Wireless Solar Charging Made Easier

19 June 2012—Anybody with a smartphone dreads the low-battery warning that initiates a mad search for an electrical outlet. But engineers at Princeton University are developing technology that could lead to widespread wireless charging stations for all our electronics. Along the way, this technology could also help build better sensors to monitor the health of both humans and buildings.

Wireless chargers operate through inductive or capacitive power transfer. An alternating current creates an oscillating electrical or magnetic field, which induces power at the receiver. “We’re looking for an opportunity to create ubiquitous charging stations,” says Naveen Verma, an assistant professor of electrical engineering at Princeton.

Verma and his team presented the work last week at the IEEE Symposia on VLSI Technology and Circuits, in Hawaii. The research focuses on using the same material—thin films of amorphous silicon—both to make solar cells and, for the first time, to build circuits to handle the electricity the solar cells produce. The combined solar cells and circuitry could be made on large sheets of plastic that could be molded or wrapped around everyday objects, from buildings to patio umbrellas. Amorphous silicon has its limitations. For one, it’s not as efficient at converting light to electricity as crystalline silicon is. But unlike crystalline silicon, it can be processed at relatively low temperatures, allowing production over large areas on plastic substrates. Amorphous silicon also produces transistors with much lower performance than crystalline silicon. The reduced speeds result in low-quality inductors, which are typically a key component in creating the oscillating fields used in wireless power transfer. What’s more, it is usually possible to build only n -type thin-film transistor (TFT) devices, but not both n- and p-type at the same time, as needed in the complementary logic of computers.

So Verma’s team designed a circuit containing two solar cells, capacitors, and n-type TFTs, skipping the p-type TFTs and inductor. The TFTs switch the current so it flows to the capacitors first from one solar cell and then the other (which is wired in reverse), thus turning the direct current produced by the cells to the desired alternating current.

Verma says the charging system can provide a device with up to 120 microwatts of power at a transfer efficiency of up to 22 percent under indoor lighting. An iPad, which uses power in the tens of milliwatts, wouldn’t benefit much from that, but there should be ways to increase the charger’s capability. A larger energy-harvesting surface can provide more power, and larger capacitors raise both power and transfer efficiency; increasing their area from 5 by 5 centimeters to 10 by 10 cm increases power by a factor of four. Verma is also interested in replacing the amorphous silicon circuits with metal oxide semiconductors, such as zinc oxide, which may work better and is compatible with the silicon processing.

In the meantime, he says, “there are a lot of devices that consume very little average power.” Some medical sensors, such as those worn on the body to monitor heart rates or other signals, need only a few tens of microwatts. And in other research presented at the conference, Verma and his colleagues propose combining thin-film solar cells with thin-film electronic circuitry for power management, readout, processing, and communications in a new type of structural sensor for buildings.

Today, sensors for monitoring strain in buildings and bridges often consist of an optical fiber connected to a detector. If a bridge bends more than a certain amount, that bends the fiber, which alters the light hitting a detector at one end. Verma would like to replace that design—which senses strain in only one dimension—with an array of sensors, powered by photovoltaics. “The kind of sensors we’re envisioning are much more functionally diverse,” he says. “This technology provides sensing capability on a scale that no technology we have now could provide.” He’s hoping to install a prototype of such a system on a bridge on the Princeton campus.
About the Author

Neil Savage, based in Lowell, Mass., writes about strange semiconductors and amazing optoelectronics. In the April 2012 issue of IEEE Spectrum, he reported on molybdenum disulfide, a potential rival to graphene in nanoelectronics.