For all the promise they have shown in the lab, polymer solar cells still need to “get on a roll” so that large sheets of acceptably efficient photovoltaic devices can be manufactured continuously and economically.

New research results reported by an international team led by the National Institute of Standards and Technology (NIST) indicate that the “sweet spot” for mass producing polymer solar cells may be far larger than dictated by conventional wisdom. In experiments using a mock-up of a high-volume, roll-to-roll processing method, the researchers produced polymer-based solar cells with a power conversion efficiency of over 9.5%, just shy of the minimum commercial target of 10%.

Research is ongoing to create the means to efficiently mass-produce polymer solar cells. Image credit: DOE/EERE.Research is ongoing to create the means to efficiently mass-produce polymer solar cells. Image credit: DOE/EERE. Somewhat surprising to the researchers, their mass-produced versions exhibited molecular packing and texture that only slightly resembled those of lab-made varieties, which at their best convert about 11% of incident sunlight into electrical energy.

“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,” says Lee Richter, an 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.”

The team experimented with a coating material composed of a fluorinated polymer and a fullerene. Going by the technical name PffBT4T-2OD, the polymer is attractive for scaled production—achieving a reported power conversion efficiency of more than 11%. Importantly, it can be applied in relatively thick layers—conducive to roll-to-roll processing.

However, the top-performing solar cells were produced with the spin-coating method, a small-batch process. In spin coating, the fluid is dispensed onto the center of a disk or other substrate, which rotates to spread the material until the desired coating thickness is achieved. Besides generating lots of waste, the process is piecemeal and substrate size is limited.

So the research team opted to test commercially relevant coating methods, especially since PffBT4T-2OD can be applied in relatively thick layers of 250 nanometers and more. They started with blade coating, painting the PffBT4T-2OD onto the substrate.

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, spacing and distribution of the crystals that formed.

The substrates blade coated at 90° Celsius were the highest performing, achieving power conversion efficiencies that topped 9.5%. Surprisingly, at the nanometer level, the end products differed significantly from the spin-coated “champion” devices made in the lab. 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.

Encouraged by the results, the team performed preliminary measurements of PffBT4T-2OD coating formed on the surface of a flexible plastic sheet. The coating was applied on NIST’s slot-die, roll-to-roll coating line, directly mimicking 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.

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