For roughly two decades, the most efficient silicon solar cells in the world used a structure invented in Australia at the University of New South Wales. This week, in a packed conference room at the IEEE Photovoltaic Specialists Conference in Denver, Panasonic gave details for the first time about a new structure that allows silicon solar cells to surpass that efficiency, setting a new world record and possibly pointing the way to cheaper solar power that can compete widely with fossil fuels.
The result reflects a new surge forward for silicon solar cells, the type that account for almost all solar cells on the market. “Amazingly, the 20-year-old efficiency record was eclipsed at this conference by three companies, Panasonic, Sharp, and SunPower,” says Richard Swanson, cofounder and former president of SunPower (see “Three Questions with a Solar Pioneer”).
The new design combines elements of two of the most efficient types of solar cells available commercially: those produced by the solar company SunPower and an earlier design by Panasonic. SunPower’s cells are highly efficient in part because they do away with the front contacts that block some of the incoming sunlight. Both positive and negative contacts are on the back.
In the new design, Panasonic used a similar approach to get rid of the front contacts and eliminate that shading. The main difference is that Panasonic applies this design to its own high-efficiency cell structure, which addresses another major problem with conventional silicon solar cells. Imperfections at or near the surface of the crystalline silicon wafers used in the cells can trap electrons, decreasing current and voltage. Panasonic prevents this by applying thin films of silicon to the front and back of the silicon wafer.
The new cell converts 25.6 percent of the energy in sunlight into electricity, edging past the long-standing record of 25 percent. Such small improvements in efficiency can have a big impact on total power output from a solar cell, but the main reason the advance is important is that it shows the potential of a new way to design solar cells, which could lead to larger improvements in the future.
Although both aspects of the new design are used in commercial solar cells, it’s not clear yet that the structure that combines them can be manufactured at competitive prices. Martin Green, the professor at the University of New South Wales whose cells had held the previous record, says one drawback of all the record-setting designs is their use of high-quality silicon crystal, which is expensive. His record designs have been adapted to cheaper forms of silicon and are being adopted by several solar cell companies (see “Advanced Solar Technology”).