Researchers at Swansea University in Wales (UK) have revealed a new design for printed solar cells that for the first time replaces the common expensive gold electrodes with a conductive nickel grid, laminated with a new transparent conductive adhesive (TCA) to provide mechanical and electrical contact. The structure could be applicable to a wide range of photovoltaic and other optoelectronic devices.

The laminate is a nickel microgrid printed onto a transparent polyethylene-terephthalate (PET) sheet that is then coated with the TCA. “The transparent conductive adhesive was designed by me and is fabricated using a widely used conducting polymer, PEDOT:PSS, and an even more widely used adhesive, acrylic microemulsion, the most common type of pressure-sensitive adhesive, found in everyday tapes, labels and hardware adhesives,” says Daniel Bryant, engineering doctorate researcher at Swansea’s Sustainable Product Engineering Centre for Innovative Functional Industrial Coatings (SPECIFIC) centre in Baglan, Port Talbot.

Once applied to the grid, the laminate behaves much like common sticky tape, such as Sellotape or Scotch tape, and can be applied to the solar cell in the same way, simply by pressing it down with your fingers. “The materials used for the adhesive are cheap, readily available, compatible with the cells and can be manufactured using well established printing techniques,” Bryant confirms.

Specifically developed for these kind of applications, the adhesive conducts, adheres, is transparent and uses no metallic inclusions or particles, which are prone to corrosion over time. “It not only succeeds as a viable contact for perovskite-based solar cells but is also applicable to other types of optoelectronic devices,” Bryant says. Furthermore, due to eliminating gold and other types of metallic contacts, the fabrication of the cells no longer requires a vacuum process to deposit the metallic contacts. While the metallic contacts were opaque, the cells can now be transparent.

The innovation was born out of the classic engineering approach of having a problem — in this case, replacing the gold and making the cells transparent — and then creating a solution. “First we sourced a transparent conducting sheet embedded with the nickel microgrid that was capable of current collection over a large area,” Bryant says. “Then we needed some way to apply this to the cells that conducted from the cell to the grid, would provide a mechanical bond to maintain electrical contact but that did not compromise on transparency.”

At the time, there was no material out there that provided these properties and functions. So Bryant and his colleagues developed one. “Taking inspiration from the tapes and adhesives industry, we narrowed it down to an acrylic microemulsion, knowing that these types of materials would be well-suited to the requirements and also lend themselves well to reformulation,” the researcher continues.

As per the inventor, the performance of devices made with the laminate are comparable to ones made with gold electrodes, with minimal losses. He hopes to at least be able to match any future efficiency increases that will be achieved with third-generation cells made with metallic contacts. He also believes that this research advance opens up possibilities for new device architectures and designs.

Swansea’s novel method promises cheaper printed solar cells. Explains Bryant: “So far this method has shown to have the capability of fabricating fully printed, atmospherically produced, gold-free solar cells with only a small drop in efficiency compared to the standard method.”

The team is now fine-tuning the conditions for scaling up the continuous manufacturing process, e.g. large-scale deposition and curing. “This represents a major breakthrough in this technology, and the route to scale up this technology can now be clearly defined,” Bryant emphasises. “The critical advantage of the new room-temperature lamination method is that it is well-suited to mass production, using well established processes.”

His team is also working on optimising the conductivity of the adhesive laminate and on understanding its stability in longer-term weathering exposure.

Written by Sandra Henderson, research editor Solar Novus Today