A team of scientists from Rice University, Houston Community College and Brookhaven National Laboratory has developed flexible organic photovoltaics that could be useful where constant, low-power generation is sufficient.
Mok et al developed flexible organic photovoltaics with a chemical additive that mitigates the material’s brittle qualities without losing efficiency. Image credit: Jeff Fitlow / Rice University.
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. Organics are also thin, lightweight, semitransparent and inexpensive.
While middle-of-the-road, commercial, silicon-based solar cells perform at about 22% efficiency, organics top out at around 15%.
“There’s been an increase in efficiency of these devices, but mechanical properties are also really important. If you stretch or bend things, you get cracks in the active layer and the device fails,” said team leader Dr. Rafael Verduzco, a researcher in the Department of Chemical and Biomolecular Engineering and the Department of Materials Science and Nanoengineering at Rice University.
“One approach to fixing the brittle problem would be to find polymers or other organic semiconductors that are flexible by nature, but his lab took another tack.”
“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, Dr. Verduzco and co-authors mixed in sulfur-based thiol-ene reagents. The molecules blend with the polymers and then crosslink with each other to provide flexibility.
The process is not without cost, because too little thiol-ene leaves the crystalline polymers prone to cracking under stress, while too much dampens the material’s efficiency.
“If we replaced 50% of the active layer with this mesh, the material would get 50% less light and the current would drop,” Dr. Verduzco said.
“At some point, it’s not practical. Even after we confirmed the network was forming, we needed to determine how much thiol-ene we needed to suppress fracture and the maximum we could put in without making it worthless as an electronic device.”
At about 20% thiol-ene, the team found that cells retained their efficiency and gained flexibility.
The next step was to stretch the material.
“Pure P3HT (the active polythiophene-based layer) started cracking at about 6% strain,” Dr. Verduzco said.
“When we added 10% thiol-ene, we could strain it up to 14%. At around 16% strain we started seeing cracks throughout the material.”
At strains higher than 30%, 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%. That seems to be the sweet spot,” Dr. Verduzco said.
The study is published in the journal Chemistry of Materials.
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Jorge Wu Mok et al. Network-Stabilized Bulk Heterojunction Organic Photovoltaics. Chem. Mater, published online October 26, 2018; doi: 10.1021/acs.chemmater.8b03791
