such factors as low melting point or target cracks and particles are addressed. Indium & Gallium can be mixed with Zinc Oxide to improve CIGS (CuInGaSe) PV module performance. In addition, sputtered AZO has helped CdTe and CZTS modules use more of the IR photons which are typically reflected by ITO.
While there has been many successes, there are still coating applications that AZO/ITO cannot satisfy that may be addressed in the future. Two examples are electrochromic (EC) glass and Infrared (IR) TCOs. Beyond performance and size challenges, the relatively thick coatings required for EC glass make ITO cost a significant concern. For IR TCOs, increasing IR transparency remains an important technical challenge. There is a point at which sophisticated fine metal meshes or nano-films can exceed most current specialty oxide mixtures. However, the lower transmission of high line counts or higher absorption of non-uniform layers, may reduce yield or outpace cost versus performance metrics.
Going forward, newer requirements and the segmentation of the transparent conductive film industry will continue to drive innovation. The need for higher conductivity films that retain transparency and transmit deeper into the IR region will continue to put pressure on thin film developers. Disruptive engineered technologies will reduce the use of vacuum processes to only where absolutely necessary. Advancements in lighting and display will necessitate minimal optical losses, maximum light extraction and lower materials costs. For shielding and sensor packaging, lightweight, high performance, conformal solutions will be ever more critical. All these areas will necessitate advanced technical solutions.
At Materion, we are dedicated to using our industry-leading expertise in thin film technology to address these challenges. For more information about our coating materials and services, please contact our Applications & Materials Scientist by email: David.Sanchez@Materion.com.