Sputtering Targets Inside Solar Panel Manufacturing: What Are They, Why Do They Matter

The solar energy you might imagine in your brain, big blue wafers of silicon, eventually will be supplanted by the ability to see better performance from films that are simply invisible to the naked eye. You’ll sputter onto your panels layers that conduct electricity, layers that block moisture, and layers that protect the underlying materials so that your solar panel lasts 25 years versus 5 years.

For a thin-film solar panel manufacturer, or someone trying to optimize efficiency, all of that might require knowing about the sputtering targets that convert real metal/ceramic into functional layers on a solar cell. This essay will cover it all, including materials, sputtering process parameters, and quality metrics for sputtering targets to enable high-yield manufacturing.

Quick Primer: What is Sputtering?

Sputtering is a manufacturing process that deposits a super-thin, uniform coating onto a substrate. It’s similar in purpose to a process called evaporation (which we’ll compare against), but unlike evaporation, sputtering physically ejected atoms off of a solid source material rather than thermally vaporizing them.

The mechanism is straightforward: sputtering involves creating a vacuum chamber where energetic particles, typically ions from a plasma, are accelerated towards a solid target material. The collisions cause atoms from the target to be ejected and subsequently deposit onto the surface of a substrate, such as glass or plastic. The end result is a thin film that coats the substrate.

Sputtering is particularly useful in the context of solar photovoltaics (PV) because it can deposit consistent coatings of materials that are difficult to evaporate thermally. Sputtering can handle the deposition of insulating/oxide layers that are problematic for other approaches.

What is a Sputtering Target?

A sputtering target is the physical piece of solid source material inside the vacuum chamber that serves as the “target” in the process described above. The material that will ultimately be ejected is called the “sputtering target.” It is most often a flat plate or a cylindrical bar that’s mounted in the vacuum chamber.

The sputtering target is negatively biased, and when the process initiates, positive ions strike the target, causing atoms from the target to be ejected and deposited onto the substrate. (Yes, for those who know, “magnetron sputtering” is the common configuration, but more on that later.)

You also have the option to add a reactive gas to do reactive sputtering, and sputter depositing oxides/nitrides etc, but this introduces complexity (target poisoning). There are power supply techniques (RF, pulsed DC) that help with deposition of insulating layers, but generally, sputtering targets are used for a variety of materials that are sputtered onto solar panels.

Where Do Sputtered Layers Show Up in Solar Panels?

Solar panels are composed of complex stacks of thin films, and sputtering is commonly used to deposit certain layers with tight control. Materials like molybdenum (Mo), transparent conductive oxides (TCOs), buffer layers, and many other functional coatings can be applied using sputtering.

There’s also deposition of barrier layers to prevent moisture ingress and improve durability. The application overview is quite broad, and a materials science deep dive would be warranted to capture all the layers.

Sputtering Target Catalog:

• Mo (and other metals)
• ITO, AZO, ZnO (TCOs)
• Absorber-related layers for CdTe, CIGS, a-Si, or other thin-film stacks
• Contact layers
• Buffer layers
• Barrier layers and much more

Target Materials, Quality, Defects, Process Control, and Variation

There are a plethora of target materials related to solar photovoltaics, including transparent conductive oxides, contact metals, absorber precursors, and more. Understanding how the sputtering target material composition, purity, microstructure, and density affect the deposition process and the resulting film’s optical/electrical/barrier properties is crucial for achieving low variability, high yield, and overall process stability. The goal is to minimize variation that can affect the uniformity and performance of the sputtered film.

Caveats: What Solar Panels Use Sputtering Targets (and What Don’t)?

Not every solar panel type utilizes sputtering targets in the same way:

• Thin-film solar panels, for example, rely heavily on various sputtering-deposited layers.
• Silicon solar cells might only incorporate sputtered layers in advanced or specialized cell designs, not for the primary absorber.
• Tandem solar cell stacks might employ sputtering processes for certain layers where sputtering offers advantages in uniformity or process compatibility without damaging underlying materials.

Why Sputtering Target Quality Matters

Insight into why the quality of sputtering targets affects better or worse outcomes in uniformity, defect rates, impurity contamination, arc frequency, surface roughness, adhesion, pinhole formation, and interactions at material interfaces. How these factors impact the performance and longevity of solar panels through their layered structures. Quality matters for efficiency, yield, and reliability, including film uniformity, density, purity, defects correlation, and interface importance.

Sustainability Impact of Sputtering Targets in Solar

Exploration of how the choice and quality of sputtering targets and the resulting layers impact the sustainability of solar panel technology (durability, circularity, and efficiency). This includes:

• Durability: Longer-lasting panels = better sustainability.
• Material Circularity: Recycling and reclaiming critical materials.
• Efficiency: Maximizing sputtering target utilization efficiency and other environmental considerations.

FAQ

1. Do all solar panels use sputtering targets? 

No, it varies by technology. Thin-film relies on them heavily, while traditional silicon uses them more selectively.

1. Are thin-film solar panels “greener”? 

It’s complicated and situational, depending on energy payback time and material sourcing.

Next Steps: How to Put This Into Action When Sourcing/Specifying Sputtering Targets

Practical guidance on sourcing and specifying sputtering targets for PV manufacturing:

1. Start from the needs of the layer, not just the material name (ITO/Mo/etc.).
2. Reactive Sputtering: Yes, you can do reactive sputtering and get oxides/nitrides, but what does that imply for stability, and how do you implement it? (RF sputtering? DC pulsed?)
3. Characterization: Targets are characterized by quality properties, and the stability of the sputtering process depends on these characteristics.
4. Requirements: Build requirements based on stability, repeatability, and consistent properties that span multiple runs.
5. Acceptability Criteria: Define criteria for new sputtering target lots (dimensions, flatness, deposition properties, optical/electrical characteristics, adhesion, etc.).
6. Fault Correlation: Define how certain properties correlate to faults (arcing, particle defects, poor adhesion, moisture ingress, etc.).
7. Adhesion Layers: Discuss the role of adhesion layers like Chromium (Cr) to improve adhesion of Mo on glass (hint: we have target sources for that too).