California, USA -- In Part 1 of this article, we looked at how lab and commercial efficiencies are growing in crystalline, CIGS and amorphous solar cells. Now we'll take a look at how increasing efficiencies have boosted cadmium telluride, the thin film made by No. 1 solar manufacturer First Solar. We also examine how cadmium telluride could also help grow the market for multijunction concentrator cells -- the world's most efficient cells -- by making concentrating-photovoltaic projects more cost effective. Efficiency improvements have been accelerating in the last few years as more money, scientists and companies have entered the solar space, according to Martin Green, a professor at the University of New South Wales. Where will these efficiencies ultimately plateau?
First Solar, the company that popularized cadmium telluride, is the world's largest solar manufacturer, with 1.1 gigawatts of production last year. It owes much of its leadership to having the lowest announced per-watt costs in the industry. The company, which broke the $1-per-watt milestone back in February of last year, in the second quarter announced it's producing panels at a cost of US $0.76 per watt.
And a big part of the reason for its low costs has been its growing efficiencies, said Jenny Chase, lead solar analyst for Bloomberg New Energy Finance. "One of the reasons First Solar is so successful is because they keep improving the efficiency of their modules, which means their effective capacity increases without them spending new money on new lines," she said. "They're doing the same thing, but because they're doing it slightly more smartly, they're getting more watts and paying slightly more, but can sell for [more money] because the modules are sold per watt."
First Solar sells the most efficient cadmium-telluride panels in the world, with Photon's February overview tracking them at 10.4 percent efficiency. The company reported that its panels grew to 11.2 percent efficiency in the second quarter of this year from 10.9 percent efficiency in the same quarter last year.
Meanwhile, the top lab record for this technology is a 16.7 percent cell created by the National Renewable Energy Laboratory, which tested the cell back in 2001, according to Progress in Photovoltaics. And the theoretical efficiency hovers around 30 percent, according to an NREL report.
In an announcement in September, Sunovia Energy Technologies and EPIR Technologies claimed they’ve made a technology breakthrough that could raise that potential efficiency. They said the cadmium-telluride technology set a new world record for open-circuit voltage, which correlates to efficiency, and estimate they will be able to make two-junction cells with a production efficiency of 35 percent.
Multilayered cells with concentrators are the world's most efficient cells, bar none. And in a test in September, Spire broke the record for this type of cell. Its multijunction concentrator cell, boasting 41.3% efficiency, included layers of indium gallium phosphide, gallium arsenide and indium gallium arsenide, as well as a concentrator that magnifies the sunlight 406 times (or, in industry language, 406 suns). The company said in October that the cell resulted from an NREL Photovoltaic Incubator subcontract awarded in early 2009.
Spectrolab had set the previous record with a 364-sun concentrator cell made of layers of gallium indium phosphide, gallium indium arsenide and germanium in August of last year. Sharp also announced in September that it broke the record, but not by as much as Spire, which boasts a 42.1 percent efficiency cell developed in partnership with Tokyo University.
"Congratulations to Spire. It's certainly a good result and it reflects the progress being made in the [concentrating photovoltaic] industry on tech improvements," said Spectrolab President David Lillington, who added that the record has been broken about every six to nine months. "Fortunately, we've been able to be the world record holder for a number of years and it's probably unrealistic to expect to maintain the record 100 percent of the time, but we have a roadmap that will hopefully put us back in the lead."
These cells are targeted at concentrating-photovoltaic projects, which use mirrors or lenses to concentrate sunlight into the cells. The projects require far fewer – and smaller – cells than those used in traditional solar panels, enabling companies to use more expensive, but more efficient, materials. CPV cells use 400 times less semiconductor material than conventional PV cells, Lillington said.
Typically, Spectrolab has been able to turn lab cells into commercial cells in about two years, Lillington said. "These are more than laboratory curiosities," he said. The company is already producing cells with 38.5 percent average efficiency today, and plans to launch a new cell with 40 percent average efficiency in the first quarter of 2011 and a 41.5 percent efficiency cell in late 2012 or early 2013, he added.
These cells could theoretically reach 55 to 60 percent efficiency, but realistically, Lillington said he thinks they will top out in the 45 percent range somewhere between 2015 and 2020. The company plans to grow its production up from 40 megawatts of CPV cells this year to roughly 120 megawatts next year, he said.
When Does the Drive to Efficiency End?
The push toward higher efficiency is likely to peak at some point when efficiency gains are no longer commercially viable, said Chris O'Brien, head of market development at Oerlikon Solar. "That's always been the case with PV where you want to make sure you're not pursuing the perfect efficiency to the point where [it's no longer cost-effective,]" he said. "Many times increasing the efficiency will increase the cost of the cell of module, and you want to make sure that the added value outweighs the cost of getting that higher efficiency."