In a quest for a smaller, more self-sustaining solar power source, a UW-Madison electrical engineer has proposed a design for solar panels that can simultaneously generate power from sunlight and store power reserves for later, all within a single device.

Hongrui Jiang and his students developed the idea, published in the journal Advanced Materials June 6. Jiang is the Vilas Distinguished Achievement Professor of electrical and computer engineering at UW-Madison and specializes in microscale devices. He and his students developed the technology as an offshoot of a National Institutes of Health grant to design a self-focusing contact lens that adapts to the eyes of adults suffering from presbyopia, a natural aging process that stiffens the lens and reduces the eye's ability to focus, especially at short distances.

To power that contact lens, Jiang and his team have worked out a design that balances energy harvesting, storage and usage. "We needed a multi-functional and small-form-factor device in order to integrate it all into a single contact lens structure," says Jiang.

The top layer of each photovoltaic cell is a conventional photo electrode, converting sunlight into electrons. During that conversion process, the electrons split off into two directions: most electrons flow out of the device to support a power load, while some are directed to a polyvinylidene fluoride polymer (PVDF) coated on zinc oxide nanowires. The PVDF has the high dielectric constant required to serve as an energy storage solution. "When there's no sunlight, the stored power will come back through the nano wires to power the load."