Solar cells, technically referred to as photovoltaics, take advantage of the same electronic properties that make semiconductors so vital to the computer industry. When sunlight strikes the surface of a semiconductor, the photons transfer their energy to electrons in the material; in a working solar cell that energy is captured and put to use. A sheet of semiconductor material lies sandwiched between two layers of electrode material. A built-in electric field draws the excited electrons to the top electrode, which carries them out of the cell and into a circuit. The bottom electrode gathers electrons from the circuit to fill the “holes” that the excited electrons left behind. The bottom line is that the semiconductors transform sunlight into usable electricity.
But most of the solar panels sold today employ crystals of silicon that are expensive to manufacture. Silicon crystals may justify their cost in microprocessors, but they price solar power out of a market dominated by cheap fossil fuels. “If you want to compare purely on a dollar per watt basis, solar power is three to four times more expensive right now,” says Atul Arya, chief operating officer with Linthicum, MD-based BP Solar, a subsidiary of the large oil company and one of the world’s top producers of silicon solar cells.
Increasingly, the search for low-cost technology is leading BP Solar and other solar-cell manufacturers to abandon silicon crystals altogether. In its place, these firms and half a dozen startups are developing photovoltaics employing cheap amorphous silicon, and even semiconductor alloys, that can be quickly spread into a thin film just a few thousandths of a millimeter thick-about a hundred times thinner than the silicon crystals used in conventional solar cells. Because these thinner solar cells require less semiconductor material and are amenable to mass production, they are significantly cheaper. And while these types of semiconductors lack the electron-shuttling efficiency of silicon crystals, they compensate by absorbing more photons than silicon crystals.
Ken Zweibel, who leads thin-film development at the Golden, CO-based National Renewable Energy Laboratory complex where many of the technology advances are being made, predicts that thin films will deliver highly efficient solar cells at one-quarter to one-fifth the cost of today’s cells. Cheap, thin films of amorphous silicon or alloy that can capture as much as 20 percent of the sun’s energy (researchers can now make films in the lab with 18 percent efficiency) could make solar cells practical for homeowners, not just in sunny California, where clogged power lines deliver the country’s most expensive electricity, but in Boston, Chattanooga and Tampa Bay.
The low cost and inherent flexibility of these thin solar cells also means they can be easily applied as coatings on a range of materials, including glass and roofing tiles. To demonstrate this aspect of the technology, BP has installed translucent awnings embedded with thin-film photovoltaics at 250 of its gas stations around the world, keeping customers dry while powering the pumps. “It becomes a window pane, or it could be a shading application, or it could be a skylight application. We are doing all of those,” says BP’s Arya.
Despite such promise, however, it could take decades for thin films to transform solar power from a marginalized technology to a mainstream source of energy. Steady growth at today’s impressive rates, doubling the market every three years, will only bring the industry to 10 percent of peak power generation in 2030, according to a U.S. solar-power industry road map issued last year. What is needed to accelerate the penetration of solar power is photovoltaic materials that are really cheap-cheap as plastics.