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Wireless Solar Charging Made Easier

CORDLESS CHARGING: Oscillators built on plastic [inset] take power from two solar cells to wirelessly charge portable devices.Click on image to enlarge.
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.
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