Wearable photovoltaic cells have long been the dream of many people. After all, it could be pretty useful to have a free form of endlessly renewable energy sewn into your clothes to charge your devices on the go. The trouble is that silicon-based solar cells tend to be quite brittle, which has made them impractical as wearable materials.
But now researchers at Rice University in Houston, Texas, say they have managed to find a way to improve the mechanical stability of wearable cells’ active layers. They have done so through the incorporation of an internal elastic network, which relies not on unbending inorganic silicone but on flexible organic carbon-based polymers.
Their device can capture sunlight and convert it into electric currents. Better yet: the organic materials are thin, lightweight and semitransparent. They can also be manufactured cheaply. On the downside, their efficiency at converting sunlight into electricity is at around 15%, as opposed to the 22% of silicon-based solar cells.
Yet that extra flexibility in the materials is worth the loss of efficiency. “The field has been obsessed with the efficiency chart for a long time,” explains Rafael Verduzco, a chemical and biochemical engineer at the American university. “There’s been an increase in efficiency of these devices, but mechanical properties are also really important, and that part’s been neglected.”
And if solar cells are to be practical as wearable electricity generators, they’d better be flexible “If you stretch or bend things, you get cracks in the active layer and the device fails,” Verduzco adds. “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 Rice researchers say they have mixed in sulfur-based thiol-ene reagents. The molecules blend with the polymers and then crosslink with each other to provide flexibility. The amount of thiol-ene used determines the level of flexibility and influences the cells’ conductivity. With too little thiol-ene in the mix, the crystalline polymers can crack more easily under stress; with too added, however, the material loses some of its efficiency.
The magic formula, the researchers say, is a 20% content of thiol-ene, which makes the material both flexible enough and efficient enough. “If we replaced 50 per cent of the active layer with this mesh, the material would get 50 per cent less light and the current would drop,” Verduzco said. “We found there’s essentially no loss in our photocurrent up to about 20 per cent. That seems to be the sweet spot.”
