Inspired by the mechanisms used by plants for holding onto the energy they capture from the sun, UCLA researchers have created predecessors of solar cells that could one day store the energy they produce until it is most needed. If panels can be made in this way at commercial prices and efficiencies, the technology could remove the need for additional storage systems under many circumstances.
Existing solar panels, whether they are made of silicon or one of the newer alternatives, store energy for mere microseconds, if at all. Plants, however, don't have the luxury of battery systems or alternative power sources for the nighttime. "In photosynthesis, plants that are exposed to sunlight use carefully organized nanoscale structures within their cells to rapidly separate charges – pulling electrons away from the positively charged molecule that is left behind, and keeping positive and negative charges separated," Professor Sarah Tolbert said in a press release. "That separation is the key to making the process so efficient."
Tolbert is the senior author of a paper in Science, announcing the creation of poly(fluorene-alt-thiophene) (PFT) plastics that mimic this capacity. As with other photovoltaic polymers, the one Tolbert is working with is potentially much cheaper than silicon, but also currently far less efficient.
"Modern plastic solar cells don't have well-defined structures like plants do because we never knew how to make them before," Tolbert said. "But this new system pulls charges apart and keeps them separated for days, or even weeks. Once you make the right structure, you can vastly improve the retention of energy."
Polymer solar cells consist of long, thin polymer donors, which release electrons when struck by light, and acceptors, which, in the case of the material with which Tolbert was working, are carbon-iodide fullerenes. Tolbert delighted pastafarians by comparing previous arrangements to plates of pasta, with spaghetti-like polymers randomly mixed with round fullerenes standing in for meatballs. Unfortunately, in the disorder, electrons can hop back from an acceptor to a polymer, wasting the energy.
Tolbert has constructed polymers that self-assemble in water to become neatly bundled like uncooked spaghetti, with a balance of fullerenes inside and outside the bundles. The fullerines inside the polymers absorb electrons but then transfer them to others located outside from the bundle. Located safely on the outside, the fullerenes can hold onto the electrons, while the polymers remain positively charged for days or even weeks without recombining.
Plenty of steps are needed before you can install solar panels made in this way on your roof. “We don't have these materials in a real device yet; this is all in solution,” said coauthor Professor Yves Rubin. “When we can put them together and make a closed circuit, then we will really be somewhere.”