Conventional solar cells are bulky and rigid, but building lightweight, flexible cells has come with trade-offs in efficiency and robustness. A new method for making flexible arrays of tiny silicon solar cells could produce devices that don’t suffer these trade-offs. Arrays of these microcells are as efficient as conventional solar panels and may be cheaper to manufacture because they use significantly less silicon. The tiny solar cells could be incorporated into, among other applications, window tinting, and they might be used to power a car’s air conditioner and GPS.
Researchers led by John Rogers, a professor of materials science and engineering at the University of Illinois in Urbana-Champaign, used a combination of etching and transfer printing to create arrays of silicon cells that are one-tenth the thickness of conventional cells. They demonstrated multiple possible designs for solar panels incorporating the microcells, including dense arrays flexible enough to bend around a pencil. “You could roll them up like a carpet, transport them in a van, and unfurl them onto a rooftop,” Rogers says.
The process builds on techniques for making flexible electronics that Rogers has been developing over the past few years. First, the Illinois researchers etch bars about 1.5 millimeters long, 50 micrometers wide, and 15 micrometers thick from a wafer of monocrystalline silicon. They use a stamp made of a soft polymer to pick up the microbars and place them on a substrate, which may be glass or a flexible plastic, and then fabricate interconnects. Rogers found that a cell thickness of 15 to 20 micrometers struck a good balance: thin enough to be flexible, but thick enough to be mechanically stable and efficient. Conventional solar cells use a layer of silicon 150 to 200 micrometers thick.
Arrays of the cells have about a 12 percent efficiency. The Illinois researchers increased the arrays’ power output by about two and half times by adding concentrators in the form of a layer of cylindrical microlenses. The best solar cells on the market convert more than 20 percent of the sunlight that falls on them into energy.
“This is a nice start at using silicon wafers more efficiently,” says Howard Branz, principal scientist in the silicon materials and devices group at the National Renewable Energy Laboratory, in Golden, CO. With their transfer-printing approach, says Branz, Rogers and his group have for the first time demonstrated how such thin cells could be manufactured on large areas.