Flexible solar cells on a roll
The team, led by the US National Institute of Standards and Technology (NIST), used a mock-up of a high-volume, roll-to-roll processing method to produce cells with a power conversion efficiency of 9.5%. This is just short of the minimum commercial target of 10%.
The mass-produced versions showed molecular packing and texture significantly different from spin-coated cells developed in the lab with around 11% efficiency. While this is a lower efficiency that cells on a solid substrate which approaches 30% (see below), the lower manufacturing cost and ease of use for the flexible polymer cells is attractive.
“The ‘rule of thumb’ has been that high-volume polymer solar cells should look just like those made in the lab in terms of structure, organization and shape at the nanometer scale,” said Lee Richter, a NIST physicist who works on functional polymers. “Our experiments indicate that the requirements are much more flexible than assumed, allowing for greater structural variability without significantly sacrificing conversion efficiency.”
“Efficient roll-to-roll fabrication is key to achieving the low-cost, high-volume production that would enable photovoltaics to scale to a significant fraction of global energy production,” added He Yan, a collaborator from Hong Kong University of Science and Technology.
The team experimented with a coating material composed of a fluorinated polymer and a fullerene (also known as a “buckyball”). This PffBT4T-2OD polymer achieved power conversion efficiency of more than 11% in the lab and can be applied in relatively thick layers of 250nm for roll-to-roll processing.
A series of X-ray-based measurements revealed that the temperature at which the PffBT4T-2OD was applied and dried significantly influenced the resultant coating’s material structure–especially the orientation, spacing and distribution of the crystals that formed.
The substrates blade-coated at 90 degrees C were the highest performing, achieving power conversion efficiencies that topped 9.5%. Detailed real-time measurements during both blade-coating and spin-coating revealed the different structures arose from the rapid cooling during spin-coating versus the constant temperature during blade-coating.
“Real-time measurements were critical to developing a proper understanding of the film formation kinetics and ultimate optimization,” said Aram Amassian, a collaborator from Saudi Arabia’s King Abdullah University of Science and Technology (KAUST).
Applying PffBT4T-2OD on a flexible plastic sheet using a slot-die roll-to-roll coating line directly mimicked large-scale production. Measurements confirmed that the material structures made with blade-coating and those made with slot-die-coating were nearly identical when processed at the same temperatures.
“It’s clear that the type of processing method used influences the shape of the domains and their size distribution in the final coating, but these distinctly different morphologies do not necessarily undermine performance,” said Harald Ade, a collaborator from North Carolina State University. “We think these findings provide important clues for designing polymer solar cells optimized for roll-to-roll processing.”