[Image above] Rice University researchers created flexible, organic solar cells that could be useful in situations requiring constant, low-power generation. Credit: Rice University, Jeff Fitlow
When you think of solar cells, you might imagine the large black panels covering the roof of a house. These types of solar cells are made of silicon and other inorganic materials, and can be heavy, stiff, and expensive to fabricate.
In contrast to inorganic solar cells, organic solar cells are lightweight, flexible, and less expensive to fabricate. While organic cells do not convert sunlight into energy as efficiently as inorganic cells—about 15 percent efficiency versus about 22 percent—organic solar cells could be ideal for low-power situations and portable objects, such as camping gear or a phone charger. But there is a hurdle to organic cells’ integration into transportable materials: organic cells can be brittle.
There is no one component of an organic solar cell that makes it brittle—the electrodes, the substrate, and the active layer all fall victim to this issue. For organic solar cells to become less brittle, researchers must find ways to increase the flexibility of each individual part.
A number of studies have looked at improving the flexibility of the electrodes and substrate. Researchers at Gwangju Institute of Science and Technology, Georgia Institute of Technology, and Jiangxi Science and Technology Normal University used the polymers PEDOT:PSS and polydimethylsiloxane (PDMS) and other additives to make flexible electrodes and substrates, while MIT researchers used graphene instead of the conventional material, indium tin oxide (ITO), to create flexible electrodes.
Creating a flexible active layer, however, has proven more of a challenge. Active layers made of an organic semiconductor blend perform multiple functions in the organic solar cell, including absorbing light and transporting both holes and electrons to the electrode. Because the active layer is a blend of materials rather than a single material, additives or compositional changes used to improve one property of the active layer—like the flexibility—could easily result in degraded performance in other areas, like the ability to absorb light.
Rafael Verduzco, chemical and biomolecular engineer, and his team at Rice University decided to use an alternative approach to improving the active layer’s flexibility, one that did not require introducing polymeric additives. “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,” Verduzco says in a Rice University news release.
In the article detailing their research, Verduzco and his team mixed sulfur-based thiol-ene reagents into the active layer, which was then placed on both ITO glass substrates and PDMS substrates for testing. They found there was a “Goldilocks Zone” for amount of thiol-ene: too little thiol-ene left the active layer prone to cracking under stress, while too much thiol-ene dampened the active layer’s energy conversion efficiency. A thiol-ene mixture of about 20 percent balanced flexibility with efficiency.
The current research focused on P3HT:PCBM organic solar cells, so Verduzco says they expect to try different organic solar cells going forward to see if they can further optimize the thiol-ene network.
Watch the video below to learn more and see these stretchy organic solar cells in action.
Credit: Rice University, YouTube
The paper, published in Chemistry of Materials, is “Network-Stabilized Bulk Heterojunction Organic Photovoltaics” (DOI: 10.1021/acs.chemmater.8b03791).