Since the beginning of this century, increasing numbers of people have installed solar panels on the roofs of their houses. As most of the panels have been produced relatively recently, there is not so much data available on the lifetime, but their energy generation is known to reduce with age. At some point these solar panels will have to be removed, a process which may begin in the next ten years. But how will they be recycled?
The EU-project CU-PV currently underway at imec, a nanotechnology research center based in Belgium, tackles this problem: How to recycle today’s solar panels, and one step before this, how to design solar panels which are more recyclable.
This project aims to maximize energy production, while also providing a framework for reducing the environmental footprint in manufacturing and recycling of photovoltaic devices. The means of doing so involves limiting the materials consumed, avoiding use of rare or toxic materials, combined to yield high efficiency devices. Back-contacted (BC) solar cells enable such high conversion efficiencies which makes them ideally suited for such applications. Today, such cells comprise 5% of the PV market share, but this is expected to increase beyond 20% over the next ten years. Imec started working on back contacted silicon solar cells in 2007, from which time the focus was on attaining high efficiencies. Now, we are working on large-area BC solar cells, and simplifying the process so that it can be used by industry.
There are three main measures that can be taken to make solar cells more recyclable:
- Replace silver with copper
- Reduce thickness of cells
- Remove lead
Replace silver contacts by copper contacts
Many of today’s solar cells are contacted on the front-side using thin layers of silver. However, silver is a precious and rare material, and consequently constitutes a significant amount of a solar cell cost, which can vary with market conditions. To make solar cells more sustainable (and cheaper), techniques are being explored to use copper-based contacts instead of silver. Imec has developed a Ni/Cu/Ag plating process to make copper contacts at the front of solar cells. First, Ni is deposited as contact layer to the underlying silicon. It also serves as diffusion barrier for the plated-Cu, before finally, the Ni/Cu contacts are sealed with a very thin Ag ‘flash’ layer to prevent oxidation of Cu. All three materials (Ni/Cu/Ag) are applied in one integrated plating sequence. This process step has already been tested in double-side contacted cells.
In a next phase, imec will also extend this technology, and apply it for back-contacted cells. In this case, a slightly different process is required: firstly, a barrier and seed layer are sputtered, on which a pattern is defined using a polymer paste (resist), prior to Cu plating. The resist layer prevents plating on non-desired areas, and ensures that the two dopant regions can be contacted independently. The polymer is then removed, and the thin seed and barrier layers can be etched.
Finally, oxidation of the copper layer is prevented by applying a thin cap layer.
Reduce the thickness of solar cells
In the former measure, the focus was limiting the use of toxic and scarce materials in solar cells and modules. To achieve the projects aim of reducing the environmental impact, together with cost reduction, reducing material consumption becomes critical, even for an abundantly available material such as silicon. Current solar cells use wafers with thickness around 170µm. The goal is to reach solar cells with a thickness of 100µm or less over the next ten years. Of course, in this case it becomes challenging to process the cells without breaking, and to integrate them into a module.
Remove lead as interconnection material
In many cells produced today, electrical connection is required on both the front and rear side. Individual solar cells are incorporated in series into PV modules using tin/lead-coated copper ribbons and lead-based soldering. Researchers and companies are looking for ways to avoid using lead in solar panel production. The approach being pursued in this project involves the use of back-contacted solar cells, which makes it possible to replace the soldered ribbons and instead use a patterned backsheet onto which the cells are placed, thereby forming an interconnected module. Cells are usually connected to the backsheet by conductive adhesives so the use of lead can be removed from the manufacturing process, in a technology that can handle wafers significantly thinner than the current industry standard.
Small-area interdigitated back contact solar cells (IBC) with 23,3% efficiency
About the project
The CU-PV project is part of the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No 308350. The project is coordinated by ECN in the Netherlands.
This video gives an overview of what the other partners in the project are doing.
Written by Barry O’Sullivan, a member of imec’s PV group, working on back-contacted solar cells.