Engineers bend organic solar cells to their will

Organic solar cells that can be painted or printed on surfaces are increasingly efficient, and now show promise for incorporation into applications like clothing that also require them to be flexible.

To this end, the Rice University lab of chemical and biomolecular engineer Rafael Verduzco has developed flexible organic photovoltaics that could be useful where constant, low-power generation is sufficient. This involved Verduzco and his team incorporating a network of elastic additives into an organic photovoltaic material to make it less brittle with little to no loss of current flow. They report their research in a paper in Chemistry of Materials.

Organic solar cells rely on carbon-based materials like polymers, as opposed to hard, inorganic materials like silicon, to capture sunlight and translate it into current. Organics are thin, lightweight, semi-transparent and inexpensive. But while middle-of-the-road, commercial, silicon-based solar cells perform at about 22% efficiency – the amount of sunlight converted into electricity – organics top out at around 15%.

"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."

According to Verduzco, 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," he said.

Rather than make an elastic mesh and pour in the semiconducting polymer, the Rice researchers mixed sulfur-based thiol-ene reagents with the polymer. These molecules blend with the polymer 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.

Testing helped the lab to find its Goldilocks Zone. "If we replaced 50% of the active layer with this mesh, the material would get 50% less light and the current would drop," 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, they found that cells retained their efficiency and gained flexibility. "They're small molecules and don't disrupt the morphology much," Verduzco said. "We can shine ultraviolet light or apply heat or just wait, and with time the network will form. The chemistry is mild, fast and efficient."

The next step was to stretch the material. "Pure P3HT (the active polythiophene-based layer) started cracking at about 6% strain," 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 thiol-ene concentrations 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%," he said. "That seems to be the sweet spot."

Damage under strain affected the material even when the strain was released. "The strain impacts how these crystal domains pack and translates to microscopic breaks in the device," Verduzco said. "The holes and electrons still need paths to get to the opposite electrodes."

He said the lab expects to try different organic photovoltaic materials while working to make them more stretchable with less additive for larger test cells.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.