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CIGS Solar Panel Recycling + Industry and Customer Implications

1954 is the year most commonly recognized as the beginning of the modern solar industry. This is when Bell Labs in the U.S. invented the first practical silicon solar cell.

Since then, the solar industry has flourished, and many new solar cell technologies have emerged, among which is the CIGS solar cell. It is a dominant type of thin-film solar cells that offer high efficiency, competitive product costs, a low carbon footprint, and more.

As CIGS solar cells are made of several elements that are not as abundant as silicon, the recycling of these cells and panels has captured increasing attention within the industry.

In the following sections, we will explore CIGS solar panel recycling, along with some insights and predictions for this particular market segment.

CIGS Solar Panel Recycling
"CIGS Solar Panel Installation" (modified) by Ken Fields, licensed under CC BY-SA 2.0 DEED.

Laying the ground: c-Si solar panel recycling

While the industry itself has been around for about 70 years, the subject of ‘recycling’ only gained significant awareness starting in the 2000s.

The primary material for conventional crystalline silicon (c-Si) cells is silicon, which is the 2nd most abundant material on earth, right after oxygen. These cells are made of silicon atoms connected to one another to form a crystal lattice, providing an organized structure that facilitates the photovoltaic process.

Beyond silicon forming the cells, a complete c-Si solar panel also includes several key components: anti-reflective coating, encapsulant, tempered glass, metal contacts, backsheet, and metal frame. 

Across the segment of c-Si panel recycling, there are two primary methods. One is to remove the junction box and aluminum frame, then crush the module and use it as mixed glass cullet.

On the contrary, the other is a refined and systematic method that involves a series of mechanical separation and ‘thermal-mechanical-chemical’ processes. 

Upon arrival at the recycling facility, the aluminum frame and junction box are removed to segregate different materials for subsequent recycling. After the tempered glass is detached, a combination of thermal, chemical and mechanical techniques are used to remove the encapsulant layer. This step exposes the underlying cells for further material separation, including the extraction of valuable semiconductor materials and metals through chemical etching or melting. Then the separated materials are further processed and purified for reuse.

It’s inspiring that some advanced technologies can now retain around 95% of the value of materials in the panels. Many other improved methods are being studied to propel the recycling segment.

CIGS solar panel recycling: State and hurdles

A significant amount of research and development efforts in the late 20th and early 21st centuries has led to CIGS solar cells reaching efficiencies competitive with conventional c-Si solar cells.

Alongside their increasing market share, early CIGS panels’ approaching end-of-life (EoL) also raises attention to the recycling of these panels.

Recapping the materials and structure

CIGS Solar Cell Structure
Source: ResearchGate

The full name of CIGS is Copper Indium Gallium Selenide. At the heart of a CIGS cell is the CIGS absorber layer, composed of copper (Cu), indium (In), gallium (Ga) and selenium (Se). Other layers within the structure also include:

  • Substrate: It can be made of polymer, glass or metal, depending on the desired configurations in relation to flexibility, stability and cost-effectiveness.
  • Back Contact: It’s also called conductive sheet, which is directly placed on top of the substrate, before placing the absorber material. Molybdenum (Mo) is chosen for its good electrical conductivity and its compatibility with the CIGS layer.
  • Buffer Layer: A thin layer of cadmium sulfide (CdS) is deposited on top of the CIGS, used as a buffer layer between the CIGS layer and the front contact and forming a p-n junction.
  • Window/Protective Layer: Also called Transparent Conductive Oxide (TCO) layer, this layer is made of a highly transparent material such as zinc oxide (ZnO) that protects the buffer from external damage.
  • Front Contact: Materials like indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO), which excel at transparency and electrical conductivity, are generally used for this contact.

As a whole, all these materials, though uncommon, are not particularly rare. However, they have much higher recycling value than the ubiquitous silicon, not just because of economics but also the hazardous potential of some elements if not handled properly.

Current recycling methods for CIGS panels

For CIGS thin-film solar panels, the recycling as well as material reuse are still at their early stages. Most of the current practices are carried out within laboratories.

The methods differ from those used for c-Si panel recycling, although both involve mechanical, thermal, and chemical processes. But one pronounced feature of CIGS panel recycling is that it often relies more on the chemical. 

A team at the Sweden-based Chalmers University of Technology first separated selenium in the cell by oxidation of the CIGS material at elevated temperatures, achieving over 99% recovery. Afterwards, selective electrodeposition was adopted for the separation of the remaining elements. Notably, electrodeposition of copper and indium, using different potentials, led to a near-complete separation of the elements.

The team also tried out high-temperature chlorination and solvent extraction for separation. But it was ultimately concluded that electrochemical separation can yield the best results in the fewest number of steps.

In 2022, a research project funded by the Swedish Energy Agency investigated the recycling of CIGS solar cells focusing on the leaching of valuable metals like silver and indium under mild conditions. The research team successfully achieved complete recovery of silver and 85% recovery of indium using 2 M HNO3 with specific surface-to-liquid ratio (A:L) after 24 hours at room temperature. 

The researchers also pointed out that their method reduces environmental risks and lowers costs compared to other methods. The team also highlights the effectiveness of selective leaching and the need for further optimization on this technique.

Conclusion

While substantial progress has been made, there remains a considerable distance to go before widespread adoption of these recycling technologies.

Nonetheless, CIGS solar panels are demonstrating a promising future.

Apart from the merits outlined at the beginning, CIGS panels also hold other unique strengths over c-Si panels, such as great flexibility, being lightweight, superior visual appeal and better performance in certain circumstances.

With an increase in impetus towards achieving three key objectives: 1) advancing sustainable solar applications, 2) enhancing the economic utilization of this technology, and 3) fortifying regulatory frameworks, more eco-friendly and efficient recycling methods for CIGS solar panels can be expected in the near future.

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