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.