In solar, how high is high?
With a worldwide glut of factory capacity, solar cell manufacturers will likely increasingly turn toward improving the efficiency of their products to improve profitability. Manufacturing costs, which are measured in dollars per watt, fall when the same production line rolls out cells that could produce more electricity. More efficient cells produce more power, which helps to lower the cost of producing electricity and provides a quicker return on investments for homeowners or power plant developers.
There is, however, a theoretical maximum efficiency for cells categorized by the makeup of their semiconductor compounds and some types of solar cells are closer to the ceiling than others. Which technologies can move smoothly toward those ceilings and which get hung up could become one of the big issues of the decade.
Efficiency also isn't cheap. In August, the University of New South Wales set a record with a solar cell that could convert 43 percent of the light that struck it into electricity. But don't expect that device-with layers of silicon, indium, gallium and other materials-to appear on rooftops anytime soon.
Suntech Power Holdings provided an interesting example into these issues last year with Pluto, the company's new high efficient solar cell. Pluto will allow Suntech to challenge SunPower, long the leader in mass manufactured high efficiency cells. Despite breaking a record, Suntech had to drop its production forecasts for Pluto production because of automation issues in the factory.
"I do think it is important and has many knock-on effects of improved economics all the way through the installation," said Travis Bradford, president of the Prometheus Institute. "My only point is that it isn't going very far. Today's technologies will improve only a couple of percentage points at best, increasing their watts/dollar by 20 to 40 percent at most."
Silicon cells have a hypothetical limit of about 33 percent, said Martin Green, the executive research director at the University of New South Wales' ARC Photovoltaics Centre of Excellence. But researchers often use 29 to 30 percent as the theoretical maximum efficiency for silicon cells because the material has inherent properties that render the 33 percent impossible to reach, Green said.
Researchers calculate the theoretical maximum efficiency by using each semiconductor's band gap and rely on the model published by William Shockley and Hans Queisser in 1961. The band gap refers to the amount of energy (photon) required to push electrons around, and that determines a material's ability to absorb and convert light to electricity.
Semiconductors with band gap between 1.1 electron-volt (eV) and 1.4 eV have efficiencies around 33 percent, Green said. In addition to silicon, this range of bandgaps also applies, in general, to cadmium-telluride and some organic cells, he added.
But the limit isn't absolute for all solar cells made with these semiconductors. The maximum efficiency could vary depending on how you measure band gaps, the proportions of the semiconductors in a compound and other factors that could be thrown into the calculations to determine that limit.
The 29-30 percent figure has been used to describe the upper limit for cadmium-telluride and CIGS cells as well.
Determining the limit for CIGS, in particular, can be tricky. It's a mashed up of four semiconductors, and the proportions of these materials could create a band gap from 1 eV to 1.7 eV, said Lin-Wang Wang, a material scientist at Lawrence Berkeley National Laboratory.A research
published by researchers at the University of Stuttgart in Germany put the upper limit for CIGS at 26 percent, and said it could be even lower.
"The theoretical limits are interesting consideration. If you are anywhere near the right band gap then it's an entry card into the playing field. Now the game starts," said Emanuel Sachs, chief technology officer of 1366 Technologies and a professor at the Massachusetts Institute of Technology.
So who are some of the world's record holders? Besides its 43 percent multi-material cell, USNW produced a monocrystalline silicon with a 25 percent efficiency.
The most efficient multicrystalline silicon cell has come from the Fraunhofer Institute in Germany, however. The tiny cell boasts around 20.4 percent efficiency, according to the journal Progress in Photovoltaics: Research and Applications, which tracks efficiency records.
The best cadmium-telluride cell, measured at 16.7 percent, came from the National Renewable Energy Laboratory in Colorado. The same lab also fabricated the highest performing cell of copper, indium, gallium and selenium (CIGS), which achieved 19.4 percent efficiency.