Rice University chemists have developed a new class of flexible organic photovoltaics that can be painted or printed on surfaces.
The photovoltaics developed in the lab of Rafael Verduzco were incorporated with a network of elastic additives that make the electrically active material less brittle with little to no loss of current flow.
Organic solar cells rely on carbon-based materials including polymers, as opposed to hard, inorganic materials like silicon, to capture sunlight and translate it into current.
They are also thin, lightweight, semitransparent and inexpensive. However while silicon-based solar cells perform at about 22 percent efficiency, organics top out at around 15 percent.
“The field has been obsessed with the efficiency chart for a long time,” Verduzco said. “There’s been an increase in efficiency of these devices, but mechanical properties are also really important, and that part’s been neglected.
“If you stretch or bend things, you get cracks in the active layer and the device fails.”
Verduzco said one approach to fixing the brittle problem would be to find polymers or other organic semiconductors that are flexible by nature.
The chemist however chose a different path: “Our idea was to stick with the materials that have been carefully developed over 20 years and that we know work, and find a way to improve their mechanical properties.”
Rather than make a mesh and pour in the semiconducting polymers, the researchers mixed in sulfur-based thiol-ene reagents.
Verduzco and his team had to determine how much thiol-ene was needed to suppress fracture and the maximum they could put in without making it worthless as an electronic device.
At about 20 percent thiol-ene, they found that cells retained their efficiency and gained flexibility. The next step was to stretch the material:
“Pure P3HT started cracking at about 6 percent strain,” Verduzco said. “When we added 10 percent thiol-ene, we could strain it up to 14 percent. At around 16 percent strain we started seeing cracks throughout the material.”
At strains higher than 30 percent, the material flexed just fine but became useless as a solar cell. “We found there’s essentially no loss in our photocurrent up to about 20 percent,” he said. “That seems to be the sweet spot.”
Image, video and content: Jeff Fitlow/Rice University