Alta Devices' solar panels achieve 28.2% photoelectric conversion rate

According to Reuters, Alta Devices, a solar panel manufacturer in Santa Clara, California, announced today that the photovoltaic conversion rate of its solar panels reached 28.2%, breaking the previous record of 27.6% set by the company last year. Alta Devices said its innovative technology can be used for large-scale production.

Alta Devices says that silicon-based solar panels have a theoretical conversion rate of 33.5%. The company may be developing solar panels that can reach that level. Thin-film solar panels like those produced by First Solar do not have the conversion rate of silicon-based panels. Pam Hegarty from First Solar says that thin-film solar panels perform better than silicon-based panels in diffuse light and high temperature environments. Although thin-film solar panels take up more space and cost more to install, they can generate more electricity in low-light environments.

Pam Hegarty said: "Everything has its two sides. Silicon-based solar panels have a higher conversion rate, but when the sun is not in direct sunlight, thin-film solar panels can absorb more sunlight and produce more electricity."

Researchers at Alta Devices studied gallium arsenide, improving its intrinsic properties to make it "almost an ideal solar cell material," said Harry Atwater, co-founder of Alta and professor of applied physics at Caltech. When a solar cell absorbs sunlight, some of the energy creates electrons in the sunlight, which are quickly extracted from the cell to form electricity. If the electrons are not extracted quickly enough, they decay, releasing energy as heat or light. Losing this energy as heat reduces the voltage and current generated by the cell, also reducing power output. But if the electron instead creates light, that light can be absorbed again by the solar cell to create another electron, which provides another opportunity for energy in the sunlight to become electricity.

Atwater said the company is working to improve epitaxial layer exfoliation and develop better ways to grow crystalline layers on wafers. The current method, called chemical vapor deposition, is too slow to make cheap solar cells.

In gallium arsenide, almost all of the electrons are generated as light, not heat. In the highest-quality samples of the material, the light production and reabsorption cycle can occur 100 times for every incoming photon, providing a high probability that the generated electrons will eventually be directed to generate electricity. To achieve the record solar cell efficiency, researchers at Alta Devices developed chemical treatments to apply to cracks in the material that would otherwise have a tendency to snag electrons and cause them to release their energy as heat. They also worked to improve the back of the cell, ensuring that the generated photons are reflected back into the material, making it more likely that they will generate electricity.

Improving efficiency is essential if solar power is to compete with fossil fuels. Increasing the output of individual solar cells would reduce the number of cells needed per installation and cut costs such as installation, wiring and some electronics required to connect panels to the grid.

Remarkably, the researchers were able to achieve high efficiencies using techniques that Atwater says could be used for cheap manufacturing. Gallium arsenide solar cells have always been very expensive to produce because they are made from extremely high-quality semiconductor wafers, similar to those used for computer chips. To cut costs, Alta Devices is using a technique called epitaxial liftoff, an early version of which was pioneered by Alta Devices co-founder Eli Yablonovitch in the 1980s. In the latest version of the technique, the wafer is used as a seed template to grow very thin layers of crystalline material to form the solar cell. Those layers are then lifted off the wafer using chemical etching, allowing the wafer to be used again.

Although still in the start-up stage, Alta Devices has recently received $72 million in a new round of financing. According to relevant sources, the company intends to develop solar cells with a photoelectric conversion rate of 30%, and the cost of producing 1 watt of electricity will be only 50 cents.

According to Green Tech Media, the Santa Clara, California-based company plans to move a little further north to Sunnyvale. The company currently has 55 employees and 35 patents. Alta Devices has also received a $3 million government grant from the U.S. Department of Energy to develop low-cost, high-efficiency solar cells.

The high-efficiency cells, which Alta already makes, are small, disposable devices. The company's larger solar modules have lower efficiencies, about 21 percent. Typically, when companies move to large-scale production, efficiency drops by a few percentage points. Alta also needs to prove that the high-efficiency cells can last for decades and weather the elements without significantly degrading.