A Solar Junction wafer of high-efficiency solar cells. Each individual chip can produce about 20 watts.

Solar Junction, a Silicon Valley solar start-up, said today that its solar cell has been measured at a peak efficiency of 43.5 percent, topping previous records.

The San Jose, Calif.-based company said that the efficiency mark was verified by the National Renewable Energy Laboratory's Measurement and Characterization Laboratory.

To achieve the 43.5 percent mark, Solar Junction designed a multijunction cell made of multiple layers of photovoltaic material. Each layer is optimized to convert a different portion of the light spectrum to electrical energy.

To boost the amount of sunlight that goes into the very small, 5.5-millimeter square cells, light is concentrated with mirrors. The 43.5 percent efficiency was reached at a concentration of 400 suns and maintained that level up to 1,000 suns, Solar Junction said.

Multijunction solar cells with concentrated sunlight have been used for decades in spacecraft and to make electricity in desert areas. Solar Junction intends to supply its cells to companies which manufacture concentrating solar photovoltaic (CPV) collectors, such as Amonix and Concentrix Solar, for utility-scale solar projects.

The company expects that its multijunction cells, which are layered onto a gallium arsenide substrate, will continue to improve in efficiency, which will make CPV more attractive to utility-scale solar developers. Competing technologies are less efficient flat solar panels or concentrating solar thermal technology.

The 4-year-old company is seeking a Department of Energy loan guarantee to expand its production to 250 megawatts per year and to start shipping commercial cells within a year.